Comparison of abrasive tests for transparent optical coatings

Sometimes, optical interference coatings have to provide wear durability. This study reports on two standardised methods that can be applied for abrasion resistance testing of transparent metal oxides: the oscillating sand method as a hard scratching test and the sand trickling as an impact test. Both tests were carried out with silica and corundum sand to reveal the geometric influence of the test medium. Appropriate cleaning procedures, based on the norms, are discussed and developed. It is shown that water purging has to be complemented by wiping or ultrasonic cleaning. The abrasion is qualified by haze and ellipsometry measurements as well as optical microscopy. The results of Al2O3, Nb2O5, SiO2 and Ta2O5 are discussed with respect to the hardness order of the thin films and compared to two glass types.