Microstructural Evolution of H-BN Matrix Composite Ceramics With Amorphous La2O3-Al2O3-SiO2 Glass Phase During Hot Pressing

BN-La 2 O 3 -Al 2 O 3 -SiO 2 composite ceramics were fabricated by hot press sintering using h-BN, La 2 O 3 , Al 2 O 3 and amorphous SiO 2 as the raw materials. The effects of sintering temperature on the microstructural evolution, bulk density, apparent porosity, and mechanical properties of h-BN composite ceramics were investigated. The results indicated that ternary La 2 O 3 -Al 2 O 3 -SiO 2 liquid phase was formed during sintering process, which provided an environment for the growth of h-BN grains. With increasing sintering temperature, the cristobalite phase precipitation and h-BN grain growth occurred at the same time, which had the signicant inuence on the densication and mechanical properties of h-BN composite ceramics. The best mechanical properties of BN-La 2 O 3 -Al 2 O 3 -SiO 2 composite ceramics were obtained under sintering temperature of 1700 °C, and the elastic modulus, exural strength, and fracture toughness were 80.5 GPa, 266.4 MPa and 3.25 MPa·m 1/2 , respectively.


Introduction
Hexagonal Boron Nitride (h-BN) and its matrix composite ceramics are typical structural-functional ceramics that have been widely used in many elds, such as aerospace, machinery, metallurgy, energy, and electronics [1][2][3][4][5][6][7]. Compared with Alumina, Zirconia, Silicon Carbide, Silicon Nitride ceramic which have the high hardness and high strength, h-BN presents the relatively low hardness and good machinable properties, because of its hexagonal layered crystal structure similar to that of graphite [8][9][10][11]. Furthermore, h-BN ceramics are di cult to sintering densi cation, so the low melting point sintering additives and/or second phase are usually added to improve the properties of h-BN composite ceramics [12][13][14][15][16].
There have been some researches on the microstructural evolution during sintering and the properties of h-BN ceramics [17][18][19][20][21]. Zhuo Zhang et al. obtained h-BN powders composed of amorphous and nanocrystalline BN by ball milling, and then sintered them under different temperatures and pressures. Higher sintering pressure was more favorable to the preferred orientation growth of the in-plane direction of h-BN grains along the pressure direction, and higher sintering temperature promoted the mass transfer and grain growth. They referred that the structural uctuation of amorphous BN resulted in the t-BN phase formation during the sintering process, and stacking faults usually existed in the as-grown h-BN grains [22]. Haotian Yang et al. discovered a method on low temperature self-densi cation of bulk h-BN. During the sintering process, cubic boron nitride (c-BN) particles incorporated into the h-BN ake powders transformed into BN onions with a volume increase, thus lling in voids among the h-BN akes and densifying the h-BN bulks, nally the dense bulk h-BN with 97.6% theoretical density was achieved by SPS sintering under 1700 °C [23].
For h-BN matrix composite ceramics, there also have been some research results which can reveal the microstructures changing mechanisms [24][25][26][27][28]. Bo Niu et al. investigated the effects of raw h-BN particle size on the textured microstructures of BN-MAS (magnesium aluminosilicate) composite ceramics. With the increase of raw h-BN particle size, h-BN grains tended to orientate with the direction perpendicular to the hot pressing direction, while the densi cation effect of MAS phase on BN-MAS composite ceramics decreased with increasing raw h-BN particle size due to the uneven dispersion of MAS phase [26,29]. Delong Cai et al. researched the in uence of sintering process on the BN-MAS composite ceramics. The sintering pressure had a great in uence not only on the mechanical properties of composite ceramics, but also on the crystallization of MAS and structural order of h-BN. The nucleophilic attack of N on M (M = Mg 2+ , Al 3+ and Si 4+ ) and the electrophilic attack of B on O were the crucial factors on the formation of amorphous MAS phase. In addition, chemical bonding was formed between h-BN and MAS and the matrix atoms diffused into the h-BN layer, leading to a strong bonding interface [30][31][32].
Ternary amorphous La 2 O 3 -Al 2 O 3 -SiO 2 glass phase have been reported on promoting sintering densi cation and improving the room/elevated-temperature mechanical properties of h-BN matrix composite ceramics [33][34][35][36][37]. But the microstructural evolution during sintering process and its effect on properties of this material systems have not been fully revealed yet, which also has important implications for guiding the composition design and process optimization of composite ceramics.
In this study, BN-La 2 O 3 -Al 2 O 3 -SiO 2 composite ceramics were sintered under different temperatures from 1500 °C to 1900 °C, meanwhile the phase composition, nanocrystalline precipitation and grain growth were systematically investigated. The corresponding mechanical properties were tested to reveal the in uence of microstructural evolution on the performance of composite ceramics. atmosphere. The heating rate was 15 °C·min − 1 , and the samples cooled down to room temperature in the furnace spontaneously.

Materials characterization
Phase compositions were identi ed by X-ray diffractometer (XRD, D/max-γB CuKα, Rigaku Co., Japan) with a scanning speed of 4 °·min − 1 . The detailed microstructures were investigated by transmission electron microscope (TEM, Talos F200X, FEI Co., USA). The bulk densities and apparent porosities of samples were measured by Archimedes drainage method. Flexural strength was measured by three-point bending method using a universal testing machine (Instron-5569, USA), meanwhile Young's modulus was obtained through the stress-strain curve. The sample size was 3 mm × 4 mm × 36 mm with a span of 30 mm and the crosshead speed was 0.5 mm·min − 1 . Fracture toughness was measured using the single edge notched beam (SENB) method. The sample size was 2 mm × 4 mm × 20 mm with a notch of 2 mm, and the crosshead speed was 0.05 mm·min − 1 . Fracture morphology was observed using scanning electron microscope (SEM, NanoLab 600i, FEI Co., USA). Considering the total volume content of the adding La 2 O 3 -Al 2 O 3 -SiO 2 was about 30%, which had exceeded the minimum threshold of XRD detection. Thus, we inferred that the amorphous glass phase was formed during hot press sintering process, which was di cult to be characterized by XRD.

Results And Discussion
Comparing with the peaks of h-BN in different composite ceramics, with the increase of sintering temperature, the relative peak intensity of corresponding (002) lattice plane increased gradually. Graphitizing Index (GI) is an indicator of crystallization degree of graphite and similar crystalline structure materials [38], and it is calculated by the following formula: where Area(100), Area(101) and Area(102) denote the integral intensity of the corresponding (hkl) re ex 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. In Fig. 2(a-c) Fig. 2(d). The h-BN grains showed typical lamellar morphology and were uniformly dispersed in the all composite ceramics. La 2 O 3 -Al 2 O 3 -SiO 2 phase lled 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 uidity of the liquid phase at this sintering temperature, which could not fully ll 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 lled 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 La 2 O 3 -Al 2 O 3 -SiO 2 phase of composite ceramics sintered under different temperatures are presented in Fig. 3(a-c), and the corresponding elemental line scanning pro les from h-BN zone to La 2 O 3 -Al 2 O 3 -SiO 2 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 La 2 O 3 -Al 2 O 3 -SiO 2 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 bene cial 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 La 2 O 3 -Al 2 O 3 -SiO 2 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 bene cial to provide the good interface bonding and better performance of composite ceramics.
Precipitation nanocrystalline was also found in La 2 O 3 -Al 2 O 3 -SiO 2 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 identi ed as cristobalite, which meant the precipitated cristobalite phase and amorphous La 2 O 3 -Al 2 O 3 -SiO 2 glass phase were coexisted in the composite ceramics.   Fig. 7(a-e). The grain size increased signi cantly 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 in uence of sintering temperature on the mechanical properties of BN-La 2 O 3 -Al 2 O 3 -SiO 2 composite ceramics, which mainly included the following two points: (1) High sintering temperature facilitated heat transfer and atom diffusion in liquid phase, which were bene cial to liquid phase pore lling 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-La 2 O 3 -Al 2 O 3 -SiO 2 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, La 2 O 3 -Al 2 O 3 -SiO 2 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 signi cantly. 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 nally the h-BN, cristobalite and amorphous phase were coexisted in the sintered BN-La 2 O 3 -Al 2 O 3 -SiO 2 composite ceramics ( Fig. 8(d)).

Conclusions
The BN-La 2 O 3 -Al 2 O 3 -SiO 2 composite ceramics were hot press sintered under different temperatures to reveal the microstructural evolution mechanisms. Ternary La 2 O 3 -Al 2 O 3 -SiO 2 liquid phase was formed during the sintering process, which had a good wettability with h-BN grains and could effectively ll the pores to improve the densi cation of composite ceramics. Higher sintering temperature contributed to the growth and crystallization of h-BN grains through better heat transfer and atomic diffusion in liquid phase environment. Furthermore, cristobalite nanocrystals were precipitated from the liquid phase and also grow gradually with the increase of sintering temperature. The BN-La 2 O 3 -Al 2 O 3 -SiO 2 composite ceramics sintered under 1700 °C exhibited the best mechanical properties, which was attributed to the mutual in uence of liquid phase environment, h-BN grain size and precipitated phase.         Microstructural evolution mechanism diagrams of BN-La2O3-Al2O3-SiO2 composite ceramics.