Co-sputtering of yttrium and aluminum oxide for hard yet transmittant and low emissivity coatings on sapphire substrate

In industrial application environments such as temperature measurement and imaging of molten steel, infrared thermal imagers require detection windows to have good light transmittance, hardness, and low emissivity. In this paper, the co-sputtering of yttrium and aluminum oxide on a sapphire substrate by magnetron sputtering is studied. According to composition control, the sputtered coatings are anti-reflective and transparent to varying degrees. With the increase of Y content, coatings gradually changed from an amorphous state to a Y₂O₃ crystalline state, and γ-Al₂O₃ is produced. In mechanical properties, the coatings’ modulus and hardness increase with the increase of Al content. At Y-deficient coatings, the modulus and hardness have maximum values of 157.7 GPa and 6.7 GPa, respectively. It has been verified through experiments and simulations that the emissivity of the coatings is consistent with the change of the extinction coefficient. Co-sputtering of yttrium and aluminum effectively delays the increase of the extinction coefficient, which is beneficial in reducing the emissivity of the material.