CERAMIC COATINGS CHEMICALLY VAPOR-DEPOSITED ON HIGH-SPEED STEEL SUBSTRATES - SCRATCH ADHESION TEST PHENOMENA

The scratch test is widely used to assess the adhesion of ceramic coat ings deposited on cermet and alloy substrates by both physical vapour deposition and chemical vapour deposition (CVD). It has been observed during such tests that, depending on the coating type, complete coatin g removal from the scratch channels can be preceded by flaking of the coating at the edges of the channels. An attempt has been made to asce rtain, for CVD coated powder metallurgy (PM) high speed steel (HSS) cu tting tool inserts, whether a relationship exists between the presence /absence and type of pre-critical load costing flaking observed during scratch testing and interfacial compositional variations determined b y Auger electron spectroscopy (AES). Scratch test and AES results are presented for the following CVD coated PM HSS inserts: titanium nitrid e (TiN) coated, where no flaking has been observed; titanium carbide ( TiC) coated, where mixed adhesive/cohesive flaking of the TiC coating has been observed and multilayer aluminium oxide (Al2O3) coated, where adhesive flaking of the Al2O3 coating has been observed. In the case of the TiN and (TiC) coated inserts it is not considered that differen ces in coating/substrate interdiffusion zone thickness alone are respo nsible for the observed divergence in scratch test behaviour. Differen ces in coating micro-hardness are also suggested to be a significant f actor. The behaviour of the multilayer Al2O3 coated insert during scra tch testing is thought more likely to be singularly associated with in terfacial chemistry.