Strengthening sapphire at elevated temperatures by SiO2 films
Introduction
Sapphire is a desired material for infrared-transmitting windows and domes because of its excellent optical and mechanical properties. However, the transmission in mid-wave (3–5 μm) 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 [1], [2] 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 material and its properties needed to be improved [3], [4], [5], [6]. Research and development work for increasing the high temperature transmission and flexural strength of sapphire is a major problem with sapphire in high-speed or high-temperature applications.
It has been observed that c-axis compression causes twinning on rhombohedral crystal planes of sapphire at elevated temperature. Coatings deposited on sapphire may reduce the contact compressive stress along the c-axis of sapphire in flexure tests, which is in favor of increasing the flexure strength of sapphire. In addition, it appears most twin events begin in the near surface due to surface flaws from machining [7]. The ability to fill these micron-size voids and cracks produced during grinding and polishing with a compressive coating may remove the crack-initiating defects that lead to loss of strength.
Silicon dioxide is an important material used as low index films in multilayer optical thin films devices because of its desirable properties. SiO2 films have been deposited and characterized using many techniques for different purposes [8], [9]. In this paper, SiO2 films were prepared on sapphire by radio frequency magnetron reactive sputtering to improve its infrared transmission and flexural strength at elevated temperatures. Results of the transmission and flexural strength for uncoated and coated sapphires tested at different temperatures have been presented.
Section snippets
Experimental
Sapphire bars investigated in this study were fabricated from sapphire (99.99% Al2O3 single crystal) that was grown and annealed via the horizontal directed solidification technique (Harbin Institute of Technology, China). Chinese Science Institute polished the bars on all surfaces to an arithmetical average roughness (Ra) of less than 8.5 nm. The sides of all the samples were fine ground. The long edges of the bars were rounded (chamfered) to a radius of 0.09–0.15 mm. Crystal axes of the bars
Composition and structure
Fig. 2 displays the Narrow-scan XPS spectra of Si 2p and O 1s for the prepared SiO2 films. As shown in Fig. 2, the electron binding energy of silicon and oxygen elements is 103.3 and 533.1 eV, which is the same as those of silicon and oxygen elements in compound silica. Besides, the quantitative analysis of XPS showed that atomic concentration of silicon and oxygen was 33 and 65%, respectively. It means that the atom ratio of silicon to oxygen was near 1:2 in the deposited films.
SiO2 films as
Conclusion
Infrared transmission and flexural strength of uncoated and coated sapphires have been investigated at different temperatures. The temperature was proven to only weakly affect the absorption of the SiO2 films, whereas it greatly affected the absorption of sapphire substrate. Therefore, with increasing temperature, the antireflection capability of the deposited films suffered a little change and the coated sapphires have larger average transmission than the uncoated ones. Flexural results
Acknowledgements
The authors gratefully acknowledge the National Defense Previous Research Foundation and National Defense Basic Research Foundation.
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