Silicon nitride (Si₃N₄) ceramics have been extensively used as structural and/or functional components in various engineering fields such as filtration, aerospace, and membrane support. It possesses low thermal expansion coefficient, excellent mechanical properties, good thermal shock resistance, and high chemical stability due to its strongly covalent bonds between atoms^[1,2,3]. Several technologies can be used to fabricate Si₃N₄ ceramics, including pressureless sintering^[4,5], reactive sintering^[6,7], carbothermal synthesis^[8], and combustion synthesis (CS)^[9,10]. Among these approaches, combustion synthesis can achieve rapid and low-cost fabrication of Si₃N₄ ceramics by the self-propagating of combustion waves using Si as starting material ^[9].

Compared with Si₃N₄ ceramics, Si₃N₄-BN-SiC composites present some improved properties such as lower dielectric constant, lower thermal expansion coefficient, and higher flexural strength, which draws attractive attentions in engineering applications^[11,12]. The traditional introduction way of an additional phase is to add the required phase into the initial powders^[13]. However, the introduced phase is difficult to disperse homogeneously into the sintered product. In-situ introduction can solve this difficulty and achieve good interface bonding between the matrix and introduced phases^[14]. But investigations involving the in-situ introduction of BN or SiC into Si₃N₄ matrix are limited. Kusunose et al.^[15] reported the preparation of Si₃N₄/BN nanocomposites by hot-pressing the t-BN coated α-Si₃N₄ powders, t-BN was in situ synthesized by the reducing reaction between boric acid and urea. Zheng et al.^[16] used B₄C and Si as raw materials to prepare h-BN-SiC composites via their combustion reaction at high-pressure N₂ gas (60-120 MPa). Inspired by the above work, present study attempts to in situ introduce BN/SiC into Si₃N₄ matrix by B₄C addition. The microstructural evolution of the in-situ fabricated BN/SiC and its effects on the properties of the sintered Si₃N₄-BN-SiC composites is studied.

The initial powders were Si powder (Peixian Tiannayuan Silicon Materials Co., Ltd., Jiangsu, China; purity≥ 99.99%; d₅₀=4.1 μm), Si₃N₄ powder (Yantai Tomley Hi-Tech Advanced Materials Co., Ltd., Shandong, China; purity≥99.9%; α-phase content=42.3% (in mass); d₅₀= 22 μm), B₄C powder (Dalian Jinma Boron Technology Group Co., Ltd, Shandong, China; purity≥99.99%; d₅₀= 1.5 μm), and Y₂O₃ powder (Yuelong New Material Co., Ltd, Shanghai, China; purity≥99.999%; d₅₀= 5.04 μm). The weight ratio of initial powders was determined as Si: Si₃N₄: B₄C: Y₂O₃=40 : (60-x) : x : 2 (x=0, 5, 10, 15, 20). The samples were named SBC00, SBC05, SBC10, SBC15, and SBC20 according to the weight ratio of B₄C, respectively. To obtain homogeneous mixtures, the ceramic powders were ball-milled for 3 h in ethyl alcohol with a ball/charge weight ratio of 2 : 1. After dried and sieved through a 150 μm (100 mesh) screen, each homogeneous mixture was cold-pressed into a rectangular compact (40 mm×40 mm×10 mm) at 10 MPa. The obtained compact was immersed into a powder bed (homogeneous mixture of 40% Si and 60% Si₃N₄ (in mass)) and ignited under 5 MPa N₂ atmosphere. Schematic diagram of the reactor and detailed preparation process was mentioned in previous work^[17].

The reaction temperature was obtained from the W-Re5/26 thermocouple immersed into the powder bed. Rectangular bars with the dimensions of 3.0 mm×4.0 mm×36.0 mm were prepared to measure the bending strength and Young's modulus by three-point bending method (Instron-3443, Instron, USA). Fracture toughness was tested by single-edge notched beam method (SEBN) on pre-notched bars (3.0 mm×6.0 mm×30.0 mm). The microstructure of the sample was observed by scanning electron microscope (SU-1000, Hitachi, Japan) and transmission electron microscope (JEM-2100F, JEOL Company, Japan). The phase composition of the sample was performed by XRD (Diffractor meter D8, Bruker, Germany), and the content of each crystalline phase was calculated based on the XRD results. The open porosity of sintered sample was determined by the Archimedes method in the distilled water. The total porosity (P) was calculated from the measured bulk density (ρ_b), theoretical density (ρ_, calculated based on the phase content of each phase) using following equation: P=1-ρ_b/ρ.

The possible reactions during the fabrication process are shown in Eq. (1-2). Both the reactions are exothermic, but the adiabatic temperature of reaction (2) is reported to be lower than that of reaction (1)^[18,19]. It meets the experimental results as shown in Table 1, the measured reaction temperature decreases from 1850 ℃ to 1765 ℃ with the increase of B₄C content. Meanwhile, the reaction time increases with the B₄C content increasing, it can be ascribed to the generation of SiC and BN which restrain the propagating of combustion wave as an inert phase.

Fig.1 displays the phase identification of the obtained composites. The detected crystalline phase of sample SBC00 without B₄C is β-Si₃N₄. With the increase of B₄C content, the content of BN and SiC increase evidently. Besides, it is worth noting that peak broadening is observed for BN, which reveals that its crystallization is unsatisfactory in such a rapid combustion process. When B₄C content is equal to or greater than 10% (in mass), residual Si is detected, which reveals that the nitridation of Si is suppressed by the newly formed BN and SiC. Meanwhile, residual α-Si₃N₄ is also detected for samples prepared with high B₄C addition. The fabrication mechanism of Si₃N₄ ceramics is primarily controlled by the dissolution of α-Si₃N₄ and precipitation of β-Si₃N₄^[20]. The decreasing reaction temperature as shown in Table 1 is unfavorable to the phase transition from α-Si₃N₄ to β-Si₃N₄, which results in the residual α-Si₃N₄ in materials. Besides, the formed BN and SiC disperse in the liquid phase and restrain the mass transport, which is also a significant factor leading to the residual α-Si₃N₄.

Fig. 2 shows the microstructure of green mixture of SBC20, and fracture-surface microstructure of the CS- fabricated specimens with varied B₄C contents. It could be seen that the microstructure varies evidently after CS process. Before CS process, the green mixture is particulate. After CS process, sample SBC00 without B₄C is composed of interlocking β-Si₃N₄ grains with high aspect ratio. As B₄C is added to the raw materials, the development of β-Si₃N₄ grains is evidently inhibited. Average aspect ratio of β-Si₃N₄ grains decreases significantly with the increase of B₄C content, almost no elongated grains can be observed when B₄C content is 20% (in mass). Furthermore, hollow spherical microstructure is observed for the samples prepared with B₄C addition. The hollow spheres have very thin wall and evident micropores on their surfaces when B₄C content is 5% (in mass). With the increase of B₄C content, the thickness of the wall increases and closed hollow sphere is gradually formed. This hollow spherical microstructure is different from that of flaky BN-SiC composites prepared at 60-120 MPa N₂ gas^[16], which illustrates that the microstructure of Si₃N₄-BN-SiC composites is evidently influenced by the N₂ gas pressure.

The properties of the sintered samples are shown in Table 1, the open porosity of sample SBC00 is 51.8%. With the increase of B₄C content, the open porosity of the sample decreases apparently. When the B₄C content is 20% (in mass), the open porosity of the obtained Si₃N₄-BN-SiC composites is 37.7%. The significant decrease in porosity can be attributed to the higher volume expansion (170%) of reaction (2) than that of nitridation of Si (21.2%)^[16], more pores are filled by the generated SiC and BN grains. However, the calculated total porosity based on the XRD results is higher than open porosity, especially for samples prepared with higher B₄C content. On one hand, closed pores are formed with increasing addition of B₄C as discussed above. On the other hand, B₄C might form glass phase with native SiO₂ film and Y₂O₃ during the high-temperature CS process, the theoretical density calculated based on the XRD results is higher than the actual value of sample, thus leading to the increasing total porosity. This behavior could be proven from the calculated content of each phase by XRD. According to the law of conservation of mass of reaction (2), the content of the generated BN and SiC should be higher than the calculated content. It illustrates that B₄C partially forms glass phase instead of BN and SiC after CS process, which could not be detected by XRD.

To investigate the reaction mechanism of the CS process, transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM), energy dispersive spectroscope (EDS) analysis, and selected area electron diffraction (SAED) are conduct on sample SBC10 and the results are shown in Fig. 3. The results demonstrate that the hollow sphere is a mixture of polycrystalline phase and amorphous phase. The crystalline phase should be BN and SiC combining the XRD analysis in Fig. 1. The amorphous phase consists of multiple elements including B, C, Si, N, and a little amount of O. On one hand, it has the characteristics of SiBCN ceramics^[21]. It is well known that SiBCN ceramics have two compositions of amorphous SiC_xN_4-x (x=1-4) and graphite-like BN(C)^[22]. The above-mentioned broadening of BN peak derives from the formation of amorphous BN(C). On the other hand, the amorphous phase contains evident glass phase combining the SEM image. The formation of the hollow spherical microstructure may originate from the comparatively low N₂ gas pressure and the formation of glass phase. At the initial stage of the combustion reaction, eutectic liquid phase, and small BN and SiC particles are formed and cover the surfaces of raw particles, but the small BN flakes could not grow up because of the low N₂ gas pressure and restriction of liquid phase. As the reaction proceeding, the newly formed products continue to cover the surfaces thus forming hollow spheres when raw particles are depleted. Ultimately, eutectic liquid phase forms glass phase during the rapid cooling of CS process.

As listed in Table 1, the mechanical properties of the obtained Si₃N₄-BN-SiC composites fluctuate with different contents of B₄C addition. Compared to the monolithic Si₃N₄ ceramics, the composites prepared with 5% (in mass) B₄C has higher bending strength of 144 MPa and higher Young's modulus of 54.5 GPa. These improvements mainly result from the decrease of porosity of the sample according to the well-known negative relationship between bending strength and porosity of porous material^[23]. Additionally, the generated BN and SiC grains may also benefit the mechanical properties of composites because of their pinning effects within the grain boundary. But the fracture toughness of composites doesnot show apparent increase. It could be ascribed to the introductions of BN and SiC grains, resulting in more lattice defects in Si₃N₄ grains. The elongated Si₃N₄ grains become new crack sources, which is unfavorable to the fracture toughness of composites. With the further increase of B₄C content, the porosity of the obtained Si₃N₄- BN-SiC composites decreases continuously from 47.3% to 37.7%, but their mechanical properties, including bending strength, Young's modulus, and fracture toughness, degrade sharply. These behaviors indicate that the variation of microstructure is the predominated factor degrading the mechanical properties. On the one hand, the introduction of hollow spheres instead of elongated Si₃N₄ grains presents lower mechanical properties than that of Si₃N₄ ceramics with elongated morphology. On the other hand, the growth of Si₃N₄ grain is restrained because of the introduction of B₄C and the consequent generation of BN and SiC. Therefore, the degraded average aspect ratio is also a significant factor decreasing the mechanical properties of the Si₃N₄-BN-SiC composites according to the theory of crack deflection^[24].

In this research, Si₃N₄-BN-SiC composites with hollow spherical microstructure were successfully fabricated by combustion synthesis. The microstructural evolution of the in-situ introduced BN/SiC and its impacts on the properties of the obtained Si₃N₄-BN-SiC composites were studied. As the B₄C content increases, the reaction temperature decreases and the porosity of sintered sample decreases evidently. Besides, the nitridation of Si, phase transition from α-Si₃N₄ to β-Si₃N₄, and growth of β-Si₃N₄ grains are suppressed with the introduction of B₄C. Therefore, residual Si and α-Si₃N₄ are detected for samples prepared with high B₄C content. The bending strength and Young's modulus of the obtained Si₃N₄-BN- SiC composites increase firstly and then decrease with the B₄C content increasing because of the decreasing porosity and degradation of microstructure. Optimal mechanical properties with bending strength of 144 MPa, fracture toughness of 2.3 MPa·m^1/2, and Young's modulus of 54.5 GPa are achieved when B₄C content is 5% (in mass).