The New Methods for Scuffing and Pitting Investigation of Coated Materials for Heavy Loaded, Lubricated Elements

In modern technology due to the increase of the unit pressure, velocities, and hence temperatures in the tribosystems of machines, a risk of two very dangerous forms of wear exists. These forms are scuffing and pitting. Scuffing is a form of wear typical of highly-loaded surfaces working at high relative speeds. Scuffing is considered to be a localised damage caused by the occurrence of solid-phase welding between sliding gear flanks, due to excessive heat generated by friction, and it is characterised by the transfer of material between sliding surfaces. This condition occurs during metal-to-metal contact and due to the removal of the protective oxide layer of the metal surfaces (Burakowski et. al., 2004). A typical scuffing zone of gear teeth (Michalczewski et al., 2010) is illustrated in Fig. 1.

pressed into the cracks at a very high pressure (elastohydrodynamic lubrication), making them propagate.Finally, cyclic stress results in breaking a piece of material off the surface.Examples of a gear and a race worn due to pitting (Michalczewski et al., 2010) are presented in Fig. 2. For many engineering materials, further improvement of their properties through a modification of their microstructure, chemical composition, and phase composition, is practically impossible.In this situation the most effective way of improving mechanical properties of various engineering components is the modification of surface properties by the deposition of PVD/CVD coatings (Michalczewski, 2008).One of the most important characteristics of these coatings is the fact that its thickness, usually in the range from 1 to 5 µm, is located in the field of dimensional tolerances of typical machine elements.
There are many successful applications of thin hard PVD/CVD coatings in various technical devices like engines, pumps, compressors.However the problem of application of such coatings for heavy-loaded friction parts (e.g.gears, bearings) is still open -the share of mechanical components that are coated is extremely small (less than 2%).Why?The service life of heavy-loaded machine parts is essentially determined by two types of tribological failures: scuffing which is a severe form of mechanical wear, and pitting which is a surface fatigue phenomenon.Up to now, there was a lack of verified laboratory test methods intended for correlated determination of coating material and lubricating media on scuffing and pitting resistance of heavy-loaded system.So, the selection of coating material and technology was realised mainly basing on very expensive and long-term practical component research and the results are frequently contradictory (Szczerek, Michalczewski, & Piekoszewski, 2009).(Szczerek, 1996).The SBT system was developed on the basis of the combinatorics that enables to reduce the tendency which is widely known as "testing rush".

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For the purpose of the tribological research in the areas mentioned above, two tribological devices have been developed: - The T-02U Universal Four-Ball Testing Machine, - The T-12U Universal Back-to-back Gear Test Rig.The set of methods and devices intended for the comprehensive tribological evaluation of PVD/CVD coatings is presented in Fig. 3.

COMPONENT TESTS MODEL TESTS T-02U
T-12U SCUFFING SCUFFING PITTING PITTING Fig. 3.The tribological methods and devices developed intended for comprehensive tribological evaluation of elements covered with PCD/CVD coatings By means of this set, low-friction and antiwear PVD/CVD coatings can be evaluated from micro to macroscale in model and component tests (Antonov et al., 2009).Using new devices, five test methods, giving the possibility of comprehensive testing of various low-friction and antiwear PVD/CVD coatings intended for machine elements, were developed.They are as follows: - The model method for the evaluation of scuffing in the four-ball tribosystem, - The model method for the evaluation of scuffing in the cone-three balls tribosystem, - The model method for the evaluation of pitting wear in the cone-three balls tribosystem, - The component gear method for the evaluation of scuffing resistance of gears, - The component gear method for the evaluation of pitting wear of gears.The new test methods and the new devices for the experimental evaluation of friction and wear of low-friction and antiwear PVD/CVD coatings are described below.The three stationary, bottom balls (2), having a diameter of 0.5 in., are fixed in the ball pot (4) and pressed against the top ball or cone (1) at the continuously increasing load P. The top ball/cone is fixed in the ball chuck (3) and rotates at the constant speed n.The tribosystem is immersed in the tested lubricant.During the run the friction torque is observed until seizure occurs.The test conditions are as follows: rotational speed: 500 rpm, speed of continuous load increase: 409 N/s, initial applied load: 0 N, maximum load: 7200 ± 100 N. The methods are described in detail in works (Szczerek & Tuszynski, 2002) and patented (Polish Patent No. 179123 -B1 -G01N 33/30).A friction torque curve (M t ) obtained at the continuously increasing load (P) is shown in Fig. 5. Scuffing initiation occurs at the time of a sudden increase in the friction torque -point 1.The load at this moment is called the scuffing load and denoted P t .

Model methods and T-02U Universal
According to the new test method, the load still increases (over a value of P t ) until seizure occurs (i.e.friction torque exceeds 10 N m -point 2).The load at this moment will be called the seizure load and denoted P oz .If 10 Nm is not reached, maximum load (c.a.7200 N) is considered to be the seizure load (although in such a case there is no seizure).For every tested lubricant the so-called limiting pressure of seizure (denoted p oz ) should be calculated.This value reflects the lubricant behaviour under scuffing conditions and is equal to the nominal pressure exerted on the wear scar surface at the moment of seizure or at the end of the run (when seizure has not appeared).The limiting pressure of seizure is calculated from the equation (1): where: p oz -limiting pressure of seizure, N/mm 2 , P oz -seizure load [N], d -average wear scar diameter measured on the stationary balls, mm.The 0.52 coefficient results from the force distribution in the four-ball tribosystem.The higher p oz value, the better action of the tested lubricant under scuffing conditions is.
The developed test methods were successfully used for testing the scuffing resistance of components with thin hard coatings (thickness of 2 µm) deposited by PVD/CVD method.The example of their application (Michalczewski et al., 2010) is presented in Fig. 6.Wear scars images on lower balls from scuffing tests for steel-steel and CrN-CrN tribosystems are presented in Fig. 7.
The developed test methods have the resolution, not achieved by the other methods, good enough to differentiate between coatings, engineering materials and lubricants (Piekoszewski, Szczerek & Tuszynski, 2001).What is more, they are fast and inexpensive.So, these test methods can be effectively used to select the optimum substrate-coating-lubricant combinations best suited for highly loaded machine components (Michalczewski et al., 2009a).The results indicate beneficial impact of low friction coatings on pitting wear (e.g.MoS 2 /Ti coating).
The presented method for testing pitting in cone-three balls tribosystem can be applied to testing fatigue wear of various materials, surface coatings as well as various lubricants.In comparison to other existing methods the new method gives better resolution and is timeand cost-effective.

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The New Methods for Scuffing and Pitting Investigation of Coated Materials for Heavy Loaded, Lubricated Elements 313

T-02U Universal Four-Ball Testing Machine
The methods for evaluation of pitting and scuffing resistance of PVD/CVD coatings is realised by means of T-02U Universal Four-Ball Testing Machine (Michalczewski et al., 2009b).The photo of the machine is presented in Fig. 11.The tribotester is equipped with a computer-aided system of control and measurements.

Component methods and T-12U Universal Back-to-back Gear Test Rig for evaluation of scuffing resistance and rolling contact fatigue of PVD/CVD coated gears
In research where high reliability is at stake, there is a tendency to use such test specimens that are similar to real machine components.The gear testing is incomparably more expensive and time consuming than tests carried out on simple specimens.But the main advantage is better reliability of the results obtained.
Concerning the most dangerous kinds of wear of gear wheels, two types can be specified: scuffing and pitting.These forms have been described previously in this study.

Component method for evaluation of scuffing resistance of gears
The test method for the evaluation of scuffing resistance of gears has been originally developed by FZG (Gear Research Centre) at the Technical University of Munich.This method was adapted for investigation of PVD/CVD coated gears at ITeE-PIB.All test gears are case carburised, with HRC 60 to 62 surface hardness and case depth of 0.6 to 0.9 mm."A" test gears are cross-Maag's ground, and their tips are especially shaped to achieve high sliding velocities, hence the tendency to scuffing.The tested PVD/CVD coating can be deposited on one or both gears -Fig.12.The only limitation is the deposition temperature that should be below 180°C, which is connected with thermal stability of gear material.Special coated gears (e.g.A20 type) are run in the test lubricant, at constant speed for a fixed time, in dip lubrication system.From load stage 4 the initial temperature is controlled.The oil is heated up to 90°C.Loading of the gear teeth is raised in stages.During the running time of each load stage the oil temperature is allowed to rise freely.After load stage 4 the pinion gear teeth flanks are inspected for damage and any changes in tooth appearance are noted.The maximum load stage is 12.If the summed total width of the damaged areas on all the pinion gear teeth faces is estimated to equal or exceed one gear tooth width then this load stage should be taken as the failure load stage (FLS).Additionally the oil temperature, vibration level and motor load during the test can be measured.
The main advantage of the method is the possibility of scuffing testing of various materials, surface coatings as well as various lubricants intended for heavy-loaded friction joints.The test method has been successfully used for extensive research to determine the effect of ecological gear oils on scuffing resistance of coated gears and for the selection of coating types for gear applications.An example of the research on gear oils is presented below.The method has been applied for selecting a proper DLC coating for increasing the scuffing resistance of gears.The results from gear tests are presented in Table 1 and Fig The presented component method for evaluation of scuffing resistance of gears have been applied for developing a new solution for manufacturing steel heavy-loaded machine components covered with low friction coatings that enables increase service life of components and allows lubricating with environmentally friendly oils.This will increase the reliability of machines and reduce pollution of the environment by oil.The result of the tests is the LC 50 fatigue life, related to 50% probability of failure.LC 50 is defined as the number of load cycles when the damage area of the most damaged tooth flanks exceeds 4% (about 5 mm 2 ).The total test time of each run is limited to 40 millions load cycles at pinion (300 operating hours).In some cases other criteria can be used.At least three valid runs are necessary to calculate the LC 50 parameter.
The main advantage of the method is the possibility of comprehensive testing on various low-friction and antiwear PVD/CVD coatings intended for heavy-loaded machine elements.
The method is realised by means of the worldwide popular back-to-back gear test rig.
The test method has been successfully used for extensive research to determine the effect of low-friction and antiwear coatings on pitting wear.An example of the research on gear oils is presented below.
The results indicate that for the coated/coated pair (pinion and wheel coated) and coated pinion/steel wheel pair a significant decrease in the fatigue life compared to the uncoated gears was obtained -Fig.16.The best results were obtained in the case of the steel pinion/W-DLC coated wheel -even fourfold increase in the fatigue life was observed.This shows a very high potential of the application of DLC coatings for gears.Thanks to the component gear method for the evaluation of pitting wear of gears, it was possible to overcome the main factor hampering application of thin coatings on heavy loaded elements for many years i.e. their poor behaviour under cyclic stress conditions.This new method will allow for selection of low-friction and antiwear PVD/CVD coatings intended for manufacturing of steel heavy-loaded machine components.This will increase the service life of components and allow for the application of environmentally friendly oils.This will increase the reliability of machines and reduce environmental pollution.

T-12U Universal Back-to-back Gear Test Rig
The T-12U Universal Back-to-back Gear Test Rig makes it possible to investigate both aforementioned forms of wear.The photo of the tester is presented in Fig. 17  The tribotester is equipped with a microprocessor-aided controller and as an option, it may also be equipped with a computer-aided measuring system.A very wide range of lubricants can be tested using the T-12U Test Rig, e.g.: gear oils, hydraulic-gear oils, eco-oils, non-toxic oils, and new EP additives.What is more, there is a possibility of testing modern engineering materials and surface coatings intended for gear manufacturing.Many test methods described in international and national standards can be www.intechopen.com The New Methods for Scuffing and Pitting Investigation of Coated Materials for Heavy Loaded, Lubricated Elements 319 performed -ISO 14635-1, 14635-2, 14635-3, CEC L-07-A-95, L-84-02, DIN 51354, IP 334, ASTM D 5182, D 4998, PN-78/C-04169, FVA information sheets: 2/IV (1997), 54/7 (1993), 243 (2000).For the last few years, the T-12U Rig has been successfully used at ITeE-PIB for the extensive research to determine an effect of modern gear oils (including ecological oils) on different forms of gear tooth wear, as well as possibility of improving the gear life by the deposition of low-friction coatings.

Conclusion
Presented methods give the possibility of comprehensive testing on various low-friction and antiwear PVD/CVD coatings intended for machine elements.All the presented methods and both tribotesters i.e.T-02U Universal Four-Ball Testing Machine, T-12U Universal Backto-back Gear Test Rig have been implemented at the Tribology Laboratory of ITeE-PIB and successfully verified.They are employed to perform various kinds of projects e.g.grants, R&D projects, ordered by the Polish government and international projects (COST Actions, 6th EU Framework Programme).They are also used to realise research orders from Polish industry (especially small and medium size enterprises) and the scientific sector (research institutes, technical universities).The new methods exhibit very good resolution and precision comparable to standardised test methods and are time and cost effective.Furthermore the cone-three ball method gives the possibility of testing fatigue wear of any coating and substrate material.Basing on the elaborated methods the optimal selection and development of PVD/CVD technologies applied for extension of the life of the heavy-loaded friction joints as well as the elimination of toxic lubricating additives have been obtained.The further development of tribological devices is performed in the frame of Strategic Programme "Innovative Systems of Technical Support for Sustainable Development of Economy," which is currently realised at the Institute for Sustainable Technologies-National Research Institute (ITeE-PIB) in Radom, in Poland.The Programme is realised within the framework of the Innovative Economy Operational Programme co-funded from European structural funds.The greatest emphasis is put on the development of advanced machines for testing spur gears and rolling bearings under extreme conditions.

Fig. 2 .
Fig. 2. The pitting wear: a) on a pinion gear, b) on a bearing race The evaluation of friction and wear characteristics of PVD/CVD coatings is only possible on the way of experimental research.The experimental research of friction and wear of interacting surfaces is realised by means of a special device called tribotester.The new test methods and testing machines have been developed based on the achievements of the System for Tribological Research (SBT) implemented in the Tribology Department at the Institute for Sustainable Technologies -National Research Institute (ITeE-PIB), Radom, Poland Fig. 4. Model tribosystems for testing scuffing: a) four-ball tribosystem: 1-top ball, 2-lower balls, 3-ball chuck, 4 -ball pot, b) cone-three balls tribosystem; 1 -top cone, 2 -bottom balls, 3 -ball chuck, 4-ball pot

Fig. 12 .
Fig. 12. Coated test gears used for testing scuffing -type A

Fig. 13 .
Fig. 13.Failure load stages for uncoated steel gears and for teeth coated with DLC coatings lubricated with eco-oil (A/8.3/90method)

Fig. 14 .
Fig. 14.The photographs of teeth surfaces after tests for various DLC coatings (gears lubricated with eco-oil) www.intechopen.comTheNew Methods for Scuffing and Pitting Investigation of Coated Materials for Heavy Loaded, Lubricated Elements 3173.2Component gear method for evaluation of pitting wear of gearsSimilarly to scuffing gear tests, the method for evaluation of pitting wear of gears has been originally developed by FZG (Gear Research Centre) in the Technical University of Munich.This method was also adapted for the investigation of PVD/CVD coated gears at ITeE-PIB.The experiments are performed using the single-stage pitting test procedure (PT C/10/90) in an FZG type gear test rig, using C-PT gears -Fig.15.Special coated gears (C-PT type) are run in the lubricant test, at constant speed for a fixed time, in dip lubrication system.The load stage is 9 or 10 giving 302 Nm and 372 of torque respectively.The oil is heated up to 90°C.The oil temperature is controlled and kept at constant level.The inspection of gears is performed every 7 or 14 hours.

Fig
Fig. 16.Fatigue life LC 50 for various pinion/wheel gear material .

Fig. 17
Fig. 17.T-12U Universal Back-to-back Gear Test Rig The New Methods for Scuffing and Pitting Investigation of Coated Materials for Heavy Loaded, Lubricated Elements

Table 1 .
Furthermore the test can be realised by means of the worldwide popular back-to-back gear test rig manufactured by many producers.The teeth failure at load stage for various DLC coatings (gears lubricated with eco-oil)