where Area(100), Area(101) and Area(102) denote the integral intensity of the corresponding (hkl) reflex of h-BN. Theoretically, the GI value of ideal h-BN crystal is about 1.6, and a higher GI value indicates a greater disorder degree in crystal.
Figure 1(b) shows the calculated GI values of h-BN grains in composite ceramics. With the increase of sintering temperature, GI values showed a decreasing trend, which basically conformed to the change rule of exponential function. From 1500 °C to 1700 °C, the GI values decreased rapidly from 13.7 to 3.4, while from 1700 °C to 1900 °C, the GI values decreased slowly from 3.4 to 2.4. Sintering temperature had a significant influence on the crystallization growth of h-BN in composite ceramics. Higher sintering temperature was conducive to heat and mass transfer in liquid phase environment and the better h-BN grains growth during hot press sintering.
In Fig. 2(a-c), TEM characterization was used to investigate the detail microstructures of BN-La2O3-Al2O3-SiO2 composite ceramics hot pressed under 1500 °C, 1700 °C, 1900 °C, and the corresponding element distributions of B, N, O, Al, Si, La were shown in Fig. 2(d). The h-BN grains showed typical lamellar morphology and were uniformly dispersed in the all composite ceramics. La2O3-Al2O3-SiO2 phase filled in the space between h-BN grains and had a good combination with h-BN grains, and there were few obvious interfacial cracks. It could be obviously observed that the grain size of h-BN became bigger with increasing sintering temperature, which was because the liquid phase had better heat and mass transfer effect at higher temperature, promoting the growth of h-BN grains.
Some pores were observed in the sample sintered at 1500 °C, this was due to the relatively low fluidity of the liquid phase at this sintering temperature, which could not fully fill the gaps between the h-BN grains. While in the sample sintered at 1900 °C, a small number of pores were also found, this was because the grown h-BN grains overlapped each other to form closed pores, which could not be filled by liquid phase. By contrast, no obvious pores were found in the sample sintered at 1700 °C, indicating this sintering temperature was favorable for obtaining composite ceramics with the high relative density.
Interface microstructures between h-BN grains and La2O3-Al2O3-SiO2 phase of composite ceramics sintered under different temperatures are presented in Fig. 3(a-c), and the corresponding elemental line scanning profiles from h-BN zone to La2O3-Al2O3-SiO2 zone are shown in Fig. 3(d-f). No defects such as crack could be observed at the phase boundary, indicating a good wettability between La2O3-Al2O3-SiO2 amorphous phase and h-BN grains. The changes of elemental contents were continuous, La, Al, Si and O content of amorphous phase increased, whereas B and N content of h-BN phase decreased gradually along the arrow direction. Comparing with three samples, when the sintering temperature increased from 1500 °C to 1900 °C, the width of the diffusion zone at the two phases interface increased from about 38 nm to more than 55 nm, indicating higher sintering temperature were more beneficial to the element diffusion in the phase interface region during hot press sintering process. In Fig. 3(g), high-resolution transmission electron microscopy (HRTEM) results exhibited the detailed interface zone formed by atom diffusion between La2O3-Al2O3-SiO2 phase and h-BN phase, which showed a gradual transition from order to disorder arrangement. On the whole, continuous, defect-free and interdiffusion grain boundary was beneficial to provide the good interface bonding and better performance of composite ceramics.
Precipitation nanocrystalline was also found in La2O3-Al2O3-SiO2 phase, and with the increase of sintering temperature, the size of these precipitated grains showed a gradual increasing trend, as shown in Fig. 4(a-c). High sintering temperature was more likely to form precipitation phase with bigger size. Through selecting electron diffraction analysis as shown in Fig. 4(d), the precipitated phase was identified as cristobalite, which meant the precipitated cristobalite phase and amorphous La2O3-Al2O3-SiO2 glass phase were coexisted in the composite ceramics.
Figure 5 shows the bulk densities and apparent porosities of BN-La2O3-Al2O3-SiO2 composite ceramics sintered under different temperatures. With the increase of sintering temperature, bulk density of h-BN composite ceramics first increased and then decreased, whereas apparent porosity exhibited the opposite tendency. The composite ceramic sintered at 1700 °C had the highest bulk density and the lowest apparent porosity. With the increase of sintering temperature, the liquid phase had better fluidity and wettability, and could well fill into the voids formed by the overlap of h-BN grains, which contributed to the improvement of densification. However, with the further increase of sintering temperature, h-BN grains had obvious growth, which led to the larger pores in the mutual framework by the large h-BN grains, resulting in the decrease of relative density.
Figure 6 shows mechanical properties of BN-La2O3-Al2O3-SiO2 composite ceramics sintered under different temperatures, including flexural strength, elastic modulus, and fracture toughness. With the increase of sintering temperature, the mechanical properties presented a small increase and followed by a rapid decrease, which was consistent with the tendency of bulk density. The best mechanical properties of BN-La2O3-Al2O3-SiO2 composite ceramics were obtained under sintering temperature of 1700 °C, and the elastic modulus, flexural strength, and fracture toughness were 80.5 GPa, 266.4 MPa and 3.25 MPa·m1/2, respectively.
Fracture morphology of BN-La2O3-Al2O3-SiO2 composite ceramics sintered under different temperatures are shown in Fig. 7(a-e). The grain size increased significantly with increasing sintering temperature, and the statistically average values are listed in Fig. 7(f). As the sintering temperature changed from 1500 °C to 1900 °C, the average size of h-BN grains increased from 0.35 µm to 2.5 µm. In addition, some pores with the size of several micrometers were found in the fracture morphology of the samples sintered at 1800 °C and 1900 °C, which was caused by the overlap structure of large h-BN grains. In contrast, the samples sintered at relatively low temperatures did not show the obviously large pores.
From the above results, we comprehensively analyzed the influence of sintering temperature on the mechanical properties of BN-La2O3-Al2O3-SiO2 composite ceramics, which mainly included the following two points: (1) High sintering temperature facilitated heat transfer and atom diffusion in liquid phase, which were beneficial to liquid phase pore filling to increase the relative density and improve the mechanical properties; (2) The grain sizes of h-BN increased rapidly with the increase of sintering temperature, and when h-BN grains grew to larger size, the porosity of composite ceramics became higher, resulting in an adverse effect on the densifying process and mechanical properties.
The microstructural evolution process of BN-La2O3-Al2O3-SiO2 composite ceramics during hot press sintering can be illustrated as Fig. 8 shown. Firstly, the four raw powders were uniformly mixed and heated gradually in the graphite mold (Fig. 8(a)); When the sintering temperature increased, La2O3-Al2O3-SiO2 liquid phase was formed and h-BN grains were uniformly distributed in the liquid phase environment (Fig. 8(b)); With the further increase of sintering temperature, the heat and mass transfer ability of the liquid phase was enhanced, and the h-BN grains began to grow significantly. At the same time, the cristobalite phase nanocrystalline was also precipitated in the liquid phase (Fig. 8(c)); At the case of sintering temperature increasing or holding time extending, the grain size of h-BN and precipitated cristobalite phase continued growing, and finally the h-BN, cristobalite and amorphous phase were coexisted in the sintered BN-La2O3-Al2O3-SiO2 composite ceramics (Fig. 8(d)).