Preparation And Properties Of C_f/MC-SiC (M=Zr,Hf) Composites

To meet the urgent requirement of thermal protection materials with the features of high-temperature durability,anti-ablation and high oxidation resistance for hypersonic vehicles,in this paper,the synthesis and ceramic transformation of ultra-high-temperature ceramic?UHTC?precursors,moreover,the preparation and properties of C_f/MC-SiC?M=Zr,Hf?ceramic matrix composites were investigated and discussed.The modified Pechini method was used to synthesize UHTC ceramic precursors,and their transformation to the final UHTC products was investigated.An aqueous polymerizable complex route using citric acid as the ligand,the ethylene glycol as the crosslinker,MOCl₂·8H₂O?M=Zr,Hf?as the metal source,has been successfully applied to synthesize zirconium carbide,hafnium carbide and their ternary carbides nanoparticles.In this aqueous process,citric acid was used to chelate the zirconium ion and ethylene glycol to form a polymerized complex resin.We suggested that,based on the results of FT-IR and ¹³C NMR spectroscopies,a very stable metal–citric acid chelate complex formed in the starting solution,which was polymerized with ethylene glycol to form the polymer in the following heat treatments.Immobilization of the zirconium ion in a rigid polymer was thus achieved and can largely guarantee the in-situ charring,resulting in carbon adjacent to the metal oxide in the pyrolysed product.The contiguous carbon and metal oxide led to in-situ reaction?1100°C?with a minimum of diffusion.After heat treatments of 1300°C,the precursor has been completely transformed into ZrC nanoparticles?¹00 nm?.Meanwhile,the submicrometer ZrB₂,HfB₂ and Hf_0.5Zr_0.5B₂ powders were also synthesized by this similar method.These carbide ceramic precursors showed high stability,a low transformation temperature?1300°C?,a favorable viscosity?40-45 mpa·s?,thus possessed a better applicability for PIP process.The formation mechanism of ZrC and ZrB₂ was clarified.As for ZrC,we came up with a new multi-reaction sites theory,which firstly involved generation of huge numbers of highly disordered metastable phases?ZrC_xO_y?with size of several nanometers.After high temperature heat treatments?1300°C?,well-defined carbide nanoparticles were formed through localized particle coarsening by Ostwald ripening.Investigation of the formation mechanism of ZrB₂ indicated that ZrC was the intermediary phase.In the whole conversion process,ZrC formation by carbothermal reduction was a fast reaction,while the direct reaction of ZrC with B₂O₃ to form ZrO₂,CO and ZrB₂ was the rate-limiting step.The preparation,structure,composition and properties of C_f/ZrC-SiC composites were investigated.C_f/ZrC-SiC composites were prepared by PIP process,which improved the densification of porous ZrC matrix.The effects of CVD PyC and SiC interphase,carbon fiber preform structure and parameter on the properties of C_f/ZrC-SiC composites were compared and discussed.The result showed that good mechanical properties were achieved for the composites with a¹.0?m thick PyC coating,meanwhile,3D5d-2,3D4d-1 C_f/ZrC-SiC composites had the comprehensive properties.Next,3D5d-2 and 3D4d-1 C_f/ZrC-SiC composites were prepared by PIP and effects of SiC:ZrC volume ratio on the ablation and mechanical properties were investigated systematically.As to 3D4d-1 C_f/ZrC-SiC,with the increase of SiC:ZrC volume ratio,the mechanical properties were improved.S9-C_f/ZrC-SiC with SiC:ZrC volume ratio of 23.4/12.5 had the high mechanical properties and the flexural strength and fracture toughness were 330.5±86.4 MPa,12.7±1.2 MPa·m^1/2,respectively.The mechanical properties of 3D5d-2 C_f/ZrC-SiC followed the similar relationships vs.SiC:ZrC volume ratio.S9-C_f/ZrC-SiC?SiC:ZrC=24.1/11.9?had the highest flexural strength of 431.6±27.2 MPa.As to the ablation properties,with the increase of SiC:ZrC volume ratio,both mass and linear ablation rates dropped firstly until the minimum value and then increased afterwards.S5-C_f/ZrC–SiC?3D4d-1?had the minimum mass and linear ablation rates of 0.58±0.01 mg·cm^-2·s^-1 and 0.008±0.0004 mm/s.Since the C_f/ZrC-SiC composites by PIP had a high open porosity?>12%?and low thermal conductivity(¹ W·m^-1·K^-1),PIP combined with VSI was used to fabricate the3D5d-2 C_f/ZrC-SiC composites.All composites had an open porosity below 7%.The thermal conductivity was in the ranges of 12.7-14.6 W·m^-1·K^-1,much higher than the composites by PIP.However,the mechanical properties decreased and the highest flexural strength?15-C_f/ZrC-SiC?only reached 182.9±11.9 MPa.Besides,the mass and linear ablation rates?17-C_f/ZrC-SiC?were 0.44±0.02 mg·cm^-2·s^-1 and 0.008±0.0005mm/s,and close to the composites by PIP.The new C_f/HfC-SiC composites were prepared and characterized.3D4d-1,3D5d-2 C_f/HfC-SiC composites were fabricated by PIP process.The mechanical properties of 3D4d-1 C_f/HfC-SiC were improved with the increase of SiC:HfC volume ratio,so S9-C_f/HfC-SiC?SiC:HfC=22.5/11.8?had the highest flexural strength of386.2±50.0 MPa and fracture toughness of 15.9±0.1 MPa·m^1/2.The 3D5d-2 C_f/HfC-SiC had the similar relationships vs.SiC:HfC volume ratio.S9-C_f/HfC-SiC?SiC:HfC=20.6/11.4?had the highest flexural strength of 377.5±25.1 MPa,elastic modulus of 63.3±5.9 GPa and fracture toughness of 14.2±2.1 MPa·m^1/2.The S3-C_f/HfC-SiC?SiC:HfC=15.7/13.7?has the lowest mass ablation rate of 0.27±0.02mg·cm^-2·s^-1 and linear ablation rate of 0.003±0.0002 mm/s.Both mass and linear ablation rates were improved with the increase of SiC:HfC volume ratio.In order to overcome the given shortcomings of PIP process,PIP+VSI was selected to prepare 3D5d-2 C_f/HfC-SiC composite.The porosity,thermal conductivity and anti-ablation properties were superior to the composites by PIP.The composite had a density of 2.73 g/cm³,an open porosity of 5.4%and a thermal conductivity of 21.4W·m^-1·K^-1.Besides,the flexural strength and toughness reached 328.9±58.4 MPa and10.2±1.2 MPa·m^1/2,respectively.The mass and linear ablation rates were 0.33±0.02mg·cm^-2·s^-1 and 0.001±0.0001 mm/s.The effects of matrix on the mechanical properties of C_f/MC-SiC?M=Zr,Hf?by PIP were investigated and the corresponding mechanism was clarified.MC showed a porous state,while SiC matrix was compact and had good mechanical properties.Therefore,the introduction of SiC matrix reduced the porosity,improved the stiffness and the load carrying capability of MC-SiC matrix,meanwhile,the compact MC-SiC matrix also had benefits in effective load transfer in the interface domains and toughening of carbon fibers.Moreover,C_f/MC-SiC composites with high SiC:MC volume ratio meant relative low MC content and would result in a lower degree of reaction damage to the carbon fiber and PyC interphase.Therefore,compared with C_f/MC,the mechanical properties of C_f/MC-SiC were intensively improved,especially for the C_f/MC-SiC composites with higher SiC:MC volume ratio.The oxyacetylene torch ablation behavior and mechanism of C_f/MC-SiC?M=Zr,Hf?composites were investigated.The ablation mainly occurred in the central ablation zones.In these areas,the material dissipation of carbon fiber and SiC by chemical reaction and the spallation of surface oxide scale by high temperature and velocity gas flow were the two main ablation mechanisms.The surface MO₂ layer acted as a component against ablation in the central ablation zones and the SiC contributed to the good oxidation resistance of remaining parts in the medium and low temperature regions.Finally,C_f/MC-SiC?M=Zr,Hf?leading edge and nose tip were fabricated by PIP process.The C_f/ZrC-SiC nose tip was used for inductively coupled plasma wind tunnels test and the peak surface temperature was 2389°C.The nose tip possessed excellent thermal shock resistance and anti-ablation properties,and the linear ablation rate was as low as 3.33×10^-4 mm/s.After ablation,the dense ZrO₂ formed on the windward side,which had a high bonding strength to the remaining body and provided the sustainable anti-oxidation and anti-ablation protection.