Fabrication Of Ultra-high Temperature Ceramic Matrix Composites By Homogeneous Ceramization Of Nanoporous Carbon And Their Properties

The external surface and propulsion system of the high-speed aircrafts face harsh thermomechanical environments during flight.Ultra-high temperature ceramic matrix composites(UHTCMCs)are the main candidate materials for thermal protection systems of the high-speed aircrafts due to their excellent ultra-high temperature properties.Reactive melt infiltration is one of the main fabrication methods of UHTCMCs,and it has many advantages including low cost,short cycle of production and high density of the obtained products.However,carbon matrixes,such as pyrocarbon(PyC),resin carbon(ReC)and pitch carbon,in the conventional C/C preforms are difficult to be completely transformed into ceramic matrixes,and a large quantity of unreacted carbon matrixes remain in the as-obtained composites.Meanwhile,excessive melt tends to be left in large pores and cracks in the as-obtained composites and thus corrodes some carbon fibers.The above phenomena result in the deteriorated mechanical and anti-ablation properties of the composites.To address the issues in the conventional RMI process,the nanoporous carbon(i.e.,carbon aerogel,CA)is employed to prepare carbon fiber reinforced CA(C/CA)preforms by replacing the conventional carbon matrixes.A series of CMCs,such as C/SiC,C/SiC-ZrC,C/SiC-HfC,C/SiC-(Hf,Ti)3SiC2-(Hf,Ti)C and C/SiC-(Hf,Zr)C,were obtained by high-efficiency infiltration and reaction of different melts.Their phase compositions,microstructures,mechanical properties and ablation behaviors were investigated as well as the relevant mechanisms of CA ceramization and CMC ablation.The major research contents and results are listed as follows:1.Three kinds of C/SiC composites were prepared using C/C preforms with CA,PyC and ReC,respectively.The comparative study suggests that the C/SiC derived from C/CA has dense SiC matrix and uncorroded carbon fibers.It has therefore the highest flexural strength(218.1 MPa),the lowest linear ablation rate(0.168 μm s-1)and relatively low mass ablation rate(0.078 mg cm-2 s-1).In contrast,the unreacted carbon matrix,excessive silicon and corroded fibers are left in the C/SiC composites derived from C/PyC and C/ReC,which results in their lower flexural strengths of 170.5 MPa and 128.0 MPa,respectively.In addition,the two C/SiC composites possess the poor ablation resistance with the respective linear ablation rates of 2.836μm s-1 and 1.648 μm s-1.The discrepancies in structures and properties of the C/SiC composites are determined by the different ceramization processes of the three carbon matrixes.CA has the connected three-dimensional nano-reticular channels and the stacking nano-sized carbon skeleton particles.In the infiltration process of liquid silicon,CA is transformed completely into dense SiC with the almost full consumption of inspersed silicon.In contrast,PyC and ReC have thick carbon walls of several microns to tens of microns.The continuous dense SiC layer formed on the carbon surface during Si-C reaction can inhibit the further reaction.Therefore,the abundant unreacted carbon and silicon still remain in SiC matrixes,and some carbon fibers are greatly corroded by the residual silicon.2.C/SiC-ZrC and C/SiC-HfC composites were prepared by impregnating molten ZrSi2 and HfSi2 into the C/CA preforms.Compared to the C/SiC with same interface structure,C/SiC-ZrC and C/SiC-HfC possess the superior ablation resistance and mechanical properties.They have respective flexural strengths of 287.9 MPa and 251.2 MPa,19%and 4%higher than that of C/SiC.Their linear ablation rates after the ablation at 2200℃ are 0.432 μm s-1 and 0.642 μm s-1,respectively,68.7%and 53.5%lower than that of C/SiC.Their linear ablation rates at 2500℃ are 1.355 μm s-1 and 0.672 μm s-1,respectively,47.5%and 74.0%lower than that of C/SiC.The linear ablation rates of C/SiC-ZrC and C/SiC-HfC are determined by the denudation rate of the oxides formed in the ablation center region.The former has a lower linear ablation rate at 2200℃ compared to the latter,but it has a much higher value at 2500℃The different ablation rates can be attributed to the discrepancies in compositions and physicochemical properties of HfO2,ZrO2 and glass phases.HfO2 formed after ablation of C/SiC-HfC has lower oxygen vacancy concentration and higher content of Si dopant than ZrO2.In addition,Hf has a better effect on the thermal stability of SiO2 than Zr.Consequently,HfO2-SiO2 layer has lower oxygen diffusion coefficient and fewer glass phase defects,which results in the lower oxygen flux through the layer and insufficient supplement of glass phases for the oxide layer.The linear ablation rates of the composites are determined by the oxide denudation rates in the ablation center region.In the ablation process at 2200℃,the fewer glass phases in the oxide layer on the C/SiC-HfC surface results in a weak adhesion between the oxide layer and the unoxided substrate.Therefore,the linear ablation rate of C/SiC-HfC is higher than that of C/SiC-ZrC.On the other hand,in the ablation process at 2500℃,HfO2 has a superior resistance to denudation due to its higher melting point and lower sintering activity.As a result,C/SiC-HfC exhibits the lower linear ablation rate than that of C/SiC-ZrC.3.C/SiC-(Hf,Ti)3SiC2-(Hf,Ti)C and C/SiC-(Hf,Zr)C composites were prepared by impregnating molten TiSi2-HfSi2 and ZrSi2-HfSi2 into the C/CA preforms,whose flexural strengths are 273.6 MPa and 260.7 MPa,respectively.In the ablation process at 2200℃,the formed HfTiO4 phase on the C/SiC-(Hf,Ti)3SiC2-(Hf,Ti)C surface increases the adhesion strength between the oxide layer and substrate,which further improves the denudation resistance of oxide layer.Resultantly,C/SiC-(Hf,Ti)3SiC2-(Hf,Ti)C has a linear ablation rate of 0.400 μm s-1 and a mass ablation rate of 0.095 mg cm-2 s-1,suggesting superior ablation resistance than C/SiC-HfC.However,in the ablation process at 2500℃,the decomposition of HfTiO4 results in the significantly decreased denudation resistance of the oxide layer.The linear ablation rate and mass ablation rate of C/SiC-(Hf,Ti)3SiC2-(Hf,Ti)C increase to 2.422 μm s-1 and 0.195 mg cm-2 s-1,respectively.C/SiC-(Hf,Zr)C composite exhibits excellent ablation resistance at 2200℃ and 2500℃ due to the formed multiphase oxide network structure supported by high-melting-point solid oxides and filled by sufficient glass phases with low oxygen diffusivity.Its linear ablation rate and mass ablation rate are 0.448 μm s-1 and 0.146 mg cm-2 s-1 at 2200℃,0.635 μm s-1 and 0.202 mg cm-2 s-1 at 2200℃.