EFFECTS OF CRYSTAL ORIENTATION AND TEMPERATURE ON THE STRENGTH OF SAPPHIRE

The flexure and compressive strengths of sapphire are dependent on cry stal orientation and temperature. Most notably, the c-axis compressive strength decreases below the tensile strength at temperatures >400 de grees C and falls to 2% of the room-temperature compressive strength a t 800 degrees C. Loss of compressive strength complicates the interpre tation of flexure tests. Four-point flexure specimens with no componen t of c-axis compression increase in strength at temperatures >500 degr ees C; however, specimens that have c-axis compression decrease in str ength. It has been observed that c-axis compression causes twinning on rhombohedral crystal planes. Intersection of twins on different rhomb ohedral planes causes fracture that leads to mechanical failure.