Effects of surface coating materials on cutting forces and ductile-to-brittle transition in orthogonal cutting of monocrystalline sapphire

Abstract

Ductile regime cutting has been studied to process brittle materials without cracking. The predominant material response during cutting can be tuned to the ductile range under certain conditions. This study aims to explore and reveal the effects of workpiece surface coatings on promoting the ductile cutting regime. Despite of several attempts on applying surface coatings, the effects of coating properties on material behaviors and cutting forces are not yet well-understood. Here, the effects of surface coatings on the cutting forces during ductile regime cutting of monocrystalline sapphire were investigated from the perspectives of types of coating materials and crystal orientation. Flexible elastomers, hard coatings, and a lubricant were applied to the surface, and the cutting force behaviors were observed. Coatings effectively promoted ductile cutting regime, but the effects varied by coating type and crystal orientation. The cutting forces and ductile cutting regime were significantly influenced by types of coating materials and dominant deformation/fracture mechanisms of sapphire crystal. The machining characteristics were analyzed based on the modified slip-fracture model; this explained the varying effects per different cutting direction. This study aids identification of effective coatings under various cutting conditions and is expected to contribute to wider utilization of ductile regime cutting in processing of brittle materials.