Crack generation and adoidance during the growth of sapphire domes from an element of shape

摘要

A failure mechanism is proposed in order to explain the generation and propagation of cracks during the growth of sapphire domes from an element of shape. According to this model, 10 um gas bubbles expansion is induced by glide dislocations (Orowan's model), up to a critical size for which the crack is initiated. Numerical simulation of stresses during the growth explains how cracks propagate first vertically (V-type cracks) then horizontally (H-type cracks). A critierion based on plastic strain relaxation is defiend in order to determine the growth parameters (pulling and rotation rates) as a function of the measured thermal gradients in the crystal. This led to the growth of crack-free sapphire hemisphere up to 50 mm in diameter that can be used for infrared dome blanks.