Monocrystalline Sapphire Stress Field Analysis with Orthorhombic Crystal Orientation Under Vickers Indentation

As an irreplaceable optical ceramic material in energy, aviation, and commerce, sapphire is making a further expansion of its application boundaries. Owing to the anisotropy of sapphire, the material properties analysis in the fabrication process is hard but essential. Hence, aiming at investigating the damage behavior of sapphire with different crystal orientations during machining, the nucleation and propagation of cracks in the orthogonal a, c, and m orientations of sapphire under Vickers indentation were explored experimentally and numerically. Firstly, the indentation morphology and indentation cracks of sapphire with different crystal orientations under different loads were studied based on a Vickers indentation tester. In general, the relative errors of the three characteristic parameters, including the half-length of indentation diagonal, the length of crack, and the maximum depth of indentation, are all within 20% between the simulation model and the indentation test results. Then, the nucleation critical loads of different cracks in sapphire under Vickers indentation are determined on the basis of the ceramic materials’ fracture mechanics theory. The critical load value of the median crack of sapphire in both A- and M-planes is less than 0.1 kgf experimentally and simulatively, while C-plane sapphire is between 1 kgf and 2 kgf. Finally, the stress field, displacement–load curve, plastic piling-up height, and dynamic propagation process during Vickers indentation are analyzed, combining the experimental results with a numerical calculation approach.