Bohan Wang

Ceramics in miniaturized devices are susceptible to high localized stresses, which can drive microstructural evolution. However, conventional mechanical testing techniques lack the spatial resolution and stress concentration capability required to replicate these conditions. Owing to the highly localized stress field it generates, nanoindentation provides an excellent platform for investigating microstructural evolution of materials under high‐stress conditions. In this study, we investigate the microstructural evolution of a ZrO ₂ –SiO ₂ nanocrystalline‐amorphous dual‐phase ceramic (NADC) using nanoindentation coupled with scanning transmission electron microscopy (STEM). The studied ceramic consists of monocrystalline ZrO ₂ nanograins (NGs) embedded in an amorphous SiO ₂ matrix. The presence of ∼43 vol.% ZrO ₂ NGs in the NADC does not impair the intrinsic deformation capacity of the amorphous SiO ₂ matrix. Nanoindentation induces multiscale microstructural alterations: at the micrometer scale, densification and plastic flow of the amorphous SiO ₂ matrix jointly contribute to the irreversible deformation of the ceramic, while simultaneously increasing the local packing density of ZrO ₂ NGs; at the nanometer scale, subgrain formation, crystal‐to‐amorphous transitions within ZrO ₂ NGs, and stress‐induced tetragonal‐to‐monoclinic phase transformation accommodate high localized stresses and strains; and at the atomic scale, dislocation activation and lattice distortion within ZrO ₂ NGs further facilitate stress relaxation and energy dissipation. These mechanistic insights provide a foundation for understanding the microstructure evolution of dual‐phase ceramics containing both crystalline and amorphous phases under high stress.

α‐SiAlON, as a solid solution of α‐Si ₃ N ₄ , exhibits excellent mechanical properties, making it attractive for structural applications at both room and elevated temperatures. α‐SiAlON ceramics stabilized by Li, Mg, Ca, Y, Mn, and rare‐earth cations have been extensively investigated. In contrast, Na‐stabilized α‐SiAlON has received little attention, and the synthesis of single‐phase Na α‐SiAlON has not yet been reported. In this work, nearly single‐phase Na α‐SiAlON powders were prepared by combustion synthesis for the first time. The effects of starting composition and α‐Si ₃ N ₄ diluent content on the phase composition and microstructure of the synthesized products were systematically investigated. Increasing the m and n values promoted the formation of α‐SiAlON and suppressed the formation of β‐SiAlON. The α‐Si ₃ N ₄ diluent content strongly influenced the phase evolution and microstructure of the products. Nearly single‐phase Na α‐SiAlON powders were obtained at an appropriate α‐Si ₃ N ₄ diluent content. The Na α‐SiAlON powders mainly consisted of rod‐like and equiaxed grains. Based on the experimental results, a dual‐pathway formation mechanism was proposed, involving the direct precipitation of α‐SiAlON crystal from a supersaturated Na–Si–Al–O–N liquid phase and heterogeneous nucleation on undissolved α‐Si ₃ N ₄ particles. This work establishes combustion synthesis as an effective route for the preparation of high‐purity Na α‐SiAlON powders and advances the understanding of their formation mechanisms.

The in situ formation of carbon fillers has been demonstrated as an effective strategy to eliminate the agglomeration of carbon fillers in ceramic composites. In this study, combustion synthesis was employed to in situ incorporate carbon into a ZrO ₂ –SiO ₂ ceramic powder using CO ₂ gas as the carbon source and magnesium powder as the exothermic medium. The resulting disordered carbon is uniformly distributed within the powder matrix. Furthermore, magnesium element is simultaneously incorporated into the powder as a byproduct. The co‐presence of carbon and magnesium elements synergistically enhances the solid‐state reaction between ZrO ₂ and SiO ₂ during sintering, facilitating the formation of ZrSiO ₄ at a significantly reduced temperature of 1080°C. Meanwhile, the solid‐state reaction significantly promotes the densification of the ceramic composite. The incorporated carbon exhibits strong bonding with the ceramic matrix and remains stable within the matrix up to at least 1000°C. Compared with conventional ZrSiO ₄ , the ZrSiO ₄ phase formed in this study exhibits much lower decomposition temperature in range of 1000°C–1200°C. Molten copper exhibits pronounced corrosion toward the ceramic composite, primarily through reactions with the SiO ₂ phase released from the decomposition of ZrSiO ₄ .

Si ₃ N ₄ –BN composites were fabricated by spark plasma sintering, and the effects of starting BN particle sizes on microstructure, mechanical properties, and thermal shock resistance of the composites were systematically investigated. The samples with submicron BN effectively refined β ‐Si ₃ N ₄ grains, resulting in enhanced mechanical properties, including flexural strength and hardness. Among all samples, the 800‐S7B2 sample exhibited the optimal combination of properties, with a flexural strength of 1008 ± 49 MPa, fracture toughness of 6.8 ± 0.1 MPa·m ^1/2 , and hardness of 8.1 ± 0.1 GPa. Thermal shock tests indicated that both the samples with submicron BN and the samples with microscale BN retained approximately 90% of their residual flexural strength after thermal shock at 1200°C. Notably, the samples with microscale BN exhibited higher R ^IV and α _cr values due to their greater fracture toughness‐to‐strength ratio, enhancing resistance to crack propagation and resulting in superior thermal shock performance, with residual flexural strength exceeding 90% after thermal shock at 1200°C. These results demonstrate that controlling the BN particle size is an effective strategy to optimize the microstructure, mechanical performance, and thermal shock resistance of Si ₃ N ₄ –BN composites.

HfC _x N _y solid solution powders were synthesized by combustion synthesis using Hf and carbon black as reactants, with HfC or HfN as diluents under a nitrogen atmosphere. The effects of processing parameters on phase compositions and microstructures were systematically examined, and the reaction mechanism in the combustion synthesis of HfC _x N _y was elucidated. The results demonstrated that both phase compositions and microstructures were strongly dependent on the processing parameters. For undiluted samples with high Hf/C molar ratio and diluted samples with low HfC content, the microstructure featured HfC _x N _y crystals embedded in the unreacted molten Hf. At moderate HfC contents, HfO ₂ formed and replaced molten Hf as the bonding phase. At high HfC contents, the microstructure evolved into agglomerates composed of HfC _x N _y crystals. In contrast, the introduction of HfN as a diluent led to more complex phase compositions. Based on thermodynamic calculations and experimental characterization, four reaction pathways for the formation of the HfC _x N _y solid solution have been proposed. In path 1, HfC _x N _y crystals are directly precipitated from Hf‐C‐N melt. In path 2, HfC crystals precipitate from Hf‐C melt, followed by the formation of HfC _x N _y phase through the reaction between HfC and N ₂ . In path 3, HfC _x N _y phase forms through a solid‐state reaction between HfC and HfN. In path 4, HfC _x N _y phase forms via a solid‐state reaction between HfN and C. This study provides mechanistic insight into HfC _x N _y formation in combustion synthesis and offers guidance for tailoring high‐quality powder.