Characterizing crack morphology toward improving ductile mode cutting of calcium fluoride

Abstract

Delaying the onset of brittle mode cutting does not only involve the augmentation of slip deformation but could also be tackled from the perspective of suppressing the leading causes for crack formation. This work studies the micro-cutting of calcium fluoride single crystals with diamond turning and orthogonal plunge-cutting. An applied coating shows promise to enhance ductile mode cutting during diamond turning and also alters the crack morphology during cutting in the brittle regime. Conventionally, calcium fluoride exhibits anisotropic pyramidal cracks that were associated with the activation of the {111} cleavage plane while the adoption of the coating prior to micro-cutting alters the cracks to become laminar in nature associated with the (1‾11‾) cleavage plane. The resistance to deformation of the coating is proposed to create reaction stress along the shear plane of the workpiece to reduce the effective tensile stresses acting on cleavage planes. Crystal plasticity finite element method modelling demonstrates the relationship between the coating stress distribution and the intensified concentration of shear stresses in the workpiece, which confirms the notion of the reaction stress acting along the shear plane. Moreover, the simulations with the coating present the affected tensile stresses acting on a majority of the {111} cleavage planes with the exception of the (1‾11‾) cleavage plane, which supports the crack orientations in brittle mode cutting.