Adhesion of diamond coatings on tungsten substrates

Adhesion data between diamond coatings and tungsten substrates as a function of the tungsten substrate surface treatment prior to diamond deposition are presented. Mirror-polished tungsten substrate surfaces were scratched with different size natural diamond grits and then characterized for average roughness (Ra), chemical composition, and bonding of the residual carbon on the surface. Diamond coatings were deposited on these surfaces by a hot-filament chemical vapor deposition method under identical conditions. The deposited coatings were characterized for growth rate, morphology, crystallinity, amount of non-diamond carbon, texture, and strain. Coating-substrate adhesion was measured by the tensile pull method. The surfaces of the coatings and the substrates produced as a result of the tensile pull testing were characterized for morphology, roughness, chemical composition, and type of carbon bonding. The results indicate that the diamond coatings deposited on differently prepared substrates have roughly similar morphology, crystallinity, non-diamond carbon fraction, and strain. During adhesion testing, failure occurs primarily near an interface between diamond-like-carbon-rich and crystalline diamond-rich regions or in the diamond-like-carbon-rich region. The measured adhesion values correlate rather well with the substrate surface roughness under the pulls. Initially adhesion increases with increasing surface roughness and then decreases with further increases in surface roughness beyond 0.1 μm.