Relationship between subsurface damage and surface roughness of optical materials in grinding and lapping processes

Subsurface damage generated in manufacturing processes directly influences performances of optical elements, however, the rapid inspection of subsurface damage is unresolved for it is covered under surface. For the purpose of rapid, accurate, and non-destructive measurement of subsurface damage produced in optical grinding and lapping processes, a theoretical model of relationship between subsurface damage and surface roughness was established through investigating median and lateral crack system in brittle surface induced by sharp indenter, and contribution of elastic stress field to the median crack propagation was also considered in the loading cycle. With this model, subsurface damage depth can be predicted accurately via measuring surface roughness of manufactured optical elements. The subsurface damage depth and surface roughness of ground and lapped BK7 glass were measured by MRF (magnetorheological finishing) spot technique and contacting profilometer, respectively, in order to verify the validity of the relationship model. The results show that the relationship model is convenient and accurate in predicting the subsurface damage depth. And there exists monotone increasing non-linear correlation between subsurface damage depth and surface roughness (image value) in optical grinding and lapping processes. At last the relationship between subsurface damage depth and process conditions in grinding and lapping processes and material mechanical properties is discussed.