Abstract
Scratch tests were carried out on TiC and TiN coatings formed by PVD (Physical Vapor Deposition) and CVD (Chemical Vapor Deposition) techniques. The critical load L_c detected by both AE signal and frictional force curve coincided with that at which the local delamination of coating was observed on the scratch track. In order to study damage process of coating during scratch test, scratch track were observed. Hair cracks parallel to the scratch track were observed before local delamination occurs. Other cracks perpendicular to the scratch direction were observed in the scratch track at L_c. Local delamination also occurred at the edge of scratch track at L_c. Shear stress at the interface between coating and substrate induced this local delamination. Stress distribution at the interface was estimated by FEM (Finite Element Method) analysis. Shear stress at the interface depended on the tangential force. Critical load L_c decreased with decreasing of tangential force due to increase of frictional coefficient. Indentation test without tangential force was also carried out. The critical load was high in indentation test compared to that in scratch test. However, the critical shear stresses at the interface for both scratch and indentation tests coincided each other. It is concluded that the critical shear stress at the interface can be the criterion for the cohesive strength between coating and substrate.
