Tribological characterisation of ta-C coatings under lubricated conditions

The friction and wear behaviour of Ti-6Al-4V can be improved significantly by protecting the surface with a ta-C coating. The friction is low for all tested liquid media and is about 0.05 for water based sodium chlorine solutions and between 0.05 and 0.08 for different oils. The wear resistance (reduction of specific wear rate) depends on the liquid and leads to very low specific wear rates of the same order of magnitude (0,06 to 0,22x10(exp -6) mm(exp 3)/Nm) for all liquids. The ta-C coating thus provides an improvement of tribological behaviour and also shows a reduced sensitivity against changes of the lubricating medium. Tests with oscillating sliding provide an economic way for the determination of friction and wear behaviour of coatings. Owing to the small size of wear scars many tests can be performed with one single specimen and the volumetric wear can be determined with a rather high accuracy for both bodies in contact even for materials couples with high wear resistance. The results of this type of model test with constant parameters, however, have always to be validated in field tests, but provide information that helps for the selection of candidate materials and might also be used for the improvement of coating techniques.