## 4 Discussion
These findings may indicate a negative impact on the core ceramic driven by greater than a certain limit of residual stress resulting from the thermal mismatch of veneering porcelain to core ceramic [24]. In addition, the core ceramic specimen was found to demonstrate numerous microcracks after being subjected to porcelain firing four times. Porcelain fracture can occur in the area of functional cusps or marginal ridges that may lack a support by the core substrate [12]. In addition, the micro-gaps existing in the interface of the bonding of a veneering porcelain to core ceramic may play a role as concentrators of high tensile stress. A fracture may develop at the point of the most vulnerable defect. However, the defects are scarce at the interface of chemical bonding established through the chemical interaction of veneered porcelain to core ceramic. According to a previous study, the level of stress was reduced by enhancing the characteristics of fracture resistance when the interfacial bonding was established through chemical interactions of a veneering porcelain to core ceramic [25]. In the present study, the veneered porcelain was fired multiple times to a lithia-based disilicate core ceramic to investigate the impact of porcelain firing on the characteristics of the interfacial bonding. Interestingly, the bond strengths of each group of the investigation were inconsistent by varying the number of firing cycles. The bonding of the Insync-Amber Press group was most stable (28.5 \(\pm\) 8.6 MPa) when it underwent four firing cycles, whereas the Insync-IPS e.max Press group showed the greatest bond strength (28.3 \(\pm\) 9.4 MPa) when the veneered porcelain was fired twice. In general, multiple firing rounds may adversely affect the fracture characteristics of veneered ceramic restoration [26]. The reaction layer was thicker when the number of firing cycles was increased. There were a greater number of pores in the thicker reaction layer, compromising the interfacial bonding and reducing the fracture resistance of a veneered posterior ceramic crown. Axial loading was applied by means of a steel ball placed in the central fossa of the posterior ceramic crown specimen. The fracture load and the initial chipping strength of the Insync-Amber Press was higher than the Insync-IPS e.max Press group. This suggests a significant difference in the failure resistance between the two groups. Also, the reaction layer of the Insync-IPS e.max Press group was thicker, which resulted in increased residual tensile stresses and consequently lower fracture resistance. Within the limitations of the testing environment, no veneered chipping was observed in the Insync-Amber Press crown group. In contrast, the Insync-IPS e.max Press crown group exhibited a pronounced pattern of veneer chipping, even under relatively low levels of loading. In other words, the reaction layer in the Insync-IPS e.max Press group was thicker, which resulted in a higher residual tensile stress, and this stress concentration leads to brittleness and an increased risk of fracture under load [15]. Various characteristics of ceramic failure are related to residual tensile stresses accumulated in the core ceramic adjacent to the bonded interface, and it was confirmed that thermal expansion mismatch and bonding properties affect the performance of the ceramic restoration [27]. The actual force that human teeth can withstand varies depending on the location of the teeth. On average, the occlusal force (the force that teeth receive when chewing) of adults is about 200 to 500 N, and it can reach up to 800 N, especially in the molar area where food is ground [28]. It is said that maxillary premolars can fracture under a load of 1121 N, and sound molars can fracture under a load of about 2500 N [29]. In this study, the fracture strength of the lithia-based disilicate core ceramic material was more than 3140 N (Insync-Amber Press), which is sufficient to withstand the force applied to molars.