The effect of MgO particle size on the fracture toughness of the cutting insert is shown in Fig. 6. Fracture toughness decreases with finer MgO particle size. The result of fracture toughness is closely related to the micrograph observation in Fig. 4, where samples with smaller Al2O3 grain sizes tend to exhibit lower values of fracture toughness. The maximum fracture toughness value is obtained with the use of 7000 nm MgO as additives i.e. 3.62 MPa∙m1/2. This is due to the large Al2O3 grain size in the microstructure shown in Fig. 4(d). A study conducted by Riu et al. explained that the Al2O3 fracture toughness and flaw tolerance will improve with increasing Al2O3 grain size; due to crack bridging by the large platelike grains .