Optimization of nanocrystalline γ-alumina coating for direct spray water-cooling of optical devices

In this study, aluminium oxide films were deposited on BK7 glass substrates using radio frequency mag- netron sputtering. The purposes of this study are to clarify the influence of O2 flow as reactive partial gas, which is necessary to form Al2O3 films, and then the influence of substrate temperature on structure and rigidity of coat- ings towards water injection. The fabricated metal oxide films were characterized using techniques such as atomic force microscopy (AFM), X-ray diffraction (XRD), spectrophotometry, ellipsometry and Rutherford backscatter- ing (RBS) analysis. Modifications of the partial gas percentage influences the optical properties and composition of the deposited aluminium oxide, the best samples being those deposited with 5% and 8% oxygen. The substrate temperature affects the structure and crystallization of the films. Nanocrystalline γ -Al2O3 has been observed at temperatures above 300 ◦ C with the grain size of 25 nm. After water injection, there was a large diversity in the surface roughness of samples with different substrate temperature. Experiments have shown that the best resistance against water injection occurs for the sample deposited at 350 ◦ C with 5% partial gas. We conclude that the rigidity of nanocrystalline γ -Al2O3 coatings can be explained by both Hall-Petch and Coble creep mechanism. In this case, there is an optimum grain size of around 42 nm against water spray.