ADHESION EVALUATION OF THIN CERAMIC COATINGS ON TOOL STEEL USING THE SCRATCH TESTING TECHNIQUE

Coatings of carbides (TiC and TiC/CrC) and nitrides (TiN and CrN) were deposited onto two types of cold work tool steel using physical vapou r deposition (PVD) and chemical vapour deposition (CVD). The adhesion strength was measured using two versions of the scratch testing techni que (stepwise and linearly increasing load). The adhesion strength rep resented by the critical load for coating removal L(c) and the coeffic ient of friction at the critical load mu(c) was determined. The stepwi se increasing load resulted in lower L(c) and higher mu(c) compared wi th the linearly increasing load. The formation of different regular cr ack patterns in the coatings during the contact between scratch indent er and exposed coating surfaces is presented schematically and the for mation mechanism is explained and analysed. Higher residual stresses w ere observed in PVD coatings deposited onto substrates of greater roug hness, owing probably to the high grain boundary constraint during gro wth. For the CVD coatings, the surface roughness of the substrate did not affect the coating's residual stresses because stress formation is due to the difference in the coefficients of thermal expansion. Model s of the work of adhesion between the coating and substrate are review ed critically and expanded.