B-doped nanodiamond composite hard coatings deposited on cemented carbide: Mechanical, structural, and tribological properties

Abstract

Herein, the effects of B doping on the mechanical, structural, and tribological properties of nanodiamond composite (NDC) coatings deposited on cemented tungsten carbide-based Co binder substrates via coaxial arc plasma deposition are studied. The results reveal decreasing films hardness 35, 27, and 25 GPa with increasing B contents of 1, 5, and 10 at.%, respectively, which is attributed to film graphitisation. Graphitisation is induced by the catalytic effects of Co atoms, which diffuse proportionally from the substrate surface into the films with increasing B content. However, Co diffusion hardly occurs during the deposition of undoped films. The 1 at.% B-doped films with an undoped NDC buffer layer exhibit enhanced hardness from 35 to 58.3 GPa owing to a decrease in the catalytic effects of Co, thereby resulting in the formation of a high amount of CC sp3 bonds at the expense of CC sp2 bonds. The adhesion of the doped films is improved by a factor of three compared with the undoped films owing to enhanced film hardness and toughness as B doping tends to release internal stress by forming long CB bonds. The sliding behaviour remains at a friction coefficient of approximately 0.08 owing to the presence of sacrificial B2O sliding layers and graphitic self-lubrication phases. The B-doped NDC films are promising hard coatings for machining difficult-to-cut materials.