Crack behaviors of optical glass BK7 during scratch tests under different tool apex angles

Abstract

Crack initiation, propagation and interaction during single and double scratch testing are investigated at a constant load under different tool apex angles to understand the influence of tool apex angle on the material removal process of optical glass BK7. During the single scratch test, a large number of fragmentized and rambled shallow lateral cracks and radial cracks occur when the tool apex angle is small. As the tool apex angle increases, the number of shallow lateral cracks and radial cracks decreases, and the lateral cracks become more continuous. Radial cracks have curved paths under the surface and can lead to chippings independently. In addition, radial cracks can facilitate the deeper spreading of shallow lateral cracks. During the double scratch test, the interaction between radial cracks and the 2nd lateral cracks in the middle zone and three kinds of unique cracks, namely, extended lateral cracks, shallower 2nd lateral cracks and deeper radial cracks, are found when the tool apex angle and separation distance are both small. Finally, the material removal volume in the double scratch test is measured in terms of different tool apex angles and separation distances.