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With the growing demand for the fabrication of microminiaturized components, a comprehensive understanding of material removal behavior during ultra-precision cutting has become increasingly significant. Single-crystal sapphire stands out as a promising material for microelectronic components, ultra-precision lenses, and semiconductor structures owing to its exceptional characteristics, such as high hardness, chemical stability, and optical properties. This paper focuses on understanding the mechanism responsible for generating anisotropic crack morphologies along various cutting orientations on four crystal planes (C-, R-, A-, and M-planes) of sapphire during ultra-precision orthogonal cutting. By employing a scanning electric microscope to examine the machined surfaces, the crack morphologies can be categorized into three distinct types on the basis of their distinctive features: layered, sculptured, and lateral. To understand the mechanism determining crack morphology, visualized parameters related to the plastic deformation and cleavage fracture parameters are utilized. These parameters provide insight into both the likelihood and direction of plastic deformation and fracture system activations. Analysis of the results shows that the formation of crack morphology is predominantly influenced by the directionality of crystallographic fracture system activation and by the interplay between fracture and plastic deformation system activations. 

In this paper, material deformation during ultra-precision machining (UPM) on the C-, R-, and A-planes of sapphire was investigated using the slip/fracture activation model where the likelihood of activation of individual plastic deformation and fracture systems on different crystallographic planes was calculated. The stress data obtained from molecular dynamics (MD) simulations were utilized, and the slip/fracture activation model was developed by incorporating the principal stresses in calculating the plastic deformation and fracture cleavage parameters. The analysis methodology was applied to study material deformation along various cutting orientations in sapphire. The stress field at crack initiation during UPM on C-, R-, and A-planes of sapphire was calculated using molecular dynamics (MD) simulations. An equation describing the relationship between crack initiation and its triggering parameters was formulated considering the systems’ plastic deformation and cleavage fractures. The model can qualitatively predict the crack initiations for various cutting orientations. The proposed model was verified through ultra-precision orthogonal plunge cut experiments along the same cutting orientations as in the MD simulations. 

In this paper, the modified slip/fracture activation model has been used in order to understand the mechanism of ductile-brittle transition on the R-plane of sapphire during ultra-precision machining by reflecting direction of resultant force. Anisotropic characteristics of crack morphology and ductility of machining depending on cutting direction were explained in detail with modified fracture cleavage and plastic deformation parameters. Through the analysis, it was concluded that crack morphologies were mainly determined by the interaction of multiple fracture systems activated while, critical depth of cut was determined by the dominant plastic deformation parameter. In addition to this, by using proportionality relationship between magnitude of resultant force and depth of cut in the ductile region, an empirical model for critical depth of cut was developed.