AbstractScratch testing using hemispherically tipped diamond styluses of tip radii 100 and 250 Jim has been used to study the surface deformation and frictional behaviour of thin (2–5μm thick) coatings of TiN and TiC deposited on steels by proprietary CVD and PVD processes. For both the CVD and PVD samples, the coefficient of friction was found to increase with increasing normal load. Analytical evaluation of the data indicates that plastic ploughing appears to dominate the deformation of the coating with adhesion playing little part in the friction process. The dominance of ploughing was confirmed by SEM observations. Further, analysis of the data for scratch tracks contained wholly in the coating showed that, depending on normal load, plastic flow in the surface layers of the substrate due to stresses transmitted through the coating could also significantly influence frictional behaviour.