Novel Method for Strengthening Sapphire at Elevated Temperatures

Aspen Systems proposes to develop a novel process to increase the strength of sapphire at elevated temperatures. The resulting strength increase will alleviate the adverse effects of frictional heating of sapphire windows. Frictional heating can initiate a c-axis strength loss, leading to premature failure. The Aspen process will uniformly implant silicon atoms through the entire window, as well as, result in the production of uniform point defects throughout the crystal structure. These mechanisms will pin the crystal lattice, resulting in a structure more resistant to rhombohedral twinning and thermal shock failure. Preliminary tests conducted by Aspen Systems have demonstrated that this process can be applied to high purity sapphire with no adverse effects on the transmission characteristics in the Midwave (3-5mm) region. For the Phase I effort we will demonstrate proof-of-principal that our process can increase the high temperature strength of sapphire without degrading its optical properties. During the Phase I Option and Phase II effort, we will optimize and tune the process to maximize the high temperature strength of the resulting sapphire. A statistically significant number of sapphire coupons will be processed and tested to verify the mechanical properties of the modified dome material. The processing method used in Phase I and Phase II can readily be scaled to meet both military and civilian needs.