## 3 Results and Discussion
### Diametrical Tensile Strength (DTS) and Volume Shrinkage of MK/HAp Bio-Ceramic Body  
Diametrical tensile strength (DTS), also known as the Brazilian method [53], offers the simplicity of testing brittle materials, in which the ultimate tensile strength of a brittle material is determined through compressive testing. The maximum tensile stress (\(\sigma_{\text{max}}\)) along the diameter of the load, which was localized at the center of the specimens, decreased towards the periphery of the disc and changed its sign (compression). Only compressive stress existed in the vertical direction (\(y\)-axis) in which the magnitude became very great under the load. When the contact angle increases, the compressive stress can dramatically decrease. Therefore, failure may be initiated in the region with the higher compressive stress. Compressive stress induced tensile stress, perpendicular to the loaded diameter of the specimen, which is constant over a wide region around the center of the disc, and the rupture began at the point of maximum tensile strength. The specimen was then broken in two according to this diameter, as shown in Figure 7 [54].  
Figure 6: Micrographs of sintered MK/HAp ceramic specimens: (**a**) HM10-1.25, (**b**) HM20-1.00, (**c**) HM30-1.25 and (**d**) HM30-1.50.  
Figure 8 represents the results of the (DTS) test conducted on all the specimens in conjunction with the as-sintered HAp ceramic as the control. As can be observed from the results, the strength of all MK/HAp specimens increased as the added amount of metakaolin increased. Meanwhile, the HAp ceramic sintered at 900 \({}^{\circ}\)C showed the lowest tensile strength.  
HM30-1.50 obtained the highest tensile strength of 16.5 MPa. However, this specimen did not exhibit the structural phase formation of hydroxyapatite (please refer to Figure 4), and thus it will not be further discussed in this paper. The specimen with the next highest tensile strength (12.5 MPa) and which recorded the formation of hydroxyapatite phase was HM30-1.25, which was initially prepared at the L/S ratio of 1.25. As discussed previously, HM30-1.25 was a unique specimen because it showed the formation of the monoclinic phase alongside the hexagonal phase, unlike others. This condition led to a significant increase in the strength due to the thermo-stable nature of the monoclinic structure. According to the study by [PERSON] [45], compared to the HAp hexagonal phase, the monoclinic is thermodynamically more stable and provides better mechanical properties. Meanwhile, [PERSON] [36] applied kaolin into the HAp to improve the mechano-biological properties of the bioceramic in which the material was found to be suitable for the human trabecular bone as the proposed scaffolds were endowed with an improved mechanical strength that matched the bearable range of the trabecular bone (2-12 MPa). Further to that, the contribution of the increased amount of metakaolin in the mixture to the increased strength is also in agreement with previous work [55]. This is because the composition of metakaolin  
Figure 8: Diametral tensile strength (DTS) and the volume shrinkage of different mix design inputs of MK/HAp ceramics.  
Figure 7: DTS Test.  
consisting of Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\) species influenced the strength of the specimens. Furthermore, HM30-1.25, which exhibited complete densification, also contributed to a higher tensile strength (please refer to Figure 6). This can also be related to the higher volume shrinkage of the HM30-1.25 specimen due to the complete densification, where fewer pores can be observed in this specimen.