In Fig. 14b, there is hardly crack coming into being on M2B. To explain the phenomena in Figs. 13 and 14, a corresponding physical model is established (Fig. 15). As shown in Fig. 15a, when abrasive particle impresses into M2B, cracks will occur on boride under external pressure if fracture toughness of the boride is too poor. Moreover, when abrasive particle scratches the specimen surface under high load, cracks will also come into being in M2B grains due to friction force and extrusion force from abrasive particles (Fig. 15a). On the other hand, if the boride possesses a better fracture toughness, nearly no crack will appear on M2B grains during the processes of indentation and micro cutting by abrasive particles, as shown in Fig. 15b.