ICMCTF2011 Session E1-2: Friction and Wear of Coatings: Lubrication, Surface Effects, and Modeling

Improvement of the life of components which undergo a combination of rolling and sliding in service, such as gears, remains an engineering challenge in particular as components get smaller and loads and slide-roll ratios increase. The major failure mode in such cases is micropitting originating from cracking generated by surface contact fatigue. Reducing the frictional tractions at asperity contacts should reduce the stresses driving such behaviour and reduce micropitting and there has been considerable work aimed at developing lubricant additive packages to achieve this. The most successful of these packages contain chemical constituents with high environmental impact and indeed several are potent carcinogens so there is a need to identify more benign solutions to the problem. Recently it has been shown that inorganic fullerene materials (IFLM) based on WS₂ have the potential to produce low friction transfer layers when added to lubricants and coatings. However, the majority of work to date has been carried out under pure sliding conditions. Thus, the objective of the work presented here was to assess the influence of IFLM in hard coatings on the fatigue and wear performance of tribological contacts subjected to rolling/sliding motion such as gears.

Preliminary wear tests were carried out using a double disc rig with specimens manufactured from carburized S156 steel in the uncoated form and coated with CrN containing WS₂ fullerenes. The results show that significant increase in the operational life of rolling/sliding components can be achieved by using coatings incorporating a sufficient volume fracion of inorganic fullerene-like materials. The IFLMs produced a low friction transfer layer on the surface which was responsible for the good performance. Coatings were then developed to produce a similar low friction transfer layer based on the combination of a hard coating and a metal sulphide. Full scale back-to-back gear tests were then performed to see if the improvements could be transferred to the component scale. Good results were achieved when the transfer layer was also formed in this case. The factors affecting transfer layer formation in service will be discussed in this presentation.

Improvement of fuel economy and component durability along with reduced emissions are important concerns for future engine systems. On other hand, the operating conditions are becoming more severe due to continual need of high power as well as light weight engines. Conventional lubrication (ZDDP and MoDTC) were optimized for conventional material surface (steel) thus, gave good tribological performance but had concerns associated with emissions. On other side, advanced surfaces like Diamond-like-Carbon offered improved tribological performance for conventional lubricants than the steel surfaces. Solid lubricants like Graphite, MoS₂, WS₂, PTFE, BN, Boric acid offered high bearing-load and low friction due to their lamellar structure orient parallel to the sliding surface along with low toxicity. Colloidal lubricant of nano-particles of solid lubricants like MoS₂, WS₂, onion-like-carbon (OLC), BN, Boric acid offered good tribological performance on steel.

The current work presents tribological performance of novel Boron based Lubricant on the ta-C coated steel surface. High frequency reciprocating rig (HFRR) tribometer was used to evaluate the friction and wear behavior of these lubricants. The performance of the lubricants was investigated under boundary lubrication regimes. High temperature and high contact pressure conditions on the HFRR tribometer. Optical profilometry was used to analyze the wear on the contact surfaces.

It was observed that novel Boron additive tested on ta-C surface gave low friction and extremely low wear. In order to understand the lubrication mechanism for the observed novel boron based additive on ta-C surface, the chemistry of the tribofilm formed on the contact surfaces were characterized by ToF-SIMS (Time of Flight - Secondary Ion Mass Spectroscopy), XPS (X-ray Photoelectron Spectroscopy) and FIB-TEM (Focused Ion Beam – Transmission Electron Microscopy). The post-test surface analysis validates the influence of the novel Boron additive that form reaction film on ta-C surface with super lubricious characteristics and hence gave good performance.

Reducing friction in the engine is a key factor towards lowering the automotive CO₂ emissions. A 25 to 30% friction reduction is commonly observed in a a-C:H coated tappets valvetrain system at low rotational speed when the system operates in boundary lubrication regime. Such tests are both time consuming and expensive. The purpose of the study is to define a simple test where the friction reducing is correlated to the one observed in car engine. Particularly, the purpose is to define the effect of oil, additives and coating nature on friction reduction. To ensure representativeness in terms of interaction between coating and oil, it must be operated in boundary lubrication condition all along the test.

Friction tests were carried out using a ball on disk configuration. The hard coating was deposited on the flat sample. The ball wear is correlated to small variations in coating roughness as determined by AFM. The stabilised friction coefficient is correlated to the wear of the ball and the resulting decrease of pressure at the end of the test. The lubrication regime could change along the friction test from boundary to elastohydrodynamic regime.

To overcome this problem, the tribology configuration was changed. The coating was deposited on the ball. The wear on the ball is negligible and the contact pressure remains constant even after 25000 cycles. The friction reducing was similar to the one observed in an engine test. That configuration enabled to test oils such as SAE 5W30, 0W20, Polyalphaolefine (PAO) and PAO with 1% Glycerol Monooleate (GMO). Using PAO oil, the stabilized friction coefficient of a-C:H coating was drastically reduced (0.060) compared to the friction coefficient using fully formulated engine oils (0.085). The experiments have shown that the degradation of friction is correlated to the formation of the anti-wear film on the steel. When using fully formulated oils, ZnDTP decomposes and forms a coloured film on steel, mainly composed of O, P and Zn. To demonstrate the detrimental effect of the anti-wear film building up on steel, a test was carried out using 5W30 oil. When the anti-wear film was built, the 5W30 oil was rinsed with solvents and replaced by PAO oil. Instead of decreasing to 0.06, the friction coefficient was drastically increased to 0.13. Changing the friction track to a radius where the DLC coated ball was slide against steel without anti-wear film, the friction coefficient immediately started at 0.060, thus evidencing that anti-wear additives did not lead to film build up on the coating. The use of GMO addition to PAO produces a further reduction of friction to 0.05.

Nanocomposite Ti-Ni-C coatings are investigated for their electrical and tribological properties. It has been shown that nanocrystalline TiC in an amorphous carbon matrix has a contact resistance comparable to metallic coatings but preferable tribological properties. The addition of a non-carbide forming metal helps to ensure the presence of a lubricating layer in the contact for compositions with little carbon matrix.

The intended application is a type of contact brush for signal and power transmission where the coating deposited on a spring steel wire slides against a rotating metal-graphite composite. In such metal-graphite composites, graphite provides a good conductivity and it is a well known solid lubricant and the metal is added to further enhance the conductivity. The accuracy of the power- and signal transmission is of highest priority in the application but if the wear rate could be lowered, possibly by the use of a Ti-Ni-C coating, the life time of the carbon contact brush may be improved.

Different compositions of the Ti-Ni-C coatings are investigated in order to find the optimal composition with balanced low wear of both the coating and the metal-graphite. The amount of amorphous carbon matrix in the coating affects the hardness which is of importance for the wear of the system, where the coating is by far the hardest component. The amount of matrix also affects the friction as it acts as a solid lubricant.

The wear rate of the metal-graphite, without current passing through it, is measured for different spring loads and rotational speeds. The coated spring is examined after 20 million revolutions using SEM and EDS to study the wear and the formation of tribofilms. For one certain spring load and rotational speed it is investigated how the current affects the wear rate of the metal-graphite and the formation of tribofilms in the contact. The transmitted current is monitored to investigate the stability over time.

A second experimental setup with cylinders in reciprocal motion allows for investigation of the contact resistance at a higher contact pressure. The coating is examined both self-mated and against the metal-graphite.

The usefulness of the nanocomposite Ti-Ni-C coating for this particular electrical contact application as well as for electrical contacts in general is evaluated and discussed.

Ultra-low carbon (ULC) steels show low yield strength and excellent formability. Plasma Assisted Physical Vapor Deposition (PAPVD) could be a potential coating method for enhancing mechanical resistance (MATTHEWS, 1995). However, in low mechanical resistance alloys PAPVD coatings may undergo premature failure if the substrate plastically deforms under heavy loading. An extra loading support is necessary for hard coatings to perform satisfactorily. Combined treatments involving plasma nitriding and PAPVD coating have been used to improve the load-bearing capacity of hard films ( Avelar-Batista, 2006). This work describes the characterization and micro-abrasive wear behavior of Ti-stabilized ULC steel after surface modification by D.C Triode Plasma Nitriding (DC-TPN) and sequential coating with Cr-Al-N by Electron Beam Plasma Assisted Physical Vapor Deposition (EB-PAPVD). The Ti-ULC substrate, the nitrided steel and the Ti-ULC duplex systems were investigated for chemical composition and via characterization techniques as SEM, EDS, XRD, micro hardness, instrumented indentation hardness and stylus profilometry. Micro-abrasive wear tests were performed in fixed-ball configuration up to 1350 revolutions (in 150 revolutions intervals) using SiC abrasive slurry and 25mm - 52100 steel ball. Micro-abrasion mechanisms are discussed. When compared to untreated Ti-ULC, nitrided steel is 2.7 times harder while the duplex system is 3.6 times harder (HV0.1). The wear coefficient for the nitrided steel is 22% lower than the one for the steel substrate, both them calculated for the steady state. Regression technique was used for calculating both substrate (kc) and coating (ks) wear coefficients of the duplex system, the latter being 6.6 times lower than the one for the nitrided steel. Coating thickness (3µm max.) was determined from inner and outer diameters of the wear craters. The results indicate that it is feasible to manufacture duplex Ti-ULC steel via PAPVD, with wear resistance improvement from the Ti-ULC to the nitrided steel and especially for the duplex system.

The selection of low friction coatings is of great interest to industrial applications. Nevertheless, regarding the lubricant lifetime of the coatings, the selection criteria often depend on the experimental apparatus and contact configuration and then cannot be directly applied to real cases. In this study, we use a model based on the local dissipated energy due to friction under gross slip conditions in fretting wear. Indeed, the maximum value of the local dissipated energy is a unique parameter that takes into account the two major variables in fretting wear experiments: the normal force and the sliding amplitude. Hence by plotting the “lifetime” versus the “local energy density” a single master curve defining the intrinsic endurance of the coating can be defined. To identify the intrinsic “energy density capacity” variable, characterising the coating fretting wear endurance (i.e. used to model the endurance master curve), a flat on flat contact configuration, allowing constant pressure conditions, is applied. This approach is considered to investigate various commercially available polymer bonded MoS2 solid lubricant film used in aeronautical applications to protect titanium surfaces in contact. The effects of contact pressure, displacement amplitude, frequency and temperature are investigated. The results show that, the local energy wear approach is suitable to characterize the lubricant performance for variable mechanical loading conditions (i.e. sliding amplitude and pressure). By contrast, elevate sliding frequencies and temperatures, by activating severe tribo oxidation processes, sharply modify the wear processes so that the endurance values, which are significantly reduced, can not be transposed on the fretting wear master curve. Using this friction energy concept, a first quantitative description is nevertheless provided to formalise the solid lubricant endurance reduction induced by severe thermal exposures.

Thin film coatings are widely used in many tribological applications. So far the selection of coating properties is done mainly by trial and error approach. For example, optimization of the thickness of coating for minimum friction and wear or for maximum electrical conductivity, based on scientific theory, does not yet exist. Failure mechanisms of coatings such as delamination, for example, are not well understood. The main goal of the present theoretical study is to analyze the effect of material properties of substrate and coating, as well as asperity tip curvature and coating thickness on the onset of plastic yielding in a typical asperity contact of a coated rough surface subjected to normal loading. This may enable optimization of the coating thickness for best performance in various applications.