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

Highlights

• Epoxy resin coating can enhance ductile-mode cutting of CaF₂ single-crystal.

• Ductile–brittle transitions increase by at least 27.5 % with epoxy coating.

• An anisotropic ductile–brittle transition predictive model is proposed.

• The coating increases ductile-mode energy dominance to improve machinability.

• Additional stress induced in the workpiece is a function of coating hardness.

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.

Introduction

Ultra-precision machining technology thrives in micro-optical applications, and enhanced productivity in manufacturing is necessary to meet industrial demands. Unique to brittle optical materials, the productivity rates are mainly determined by the critical uncut chip thickness, otherwise known as the ductile–brittle transition cutting depth, which typically ranges in the submicrometric scale. While the single-point micro-cutting process has been proven successful in machining brittle materials, it is still limited by the submicrometric ductile–brittle transition. Positive improvements to the ultra-precision machining field have been presented to enhance ductile-mode machining of difficult-to-machine materials, where the critical cutting depth is typically increased using complex and expensive equipment. These include the adoption of elliptical vibration-assisted machining [1], ion doping [2], and thermally assisted machining [3,4] either to achieve higher cutting depths or to reduce the subsurface damage.

An economical solution to augment the micro-cutting process involves the application of epoxy resin prior to machining. Cheng et al. [5] applied epoxy resin prior to the micro-grinding of glass and reported that ductile-mode machining was enhanced because of the restraining effect of the resin, which held cracks in place, and higher forces were required for fracture to occur. In these micro-grinding tests, the material removal orientation of a single grit, analogous to a single-point diamond tool, was parallel to the plane of the applied epoxy coating, as illustrated in Fig. 1(a). In this scenario, crack origins that are directly in contact with the epoxy might be suppressed, as proven by Cheng et al. [5] in their assessment of the size of the crack forming along the uncut shoulder of the tool path. However, this assessment of ductile-mode cutting was not considered for two areas: (i) the overall machined surface quality that accounts for all surfaces that were in contact with the tool, particularly the bottom surface, and (ii) an alternative coating configuration where the material removal orientation is perpendicular to the plane of the applied coating. This work addresses these issues by assessing crack formations at the bottom of the tool path on the machined surface, which is not in contact with the coating, and evaluates the alternative coating configuration, as illustrated in Fig. 1(b).

Moreover, Cheng et al. [5] proposed the inclusion of additional energy requirements for brittle fracture of the workpiece to occur under the influence of the restraining effect of the coating, but briefly explained it as the energy to offset the crack formation energy by holding cracks in place and delaying propagation. It is essential to identify the parameters which determine the additional energy required for brittle failure to occur when building a model to understand the mechanical phenomenon and its predictability.

The machinability of a brittle material with the applied coating was evaluated in micro-cutting, where plunge-cutting experimental tests served as the primary method for characterisation of the ductile–brittle transition. In addition to the experimental evaluation, an analytical model was introduced to predict the ductile–brittle transition under the influence of the coating. An energy-based approach was adopted for the predictive model by assessing the dominance between the ductile- and brittle-mode cutting energies. The cutting modes are correlated with the machined surface quality, where smooth defect-free surfaces are defined as the ductile regime, and micro-crack formation is characterised under the brittle regime. In the ductile mode, the elastic strain energy induced during cutting is released into plastic deformation through a periodic elastic–plastic cycle of building up and releasing compressive stresses to sustain plastic flow. In the meantime, tensile stresses are also building up in the work material, which can be released with the formation of new surfaces when the critical tensile stress for crack formation is reached (i.e., the elastic–brittle occurrence) [6]. Therefore, the ductile–brittle transition can be evaluated by comparing the elastic–plastic and elastic–brittle dominance with a progressively increasing uncut chip thickness.

Arif et al. [7] detailed such a methodology to predict the critical uncut chip thickness by considering the elastic recovery of the material in the tertiary deformation zone on the machined surface. However, the model did not account for single-crystal anisotropy, which is a critical factor to consider when evaluating the machinability of single crystals. The model proposed in this paper incorporates anisotropic changes to the material property definition to analyse the directional differences in ductile–brittle transition. The present model also incorporates the addition of a coating layer to predict the enhanced critical uncut chip thickness.