## 3 Results and Discussion  
To optimize the sintering temperature of the laminated ceramics, pure-matrix samples were sintered at different sintering temperatures. The relative density of the pure-matrix ceramic at different sintering temperatures is shown in Figure 4. The relative density increased from 95.7% to 98.1% and the temperature increased from 1840 \({}^{\circ}\)C to 1880 \({}^{\circ}\)C and then slightly decreased to 97.2% at 1900 \({}^{\circ}\)C. The decrease of the density at a higher temperature was likely due to the evaporation of the sintering additives [6; 15]. The liquid phase overflow was also observed at 1900 \({}^{\circ}\)C. The highest relative density of the ceramics, 98.1%, was achieved at the sintering temperature of 1880 \({}^{\circ}\)C. The samples prepared for ballistic testing were sintered at this temperature. The X-ray spectrum of the pure-matrix sample is shown in Figure 5. \(\alpha\)-SiC was the main phase in the final product. YAG phase was also observed, which was formed from the additives during sintering following the reaction:  
\[3\text{Al}_{2}\text{O}_{3}+5\text{Y}_{2}\text{O}_{3}\to 2\text{Y}_{3} \text{Al}_{5}\text{O}_{12} \tag{1}\]  
This result was consistent with other research [6]. Figure 4: The relative density of the sintered uniform-layered sample under different temperatures. Figure 5: The X-ray spectrum of the sintered laminated SiC ceramics. The mechanical performance of the samples is shown in Table 2. The flexural strength of the uniform-layered, gradual-layered, and pure-matrix sample was 348 MPa, 396 MPa, and 640 MPa, respectively. The toughness of the uniform-layered sample was the highest at 7.4 MPa\(\cdot\)m\({}^{1/2}\). The toughness of the gradual-layered and the pure-matrix sample was 6.8 and 6.9 MPa\(\cdot\)m\({}^{1/2}\), respectively. The hardness of the matrix layer for the laminated SiC was 22 GPa. The SSIC sample had a flexural strength of 414 MPa and a toughness of 4.2 MPa\(\cdot\)m\({}^{1/2}\). The hardness of the SSIC was 26 GPa, higher than that of the laminated SiC. The microstructures of the laminated ceramics are shown in Figure 6. The thickness of the matrix layer in the uniform-layered sample was approximately 800 \(\upmu\)m and the thickness of the interface layer was approximately 20 \(\upmu\)m after sintering. The matrix layer thickness in the gradual-layered sample was gradually changed: from the bottom (i.e., the strike face in the ballistic test) to the top were 800 \(\upmu\)m, 450 \(\upmu\)m, and 250 \(\upmu\)m, respectively. The interface layer of the gradual-layer sample was also 20 \(\upmu\)m. The detail of the interface layer is shown in Figure 6c and the SiC particles and the BN plates were observed. The fracture surface of the matrix layer is shown in Figure 6d. The fracture mode was a combination of inter-granular and trans-granular fractures. The grain was fine with a size of 1-3 \(\upmu\)m. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample Type** & **Flexural Strength 1 (MPa)** & **Toughness 2 (MPa-m\({}^{1/2}\))** & **Vickers Hardness 3 (GPa)** \\ \hline Uniform-layered & 348 & 7.4 & 22 (matrix) \\ Gradual-layered & 396 & 6.8 & 22 (matrix) \\ Pure-matrix & 640 & 6.9 & 22 \\ SSIC & 414 & 4.2 & 26 \\ \hline \hline \end{tabular}  
* \({}^{1}\): data obtained by three-point bending test; 2: data obtained by the single-edge notched beam method; 3: the testing load was 1 kg.  
\end{table} 
Table 2: Mechanical performance of the samples. Figure 6: The microstructure of the laminated ceramics: (**a**) uniform-layered; (**b**) gradual-layered; (**c**) high magnification of the interface layer; (**d**) the fracturing surface of the pure-matrix laminated ceramic. The laminated SiC without the interface layer (pure-matrix) showed a high strength as well as toughness compared to the solid-state sintered SiC, SSIC.