Particle erosion of infrared materials

Erosion test of some infrared (IR) optical crystals (Ge, ZnS, MgF₂, and quartz) was conducted with a number of different erodents (glass bead, and angular SiC, SiO₂, Al₂O₃) by a homemade gas-blasting erosion tester. The influence of impact angle, impact velocity, erodent, and erosion time on the erosion rate and the effect of erosion on their IR transmittance were studied. The dam- aged surface morphology was characterized by scanning electron microscopy, and the erosion mechanism was explored. All of the materials show the maximum in wear versus impact angle at 90°, confirming their brittle failure behavior. It is found that the erosion rate is dependent on the erodent velocity by a power law, and it is highly correlated to the hardness of the erodent. The erosion rate-time curves do not show an incubation state, but an accelerated erosion period followed a maximum erosion (steady state). The decrease of IR transmittance is direct proportion to the erosion rate. Although the material loss occurs primarily by brittle process, ductile behavior is clearly an important feature, especially for MgF₂ and ZnS.