## 3 Results and Discussion
[22] reported that a higher hardness would benefit the ballistic performance when using 7.62 mm M993 (WC-Co core, designed to target light armor vehicles). However, in this study, the type-87 5.8 mm steel core is not designed as armor-piercing (AP) ammunition and the steel core had a much lower hardness than that of the WC-Co core. The hardness of the matrix SiC layer for the laminated ceramics, 22 GPa, was likely sufficient, and further increasing the hardness of the ceramic might not contribute to the ballistic performance [23]. High flexural strength would be beneficial to resist the tensile wave generated by the impact and the toughness would resist the penetration through energy absorption [24]. With a higher flexural strength and a higher toughness, the pure-matrix sample would have better energy absorption ability and thus less BFS depth compared to the SSIC sample. However, the mechanical performance of the laminated ceramics was not directly related to their ballistic performance of the CA test. The least BFS depth was obtained by the gradual-layered sample which had a lower strength and a close toughness compared to the pure-matrix sample. This indicated that the energy absorption effect by the layered structure was the dominating factor. Due to different mechanical properties between the matrix layer and the interface layer, the stress wave in the laminated ceramics or composites would be reflected at the interface and would be attenuated [14; 25; 26]. [PERSON] et al. [25] showed that laminated ceramics could have better impact resistance due to the multi-reflection and attenuation of the stress wave at the interfaces. The reflection and attenuation of the stress wave also reduced the wave propagation speed within the material and thus enhanced penetration resistance [14; 26]. The gradual-layered sample, with more interface layers at the back face side, had a decreased BFS depth compared to the uniform-layered sample. This was in agreement with the studies mentioned above in that the introduction of the interface layers helps to improve the ballistic performance. The ballistic performance of the laminated ceramics with interface layers not only depended on the layer structure of the ceramics itself but was also highly dependent on the testing conditions. The BFS depths of the uniform-layered, gradual-layered, and pure-matrix samples were similar in the SA test, while these ceramics had different BFS depths in the CA test. Of note is that the ballistic performance was highly related to the testing conditions as well as the armor itself, e.g., the back support [27], the confinement [28], or the pre-stress applied to the armor plate [29; 30]. In the CA test, the armor plate was attached to a flexible armor jacket which provided stronger back support than the armor plate for the SA test. The stress wave generated by the ballistic impact was reflected at the ceramic/backing interface due to the mechanical impedance [2]. A theoretical study by [PERSON] [27] also showed that a thicker back-plate would help to offset the tensile stress at the ceramics and support the ceramic for a longer time, preventing the premature failure of the ceramics. Compared to the laminated SiC ceramics for the CA test, the laminated SiC ceramics in the SA test had a weaker back support, thus they likely failed before the laminated structure could play a role. As a result, the BFS depths of the three laminated SiC ceramics in the SA test were close. It is still unclear whether there is a threshold for the laminated ceramics with interface layers to be effective, e.g., the critical impact energy or velocity of the wave. Additional experiments, as well as theoretical studies, are needed to explain this phenomenon.