Understanding crack initiation and propagation mechanisms of ZnO crystals induced by nanoindentation and nanoscratch

To understand crack initiation and propagation mechanisms of ZnO crystals involved in abrasive machining, stress-field models induced by nanoindentation and nanoscratch were developed by considering geometric features of the indenter. Then, the mechanical properties of ZnO single crystals were determined through the nanoindentation and nanoscratch tests, providing essential parameters for the calculation of stress fields. Simulated results of nanoindentation induced stress field revealed that cracks initiated at the edge of the indentation and propagated along the indentation direction during loading process. Additionally, cracks also initiated at the indentation bottom and propagated along the edge direction of the indenter. Simulated results of scratch induced stress field indicated that median crack was induced by the compressive stress in front of the indenter, and the residual tensile stress promoted the radial cracks on the scratched surface. The residual tensile stress in normal direction initiated and propagated lateral cracks, while radial cracks were induced by tensile stress σ_x and perpendicular to the scratching direction. In addition, all radial cracks only occurred at the groove bottom, with no cracks forming at groove edges. Finally, the surface morphologies of the indentations and scratch grooves were characterized using scanning electron microscopy (SEM). The observed results showed good consistency with the theoretical predictions, thus further validating the reliability and accuracy of the models. These results not only enhance the understanding of crack initiation and propagation mechanisms of ZnO crystals induced by nanoindentation and nanoscratch, but also facilitate the mitigation of brittle damages through strategic load adjustment.