Toughening Mechanism And Oxidation/ablation Behaviors Of HfC-based Ceramics With The Incorporation Of Silicides And Borides

Hafnium carbide(HfC)ceramic is a promising candidate for applications in rocket nozzles,nuclear reactor rods and space/air craft.However,it is very brittle and easy to be oxidized/ablated,which limit its applications as high-temperature structural materials in extreme environments.Inspired by the composite concept,fabricating the multi-length composite structures with the incorporation of secondary phases is an effective approach to solve this problem.Silicides and borides are promising candidates for toughening HfC as a result of their unique microstructures,together with improving its oxidation/ablation resistance due to their excellent oxidation resistance.However,their inherent brittleness leads to a limited contribution to providing toughening effect.To address this issue,the two-component silicon carbonitride(SiCN)and silicon hexaboride(Si B₆)are incorporated into HfC as reactants,sintering additives,and secondary phases.The micro-and nano-scale multi-length composite structures of HfC-based composites with optimized grain boundaries can be successfully prepared through vacuum carbothermal reduction,solid-solution and in-situ reactive sintering.The fracture toughness and oxidation/ablation resistance of the HfC-based composites are significantly enhanced.The main research results and conclusions are listed as follows:(1)The uniform and ultra-fine HfC-C nanoparticles(?300 nm)are synthesized at1450�C by vacuum carbothermal reduction with the glucose-derived hydrothermal precursor as carbon source and template.An approach to improve the sintering ability and the fracture toughness of HfC ceramic by designing a unique composite microstructure is proposed.HfC-C ceramic sintered with 15 vol.%SiCN sintering additive possesses a beneficial three-dimensional network microstructure composed of inter-penetrating phases of carbon,Si C and HfC,which exhibits the varied stoichiometry at multi-length scales.The HfC-C-15 vol.%SiCN sample exhibits a higher fracture toughness of 5.5 MPa m^1/2,which can be attributed to the unique composite structure that is able to promote stress releases in the crack tip and enhance the resistance to crack propagation.(2)Based on a two-component SiCN(silicon carbonitride,SiCN)containing SiCN and turbostratic carbon,a dense HfC-SiCN composite is fabricated by incorporating SiCN sintering additive using spark plasma sintering(SPS).The addition of the SiCN not only enhances the density(up to?97%)of the final product,but more importantly results in an increase of the toughness from 4.3 MPa m^1/2 to 8.5 MPa m^1/2,with only a10%reduction in the flexural strength.The improvement has been attributed to the unique microstructure of the obtained samples,exhibiting several important characteristics:(i)SiCN and C secondary phases homogeneously disperse in the HfC matrix to control grain size;(ii)HfC grains contain different levels of Si,O and N to form solid solution;(iii)enrichment of free carbon occurs at grain boundaries and triple junctions.The toughening and strengthening mechanisms of HfC-SiCN composites are revealed through analyzing the crack behaviors and fracture modes.(3)To improve oxidation resistance of HfC ceramics,the introduction of SiCN into HfC can form the HfC-SiCN composite system,which exhibits good oxidation resistance at 1500�C in air.It can be found that the thickness of oxide layer in HfC-SiCN system exhibits a decrease trend from?1560?m in the sample without SiCN to?200?m in the sample with 10 vol.%SiCN after oxidation for 4 h,and then shows a slow increase trend with increasing oxidation time at 1500�C.The dense and homogeneous oxide layer is completely formed in the sample with 10 vol.%SiCN after 2 h oxidation,in which the Hf O₂ phase forms a framework,and silicates fill the spaces and heal the defects.The formation of silicates can restrain the volatilization of silica at high temperatures.(4)To solve the problem that the porous oxide layer on the HfC-based ceramics is easy to mechanically scour during ablation,the incorporation of SiCN induces the formation of the unique and homogeneous oxide layer with triple-layer structure on the HfC-SiCN sample,which has the better ablation resistance during ablation at 2500�C.It is attributed to its good thermal stability and damage tolerance for retarding the mechanical scouring and oxidation.Significantly,the triple-layer oxide layer exhibits a lower oxygen permeability due to the homogeneous silica distribution in oxide scales.A few of HfC and Si OC grains in the porous intermediate layer are retained and embedded in the Hf O₂ framework structure,which can reinforce the oxide layer against mechanical scouring during ablation.(5)Motivated by the composite concept of carbides and borides for tailoring their mechanical properties,the strong and tough HfC-HfB₂ composites with the Si solid solution are fabricated by one-step in-situ reactive SPS through incorporating a novel Si B₆ additive that functions both as the boron source and sintering additive.The incorporation of the Si B₆not only promotes the densification of HfC(up to?99%),but also significantly enhances the toughness from 4.3�0.5 MPa m^1/2 to 14.2�1.4 MPa m^1/2.The flexural strength of the HfC-HfB₂ composites is simultaneously improved to 529�48MPa,which is about 1.4 times higher than that of HfC.The grain boundaries of HfC/HfB₂and HfB₂/HfB₂ are cleaner without the segregation of oxygen impurity,which is not only beneficial for the sintering densification but also leads to a mixed mode of transgranular and intergranular cracking.This mixed mode of cracking,together with the improved densification and the strengthening by Si solid solution and secondary phase,is believed to be responsible mechanisms for the superior mechanical properties.