Microstructure And Properties Of ZrC-SiC-LaB₆Ceramic Composites

As a typical ultra high temperature ceramic material, ZrC has received attention from researchers in aviation, aerospace, military, energy and other fields due to its high melting point, high hardness, retained strength and other excellent performance. In this paper, the relations between the content of SiC whisker(SiC w), SiC nanoparticles(SiCn) and LaB6powder and microstructure, relative density, mechanical properties and ablation/oxidation resistance were investigated.Based on SiC whisker, SiC nanoparticle, LaB6content of optimization design, ZrC-20SiCw-0SiCn-10LaB6composites, ZrC-15SiCw-10SiCn-10LaB6composites, ZrC-10SiCw-15SiCn-10LaB6composites and ZrC-15SiCw-15SiCn-10LaB6composites were preparaed respectively by hot pressing sintering process. The phase constitute, relative density, microstructure, mechanical properties and ablation/oxidation resistance were tested, and the influence of additions content on the microstructure, relative density, mechanical properties and ablation/oxidation resistance was discussed deeply, and the strengthing and touguening mechanisms and ablation/oxidation mechanism of the ZrC-SiC-LaB6composites are analyzed.The relative density of four groups ZrC-SiC-LaB6composites were over98%, especially the fourth group:ZrC-15SiCw-15SiCn-15LaB6composites was the highest in four groups of specimens, reaching the maximum value99.77%. The mechanical performance of the four groups ZrC-SiC-LaB6composites were tested. The maximum flexural strength was556.92MPa, and the maximum fracture toughness was4.79MPa-m1/2. The transgranular fracture mode of ZrC-SiC-LaB6composites was the combination of transgranular fracture and intergranular fracture. The strengthing and touguening mechanism mainly included:crack deflection, whisker load, whisker fracture, whisker pull-out, fine grain strengthening, elastic modulus and thermal expansion coefficient mismatch, residual stress and so on.The ablative performance of four groups ZrC-SiC-LaB6composites were evaluated by oxygen-acetylene flame and the ablation mechanism was analyzed. Results show that mass ablation rate and the linear ablation of the four samples are low. After180s oxygen-acetylene flame ablation, the mass ablation rate and the linear ablation of the second group ablation sample were only0.74X104g/s and15.00X104mm/s. B2O3, SiCh, La2Zr2O7glass phases with certain viscosity and liquidity were formed and covered on the ablation surface of specimen at elevated temperatures, which could bounded ZrCh tightly. SiC whisker linked ZrCh grains, preventing oxidation layer from erosion by the high-speed flow. ZrCxOy transition layer between surface oxide layer and the matrix layer with higher density further inhibited the inward diffusion of oxygen. The ablation resistance of ZrC-SiC-LaB6composites was improved owing to the protection of surface oxide layer and transition layer.