Nanoscale Plastic Deformation and Mechanical Properties in R, A, and M Planes of Sapphire Single Crystal During Nanoindentation

Aluminum oxide (Al2O3) in both single crystals (sapphire) and dense polycrystals (α- Al2O3) has recently found a wide range of new important applications in the fields of nanoelectronics, laser hosts (titanium-sapphire), high-temperature structural products, and optoelectronic devices. These novel crystals exhibit a range of technologically interesting properties including high hardness, chemical inertness, superior mechanical performance, and good thermodynamic stability. However, the technological application of sapphire is somewhat limited due to low toughness, poor plasticity, and inferior thermal shock resistance. In this project, nanoscale plastic deformations and stress distributions in R, A, and M planes of sapphire single crystals will be investigated by a combination of depth-sensing nanoindentation and Hertzian elastic theory. Multiple pop-in events in load-displacement curves will be observed during indentation loading. The initial elastic-plastic transitions and deformation behaviors of R, A, and M planes will be analyzed under the anisotropic elastic characteristic and small indentation depth. The distributions of elastic principle stresses and resolved shear stress in crystals will be determined by Hertzian stress analysis. Possible slip or twinning-dominated mechanisms governing the onset of plasticity will be also identified. We believe that this work extends the research effort in this field to reach a better understanding of nanoscale deformation mechanisms of sapphire indented on R, A, and M planes by a combination of nanoindentation and Hertzian elastic theory.