Thermal Characteristics and Tribological Performances of Solid Lubricants: A Mini Review

Abstract

Solid lubricants separate two moving surfaces and reduce wear. Materials’ ability to act as solid lubricants depends on their characteristics relative to contact surfaces. Chemically stable fluorides (BaF2, CaF2), boron nitride (h-BN), transition metallic sulphides (MoS2, WS2), soft metals (Au, Ag), binary and multi-component oxides, such as silver-containing sulphates, chromates, and oxides, and MXenes are effective solid lubricants. Solid lubrication depends on the material’s structure. Structure, mechanical properties, chemical reactivity, and kind of substance characterise these materials (refractories, ceramics, glass, etc.). High temperatures (>300°C) are obtained at asperities due to frictional heat produced when two surfaces rub. High temperatures can breakdown lubricants, but the resulting compounds must be lubricants; otherwise, corrosive vapours or abrasive solids can occur. High thermal conductivity helps lubricants remove heat generated by rubbing. Lubricants must not be melted, as the solid will lose strength and distort or be removed like liquid. Tensile strength, compressibility, and hardness are significant mechanical qualities for solid lubricants in extreme conditions. This chapter discusses solid lubricants and their structure. Also discussed are solid lubricants’ mechanical and thermal properties. The lubricating mechanism and conclusion are also conferred.

1. Introduction

Solid lubricants are defined as solid materials that are consciously added to or naturally generated on contact surfaces while they are in motion to decrease wear and friction and offer protection from damage. These are primarily used in harsh conditions, i.e., high temperatures, abrupt changes between vacuum and moist air atmosphere, heavy loads, high speeds, chemically reactive environments, and severe thermal shock circumstances [1, 2, 3, 4, 5]. Severe wear and oxidation, high friction, and premature failure are unavoidable in the absence of a lubrication mechanism under the aforementioned operating circumstances. The self-lubricating parts are necessary for a variety of industries, including hot metal processing, power generation, automotive, aviation, and aerospace to operate consistently and dependably. The relative motion between rolling/sliding surfaces makes sense for solid lubrication research to focus on minimising and eliminating material and energy losses. When exposed to harsh environments, effective solid lubrication will help to decrease the tendency to failure of components, to enhance the material and energy utilisation by increasing the efficacy and overall performance. There is a sizable demand for lubricating ceramic, metallic, and polymeric parts in rolling and sliding contacts, including, cylinder wall/piston rings, variable stator vane bushings, bearings for space satellites [1, 2, 6, 7, 8]. The tribological design primarily concerns about minimization of unfavourable wear and in and contemporary engines, seals, and bearings for next-generation propulsion systems is the. To further improve engine efficiency and lower NO_x and CO₂ emissions, these components’ friction coefficients and wear rates should be lower than 0.2 and less than 10⁻⁶ mm³N⁻¹ m⁻¹, respectively, and independent of ambient temperature, applied load, and sliding velocity. It has been challenging to develop low friction and wear exhibiting materials over a broad temperature range [9]. Currently used solid lubricants face significant challenges, particularly in terms of chemical stability and high-temperature structural or intermittent but reliable lubricity of surface. Ballistic missiles, hypersonic transportation, and other motion systems also need an atmosphere with exceptionally high working temperatures [10]. Multi-component oxides, binary oxides (V₂O₅, B₂O₃, and PbO), layered materials (transition metal dichalcogenides, hexagonal boron nitride, and graphite), soft metals, polymer composites, and alkaline earth fluorides are all taken into consideration as solid lubricants and self-lubricating materials/coatings in this chapter (Ti₃SiC₂, Ti₂AlC, etc.). It is feasible to directly apply high-temperature solid lubricants to the surfaces of machinery parts by using straightforward techniques like painting and burnishing. Examples of methods for creating self-lubricating materials or coatings at high-temperature include additive manufacturing, hot isostatic pressing, laser cladding, chemical vapour deposition, electrodeposition, thermal/plasma spraying, magnetron sputtering, spark plasma sintering, pulsed laser deposition, hot pressing, and pressureless sintering [2, 8, 11, 12, 13, 14].

The purpose of this chapter is to deliver a theoretical framework for the tribological properties of solid lubricants. Tribological properties of different types of solid lubricants, such as chemically stable fluorides (BaF₂, CaF₂), boron nitride (h-BN), transition metallic sulphides (MoS₂, WS₂), soft metals (Au, Ag), binary and multi-component oxides, such as silver-containing sulphates, chromates, and oxides, mixtures of various solid lubricants and MXenes has been discussed in the following section.

2. Tribological behaviour of different solid lubricants

The most often used solid lubricants are molybdenum disulfide, graphite, soft metals, and polytetrafluoroethylene (PTFE) [1, 2, 3, 4, 5, 10, 15]. Despite the fact that each of these lubricants has some limits, they have all been widely utilised, either individually or in various combinations. New self-lubricating compounds and composites/coatings have been developed as a result of the shortcomings of conventional lubricating solids. Numerous polymers are often used at cryogenic temperatures or in vacuum because of high chemical stability, excellent machinability, and low densities and friction coefficient [10]. The following subsections discuss the tribolological properties of each solid lubricant in brief.

2.1 Polytetrafluoroethylene (PTFE) and polyimides

Currently, polytetrafluoroethylene (PTFE) and Polyimides are used as solid lubricants because of their thermal stability in broad range of environment change and superior tribological properties. However, these have some limitations such as poor radiation and dimensional stability, low strength, high thermal expansion, and poor thermal conductivity, which limit their applications [10]. For these molecules, the hexagonal structure with lattice constant 0.562 nm is laterally packed [15]. The ease of gliding parallel to the c axis of these rod-like molecules results in PTFE’s low friction feature. Since PTFE, including graphite and MoS₂, exhibit the lowest coefficient of friction, can be used as a bearing material and retains its lubricity and mechanical properties up to 260°C [10]. PTFE typically has a friction coefficient of 0.04 against steel, but under exceptionally demanding conditions, it can reach 0.016 [10]. The PTFE coating results in decreasing the wear rate by 39% of finger seals [16].

PTFE is generally used with additives such as graphite, fluoride, MoS₂, or powdered graphite in order to minimise the cold flow under high speed and heavy load circumstances. These powdered additions typically decrease a material’s transport capacity even if they might improve its tribological qualities. The application of fibre reinforcement in this situation satisfies the requirement for maximum load capacity [17]. In aviation bearings and those bearings that are subject to heavy loads, woven fabric fibres are typically utilised to increase the bonded PTFE liners’ creep resistance. Non-metallic plain cylindrical bearings with a load capacity of more than 207 MPa and minimal wear and friction up to 121°C were tested. The bearings are constrained, though, by creep deformation at higher temperatures [1].

A collection of polymeric synthetic resins with the imide group that are resistant to high temperatures, corrosion, and abrasion are known as polyimides. It is generally used in films and coatings. The polyimide varnish shows reduction of wear and friction of coating up to 500°C by addition MoS₂ or CF_x solid lubricants. Polyimide-bonded CF_x films have been shown to be efficient gas bearing lubricants up to 350°C. Alkalies quickly degrade polyimides, despite their resistance to the bulk of conventional solvents and chemicals. Polyimides were frequently used in mechanical components, seals, gears, bearings, and at Rolls Royce, Pratt & Whitney, GE Aircraft Engine, etc. One such illustration is the DoPontTMVespel CP-8000 material utilised for the stator bushings in the compressor of the BR710 engine. Fibercomp has a compressive strength of 172 MPa at 260°C and a friction coefficient of 0.1 to 0.2. Surface brittle fracture from polyimides’ susceptibility to brittleness always results in wear damage [10]. It’s interesting to note that the UV-assisted direct ink writing (DIW) technique can create polyimide 3D architectures with only about 6% volume shrinkage [18]. The superior tribological behaviour showing self-lubricating devices using digital light processing and post-heat treatment was printed using PTFE-filled photosensitive polyimide (PSPI). The PSPI-7 weight percent PTFE composites that were 3D printed have strong mechanical characteristics, such as improved interlayer bonding, thermal stability up to 390°C, and tensile strengths more than 90 MPa. Furthermore, wear rates are lowered by 98%, while friction coefficients are reduced by 88%. For instance, at 20N, the surface lubrication and alternate lubrication friction coefficients are each lowered to 0.09 and 0.04, respectively. A significant target-region lubricating bearing made in 3D printing was successfully demonstrated.

It can be summarised that PTFE is solid lubricant with numerous of characteristics that includes self-cleaning capability, low friction, high electrical resistance, heat resistance to a wide range of temperatures, corrosion resistance, durability, and non-flammability. This type of film lubricant lowers friction between two surfaces without the use of oil or grease. Low-friction, corrosion resistance, and dry lubrication are some of the applications for PTFE lubricant.

2.2 Soft metals

Hard Metals Pure metals like Pt, Au, Zn, Pb, Sn, Ag, and In, some of which function as solid lubricants due to their sufficient softness. The formation of a shear-simple tribo-layer and increased ductility are the primary mechanisms of lubrication of soft metals. The low melting point having soft metal films of Zn, Pb, Sn, and In related alloys feature multi-slip systems that can effectively correct for microstructural faults through frictional heat during sliding when operating at low temperatures and lightly loaded conditions. However, the Mohs hardness of Ag, Au, and Pt is low, and their melting temperatures are high. Table 1 displays the tribological behaviour of materials that self-lubricate and contain soft metals [19, 20, 21, 22, 23].

Material	Synthesis method	Wear test parameters					Results
		Type	CB	Load	Vel.	Temp.	COF	WR
NiMoAl-Al2O3-Ag [19]	Plasma spraying	Ball-on-disk	Al2O3 ball	12 N	0.1 ms^−1	RT	0.53	1.47 × 10^−5
						500°C	0.36	8.84 × 10^−5
						900°C	0.17	3.35 × 10^−5
NiMoAl-Ag [20]	High-velocityoxy-fuel spraying	Ball-on-disk	Si3N4 ball	5 N	0.1 ms^−1	20 °C	0.3	1.4 × 10^−5
						600°C	0.3	7.7 × 10^−5
						800°C	0.09	6.0 × 10^−5
TiN-In [21]	Sputtering deposition	Pin-on-disk	Al2O3 ball	1 N	0.1 ms^−1	150–1200°C	0.5–0.6	—
Ta-Ag [22]	Magnetronsputtering deposition	Ball-on-disk	Si3N4 ball	2 N	0.128 ms^−1	600°C	0.2	5.2 _ 10^−5
Al2O3-DLC-Au-MoS2 [23]	Magnetron-assisted pulsed laser deposition	Ball-on-disk	M50 steel ball	1 N	0.2 ms^−1	RT (air)40% RH	0.13–0.14	—
						RT (N2) < 1%RH	0.02–0.03
			Si3N4 ball			500°C(air)	0.1

Table 1.

Tribological behaviour of soft metals.

Note: CB = counter body material; vel. = velocity; temp. = temperature; COF = coefficient of friction; WR = wear rate during the wear test; RT = room temperature; and RH = relative humidity.

The coatings of binary Sn-Co alloy are frequently utilised in engineering applications to replace hard chromium coatings. Ion-plated Pb coatings outlast sputtered MoS₂ coatings in rolling contact bearing applications due to the production of lubricious PbO inside the coatings. These coatings were developed for use on rolling elements that rotate slowly, such as those used in space mechanics. Pb-Sn-Cu coating on steel is used for decades to prevent rust. Lubricious Ag-Cu-Pb-S and Pb-Sn-Ag additives were equally dispersed throughout the TiC-reinforced high-speed steel pre-forms to establish an interpenetrating network microstructure for minimising friction and wear at extreme temperatures [24]. At 300–700°C, the wear rate can even be decreased by two orders of magnitude. Ag, Au, and Pt are examples of soft metallic coatings that have been produced utilising electrodeposition and physical vapour deposition methods. These coatings are especially helpful in hostile conditions and at extremely high temperatures, like in spacecraft. It is feasible to avoid undesirable subsurface cracking, extreme wear, and friction by utilising oxygen-ion assisted screen cage ion plating to bond Au and Ag lubricating coatings. In this case, low and constant friction coefficients and little wear were achieved principally due to the high adhesion of lubricious Ag and Au films to alumina [25]. Self-lubricating Au and Ag films can be used in advanced jet engines, spaceships, and fast-moving equipment with light loads. An Au-Co alloy coating was created to efficiently lubricate the roll ring assembly on the space station. Examples include silver [22] or MoS₂ [26] which have hard surface texture to reduce wear and friction. Additionally, a number of metals, such as Cr, Mo, Ni, Cu, and Fe, have high friction at room temperature, but as they slide above their oxidational temperatures, their coefficients of friction significantly improve and are reduced to just about half of their initial values. The improvement in sliding friction is related to the development of oxidised products in the wear track. For tribological applications, like turbomachinery parts of fretting interfaces, seals, and bearings, up to 580°C, films made by ion bombardment assisted deposition (IBAD), magnetron sputtering, plasma spraying, pulsed laser deposition, and AlCuFeCr, Cu/Mo, Zn/W, Ni/Ti, and Au/Cr multi-layered techniques have been created recently.

It can be concluded that soft metal such as silver, tin, and lead acts as self-lubricating film on a hard substrate because of their low melting temperature and shear strength. Tin and lead have been coated on the piston skirt surface as these can be used as the self-lubricant overlay. As aforementioned, lead has been widely applied in internal combustion engines as an overlay for journal bearings. However, polymer film having solid lubricants like PTFE, graphite and MoS₂perform lower friction properties and has been applied as the overlay substituting the soft metals.

2.3 Molybdenum disulphide

MoS₂ has an important place among the mostly used solid lubricants [27], which is obtained from earth’s crust in the form of Molybdenite. It is commercialised as inclusions, films, suspensions, or fine particles in the composites after refinement and suitable treatments. The compound’s sulphur lamellae are thought of as a laminar solid, and their weak van der Waals bonds make the shearing events that cause layer configurations during sliding easier. Furthermore, the lamellae have the necessary resistance to asperity penetration due to the strong covalent interaction between sulphur and molybdenum [27]. In Figure 1, the laminar structure is displayed. At ambient temperature and up to 300°C, the typical COF of unaltered MoS₂ is around 0.08. Depending on a number of variables, including the load, the operating circumstances, and the sliding speed, MoS₂ can provide enough lubrication in a vacuum up to 1000°C [28]. Oxidation significantly reduces MoS₂’s efficacy [29, 30]. When MoS₂ is oxidised, molybdenum oxide (MoO₃) is produced, which increases friction and wear. MoO₃has been observed to act abrasively with numerous alloys [27, 31]. Contrarily, MoO₃ additions to MoS₂ were found to improve tribological behaviour. Powder particle size, air accessibility, inclusion type, and composition are all factors that affect MoS₂ oxidation [32]. However, using inclusions with MoS₂ to prevent oxidation is demonstrated [27], which eliminates air from the particles. With a constant decrease in friction as temperature increased, NiAl-based composites with a 5 wt%Ti₃SiC₂–5 wt%MoS₂ composition had excellent tribo-behaviour [33]. At 400°C, there was a substantial reduction in the wear rates and COF. In addition to the development of a self-lubricating layer, it is believed that the production of TiO₂ and SiO₂protective oxides will reduce COF and wear. The lowest wear rate and COF were observed at 200°C [34] for ZrO₂/Y₂O₃ composites containing 10 wt% CaF₂ and 10 wt% MoS₂. MoS₂oxidises to MoO₃ at higher temperatures, which is a less lubricious [35]. For YSZ coating with Mo, the wear and COF was reduced up to 300°C [36]. However, a coating breakdown happened at temperatures higher than 300°C. Despite this, MoS₂ incorporation reduced the COF up to 700°C, however the onset of oxides and MoS₂’s non-lubricious impact were taken into account. Another tribological investigation [37] on composite coatings made of Ni₃Si and different amounts of MoS₂, BaF₂, and CaF₂ revealed that the MoS₂ broke down into Mo₂S₃, which enhanced wear and friction. Due to the low quantity of solid lubricant, the composite was also shown to have poor tribological properties at high temperatures. However, as the temperature above 400°C owing to the creation of a glazed layer, an admirable self-lubricating property was seen upon a greater solid lubricant concentration (15 wt% MoS₂ and 10 wt% BaF₂/CaF₂). At 350°C, MoS₂ PVD films showed a low and steady COF of 0.15. At around 370°C, MoS₂ was observed to degrade to MoO₃, albeit [38, 39]. Figure 2a shows the relationship between relative wear rates for different MoS₂ based solid lubricants [33, 34, 35, 36, 37] and Figure 2b shows the influence of sliding temperature over COF. The wear rate values at the stated temperature are used to determine the related wear rates after dividing with the wear rates at room temperature.

Figure 1.

Crystal structure of MoS₂.

Figure 2.

(a) COF, and (b) wear vs. sliding distance for MoS₂-based composites and coatings.

2.4 Graphite

Graphite has a hexagonal configuration, and is made up of planes of polycyclic carbon atoms, of which the basal planes have weaker bonding. When sliding against graphite, a metallic mating surface exhibits coefficients of friction that range from 0.05 to 0.15 in the surrounding air. But perpendicular to the basal planes has a three times greater coefficient of friction than moving parallel [15]. When tested in a plane that is perpendicular to graphite’s basal planes, it is discovered that graphite is soft and lubricates, but when tested in a vacuum or at high altitudes, it is found to be hard and wears out quickly. Dry nitrogen or a vacuum have a coefficient of friction that is typically 10 times greater than that of air. It is necessary to treat graphite with water or another condensable vapour, such as hydrocarbons, in order for it to acquire the lubricious characteristics that it possesses. Because water is absorbed by graphite’s hexagonal planes, the bonding energy that holds them together is less when the environment is humid. In reality, due to its high affinity for hydrocarbon lubricants, graphite shows superior lubricity in boundary conditions. Graphite is an effective lubricant in an oxidising environment up to 450°C before failing due to structural deterioration by oxidation. In order to operate over 650°C in military aircraft, and rolling-contact bearings were fitted with powdered graphite lubricants. Impregnated graphite parts are frequently utilised in high-temperature applications like fuel chamber liners, heat shields, and missile nozzle inserts but are not suitable for situations where loading or mechanical shock is rather extreme.

Graphite is frequently utilised in electrically conducting motor and generator brushes and the mechanical seals’ rubbing component. Amorphous carbon predominates in situations where a covering with more thermal insulation and less lubricity is needed. Amorphous carbon and graphite can be blended in this situation to fully utilise each material’s advantages and disadvantages. Carbon-graphite components are robust, durable, and have little friction. For applications involving harsh chemicals, carbon-graphite bearings that can operate for an extended period of time at temperatures higher than 580°C are perfect.

It can be summarised that the lubricating mechanism of graphite is mechanical in origin and arises from the sliding of one graphite particle over another. Graphite can be used as a dry lubricant or mixed with lubricating oil. For enhanced lubrication, graphite particles may also be included into a grease product. Due of graphite’s reputation as a great lubricant, many grades of graphite have been evaluated with a variety of water-based drilling fluids. It was determined that adding dry graphite to a water-based drilling fluid had a negligible effect on friction. Due to the failure of all tests utilising dry graphite, it was determined that the surface of the graphite was hydrophobic or organophilic.

2.5 Graphene

Graphene, a honeycomb structure containing two-dimensional carbon allotrope, is expected to have superior friction-reducing properties. MoS₂ and graphite both employ the same method to lessen slide friction. But unlike graphite, graphene exhibits lubrication in a dry atmosphere. Its usage is well acknowledged in solid or colloidal liquid-based lubricants [40, 41]. Also, it is frequently utilised in the electronics and mechanical sectors because of its remarkable mechanical, electrical (~10² S/m), and thermal qualities. It is significant solid lubricant because to its high strength, simple shearing ability, and chemical inertness. Additionally, it is frequently employed in nano and micro mechanical systems because of its ultra-thin dimension [42]. Due to the creation of a lubricious tribo-layer up to 400°C, which considerably decreased wear and CoF, graphene nano platelets (1.5 wt%) reinforced NiAl matrix demonstrated outstanding tribological performance [43]. However, when sliding temperature increases up to 500 C, graphene nano platelets (GNPs) lose their protective function owing to oxidation, which causes delamination and extensive adhesive wear. Nevertheless, unstable friction and increased wear were caused by a reduction in lubrication over 600°C as a result of its oxidation. Graphene layers and its oxide were studied for wear behaviour. Graphene layer offered the superior wear protection, and the wear was reduced as compared to steel sliding surfaces by 3–4 orders. However, Graphene oxide showed increased wear rate as compared to graphene layers by 1–2 orders [44]. The majority of research on graphene demonstrates the transitional period of increasing friction and wear over 550–600°C.

2.6 Hexagonal boron nitride (h-BN)

The “white graphite” known as h-BN is made up of stacked (BN)₃ rings. With preferred shear perpendicular to the c-axis, it has anisotropic shear characteristics [10]. The main source of lubrication at high temperatures is lamellar slide along the basal plane. Low strength and poor quality composite materials are caused by h-weak BN’s adherence to the majority of metals and ceramics and difficult sintering [1, 10]. Because van der Waals forces are stronger in h-BN layers than in graphite or MoS2, they perform less well in tribological tests. It operates better in high-temperature and humid circumstances due to its improved thermal stability and oxidation resistance, due to which it gets easily sintered. The composites and ceramics achieve better tribological by h-BN. In normal air, h-BN possesses coefficient of friction in the range of 0.2–0.25, and in humid air, less than 0.1 [45, 46, 47]. h-BN is added to lubricating oil [48] or water [49] or impregnated into porous surfaces [50, 51, 52, 53] as lubricating micro-particles. Although h-BN is more thermally stable than graphite and MoS₂, its coefficient of friction is high at ambient temperature; however, it decreases to 0.15 at 600°C. On a ball-on-disk tribometer, pure h-BN and h-BN-10 wt% CaB₂O₄ were tested against Si₃N₄ from room temperature to 800°C [46]. Cutting tools made of Si₃N₄ break down under fatigue loading and high temperatures, especially when hard materials are being machined at high speeds [54]. A Fe₂O₃, SiO₂, and B₂O₃ tribo-film forms when h-BN is applied to silicon nitride, considerably improving the tribo-pair of austenitic stainless steel. Al₂O₃, TiB₂, and B₄C use h-BN as a solid lubricant at high temperatures due to its oxidation resistance and chemical stability [55]. Due to low diffusion coefficient and flaky structure, h-BN is challenging to use as a cutting tool additive [56]. In Al-forming, h-BN substitutes soiled graphite or MoS₂ without leaving any stains. Surface quality and tribological performance, such as lubrication-film stability, are determined by the concentration and particle size h-BN powder [57]. A pin-on-disc high-temperature tribometer was used to conduct wear testing from ambient temperature to 400°C [58]. Understanding third body development and velocity accommodation is necessary for controlling wear. Microplastic deformation, brittle fracture, and Grooves are characteristics of BN-based composites [59]. For high-temperature applications, C/C-h-BN-SiC composites that were moulded, carbonised, and infiltrated with liquid silicon shown outstanding self-lubrication, self-healing, and oxidation resistance. At very high braking speeds, adding h-BN to C/C-h-BN-SiC improves friction and wear without clamping stagnation [60]. Titanium alloys were coated with a 15 m thick h-BN coating for high temperature tribological applications, and they were then rapidly thermally annealed in a furnace using infrared radiation. At 360°C, sliding against paired 15-5PH stainless steel cylinders lowered the friction coefficient from Ti-alloys to Ti h-BN from 0.72 to 0.35 [52, 61]. At room temperature, Ni-P-35 vol.% h-BN autocatalytic composite coating exhibits a 106 mm³/(Nm) wear rate and a 0.2 friction coefficient when applied to an AISI52100 steel ball [53]. From room temperature to 600°C, nickel-based composites are self-lubricated by silver and h-BN nanopowders [60]. On non-metallic substrates, we also evaluated the mechanical durability and tribological behaviour of h-BN films deposited using an ion beam. Although BN coatings on Si and SiO₂ had friction coefficients <0.1, they could shatter when placed on nonmetallic surfaces [48]. Table 2 represents the tribological behaviour of h-BN at different wear test parameters.

Material	Synthesis method	Wear test parameters					Results
		Type	CB	Load	Vel.	Temp.	COF	WR
h-BNh-BN-10 wt.% CaB2O4 [46]	Hot pressing	Ball-on-disk	Si3N4 ball	1.5 N	0.188 ms^−1	RT (in air)	0.18	—
						400°C (in air)	0.58
						800°C (in air)	0.38
						RT (in H2O)	0.08
						400°C (in H2O)	0.25
						800°C (in H2O)	0.22
Cu-based composites (Cu-Sn-Al-Fe-h-BNGraphite-SiC) [51]	Hot pressing	Block-on-ring	AISI52100 bearing steel	50–125 N	1.04–2.6 m/s	RT	~0.5	1.3 × 10^−5–4.3 × 10^−5 mm^3/Nm
B4C-h-BN [55]	Hot pressing	Pin-on-disk	B4C pin	10 N	0.656 m/s	RT	0.591–0.321	2.07 × 10^−5–1.94 × 10^−4 mm^3/Nm
NiCr/Cr3C2-NiCr/h-BN [49]	Plasma spraying	Ball-on-disk	Si3N4 ball	9.8 N	0.188 m/s	20°C	0.65	5.3 × 10^−5 mm^3/Nm
						800°C	0.55	1.15 × 10^−5 mm^3/Nm
Ni-P-h-BN [49]	Electroless plating	Pin-on-disk	AISI52100 steel ball	2 N	0.1 m/s	RT	0.2	1.24 × 10^−6 mm^3/Nm
NiCrWMoAlTi-h-BNAg [62]	Hot pressing	Ring-on-disk	AISI52100 steel ball	20 N	1 m/s	RT	0.54	7 × 10^−5 mm^3/Nm
						600°C	0.37	7 × 10^−4 mm^3/Nm

Table 2.

Tribological behaviour of h-BN.

Note: CB = counter body material; vel. = velocity; temp. = temperature; COF = coefficient of friction; WR = wear rate during wear test; RT = room temperature; and RH = relative humidity.

2.7 Transition metal dichalcogenides (TMD)

TMD solid lubricants are hexagonal layered compounds created by joining transition metals like molybdenum, tungsten, and niobium with chalcogenides like sulphur, selenium, and tellurium [27, 63]. The production of easily-shearable lamellas is caused by the weak adhesion forces (van der Waals) between atoms with sulphur-like characteristics. Strong transition metal and chalcogenide linkages make up the lamellar structures of Mo-, W-, and Nb-disulfides as well as Nb-diselenide. Particularly for vacuum applications, good intrinsic solid lubricants include MoS₂ and WS₂. Adsorbed substances or additives are not necessary for lubrication. The lubricity rapidly deteriorates as chemicals from the environment, such as H₂O, are absorbed, which affects the lamellas’ ability to slide. MoS₂ films are less susceptible to moisture when Pb, Au, and polytetrafluoroethlene are added (PTFE). In humid air, carbon enhances the performance of burnished and bonded MoS₂ [64]. MoS₂ films are less susceptible to moisture when Pb, Au, and polytetrafluoroethlene are added (PTFE). In humid air, carbon enhances the performance of burnished and bonded MoS₂ [3]. In non-oxidising conditions, MoS₂ is thermally stable up to 1100°C, but in air, it oxidises at 350°C. The maximum-use temperature is constrained by the oxidised by product MoO₃, which is thought to be abrasive. MoS₂ fails early due to a slow oxidative deterioration brought on by water vapour and oxygen in atmosphere. Surface oxidation at 100°C is indicated by a low W⁶⁺ content in WO₃ peaks. Sulphur virtually vanishes at 500°C, and WS₂ transforms into WO₃, as shown in Figure 3 [65]. Closed-cage WS₂ nanoparticles have been researched for improved lubrication in challenging environments [66].

Figure 3.

High temperature self-lubricating properties of magnetron-sputtered WS₂ coatings [65].

As lubricants or additives, TMD compounds decrease wear and friction. TMD powders are used in relay and switch contacts, threaded parts, sleeve bearings, and metal-forming dies. Materials containing TMD are employed in bulk composites made of powder metal, PVD thin films, and burnished thick coatings. It does not matter how a material is made; what matters is that it has low friction and low wear over a wide temperature range [2]. For reduced friction in bearings and other sliding/rolling applications, burnished MoS₂ or WS₂ coatings can be made with the basal planes of the MoS₂ or WS₂ crystallites parallel to the sliding direction. With proper process control, more uniform coating can be generated using spraying, such as the superior lubrication of SSRMS gears that have last several million cycles without wearing out. MoS₂ solid lubrication is used in space for release mechanisms, gears, slip rings, pointing mechanisms, and ball bearings. For upcoming space missions like the ESA’s Bepi Colombo mission to Mercury, the rising solid lubrication of MoS₂ must function over the wide temperature range with stable and reliable performance [67]. For Hubble and JWST, MoS₂ was selected as the solid lubricant [27]. Resin-bonded MoS₂and sputtered coatings are used on satellites and the space shuttle. Depending on humidity and sliding circumstances, resin-bonded spray coatings with heat curing have coefficients of friction over 0.06–0.15 and better wear life. Silicates (such as Na₂SiO₃) and phosphates (such as AlPO₄) are often the inorganic-bonded MoS₂ coatings for launch vehicle bearings and gears because they can withstand relatively high temperatures over 650–750°C. Softening or deterioration may result from water or humidity. MoS₂ that has been phosphate-bonded lubricates the main differential pivot on the Mars Science Laboratory (MSL) Curiosity Rover. Component friction is decreased via sputter-deposited MoS₂ coatings, either with or without soft metals and other compounds. With sputtered coatings, vacuum deposition on big objects is challenging [68].

It can be concluded that the possible alternative of soft metal lubricant is transition metal dichalcogenides (TMDCs), which are semiconductors of the form MX₂, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se, or Te). Due of its durability, MoS₂ is the most researched material in this family. TMDCs are attractive for fundamental studies and applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing, and personalised medicine due to their unique combination of atomic-scale thickness, direct bandgap, strong spin–orbit coupling, and favourable electronic and mechanical properties.

2.8 Binary metallic oxides

Superior wear resistance is seen in alumina, zirconia, and mullite, although high friction results in debris and cracks in dry air [69]. Some oxides make effective solid lubricants because of their exceptional thermal stability in air, even at high temperatures. They have not been investigated for room-temperature solid lubrication because of their brittleness. They are unable to produce smooth transfer layers on worn surfaces at room temperature because they refuse shear or deform. Debris from oxide wear is abrasive. In dry air, oxide surfaces are inert and do not produce powerful adhesive bonds like tribological materials. For solid lubrication at high temperatures, soft oxides have been investigated. B₂O₃, Bi₂O₃, Ag₃VO₄, and PbOAg₂MoO₄ are high-temperature lubricants that are thermally stable and efficient. They cannot lubricate when at normal temperature. Above a particular temperature, the brittle-to-ductile transition results in different friction and wear properties. Above 0.4 to 0.7 T_m, oxides become softer (in K) and to create the connection between ionic potentials and friction coefficient, a crystal-chemical model was proposed [9]. Low melting temperatures and strong ionic potentials are properties of Re₂O₇, B₂O₃, and V₂O₅. Lower binary oxide friction coefficient is caused by oxides with higher ionic potential. This lessens shear strength and explains why vanadium or molybdenum oxides behave lubriously. Contrary to the theory, Bi₂O₃ and PbO have low friction and low ionic potentials [70]. Oxides are categorised as highly basic, basic, oracidic based on interaction parameter, optical basicity, binding energy, and average ionic polarizability [71]. Increased ionicity and high unshared electron density are associated with low interaction parameters in very basic or basic oxides with low binding energy and high polarizability [70].

It may be possible to understand the complex friction behaviour of binary/mixed oxides based on polarizability by considering how the formation of vacancies and ion hopping at oxide surfaces affect frictional behaviour at high temperatures [71]. To better understand experimental results and generate predictions, computational modelling and simulations are utilised to look at the relationship between oxide crystal structure and frictional behaviour. DFT and MD simulations are used the most often in atomic-scale modelling techniques. DFT calculations are time-consuming and ineffective since they can only be performed on static, nanometre-scale systems. To better understand tribological behaviour on experimentally relevant length scales, molecular dynamics simulations look at the behaviour of moving atoms in a series of system configurations. Me_nO_2n−1, Me_nO_3n−1, and Me_nO_3n−2 are examples of substoichiometric transition metallic oxide compounds that have planar lattice faults that could lead to crystallographic shear planes with less binding forces. At high temperatures, TiO_x, VO_x, MoO_x, and WO_x deform due to plastic flow [72]. Good selflubricity is exhibited by vanadium-based Magneli phases in high-temperature hard nitride coatings [73]. Until the V₂O₅ phase melts at 690°C, these hard coatings reduce friction coefficients from 100 to 700°C [74]. Because lubricious Magneli phases are produced in sliding contact, hard nitrides or carbides, including Wor Mo components, offer higher high-temperature tribological qualities [75].

In order to create compounds or systems with low melting points, which have decreased hardness and shear strength at high temperatures, the difference in ionic potential can be increased [9]. Up to 1000°C, YBa₂Cu₃O_y exhibits a coefficient of friction over 0.20–0.50, making it a potential high-temperature solid lubricant. From cryogenic to high temperature, the mechanical and tribological characteristics of YBa₂Cu₃O₇-delta/Ag composites were assessed [76]. Strongly lubricious and sticky thin oxide layers can be produced in situ during wear or directly using coating techniques such as reactive magnetron sputtering in an oxygen-containing environment. The modern hard coatings have multiple uses and are highly hard, tough, temperature stable, oxidation resistant, low friction, and wear resistant [77]. Vanadium inclusions lower friction coefficient by self-adapting the hard coating while sliding, and nitride coatings provide excellent hardness and wear resistance [73]. When applied to worn surfaces above 400°C, lubricious V₂O₅ is produced by the V-containing nitride [78]. Reactive cathodic arc ion-plated (V, Ti) N coatings underwent reciprocating wear tests from room temperature to 700°C. The formation of TiO₂ and V₂O₅ oxides at 500°C lowered the friction coefficient of (V, Ti) N coatings. To minimise friction coefficient [79], Magneli phase V₂O₅ oxides were melted over the worn surface while sliding at 700°C. Under high-temperature oxidation conditions, hard nitride coatings produce lubricious metallic oxides. The reduction in friction between 450 and 650°C, slightly below the melting point of V₂O₅, is significantly associated with V₂O₅ and related Magneli phases. This conforms to the adaptive lubricating processes described by Voevodin et al. [73, 80]. As a result of in situ lubricious oxide formation, some Ni-Cu-Re, Fe-Re, and Cu-Re alloys exhibit friction coefficients ranging from 0.2 to 0.3.

It can be concluded that adaptive mechanisms have improved the solid lubrication of hard coatings at a range of temperatures. Future research will focus on binary oxides because it is believed that, with the right surface design and composition, they can act as lubricants under certain conditions, despite the fact that the majority of them only maintain low shear strengths over a narrow temperature range, typically at high temperatures. The oxide material is resistant to high temperatures, moist air, and vacuum.

2.9 Ternary metallic oxides

2.9.1 Molybdates

At high temperatures, lubricious materials include Ag₂Mo₂O₇, Ag₂MoO₄, CaMoO₄, K₂MoO₄, BaMoO₄, CoMoO₄, SrMoO₄, and ZnMoO₄. At room temperature, PbMoO₄ and CaMoO₄ exhibit low Mohs hardness-3.0 and 3.5, respectively. BaMO4, a scheelite-type tetragonal molybdate, is used in photocatalysts, solid-lubrication, photoluminescence and solid-state lasers. BaMoO₄ and SrMoO₄ powders are produced using various methods, including electrochemical method, intricate polymerisation method, micro-emulsion route, hydrothermal approach, microwave-assisted synthesis [81]. Tetragonal SrMoO₄ has lattice constants of a = b = 0.539 nm and c = 1.202 nm. The layered microstructures of Ag₂MoO₄ and Ag₂Mo₂O₇, which are similar to WS₂, may lessen friction at high temperatures [82, 83]. The lattice parameters of B-Ag₂MoO₄ are 0.9318 nm, and it features a characteristic AB₂O₄ cubic spinel structure with exceptional high-temperature stability. Pure molybdenum coatings and plasma-sprayed silver underwent in situ silver molybdate production analysis [84].

When compared to an unaltered Ni-based alloy, plasma-sprayed coatings at 600°C and 800°C reduced friction and wear. Fe-Mo alloys with CaF₂ added develop a surface glaze with MoO₃, Fe₂O₃, CaF₂, and CaMoO₄ after 600°C sliding wear tests. PbMoO₄ films made by pulsed laser deposition performed excellent at 700°C, but at ambient temperature, they degraded rapidly [85]. The tribological characteristics of hot-pressed nickel-chromium matrix composites with BaMoO₄ were investigated up to 600°C. The NiCr-20 wt% BaMoO₄ composite, which displays a lower friction coefficient and almost an order of magnitude lower wear rate at 600°C than unmodified Ni-Cr composite, demonstrates the greatest tribological performance. This is a consequence of the smooth, thick oxide layer that exhibits strong Raman peaks related to BaMoO₄ [86]. Because of the cooperative lubrication of Ag and the barium salts BaCrO₄ and BaMoO₄, Ni₃Al matrix composites self-lubricate from ambient temperature to 800°C [87]. Non-lubricious BaAl₂O₄ must be avoided during manufacturing.

2.9.2 Tungstates

ZnWO₄, CoWO₄, CaWO₄, BaWO₄, and SrWO₄ all lubricate well at high temperatures. At 600 to 800°C, CoWO₄ exhibits frictional properties between 0.25 and 0.25. Solid electro-optical and lubricant properties are exhibited by AWO₄ (A = Ca, Ba, and Sr). BaWO₄ is created through solid-state reactions, hydrothermal-electrochemical processes, and high-temperature flux crystallisation. Using a hydrothermal process, BaWO₄ powders with whisker-like, flake-like, and olive-like structures have been produced [88]. Solution approaches, employing organic templates and moderate hydrothermal temperatures, polymer, or micro-emulsions, were used to produce dendrite-like, hollow, and organised BaWO₄ structures [89]. SrWO₄ shows a tetragonal structure. The performance of Ag₂WO₄ was investigated at elevated temperatures as a tribological material using ab-initio MD simulations [83].

Using a ball-on-disc high-temperature tribometer, powder metallurgy was used to manufacture Ni₃Al-based composites with W, BaF₂-CaF₂, and silver. These materials were then tested up to 800°C for tribological performance. CaWO₄ and BaWO₄, which provide steady friction and little wear, are formed during high-temperature sliding [90]. Lubrication at high temperatures is provided by thin WS₂-ZnO composite coatings. ZnWO₄ was produced via the reaction of zinc oxide and tungsten disulfide at high temperatures [91]. A unidirectional ballon-disc wear and friction tester is used to assess the high temperature performance of WS₂ and ZnO burnished films [92]. The lubricious ZnWO₄ oxide layer formed at 500°C results in reducing the wear and friction. Zinc oxide created a high-temperature lubricant in these composite coatings, whereas WS₂ supplied low-temperature lubrication. When subjected to prolonged thermal cycling, however, WS₂’s low-temperature lubrication will be lost as a result of an irreversible reaction.

2.9.3 Vanadates

Although they are often ineffectual at generating self-lubricity at ordinary temperature, lubricious ternary oxides ofBi₄V₂O₁₁, BiVO₄, AgVO₃ [93], Ag₃VO₄ [82, 83], and binary V₂O₅ are effective and thermally stable at high temperatures. Depending on the quantity of oxygen ligands around the vanadium atom, the pentavalent monomer of vanadium oxide can be found in either the meta- or ortho-form. At room temperature, hydrothermal and wet precipitation techniques have been used to create vanadate powders with a variety of particle size distributions and morphologies. When tested with a ball-on-disc friction analyser, VN thin films of varying Ag contents show favourable frictional behaviour up to 1000 _C, which is attributable to the in-situ synthesis of lubricious Ag₃VO₄ and Ag-vanadate during sliding [82]. Ag/VN thin films were pulsed-laser deposited to produce Ag₃VO₄, AgVO₃ and V₂O₅ [82], which similarly showed enhanced tribological behaviour when slid against an alumina ball over 700–900°C. Due to the development of AgVO₃ and Ag₃VO₄ over 600–800°C [94], laser-clad NiCrAlY–based coatings with V₂O₅ and Ag₂O solid lubricants show enhanced wear resistance. To produce continuous self-lubricity over a wide temperature range, additional inorganic compounds, Ag₃VO₄, and soft metals were imbedded into textured hard surfaces [95].

2.9.4 Tantalates

(Cu, Ag)-Ta(Mo, V)-O based ternary metal oxides have minimal friction at high temperatures and are structurally and chemically inert. A layered structure containing-silver tantalate (AgTaO₃) with a layered structure can be slid at high temperatures to produce a silver-containing phase, which is a soft metallic phase [96]. Layered AgTaO₃ melts and transforms into structural phases at 1172°C [97]. The temperature dependency of tribological and mechanical sliding processes is related to variations in AgTaO₃ structural properties.

A number of processing techniques were looked into for coatings that need lubricious silver tantalate coatings at very high temperatures, including (1) powder films that have been burnished into the substrate (2) monolithic silver tantalate films made by magnetron sputtering (3) coatings made of an adaptive silver nanocomposite /tantalum nitride that, while sliding, forms a lubricious silver tantalate layer on its surface. The friction coefficients of this coating ranged from 0.06 to 0.15 when dry sliding across Si₃N₄ counter-faces at 750°C [96]. Through friction, heat, and shear stress, a nanocrystalline layer of Ag, Ta₂O₅, and AgTaO₃ was mechanically connected. Silver clusters lessen friction on the sliding surface [98]. Because of the silver’s high mobility and quick surface diffusion, AgTaO₃ has a low wear resistance. The random movement of silver particle clusters, which occurred at high temperatures and caused system failure in sliding components, was a notable phenomena [98]. Recently, using DFT and MD simulations with newly discovered empirical potential parameters and experimental results, the wear and friction mechanism of three ternary oxides-CuTa₂O₆, CuTaO₃, and AgTaO₃ were demonstrated. Experimentally, the composition of the film after sliding is compared with film before sliding, and the growth of Cu or Ag clusters throughout the film development is observed in DFT and MD energy barriers for atomic movement on the surface. Theoretical as well as experimental outcomes results confirmed the effect of metal (or metal oxide) clusters on the sliding surface on wear and friction mechanism [99].

2.9.5 Alkaline earth metallic chromates

As solid lubricants for self-lubricating metallic or ceramic matrix composites, MCrO₄, MCr₂O₄, and MCrO₃ (M = Ba, Sr., and Ca) between chromium sesquioxide and alkaline earth metallic oxides have been explored [11, 100]. M represents +2 alkaline earth metals in MCrO₄ oxometallates. Ba²⁺ cations with a coordination number of 12 and [CrO₄]₂ tetrahedra make up the constituents of BaCrO₄. BaCrO₄ has an orthorhombic structure. The crystal structure of BaCrO₄ is shown in Figure 4. BaCrO₄ has a compression coefficient of 0.0357 GPa and a bulk modulus of 28.1 GPa. BaCrO₄ is a high-temperature solid lubricant and an oxidising agent, which accelerate vapour-phase oxidation processes by acting as a catalyst. Moreover, it is a model system for researching the morphological regulation of inorganic minerals [11, 101].

Figure 4.

Schematic of BaCrO₄ crystal structure [102].

Oxometallates with the formula M²⁺Cr₂³⁺O₄²⁻ are made up of the oxides of bivalent and trivalent chromium metallic elements. The M²⁺ to Cr³⁺ratio radius determines the morphology of their crystals. The majority of MCr₂O₄ compounds with a spinel structure (M = Co, Ni, Zn, Mg, Cu, Mn, and Fe) are thermally stable. Alkaline earth oxides BaCr₂O₄, SrCr₂O₄, and CaCr₂O₄ are known as chromates [102]. These chromates have multilayer architectures with M²⁺-separated triangular CrO₂ sheets [103]. In the inert (Ar) environment, BaCr₂O₄ is thermally stable. At 1400°C, it is steady. BaCrO₃ has 4, 6, 12, 14, and 27 layers, with c/a ratios of 1.654, 2.433, 4.901, 5.752, and 11.101, respectively. Chemical co-precipitation was used to create BaCrO₄ particles with a variety of crystallographic morphologies and sizes for preparing solid lubricants at high-temperature [101]. It was investigated if BaCrO₄ particles made using the aqueous solution technique were thermally stable. BaCrO₄ has been shown to be thermally stable up to 1400°C by DTA-TG and X-ray diffraction. BaCrO₄ breaks down in two phases in vacuum. BaCrO₄ breaks down into Ba₃(Cr⁶⁺Cr⁵⁺)₂O_9−x with pentavalent Cr⁵⁺ and hexavalent Cr⁶⁺ cations, and BaCr₂O₄ with trivalent Cr³⁺ cations after vacuum heat treatments [104]. Due to its tendency to turn green on polished surfaces, BaCrO₄ is not thermally stable during vacuum sintering [105].

Thermal stability is one of the most important parameters for lubricants to function in a variety of atmospheres and at high temperatures. With oxygen, BaCrO₄ breaks down into Ba₃(CrO₄)₂ and BaCr₂O₄. BaCrO₄ breaks down into BaO Cr₂O₃ CrO₃ and BaCr₂O₄above 900°C in a non-oxidising environment, as per the Ba-Cr-O phase diagram. By using Cr₂O₃ and BaCO₃powders in stoichiometric ratio in a solid-state reaction, microsized BaCr₂O₄ particles were created [106]. A prior study found that BaCr₂O₄ is unstable in air at high temperatures. BaCr₂O₄oxidises to BaCrO₄ and Cr₂O₃ at 790.2°C. In high-temperature wear testing, an oxidation reaction could aid in self-lubrication. The wear track showed spread BaCrO₄ at high temperatures and is easily sheared [100]. At 800°C, dry sliding wear was investigated against an Al₂O₃ ball. BaCr₂O₄crystallises with [BaO₄]-chains and edge-shared CrO₆-octahedra, reducing wear and friction. BaCr₂O₄ ceramics showed low wear and friction over 400–600°C. When BaCr₂O₄oxidises in air, a self-lubricating layer forms on a worn surface, reducing wear and friction. The relative density of pure BaCr₂O₄ ceramics decreases with severe oxidation, speeding up wear [100].

Alkaline earth metallic chromates can be used to create self-lubricating materials [11, 100, 105, 106]. By combining BaCrO₄ and BaCr₂O₄ with a metallic or ceramic matrix, electrodeposition [100], low-pressure plasma spraying [11], and powder metallurgy [105] can create self-lubricating composites or coatings. Spark-plasma-sintered ZrO₂ (Y₂O₃) matrix composites with BaCrO₄ had wear rates of 106 mm³/Nm and friction coefficients of 0.29–0.32 from room temperature to 800°C [105]. At 800°C, barium chromate softens and forms a self-lubricating coating of fine grains on sliding surfaces subjected to tribo-stress. The in-situ growth of ultrafine nanograin surface glazing brought on at high temperatures by thermo-mechanical recrystallization/deformation is a lubricating process. At high temperatures, plastic smearing and self-lubrication are made possible by grain rotation and grain boundary sliding in the glaze layer. At high temperatures, the ZrO₂-BaCrO₄ coating developed using plasma-sprayed technique at low-pressure proved lubricious [11].

2.10 Alkaline earth metallic sulfates

High-temperature solid lubricants, papermaking, cosmetics, electronics, pigments, and ceramics all use alkaline earth metallic sulphates like anhydrite, celestite, and baryte [107]. Baryte and celestite’s exceptional lubricating properties are closely correlated with their morphologies and structural makeup. At normal temperature, the lubricating processes involve sliding along the (001) basic plane, and at high temperatures, in situ production of ultrafine nanograin surface glaze. Similar to BaCrO₄, SrSO₄ is made up of Sr²⁺ cations and [SO₄]₂ tetrahedra. Seven [SO₄]₂ tetrahedra form the coordinates for each Sr²⁺ cation. SrSO₄ crystal planes (002) and (210) are seen in Figure 5. Controlled nucleation and growth were used to create alkaline earth sulphate particles with distinct and well-defined crystallographic morphologies [108].

Figure 5.

Schematic of atomic arrangements at planes of (a) (002), and (b) (210) in SrSO₄ crystal [102].

With lattice values of 0.8359, 0.5352, and 0.6866 nm for SrSO₄; and 0.8881, 0.5454, and 0.7157 nm for BaSO₄, respectively, these substances have orthorhombic structures. With just an orthorhombic to a monoclinic phase change (i.e., structural change) at 1100°C and almost minimal weight loss up to 1300°C, baryte, celestite, and their sulphate solid solutions are thermally resistant. Surface energy, supersaturation, and reaction diffusion are three factors that have an influence on crystal formation and make it challenging to form sulphate hierarchical structures [107, 108]. Without the use of surfactants or templates, SrSO₄ nanocrystals with a range of features, from a needle-like to a tablet-like shape, were produced using a simple aqueous solution method [107]. The (020) and (210) planes affect the crystalline morphology of SrSO₄. Monodispersed peanut-type SrSO₄ particles with average length of 1.7 m and aspect ratio of 1.4 were created at room temperature. The mean pore size and BET surface area of these peanut-shaped SrSO₄ particles were 34.3 nm, and 20.9 m²/g, respectively. The chemically precipitated Ba_xSr_1−xSO₄ solid solution nanocrystals are characterised by orthorhombic structure and ellipsoidal shape. Ba_xSr_1−xSO₄ solid solutions are indexed as a single orthorhombic phase with the space group Pbnm (62) and changing composition parameters [109].

Moreover, baryte-like structure containing-alkaline earth sulphates exhibit exceptional self-lubricity, thermochemistry stability, and innocuity. At high temperatures, these sulphates lubricate. BaSO₄-impregnated hard surfaces with a texture can self-lubricate [95]. Only temperature shows a considerable effect on the tribological properties of baryte, and material is widely accessible and inexpensive. A lubricant for brake pads is baryte. Brake pad baryte is increased to minimise friction and wear. Variations in sliding velocity and rubbing pressure have minimal impact on the brake pads’ friction that contain baryte. BaSO₄ is added in greater amounts to friction materials to achieve excellent coefficient stability and fade resistance. Baryte can withstand high temperatures and does not alter much at 300°C [110]. To create self-lubricating, sulfate-containing composites, several techniques such as spark plasma sintering, hot pressing, electrodeposition, physical vapour deposition, plasma spraying, etc. were developed.

By using burnishing [111], electrodeposition [112], pulsed laser deposition [113], and powder metallurgy [69, 106],(Ba, Sr) SO₄, SrSO₄, and, BaSO₄may be injected or generated to create self-lubricating composites or coatings. At high temperatures, ZrO₂ (Y₂O₃)-Al₂O₃-50BaSO₄ composites show superior friction and wear characteristics than unmodified ZrO₂(Y₂O₃)-Al₂O₃ ceramics. The frictional behaviour of composite against an alumina ball as a function of wear cycle and temperature is shown in Figure 6 and worn surface at 800°C is shown in Figure 7. At 800°C, BaSO₄ composite has a 0.33 friction coefficient and a 4.72106 mm³/(Nm) wear rate. High temperatures cause a layer of self-lubricating fine-grained BaSO₄ to be form on sliding surfaces, avoiding direct balls to oxide ceramic tribo-contact. At high temperatures, lubricating operations result in a surface glaze with ultrafine nanograins because of thermo-mechanical recrystallization/deformation. Plastic smearing and self-lubricity are caused by the BaSO₄ nanograins’ grain boundary sliding and rotating in the glaze layer [69].

Figure 6.

Friction coefficients of ZrO₂(Y₂O₃)-Al₂O₃-50BaSO₄ composite at different temperatures [102].

Figure 7.

Worn surface view of ZrO₂(Y₂O₃)-Al₂O₃-50BaSO₄ composite at (a) 10 μm, and (b) 1 μm after wear test performed at 800°C [69].

Coatings that include alkaline earth sulphate boost tribology. For SrSO₄-Ag or SrSO₄ coatings on silicon nitride or ZrO₂(Y₂O₃)-Al₂O₃ceramics throughout a broad temperature range, chemical precipitation verifies low wear as well as friction coefficient [114]. When covered with silver, CaSO₄ films produced using a pulsed laser are flexible and readily malleable, lubricating more effectively than CaF₂ [113]. Using a high-frequency reciprocating ball-on-block tribometer with induction heating, the wear and friction properties of Al₂O₃ and SUS316 stainless steel coated with powder films are studied. Up to 800°C in air, Al₂O₃ coated with chemically precipitated SrSO₄ and BaSO₄ powder exhibit low friction coefficients. Flake-shaped BaSO₄ powder coatings over alumina have lower friction coefficients than lump-shaped films. Between ambient temperature and 800°C in air, coatings of BaSO₄-10mass% Ag on SUS316 show typical friction coefficients of 0.2–0.4 [111].

2.11 Silicates

Each O₂ ion is linked with two Si⁴⁺ ions to form the (SiO₄)⁴⁻ units that make up silicates. SiO₄ tetrahedra may be combined to develop rings/chains. These can be converted to sheets or double chains. Silicates have the ability to interchange cations that are not SiO₄ tetrahedra, such as Si⁴⁺ and Al³⁺, without compromising oxygen coordination. Layered micas may easily cleave in a plane while being hard. The layer-to-layer Van der Waals coupling is weak. Commercial ceramics’ machinability is improved by the use of micas and other minerals like serpentine and attapulgite. Tetrahedral SiO₄ group condensation occurs repeatedly and produces chains, cyclic, and larger polymeric structures [115]. Metal rubbing surfaces react with sodium or potassium silicate to produce a lubricating layer. Aluminium-magnesium silicate and other silicate-based materials like Al₄ [Si₄O₁₀](OH)₄ shield engine surfaces and reduce friction and wear. Silicate sliding contacts can fix themselves [116]. The tribofilm’s diverse mineral composition demonstrates that the mechanical damage can be self-repaired at sliding surfaces using these additives. A sticky melt layer in silicates causes them to lubricate rubbing contacts at high temperatures. Low-temperature silicates behave like hard solids, where friction is mostly unaffected by deformation strain rate. Tool performance and durability are enhanced by lubrication. At 920°C, inorganic sodium metasilicate lowers wear rate and friction coefficient (by half) [117]. A covering made of a Sb₂O₃nanocomposite, MoS₂, and magnesium silicate hydroxide was created by Wang et al. This 150–250 nm thick composite coating is created by burnishing powders of antimony trioxide, molybdenum disulfite, and lamellate magnesium silicate hydroxide onto a copper substrate. At 400°C, a super-lubricity condition is achieved by the composite coating, in which the friction coefficient lowers to less than 0.01 within 100 revolutions. This is because the magnesium silicate hydroxide, molybdenum disulfide, and antimony trioxidephase all act as lubricants, allowing for straightforward shearing. In the case of magnesium silicate hydroxide, the sliding motion releases O-H-O, OH-, O-Si-O, OH-Mg-OH, and Si-O-Si groups from its layered structure [118]. The nickel superalloy substrate is subjected to coated by burnishing magnesium silicate hydroxide-C-Sb₂O₃ and demonstrates high-temperature super-lubricity as a result of the formation of a silicate-containing carbon layer with an easily shearable composition [119].

2.12 Caesium oxythiomolybdate

The high-temperature solid lubricants (Cs₂MoOS₃, ZnMoOS₃, and Cs₂WOS₃) have been studied for ceramic bearings in single-use engines. The US Air Force Research Laboratory developed the complex chalcogenide Cs₂MoOS₃ in 1987 at Wright-Patterson Air Force Base (WPAFB). The purpose was to lubricate silicon nitride bearings in air at temperatures, speeds, and loads as high as 760°C, 1.2 million DN, and 890 N, respectively for 5–6 hours. To avoid failure and ensure a long wear life, burnished caesium oxytrithiomolybdate-based lubricants must be replenished. By reproducing the target chemistry, pulsed laser deposition makes Cs₂MoOS₃ films that are very adherent. These films were developed for silicon nitride bearings in order to interact with the environment and produce new lubricious phases at high temperatures [120, 121]. At 600°C, this adaptable lubricant exhibits 0.03 as a friction coefficient. Additionally, 700°C and room temperature are OK; however, 300°C and 800°C are not. As the test temperature increases, components interact with O2 and one another to generate a high-temperature lubricious phase. At 650°C, the friction coefficient of caesium oxytrithiomolybdate (Cs₂MoOS₃) covered with sodium silicate is less than 0.2 [121]. Over 200°C, Cs₂MoOS₃ becomes an unstable lubricant. Powdered Cs₂MoOS₃ is oxidised at temperatures between 600 and 800°C to produce Cs₂MoO₄, caesium oxides, Cs₂SO₄, etc. At 300 to 600°C, the oxidation of Cs₂MoOS₃ produces lubricious oxides such as MoO₃ and Cs₂MoO₄on ceramics. The friction coefficients of Cs₂MoOS₃ coatings on alumina and zirconia substrates are low. With a friction coefficient of less than 0.1, Cs₂MoOS₃films are effective lubricants on silicon nitride and silicon carbide in the temperature range of 600–750, and 500–600°C, respectively. Production of caesium silicate glass and the softening of oxides are necessary for lubrication [120]. For Si₃N₄bearings at high temperatures, Cs₂WOS₃ and ZnMoOS₃ are the thermodynamically stable lubricants [6, 120]. Rosado et al. [122] proposed that silicon nitride bearings with caesium tungsten (Cs₂WOS₃) bonded coatings might be lubricated with low shear strength glass. At 650°C, lubricious Cs-based compounds’ tribological properties and high-temperature rolling contact persistence were examined on Si₃N₄ balls [6]. The best outcomes came when an in-situ produced caesium silicate reaction layer was paired with a hydrated caesium silicate-bonded covering. Combining these factors resulted to rolling friction coefficients below 103 and low wear coefficients, resulting in very extended endurance lives despite high contact loads. The low wear coefficients allowed for this to happen. Adding alkali ions to a combination of silicate glass can make the glass more fluid. These glasses have been used as steel lubricants for the over 70 years. Hot extruded steel surfaces can be lubricated by glass lubricants at 600°C by reducing area and increasing length significantly [123]. For one-time use and brief periods of time, thin PLD films work well. It is necessary to assess the tribological performance and dependability of Si₃N₄ piston pins, high-temperature seals, intake and exhaust valves, roller followers, cam lobes, camshafts, etc.

2.13 MXenes

The conflict between people and energy has grown increasingly acute since the dawn of the twenty-first century, and humanity now faces a significant energy crisis as a result of high population growth and extensive fossil fuel exploitation. Every year, 20% of the world’s energy is used to overcome friction, and it is possible to decrease this energy loss by using new system design to create innovative materials, efficient lubricants [124, 125, 126]. For the energy crisis to be slowed and energy loss to be minimised, an efficient lubricant is essential. MXenes are carbonitrides, nitrides, or carbides of transition metals [127]. Early transition metals are denoted by M (such as Ti, V, Zr, Ta, V, Mo, orNb), X (such as C and N), and Tx (such as surface functional groups like -O,-F, and-OH) [91, 128, 129] (Figure 8a). Different MXene surface terminations are produced by various synthetic techniques. MXenes are produced from MAX phase via HF etching [131]. As shown in Figure 8b, etching techniques [132] produce multilayer MXenes by selectively removing A-atom layers. A SEM image of Ti3C₂T_xMXene in accordion form is shown in Figure 8c. Single-layer MXenes may be created using intercalation and delamination [133]. The terminations of HF-etched MXenes are -F, -OH, and -O. Fluorine-free MXenes have been produced using Fenton methods, electrochemical, and hot alkali [134]. MXenes have applications in many different industries, such as biology, catalysis, sensors, electronics, electromagnetic, and tribology, and energy storage. MXenes can travel between layers when under pressure because they have weak interlayer connections [135, 136, 137]. Because MXenes have a large specific surface area, it is simpler to create lubricating or transfer films, which reduce wear and friction. Their voluminous surface groups facilitate control and modification while increasing polymer affinity.

Figure 8.

(a) element compositions of MAX and MXene, (b) the preparation process of MXene, and (c) SEM image of multilayer Ti₃C₂T_xMXene [130].

MXenes are possible lubricant choices in a variety of tribological applications due to simplicity of modification, easily film manufacturing, having adjacent layers with low shear strength and improved interaction with polymeric matrixes. Element compositions of MAX and MXene is shown in Figure 9a. The growth rate of the tribology literature reports on MXenes has significantly outpaced that of MXenes in 2021, as shown in Figure 9b. These reports continue to rise every year. As lubricants, MXenes are underappreciated. The development of MXenes’ tribology is summarised in Figure 9c. Ti₃C₂T_x, the first MXene, was discovered in 2011 [127], although it wasn’t employed in liquid lubrication until 2014 [138].

Figure 9.

(a) applications of MXenes, (b) statastics of MXenes publications from 2015 to 2021, and (c) the development of MXenes in tribology [130].

In 2016 [39], Ti3C2Tx MXene was used for the first time as a polymer reinforcement in solid lubrication. The composites demonstrated exceptional tribological and mechanical properties. MXene was sprayed on copper discs as a solid lubricant in 2018 [40]. Recent years have seen a rise in MXene research. MXenes attained superlubricity in both solid and liquid forms in 2021 [41, 42, 43]. MXene is a fantastic lubricant, according to a recent research on friction. A thorough review article is required to comprehend the present state and issues with MXene-based lubricants since no focused study consistently summarises and analyses MXenes in the course of tribology research. From mechanical behaviour to simulation findings, this paper explains the lubricating potential of MXenes before summarising, analysing, and contrasting their tribological characteristics. Solid lubricants, lubricant additives, and reinforcing phases are the three tribological uses of MXenes (Figure 10). A summary of previous studies on MXenes and MXenes-based composites is provided, including information on preparation techniques, the characterisation of the materials, friction and wear tests, and lubrication mechanisms. Applications for MXenes in tribology encounter several challenges. We propose workable solutions and future research prospects for MXenes based on the research state and barriers. This paper provides a thorough summary of the state of the MXenes lubrication research, closes a gap in MXenes tribology, suggests research directions based on unsolved issues and real-world applications, and stimulates MXenes research.

Figure 10.

A categorisation of MXenes’ tribological applications [130].

Concluding the above discussed MXenes as solid lubricants, it can be demonstrated that although these have shown greater lubricity in solid lubrication, it needs a lengthy break-in time. Fast superlubricity is achieved in GO, and attempts are required to be taken to accomplish the same in MXene. In addition, MXene’s superlubricity technology is in its infancy, and its lubrication processes have not yet been confirmed, necessitating more research. MXene-based lubricants have scope in various industries, such as, spacecraft components, automotive industries, NEMS, machining industries, etc. These lubricants can decrease the environmental pollution and energy consumption significantly. These lubricants are expected to cope-up with the energy crisis and to achieve sustainable development.

3. Solid lubrication mechanism

In the absence of any liquid, gas, or lubricant, there is a significant amount of adhesion between two solid surfaces that are in perfect condition (ultra-high vacuum or new metallic surface). Chemical or physical forces can adhere [15]. In sliding and spinning equipment, adhesion leads to wear and friction. When exposed to extreme conditions such as high velocity, load and temperature, strong adhesion at tribo-stressing surfaces results in scuffing, cold welding, friction damage, or even disintegration. Adhesion is affected by surface characteristics such cleanliness, temperature, velocity, atmosphere, contact time, and loadas well as interface factors like mutual solubility, crystallographic orientation, chemical activity, and charge separation [15]. Components are exposed to severe wear and fatigue, high velocity, and high mechanical dynamic and thermal loads due to plastic deformation during hot metal forming [4]. During forming at high temperatures, the proper lubrication is required for decreasing the friction stresses, and avoiding seizures and galling. Tool wear occurs during high-speed dry machining due to abrasive, chemical, adhesive, and electrochemical wear [4, 5, 15]. Pantograph contact strips [4] deteriorate from arc discharge attack-induced adhesion and high-temperature mechanical impact. For casting thin-strip steel at high temperatures, refractory side dams withstand mechanical stress, corrosion, wear, and thermal shock. The lubrication mechanism differs with temperature variation under the application of friction force, which can be understood taking the solid-lubricated Ni₃Al composite [4]. Figure 11 shows different lubrication mechanism over wide range of temperature under sliding conditions.

Figure 11.

The lubricating mechanism of solid lubricated Ni₃Al matrix over wide temperature range [4].

The gas turbines must have abradable seals in compressor and turbine sections that can withstand abrasion, thermal corrosion, fatigue wear, and blistering. Extreme adhesive wear, also known as scuffing or smearing, and fatigue spalling during cyclic contact strain are the two main causes of rolling-contact bearing failure [139]. Significant anisotropy is seen in lamellar solids with poor interplanar cohesion. Cleavage occurs at low shear stresses and lowers sliding friction in materials with anisotropic mechanical properties. These lamellae self-lubricate as a result of crystallographic slip under light shear pressures [140]. Traditional solid lubricants (MoS₂, graphite, etc.) have layered structures that are simple to shear and provide self-lubricity; however, they lose their effectiveness at high temperatures (MoS₂at 350°C and graphite at 450°C) due to structural degradations brought on by oxidation. When applied as a thin coating on rubbing surfaces, chemically stable fluorides and non-lamellar soft solids like indium, lead, tin, silver, gold can lessen wear and friction.

The second solid lubrication method entails the constant, ongoing formation of soft solid layers while sliding. Although the hard substrate controls the contact area, the thin solid sheet dictates the shear strength of contacting asperities. The contact area and asperity shear strength determine the frictional force in these circumstances. The contact surface between efficient soft solid lubricants develops a strongly adherent transfer layer after a limited run-in period. While thin lead transfer films offer sliding lead-based alloys self-lubrication, the composite coatings of plasma-sprayed Ni₈₀Cr₂₀-Cr₂O₃-Ag-CaF₂/BaF₂with fluoride and silver eutectic components have a low shear strength layer (PS₃O₄) materials for low-friction bearings and seals [1, 7, 15]. For sliding and rolling contact components, soft films and lamellar solids are frequently used. To create lubricious tribo-chemical coatings, intermetallics, metals, and ceramics react with water vapour or air. Metal or ceramic surfaces are kept out of direct touch by these tribo-chemically reacted coatings’ low friction and shear strength. Vanadium or chromium as alloying elements produce tenacious and lubricious oxide layers in nitride or metal coatings that minimise friction at high temperatures [2].

Extremely high-temperature lubrication may be offered by microstructurally designed thermally stable oxides, particularly in oxidising conditions. Seven lubrication methods exist for lubricious oxides. (1) The ability to shear readily due to the crystal-chemical concept of cation screening by surrounding anions [9, 141]; (2) Oxides soften in the temperature range of 0.4–0.7 T_m (K), which is the ductile-to-brittle transition temperature. Upon meeting the operating temperature to a critical temperature for typical soft oxides, the lubricious behaviour is contributed for by material softening and plastic smearing [142]; and (3) The process of viscous flow originating from very thin liquid films, such as glass lubrication in hot metal formation, is analogous to the low friction characteristic of melting wear for the oxides by surpassing T_m [79, 103, 140].

Environmentally assisted oxidation results in the formation of ternary oxides with low melting points such as tantalates, tungstates, vanadates, niobates, and, silver molybdates, which get easily sheared. The nitride hard coatings follows a self-adaptation process through the lubricious oxides formation at high temperatures with the vanadium addition to it, due to which the wear resistance increases and these coatings can be application for metal cutting applications [73]. Lubricants made of polycrystalline oxide with nanometre-sized grains may become more ductile. The grain rotation and grain boundary sliding at low temperatures results in deforming the nano-sized oxides during tribo-stressing. On rubbing surfaces, adaptable and the introduced/created lubricious oxide nanofilms may show significant viscous flow and plastic deformation to increase the operating temperature range [143, 144].

From the previous research work, it can be concluded that for sliding friction the friction force has two contributing sources. First, an adhesive force is created at the actual contact area between the surfaces (the asperity junction). Second, a force of deformation is required to plough or slice the asperities of the harder surface through the softer. The resultant friction force is equal to the total of the two contributing sources: adhesion friction and deformation and/or fracture friction. The adhesion is caused by the attractive forces between the contacting surfaces. This model provides a foundation for comprehending how thin surface layers can reduce friction and provide lubrication. However, it should be understood that one of the contributing sources frequently influences the other. In other words, the two sources cannot be considered entirely separate. Also, Solid lubrication is accomplished by introducing a solid or self-lubricating substance with low shear strength and strong wear resistance between the interacting surfaces in relative motion. To reduce the coefficient of friction, the shear strength of the interface, the surface energy, the real area of contact, and the ploughing or cutting contribution must be minimised. In general, reducing wear requires limiting these characteristics while enhancing the hardness, strength, and toughness of interacting materials. In order to lower the coefficient of friction and wear rate of materials, surface design and engineering of solid lubricants can be implemented.

4. Conclusion

Many industrial applications, such as advanced propulsion systems in aerospace and aviation, metal processing, automotive, nuclear power engines, electric railways, etc., depend on an understanding of the tribological (wear and friction) behaviour of self-lubricating composites and solid lubricants in harsh environments. Research has been done on solid lubricant compounds that are environmentally friendly. A few examples of solid lubricants are laminar solids, polymers, chemically stable fluorides, soft metals, binary or ternary oxides, chromates, sulphates, and combinations. (1) Noble metals (Ag and Au) with increased ductility and plastic deformation, provide decent lubricity. Polymer composites made with polyimides or PTFE can lubricate up to 350°C. (2) Graphite and DLC transfer films lubricate in wet air, while MoS₂/WS₂ transfer films are good lubricants in dry N₂ and vacuum. At specific temperatures, layer-lattice solid lubricants (Graphite fluoride, MoS₂, etc.) can oxidise or dissociate, as can the complex chalcogenides ZnMoOS₃, Cs₂WOS₃, and Cs₂MoOS₃. In oxidising circumstances, the non-layered inorganic compounds CaF₂ and BaF₂/CaF₂ eutectic are chemically stable. They provide high lubricity from 500 to 900°C and have low shear strength and facile film-forming ability. Over a wide temperature range, the superior thermal stability and lubricity is exhibited by alkaline earth chromates (BaCrO₄ and BaCr₂O₄), sulphates (SrSO₄ and BaSO₄), and their solid solutions. (3) For extremely high temperatures, oxide lubrication will be investigated. To lessen brittleness at low temperatures, lower the oxide particle size. Rather than dislocation activity in ultrafine grains, sliding or rotation of grain boundaries in this instance causes plastic deformation. Powder metallurgy, thermal spraying, electroplating, laser cladding, physical/chemical vapour deposition, and additive manufacturing have all been used to create self-lubricating composites and coatings. (4) Solid lubricants’ synergistic effects related to tribology for vacuum, humidity, temperature, or load are heavily studied. Adaptive processes are designed to achieve low-melting-point and easy-to-shear binary and ternary compounds considering some parameters such as, melting of soft metals or temperature-activated diffusion, interfacial tribo-reaction or environmental-assisted oxidation. (5) The tribological surfaces for reversible temperature, humidity, load, or vacuum are necessary for wide-range solid lubrication through a number of heat cycles. Micro-laminate structures are utilised to create temperature-adaptive composites or coatings with lubrication that limits tribo-reaction, melting, oxidation, or diffusion across a number of heat cycles. (6) A breakthrough that might extend the life and performance of air and spacecraft is adaptive solid lubrication, which can operate from ambient temperature to 1000°C or more. The applications are the seal parts and contact bearings (sliding and rolling) for high-speed dry machining, side dams for thin-strip steel casting, high-tech propulsion systems, pantograph contact strips for electric trains. For better lubrication systems at low/high temperature, and high vacuum, pressure and chemical reactivity, the microstructurally engineered solid lubricants are crucial.