Micron-size cracks and voids produced during grinding and polishing may be responsible for sapphire's loss of flexure strength at elevated temperatures. The ability to fill these micron-size voids and cracks with a compressive coating may remove the crack-initiating defects that lead to loss of strength. It is well known that compression of c-axis sapphire can cause twinning on rhombohedral crystal p;lanes, especially at elevated temperatures. If twins on two different planes intersect, a crack will form and the sapphire will fail in tension. In ring-on-ring biaxial flexure test, microtwins could form where the load rings contact the c-axis sapphire. A coating should mitigate the very high compressive stress produced at the surface of the sapphire by the load rings. Results presented here for ring- on-ring biaxial flexure test show that compressive coatings increase the fracture strength of c-axis sapphire by a factor of about 1.95 at 600 degrees C. Additional results show that the coating do not significantly strengthen sapphire at ambient temperature. This is not surprising since sapphire already is very strong in compression and tension at ambient temperature.