Characteristics of silicon dioxide films prepared on sapphire

Silicon dioxide (SiO2) films 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. The transmittance of the uncoated and coated sapphire was measured using a Fourier transform infrared (FTIR) spectrometer. Effects of the coatings on flexural strength of sapphire at elevated temperatures were investigated and rain erosion tests of the uncoated and coated sapphire were performed for 211 m/s impact velocity with an exposure time ranging from 1 to 8 min on a whirling arm rig. The results show that the designed SiO2 films can improve the transmission of sapphire in mid-wave IR and strengthen sapphire at high temperature. From three-point bending tests, it was indicated that SiO2 coatings increased the flexural strength of c-axis sapphire by a factor of about 1.5 at 800 °C. After the rain erosion tests, the decreases in normalized transmission were less than 1% for the coated sapphires.