Investigation Of Improving Strength And Transmission For Sapphire

Sapphire is the best one of all infrared windows and domes materials used for 3 5μm mid-wave infrared waveband for its optics,mechanics and calorifics. However, the strength of sapphire decreases rapidly at elevated temperature and its optical transmission cannot satisfy the demand of applications and design either. The most efficient way to solve the problem is to prepare anti-reflective and protective films on sapphire. Silicon dioxide (SiO₂) and silicon nitride (Si₃N₄) are promising anti-reflective films for sapphire for their good physical and chemical properties and good adhesion to sapphire. Great progress has been made in the researches about SiO₂ and Si₃N₄ anti-reflective and protective films overseas, but no domestic work has yet been done. Researches of this paper concentrate mostly on the design and preparation of anti-reflective and protective films on sapphire based on SiO₂ and Si₃N₄, the preparation processing of SiO₂ and Si₃N₄ and the effects of the processing on the properties of the films, and the preparation of SiO₂ and Si₃N₄ on sapphire domes used as anti-reflective and protective coatings as well. The main contents and results are as follows:(1) With the help of OPFCAD software, anti-reflective and protective films of SiO₂, SiO₂/Si₃N₄, SiO₂/Si₃N₄/SiO₂ are designed on sapphire substrates. The results show that after SiO₂, SiO₂/Si₃N₄, SiO₂/Si₃N₄/SiO₂ films deposited on the surfaces of sapphires the average transmittances over 3⁵μm waveband can exceed 97%, which can meet the requirements of missile dome.(2) Si₃N₄ films are prepared by RF reactive magnetron sputtering on Si and sapphire substrates. The effects of the depositing parameters on the depositing rates of the films are studied systematically, and thus the parameters are optimized. The prepared Si₃N₄ films are analyzed by XPS and XRD, which show that the deposited Si₃N₄ films are amorphous compound.(3) The designed films of SiO₂/Si₃N₄ are prepared on sapphire by RF reactive magnetron sputtering. The average transmittance of sapphire over 3⁵μm waveband can reach 92.21% after deposited films on both sides. The high temperature strengths of both coated and uncoated sapphires are tested, and the results show thatSi3N4 films can evidently improve the high temperature strength of sapphire which can satisfy the demand of window and dome.(4) The high temperature transmittance of both coated and uncoated sapphires are measured. The transmittance of them decline as the temperature rise.(5) The anti-reflective and protective films are successfully prepared on the dome simulator and sapphire dome by RF reactive magnetron sputtering.The average transmittance and uniformity can meet the demand of dome.