## 4 Discussion  
In this study, lithtia-based heat-pressed ceramic copings of Amber Press MO1 and IPS e.max Press MO1 were layered with Inync veneering porcelain to produce a double layered structure. The double layers were found to establish a chemical bonding, as indicated by a reaction layer at the interface of the bonding. The reaction layer area of the Insync-IPS e.max Press group showed a thicker difference than that of the Insync-Amber Press group. Studies have shown that the thicker reactive layer of thermo-pressed ceramic coping results in lower heat diffusivity and lower resistance to fracture [15]. In addition, a thick reaction layer is prone to microcracks or defects and, especially under load, becomes brittle due to stress concentrations, which increases the risk of fracture [16]. A thermal mismatch may play a critical role in retaining the stability of the interfacial bonding of a veneering porcelain to core ceramic. The veneering porcelain exists as viscoelastic at a high temperature during firing and becomes solid during cooling below the glass transition temperature. Because of the phase transformation, residual stress may develop in a veneered ceramic restoration [17]. Also, the cooling of veneer ceramic restorations may not occur uniformly, as the outer surface of the ceramic exposed to the surrounding air may cool faster than the inner surface, but residual stresses may occur throughout the veneer structure of the ceramic restoration [18]. That is, a mismatch in the coefficient of the thermal expansion of a veneering porcelain may develop residual tensile stress in the core ceramic adjacent to the interface of bonding and cause a chipping or fracture under occlusal loading [19]. A sintered veneering porcelain may also demonstrate inherent defects or pores of stress concentrators at the subsurface or at the interface of bonding. According to a previous study, a slight compressive stress built in the layered structure can be conducive to suppressing the development of a crack, leading to chipping or fracture [20]. However, the criteria for an appropriate range of thermal mismatch of a veneering porcelain to core ceramic are unclear. Clinical prediction studies using the Viscoelastic Finite Element Method (VFEM) to address this issue have shown that thick ceramic layers can lead to strong stress gradients, which can easily propagate cracks due to contact damage. Therefore, the interaction between the thickness ratio and residual stress is more relevant to the susceptibility to chipping than bulk fracture [21]. The thermal expansion coefficient of Insync dentine veneering porcelain was \(8.7\times 10^{-6}/^{\circ}\)C, and that of Amber Press and IPS e.max Press core ceramics was \(10.2\times 10^{-6}/^{\circ}\)C and \(10.6\times 10^{-6}/^{\circ}\)C, respectively. Thus, the thermal mismatch was \(1.5\times 10^{-6}/^{\circ}\)C in the Insync-Amber Press group and \(1.9\times 10^{-6}/^{\circ}\)C in the Insync-IPS e.max Press group. In the present study, the veneering porcelain was fired three times to investigate the pattern of crack propagation under the Vickers indenter pressed within the range of approximately 250 \(\upmu\)m from the bonded interface. The Vickers indentation method involved in this study is widely used to characterize the residual stress of brittle materials such as small and irregularly shaped dental ceramic restorations [22,23]. In the present study, the ceramic specimen was finely polished to demonstrate a flat surface to meet the requirement of the testing method. For the Insync-Amber Press group, the pattern of a crack that formed along the edges of the indentation was symmetrical in all directions, indicating no accumulation of a residual tensile stress resulting from the thermal mismatch. However, an accumulation of a residual tensile stress was evident in the Insync-IPS e.max Press group, where the crack that developed toward the interface of bonding was longer than the other sides of the indentation.