To further examine the quality of the flash-sintered composite,the mechanical properties were measured by indentation tech-nique on the sample obtained at the optimal conditions. Fig. 5a shows a typical indentaion morphology. The hardness (H) of theceramic was estimated using the following equation,
H
= 1.8544P
d2 (1)where P is the indentation load and d is the indentation diagonal measured from the SEM images. The measured value is11.0 ± 0.3 GPa.There are two types of cracks: Palmqvist cracks and median cracks, classified by the crack-to-indent ratio. The crack with the ratio in the range of 0.25–2.5 is classified as Palmqvist cracks [26];while the crack with the ratio >2.5 is classified as median cracks.For the current material, the crack-to-indent ratio was measured to be about 1.2. Thereby, the equation proposed for Palmqvist cracks were used to estimate the fracture toughness (KIC) of the composite[26,27]: