ICMCTF2013 Session E3-2+G: Tribology of Coatings for Automotive and Aerospace Applications

Reconstructive coatings have attracted great attention in recent years thanks to the advancement of active materials. In addition to be highly wear resistant and thermally stable, reconstructive coatings are self-generated. In this presentation, we discuss about these coatings through tribofilm formation and those containing solar-activated nanoparticles. In a lubricated system, it was found that lubricants with high polarity enhanced the formation of a transfer layer on the steel surface, whereas nonpolar failed to do so. Lubricants with high polarity are effective to prevent metal hardening and to bond debris particles to the metal surface. Those with nonpolar components, on the other hand, generate abrasive nanoparticles during sliding. Active reconstructive coatings can protect and repair surfaces. An example is given on a coating containing solar-activated nanoparticles as electron donors. When encapsulated with inhibitors, the coating provides protection and repair of damaged surfaces with improved triboloical properties.

Automotive industry thrives to discover new practical technologies in order to reduce total weight and reduce fuel consumption. Aluminum alloys are a good choice in terms of weight reduction however they are susceptible to wear and corrosion once introduced to the combustive environment of the engine. Many methods has been used and applied to tackle these disadvantages however each have their own drawbacks. Plasma Electrolytic Oxidation (PEO) treatment is an alternative technique for surface modification of aluminum silicon cylinder bores. The goal of this paper is to study the coating composition and surface profiles of PEO treated Al 319 cylinders and their influence on coefficient of friction (COF) on the surface of the bores and the wear rate on the counterface piston rings. Thin coatings (< 10 µm) on cylinder bores with different surface roughness were produced. The coated bores were then machined by a brushing technique. Tribotests on the brushed coatings were conducted under designed lubrication modes. The correlation of coating surface profiles vs. COFs and piston ring wear were discussed.

Future engine development needs the selection of new coating materials which support the functional design of complex tribological systems. PVD coatings may be utilized to control wear in low viscosity lubricants, to protect standard materials in high temperature and oxidizing environment and to adapt the wear between different materials. Coating design must consider possible chemical reactions with formulated oils and additives in lubricants. Reactive cathodic arc evaporation has the flexibility to synthesize new and very different thin film materials. Mo-N, Mo-O-N and other MoN-based coatings were produced at different substrate types and were characterized with respect to their stoichiometry (RBS,EDX), morphology (SEM, TEM), mechanical properties (Nanoindentation) and the microstructure of the layers was investigated by X-ray diffraction. The reciprocating wear test was used to compare the wear behavior for these coatings under dry and lubricated condition with 100Cr6 as counter-part. The test allows a classification of the coatings with respect to material transfer similar to scuffing or fretting and may be utilized for a pre-selection of coatings for engine tests. Very low friction coefficient and very low counter-part wear were observed under lubricated conditions.

Aluminum matrix composite coatings containing 0, 10, and 22 wt.% Al₂O₃ particulate reinforcements were deposited via the cold spray process. Reciprocating dry sliding wear experiments were conducted on the coatings using a custom-built in situ tribometer, such that the third body activity at the sliding interface was directly observed through the transparent sapphire counterface. Higher Al₂O₃ contents in the coatings led to greater stability of the transfer films (third bodies adhering to the slider), as well as greater stability of the friction coefficient, lower mean friction coefficient, and lower wear rates.

Wear tracks and transfer films from 0 and 22 wt.% coatings were analyzed ex situ via optical interferometry, scanning electron microscopy, focused ion beam cross-sectioning, x-ray diffraction, and nanoindentation to reveal the morphological, microstructural and mechanical properties associated with the change of transfer behavior. Third bodies in the wear tracks of the 0 wt.% Al₂O₃ coating exhibited near-surface grain refinement and cracks corresponding to the formation of wear debris particles. Wear tracks on the 22 wt.% Al₂O₃ coating consisted of a coherent, nanocrystalline third body, typically referred to as a mechanically mixed layer (MML). The MML exhibited elevated oxygen content, grain sizes below 100 nm, and higher hardness than the 0 wt.% coating wear track third bodies. Transfer films formed during wear of the 0 wt.% Al₂O₃ coating exhibited bimodal microstructures consisting of patches of aluminum with grain sizes of 0.2 - 1 μm intermixed with lamellae of nanocrystalline aluminum with grain sizes below 100 nm. Transfer films formed during wear of the 22 wt.% Al₂O₃ coatings consisted of nanocrystalline material similar to that found in the MML in its wear track.

In situ tribometry observations revealed that the dynamic changes in the transfer films contributed to increased wear and friction, and played a greater role than is commonly reported for metals tribology. Reduced wear in the 22 wt.% Al₂O₃ coating was attributed to mechanical stability of the transfer film, which was also connected to more uniform microstructures in the transfer film and wear track third bodies. The Al₂O₃ particles allowed for localization of the deformation and modified the third body flows, consequently leading to the formation of uniform microstructures. From this study, a new understanding was gained regarding the role of third bodies in the tribology of cold sprayed Al-Al₂O₃ composites, and the mechanisms described may have implications for the tribology of metals and metal matrix composites beyond the present system.