Mechanisms of ductile mode machining for AlON ceramics

Abstract

This paper aims to reveal the mechanisms of ductile mode machining for AlON ceramics. The removal characteristics during machining were studied through ultra-precision grinding experiments. The machined surface consists of fractured and smooth areas, which were generated by brittle and ductile removal, respectively, of the individual AlON grains. The material removal mode has a determining effect on the surface/subsurface quality. The proportion of the fractured areas on the ground surface decreased gradually with a decrease in the depth of cut. The crystal indices of the grains most prone to brittle removal on the workpiece surface were determined using micro-area X-ray diffraction (μXRD) analysis performed using a beam with 50 μm diameter. The results showed that the ductile removal of the {111} planes is critical for the ductile mode machining of AlON. Nanoindentation tests based on electron back-scattered diffraction (EBSD) indicated that AlON shows strong anisotropy in its mechanical properties and machinability. The (111) plane has the highest hardness and lowest fracture toughness, at 22.91 GPa and 1.8 MPa m1/2, respectively. The material removal mechanism during the grinding of AlON was discussed in detail. The minimum and maximum dc(hkl) values must be known for classifying whether the removal mode of the workpiece is brittle or ductile. A damage-free surface could be obtained during ductile mode grinding by controlling hmax to be less than dc(111). The subsurface deformation mechanism during ductile mode grinding was analysed. An amorphous layer was observed close to the ground surface. Further, dense dislocations with no particular orientation were present beneath this amorphous layer. As the crystal structure became clearer with an increase in the depth, the plastic deformation shifted to stacking faults parallel to the {111} planes.