Abstract
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. In this paper, double layer films of SiO 2 /Si 3 N 4 were prepared on sapphire (α-Al 2 O 3 ) by radio frequency magnetron reactive sputtering in order to increase both transmission and high temperature mechanical performance of infrared windows of sapphire. Composition and structure of each layer of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), respectively. Surface morphology and roughness of coated and uncoated sapphire have been measured using a talysurf. Flexural strengths of sapphire sample uncoated and coated with SiO 2 /Si 3 N 4 have been studied by 3-point bending tests at different temperatures. The results show that SiO 2 /Si 3 N 4 films can improve the surface morphology and reduce the surface roughness of sapphire substrate. In addition, the designed SiO 2 /Si 3 N 4 films can increase the transmission of sapphire in mid-wave infrared and strengthen sapphire at high temperatures. Results for 3-point bending tests indicated that the SiO 2 /Si 3 N 4 films increased the flexural strength of c -axis sapphire by a factor of about 1.4 at 800 °C.
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