INTERLAYERS FOR DIAMOND-COATED CUTTING TOOLS

Boron-containing chemical vapor deposition (CVD) films are being explored for use as interlayers between WC-Co substrates and CVD diamond coatings to enhance the adherence between these materials. The wear resistance and hardness of diamond coatings on tough WC-Co substrates gives an ideal coated tool material for many machining applications. CVD diamond applied directly onto WC-Co exhibits very poor adherence. Factors contributing to this problem include the solubility and diffusivity of carbon in cobalt, resulting in limited diamond nucleation and precipitation of graphite, and the large thermal expansion mismatch between WC-Co and diamond. Boron-containing films act as diffusion barriers to carbon diffusion into the cobalt and enhance chemical bonding at the coating-substrate interface. Compositional grading of the interlayer maximizes its effectiveness as an aid to adherence. The experimental results leading to these observations are discussed in this paper.