The erosion of diamond, sapphire and zinc sulphide by quartz particles

In this paper we study the degradation of the surfaces of three nominally b rittle materials when impacted with small, fast-moving particles of quartz. The localised high pressures generated by such events can cause deformatio n and fracture at the site of impact, often extending some way into the mat erial. Crack intersections or deviations up to the surface then sever regio ns of material from the bulk, Leading ultimately to erosion. In many applic ations, the prime consideration is how quickly this process causes material to be worn away. However, for imaging applications, such as used in the ae rospace industry, optical components are subjected to collisions with airbo rne dust and sand, and degradation of the material to even a small degree c an be significant, as surface pitting and cracking causes light to be scatt ered and resolution to be lost. The degradation of three such (infrared) op tical window materials is investigated experimentally: free-standing chemic al-vapour-deposited (CVD) diamond, sapphire (single-crystal Al2O3) and poly crystalline zinc sulphide. The former of these materials is generating much interest at present as a potential window material, due to its extreme har dness, strength and broadband transmission in the infrared. Sapphire is a c urrent 3-5 mu m window material which also possesses high mechanical hardne ss and strength. Zinc sulphide (3-5 and 8-12 mu m) has been used for many y ears in such applications because of its excellent optical properties, howe ver, it is mechanically weak. The effect of particle size and impact veloci ty on both the optical degradation and erosion of these materials is discus sed and the effect of grain size and crystal orientation is considered for the erosion of CVD diamond. (C) 1999 Elsevier Science B.V. All rights reser ved.