Sub-surface crack formation in ultrasonic vibration-assisted grinding of BK7 optical glass

Due to the inherent properties of high brittleness and low fracture toughness of optical glass materials, sub-surface cracks would be inevitably induced into the ultrasonic vibration assisted grinding process. Knowledge of the formation mechanisms of sub-surface cracks plays a key role in implementing high efficiency and precision machining of this kind of materials. In this work, the ultrasonic vibration assisted grinding experiments of BK7 optical glass were carried out. Processed by cross-sectional polishing and assisted by HF acid etching, quite a few sub-surface cracks with different shapes were observed beneath the machined surfaces. Categorized by the shapes and formation mechanisms, four kinds of sub-surface cracks (i.e., straight median sub-surface crack, arc median sub-surface crack, lateral sub-surface crack, and bifurcated sub-surface crack) and their corresponding formation mechanisms were both clarified. Experimental and analytical results suggest that the arc sub-surface cracks were formed by the relative shear stresses parallel to the plane of median crack frontiers, which are generated by nonsymmetric contacting between abrasive grains and glass specimen in ultrasonic vibration grinding. The bifurcation cracks were formed by the distortion in frontier fields caused by the impact effect provided by ultrasonic vibration as the abrasive grains vibrate down to the glass material. Effects of grinding and ultrasonic vibration parameters on the maximum depth of sub-surface cracks were also investigated in this work. The value of the maximum depth of sub-surface cracks showed a great dependence on the processing parameters.