Design, Preparation And Properties Of Middle-Wave Infrared Antireflective And Protective Films On Sapphire Substrates

Domes are important components of infrared guide air-to-air missiles. Sapphire has many excellent properties that it is superior to other current and emerging materials for window and dome applications. However, the transmission in mid-wave IR of sapphire limited by the level of preparation and fabrication process can not reach what we expected and it decrease at harsh environments. In addition, the decrease in the mechanical strength of sapphire with increasing temperature is unusually rapid, in comparison to that in most ceramic materials. Resistance to thermal stress is limited by a loss of mechanical strength at elevated temperature. Hence, sapphire is a new window and dome material that needed to be refined and improved. Depositing IR antireflective and protective thin films on sapphire is an effective method to improve its IR transmittance, high temperature strength and durability. Silicon Dioxide films has been used as antireflective and protective thin film of sapphire because of its desirable properties, such as small refractive index, transparency over broad wave band, low absorption, and chemical stability, etc. Silicon Nitride has big refractive index of～1.99, and Si₃N₄ film can be used as antireflective film system by combination with SiO₂ film which has low refractive index.In this article, SiO₂ coatings and double layer films of SiO₂/Si₃N₄ are used as IR antireflective and protective film systems for sapphire. Single layer films of SiO₂ and two-layer films of SiO₂/Si₃N₄ have been designed and prepared on sapphire (α-Al₂O₃) by radio frequency magnetron reactive sputtering. The depositing processes and properties of the films are studied systematically. The main contents and results are listed as follows:Anti-reflective and protective films of SiO₂, SiO₂/Si₃N₄, and SiO₂/Si₃N₄/SiO₂ are designed on the sapphire substrate. The design results explain that sapphire coated on both sides can transmit over a wide waveband from 3 to 5urn for all the designs, which can meet the transmission requirements of missile dome in infrared application.SiO₂ and Si₃N₄ films have been prepared on sapphire substrate by radio frequency magnetron reactive sputtering. Preparations of SiO₂ films with RF magnetron reactive sputtering are studied systematically, influences of processing parameters on the deposition rate, adherence, internal stress, and infrared transmission of SiO₂ film are discovered, and optimized parameters are obtained. The influences of processing parameters on the oxygen content and the infrared spectra of sapphire coated with Si₃N₄ films were measured using a Fourier Transform Infrared (FTIR) spectrometer. Composition and structure of Si₃N₄ films was analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), respectively. The pure Si₃N₄ films have been obtained.According to the films design, sapphires have been coated with single layer films of SiO₂ and two-layer films of SiO₂/Si₃N₄ on two sides. The average transmittance can be increased 9.0% at a wavelength from 3 to 5μm for all the coated sapphires. The transmission of coated sapphire can meet the requirements of missile dome in infrared application. Preparing films on sapphire domes have been studied. The SiO₂ films were firstly deposited on the dummy dome and sapphire dome by RRFS. The average transmittance in 3～5μm waveband of coated dome can increase 4.0ï¼…and the transmittance variation in whole dome area is less than 2.0ï¼…, all these meet the requirements of sapphire domes in infrared application.Rain erosion experiments were carried on a whirling arm rig provided by Chinese Aerodynamics Research Institute. Whirling-arm rain erosion results express that SiO₂ coating was strongly bonded to the sapphire substrate during impact. After rain erosion, the decreases in normalized transmission were less than 1ï¼…for designed SiO₂ films, and sapphires coated with SiO₂ films had a higher transmittance than uncoated ones. It explains that SiO₂ films can satisfy the rain-erosion-resistant requirements of sapphire substrate in use.For the two films design, the flexure strength of coated and uncoated sapphire samples has been studied by 3-point bending tests at room temperature and high temperatures. The designed single layer of SiO₂ film and two-layer of SiO₂/Si₃N₄ films can strengthen sapphire at the same test temperatures. 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℃.The mechanism that films can strengthen sapphire were discussed and analyzed. Two factors could contribute to the effect of films increased the strength of sapphire at ambient temperature. The coatings can improve the surface morphology and reduce the surface roughness of sapphire substrate. A second effect could be that the stress state of sapphire surface is changed from tensile to compressive by the coatings. Films can improve the surface quality and reduce the contact compressive stress along the c-axis of sapphire samples at elevated temperatures, which maybe restrain the twins to be produced so as to increase the flexure strength of sapphire at high temperatures.