FAILURE MODE ANALYSIS OF TIN-COATED HIGH-SPEED STEEL - INSITU SCRATCH ADHESION TESTING IN THE SCANNING ELECTRON-MICROSCOPE

A scratch test apparatus for in situ testing of coating adhesion in the scanning electron microscope has been designed. Scratch tests were performed on TiN-coated high speed steel substrates with various coating thicknesses and substrate hardnesses. Four groups of coating damage and detachment mechanisms were identified: deformation, crack formation, chip formation and flaking. In all cases, incipient coating failure was associated with a sharp increase in the friction force readings. The maximum normal force that the coating-substrate composite could sustain increased significantly with increasing coating thickness and substrate hardness. An interesting observation was that direct adhesive failure by interfacial fracture only occurred for the combination of a thick (3 μm) coating on a hard (1000 HV) substrate. In all other cases, coating failure was due to a ductile chip formation mechanism. The detailed influence of the coating thickness and substrate hardness on the resulting coating failure modes is illustrated in the paper by the introduction of coating failure maps. The results demonstrate the great potential for in situ studies in order to obtain a better understanding of the basic coating failure mechanisms. In situ scratching allows the dynamics of the process to be studied; the coating damage and detachment mechanisms in front of the tip can be identified and the scratching events can be directly correlated to the corresponding friction force characteristics. Unfortunately, direct extrapolation of results from the in situ test to the conventional scratch test is difficult owing to the smaller tip radius (25 instead of 200 μm) used in the in situ experiments. © 1990.