Fabrication And Properties Of C_f/HfC-SiC Ultra-high Temperature Ceramic Matrix Composites

Continuous carbon fiber reinforced ultra-high temperature ceramic composites(C_f/UHTCs)are endowed with light weight,excellent mechanical properties,ablation resistance at ultra-high temperatures and outstanding design flexibility.And furthermore,the inherent brittleness and poor thermal shock resistance of bulk UHTCs are overcome.Therefore,these materials are considered as the most promising candidates for applications in thermal structures of hypersonic vehicles.Reactive melt infiltration(RMI)is an important method of rapid preparation of C_f/UHTCs with high density.However,there are problems of residual metal in the matrix owing to the reactive melt infiltration kinetics and fiber/interphase corrosion caused by high temperature melt,which lead to poor high temperature mechanical properties and reliability.In this work,a new RMI method based on preform pore structure regulation and controlling was put forward in order to solve the problems above,and fabrication and properties of RMI-C_f/HfC-SiC were studied.Firstly,porous C_f/HfC-C preforms were prepared via sol-gel processing.Then,Si was infiltrated into the preforms,and the RMI-C_f/HfC-SiC composites were obtained.The reactive melt infiltration kinetics were improved by controlling and optimizing the pore structures of the C_f/HfC-C preforms,which promoted the consumption of molten Si.Meanwhile,the processing temperature can be reduced by using Si which has a relatively low melt point,and the fiber/interphase corrosion can be effectively reduced.The main contents and results are listed as follows:(1)HfC precursor was successfully prepared,and the ceramization mechanism was revealed.The HfC precursor sol was prepared using HfCl₄ and sucrose as Hf and C sources,respectively.After dried and cured,solid state precursor can be obtained.Hf is attached on the precursor by chemisorption.In argon atmosphere,the precursor can be pyrolyzed at 600°C,and carbothermal reduction reaction can be completed at1600°C while in vacuum carbothermal reduction temperature can be reduced to1500°C,resulting in a high reduction degree with an oxygen content of<1 wt.%.The main components of the powder obtained by the transformation of precursor were HfC and residual carbon.The average particle size of HfC is<100 nm,and the unit cell parameter of HfC is smaller than the standard data.Oxygen and carbon content,average particle size and size distribution of the gained powder can be controlled by adjusting the ratio of raw materials.Gas-solid reaction mechanism can be applied to reveal the carbothermal reduction process of Hf O₂-C system obtained by the pyrolysis of precursor.(2)Based on the research on the HfC precursor sol,porous C_f/HfC-C preforms with controllable pore structure were fabricated via sol-gel processing,and C_f/HfC-SiC composites were obtained via RMI processing using Si.Porous C_f/HfC-C preforms with different pore structures were prepared by controlling the number of sol-gel cycles and other parameters.The porosities of the C_f/HfC-C preforms are27.8-33.0 vol.%and the median diameters are 2-13?m.The pore structure shows feature of multi-stage distribution consisting of pores of nano,submicron and micron scale,which is beneficial to RMI processing.Though the processing temperature can be reduced by introducing UHTC phase via sol-gel method and by using Si for RMI,fiber/interphase damages can still be observed in the preforms,with outer SiC interphase layer broken and Hf invasion appearing at the inner Py C layer.The corrosion damages were aggravated by RMI processing.The fiber/interphase corrosion damages are particularly obvious in the composites with higher HfC content because of the accumulated damages resulted from the increased number of sol-gel cycles,leading to lower mechanical properties.The HfC contents of the RMI-C_f/HfC-SiC composites fabricated by the sol-gel-RMI processing are 26-37wt.%,with porosities of<6 vol.%,densities of 2.65-3.01 g/cm³ and residual metal contents of<5 wt.%.The main compounds in the matrix are HfC,SiC,Hf Si₂,residual Si and Si-rich SiC.(3)The mechanism of fiber/interphase corrosion driven by chemical etching-diffusion&melt infiltration was revealed,and an optimized fabrication processing was applied,leading to the improvement of mechanical properties of RMI-C_f/HfC-SiC composites.It is revealed that the reactions between Hf O₂ and SiC are responsible for the fiber/interphase corrosion.Inhibiting the Hf O₂-SiC reaction by controlling carbothermal reduction temperature can reduce the fiber/interphase corrosion.The bending strengths of the composites prepared via the optimized processing are increased by?50%(from 214.1 MPa to 319 MPa).(4)Mechanical properties of RMI-C_f/HfC-SiC composites at elevated temperatures were systematically studied,and high temperature fracture mechanism was revealed.With the temperature rising,the bending strengths of RMI-C_f/HfC-SiC composites increase first and then decrease.From room temperature to 1000°C,the bending strengths increase and reach 329.5 MPa at 1000°C.Then,with the temperature further increasing,the bending strengths decrease,and only 149.1 MPa remains at 1400°C.The fracture micro-morphologies show that,at elevated temperatures,the residual metal in the matrix can melt and erode the fiber/interphase,and meanwhile,the original fiber/interphase corrosion can be aggravated.These degradations lead to shorter fiber pull-out,lower bending strength and the trend to brittle fracture.(5)Ablation behaviors of RMI-C_f/HfC-SiC composites were studied using the air plasma ablation test platform,and anti-ablation mechanism was revealed.During ablation tests with heat flux of 4 and 5 MW/m²,the surface temperatures of composites ablation center were?1820°C and?2080°C,respectively,far lower than that of composites fabricated via PIP method.The RMI-C_f/HfC-SiC composites show feature of micro ablation,with mass ablation rate of<5 mg/s and liner ablation rate of<1?m/s.After the ablation test in constant temperature mode of 2200°C,the mass and liner ablation rates are 0.45 mg/s and-0.1?m/s,respectively.During ablation test,HfC and SiC can be oxidized to form porous Hf O₂framework and Si O₂ melt.The Si O₂ melt covers the surface and fills the cracks and pores of the material,which can effectively prevent the invasion of oxygen into the material.The porous Hf O₂framework provides attachment for Si O₂,reducing Si O₂ loss from high-speed airflow scouring.Particularly,Hf O₂ can be soluble in Si O₂ to form Hf-Si-O phase,which further improves the stability of ablation surface protective layer and the ablation resistance of the material.