Preparation, Microstructure Evolution and Mechanical Properties of SiC/(HfxTa1–x)C/C Nanocomposites

Introduction HfC and TaC are typical ultra-high temperature ceramics (UHTCs), which are important candidate materials for thermal protection components in high-speed aircraft. However, their oxidation resistance at medium and low temperatures is relatively poor. Due to the similar crystal structures of HfC and TaC, and the close atomic radii of Hf and Ta (rHf = 1.585 Å, rTa = 1.457 Å, Δr xTa1–x)C (0 0.2Ta0.8C has a melting point as high as 4 300 K, which is recognized as the highest melting point carbide. Compared with single-component HfC and TaC, (HfxTa1–x)C exhibits higher initial oxidation temperature and lower oxidation rate, indicating significantly improved oxidation resistance. In this work, a series of processable liquid single-source precursors (SSPs) were successfully prepared by introducing HfCl4 and TaCl5 into allylhydridopolycarbosilane (AHPCS). Through the polymer-derived ceramic (PDC) method, SiC/(HfxTa1–x)C/C nanocomposites with core–shell structured SiC@C and (HfxTa1–x)C@C nanoparticles were obtained, and the evolution of phase composition and microstructure during the ceramic transformation process was investigated. Combining PDC and spark plasma sintering (SPS) technology, nearly dense SiC/(HfxTa1–x)C/C bulk materials were prepared, and the mechanical properties of the resulting bulk ceramic were investigated. Methods Chemically stoichiometric amounts of HfCl4 and TaCl5 were weighed and reacted with AHPCS in CHCl3 solvent at 60 ℃ to prepare a series of SSPs. The molecular structure of the synthesized SSPs was characterized using Fourier-transform infrared spectroscopy (FT-IR, Nicolet iS10, Thermo Fisher Scientific). Thermal behavior analysis of the SSPs was conducted using a thermogravimetric analyzer (TGA, STA 449 F5 Jupiter, NETZSCH). The SSPs were heat-treated at temperatures ranging from 900 ℃ to 1 600 ℃ to obtain SiC/(HfxTa1–x)C/C nanocomposite powders. The phase composition of the ceramic powders was analyzed using X-ray diffraction (XRD, SmartLab-SE, Rigaku Corporation) with Cu Kα1 radiation (λ=1.540 6 Å). The microstructure and phase composition of the samples were analyzed using transmission electron microscopy (TEM, Talos F200S, Thermo Fisher Scientific) and selected area electron diffraction (SAED). The powders were sintered using a spark plasma sintering (SPS, CXSPS-40A, Chenxin Corporation) at 50 MPa and 2 200 ℃ to prepare SiC/(Hf0.2Ta0.8)C/C bulk materials, named as LAH2T8-bulk. The surface morphology of LAH2T8-bulk was observed using scanning electron microscopy (SEM, CLARA, TESCAN). The hardness and elastic modulus of the bulk materials were measured using a nano-indenter (iNano, Nanomechanics, Inc.) with a Berkovich tip under a maximum load of 20 mN. The volume density and porosity of LAH2T8-bulk were measured using the water immersion method. Results and discussion During the synthesis of liquid SSPs, the main reactions are dehydrochlorination between metal chlorides and AHPCS. The crosslinking of liquid SSPs is achieved through further consumption of Si-H via dehydrochlorination reactions. The ceramic yield of the obtained SSPs at 900 ℃ exceeds 90%. By combining the PDC method with SPS, nearly dense SiC/(Hf0.2Ta0.8)C/C ceramic bulk with an open porosity of 4.87% were successfully prepared. In the crystalline phases, the mass fractions of β-SiC and (Hf0.2Ta0.8)C are 60.7% and 39.3%, respectively, with average grain sizes of both phases being less than 90 nm. The measured nanoindentation hardness is (7.72±1.36) GPa, and the elastic modulus is (76.71±7.89) GPa. Conclusions By altering the feed ratio of HfCl4/TaCl5 during the synthesis process of the SSPs and designing their molecular structure, the composition of the transition metal carbide solid solution products in the final SiC/(HfxTa1–x)C/C nanocomposites can be effectively controlled. After heat treatment at 1 600 ℃, core–shell structured (Hf0.2Ta0.8)C@C and SiC@C nanoparticles were in-situ formed in the ceramic matrix. The combination of the PDC approach with SPS technology results in the bulk SiC/(Hf0.2Ta0.8)C/C ceramic nanocomposites with good mechanical properties. © 2024 Chinese Ceramic Society. All rights reserved.