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Brittle materials generally exhibit size effects, and the mechanical properties of these materials degrade significantly with an increase in size. However, the mathematical law governing the attenuation degree of mechanical properties with the increase in size is still unknown. In this study, maximum loads of differently sized ceramic test strips were subjected to three point bending tests under two working conditions of equal spans and span amplifications, respectively. Subsequently, the theoretical maximum loads of materials were calculated using the finite element method (FEM). By calculating the difference between the calculated values and the actual maximum loads, the attenuation of mechanical properties of ceramic samples were observed. The results show that the theoretical mechanical properties and the performance attenuation caused by the size effect tend to increase according to the following equation: y=ax3+bx2+cx+d. Therefore, mechanical properties and performance attenuation of any sample exhibiting a size within the experimental range can be predicted by a mathematical law, which was obtained through mechanical tests results of four samples with different sizes. The obtained mathematical law holds great significance for predicting the mechanical properties of materials under size effects. [ABSTRACT FROM AUTHOR]

The application of C/C-SiC composites remains a challenge because of the poor mechanical properties induced by the liquid silicon infiltration method. In this study, a thick high textured pyrolytic carbon (HT PyC) interface manufactured by chemical vapor infiltration (CVI) was used to protect the carbon fibers and improve the flexural properties. The microstructure of HT PyC, its flexural properties, and its associated strengthening and toughening mechanisms were analyzed on the C/C-SiC composites. The results showed that the bending strength of the C/C-SiC composites was 344.74 MPa, which is nearly 31.82% higher than that of C/C composites. The fracture mode transformed from brittle fracture to pseudoplastic fracture owing to the addition of SiC, and the C/C-SiC composites maintained a better toughness compared to C/C composites. Multiple mechanisms of strength and toughness improvement were found to be responsible for the excellent performance of the C/C-SiC composites. The HT PyC interface played a critical role in the excellent flexural properties. The strong bonding among the carbon layers and high interfacial strength between the fiber and HT PyC led to the increase of strength. Besides, the improvement in toughness was attributed to the multiple effects of the carbon-layers deformation, cracks deflection and propagation caused by the bridging areas, sublayers, nano- and micro-scale cracks. [ABSTRACT FROM AUTHOR]

HfC nanowires (HfCNWs) were in-situ synthesized via CVD technique and acted as the reinforcement in the CVD-HfC coating for C/C composites. The morphology, flexural property and long-lasting anti-ablation performance of the HfC coating were researched. The deposition efficiency and compactness of the HfC coating were both elevated by virtue of in-situ growing HfCNWs. The flexural property of the coated C/C exhibited a pronounced increment on account of incorporating HfCNWs. HfCNWs formed an interlocking network in the oxide layer and inhibited the coating delamination or spalling in harsh ablation condition, so the long-lasting anti-ablation performance of the HfCNWs-reinforced HfC coating improved. After exposure to the oxyacetylene flame for 120 s, the mass and linear ablation rates of the HfCNWs-HfC-coated C/C were only 0.57 mg/s and -0.35 μm/s, which were 54.03% and 117.77% reduction in comparison with those of the HfC-coated C/C (1.24 mg/s and 1.97 μm/s). [ABSTRACT FROM AUTHOR]

C/SiC–ZrC composites were prepared by a combining slurry process with precursor infiltration and pyrolysis, and then annealed from 1200 °C to 1800 °C. With rising annealing temperature, their mass loss rate increased, and the flexural strength and modulus decreased from 227.9 MPa to 41.3 MPa and from 35.3 GPa to 22.7 GPa, respectively. High-temperature annealing, which elevated thermal stress and strengthened interface bonding, was harmful to the flexural properties. However, it improved the ablation properties by increasing the crystallization degree of SiC matrix. The mass loss rate and linear recession rate decreased with increasing annealing temperature and those of the samples annealed at 1800 °C were 0.0074 g/s and 0.0011 mm/s respectively. Taking mechanical and ablation properties into consideration simultaneously, the optimum annealing temperature was 1600 °C. [ABSTRACT FROM AUTHOR]