Mechanical and tribological properties of chemically vapour-deposited boron carbide coatings

Abstract

Boron carbide coatings of different compositions and thicknesses were deposited on TiC-coated cemented carbide by chemical vapour deposition. Rhombohedral boron carbide (B 13C 2) with a carbon content ranging from 9 to 17 at.% and amorphous boron carbide with 22 at.% C were deposited. B 13C 2 exhibits a nodular morphology with crystal facets corresponding to the grain size of the coating. The nodules are nucleated on cobalt-rich outgrowths originating from the interface. Transmission electron microscopy examination indicates the presence of high local stresses in the coating. The Vickers hardness of the B 13C 2 coatings was estimated to range from 4000 to 4500 HV, the higher value corresponding to the coating with the highest carbon content. An attempt to evaluate the adhesion of the coatings, using the scratch test, failed owing to the high hardness of the coatings. However, the adhesion of B 13C 2 to TiC appears to be comparable with or even superior to that of TiC to cemented carbide. In contrast, the adhesion of the amorphous coating to TiC is very poor. The solid particle erosion resistance of B 13C 2 was found to be far superior to that of TiC. The best performance was obtained for the high carbon content coating. In contrast, the amorphous boron carbide gave very poor performance owing to its poor adhesion. The sliding wear resistance was investigated by a simulated tool wear test (a modified pin-on-ring test with continuous introduction of a fresh counter-material surface). Using quenched-and-tempered steel and austenitic stainless steel as counter materials, no improvement in properties compared with those of TiC was obtained. The results are discussed in relation to the wear mechanisms identified.