Evaluation of subsurface crack depth during scratch test for optical glass BK7

To get insight into the deformation and fracture behavior of brittle materials during abrasive process, scratch tests with linearly increasing normal load and constant normal load were conducted on BK7 using Vickers indenters. The effect of the applied normal load on the characteristics of surface cracks as well as the square of the scratch depth was found insensitive to the two load conditions discussed here. By taking both scratch and residual depths into consideration, a model of scratch hardness for brittle material was developed. Based on this scratch hardness, the correlation between the depth of the subsurface cracks and the scratch depth was established and was found in general agreement with the experimental results.