FRICTION AND WEAR BEHAVIOR OF TIN IN AIR - THE CHEMISTRY OF TRANSFER FILMS AND DEBRIS FORMATION

Friction tests were performed on TiN-coated substrates at low speed (less than 0.1 m s-1) in air. Optical (Nomarski) and scanning electron microscopy, Auger electron spectroscopy and transmission electron microscopy (TEM) were used to characterize the transfer films and debris generated during sliding against steel and sapphire balls. Friction coefficients of steel against rougher (R(a) almost-equal-to 60-100 nm) TiN coatings started and remained relatively high (0.5-0.7) owing to wear and transfer of the steel. After the TiN coating was polished (R(a) almost-equal-to 4 nm), transfer was reduced and initial friction coefficients ranged from 0.15 to 0.2. Initial friction coefficients with sapphire balls sliding against polished TiN were even lower (0.05). However, friction coefficients with both balls increased as debris formed and transferred to the wear track. Auger analysis showed that steel balls accumulated metallic iron and/or iron oxide as well as titanium oxide layers and debris, whereas sapphire balls acquired only titanium oxide transfer layers. TEM of debris stripped from contact areas of steel and sapphire balls identified the phases of debris as a rhombohedral ternary oxide (FeTiO3/alpha-Fe2O3) and rutile (TiO2) respectively; morphologies were either thin, uniform flakes - prevalent in low friction sliding - or spherical clusters. A thermochemical basis for oxide debris formation is given and the friction behavior is interpreted in terms of an oxide wear mechanism.