Invited Paper TR-ThA6
Tribochemical and Microstructural Evolution during Friction and Wear of Nanocomposite Coatings

Thursday, November 3, 2011, 3:40 pm, Room 111

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, such as in extreme environments, attention turns to solid coatings. The focus of this talk is how contacting surfaces and subsurfaces change both structurally and chemically in order to accommodate interfacial shear for two multifunctional coating systems: nanocomposite MoS₂/Sb₂O₃/Au and Ni/TiC/graphite. It was determined that the coatings exhibit velocity accommodation modes (VAM) of interfacial sliding and intrafilm shear, as determined by advanced electron microscopy (3-D focused ion beam serial cross-sectioning, HAADF-STEM, and HRTEM) and spectroscopy (Raman, Auger and EDS wear maps) techniques.

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.

In the case of laser deposited Ni/TiC/graphite composite coatings, it was determined during sliding that a wear-induced tribochemical and structural change from microcrystalline graphite to amorphous carbon/nanocrystalline graphite hybrid layer resulted in decreased friction and wear. Other novel insights were determined from 3-D microstructural evolution during wear, such as a mechanically mixed layer developed consisting of predominately refined nanocrystalline Ni grains (~10 nm grain size) and disordered carbon below this hybrid layer. The formation of these low interfacial shear strength films and recrystallized zones were responsible for intrafilm shear VAM to achieve low friction coefficients (~0.09) and wear factors (~6.8 x 10^-7 mm³/Nm).