ADHESION EVALUATION OF METALLIC AND CERAMIC COATINGS DURING STRAININGOF A STEEL SUBSTRATE - FLAKING AND CRACK BEHAVIOR

An evaluation of adhesion and cohesion properties for coatings was att empted by studying crack initiation and behaviour during uniaxial stra ining of a coating-substrate system. Some soft and hard metal coatings and some ceramic coatings have been studied. Straining of coated spec imens was done in two ways: firstly, by uniaxial strain control tensil e testing connected to acoustic emission (AE) equipment for monitoring crack activities; secondly, by uniaxial load control tensile testing with replica tape application for crack detection on the surface. Usin g both pieces of information, the stress level, sigma(fs), at which a crack reaches the coating surface can be determined. Information on ad hesion failure (flaking) and cohesion failure (transverse and +/-45 cr acking) in the coating was obtained. It,ras found that cracks transver se to load direction appear on the surface either before or at the sub strate yield point for hard metal coatings depending on the coating de position process and thickness. For the ceramic coatings of the PVD an d CVD process, the transverse cracking always appeared in the vicinity of the substrate yield point. The cracks originated at the interface rather than at the surface. Flaking of some of the hard metal and CVD coatings was observed near the substrate fracture surface. For the sof t coatings, the transverse cracking may or may not appear depending up on its ductility. A +/-45 cracking in some of the coatings was formed during yielding or just before fracture. Micro-hardness measurements w ere made on all the coating-substrate systems, and the coating hardnes s was derived.