Deformation and removal mechanism of single crystal gallium nitride in nanoscratching

Understanding the deformation mechanism of a brittle material under mechanical loading is of value for unveiling the material removal in an abrasive machining process. In this work, instrumented nanoscratching was performed on the (0001) plane of single crystal gallium nitride (GaN) using a conical diamond indenter and the scratch-induced surface/subsurface deformation patterns were characterized by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A finite element model of scratching was established to relate stress distribution and deformation patterns. The stress threshold for the elastic-plastic deformation transition of the crystal was found being relatively high in comparison to single crystal silicon and gallium oxide. The plastic deformation in the GaN sublayer was dominated by slip, dislocation and lattice distortion under relatively low normal loads, but those defects were accompanied with grain rotation when the load was sufficiently high. The thickness of the damage layer was directly related to the ratio of lateral force to normal force during scratching. Such a force ratio can be used as an indicator for controlling the damage layer thickness during abrasive machining, thus providing a valuable guidance for developing the optimal parameters of an ultraprecision abrasive machining process.