Presence of hybrid seeds (spawn/hatchlings and fry) in hatchery populations in four Indian states (Odisha, Gujarat, Bihar and West Bengal), n = 685.
\r\n\tIn sum, the book presents a reflective analysis of the pedagogical hubs for a changing world, considering the most fundamental areas of the current contingencies in education.
",isbn:"978-1-83968-793-8",printIsbn:"978-1-83968-792-1",pdfIsbn:"978-1-83968-794-5",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"b01f9136149277b7e4cbc1e52bce78ec",bookSignature:"Dr. María Jose Hernandez-Serrano",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/10229.jpg",keywords:"Teacher Digital Competences, Flipped Learning, Online Resources Design, Neuroscientific Literacy (Myths), Emotions and Learning, Multisensory Stimulation, Citizen Skills, Violence Prevention, Moral Development, Universal Design for Learning, Sensitizing on Diversity, Supportive Strategies",numberOfDownloads:null,numberOfWosCitations:0,numberOfCrossrefCitations:null,numberOfDimensionsCitations:null,numberOfTotalCitations:null,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"September 14th 2020",dateEndSecondStepPublish:"October 12th 2020",dateEndThirdStepPublish:"December 11th 2020",dateEndFourthStepPublish:"March 1st 2021",dateEndFifthStepPublish:"April 30th 2021",remainingDaysToSecondStep:"3 months",secondStepPassed:!0,currentStepOfPublishingProcess:4,editedByType:null,kuFlag:!1,biosketch:"Dr. Phil. Maria Jose Hernandez Serrano is a tenured lecturer in the Department of Theory and History of Education at the University of Salamanca, where she currently teaches on Teacher Education. She graduated in Social Education (2000) and Psycho-Pedagogy (2003) at the University of Salamanca. Then, she obtained her European Ph.D. in Education and Training in Virtual Environments by research with the University of Manchester, UK (2009).",coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"187893",title:"Dr.",name:"María Jose",middleName:null,surname:"Hernandez-Serrano",slug:"maria-jose-hernandez-serrano",fullName:"María Jose Hernandez-Serrano",profilePictureURL:"https://mts.intechopen.com/storage/users/187893/images/system/187893.jpg",biography:"DPhil Maria Jose Hernandez Serrano is a tenured Lecturer in the Department of Theory and History of Education at the University of Salamanca (Spain), where she currently teaches on Teacher Education. She graduated in Social Education (2000) and Psycho-Pedagogy (2003) at the University of Salamanca. Then, she obtained her European Ph.D. on Education and Training in Virtual Environments by research with the University of Manchester, UK (2009). She obtained a Visiting Scholar Postdoctoral Grant (of the British Academy, UK) at the Oxford Internet Institute of the University of Oxford (2011) and was granted with a postdoctoral research (in 2021) at London Birbeck University.\n \nShe is author of more than 20 research papers, and more than 35 book chapters (H Index 10). She is interested in the study of the educational process and the analysis of cognitive and affective processes in the context of neuroeducation and neurotechnologies, along with the study of social contingencies affecting the educational institutions and requiring new skills for educators.\n\nHer publications are mainly of the educational process mediated by technologies and digital competences. Currently, her new research interests are: the transdisciplinary application of the brain-based research to the educational context and virtual environments, and the neuropedagogical implications of the technologies on the development of the brain in younger students. Also, she is interested in the promotion of creative and critical uses of digital technologies, the emerging uses of social media and transmedia, and the informal learning through technologies.\n\nShe is a member of several research Networks and Scientific Committees in international journals on Educational Technologies and Educommunication, and collaborates as a reviewer in several prestigious journals (see public profile in Publons).\n\nUntil March 2010 she was in charge of the Adult University of Salamanca, by coordinating teaching activities of more than a thousand adult students. She currently is, since 2014, the Secretary of the Department of Theory and History of Education. Since 2015 she collaborates with the Council Educational Program by training teachers and families in the translation of advances from educational neuroscience.",institutionString:"University of Salamanca",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"0",totalChapterViews:"0",totalEditedBooks:"0",institution:{name:"University of Salamanca",institutionURL:null,country:{name:"Spain"}}}],coeditorOne:null,coeditorTwo:null,coeditorThree:null,coeditorFour:null,coeditorFive:null,topics:[{id:"23",title:"Social Sciences",slug:"social-sciences"}],chapters:null,productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"301331",firstName:"Mia",lastName:"Vulovic",middleName:null,title:"Mrs.",imageUrl:"https://mts.intechopen.com/storage/users/301331/images/8498_n.jpg",email:"mia.v@intechopen.com",biography:"As an Author Service Manager, my responsibilities include monitoring and facilitating all publishing activities for authors and editors. From chapter submission and review to approval and revision, copyediting and design, until final publication, I work closely with authors and editors to ensure a simple and easy publishing process. I maintain constant and effective communication with authors, editors and reviewers, which allows for a level of personal support that enables contributors to fully commit and concentrate on the chapters they are writing, editing, or reviewing. I assist authors in the preparation of their full chapter submissions and track important deadlines and ensure they are met. I help to coordinate internal processes such as linguistic review, and monitor the technical aspects of the process. As an ASM I am also involved in the acquisition of editors. Whether that be identifying an exceptional author and proposing an editorship collaboration, or contacting researchers who would like the opportunity to work with IntechOpen, I establish and help manage author and editor acquisition and contact."}},relatedBooks:[{type:"book",id:"6942",title:"Global Social Work",subtitle:"Cutting Edge Issues and Critical Reflections",isOpenForSubmission:!1,hash:"222c8a66edfc7a4a6537af7565bcb3de",slug:"global-social-work-cutting-edge-issues-and-critical-reflections",bookSignature:"Bala Raju Nikku",coverURL:"https://cdn.intechopen.com/books/images_new/6942.jpg",editedByType:"Edited by",editors:[{id:"263576",title:"Dr.",name:"Bala",surname:"Nikku",slug:"bala-nikku",fullName:"Bala Nikku"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"1591",title:"Infrared Spectroscopy",subtitle:"Materials Science, Engineering and Technology",isOpenForSubmission:!1,hash:"99b4b7b71a8caeb693ed762b40b017f4",slug:"infrared-spectroscopy-materials-science-engineering-and-technology",bookSignature:"Theophile Theophanides",coverURL:"https://cdn.intechopen.com/books/images_new/1591.jpg",editedByType:"Edited by",editors:[{id:"37194",title:"Dr.",name:"Theophanides",surname:"Theophile",slug:"theophanides-theophile",fullName:"Theophanides Theophile"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"3092",title:"Anopheles mosquitoes",subtitle:"New insights into malaria vectors",isOpenForSubmission:!1,hash:"c9e622485316d5e296288bf24d2b0d64",slug:"anopheles-mosquitoes-new-insights-into-malaria-vectors",bookSignature:"Sylvie Manguin",coverURL:"https://cdn.intechopen.com/books/images_new/3092.jpg",editedByType:"Edited by",editors:[{id:"50017",title:"Prof.",name:"Sylvie",surname:"Manguin",slug:"sylvie-manguin",fullName:"Sylvie Manguin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"3161",title:"Frontiers in Guided Wave Optics and Optoelectronics",subtitle:null,isOpenForSubmission:!1,hash:"deb44e9c99f82bbce1083abea743146c",slug:"frontiers-in-guided-wave-optics-and-optoelectronics",bookSignature:"Bishnu Pal",coverURL:"https://cdn.intechopen.com/books/images_new/3161.jpg",editedByType:"Edited by",editors:[{id:"4782",title:"Prof.",name:"Bishnu",surname:"Pal",slug:"bishnu-pal",fullName:"Bishnu Pal"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"72",title:"Ionic Liquids",subtitle:"Theory, Properties, New Approaches",isOpenForSubmission:!1,hash:"d94ffa3cfa10505e3b1d676d46fcd3f5",slug:"ionic-liquids-theory-properties-new-approaches",bookSignature:"Alexander Kokorin",coverURL:"https://cdn.intechopen.com/books/images_new/72.jpg",editedByType:"Edited by",editors:[{id:"19816",title:"Prof.",name:"Alexander",surname:"Kokorin",slug:"alexander-kokorin",fullName:"Alexander Kokorin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"1373",title:"Ionic Liquids",subtitle:"Applications and Perspectives",isOpenForSubmission:!1,hash:"5e9ae5ae9167cde4b344e499a792c41c",slug:"ionic-liquids-applications-and-perspectives",bookSignature:"Alexander Kokorin",coverURL:"https://cdn.intechopen.com/books/images_new/1373.jpg",editedByType:"Edited by",editors:[{id:"19816",title:"Prof.",name:"Alexander",surname:"Kokorin",slug:"alexander-kokorin",fullName:"Alexander Kokorin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"57",title:"Physics and Applications of Graphene",subtitle:"Experiments",isOpenForSubmission:!1,hash:"0e6622a71cf4f02f45bfdd5691e1189a",slug:"physics-and-applications-of-graphene-experiments",bookSignature:"Sergey Mikhailov",coverURL:"https://cdn.intechopen.com/books/images_new/57.jpg",editedByType:"Edited by",editors:[{id:"16042",title:"Dr.",name:"Sergey",surname:"Mikhailov",slug:"sergey-mikhailov",fullName:"Sergey Mikhailov"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"371",title:"Abiotic Stress in Plants",subtitle:"Mechanisms and Adaptations",isOpenForSubmission:!1,hash:"588466f487e307619849d72389178a74",slug:"abiotic-stress-in-plants-mechanisms-and-adaptations",bookSignature:"Arun Shanker and B. Venkateswarlu",coverURL:"https://cdn.intechopen.com/books/images_new/371.jpg",editedByType:"Edited by",editors:[{id:"58592",title:"Dr.",name:"Arun",surname:"Shanker",slug:"arun-shanker",fullName:"Arun Shanker"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"878",title:"Phytochemicals",subtitle:"A Global Perspective of Their Role in Nutrition and Health",isOpenForSubmission:!1,hash:"ec77671f63975ef2d16192897deb6835",slug:"phytochemicals-a-global-perspective-of-their-role-in-nutrition-and-health",bookSignature:"Venketeshwer Rao",coverURL:"https://cdn.intechopen.com/books/images_new/878.jpg",editedByType:"Edited by",editors:[{id:"82663",title:"Dr.",name:"Venketeshwer",surname:"Rao",slug:"venketeshwer-rao",fullName:"Venketeshwer Rao"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"4816",title:"Face Recognition",subtitle:null,isOpenForSubmission:!1,hash:"146063b5359146b7718ea86bad47c8eb",slug:"face_recognition",bookSignature:"Kresimir Delac and Mislav Grgic",coverURL:"https://cdn.intechopen.com/books/images_new/4816.jpg",editedByType:"Edited by",editors:[{id:"528",title:"Dr.",name:"Kresimir",surname:"Delac",slug:"kresimir-delac",fullName:"Kresimir Delac"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}}]},chapter:{item:{type:"chapter",id:"71911",title:"Hybridization in Carps and Early Detection of Carp Hybrids Using PCR-Based Kit",doi:"10.5772/intechopen.91946",slug:"hybridization-in-carps-and-early-detection-of-carp-hybrids-using-pcr-based-kit",body:'Hybridization is defined as the mating of genetically differentiated individuals or groups and may involve crosses within a species (also known as line crossing or strain crossing) or crosses between separate species [1]. Issues related to hybridization are complex, making the job of conservation biologists tougher. Interspecific and intergeneric hybridization do happen naturally, and it is considered to play an important role in evolution process [2]. Indian major carps (IMCs) comprising of Labeo rohita, Catla catla, and Cirrhinus mrigala, owing to their fast growth and taste, enjoy a prime position in the Indian aquaculture scenario. These carps attain a marketable size of 800–1000 g in less than a year and are generally propagated on an extensive and/or intensive scale in a polyculture system [3]. Among the three IMCs, catla and rohu are generally chosen for freshwater aquaculture due to their faster growth. IMCs, though originally inhabitants of the Ganga River network in North India and the rivers of Pakistan, Bangladesh, Nepal, and Burma, are also transplanted into other rivers in central as well as peninsular India and also in aquaculture systems of Nepal and Sri Lanka. In 2005 global carp production reached 28.8 million tons, accounting for 37.5% in quantity and 25.6% in value of total aquaculture production [4]. China with 21.05 and India with 3.9 million tons were the top carp producers in the world. Indian major carps catla (2.76 million tons), rohu (2.91 million tons), and mrigal (0.47 million tons) were among 29 species with production over 100 t in 2005 [4].
It was noted that the growth of IMCs affected the grow-out culture phase and the profitability in carp farming is decreasing [5, 6]. The growth of carps is affected largely due to poor quality and mixed seed of carps produced by breeding carps of different species at the same time in the spawning pools of carp hatcheries that make easy hybrids. To make more profit, hatchery managers practice this in breeding programs in hatcheries when there is paucity of brood fish of desired species that is either males or females. This practice has been rampant in many parts of the Indian subcontinent. Unlike other agricultural crops and domestic land animals, the hybrids did not grow better than their native natural parents. Good quality seed of catla and rohu are in great demand in the Indian subcontinent. Seed is one of the indispensable resources needed for aquaculture. Taking the advantage of seed demand of these two species, many hatchery owners and seed producers supply hybrid seed (catla × rohu or rohu × catla) of these two species in the name of pure rohu or catla during the young stages. These hybrids cannot be easily differentiated from each other morphologically at early stages of development, e.g., hatchling and early fry stages. Among the most pressing issues concerning seed in global aquaculture development include inadequate and unreliable supply of quality seed, genetic quality, and inadequate hatchery technology.
For production of quality seed in aquaculture, many methods were tried in the past, including intergeneric and interspecific hybridization. Hybridization technique was used by aquaculturists in the hope of producing aquatic organisms with specific desirable traits or general improvement in performance. Intergeneric and interspecific hybridization programs have been applied in fish farms with the purpose of producing animals that perform better than the parental species (hybrid vigor) [1].
Nevertheless, many species are jeopardized by hybridization and genetic introgression, and these are particularly prevalent threats to the diversity of freshwater fish [7, 8, 9, 10]. If fertile, hybrids can genetically contaminate natural and farmed stocks through genetic homogenization. They may also compete in several ways with the native parental lineages [2, 11, 12].
Currently, several genetic markers have been developed for different species that are used in aquaculture programs [13, 14]. PCR-RFLP and multiplex PCR are considered fast, simple, and inexpensive, but they have rarely been used in the characterization of hybrids or in aquaculture in general. These techniques are important tools in species identification, especially in studies related to the biological conservation and forensic identification [15, 16]. Natural occurrence of both interspecific and intergeneric hybrids of Indian major carps has been reported mostly from reservoirs and other natural ecosystems. From natural ecosystems such as reservoir and dry bundhs, several hybrids have been recorded [17]. Many of these hybrids were found to be intermediate in characters of the parent species. Only a few hybrids, both artificially produced and naturally occurring, have been studied in detail for their cultural qualities and adaptability to various environments. Several interspecific and intergeneric hybrids of Indian major carps are Catla catla, Labeo rohita, Cirrhinus mrigala, and Labeo calbasu [18, 19], and those of Indian major carps with exotic carps, viz., common carp [20] and silver carp [21], have been artificially produced through hypophysation. These hybrids were not popular due to poor survival and many undesired traits in aquaculture.
Quality seed is a fundamental prerequisite for sustainable and successful aquaculture, be it small-scale or commercial farming. Inadequate supply of quality seed is often suggested as a major constraint for aquaculture in many parts of the world [22]. The issue of quality comes to the attention of producers only after a certain period of time when performance indicators (e.g., growth, production, survival, and disease) consistently point a finger towards seed quality. Factors which contribute to production of poor quality seed would have become established as a normal practice in the system.
Field level morphological identification of seeds of commercially important carp species, viz., catla and rohu, at early stage is a difficult task as experience has shown. Many times fish farmers get cheated by unethical practices by hatchery owners as they sell these hybrids in the name of pure rohu or catla seed. At that point of time, it is almost impossible for the farmers to recognize the differences. So after nursery rearing, poor survival and growth of these seeds become a bane for them. Even though they approach the fishery officials for law enforcement, they also feel helpless in the absence of a genuine identification method at this early stage.
An attempt has been made to review hybridization in carps (Cyprinidae), focusing on the advantages and disadvantages of human-mediated hybridization in cultured species of carps and detection methods of carp hybrids. Further, the development of a novel multiplex PCR-based approach for the identification of seeds of rohu and catla and their hybrids (parents and hybrids), using some molecular markers, and its field testing with hatcheries in four Indian states are described.
The major purpose behind human-mediated hybridization is genetic improvement in cultured species. This is expectedly achieved through combining desirable traits of parental species, resulting in heterosis or hybrid vigor in the progeny. In fishes human-mediated hybridization is around 50% [23]. In natural hybridization reproductive barriers prevent introgression to happen. That is, chances of F1 hybrid mating with parental species are remote. On the other hand, in human-mediated hybridization in which the rate is higher and the number of escapees is more, the reproductive barriers are broken down, leading to introgression which may eventually lead to genetic extinction (Figure 1).
Schematic representation of natural and human-mediated hybridization.
Hybridization is more widespread among members of Cyprinidae than any other groups of freshwater fishes [23]. Several permutation and combinations of interspecific and intergeneric crosses were carried out among Indian major carps (Catla catla, Labeo rohita and Cirrhinus mrigala) and exotic carps (Hypophthalmichthys molitrix, Ctenopharyngodon idella, Aristichthys nobilis and Cyprinus carpio) with the major objective to achieve hybrid vigor in economic traits [24, 25, 26, 27, 28, 29, 30, 31]. The Indian major carp species are known to be able to hybridize, and hybrids are fertile and can be backcrossed to the parental species [32, 33, 34]. Hybridization has been shown to have a significant impact on production-related traits (notably growth), with some studies reporting a growth rate of F1 hybrids intermediate between that of the parent species [35, 36]. Hybrids are reported to have lower rates of growth than either of the parental species of Indian carps, other studies reporting growth rates lower than for either of the parental species [37]. Intergeneric hybrids between catla (Catla catla) and fimbriatus (Labeo fimbriatus) were produced by employing the technique of hypophysation and dry stripping. Detailed investigations on their embryonic and larval development, taxonomic characters, and aquaculture potential in terms of growth, feed utilization, body carcass composition, meat yield, etc. were carried out [28].
A catla-rohu hybrid produced by hypophysation was found to be intermediate in general appearance to the parent species. Gut content analysis revealed that the hybrid was mainly phytophagous in its diet. Growth rate was observed to be faster than in rohu. It matured within 3 years and was equally responsive to hypophysation [38]. In other cyprinid species, also such efforts of hybridization were carried out [23, 39].
It has long been recognized that hybridization can have a variety of evolutionary outcomes, including outcomes that maintain or increase diversity such as stable hybrid zones, the evolutionary rescue of small inbred populations, the origin and transfer of adaptations, the reinforcement of reproductive isolation, and the formation of new hybrid lineages [40, 41].
Hybridization in carps was being carried out to increase growth rate, transfer desirable traits between species, combine desirable traits of two species into a single group of fishes, reduce unwanted reproduction through production of sterile fish or mono-sex offspring, take advantage of sexual dimorphism, increase harvestability, increase environmental tolerances, and increase overall hardiness in culture conditions. Hybrids constitute a significant proportion of some countries’ production for certain taxa, for example, hybrid striped bass in the USA, hybrid clarid catfish in Thailand, hybrid characids in Venezuela, and hybrid tilapia in Israel. Hybridization has been used in tandem with polyploidization to improve developmental stability in hybrid progeny [1]. Intergeneric hybrids between catla (Catla catla) and fimbriatus (Labeo fimbriatus) combined desirable qualities such as the small head of the fimbriatus and the deep body of the catla and exhibited heterosis in terms of meat yield with higher flesh content than either of the parents. Hence the hybrids appear to be of considerable importance to aquaculture [28]. It is also believed that hybrids of parental genotypes might be able to explore ecological niche unavailable to the latter. It can lead to adaptation through creation of novel genotypes and morphologies. Further, hybrid vigor or heterosis can also occur [39]. Interspecific hybrids are created purposely to enhance productivity of aquacultural strains [42]. Hybridization is done also to enhance recreational angling opportunities [43].
Evolutionary evidence that hybridization is a constructive process was mentioned, as (a) reproductive barriers, both pre- and post-zygotic, between described species appear incomplete among many fishes, (b) permanent transfer of genetic information apparently is possible even when hybrids and backcrosses are under negative selection, and (c) genetic exchange through introgressive events may have significant effects on the genetic composition of a species, and thereby, actually contribute to diversity within taxa [7]. Studies of interspecific hybridization between the Siberian sturgeon and Russian sturgeon showed that the hybrids had higher survival and growth than the purebreds under provided hatchery conditions. The hybrid crosses displayed positive average heterosis in most of the assessment times for growth and survival traits, and better fitness-related traits than purebreds, thus suggesting that interspecific hybridization provides a survival advantage to sturgeons during their evolutionary period [44].
The harmful effects of hybridization, with or without introgression, have led to the extinction of many populations and species in many plant and animal taxa. Hybridization is especially problematic for rare species that come into contact with other species that are more abundant [2]. Hybridization can decrease diversity through the breakdown of reproductive barriers, the merger of previously distinctive evolutionary lineages, and the extinction of populations or species. There are two main mechanisms by which hybridization can lead to extinction. If hybrid fitness is strongly reduced relative to that of parental individuals (i.e., outbreeding depression), and hybridization is common, population growth rates of one or both parental lineages may decline below replacement rates due to wasted reproductive effort, leading to extinction [41, 45]. Hybridization involving captive-bred individuals can have harmful consequences beyond the loss of genetic integrity [46]. In many cases, the stocked individuals differ genetically from the target population, which can result in outbreeding depression following hybridization [45].
Inadvertent hybridization and backcrossing can lead to unexpected and undesirable results in hybrid progeny, such as failure to produce sterile fish, loss of color pattern, and reduced viability. Uncontrolled and unintentional hybridization could undermine the performance of cultured stocks and restrict future use of the contaminated stocks as broodstock. The level of unintentional or accidental hybridization has important considerations for the conservation of aquatic biodiversity and will influence risk assessment on the use of hybrid fishes in aquaculture and fisheries [1]. Continued hybridization may eventually lead to a breakdown of species barriers, thereby compromising the genetic integrity of the species in the wild and leading to production losses in aquaculture [30]. Hybrid introgression in major carp species is very likely to have negative consequences, as a result of loss of distinct feeding strategies of the pure species, which are the basis of successful polyculture systems [46].
In Indian major carps, inadvertent production of hybrids out of mixed spawning of species has been well documented. Actually the farmers are on the lookout for “mixed seeds,” meaning a certain proportion of catla, rohu, and mrigal along with other exotic carps for polyculture [47]. For the sake of time and economy, the hatchery producers keep broodfish of different species, particularly rohu and catla, in the same breeding pool, resulting in unintentional production of the hybrid seeds [34, 48]. The intergeneric hybrids are fertile, and they can breed (backcross) with parental species to produce introgressed F2 hybrids. The thoughtless and injudicious ways of fish breeding are likely to affect the “gene pools” of these prized food fishes badly [27].
Silver carp and bighead carp sometimes are hybridized inadvertently because of their similar appearance and because of shortage of “the correct” species at spawning time due to differences in maturation times between male and female carp. This hybridization often results in a fish that does not feed efficiently as its gill rakers are intermediate in shape between those of the silver carp that eats phytoplankton and those of the bighead carp that consumes zooplankton [1]. The rohu-catla reciprocal hybrids are reported to have limited economic value [27]. These hybrids are also reported to be more susceptible to parasitic infection than the parental species [49]. Hybridization between silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis) suggests further generations of hybridization or introgression between the species in hatcheries, with potentially damaging consequences for the integrity of these stocks and their performance in aquaculture [50]. Pecos pupfish (Cyprinodon pecosensis) is threatened with replacement by its hybrids with sheepshead minnow (C. variegatus) [12]. Continued hybridization between invasive bigheaded carps (Hypophthalmichthys nobilis) and silver carp (Hypophthalmichthys molitrix) has indicated reduced nutritional performance of their progeny [29].
Accurate identification of hybrids is important not only for sustainable aquaculture development, guiding aquaculture domestication efforts, assessing aquaculture production, and identifying useful crosses, but also to allow for a better understanding of biodiversity issues. It would be unfortunate to experience a widespread loss of pure species in aquaculture as happened with tilapia as a result of widespread introduction and subsequent hybridization; it would be also a significant cause for concern if hybrid Thai catfish or the hybrid Venezuelan characids pose more of a threat to local species than the pure species [51].
Before 1966 only morphology-based methods were used to identify hybrids. Subsequently followed by morphology (45%), allozymes (35%), mtDNA (12%), nDNA (4%), and karyology (2%) were used till the late 1990s. Genetic markers and population genetic theory have provided powerful tools facilitating the description of hybridization events and serve as sources of evidence for factors underlying occurrence, direction, and extent of introgression between fish taxa [23]. VNTR minisatellite and microsatellite loci, SINE’s, RAPD, AFLP, and ISSR assume dominance, whereby individuals are characterized by the presence or absence of amplification products of specific size. The number of alleles producing a product (one for heterozygotes and two for a homozygote) cannot be directly determined. Thus, the per-locus information context of dominant markers is less than for codominant loci. Mitochondrial DNA cannot be used alone to detect hybrids because of the marker’s haploid and matrilineal mode of inheritance. However, mtDNA can be a powerful tool to establish directionality when used in conjunction with nuclear genetic markers [23]. Genetic markers, such as allozymes, mtDNA, and nuclear DNA, were used to confirm hybrid status and to determine directionality of the hybridization event [12]. Multiple markers were employed to determine if an Icelandic population of eels (Anguilla anguilla) included hybrid individuals from matings of parents originating from populations in North America and Europe [43].
Historically, meristic and morphometric measurements were the primary means of identifying naturally occurring hybrids. The introduction of allozyme electrophoresis provided a methodology whereby individuals of most species could be assayed for biochemical markers with a demonstrable heritable basis [52]. The use of mtDNA was first cited in the surveyed literature in analyses of fish hybridization in the mid-1980s [4]. In the late 1980s, nuclear DNA was started to be used for identifying hybridization process [53, 54].
Documentation of hybridization often has been based on meristic or morphological criteria that can be misleading when used as the sole source of inference, particularly for hybrid individuals beyond the F1 generation [55]. Morphology, allozymes, and mtDNA were used in the analysis of Notropis chrysocephalus and N. cornutus hybrid zones in Michigan drainages [56]. Genomic RFLP was used to detect hybrids of Indian major carps, and the results indicated that intergeneric hybridization did occur during “mixed spawning” of these carps and the hybridization frequency was appreciable, at about 10% [34]. Utilizing an integrated approach, which incorporates geometric morphometrics, life history, and molecular genetic analyses, the levels and processes of hybridization in two species of cyprinids were determined [39]. The extent of intergeneric hybridization in Indian major carps was studied using allozymes [30]. DNA fingerprinting using RAPD-ISSR assay was used to detect hybridization in Indian major carps [57].
For the parental lineages, 50 individuals of Labeo rohita (rohu) and 50 individuals of Catla catla (catla) were genetically analyzed. Crosses performed by mating females of catla and males of rohu and vice versa resulted in the intergeneric hybrid (Figure 2). Twenty-four hybrid individuals were included in the genetic analysis. Spawns of reciprocal hybrids were collected for further genetic analysis. DNA was extracted from the fin clips of adults of parental species using standard phenol-chloroform method [58].
Catla (PCc), rohu (PLr) parents, F1 hybrid (CcLrF1Hy), and F2 backcross hybrids (bcF2Hy).
Total genomic DNA from spawn was isolated using DNeasy blood and tissue kit, Qiagen. DNA quantity was determined against a molecular marker standard (λ-DNA 25 ng, Fermentas) by electrophoresis in a 0.8% agarose gel. β-actin sequences of carps available in GenBank were downloaded (Accession numbers: AF415205, M24113, GU338376, AY531753, AF415206) and aligned using Clustal W program implemented in the software Bioedit version 7.0.5.3 [59], and conserved primers for the amplification of a fragment size of ~ 1000 bp in Indian major carps and minor carps were done manually. Genomic DNA (~20–100 ng) from both species of IMCs was amplified in a 25 μl PCR volume containing 10 picomoles of each conserved primer, 2.5 mM of each dNTP, and 0.25 U of Taq polymerase with a thermal regime of 94°C (5 min), 35 cycles at 94°C (0.5 min), 60°C (0.5 min) and 1 min at 72°C (1 min) and final extension of 72°C (5 min). PCR products were purified using Qiagen PCR purification kit followed by bidirectional cycle sequencing on ABI 3100 PE automated capillary sequencer.
A total of 20 sequences of both the species (10 Labeo rohita and 10 Catla catla) were aligned using Clustal W program in Bioedit software. Species-specific reverse primers for both species were designed, taking the species-specific mutation into account. A touchdown PCR was carried out with a 25 μl PCR volume containing 10 picomoles of each species-specific reverse primers (one rohu and one catla) and 20 picomoles of universal forward primer, 2.5 mM of each dNTP, and 0.25 U of Taq polymerase with the PCR condition of 94°C (5 min), 2 cycles at 94°C (0.5 min), 68°C (0.5 min) and 1 min at 72°C (1 min), 2 cycles at 94°C (0.5 min), 66°C (0.5 min) and 1 min at 72°C (1 min), 2 cycles of at 94°C (0.5 min), 64°C (0.5 min) and 1 min at 72°C (1 min), 25 cycles at 94°C (0.5 min), 62°C (0.5 min) and 1 min at 72°C (1 min) and final extension of 72°C (5 min). The PCR products were checked in a 2% agarose gel.
Partial sequences of the nuclear β-actin gene amplified using a set of primers BAF (5′GTAGGCACGACATTGAATGGG3′) and BAR (5′AGACAAAGGAAGTCCCTCTGC3′) generated a total of 820 bp which revealed some differences in the nucleotide composition between Labeo rohita and Catla catla. Single-nucleotide polymorphism was found between the species which were used to design species-specific internal primers.
Two primers were designed specific for each species considering the polymorphic sites in the sequences. Both primers designed were in the reverse direction: primer BALRR (5′-CTTGAAAACTGTACAATCACGTTC-3′) was specific for Labeo rohita, and BACCR (5′-GCTAGCTAATAGACGTAATCATTTAG-3′) was specific for Catla catla. Amplification of these primers (BAF, BALRR, and BACCR) established different electrophoretic banding patterns when run in a 2% agarose gel. The result revealed one band at about 100 bp specific for L. rohita and another band at 300 bp specific for C. catla. In the rohu × catla hybrid, a heterozygote pattern was observed with two diagnostic bands, with each one inherited from one parental strain. Using these species diagnostic primers, a PCR-based rohu-catla hybrid identification kit was developed which has received provisional Indian patent number “343/KOL/2013 of 26.3.2013.” For the validation of the developed kit, a total number of 685 samples from different places were screened which revealed that 54 out of them were hybrids (Figure 3).
PCR test of hatchery spawn samples (300 bp marker in catla, 100 bp marker in rohu, and both markers in hybrid).
The use of a multiplex PCR marker in the present study revealed a distinct electrophoretic pattern between rohu and catla and their hybrid. The advantage of multiplex PCR is that it does not require the additional step of restricted enzyme digestion and can thus eliminate any post-PCR analyses as well as additional time and costs. On the other hand, there are limitations to the primer designs that should be taken into consideration. The primers should be specific and reliable in binding. This study can serve as a basis for further study on the introgression of these hybrids with their parental species. Genetic monitoring of mixed spawning and unintended hybridization of Indian major carps in hatcheries can be carried out with the help of the kit (Figure 4).
Rohu-catla early hybrid identification kit.
Contents of the kit are mentioned below:
Species-specific primers
Universal primer
dNTP mix (2.5 mM each)
10X Taq DNA buffer
Taq DNA polymerase 3 U/μl
Positive control DNA 100 ng
Nuclease-free water
100 bp ladder
Advantages and utility of this kit are summarized below:
PCR and agarose gel-based detection kit
No sequencing required
Takes only 4–6 h to get the results
Highly sensitive and specific for rohu × catla hybrids and reciprocal crosses
Useful for screening of hatcheries for genetic contamination
Potential for seed certification hatchery accreditation
An essential tool for government/private agencies to ensure purity of seed
A total of 685 samples of different life stages (spawn and fry) from different hatcheries of four Indian states, Odisha, Gujarat, Bihar, and West Bengal (Table 1), were collected and tested using the species-specific designed primers with the same thermal cycler condition [60].
State | Details of hatchery | Samples (n) | Hybrid (n) | Hybrid (%) |
---|---|---|---|---|
Odisha | State Fisheries Farm, Kausalyaganga | 150 | 6 | 4 |
Odisha | State Fisheries Farm, Bhadrak | 30 | 0 | 0 |
Odisha | State Fisheries Farm, Balasore | 30 | 2 | 7 |
Odisha | Balakati Private Hatchery | 25 | 0 | 0 |
Odisha | Balakati Private Hatchery | 25 | 3 | 12 |
Gujarat | Private Hatchery | 45 | 23 | 51 |
Bihar | Shri Tripura Chaudhary, Matsya Farm, Vaishali | 30 | 0 | 0 |
Bihar | Asha Fish Breeding Centre, Baheri Block, Darbhanga | 30 | 3 | 10 |
Bihar | Kamla Fish Hatchery, Jhajarpur, Madhubani | 30 | 2 | 7 |
Bihar | Koshi Fish Hatchery, Madhubani | 30 | 5 | 17 |
Bihar | Ganga Fish Hatchery, Madhubani | 30 | 4 | 13 |
Bihar | Yamuna Fish Hatchery, Madhubani | 30 | 6 | 20 |
West Bengal | Naihati Market | 200 | 137 | 68.5 |
Presence of hybrid seeds (spawn/hatchlings and fry) in hatchery populations in four Indian states (Odisha, Gujarat, Bihar and West Bengal), n = 685.
There have been numerous studies on hybridization of fishes, and certainly not all of the hybrids reported are contributing to commercial aquaculture production. Accurate identification of hybrids is important not only for sustainable aquaculture development, guiding aquaculture domestication efforts, assessing aquaculture production, and identifying useful crosses, but also to allow for a better understanding of biodiversity issues.
Intergeneric hybrids between the species rohu (Labeo rohita) and catla (Catla catla) are being produced in Indian carp hatcheries without any monitoring. The parental species belong to the more representative genus of the family Cyprinidae, which is an important freshwater fish group that is widespread throughout India, and are important fishery resources to specific communities. Some of the hatcheries are practicing multispecies breeding (mainly rohu and catla) in the same breeding pool and at the same time leading to interspecific hybridization. There is every chance of these hybrids escaping to natural waters which would lead to pollution of the genetic material in the wild, leading to non-availability of the pure strains of the carps in the future. There seems to be a misunderstanding regarding the culture of mixed species for composite fish culture with mixed breeding of carps by the hatchery managers and fish farmers. When the farmers are demanding mixed seeds (mixture of pure rohu and catla), the hatchery managers are mixing the spawners to produce hybrids.
Hybrid identification based on morphology, ecology, and behavior can be difficult and, most of the time, confusing and uncertain. Multiplex PCR strategy has proven to be an efficient methodology that could be quickly and inexpensively executed, which would allow diagnoses through simple PCR based upon single-nucleotide polymorphisms.
Morphological differences of hybrids are only minor and need close examination by experienced workers, and it is difficult to verify the genuine hybrids from interspecific hybridization. Allozymes were used to detect the genetic difference between the hybrids and their parental species; the use of allozyme loci failed to provide a sufficient genetic basis of hybrids, probably due to the less informative nature of allozyme loci (i.e., limited number of polymorphic loci available and low level of polymorphisms). Mutation at the DNA level that causes a replacement of a similarly charged amino acid may not be detected by allozyme electrophoresis, although allozymes represent actual gene products. Molecular techniques have been applied in the worldwide aquaculture, allowing for an adequate management of several cultivated species and providing a huge number of molecular markers that have been applied successfully for hybrid identification and detecting genetic introgression in fish. Nuclear molecular markers have supplied valuable information in the detection of hybridization events as well as the identification of reciprocal hybrids. Since mitochondrial DNA in animals has the characteristic of maternal inheritance [61], they are not suitable to detect hybrids. On the other hand, nuclear DNA serves as an efficient tool for hybrid identification.
The use of multiplex PCR marker revealed a distinct electrophoretic pattern between Labeo rohita, Catla catla, and their hybrid. The advantage of multiplex PCR is that it does not require the additional step of restriction enzyme digestion, which can eliminate any post-PCR analyses as well as additional time and costs. On the other hand, there can be limitations to the primer designs that should be taken into consideration, where these mainly reflect the ability of the primers to have good specificity and reliability in the application [62].
Finally based on the personal experience and inferences from other related studies, the following policy guidelines are recommended:
Natural hybrids should be eligible for protection.
Human-induced hybridization undesirable in majority of situations is a threat to conservation of parental species and low/intermediate heterosis. It causes extinction of pure taxa by replacement and genetic mixing. Hence it needs to be strictly regulated/eliminated.
Human-mediated hybrids shall be protected only in exceptional circumstances, such as when hybrids contain the only remaining genetic information from a taxon that has otherwise been lost by genetic mixing or when the circumstances of their origin are unclear.
In Indian major carps, mixed spawning of different species in hatcheries is to be stopped.
Good hatchery practices are paramount, and genetic monitoring of hatchery stocks on a regular basis is entailed to maintain the quality of fish seeds.
Unambiguous and rapid detection of hybrids at hatchery level is essential. The carp reciprocal early hybrid identification kit is useful for this purpose. Further, it is potentially useful tool for seed certification and hatchery accreditation.
I am thankful to my collaborators (Section 7) Mohanty M, Sahoo L, Das P, Das BK, Verma DK, and Routray P. This study was conducted when I was the Director of ICAR-CIFA, Bhubaneswar, during 2012–2017.
It is certified that there is absolutely no conflict of interest.
Plastics and materials based on plastic have become an acceptable replacement of metallic materials and, as a consequence, they have to face the challenge of having also a good tribological behavior, implying a set of characteristics favorable to a reliable functioning of the application.
\nIssues that an engineers (both designers and users) have to pay attention when using polymeric materials in tribological applications include dimensional stability. These materials have higher thermal expansion coefficients, shorter durability, sensitivity and particular behavior to high and low temperatures. As they are characterized by lower hardness, they are not prone to be introduced in rolling contacts, will few exceptions (here including car tires and gears), most applications being for sliding motion (belt, sliding bearing, seals, brakes etc.).
\nThe advantages of using polymeric materials (polymers, blends and composites) [1, 2, 3, 4, 5] include self-lubricity, lower density as compared to metallic materials, resistance to tribocorrosion [6] or general oxidation, non-toxic nature and potential processing to final shape, usually, by injection molding. But their favorable properties come in a package with disadvantages. One is that a slight change in working conditions (load, velocity, temperature etc.) could substantially modify tribological characteristics [7], especially wear rate and low friction is not related to low wear rate. Also, negative temperatures have different influences on polymeric materials (some become brittle, some resist without problems and some are conditioned by the working conditions and environment).
\n\nFigure 1 presents materials based on polymers and elastomers that could be used in tribological applications.
\nMaterials based on polymers and elastomers, involved in tribological applications [1, 2, 3, 4, 5].
When using polymeric materials, the designer should pay attention how the component will obey design requirements, if it has dimensional stability, mechanical characteristics with reliable values, if issues related to aging are acceptable for the component durability (life time). The design should be done so that the working conditions will vary in narrow ranges (temperatures, load, velocity, material composition and morphology) [8].
\nThe majority of tribological applications with polymeric materials are involving couples with one element made of metallic materials, the other being polymeric. Sometimes, the polymeric material is moving against a body made of the same materials, an example being gear transmissions.
\n\nFigure 2 summarizes the main aspects of tribological performance when using polymer-based materials.
\nA chart of significant aspects related to tribological performance implying polymeric materials.
\nTable 1 presents the polymers used in tribological applications, several features and usual components made of them.
\nPolymer | \nTribological characteristics | \n
---|---|
PTFE | \nLow friction, but high wear rate. Used both neat, as matrix and as solid lubricant. More recently, added in polymers, resulting polymer blends; in composite as solid lubricant or matrix in composite with reinforcements as glass fibers, carbon fibers, metallic powder as copper. High working temperature [9, 10] | \n
PA | \nModerate friction coefficient, low wear rate, but too sensitive to water and humidity. Working temperature quite low [11]. | \n
POM | \nSimilar performance as PA. Good durability in rolling contacts. | \n
PEEK | \nPolyetheretherketone, semicrystalline High working temperature and very good chemical resistance. Accept higher contact pressure but high friction coefficient as neat polymer [12, 13, 14] | \n
UHMWPE | \nVery good wear resistance, especially against abrasion, even in water. Moderate friction coefficient. Modest working temperature. | \n
PU | \nGood wear resistance in rolling contacts. Relatively high friction coefficient in sliding. | \n
PI | \nHigh performance polymer with very good behavior in high contact pressure. Higher friction coefficient. | \n
PBT | \nA reliable behavior in sliding contact, lower wear as PA, but more restrictive condition in molding. Usually with a solid lubricant or reinforcement [15, 16] | \n
PEI | \nAmorphous thermal stability, very good mechanical and physical properties, easy processability, applicability and possibility of recycling and repair, thermosetting polyimides, blended with PEEK [13] | \n
PES | \nAmorphous [17] | \n
PPS | \nSemicrystalline, polyphenylenesulphide, water lubrication high glass transition and high melting temperature and high mechanical strength, high COF on steel in dry regime (0.4...0.5), PPS + SWCNT (0.5 wt.%) + WS2 (1.5 wt.%) [18, 19] | \n
PPP | \nPolyparaphenylene, semicrystalline, very high mechanical stability at room temperature, poor wear resistance [12] | \n
PBI | \nPolybenzimidazole semicrystalline, high heat resistance and mechanical property retention, even under high temperatures [12] | \n
Epoxy and phenolic polymers | \nUsed especially as binder agents in composites, they induce high friction, but constant. Their brittleness induces wear by micro-detaching harder particles (as a dust) that could damage the smooth functioning of the tribosystem. The composites with these resins usually are designed for frictional applications (high and constant friction coefficient, with controlled wear evolution in time) | \n
Semi-crystalline polymers can be used even above their glass transition temperature (Tg), another added advantage against chemical constancy.
\nVarious inorganic nanofillers, e.g., from metals (Cu, Fe), metallic and non-metallic oxides (CuO, ZnO, TiO2, ZrO2, SiO2) and salts as silicon nitride (Si3N4), have been proved to not only enhancing mechanical properties, but also to lowering the friction coefficient and the rate of wear under various sliding circumstances. In particular, PEEK, PPS, and PTFE are the most widely studied polymers for different tribological applications and they are often blended with TiO2, SiC, Si3N4, and carbon fiber fillers [19]. Nevertheless, it is also noted that there are no single or combined polymers or fillers that provide the best tribological performance in all conditions. Being a result of “system responses”, friction and wear always depend on both the intrinsic material properties and the external environmental conditions. The beneficial effect of adding a certain material in a polymeric matrix is exemplified by tests did by Kurdi et al. [21] (Figure 3), 5–15% of TiO2 reducing friction and wear at room temperature, but not at elevated temperature. Thus, functioning conditions are tremendously important when selecting a pair of materials for a good or at least acceptable tribological behavior.
\nInfluence of percentage of TiO2 on (a) friction coefficient and (b) specific wear rate, for a pin-on-disk configuration, in sliding at v = 0.1 m/s, average pressure p = 1 MPa, for 2 hours [21].
Hanchi et al. [13] reported results on friction and wear under dry sliding of injection molded blends of PEEK and PEI, at temperatures from 20–232°C, on a pin-on-disk tribotester. It was found that tan δ peaks corresponding to α transitions occurring in the vicinity of the glass transition temperature (Tg) coincided with catastrophic tribological failure in the case of PEI and the amorphous PEEK/PEI blends. PEEK and the annealed 70% PEEK/30% PEI blend exhibited marked increases in friction and wear above the Tg. The absence of catastrophic tribological failure in PEEK and the annealed 70/30 blend in the vicinity of Tg corresponded to a transition of significantly lower strength those observed in PEI and the amorphous blends. Between 90°C and 105°C for PEI and 45°C and 70°C for the PEEK/PEI 50/50 blend, severe to mild friction and wear transitions were observed. It appeared that a substantial change in ductility associated with these β transitions resulted in the transitional tribological behavior.
\nUnal et al. reported the influence of test speed and load values on the friction and wear behavior of PTFE, POM and PEI, on a pin-on-disc tribotester. Tests were carried out at room temperature, under 5 N, 10 N and 15 N and at 0,5 m/s, 0,75 m/s and 1 m/s. The specific wear rates were deduced from mass loss. The results showed that, for all tested polymers, the coefficient of friction increases linearly with the increase in load. For the load and speed range of this investigation, the wear rate showed very low sensitivity to the applied load and large sensitivity to speed, particularly at high load values [22].
\nWhat do the engineers want from polymeric materials when introducing in tribological applications? A set of characteristics including thermal, mechanical and tribological ones:
higher softening temperatures, sometimes obtained by adding short glass fibers;
higher toughness; reinforcement could rise the flexural modulus till 11,000 MPa, a value that is overpass only by PPS in the thermoplastic polymers;
low or acceptable friction and high wear resistance;
good strength al negative temperature, including impact resistance;
no or very less liquid absorption (including water)
chemical resistance at fluids circulated in application (as lubricant or/and environment);
good dimensional stability; low thermal expansion;
good ability for compounding (mixing), when adding materials for reinforcement, solid lubricants, anti-ignition agents etc.,
good processing capability (uniform flow, fast solidification and acceptably low cost and improvement by treatment).
Based on important works on tribology of polymer-based materials [3, 20, 23, 24, 25]. Figure 4 presents a classification of adding materials taking into account the function of these materials in polymers. Generally, reinforcements [24, 25, 26, 27] and solid lubricants in polymer-based materials improve their tribological behavior, but it is not a rule and the new recipes should be tested at laboratory scale and then the designed components at actual scale and under functioning conditions. Some solid lubricants, especially with sheet-like aspects (graphite, graphite, sulphides etc.) weaken the bulk materials as they reduce the superficial energy, but the mechanical properties are diminishing. Reinforcements in polymers make their resistance greater, but generate a more intense abrasive wear on the counterpart surface and the friction coefficient is higher and the surface quality of both rubbing surfaces becomes worse. Reinforcements reduce or even damage the protective transfer films [28]. They could generate a sliding regime characterized by severe and third body wear [29]. For instance, the composite PA + 50% glass beads [11] exhibit a third body friction and wear, especially at low velocity (see Figures 35b and c).
\nA classification of materials in polymeric-based materials.
\nFigure 4 points out that adding materials in polymers have different roles (sometimes, they could act in two or more directions) and the influence of the set added in the basic polymer could be synergic [30], difficult to enclose in formula, thus, testing is a necessity. Figure 5 presents several reinforcements: a) micros glass beads with large dispersion of the bead radius (this is a favorable aspect because this large distribution allow for the small beads to fix the matrix next the bigger ones and wear is considerably reduced), b) short glass fibers with diameters of 8…20 μm and length of hundred microns [31] (similarly, carbon fibers are added in 10…20% wt), c) aramid fibers [16, 32] (more flexible and with nail-shape ends that help them to fix the polymer matrix).
\nAspect of reinforcements in polymers. (a) Glass beads used in [11, 15]. (b) Short glass fibers from LANXESS [31]. (c) Short aramid fibers Twaron [16].
Harder polymers and polymer composites with hard components are helped to reduce friction by adding solid lubricants with plaquette-like shape (graphite [33], graphene, disulphides [33], several examples being given in Figure 6) or polymers as PTFE, with more uniform transfer and having very low friction coefficient.
\nAspect of several solid lubricants introduced in polymers. Graphite [33]. Hexagonal plates of WS2 [34]. Hexagonal boron nitride (h-BN) [33].
Tribologists considers that short fibers are more beneficial for tribological application, but recently, the polymers with long fibers were also introduced as materials for moving parts due to the advances in fibers and polymer technology. There is a short discussion about fiber architecture. Usually, short and tangle fibers are randomly organized in the material, they rarely could be oriented, but the cost will increase. Long fibers could be organized in woven, unidirectional, multi-axial, depending on the other requirements besides the tribological one. Being organized, the wear of materials of long fibers is usually in steps, characterizing the damage of each layer of fibers. As fibers could have 5 to 50 microns, the wear of the first layers or two ones will end the life of the triboelement. The nature of the fibers is natural, synthetic or combination. For tribological application, there are used carbon fibers, carbon nanotubes, glass fibers (if short, from tens microns to hundreds of millimeters but more efficient being those of several hundred microns to several millimeters), polymer fibers, more recently, aramid fibers [16]. Particles as reinforcement could have different shapes, from almost spherical (as for glass beads) to sheet-like or plaquettes (one dimension being very small as compared to the other two). A particular aspect of wearing polymeric composites or blends is the initially preferential wear of the softer material, the result being an increase concentration of harder particles or fibers; then the counterpart body will “attack” these harder materials; they could be fragmented and embedded into the soft matrix or they are torn off becoming wear debris, “traveling” in contact and induces oscillations of friction coefficient, but when their concentration increases, the component of abrasive wear becomes dominant and wear is greater; when the tribolayer loses its hard particles, the cycle is repeating. Thus, wear is a dynamic process, in steps, depending on local concentration of material constituents [9].
\n\nFigure 7 presents a process of consolidation of the tribolayers by embedding the fragments broken from short glass fibers a) PTFE +25% glass fibers, water lubrication, partial bearing (Ø60 mm, 30 mm width) and steel shaft: some glass fibers within the superficial layer cannot bear the local load and were broken; the fragments are embedded into the PTFE matrix [9].
\nConsolidation of the soft polymer matrix by glass fiber fragments, water lubrication, composites PTFE+glass fibers, large contact, partial bearing (120°) (Ø 60 mm x 30 mm width) [9]. (a) PTFE+15% glass fibers. (b) Detail of (a). (c) PTFE+25% glass fibers.
Sometimes, adding materials in polymers could worsen the tribological behavior. For instance, too much concentration of glass fibers increases both and friction coefficient and wear (especially abrasive wear on both surfaces in contact). A relation between mechanical characteristics in tensile tests and tribological one could be triky. Tensile strength could be improved by adding reinforcements, but strain at break is usually decreased. In sliding contact, a deformability ensures the contact conformability and in fluid lubrication helps generating the fluid film. But, even from 1979, Evans and Lancaster [35] reported that fibers in polymers have beneficial effects on wear and only rarely worsen this parameter. Some adding materials could have the role of a reinforcement but also could help for heat evacuating. A greater interest in using polymer composites and blends pointed out that the designer of the material has to do compromises that have to be accepted only by experimental results, models for predicting tribological behavior being difficult to establish in quantities [36].
\nThe addition of short carbon fibers (SCF) in a concentration from 5% to 20 vol% can improve the wear resistance of neat PEI remarkably, especially at high temperature and under high working pv-factor. The increased test temperature from room temperature to 150°C leads to a seven times increase in the wear rate of neat PEI and five times for the composites. SCF/PEI can withstand much higher pv-factor than that of neat PEI. When the pv-factor increased from 1 to 9 MPa m/s, the time-related wear rate of SCF/PEI almost linearly enhanced from 1.5 × 10−3 to 7 × 10−2 m/h. However, the wear rate of neat PEI increased from 0.214 to 3.42 m/h when the pv-factor was only increased from 0.25 to 3 MPa m/s. The micrographs of the worn counterface and specimens indicated that the sliding of neat PEI against metal counterface did not form a transfer film, and wear mechanisms varied from fatigue wear to plastic plowing at the increased temperatures. The presence of short carbon fibers helped generating transfer films both on the counterface and worn surface of specimens. The transfer film became more continuous with the increased test temperature. The composite wear was mainly undertaken by fibers [37].
\nEven if the process of wearing the polymeric composites comprises same stages, the aspect, dimensions and the concentration of added materials make the aspects of worn tribolayers very different. When sliding two bodies one against the other, the matrix is more deformable and the adding materials are like pebbles in the bottom of a shallow river. A partial detaching between matrix and particles/fibers could happen, the fibers change their position and the particles could roll or be dragged on the surface. The space left behind the hard element accumulate fine wear debris from both bodies or even from lubricant (when lubricating), stiffening the tribolayer. The random position of the hard materials and their agglomeration by wearing the soft material increase the probability of detaching conglomerates. This is why an optimum concentration of hard reinforcement in polymer-based material is around 15…25% and depends on the nature of reinforcement. For instance, 20…25% wt is an optimum in PTFE [9, 38, 39], but short aramid fibers are usually added at 10% wt due to the difficulty of injection molding as they block the injection nozzle [16]. As for particles with similar dimensions in all directions Georgescu [15] and Maftei [11] proved that 20% is the optimum concentration for glass beads of micron size.
\nIf one analyzes the soft phases introduces in polymer-based materials, usually a solid lubricant, and with particular reference to PTFE [40] as solid lubricant, this concentration varies from 5 to 15% wt depending on the nature of the involved material. In PBT, the best concentration of PTFE was 5…10% the preferred criterion being the wear rate of the polymeric blend on steel [15].
\nLaboratory tests, on simplified specimens, are useful for ranking materials, but these results could not be extrapolated to actual component, especially for polymeric materials.
\nTest campaign has to answer how the material pair behaves in a series of parameters
lubrication regime,
environment
working regime (load, speed etc.),
family of tested pairs of materials
In the ISO standard collection, the word wear is mentioned in 118 items, the test methology being adapted to the application, as, for instance, road and tire wear, implants, but for testing plastics there is.
\nISO 7148-2:2012 Plain bearings — Testing of the tribological behavior of bearing materials — Part 2: Testing of polymer-based bearing materials.
\nISO 6601:2002. Plastics — Friction and wear by sliding — Identification of test parameters.
\nISO 20329 Plastics — Determination of abrasive wear by reciprocating linear sliding motion.
\nISO 9352:2012 Plastics — Determination of resistance to wear by abrasive wheels.
\nISO/DIS 7148–2 Plain bearings — Testing of the tribological behavior of bearing materials — Part 2: Testing of polymer-based bearing materials.
\nISO/TR 11811:2012 Nanotechnologies — Guidance on methods for nano- and microtribology measurements.
\nThe selection of tests necessary for assessing tribological behavior of a material pair including polymer-based materials depends on
the research level (laboratory, application under development, design of new materials, failure investigation),
the characteristics of the tribosystems, distinct regimes of sliding wear are “severe” and “mild”.
the actual working conditions
Many different approaches could be seen in literature for assessing the tribological behavior of a system, differentiate in scale and complexity of the tested system. A logical order will be.
\nLaboratory tests → Model tests→ Component bench tests → System bench tests→ Machine bench test→ Machine field test [41, 42].
\nIn the same direction there are increasing complexity and costs, but first types of tests have increasing control and scale investigation and flexibility.
\nDepending of the novelty degree of the solution, one or more of the stages mentioned above could be omitted. New materials and original design solutions ask for all, but they have to be solved quite rapidly in order to gain the market.
\nA testing campaign is suggestively given in Figure 8. This plan was elaborated by Georgescu [15], but also used by Botan [16]. It was the result of consulting adapted from [42]. Polymeric blocks have the dimensions (10 mm x 16.5 mm x 4 mm). The values are quite small and it is very probably not to have actual component of such dimensions, but such a test campaign is very useful for ranking the materials and to investigate modifications in the tribolayers by the help of electron scanning microscopy, AFM, Raman microscopy as the test specimens are small.
\nCharacteristics and relevant parameters for the tribotester block-on-ring [15, 16].
The set of tribological parameters are characterizing the materials Laboratory tests, on simplified specimens, are useful for ranking materials, but these results could not be extrapolated to actual component, especially for polymeric materials.
\nWhen designing a test campaign, for assessing the tribological behavior of a material pair, the tribosystem has to be identified as one in Figure 9 [42], this simplified initial system being tested at laboratory level with as many as possible parameters closer to those from actual application.
\nTesters for assessment of tribological behavior of polymers and polymer composites [42].
The coefficient of friction is a convenient method for reporting friction force, since in many cases Ff is approximately linearly proportional to F over quite large ranges of N. The equation, known as Amonton’s law is
\nwhere the value of μ depends significantly on working regime (lubricated or not), the composition, topography and history of the tribolayers, the environment in which they are working and the loading conditions. Ashby [43] gave a suggestive diagram, positioning the polymeric materials with lowest wear rate, but wear rate values could scan o two-order of magnitude. He also suggests by this diagram that wear rate field could be extended, especially towards low values by filling the polymers. A special position is noticed for PTFE (Figure 10), unique polymer as tribological behavior (the lowest friction coefficient, high wear rate, high working temperature and very resistant in aggressive media).
\nPositioning of polymers and polymer composites in a space hardness-wear rate constant [http://www.mie.uth.gr/ekp_yliko/2_materials-charts-2009.pdf] [43].
Usually, when a component if made of polymeric material, the other is harder, made of steel, but recently contact could be between the same polymeric materials of different. Thus, friction has to be treated for these cases.
\nIn the case of harder counterpart, the friction polymer-metal has the following components: plowing as a form of abrasion with larger elasto-plastic deformation and micro-cracks and adhesion [3, 8]. These processes are severely depending on many factors including the hardness and asperity shape of the counterpart, contact load, speed, temperature. This component of friction could be reduced by introducing a lubricant in contact or/and by re-design the system to have rolling or rolling-sliding motion and by an adequate cutting (usually grinding, honing) of the metallic counterpart.
\nThe adhesion is present both in static friction and dynamic friction of polymeric materials: at the interface motion generates shear and deformation of a very thin layer of the polymeric material, directly in contact with the counterpart. As adhesion and transfer on the counter part are developed in steps, the friction loss, and consequently, the friction coefficient will vary in time, especially for sliding contacts.
\nValues of friction coefficient are given by producers, researchers but they are depending on test conditions. Thus, they could give a ranking of the tested materials under the same conditions, but they could not be the same with actual components. Sometimes, especially under low load, negative values of μ may be noticed: they are rather artificial, due to contact separation and inertia of the tester components; values of μ greater than 1 are physically logical, especially in material processing, in the interaction between a car tire and a dry road. Sampling could vary depending on the gauge measuring the resistance force. Researchers usually use a moving average to draw the curve of friction force or coefficient in time. For instance, the curve in Figure 11 was done by moving average of 200 values with sampling 2 values per second. But extreme values are also important as they limit a range that could explain failure mechanisms as adhesion or local melting, especially for polymeric materials.
\nFriction coefficient for three tests block-on-ring, with different sliding distances [15].
In most cases, a single value of coefficient of friction is not adequate. This can be seen from the examples in Figure 11, depicting the evolution of friction coefficient for three sliding distance. The aspect of evolution is kept for PBT, but these three tests gave values between 0.16…0.19, with stable evolution, a characteristic of polymer sliding as compared to metal–metal contacts.
\nThe evolution of COF in Figure 11 points out that, for polymer on steel in dry regime, it is less sensitive to time, but these conclusion has to mention the time range for which the researchers had obtained this results, here for 2500…7000 m.
\nCzichos [41] modeled the evolution of COF for a dry regime in four stages: 1- increasing trend as the surfaces accommodate by wear, 2 - shorter stage of maximum values of COF, 3 - decrease of COF by the generation of a tribolayer favorable to reduce friction, for instance, a soften or molten layer of polymer, transfer films on harder surface etc. and the abrasive wear and deformation intensities decrease, 4 - stable evolution of friction. For polymer on steel or even on themselves, the authors will add a stage, 5 - slowly or sudden increase of COF meaning worsening the surface in contact due to severe wear, fatigue etc., in many times this increase announcing the life end of at least one triboelement (Figures 12 and 13).
\nInfluence of load at the same sliding velocity (GB10 - PBT +10% glass beads, GB20 - PBT +20% glass beads) [15].
Influence of sliding velocity under the same load [15].
Too low load makes the friction coefficient to have higher oscillations as superficial layer of the polymer is not compresses and hard asperities will easier tear up micro-sheets or plaquettes. As the load increases, the tribolayer is compacting and the energy loss by tearing decreases. This phenomenon of oscillating the friction coefficient in dry contact of polymers have been notice also by Jones in 1971 [44]. Higher concentration of reinforcement increases the friction coefficient and makes its evolution wavy (high amplitude could mean an increase of the glass bead concentration in the tribolayer and low values could happen when the tribolayer is richer in polymer.
\nConvergence of the curves for higher velocity (in Figure 13, for sliding speed of 0.5 m/s and 0.75 m/s) means that friction process is similar, very possible involving a very thin soften/melted layer of polymer. This is obvious in another study [11], using pin-on-disk tribotester.
\nThis example point out the influence of the nature of polymeric materials: the composite (the composites with hard micro-particles in a PBT matrix have higher and rough aspect of the curve, the blends PBT+ PTFE having lower values even the polymer PBT, considered a polymeric blends with soft drops of PTFE in PBT matrix). Figure 14 presents the influence of sliding velocity on the friction coefficient, and the curves in Figure 15 show the friction coefficient evolution in time depending on the highest load and velocity. The last plot is given only once as it could be related both to load and velocity dependence. The abbreviations for the materials are: PF5 - PBT + 5% PTFE, PF10 - PBT + 10% PTFE, PF15 - PBT + 15% PTFE). The composition of the hybrid composite GB10 + PF10 (having 10% glass beads and 10% PTFE) makes the friction coefficient to be higher at low velocity (0.25 m/s), but for the other two tested velocity, this tribological parameter evolves in a similar manner, but with higher oscillations, probably because of hard glass beads in the tribolayer (Figure 16).
\nInfluence of sliding velocity, at F = 5 N, for PBT, PF5, PF10, PF15 [15].
Influence of load, at v = 0,75 m/s, for PBT, PF5, PF10, PF15 [15].
Evolution of the friction coefficient for PBT and a hybrid composite (PBT + 10% glass beads+10% PTFE) [15].
Wear is not only a process of material removal in moving contacts, but a more complex one, defined recently as damage of the solid bodies caused by working or testing conditions, generally involving progressive loss of material, elasto-plastic deformations, tribo-chemical reactions caused by local pressure and heat generation in friction and their synergic interactions [8, 20]. In majority cases, the relative motion is intentional: for example, in plain bearings, pistons in cylinders, automotive brake disks interacting with brake pads, or in material processing (cutting, injection, rolling or extrusion). But in some cases, there are also undesired motion(s), resulted because of particular working conditions, as in the small cyclic displacements, known as fretting, produced by vibrations, elasto-plastic and tribological behavior of components in contact. If solid particles are passing through the contact, as contaminants in lubricant or, intentionally, as abrasive material for processing, then they will have a tremendous influence on wear process and, thus, on system durability.
\nWear is a complex process, quantified by the volume or mass of removed material, from each body in contact, the change in some linear dimension after a time period of functioning. Thus, wear is obviously a function of material pair, working time and conditions and it is related to a particular tribosystem (materials, dimensions, shapes and working conditions).
\nIn some cases, material may be lost from both triboelements, or significant transfer of material may occur between the triboelements, and particular care is needed in both measuring the magnitude of wear and describing the damage it generates (material removals, abrasion, adhesion, transfer, plastic deformation, fragmentation and mixing the constituents of the tribolayer changes in the topography, the last one being investigating by the help of advanced non-contact profilometers [45].
\nThe wear of polymeric material implies an aspect that is of interest only in pairs with a polymeric material: melting wear. A part of heat generate by friction is transferred to the polymeric materials and as thermal conductivity of polymers is low, a very thin layer could soften or even melt, the material latent heat of melting imposing a temperature limit in dry contacts. Stachowiack and Batchelor [46] described the scenario of temperature evolution in contact with a polymeric material (Figure 17). Similar observations are done by Briscoe and Sinha in [8], relating the polymer softening and its nature to transfer process on the harder counterface.
\nEvolution in time of temperature of polymeric surface in sliding contact [46].
Experiment work validated this process of keeping constant the temperature in contact when a triboelement is made based on polymers. In order to support this conclusion, two studies are presented. First one is shortly presented in Figure 18. A cylindrical pin made of bearing steel is sliding against a disk made of composites PA + 10% wt glass beads +1% black carbon [11]. The thermo-image in left side presents the positions and their codes where the temperature was recorded with a thermo-camera. The temperature evolutions in time for these three points re given in the right. It is obvious the tendency of maintaining the temperature almost constant for v = 0.5 m/s and v = 1 m/s. As for the highest tested velocity, the plateau is zigzagged at almost regular time period. This could be explained by the polymer softening or even melting, followed by easier removal from the tribolayer, enrichment in glass beads of the tribolayer, with higher friction and thus, generating heat and rising the temperature. When the glass beads are embedded in the remaining matrix or removed, the temperature would reach a minimum.
\nTemperature evolution in time (a) for pin-on-disk tester, pin made of hard steel and disk made of PA + 10% grass beads + 1% black carbon, dry sliding for 10000 m and a thermal image during the test (b) (the rotation of the disk is clockwise) [11].
Another study [16] for emphasizing the importance of testing composites with polymer matrix has the results obtained on block-on-disk tribotester (Figure 19). The block is made of composite with 10% short aramid fibers (Twaron, grade, 225 μm in length see Figure 5c), with two different matrices: PA and PBT and the ring is the outer ring of a taped rolling bearing (the quality of rolling bearing ring keeps contact the influence of the counterbody in sliding). Analyzing Figure 20, the friction coefficient for PAX on steel has a steady evolution, in narrow ranges, for low loads (F = 5 N and F = 15 N) but for F = 30 N, for higher velocities, it increases and becomes steady at higher values, around 0.3. Temperature is steady for the same low loads, but it increases with different slopes for highest load. A too low load on polymer-based material - steel could rise COF and temperature in contact because the hard body does not contact the polymeric tribolayer enough and thus, the wear has a more intense abrasive component, tearing-off easier the polymer.
\nImages of thermal recordings of the temperature at the end of the test, for temperature at the contact edge, F = 30 v = 0,75 m/s (block made of PA- polyamide, PAX - polyamide +10% aramid fibers +1% black carbon, PBT - polybythylene therephtalate, PBX - PBT +10% aramid fibers +1% black carbon) [16].
Evolution of friction coefficient and temperature at the contact edge in time, depending on load, sliding velocity, for a sliding distance of L = 5000 m, block made of PBT +10% aramid fibers, L = 5000 m, block made of PAX (PA6 + 10% aramid fibers) [16].
The combined analysis of two tribological parameters could reveal a qualitative change of the working regime. For instance, analyzing COF and temperature at the contact edge (Figure 20),
a too low load and sliding velocity make the temperature rising due to abrasive wear (more intense under low load)
a higher speed makes the temperature curve higher for v = 0.75 m/s, but the COF is kept low meaning a softening process happened,
a too high load makes the temperature to have a slope, greater as velocity increases; a mild regime (thus, a favorable regime) will keep the temperature constant in contact as for tests under F = 15 N. The severe regime is marked by high oscillation of friction coefficient or even a constantly increased value and also by the same shape of the temperature curves.
Comparing curves in Figure 20, regimes with F = 30 N and high sliding velocity (v = 0.5…0.75 m/s) could be considered as severe because they do not make tribological parameters as friction coefficient and temperature in contact, stable.
\nThe composite with PBT matrix with the same adding materials (10% short aramid fibers and 1% black carbon) has a similar evolution of COF, but temperature increases only for the extreme tested regime (F = 30 N, v = 0.75 m/s).
\nThe applications involving the friction couple polymeric material - metallic counterpart are preferred by mechanical requirements of the design solution and the better tribological behavior by monitoring and measuring a set of tribological characteristics (wear, friction, temperature in contact, changes in materials’ structures etc.) as compared to sliding polymers against themselves (Figure 21).
\nEvolution of friction coefficient and temperature at the contact edge in time, depending on load, sliding velocity, for a sliding distance of L = 5000 m, block made of PBX (PBT +10% aramid fibers) [16].
Wear process of polymeric materials are characterized by a transfer film, generated when sliding against a harder surface, strongly influencing on the tribological behavior of the system [8].
\nA favorable transfer film should be continuous, very thin and regenerating without inducing troubles in the working systems. This is the ideal transfer film of a polymeric material but, actually, there are two types of polymers, those generating an almost continuously transfer film as high density polyethylene (HDPE) and ultra-high-molecular weight polyethylene (UHMWPE), and those that form lumps or islands, more or less regular. Transfer process is influenced by contact temperature and texture of the counterpart. Only few polymers have only a mechanical component of the transfer film (again, PTFE and UHMWPE have to be given as examples) and polymers that could chemically interact with the metallic surface.
\nMyshkin et al. [7] pointed out that the dependence of friction coefficient with velocity has different shapes depending on the polymer sliding on steel or on itself, and even for the same polymer, the curve depends on temperature of the environment. At low velocity (10−3…10−2 m/s), friction coefficient has an almost constant evolution, but at higher speed, its evolution could be with velocity could be parabolic, with minimum when the material is softening or has a thin melt layer, than it could increase. The conclusion of this work is that tests in the same conditions as the application are tremendously necessary for a reliable working of the tribosystem involving polymer-based materials in order to correct assess the power loss by friction and to prevent component failure by frictional heat.
\nThe wear rate can then be defined as the rate of material removal or dimensional change per unit time, or per unit sliding distance. Because of the possibility of confusion, the term “wear rate” must always be defined, and its units stated. It is usually the mass or volume loss per unit time.
\nThe Archard model of sliding wear [47] leads to the equation:
\nwhere w is the volume of material removed from the surface by wear per unit sliding distance, W is the normal load applied between the surfaces, and H is the indentation hardness of the softer surface. Many sliding systems do show a dependence of wear on sliding distance which is close to linear, and under some conditions also show wear rates which are roughly proportional to normal load. The constant K, usually termed the Archard wear coefficient, is dimensionless and always less than unity. The value of K provides a means of comparing the severities of different wear processes.
\nFor the tribotester block-on-ring the wear parameter that reflects well the behavior of the materials could be the linear wear rate
\nwhere ΔZ is the change in distance between ring and block at the end of the test, F is the normally applied load and L is the sliding distance. Figure 22 presents test parameters, as recorded by the tribometer UMT-2, including friction coefficient (COF), wear depth (Z).
\nExample of parameters monitored in actual time real on the tribotester UMT-2, block-on-ring test, block made of PBT, ring made of steel (100Cr6), F = 5 N (= Fz), v = 0,25 m/s, L = 7500 m, COF –friction coefficient, Fx – Resistant force (friction), AE – Acoustic emission, Z – Wear depth (linear wear) (linear change between ring and block), Fz – Normal load [15].
For pointing out wear parameters in a tribosystem with polymer-based material(s), the same two cases are analyzed (Figure 23).
\nLinear wear rate of the blocks made of polymer-based materials.
A study has another objective [16]: to assess the tribological behavior of two polymer matrices, PA and PBT, with the same concentration of reinforcement, 10% wt short aramid fibers (Twaron, 225 microns as average length). There were measured several tribological parameters, average values of friction coefficient (COF, Figure 24), wear rate (Figure 25) and maximum value of the temperature at the contact edge (Figure 26). Wear rate in Figure 25 was calculated as
\nwhere Δm is the mass loss of the block, L is the sliding distance and F is the applied load in contact.
\nAverage values for COF for 5000 m of dry sliding on steel (same scale for PA and PAX and PBT and PBX, respectively) [16].
Wear rate of the block as a function of load (in N) and sliding speed (m/s), obtained on block-on-ring tester, dry regime, for blocks made of polymers (Polyamide 6 - PA and Polybuthyleneterphtalate - PBT) and their composites with 10% short aramid fibers (PAX and PBX).
Temperature in contact is very important in tribosystem with one or both elements made of polymeric materials as a jump in contact temperature of less amount as for metals (even 10°C) could change their mechanical and thermal properties, could even change the chemical organization of the molecular chains; the power dissipated in the contact is given by (μ⋅F⋅v) where μ is the friction coefficient, F is the normal load and v is the sliding velocity. The local temperatures in the contact areas can therefore become much higher than the bulk temperatures. This factor needs to be considered when designing wear tests or interpreting test results.
\nIn Botan’s study [16], neat PBT had a very good tribological behavior (being analyzed, average values of COF during 5000 m of sliding on steel, low wear as compared to PA) but adding 10%wt short aramid fibers in PBT substantially improves wear resistance. Thermal monitoring of the contact edge allows for ranking the tested materials having the temperature as criterion (Figure 26).
\nMaximum value of temperature at the contact edge, for all four tested materials in [16] (material codes as in previous figure).
In study from 2012, Pei et al. [12] present the tribology of three polymers, considered as high-performance materials, introducing for evaluating the product pv (p being the average pressure in contact and v the sliding velocity). This parameter has to be used with precaution. Comparison should be done for the same tribosystem (dimensions and shapes) and under the same testing conditions. It is not recommended to extrapolate the results outside the investigated parameters. From Figure 27, one may notice that PPP grades exhibited low wear resistance as compared to PEEK and PBI had the lowest wear rate, due to its high value for heat resistance and very low decrease in mechanical characteristics under higher temperatures.
\nSpecific wear rate of polymer sliding on steel and counterpart temperature for [12].
Obviously, in dry regime friction coefficient of a polymer on steel is lower than that for steel-on-steel and long and aligned carbon chain (as in PTFE and PE, even PA) will give lower dynamic friction coefficient, around 0.2…0.3, lower for PTFE, but polymers with higher mechanical characteristics as PPS and PEEK will have this parameter higher 0.3…0.5. Wear rate exhibits values that could not be deduced from the mechanical and structural characteristics. For instance, in Figure 28, the lowest wear rate among tested polymers under the same conditions was obtained for PA6, and wear rate increases from this to PI, PPS, PE-UHMW till PEEK, but high values were obtained for POM and PTFE.
\nTwo tribological parameters for polymer in dry sliding on steel [http://www.appstate.edu/∼clementsjs/polymerproperties/$p$lastics_$f$riction$5f$w$ear.pdf]. (a) Friction coefficient. (b) Wear rate [48].
Worn surfaces and the debris resulting from wear, may be examined for several reasons:
to study the evolution of wear during an experiment, or during the life of a component in a practical application,
to compare features produced in a laboratory test with those observed in a practical application,
to identify mechanisms of wear,
(by studying debris) to identify the source of debris in a real-life application.
\nFigure 29 presents two virtual images, reconstructed with SPIP The Scanning Probe Image Processor SPIPTM, Version 5.1.11/2012, from a study done by Georgescu [15], pointing out initial surface (a) and traces as result of abrasive wear on the composite.
\nVirtual images of block surfaces made of PBT + 20% glass beads. (a) Initial surface. (b) Used surface (F = 5 N, v = 0,75 m/s, L = 7500 m) [15].
After testing, the worn surface quality of the composite with only 10% glass beads was better, meaning a lower value for Sa, Sz (Figure 30). In tribological evaluation a ratio Sz/Sa, bringing together an averaging parameter with an extreme one (Sz) is important because singular or rare high peaks have a great influence on the tribological behavior, especially for composites with hard fillers. Adding micro glass beads in PBT increases the amplitude parameters (these are plotted for v = 0 m/s, in Figure 30). Ssk has high positive values for 20% glass beads in PBT, but the polymer and the composite with only 10% glass beads have lower values, oscillating between 1 and − 1. If Ssk <0, it can be a bearing surface with holes and if Ssk > 0 it can be a flat surface with peaks. Values numerically greater than 1.0 may indicate extreme holes or peaks on the surface, as for the worn surfaces of composite PBT + 20% glass beads. For v = 0.50 m/s (Figure 30) and v = 0.75 m/s, Ssk < 0, reflecting the micro-plowing process. For Sku > 3, all worn surfaces indicated long and narrow valleys, with high peaks, the valley are dominated as result of tearing-off glass beads and maintaining the shape of the extracted beads. Smaller values of Ssk indicate broader height distributions but these polymeric materials have narrow height distribution as all values are above 3 (Table 2).
\nRoughness for worn surfaces of the block made of PBT, PBT + 10% glass beads (GB10) and PBT + 20% glass beads (GB20). (a) Sa- roughness average. (b) Sz - peak-peak height, the difference between the highest and lowest point in surface. (c) Surface skewness, Ssk, or the asymmetry of the height distribution histogram. (d) Surface kurtosis, Sku, or the “peakedness” of the surface topography [15].
Parameter | \nInformation, unit | \n
---|---|
Load (normally applied), constant or variable | \nN | \n
Sliding speed | \nm/s | \n
Pair of materials | \nComposition, phases, structures | \n
Temperature (environment and in contact) | \nThe second is difficult to measure | \n
Type or relative motion | \nSliding, rolling, combined motion, small oscillations, impact | \n
Contact type | \nConformal, non-conformal, volumes of the triboelement | \n
Particularities of tribosystem (if the case) abrasive/erosive particles | \nMaterial, shape, size and distribution | \n
Contact dynamics | \nStiffness, damping, inertial mass | \n
List of important parameters that influence the tribological behavior [42].
Components with high volume of polymeric material are less heat conductive and prone to have melt/soften contact. The solution given by research and practice: polymeric coatings, thick enough to reduce friction and to bear wear for a specified life and reliability.
\nDuring a test, many influencing factors have to be controlled. These can be grouped in
-mechanical and environmental test conditions (such as contact load or pressure, speed, motion type and environment temperature, composition), and
-specimen(s) parameters (such as material composition, microstructure, volume, shape and their initial surface finish).
Some of them could be monitored during the test (as friction force), some only at before and after test. For polymers, investigations must be done just after the test as the specimens could age and thus, altering the information.
\nResearchers have to prioritize what factors are kept constant and what factors will vary on ranges of interest.
\nA full program of testing under all combinations of these factors would be time-consuming and costly, and may not be required. Often a single factor can be identified as “key” to the material response, and in this case a good approach is to set all the other factors at constant values and vary the chosen factor in a controlled way in a series of tests. Test campaign must promote an objective, to establish variables (materials, working regime parameters, environment) and the most relevant results to be given, non-destructive investigation in order to understand and direction the damage processes during testing.
\nTribologists is now using mapping technique when two (or more) factors are changed in a controlled way (normally more coarsely than in parametric studies), the parameter of interest being the friction coefficient, wear or wear rate, temperature or durability till a particular value for wear temperature etc. are reached. The mathematical model for building the map surface is very important. For instance, maps in Figure 23 are built with double spline curves, enforcing the obtained values from the tests to be on the surface. Sharp peaks or deep zones on the maps could indicate a qualitative change in tribological processes (change in wear process balance, tribochemical reactions induced by temperature threshold etc.)
\nThe mapping technique is efficient for determining the overall behavior of a material or a tribosystem as it provides useful data about the position of transitions in wear behavior for a systematic test campaign. This comes at the expense of a reduction in the detailed knowledge of the variation of friction and wear with any one factor, but once the regime of interest is better defined through the use of maps, then a more detailed parametric study can be conducted.
\nInitially, PTFE was simply used as bushes, seals, but its low mechanical characteristics make the researchers for materials to use it as matrix in composites [9, 39], adding material in other polymers, and even metallic sintered materials, more rigid and less prone to wear.
\nBurris and Sawyer studied the blend PEEK + PTFE [49]. PEEK has wear resistance, mechanical strength and a higher working temperature as compared to other polymers, but a high friction coefficient in dry regime μ = 0,4 and low thermal conductivity. PTFE has a high wear rate, and the fact that has the lowest friction coefficient in similar conditions does not recommend it to be used simple, without blending with another polymer or reinforcements. A qualitative model of a polymeric blend could be modeled as in Figure 31a.
\nContact surface 6,35 mm x 6,35 mm, F = 250 N and alternating sliding on 25.4 mm, v = 0.05 m/s, dry sliding on stainless steel AISI 304. (a) Model proposed by [49]. (b) Wear rate as a function of PEEK concentration.
Many researchers and producers of polymeric materials recommend only 5–20% PTFE [46, 50, 51], experiments done by Burris and Sawyer [49] obtained an optimum for the wear rate using the blend 30% PEEK +70% PTFE and, thus, underlined the necessity of testing new formulated materials for tribological applications.
\nUnder 20% PEEK, wear has a sharp evolution, explained but not enough PEEK for creating a harder matrix for the soft polymer, thus the last one is easy to be deformed, abraded; the wear is supported by PTFE and not by the harder material (which has a higher wear resistance. The transfer process is more intense, and the wear debris have higher volumes. The authors suggest that preferentially lose of PTFE make the tribolayer grows rich in PEEK and the wear is reduced. At higher concentration of PEEK, the wear is dominated by fatigue cracks and the micro-reservoirs of PTFE are in reduced number and the solid lubrication of PTFE is done only on patches. Wear debris made of PEEK generate a more intense abrasive wear, even as third body, care damage the transfer films on both surfaces in contact.
\nA similar tribological behavior was noticed by Tomescu [9], when a composite copper + PTFE was tested in dry and water lubrication regime.
\nNeale admitted that wear is a complicated process and even if the mechanisms could be described, there are combinations and transitions among them that make them difficult to be understood yet and reduced [52]. Four main wear mechanisms are discussed in literature [23, 46]: abrasion, adhesion, fatigue and tribo-corrosion, with particular, mixt variants (thermal and tribofatigue, fretting etc.).
\nAspects of wear mechanisms with different adding materials in polymers are well described and interpreted in [3, 8, 20, 46]. A particular wear process of polymeric materials is the so-called delamination, that is a combined process of sub-layer crack, plastic deformation and material removal (Figure 32).
\nWear deterioration of a polymeric body in sliding against a harder material, also known as delamination [35].
Forms of abrasive wear are micro-cutting, plowing and micro-cracking with material remove are particularized for polymers that are visco-plastic materials.
\nAdhesion has particular aspects in tribosystems with polymers, including polymeric transfer on the counter surface, especially when this is made of steel.
\nAs Stachowiak and Batchelor [46] mentioned, this transfer has two extreme consequences:
beneficial, when the transfer film is thin and transform the moving contact in polymer-polymer,
not beneficial, with lump or insular transfer, that change too much the surface topography.
The solution of reducing wear of polymers is to add materials that keep the polymer into a network (random or organized) to minimize the polymer volume implied in the local deformation and detaching small wear particles instead of big ones.
\nThe research has to establish an optimum concentration of constituents that allow for having a better tribological behavior (reduced wear, permissible working temperature, low power loss due to friction and to keep the functions of the systems in an reliable range).
\nFor instance, Maftei [11] elaborated composites with glass beads in a polyamide matrix with concentration between 5% wt and 50% wt and tested them on pin-on-disk tribotester. SEM investigation revealed agglomerated glass beads, a very thin soften layer of polymer that cover like a blanket the glass beads, justifying the still low friction coefficient. The next figures (Figure 33 and Figure 34) point out differences between wear mechanisms for PA6 (a) (abrasive, fatigue with small cracks) and the composite (detaching smaller polymer debris, al lower sliding velocity the soften layer does not exists and polymer is deformed by the random small movements of the beads in the matrix, at higher velocity (d) several beads roll in the superficial layer as the polymer is less viscos.
\nSEM images on tribolayer generated from composites with PA6 matrix and different concentrations of glass beads [11].
SEM images for tribolayers: PA disk (a) and for the composite with 50% glass beads (b, c and d), dry sliding on steel (no gold coating of the samples) for SEM investigation. (a) v = 0.5 m/s, p = 1 MPa. (b) v = 0.5 m/s, p = 2 MPa. (c) v = 1 m/s, p = 1 MPa. (d) v = 1,5 m/s, p = 1 MPa [11].
Typical aspects of the failure mechanisms in polymer sliding against harder bodies are described in [53, 54, 55]: abrasive wear, adhesion wear (with transfer) and fatigue wear (Figure 35).
\nTypical aspects of the failure mechanisms in sliding on steel in dry regime (a) adhesive wear, (b) abrasive wear, (c) fatigue wear [11].
The geometry of the reinforcement makes the wear mechanism to be different for the same fibers, if the matrix is different, as one may see in Figure 36. The first line of SEM images is for the matrix of PA6, more ductile than PBT - the matrix of the composite in the second line of SEM images. All tests are done on block-on-ring tester, in dry regime. A more ductile matrix is easier worn and torn-off, the fibers remaining to bear the load and there visible the deformations (flows) induced by a higher load on the fiber ends. In a PBT matrix, more rigid than PA6, the transfer on the steel counterbody is less and the fibers are scratched under higher load.
\nBlock-on-ring, L = 5000 m (thin gold coating of the samples) [16].
Composites with reinforcing particles or fibers: dynamic wear process, in stages: 1 - low wear of polymer and enrichment of the superficial layer in harder materials, 2 - too much hard particles or fibers within tribolayer, the result being big wear particles torn up in bigger conglomerate, 3 - leveling the rough surface after detaching hard particles/fibers by the help of plastic matrix (friction coefficient has high oscillations and the process is repeating.
\nFriction materials, as for brake pads, need special attention as they have to fulfill requirements as constant friction coefficient and controllable wear (linear would be better). Manoharan et al. [55] presented a study for a composite containing nine major ingredients, including epoxy resin, reinforcement, solid and liquid lubricants etc. (this pointing out the complexity of a composite destinated for brakes). Tests done on disk-on-plate tribotester, in the presence of third body (sand), revealed that wear volume loss of composite brake pad increases with increasing sliding distance and load, but wear rate increases with applied load and decreases with increasing sliding distance. Glass fibers and hard particle fillers were effective in reducing wear rate of the composite. It is reasonable to deduce that binders would increase the adhesion of glass fibers, SiC into the formaldehyde matrix. When the load is increased, microcracks are formed, followed by fragmentation in composite brake pad. Plowing, cracking and accelerated breakage of fibers in composite are evident under higher load. This study is here given in order to underline the necessity of testing new formulated friction materials, no theoretical model being able to reliably predict the tribological behavior in terms of values for wear, friction and durability.
\nSamyn et al. [56] presented a useful review on tribology of polyimides. Temperature modifies the tribological behavior of this polymer by chemical effects.
\nThe tested sintered polyimides show two sliding regimes: between 100°C and 180°C, friction is high and wear rate increases, with a discontinuous minimum at 140°C. Raman spectroscopy motivated that hydration generates a reversion of polyimide into a precursor. A maximum hydrolysis intensity at 140°C explains the minimum wear rate with acid groups acting as a lubricant. From 180–260°C, friction decreases and wear rate become stable at mild loads, with a maximum value for the wear rate at 180°C. Wear rates increase at high loads, but brittleness is not obvious till 150 N, at high temperatures. A discontinuous platelet transfer film develops above 180°C.
\nThermoplastic polyimides show three sliding regimes that are related to a combination of chemical and thermal effects.
at 100 to 120°C, friction increases and is higher and wear rates are lower as compared to sintered polyimides; a thin transfer film develops; dark wear particles were produced by hydrolysis,
at 120 to 180°C, friction decreases and a transition to high wear rates is initiated; a patchy-like transfer film develops and the polymer surface becomes irregular and opaque due to softening and chemical modification; wear debris become brittle and act as an abrasive,
at 180–260°C, friction increases and overload wear results from melting; a thick transfer film develops, and the polymer surface smoothens. Roll-like debris are visually observed as an indication for melting. Raman investigation indicates thermal decomposition of aromatic structures into amide monomers on the polyimide surface, weakening strength and producing higher wear.
And study point out the importance of test parameters, here the two polymers, the temperature and the load. Such a study could be done for each polymer of interest, with particular values for the test parameters, as they do not have a pattern due to their diversity in chemical structures and molecular organization.
\nAgglomeration of reinforcement fibers of particles are observed even in lubricated system with polymer composites sliding against steel. A suggestive model of reinforcements agglomeration in the superficial layer of polymeric composites, due to preferential wear of the polymer matrix has been described by Blanchet and Kennedy [10] from 1992, and then developed by Han and Blanchet in 1997 [57] and experimental results given in Figure 37 sustained their model. Each worn surface after sliding in water has a similar concentration in short glass fibers, even if initially the concentrations were different.
\nImages of the partial bearings made of PTFE + short glass fibers with different concentrations, test conditions: v = 2.5 m/s and p = 4.6 MPa, water lubrication, Lx = 10,500 m [8]. (a) 15% glass fibers. (b) 25% glass fibers. (c) 40% glass fibers.
New development in processing polymer-based materials (here including polymers, polymer blends, polymer composites and stratified materials based on polymeric fabrics) make easier to replace metallic parts with ones made of polymer-based materials, at a convenient price.
\nTest campaign are running faster as the market obliged the designers and producers to give more reliable products and the new achievement in monitoring and investigating the tribological behavior help them to understand and formulate new and adequate materials.
\nAn obvious tendency for these materials is using them as coating, thick enough to fulfill an imposed reliability and durability.
\nNew technologies allow for a better dispersion of the constituents, making the resulting materials more predictable [58, 59, 60].
\nTesting is very important when using polymer-based materials. New recipes of polymer-based materials has to follow the logical chain of testing, meaning laboratory specimen - component - partial system - entire system, in order to avoid catastrophic failure of the entire system. Even if it is difficult to imagine now new tribological parameters to be monitored or calculated, variant versions could be adapted for particular applications.
\nThis work has been supported by the European Social Fund through the Sectoral Operational Programme Human Capital 2014-2020, through the Financial Agreement with the title „Burse pentru educatia antreprenoriala in randul doctoranzilor si cercetatorilor postdoctorat (Be Antreprenor!)” “Scholarships for entrepreneurial education among doctoral students and postdoctoral researchers (Be Entrepreneur!)”, Contract no. 51680/09.07.2019 - SMIS code: 124539.
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