DENG Renqing ^{1, 2}, CHEN Yufeng ¹, YU Chao ¹, DONG Bo ³, HU Jiaxun ¹,

DENG Chengji ¹, DING Jun ¹, ZHU Hongxi ¹, ZHU Qingyou ¹

(1. State Key Laboratory of Advanced Refractories, Wuhan University of Science and Technology, Wuhan 430081, Hubei, China; 2. Dongguan Maiwo Technology Development Co., Ltd., Dongguan 523931, Guangdong, China;3. China Railway 11th 

Bureau Group Co., Ltd., Wuhan 430071, Hubei, China)

Extended abstract:[Background and purposes] Recrystallized silicon carbide (R-SiC) porous ceramics are pivotal advanced refractories utilized in high-temperature applications, such as kiln furniture and thermal exchangers, due to their exceptional thermal stability, chemical inertness and mechanical strength at elevated temperatures. Recrystallization sintering process, following the evaporation-condensation mechanism without sintering aids, is favored for producing high-purity SiC ceramics. However, a significant challenge lies in balancing the material's porosity with its mechanical performance. In traditional approaches, powders with bimodal or multimodal particle size distributions were used to enhance densification, but often at the cost of complex processing and possible uncontrolled grain growth. The introduction of silicon carbide fibers (SiC_f) into ceramic matrices is a well-established strategy for toughening and strengthening. However, its application and mechanistic understanding within a single-particle-size R-SiC system, where the reinforcement mechanism differs fundamentally from dense composites, remain unexplored. This study was aimed to systematically explore the influence of SiC_f and sintering temperature on phase evolution, microstructural development and resultant mechanical properties of single-particle-size R-SiC porous ceramics, seeking to identify the optimal processing parameters and elucidate the underlying mechanisms.[Methods] The green bodies were prepared using a single-size 6H-SiC powder (D₅₀=1.07 μm) as the matrix. Silicon carbide fibers (SiC_f , diameter of 5–10 μm, length of 50–100 μm) were incorporated as a reinforcing phase at a concentration of 20 wt.%. The powder and fibers were homogenized via ball-milling in an ethanol suspension for 4 h. The mixed batches were subsequently dried, mixed with 2 wt.% of a binder solution (hydroxyethyl cellulose and polyvinyl alcohol) and uniaxially pre-pressed at 10 MPa, followed by cold isostatic pressing (CIP) at 180 MPa, to form green compacts with dimension of 40 mm×   8 mm×6 mm. The binder was removed at 600 ℃ for 3 h in Ar. The final recrystallization sintering was conducted in a high-temperature furnace in Ar, at temperatures ranging from 1600 ℃ to 2200 ℃ for 1 h. Phase composition was determined by using X-ray diffraction (XRD) with Rietveld refinement. Microstructure, grain size and sintering neck development were analyzed by using scanning electron microscopy (SEM). Apparent porosity and bulk density were measured according to the Archimedes principle. The cold modulus of rupture (CMOR) was evaluated by using three-point bending test.[Results] XRD analysis results confirmed that the primary phases were 6H-SiC, with additional 4H-SiC and 3C-SiC polymorphs. The introduction of SiC_f, which contained a fraction of the metastable 3C-SiC polytype, influenced the phase evolution during sintering. At the optimal temperature of 2100 ℃, the SiC_f-added sample (PSC2) exhibited an increase in the content of 4H-SiC and 3C-SiC, as compared with the pure SiC sample (PSC1), which was attributed to the evaporation-condensation and phase transformation of the SiC_f -derived 3C-SiC. At 2100 ℃, the addition of SiC_f resulted in a more uniform distribution of finer grains and a narrower size distribution of sintering necks, as quantified by a lower SPAN value (1.30 for PSC2 vs. 1.67 for PSC1). This indicated that SiC_f  promoted homogeneous neck development by providing additional vapor sources and nucleation sites. Consequently, the mechanical properties were significantly enhanced. The sample with 20 wt.% SiC_f sintered at 2100 ℃ demonstrated optimal performance, with an apparent porosity of (45.5±0.8)%, a bulk density of (1.80±0.03) g·cm^–3 and a CMOR of (43.44±4.75) MPa. This represented a 35.7% increase in flexural strength over the baseline sample [PSC1: (32.02±6.89) MPa] at the same temperature. However, a critical finding was the temperature-dependent behavior of SiC_f. When the sintering temperature was elevated to 2200 ℃, the beneficial effect of SiC_f was negated. Excessive grain growth, particularly abnormal or exaggerated grain growth, was observed in the SiC_f-containing sample. This microstructural degradation led to a precipitous drop in mechanical strength, with the CMOR of PSC2 decreasing to (23.62±1.47) MPa, a 45.6% reduction from its peak value at 2100 ℃ and lower than that of the pure SiC sample at 2200 ℃ [(26.85±0.82) MPa]. This suggests that, beyond a critical temperature, the enhanced mass transport facilitated by SiC_f becomes detrimental, triggering unstable grain coarsening.[Conclusions] High-porosity and high-strength SiC porous ceramics were prepared using a single-particle-size SiC powder and SiC fibers (SiC_f) as the raw materials, at sintering temperatures in the range of 2100–2200 ℃. At 2100 ℃, the decomposition of SiC_f occurred, leading to a decrease in the content of 4H-SiC and 3C-SiC polytypes. This indicates a phase transformation process from the metastable 3C-SiC to the more stable 4H-SiC and 6H-SiC polytypes. When the sintering temperature was elevated to 2200 ℃, abnormal grain growth was observed. The addition of SiC_f further promoted secondary recrystallization, which resulted in a significant degradation of mechanical properties, as evidenced by the decrease in the cold modulus of rupture from (26.85±0.82) MPa to (23.62±1.47) MPa. The optimal comprehensive performance of the recrystallized SiC porous ceramic was achieved with the addition of 20 wt.% SiC_f and sintering temperature of 2100 ℃, yielding an apparent porosity of (45.5±0.8)%, a bulk density of (1.80±0.03) g·cm⁻³ and a superior cold modulus of rupture of (43.44±4.75) MPa.

Key words: recrystallization sintering; SiC fiber; porous silicon carbide ceramics; mechanical properties; sintering neck; sintering temperature