Effect of cutting tool geometries on the ductile-brittle transition of monocrystalline sapphire

This paper investigates the effect of cutting tool geometries on the initiation of ductile-brittle transition (DBT) of monocrystalline sapphire with a series of single-grit scratching tests. According to the method of single factor experiment, four typical geometrical parameters, namely rake angle, apex angle, opening angle, and truncated section, were evaluated in 7 scratching tests. The scratching force, groove topography and critical cutting depth for DBT (DBd_c) have been measured by a dynamometer, SEM, and AFM, respectively. Furthermore, a new theoretical model of DBd_c with respect to the rake and apex angle is established. The predicted DBd_c agrees well with experimental results. The mechanism of the initiation of DBT is further investigated by the coupled SPH-FEM simulation. The research results reveal that all of the four geometrical parameters are the dominant factors with significant impact on the initiation of DBT of sapphire. Without exceeding the critical value, an increase in the negative rake angle contributes to an increased DBd_c. The apex angle and opening angle are proved to adversely affect the DBd_c and give rise to DBT at a smaller cutting depth, though the mechanisms associated with these two parameters are different. The parameter of the truncated section also has a significant effect where the DBd_c decreases abruptly and severe brittle fracture is present with an increasing dimension of the truncated section.