Improving the Properties of Sapphire by Coatings

Abstract

SiO2 coatings and double layer films of SiO2/Si3N4 have been designed and prepared on sapphire (α-Al2O3) by radio frequency magnetron reactive sputtering in order to improve the optical and mechanical properties of infrared windows of sapphire. The transmission of the coated and uncoated sapphire in the wavelength range from 2.5 to 6.7um was determined. Surface morphology and roughness of coated and uncoated sapphire have been measured using a talysurf. The flexure strength of coated and uncoated sapphire samples has been studied by 3-point bending tests at room temperature and high temperatures. The results show that the coatings can improve the surface morphology and reduce the surface roughness of sapphire substrate. What’s more, the designed single layer of SiO2 film and two-layer of SiO2/Si3N4 films can both increase the transmission of sapphire in mid-wave IR and strengthen sapphire at high temperatures. Coated sapphire transmits over 95% at wavelength from 3 to 5um for the two designs. Flexure tests reveal that SiO2 coatings and SiO2/Si3N4 films increase the strength of c-axis sapphire by a factor of about 1.5 and 1.4, respectively, at 800°C.