Solid particle erosion of infrared transmitting materials is a particular problem to the aerospace industry, where such materials are frequently used as ‘windows’ over guidance systems. They are typically weak and brittle and may suffer severe optical degradation as a result of impacts. Added durability may be provided by the deposition of a coating. An optimum coating thickness has been found, both experimentally and by theoretical modelling, taking into consideration the reduction in stress with depth below the surface of the target material due to an impact and the residual thermal stresses following coating deposition.