Two SiC-containing metal diborides materials, classified in the ultra-high-temperature ceramics (UHTCs) group, were fabricated by hot-pressing. SiC, sinterability apart, promoted resistance to oxidation of the diboride matrices. Both the compositions, oxidized in air at 1450°C for 1200 min, had mass gains lower than 5 mg/cm2. Slight deviations from parabolic oxidation kinetics were seen. The resistance to thermal shock (TSR) was studied through the method of the retained flexure strength after water quenching (20°C of bath temperature). Experimental data showed that the (ZrB2+HfB2)–SiC and the ZrB2–SiC materials retained more than 70% of their initial mean flexure strength for thermal quenchs not exceeding 475° and 385°C, respectively. Certain key TSR properties (i.e., fracture strength and toughness, elastic modulus, and thermal expansion coefficient) are very similar for the two compositions. The observed superior critical thermal shock of the (ZrB2+HfB2)–SiC composite was explained in terms of more favorable heat transfer parameters conditions that induce less severe thermal gradients across the specimens of small dimensions (i.e., bars 25 mm × 2.5 mm × 2 mm) during the quench down in water. The experimental TSRs are expected to approach the calculated R values (196° and 218°C for ZrB2+HfB2–SiC and ZrB2–SiC, respectively) as the specimen size increases.

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Resistance to Thermal Shock and to Oxidation of Metal Diborides–SiC Ceramics for Aerospace Application

Spinel-containing alumina-based refractory castables

Sustainable structural materials with light weight, great thermal dimensional stability, and superb mechanical properties are vitally important for engineering application, but the intrinsic conflict among some material properties (e.g., strength and toughness) makes it challenging to realize these performance indexes at the same time under wide service conditions. Here, we report a robust and feasible strategy to process cellulose nanofiber (CNF) into a high-performance sustainable bulk structural material with low density, excellent strength and toughness, and great thermal dimensional stability. The obtained cellulose nanofiber plate (CNFP) has high specific strength [~198 MPa/(Mg m-3)], high specific impact toughness [~67 kJ m-2/(Mg m-3)], and low thermal expansion coefficient (<5 × 10-6 K-1), which shows distinct and superior properties to typical polymers, metals, and ceramics, making it a low-cost, high-performance, and environmental-friendly alternative for engineering requirement, especially for aerospace applications.

Lightweight, tough, and sustainable cellulose nanofiber-derived bulk structural materials with low thermal expansion coefficient

[Sun, Ziqi; Zhou, Yanchun; Wang, Jingyang; Li, Meishuan] Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China. [Sun, Ziqi] Chinese Acad Sci, Grad Sch, Beijing 100039, Peoples R China.;Zhou, YC (reprint author), Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China;yczhou@imr.ac.cn

Thermal Properties and Thermal Shock Resistance of γ‐Y2Si2O7

A set of dense magnesia-magnesium aluminate spinel composites has been prepared by hot-pressing magnesia powder using 0 to 30 wt.% of spinel powder of mean particle size 3, 11 and 22 μm. Bend stength, modulus, and fracture toughness, have been measured. Strength and modulus decrease with increasing spinel content, and for a given loading, spinel particle size, as a result of microcracking caused by thermal expansion mismatch between the magnesia matrix grains and the spinel particles, the effects of which may be intensified by recrystallization of spinel. Fracture is predominantly transgranular for the pure magnesia, and intergranular for the composite materials. Possible reasons for the differences in behaviour between this system and the SiC–Al 2 O 3 system with a similar thermal expansion mismatch are examined.

Mechanical properties of magnesia-spinel composites

The effect of mullite on the mechanical properties and thermal shock behaviour of alumina–mullite refractory materials

Thermal shock parameters (R, R‴ and R‴′) have been calculated using measured strength and modulus values for model magnesia-spinel composite materials. The R‴ and R‴′ parameters vary with both spinel content (0–30%) and spinel particle size (3–22 μm). In general, the larger the spinel content, the higher the values of R‴ and R‴′. It is predicted that the coarsest (22 μm) spinel provides a significant improvement in resistance to thermal shock damage through maximised difficulty of crack propagation, with a maximum at ∼20% addition. These predictions were also matched by thermal shock testing. After quenching from 1000 °C, the 20% 22 μm spinel composite had a retained strength ∼4.5 times higher than that for similarly quenched pure magnesia.

Thermal shock parameters [R, R‴ and R‴′] of magnesia–spinel composites

The incorporation of up to 30% of spinel powder of median particle sizes 3, 11 and 22 μm into dense magnesia gives improved resistance to thermal shock as indicated both by the R ‴ thermal shock resistance parameters calculated from measured mechanical properties, and by a standard quench test. The loss of strength of the composite materials in the quench test matched satisfactorily that predicted by the R ‴ parameter. The maximum benefit from spinel additions was obtained with spinel of the largest particle size, at a loading of 20%. It is postulated that the pre-formed cracks in the composites, resulting from the thermal expansion mismatch between magnesia and spinel, inhibit the accumulation of strain energy during thermal shock.

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Papers

Mechanical properties of magnesia-spinel composites

The effect of mullite on the mechanical properties and thermal shock behaviour of alumina–mullite refractory materials

Thermal shock parameters [R, R‴ and R‴′] of magnesia–spinel composites

Thermal shock behaviour of magnesia-spinel composites

Mechanical properties and thermal shock behaviour of alumina-mullite-zirconia and alumina-mullite refractory materials by slip casting