Broad band anti-reflective coatings need a low-index material at the top position. Normally SiO2 is used. This layer is in direct contact to the environment. While a number of high- and medium-index layers with a high hardness (up to 30 GPa) have been developed in the past, the hardness of the low-index layer on the top is limited to about 10 GPa. Under hard conditions this layer is then the weak ...