Near-net-shape And Properties Of ZrB₂-SiC Based Ultra-High Temperature Ceramic Thin-walled Parts Fabricated By Plasma Spray Forming

ZrB2-SiC ultra high temperature ceramics(UHTCs)are believed to the promising candidates in the aerospace field owing to the combination of high melting point,high strength and low density.Nevertheless,intrinsic brittleness and its induced poor thermal-shock resistance and difficult machining of thin-walled parts are still obstacles to extend application fields.To this end,advanced digital manufacturing technique with combination of materials design and near-net-shape thin-walled part forming have been attracted considerable interests in materials community.On the basis of excellent mechanical properties,thermal conductivity and high temperature stability of boron nitride nanoplatelets(BNNPs),the aim of this thesis is to develop ZrB2-SiC ceramic based composites reinforced with BNNPs.More importantly,near-net-shape of plasma spray forming with characteristics of high efficiency,easy to be controlled and combination of materials design and part forming has been well recognized to be a novel near-net-shape forming technique.Uniform particle size distribution and good sphericity are necessary for the agglomerated particles.Agglomerated composite powders with uniform particle size distribution and good flowability through spray drying were employed as feed stock materials for plasma spray forming.Meanwhile,the effects of plasma spray parameters and post-treatment densification(Hot Isostatic Pressing,HIP)on the microstructure evolution,mechanical properties of the as-sprayed thin-walled ZrB2-SiC based composite parts were also investigated.The thin-walled ZrB2-SiC based composite parts were successfully fabricated by plasma spraying near net forming and followed by HIP,in which the compositions of these composite parts were ZrB2-SiC,ZrB2-SiC-lwt.%BNNP,ZrB2-SiC-2wt.%BNNP,respectively.Microstructure analysis showed that there is no phase transformation of ZrB2 after near-net-shape forming,BNNP can survive their original structure after high-temperature process including plasma spray forming and HIP.Also,it is found that the typical lamellar structure associated with plasma spray forming disappeared after HIP densification.Multi-scale mechanical tests such as instrumented micro-indentation and micro-scratch,three-point-bending were employed to evaluate the room-temperature mechanical performance and thermal shock resistance of ZrB2-SiC based composite parts,and toughening and strengthening mechanisms associated with these added BNNPs were also discussed.Results showed that the fracture toughness and flexural strength of ZrB2-SiC-2wt.%BNNPs part at ambient temperature with respect to ZrB2-SiC part were improved by 20.1%and 32.8%,respectively.BNNP pull out,crack bridge,crack deflection associated with BNNPs ascribe to the improved fracture toughness.By using the modified'shear lag' model to calculate the interface-stress distribution between BNNP and matrix,it is found that most of the added BNNP can satisfy the critical length to exhibit as-expected toughening and strengthening effects.Also,higher contract area of BNNP to matrix is believed to contribute to achieve high efficiency of load transfer and to strengthening ZrB2-BNNP.Instrumented micro-scratch experiments showed that the addition of BNNPs ascribes to reduce the friction coefficient and to improve the scratch resistance.Thermal-shock tests indicated that the critical temperature difference of ZrB2-SiC-2wt.%(460?)was improved by 10.8%than that of ZrB2-SiC(415?),especially the residual flexural strength of ZrB2-SiC-2wt.%after thermal shock at 400? was improved by up to 65.1%as compared with that of ZrB2-SiC part.Above significant improvement on the thermal shock resistance of the ZrB2-SiC based composite parts reinforced with BNNPs are thought to be ascribed to the enhanced mechanical performance at room temperature and decreased temperature gradient within the part exposed under high temperature due to high thermal conductivity with the added BNNP.The research is expected to support some fundamentals on the advanced digital manufacturing technique with combination of UHTC materials design and near-net-shape thin-walled part forming.