Investigation of SiO2/Si3N4 films prepared on sapphire by r.f. magnetron reactive sputtering

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 SiO2/Si3N4 were prepared on sapphire (α-Al2O3) 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 SiO2/Si3N4 have been studied by 3-point bending tests at different temperatures. The results show that SiO2/Si3N4 films can improve the surface morphology and reduce the surface roughness of sapphire substrate. In addition, the designed SiO2/Si3N4 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 SiO2/Si3N4 films increased the flexural strength of c-axis sapphire by a factor of about 1.4 at 800 °C.