3.3. Vickers hardness and fracture toughness
The effect of MgO particle size on the Vickers hardness of the cutting insert is shown in Fig. 5. Vickers hardness increases with finer MgO particle size and gradually decreases with coarser sized MgO. The result of the hardness values is closely related to the micrograph observation in Fig. 4, where samples with smaller Al2O3 grain sizes tend to exhibit higher values of hardness. The maximum hardness value is obtained with the use of 80 nm MgO as additives i.e. 1740 Hv.
This is due to the small Al2O3 grain size in the sample microstructures which resulted from the presence of MgO [7,8]. A study conducted by Coble revealed that the role of MgO is to accelerate the sintering rate for Al2O3, thus hindering Al2O3 grains from growing. The critical condition for Al2O3 to undergo discontinuous grain growth was averted by the enhancement of the sintering rate by the presence of MgO [7]. It is clearly shown that the effect of MgO particle size plays an important role in the development of the hardness value and microstructures of the samples.