Research On Properties Of Sapphire Single Crystal Under Irradiation/Indentation Load/Surface Patterning

With the development of space science and technology, space exploration now has to be met the more indepth and more accurate development direction. The space optical system, working as the ‘eye’ of space probe, is used to detect unknown and collect imformation. The space optical window is the key component for such a systerm, which is required to possess higher optical transmittance in a wide frequency band and excellent adaptation properties of space environment to make sure the efficiency, accuracy and reliability of imformation. As a bran-new kind of space optical window material, sapphire single crystal has begun to replace the traditional materials such as ZnS, spinel and MgF2because of its excellent optical properties, high strength, thermodynamic stability and outstanding corrosion resistance. Specially, the development of artificial preparation technology of surface pattern periodic microstructures on sapphire single crystal provide a new idea and choice to research and manufacture the space optical window. However, when sapphire single crystal is served in space optical system as optical window material will be exposed in the hostile space environments directly, the optical properties and structure properties decrease, thus, the character of service is affected, even total system lose its effectiveness. Therefore, there is important significance to study the optical and structure properties of sapphire single crystal under particle irradiation, micro loading and surface patterning conditions.The optical and structure properties before and after electron, proton and integrated current irradiation with different variety, dosage and energy were discussed through integrated radiation simulator equipment. The front and back side surface roughness of sapphire single crystal before and after irradiation was tested and discussed through atomic force microscope, it indicated that the irradiation caused surface roughness of sapphire single crystal increased, the integrated irradiation and the electron irradiation had the most and the least effect on the surface roughness of sapphire single crystal; the dose effect was accompanied by the ascent of the surface roughness, that was to say, the dose of irradiation particles increased, the surface roughness rose seriously; the energy effect was accompanied by the ascent of the surface roughness, in other words, the energy of irradiation particles increased, the surface roughness rose seriously; there was disparity between the surface roughness of front side and back side of sapphire single crystal because of the distinguishing irradiation range between different kinds of particles, the electron irradiation had the most effect on surface roughness of back side. The transmittance properties in ultraviolet, visible, near infrared and intermediate infrared band before and after irradiation were measured and discussed through UV-VI-NIR spectrophotometer and infrared spectrometer, it indicated that the transmittance of sapphire single crystal decreased after irradiation and the decreasing degree was distinguishing in different band, meanwhile, the transmittance decreased most seriously in the ultraviolet band; proton irradiation had the most effect on transmittance of sapphire single crystal in ultraviolet and visible band, the integrated irradiation had the most effect on transmittance in intermediate infrared band; the dose effect and the energy effect were also existent in the process of transmittance decline during irradiation. The surface accuracy including PV and RMS of sapphire single crystal before and after irradiation was tested and analyze through laser interferometer, the results showed that the irradiation had no effect on surface accuracy of sapphire single crystal. The mechanism of electron, proton and integrated irradiation was discussed in this paper, too.The structural properties of sapphire single crystal under the action of indentation load was discussed through nanoindentation test method. The Nix-Gao model was introduced to discuss and build mathematical modeling of indentation size effect of sapphire single crystal during nanoindentation, ultimately, confirmed that the nano hardness and nano Young modulus were31GPa and382GPa. The first pop-in event (the displacement discontinuity) of sapphire single crystal during nanoindentation was discussed and found that the maximum load, the maximum depth and the loading rate except the dimension parameters of indenter had no effect on Pcr which first pop-in event occurred and mutational displacement Δhcr, the deformation before first pop-in event occurred was discussed. The creep property of sapphire single crystal during nanoindentation was discussed and then the creep stress exponent was figured out. The finite element analysis software Ansys was used to simulate the process of nanoindentation test of sapphire single crystal, it indicated that the computational load-depth curve almost coincided the experimental curve, therefore, the finite element simulation analysis method could be used as theoretical analysis method of sapphire single crystal during nanoindentation.The optical properties of sapphire single crystal with patterned periodic microstructures on surface were discussed. The equivalent medium theory and FDTD (finite difference time domain) method were uesed to calculated the transmittance properties and reasonable dimension parameters of surface periodic microstructures are gotten; three kinds of periodic microstructures including cone shape, shell shape and trapezoid shape with same cycle of3.0μm were fabricated through photetching on surface of sapphire single crystal. The transmittance of patterned sapphire single crystal and contrast sample (without microstructures) in intermediate infrared band was tested at room temperature and discussed, the results showed that the patterned microstructures caused the optical transmittance of sapphire single crystal rose, meanwhile, the spectral red shift phenomenon was accompanied by the ascent of transmittance, the trapezoid structures had the most effect on transmittance of sapphire single crystal. The transmittance and emissivity of patterned sapphire single crystal at high temperature were measured through high temperature infrared optical test system, the transmittance of patterned sapphire single crystal increased with the ascent of temperature below300℃, afterwards, the transmittance rose again, the patterned structures had no effect on transmittance; rising of temperature caused the emissivity increased, there was patterned structures effect on the emissivity of sapphire single crystal, the trapezoid structures had the most effect on emissivity because of its large surface area.