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Microstructural evolution and high‐temperature ablative damage mechanism of Csf/SiC‐BN composites with and without polyborosilazane (PBSZ) as sintering aids that can convert to polymer‐derived SiBCN ceramics (PDCs‐SiBCN) by cross‐linking and pyrolysis during sintering were studied. The density of Csf/SiC‐BN composites increases from 2.17 to 2.25 g/cm3 with 10 wt.% sintering aids because PBSZ promotes rearrangement and redistribution of raw materials by pyrolysis during the sintering process. With the increase of carbon fiber content, ablation performance (mass ablation rate: 15.25 mg/s; linear ablation rate: 0.100 mm/s) negligibly decreases, compared to counterparts with lower fiber content (mass ablation rate: 16.53 mg/s; linear ablation rate: 0.072 mm/s) because the presence of PDCs‐SiBCN partially compensates for the detrimental effect of carbon fibers on ablation performance. Csf/SiC‐BN composites with and without PBSZ show similar phase composition mainly containing quartz, amorphous SiO2, and matrix crystallization products and morphology after ablation because of similar phase composition before ablation. The ablation mechanism is oxidative corrosion and mechanical scouring and stripping. [ABSTRACT FROM AUTHOR]

The impact of Zr addition on the solid sintering process of SiBCN multiphase ceramics was investigated under high temperatures. To gain deep insight, SiBCN‐Zr diffusion couple method (sample preparation at 1500–1800°C) was utilized in conjunction with first‐principles calculations. Significantly, the results demonstrated the formation of SiC and BN as well as ZrC/ZrB2 at the interface of the diffusion couples. Possible mechanisms have been analyzed and proposed. On the one hand, the extensive generations of defects are supposed to facilitate the formation of SiC and ZrC during the preparation of SiBCN amorphous powders. On the other hand, the presence of distinctive BN(C) structures in SiBCN hindered the formation of ZrB2, thus causing a delay in its formation. Through first‐principles calculations, remarkable variations in interatomic bonding strengths have been revealed among the different phases at the interface. Specifically, interfaces of ZrC/ZrB2 and SiC exhibit stronger bonding strengths compared to BN and SiC, implying that Zr, and its compounds may act as binding agents. [ABSTRACT FROM AUTHOR]

The layering process using phenolic resin coupled with pressureless sintering technique was explored to prepare short carbon fiber‐reinforced SiBCN composites (Csf/SiBCN). The effect of phenolic resin content on the microstructure and mechanical properties of Csf/SiBCN was mainly studied. Results suggest that the addition of phenolic resin can appropriately improve density and flexural strength and reduce total pore volume of the composites because phenolic resin can partly consolidate SiBCN powders to promote shaping via low‐temperature pressureless sintering. As the phenolic resin content increases, the fracture behavior changes from delamination fracture to monolithic fracture, thereby reducing fracture displacement. The reason for this performance is that the interlayer bonding is enhanced to hinder crack propagation along the layers. [ABSTRACT FROM AUTHOR]

B4C–MAS and B4C–TiB2–MAS ceramic composites were prepared by hot pressed sintering. The effects of particle size of B4C and TiB2 content on the microstructure and general mechanical properties of the ceramic composites were investigated by using X‐ray diffractometer, scanning electron microscope, and universal testing machine. The strengthening and toughening mechanism of ceramic composites is mainly attributed to the addition of magnesium–aluminosilicate (MAS) and thermal mismatch effect of TiB2. When the TiB2 content was 20 wt.%, the bending strength, fracture toughness, and Vickers hardness of B4C–TiB2–MAS ceramic composites were 375 MPa, 4.42 MPa·m1/2, and 23.9 GPa, respectively, and its relative density could reach 99.7%. The dynamic mechanical properties of B4C–TiB2–MAS ceramic composites were investigated at strain rates of 2000–6000 s−1 using a Separate Hopkinson Pressure Bar (SHPB) device. The results showed that the B4C–TiB2–MAS ceramic composites exhibited significant strain rate sensitivity. The dynamic compressive strength of the ceramic composites increased from 4634 to 5734 MPa with increasing strain rate in the tested strain rate range. [ABSTRACT FROM AUTHOR]

To develop materials suitable for aerospace applications, silicon nitride/boron nitride (Si 3 N 4 /BN) fibrous monolithic ceramics with varying BN contents were prepared. Employing analytical techniques such as XRD and SEM, coupled with mechanical testing equipment, the influence of BN concentration on the thermal shock resistance of Si 3 N 4 /BN fibrous monolithic ceramics was assessed. When the thermal shock differential is less than 800 °C, its residual flexural strength gradually decreases as the thermal shock differential increases. Conversely, when the differential exceeds 1000 °C, the residual flexural strength of the material increases. The residual strength of all samples reached its peak after undergoing a thermal shock assessment at a 1500 °C differential. When the BN mass fraction is 5 wt.%, the residual strength after a thermal shock at a temperature difference of 1500 °C is 387 ± 19 MPa, which is 124% higher than the original strength of the sample that did not undergo thermal shock (25 °C, 311 ± 18 MPa). The oxide layer formed on the thermal shock surface played a role in bridging defects introduced during material surface processing. [ABSTRACT FROM AUTHOR]

Density‐modified structural features and mechanical properties of the amorphous Si2BC3N are studied by ab initio molecular dynamics simulations. The chemical bonds of Si2BC3N are insensitive to the density variation, which is reflected by the negligible changes in bond lengths, peak locations in density of states and Bader charge values under different densities. Instead, the composition of chemical bonds is altered. Meanwhile, high‐density condition induces the transition of polyhedral units from the sp2‐like trigonal configuration to the tetrahedral configuration, especially for B, C, and N. This variation is the main structural responding mechanism of Si2BC3N to increased density, which is different from the stretching and/or shrinkage of bonds as occurred in crystals. The increased tetrahedron content shall further benefit the amorphous structure stability of Si2BC3N by impeding the separation of turbostratic BN(C), and enhance the second‐order elastic constants, elastic moduli and tensile/shear strengths of Si2BC3N. The high density may decrease the debonding capability of the fiber/Si2BC3N interface but will not change the preference for crack deflection at interfaces. These results suggest the promising prospect of mechanical property optimization of SiBCN ceramics through density tailoring in experiments. [ABSTRACT FROM AUTHOR]