The effect of anisotropy on the deformation and fracture of sapphire wafers subjected to thermal shocks

This paper studies the effect of anisotropy on the response of an R-plane sapphire wafer to a rapid thermal loading. The finite element method was used to analyse the temperature and stress distribution in the wafer when the environment was heated from room temperature to 800 °C, and then cooled down to room temperature. To determine the weak and strong points along the wafer edge, fracture criteria for anisotropic materials were applied. It was found that the maximum tensile stresses occur at the flat wafer edge on cooling down, and could fracture the wafer, most likely at a location of a high tensile stress and in a direction of a weak cleavage plane. The wafer appears to be most prone to fracture at its flat edge, and would crack in the weakest plane image. The strongest points along the edge are located at the sides of the flat edge, where the tensile stresses in the wafer plane are the lowest. A circular wafer subjected to the same thermal loading was also analysed for comparison, and the weakest and strongest locations and cleavage planes were determined.