Masato Yoshioka

Crack Formation Process during Indentation or Scratching on the Surface of MgO Single Crystals

Abrasive machining such as lapping or polishing is mainly used in the field of precision machining of brittle materials. However, the process of abrasive machining, especially micro-fracture mechanism, has not been made completely clear. In this paper, using newly made micro-scratching device, process of crack nucleation and propagation in MgO single crystal around scratching or indentation point is observed. With due regard to three dimensional structure of cracks, crack formation mechanism is discussed. Vertical cracks nucleated in MgO single crystal are classified into two kinds. One generates when indenta-tion weight is increasing, and is unchanged after unloading. The others grow abruptly when unloading begins. The former are nearly parallel to (100) plane, on the other hand, the latter nearly to {110} plane. It can be considered that cracks are produced by cleavage, while cracks are by “dislocation pile-ups”. On the basis of these fundamental mechanism of crack formation, cracks with other various shapes such as “oblique cracks” or “delta cracks” are also discussed.