ICMCTF2011 Session E1-1: Friction and Wear of Coatings: Lubrication, Surface Effects, and Modeling

Transition metal nitride based hard coatings deposited by plasma-assisted vapor deposition are widely used to reduce friction and wear of tools. Application temperatures may be extremely different ranging from low temperatures for deep drawing dies to above 1000°C for dry-cutting tools. These different loading conditions demand the specific development of coatings to meet the individual requirements of the given application. This contribution presents an overview on self-adaptation mechanisms available to reduce friction and wear in different temperature regimes.

Self-lubrication can be achieved by the in-situ formation of easily shearable tribo-layers on the coating surface in a sliding contact, to accommodate the velocity difference by interfacial sliding. TiC_1-xN_x hard coatings have been studied as a model system because they present a time-dependent tribological behavior with an initial running-in period marked by an elevated friction coefficient, followed by a steady-state regime with low-friction and wear at room temperature in ambient air. Tribological tests performed at different relative humidity levels reveal that a minimum value between 15 and 25 % is needed to trigger the low-friction regime. By in-situ observations of tribo-layer film growth it could be observed that third body material is formed during this running-in period by plowing of the coating and shearing of the removed material. The appearance and thickening of the transfer film marks the beginning of the steady-state low-friction regime. At this stage in the tribological test, Raman spectra indicate the presence of C–H bonds in the wear track, being responsible for interfacial sliding and thus friction reduction.

Since humidity-based lubrication mechanisms as explained for TiC_1-xN_x and other lubricant phases like diamond-like carbon or MoS₂ fail at elevated temperatures, the so-called Magnéli phase oxides have been studied as potential candidates for self-lubricious tribo-layers effective at high-temperatures. Among these Magnéli phase oxides, V₂O₅ formed in-situ by segregation and oxidation of V in elevated-temperature sliding contacts has been studied intensively for friction reduction due its low melting point of about 650°C, thus acting as low-melting oxide tribo-layer film on the V-depleted coating. This high-temperature lubrication mechanism is demonstrated for out-diffusion and oxidation of V in V-alloyed Ti_1-xAl_xN and Cr_1-xAl_xN coatings.

In summary, it can be concluded that the in-depth understanding of the coating behavior under particular loading conditions enables to tailor coatings with beneficial self-adaptive tribological properties.

Nanocomposite thin films of niobium nitride, tantalum nitride, and vanadium nitride with silver nanoinclusions were created using unbalanced magnetron sputtering to investigate their potential as adaptive, friction reducing coatings. Our hypothesis is that in the low- to mid-temperature range, silver migrates via diffusion to the surface to reduce friction. At higher temperature, oxygen, silver and the transition metals react to form potentially lubricious double oxide phases at the counterface. The coatings were tribotested against Si₃N₄ at different temperatures between 22 and 1000°C while in-situ Raman measurements were performed during heating and wear testing at 750°C to identify the evolution of phases as the coating surface and in the wear track. The chemical and structural properties of the coatings were characterized before and after wear testing using x-ray diffraction and Raman spectroscopy. The post-wear testing investigations revealed the formation of silver niobate, silver tantalate, silver vanadate, and pure silver on the surface of the coatings. Tantalum and niobium-based coatings performed better than the vanadium-based ones throughout the entire range of temperatures. The NbN/Ag coating was then subsequently doped with MoS₂ to investigate if an increase in performance of the coating was attainable by introducing a low temperature lubricant. Friction coefficients were not reduced at low temperature, however, high temperature friction coefficients were lower. We speculate that the high temperature friction reduction may be the result of metal substitutions in the crystal lattice creating weaker chemical bonds, or different phases formed in the presence of sulfur.

The tribological mechanisms during friction interaction between stainless steel and steel at high temperature are very complex and can involve simultaneously different wear modes. The use of solid lubricants reduces these effects but their friction behavior at low and high temperatures have to be investigated, in order to optimize the coating nature and process used to protect the steel surface.

Cylinder-on-flat friction tests were conducted, using a linear reciprocating tribometer, to measure the friction coefficient of a molybdenum disulfide (MoS₂) based varnish coating and to quantify its durability. The varnish (5 microns thickness) was sprayed on a polished steel flat surface and then maintained fixed to the tribometer while a AISI-304L stainless steel cylinder (50 mm diameter, 15 mm width) was sliding on it. The conditions used during the tests were as follows: reciprocating motion (2.5 mm length, frequency 5 Hz), contact temperature between 20°C and 300°C, and normal load between 100 N and 400 N.

For each test, the evolution of friction coefficient versus time allowed us to determine the lifetime of the coating in the contact. The lifetime is defined as the number of cycles before elimination of the coating in the contact, corresponding to an increase of the friction coefficient. SEM and optical micrographs of the wear track (both on the cylinder and on the flat) were obtained to characterize the wear mechanisms of the coating and counterbody. The effects of test temperature and normal load on the durability of the coating and wear mechanisms are investigated.

The MoS₂ based varnish coating shows a high friction coefficient at high temperature. Determining the lifetime of a solid lubricant coating appears to be not easy. In general, it decreases with an increase in temperature or in normal load.

Friction in every wear situation raises the temperature in the contact zone. For reliable modelling and coating optimisation for wear resistance it is essential to determine the actual mechanical and tribological behaviour at these high temperatures rather than infer from room temperature. Recently, elevated temperature nanoindentation has become a valuable addition to nanomechanical test capability, with the NanoTest capable of testing hardness, modulus and creep behaviour reliably to 750C and beyond.

Nevertheless, it can be more important to test other mechanical contact situations such as sliding, fretting or impact. Results of innovative nano- and micro-tribological experiments at high temperature will be presented for a range of hard coatings.

Friction results will be reported for both (heated probe + heated sample), and for a heated probe alone. The latter simulates a normal friction situation where heating takes place due to local heat generation, whereas the former allows diffusion and surface equilibration to occur before contact, for instance as in high temperature machining.

Friction and wear mitigation is typically accomplished by introducing a shear accommodating layer (e.g., a thin film of liquid) between surfaces in sliding and/or rolling contacts. When the operating conditions are beyond the liquid realm, attention turns to solid coatings. The focus of this talk is how contacting surfaces change both structurally and chemically in order to control interfacial shear for two coating systems: nanocrystalline ZnO and nanocomposite MoS₂/Sb₂O₃/Au. It was determined that the coatings exhibit velocity accommodation modes (VAMs) of intrafilm shear and interfacial sliding, respectively, as characterized by advanced electron microscopy and spectroscopy techniques.

In the case of nanocrystalline ZnO, sliding and rolling contact fatigue (RCF) experiments and density functional theory (DFT) calculations revealed the atomistic origins of low friction and nanocrystalline plasticity when sliding along ZnO textured (0002) nanocolumnar grains. The atomic layer deposited (ALD) sub-stoichiometric ZnO film was structurally tailored to achieve low surface energy and low growth stacking fault energy basal planes. Sliding on this defective ZnO structure resulted in an increase in both partial dislocation and basal stacking fault densities through intrafilm shear/slip of partial dislocations on the (0002) planes via a dislocation glide mechanism. This shear accommodation mode mitigated friction and prevented brittle fracture classically observed in higher friction microcrystalline and single crystal ZnO, which has potential broad implications to other defective nanocrystalline ceramics.

In the case of amorphous-based MoS₂/Sb₂O₃/Au nanocomposite sputtered coatings, the main mechanism responsible for low friction and wear in both dry and humid environments is governed by the interfacial sliding between the wear track and the friction-induced transfer film on the counterface ball. In dry environments, the nanocomposite has the same low friction coefficient as that of pure MoS₂ (~0.007). But unlike pure MoS₂ coatings which wear through in air (50% RH), the composite coatings showed minimal amount of wear with wear factors of ~1.2-1.4 x 10^-7 mm³/Nm in both dry nitrogen and air. Cross-sectional TEM of wear surfaces revealed that frictional contact resulted in amorphous to crystalline transformation in MoS₂ with 2H-basal (0002) planes aligned parallel to the sliding direction. In air, the wear surface and subsurface regions exhibited islands of Au. The mating transfer films were also comprised of (0002)-orientated basal planes of MoS₂ resulting in predominantly self-mated ‘basal-on-basal’ interfacial sliding, and thus low friction and wear.

The use of coatings is more and more widely spread in contacts in order to reduce friction and protect surfaces from wear. Nevertheless, selecting an appropriate coating (from the very numerous available coatings) for given tribological conditions is always a complex process. The objective of this work is to develop a pre-selection tool of coatings, based on a coatings database and selection criteria.

The selection criteria (and consequently the characteristics of the coatings that are put in the database) are from diverse natures:

- Tribological behavior of the coating (wear resistance for different wear modes, friction coefficient in given conditions);

- Non tribological behavior (corrosion resistance, electrical, thermal or magnetic properties, biocompatibility, toxicity);

- Non functional characteristics (cost, deposition rate, color, pollution related to the deposition method);

- Limits coming from the running conditions (environment, temperature, relative humidity, loading, motion);

- Limits coming from the substrate (size, shape, material, substrate/coating adhesion, maximum temperature) and from the counterbody (hardness, roughness, chemistry…).

The initial version of the database contains 36 deposition methods and 156 coatings (with characteristics coming from the literature). It can be enlarged by the user.

The coatings in the database are ranked after a weighted search, with the requirements and selection criteria coming from the user.

Coatings are evaluated and compared by weight point and reliability. Weight point (W) is the sum of products of weight value (k_i) and weight factor (w_i) for each requirement (n_i). The values of k are linked to the coatings characteristics related to the selection criteria. They are between 0 and 10 and may come from quantitative or qualitative evaluation depending on the nature of the criterion. The values of w are chosen by the user (between 1 and 10) depending on the importance he wants to give to each research criterion in the selection. Reliability is used based on some presumption due to the incompletion of coating information. The total reliability (R) is the product of the reliability for each requirement (r).

Finally, all the remaining coatings are ranked according to the weight point considering all the requirements. The first 10 coatings are selected as candidates coatings, and their reliability are provided.