Properties of reactive sputtered alumina-silica mixtures

Standard antireflective coatings applied to hard substrates like sapphire suffer from poor abrasion resistance. Silica is used as low refractive index layer in many multilayer systems although it has a lower hardness than the substrate. In this work an attempt was made to enhance the hardness by the addition of alumina. Magnetron sputtering was used in two different ways because it delivers dense coatings with high durability. Nanoindention hardness measurements of mixed alumina-silica films are presented in comparison to haze measurements after a sand trickling test. The hardness of silica is unexpectedly lowered by the addition of small amounts of alumina. Two different stacks were coated in which the low refractive index layers were sputtered as pure material and material mixtures. The thickness loss results after an oscillation abrasion test are presented.