Crack generation and avoidance during the growth of sapphire domes from anelement of shape

A failure mechanism is proposed in order to explain the generation and prop agation of cracks during the growth of sapphire domes from an element of sh ape. According to this model, 10 mu m gas bubbles expansion is induced by g lide dislocations (Orowan's model), up to a critical size for which the cra ck is initiated. Numerical simulation of stresses during the growth explain s how cracks propagate first vertically (V-type cracks) then horizontally ( H-type cracks). A criterion based on plastic strain relaxation is defined i n order to determine the growth parameters (pulling and rotation rates) as a function of the measured thermal gradients in the crystal. This led to th e growth of crack-free sapphire hemispheres up to 50 mm in diameter that ca n be used for infrared dome blanks. (C) 1999 Elsevier Science B.V. All righ ts reserved.