LUBRICATED SLIDING WEAR OF PHYSICALLY VAPOR-DEPOSITED TITANIUM NITRIDE

Coated components are used in conjunction with uncoated materials in many tribological environments that contain lubricants developed for the uncoated substrates. There is no reason to believe that a lubricant optimized for a steel substrate will have a similar effect on the coated surface and thus there is a need to assess the effects of lubricants on the sliding wear of coated components and identify any deleterious lubricant-coating interactions. The lubricated sliding wear performance of titanium nitride films sliding against 52 100 steel and polycrystalline sapphire counterfaces was investigated as a function of coating microstructure for a range of different physical vapour deposition (PVD) processes. Improvements in both coating wear rate and lifetime have been found, even using unformulated base oils, though variations in behaviour between lubricants have been observed which can be detrimental. When the microstructure of titanium nitride coatings on steel substrates is open (zone 1), the use of an unformulated oil can improve wear performance until it is comparable with that of a fully dense film with the zone T microstructure. Furthermore, the use of lubricants led to a substantial improvement in wear lifetime over similar unlubricated coating contacts owing to reductions in the amount of coating detachment during wear. The presence of the lubricant reduces the amount of transferred material from the sliding counterface (particularly for the steel sphere), minimizing coating-counterface adhesion and hence friction which reduces the incidence of premature failure by coating detachment. The implications of these observations are discussed for the use of TiN coated components in real tribological applications.