Beneficial stress of a coating on ductile-mode cutting of single-crystal brittle material

Abstract Innovative techniques have been proposed to overcome the challenge of the strong tendency of brittle materials to crack during machining. One of them is the use of an epoxy coating to serve as a crack formation restraint. However, this only serves to achieve ductile-mode grinding along the uncut shoulders. Therefore, this study evaluates the effect of a layer of epoxy resin on the machined surface perpendicular to the micro-cutting direction of a brittle material, single-crystal calcium fluoride. An increase in the ductile–brittle transition was observed in micro-cutting experiments on the (111) plane of calcium fluoride with the use of a 4-μm thick epoxy resin coating, ranging from 167–347 nm to 213–476 nm for the 0° rake angled tool. A similar increase was also observed with the +5° rake angle, ranging from 91–233 nm to 187–310 nm. An energy-based ductile–brittle transition predictive model is introduced, which characterises the anisotropic behaviour of the single crystal and incorporates the additional stress induced during cutting with a coating as a fraction of the coating hardness. The analytical model accurately predicts a consistent range of improvement from 69–325 nm to 257–485 nm for the 0° rake angle and from 62–285 nm to 215–460 nm for the +5° rake angle. The ductile-mode cutting energy increases to preserve its dominance over brittle-mode cutting and delays the onset of brittle-mode activation. The validity of the model extends the understanding of a surface coating as a restraining technology to include the beneficial stress acting in the deformation zone during cutting.