In this paper we study the degradation of the surfaces of three nominally brittle materials when impacted with small, fast-moving particles of quartz. The localised high pressures generated by such events can cause deformation and fracture at the site of impact, often extending some way into the material. Crack intersections or deviations up to the surface then sever regions of material from the bulk, leading ultimately to erosion. In many applications, the prime consideration is how quickly this process causes material to be worn away. However, for imaging applications, such as used in the aerospace industry, optical components are subjected to collisions with airborne dust and sand, and degradation of the material to even a small degree can be significant, as surface pitting and cracking causes light to be Ε½ . scattered and resolution to be lost. The degradation of three such infrared optical window materials is investigated experimentally: Ε½ . Ε½ . free-standing chemical-vapour-deposited CVD diamond, sapphire single-crystal Al O and polycrystalline zinc sulphide. The former 2 3 of these materials is generating much interest at present as a potential window material, due to its extreme hardness, strength and broadband transmission in the infrared. Sapphire is a current 3-5 mm window material which also possesses high mechanical hardness Ε½ . and strength. Zinc sulphide 3-5 and 8-12 mm has been used for many years in such applications because of its excellent optical properties; however, it is mechanically weak. The effect of particle size and impact velocity on both the optical degradation and erosion of these materials is discussed, and the effect of grain size and crystal orientation is considered for the erosion of CVD diamond.