Reaction-bonded Si₃N₄-SiC refractories are preferred thermally conductive materials for waste incinerators. However, their high apparent porosity limits their high-temperature wear resistance and resistance to water vapor oxidation. Promoting the growth of Si₃N₄ whiskers is an effective approach to improve the densification of Si₃N₄-SiC refractories. Related researches have mainly focused on catalytic nitridation to facilitate the direct nitridation reactions, while there are few studies on enhancing the partial pressure of SiO and promoting the indirect nitridation reactions. In addition to ultrafine silicon powders, diborides are new additives that can enhance the partial pressure of SiO and promote the densification of Si₃N₄-SiC refractories. Nevertheless, the relationship between their microstructure evolution and densification effects remains unclear. In this work, the phase composition and microstructure evolution of TiB₂ and AlB₂ micro-powders during the preparation process of Si₃N₄-SiC refractories were investigated, as well as their effects on the bulk density, apparent porosity, phase composition and microstructure of the refractories. This study is expected to enrich the nitridation mechanism system of silicon powders, and provide a theoretical basis for selecting densification additives for Si₃N₄-SiC refractories.

To investigate the oxidation and nitridation reactions of TiB₂ and AlB₂ micro-powders during the preparation process of Si₃N₄-SiC refractories, thermodynamic analysis of the Si-TiB₂-AlB₂-N₂-O₂ system was performed using HSC Chemistry software, and TG-DSC analysis in high purity N₂ atmosphere was conducted with a synchronous thermal analyzer. The phase composition and microstructure evolution of TiB₂ and AlB₂ micro-powders heat-treated at 1400 ℃ in high purity N₂ atmosphere were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). Moreover, quantitative analysis of the phase composition was implemented via the K-value method. Reaction-bonded Si₃N₄-SiC refractories were prepared with TiB₂ and AlB₂ micro-powders as additives. The bulk density and apparent porosity of refractory samples were determined via the Archimedes drainage method. A universal testing machine was used to measure the compressive strength of the refractory samples at room temperature. XRD, SEM, EDS, and TEM were employed to analyze the phase composition and microstructure evolution of Si₃N₄-SiC refractories, with particular attention on the morphology, quantity, and distribution of Si₃N₄ whiskers. Based on the above mentioned analyses, the relationship between the microstructure evolution and densification effects of diboride additives was established.

Thermodynamic analysis results show that the oxidation and nitridation activities of AlB₂ are slightly higher than those of TiB₂ and significantly higher than those of Si. According to TG-DSC analyses, the initial temperature of the nitridation reaction for TiB₂ is 150 ℃ higher than that of AlB₂, which is consistent with the thermodynamic analysis results. After heat treatment at 1400 ℃ in high purity N₂ atmosphere, over 60% of TiB₂ micro-powders remains unreacted and a glass phase surrounded structure can be observed. In contrast, the content of unreacted AlB₂ content is below 10%, and few glassy phases are observed. That's because B₂O₃ glassy phase has reacted with Al₂O₃ to form a new solid phase of Al₄B₂O₉. With an appropriate addtion, both TiB₂ and AlB₂ micro-powders improve the densification of Si₃N₄-SiC refractories, reduce the content of Si₂N₂O phase, and lower the ratio of α/β-Si₃N₄. The optimal addition amount for both additives is 0.6%. Under the same addition amount, TiB₂ exhibits a superior densification effect. When the TiB₂ addition amount is 0.6%, a large quantity of β-Si₃N₄ whiskers are generated, with an average diameter of approximately 200 nm and an aspect ratio exceeding 50. These whiskers entangle with each other to form a three-dimensional network structure, and their growth follows the gas-solid mechanism.

The microstructure evolution of diboride additives modulates the SiO partial pressure and Si₃N₄ whiskers growth during silicon powder nitridation, which in turn affects the densification of Si₃N₄-SiC refractories. For instance, the oxidation products of TiB₂ do not undergo further chemical reactions but instead form a protective glassy phase. This phase protects most TiB₂ from consumption until the nitridation reaction stage, thereby effectively reducing the partial pressure of O₂. Since the partial pressure of SiO in the SiO-N₂-O₂ system increases as the partial pressure of O₂ decreases, TiB₂ micro-powders are effective to enhance the partial pressure of SiO and promote the indirect nitridstion reactions, producing a large number of β-Si₃N₄ whiskers. Therefore, they are highly suitable as densification additives for Si₃N₄-SiC refractories.