Fracture and Microscopic Deformation on (0001) and (1010) in Sapphire Single Crystals due to Repeated Indentations

The study described in this paper concerns the microscopic deformation and fracture, such as micro-cracks, chipping and striation produced by repeated indentation loading. The repeated indentations with a diamond Vickers indenter were made onto the same point on the (0001) and (1010) faces of the Sapphire (Al2O3) single crystals under two kinds of indentation speed of 10 and 100 (cycles/min). Microscopic observations of the indented surface were made to evaluate the appearances of the radial and lateral cracks, and to examine the number of loading cycles needed to originate chipping. The effects of the indenter azimuth and crystal face on the patterns of the fracture and deformation were also examined. The conclusions obtained from the experimental results are summarized as follows: (1) The fracture on both the (0001) and (1010) faces starts with an origination of radial cracks, and with an increase in number of loading cycles, lateral cracks, chippings and striations are produced. (2) Relation between indentation load and number of cycles to chipping in Sapphire is correspondent to that between stress and number of cycles to fracture obtained under fatigue tests for metallic materials. (3) Deformations, and fracture damage on the (0001) face under 0° indentation are greater than those under 45° indentation. On the contrary, on the (1010) face, those are greater in the case of 45° indentation. (4) Twinnings are observed on the (1010) face under the load of greater than 3.0N. (5) The pile-up of the material and small plastic deformations of the block pattern are formed around an indent on the (1010) face.