Deformation mechanism and force modelling of the grinding of YAG single crystals

Abstract

YAG single crystals are the primary host materials for solid-state lasers at multi-kW scale and must be processed using ultra-precision grinding to achieve a satisfactory dimensional precision and surface integrity. However, the deformation mechanism of YAG crystals is not well understood, which has thus hindered the development of high efficiency grinding technology for the crystals. In this work, precision grinding of YAG single crystals was investigated. Ductile-like surfaces that are free of cracks and brittle-ductile surfaces that consist of fractured spots and ductile striations were found after grinding. The deformation mechanisms associated with the two types of surfaces were explored with the aid of transmission electron microscopy (TEM). The results indicated that the deformation involved in the formation of the ductile-like surface was mainly caused by the slippage of (0 0 1) crystal planes, along with the formation of dislocations and stacking faults and the distortion of atomic planes. The brittle-ductile surfaces were generated by the plastic deformation due to the formation of nanocrystals and nanovoids, combined with brittle fracture caused by the crack propagation initiated at intersections of slip lines. A theoretical model was developed to predict the grinding force in the ductile-like grinding process, which has taken the combined effect of strain rate, random distribution of abrasive radii and elastic-to-plastic transition depth into account for the first time. The key model parameters were obtained using a genetic algorithm trained using the experimental force data. The modelled force agrees well with the measured. This model enabled an in-depth understanding of the deformation mechanism of a crystal solid involved in ultraprecision grinding and the effect of strain rate on its material removal.