Improving on Surface and High-temperature Strength of Sapphire by SiO_2 Films

Sapphire is a desired material for infrared-transmitting windows and domes because of its excellent optical and mechanical properties. However, its thermal shock resistance is limited by loss of compressive strength along the c-axis of the crystal with increasing temperature. Research and development work for increasing the high temperature strength of sapphire is a major problem with sapphire in high-speed or high-temperature applications. In this paper, SiO2 coating was prepared on sapphire substrate by RF magnetron reactive sputtering. The flexure strength of sapphire sample uncoated and coated with SiO2 was studied by 3-point bending tests at room temperature and high temperatures. Surface morphology and roughness of coated and uncoated sapphire were measured. Surface residual stress of sapphire substrate and sapphire coated with SiO2 film was estimated by using pointed-indentation cracking under Vickers indenters. The results show that SiO2 coating can improve both the surface morphology and roughness of sapphire. At the same time, the SiO2 coating can change the surface residual stress of sapphire. Results for 3-point bending tests show that the SiO2 coating increases the fracture strength of c-axis sapphire by a factor of about 1.5 at 800 ℃.