Tensile Mechanical Behavior and Failure Mechanism of a Plain-Woven SiCf/SiC Composites at Room and Elevated Temperatures

Ceramic matrix composites (CMCs) are the preferred materials for solving advanced aerospace high-temperature structural components;it has the comprehensive advantages of higher temperature (~1500˚C) and low density. In service environments, CMCs exhibit complex damage mechanisms and failure modes, which are affected by constituent materials, meso-architecture and inhere defects. In this paper, the in-plane tensile mechanical behavior of a plain-woven SiCf/SiC composite at room and elevated temperatures was investigated, and the factors affecting the tensile strength of the material were discussed in depth. The results show that the tensile modulus and strength of SiCf/SiC composites at high temperature are lower, but the fracture strain increases and the toughness of the composites is enhanced;the stitching holes significantly weaken the tensile strength of the material, resulting in the material is easy to break at the cross-section with stitching holes.

Multi-scale Modeling and Finite Element Analyses of Thermal Conductivity of 3D C/SiC Composites Fabricating by Flexible-Oriented Woven Process

Thermal conductivity is one of the most significant criterion of three-dimensional carbon fiber-reinforced SiC matrix composites(3D C/SiC).Represent volume element(RVE)models of microscale,void/matrix and mesoscale proposed in this work are used to simulate the thermal conductivity behaviors of the 3D C/SiC composites.An entirely new process is introduced to weave the preform with three-dimensional orthogonal architecture.The 3D steady-state analysis step is created for assessing the thermal conductivity behaviors of the composites by applying periodic temperature boundary conditions.Three RVE models of cuboid,hexagonal and fiber random distribution are respectively developed to comparatively study the influence of fiber package pattern on the thermal conductivities at the microscale.Besides,the effect of void morphology on the thermal conductivity of the matrix is analyzed by the void/matrix models.The prediction results at the mesoscale correspond closely to the experimental values.The effect of the porosities and fiber volume fractions on the thermal conductivities is also taken into consideration.The multi-scale models mentioned in this paper can be used to predict the thermal conductivity behaviors of other composites with complex structures.

C/SiC/MoSi_2-SiC-Si multilayer coating for oxidation protection of carbon/carbon composites

C/SiC/MoSi2-SiC-Si oxidation protective multilayer coating for carbon/carbon （C/C） composites was prepared by pack cementation and slurry method. The microstructure, element distribution and phase composition of the as-received coating were analyzed by scanning electron microscopy （SEM）, energy dispersive X-ray spectroscopy （EDS） and X-ray diffraction （XRD）. The results show that the multilayer coating was composed of MoSi2, SiC and Si. It could effectively protect C/C composites against oxidation for 200 h with the mass loss of 3.25% at 1873 K in static air. The mass loss of the coated C/C composites results from the volatilization of SiO2 and the formation of cracks and bubble holes in the coating.

Effect of ZrC-SiC content on microstructure and ablation properties of C/C composites

C/C-ZrC-SiC composites with different ZrC-SiC contents were fabricated by precursor infiltration and pyrolysis. The effect of ceramic content on the microstructure and ablation resistance was investigated. Both the C/C-SiC and C/C-ZrC-SiC composites exhibited good ablation resistance under the plasma flame above 2300℃. Withtheincreaseof ZrC content, a continuous oxide layer and a solid Zr-Si-O mesophase were formed during the ablation. And the structure of the formed oxides layer closely linked with the contents of ZrC-SiC ceramics. The solid ZrO2-ZrC and Zr-Si-O mesophase could increase the viscosity of SiO2 moderately and improve the anti-scouring ability. The continuous SiO2-ZrO2-ZrC-SiC layer would serve as a thermal and oxygen barrier for preventing the substratefrom further ablation. The C/C-ZrC-SiC composites with 27.2%ZrC and 7.56%SiC shows superior ablation resistance, and the mass and linear ablation rates are-3.51 mg/s and-1.88 μm/s, respectively.

Characterization of SiC nanowires prepared on C/C composite without catalyst by CVD

SiC nanowires were prepared on C/C composite surface without catalyst by chemical vapor deposition（CVD） using CH3 SiCl3 as precursor.SEM images of the CVD-product reveal that some long nanowires have grown to tens of micrometers with some gathered as a ball.Some short nanowires agglomerate like chestnut shell with many thorns accompanied by some deposited nano-particles.XRD,Raman-spectrum and FTIR patterns indicate that the product is a typical β-SiC.TEM images show that the nanowires have a wide diameter range from 10 to 100 nm,and some thin nanowires are bonded to the thick one by amorphous CVD-SiC.A SiC branch generates from an amorphous section of a thick one with an angle of 70° between them,which is consistent with the [111] axis stacking angle of the crystal.SAED and fast Fourier transform（FFT） patterns reveal that the nanowires can grow along with different axes,and the bamboo-nodes section is full of stacking faults and twin crystal.The twisted SiC lattice planes reveal that the screw dislocation growth is the main mechanism for the CVD-SiC nanowires.

Oxidation behavior of C/C composites with SiC/ZrSiO_4-SiO_2 coating

A SiC/ZrSiO4？SiO2 （SZS） coating was successfully fabricated on the carbon/carbon （C/C） composites by pack cementation, slurry painting and sintering to improve the anti-oxidation property and thermal shock resistance. The anti-oxidation properties under different oxygen partial pressures （OPP） and thermal shock resistance of the SZS coating were investigated. The results show that the SZS coated sample under low OPP, corresponding to the ambient air, during isothermal oxidation was 0.54% in mass gain after 111 h oxidation at 1500 ° C and less than 0.03% in mass loss after 50 h oxidation in high OPP, corresponding to the air flow rate of 36 L/h. Additionally, the residual compressive strengths （RCS） of the SZS coated samples after oxidation for 50 h in high OPP and 80 h in low OPP remain about 70% and 72.5% of those of original C/C samples, respectively. Moreover, the mass loss of SZS coated samples subjected to the thermal cycle from 1500 ° C in high OPP to boiling water for 30 times was merely 1.61%.

Fabrication of Y_2Si_2O_7 coating and its oxidation protection for C/SiC composites

Yttrium silicate （Y2Si2O7） coating was fabricated on C/SiC composites through dip-coating with silicone resin ＋ Y2O3 powder slurry as raw materials. The synthesis, microstructure and oxidation resistance and the anti-oxidation mechanism of Y2Si2O7 coating were investigated. Y2Si2O7 can be synthesized by the pyrolysis of Y2O3 powder filled silicone resin at mass ratio of 54.2：45.8 and 800 °C in air and then heat treated at 1400 °C under Ar. The as-fabricated coating shows high density and favorable bonding to C/SiC composites. After oxidation in air at 1400, 1500 and 1600 °C for 30 min, the coating-containing composites possess 130%-140% of original flexural strength. The desirable thermal stability and the further densification of coating during oxidation are responsible for the excellent oxidation resistance. In addition, the formation of eutectic Y-Si-Al-O glassy phase between Y2Si2O7 and Al2O3 sample bracket at 1500 °C is discovered.

Ablation Processes for HfC-Coated 2.5D Needle-Punched Composites Used for Aerospace Engines Under Hypersonic Flight Conditions

The working environment of aerospace engines is extremely harsh with temperature exceeding 1700℃and accompanied by thermal coupling effects.In this condition,the materials employed in hypersonic aircraft undergo ablation issues,which can cause catastrophic accidents.Due to the excellent high-temperature stability and ablation resistance,HfC exhibits outstanding thermal expansion coefficient matching that of C/SiC composites.2.5D needle-punched C/SiC composites coated with HfC are prepared using a plasma spraying process,and a high-enthalpy arc-heated wind tunnel is employed to simulate the re-entry environment of aircraft at 8 Mach and an altitude of 32 km.The plasma-sprayed HfC-coated 2.5D needle-punched C/SiC composites are subjected to long-term dynamic testing,and their properties are investigated.Specifically,after the thermal assessment ablation experiment,the composite retains its overall structure and profile;the total mass ablation rate is 0.07445 g/s,the average linear ablation rate in the thickness direction is-0.0675μm/s,and the average linear ablation rate in the length direction is 13.907μm/s.Results verify that plasma-sprayed HfC coating exhibits excellent anti-oxidation and ablation resistance properties.Besides,the microstructure and ablation mechanism of the C/SiC composites are studied.It is believed that this work will offer guideline for the development of thermal protection materials and the assessment of structural thermal performance.

Oxidation behavior of CVI,MSI and CVI+MSI C/SiC composites

The oxidation behavior of chemical vapor infiltration(CVI),molten silicon infiltration(MSI)and CVI+MSI C/SiC composites at 500-1 400℃was studied.The oxidation below 900℃increased successively for CVI,CVI+MSI and MSI composites.However,the oxidation of CVI composite above 1 000 ℃was much faster thanthat of MSI and CVI+MSI composites. As active carbon atoms produced by siliconization of fibers during MSI process were oxidized first and decreased initial oxidation temperature.The initial oxidation temperature of MSI,MSI+CVI and CVI composites was 526,552 and 710℃,respectively.New active carbon atoms were generated due to the breaking of 2D molecular chains during oxidation,so the activation energy of three C/SiC composites was decreased gradually at 500-800℃with oxidation process,exhibiting a self-catalytic characteristic.

Oxidation Behavior of C/C-SiC Gradient Matrix Composites

Oxidation behavior of C/C-SiC gradient matrix composites and C/C composites were compared in stationary air. The results show that oxidation threshold of C-SiC materials increases with the amount of SiC particles in the codeposition matrix. Oxidation rate of C/C-SiC gradient matrix composites is significantly lower than that of C/C material. The micro-oxidation process was observed by SEM.

DIELECTRIC PROPERTIES OF NANO Si/C/N COMPOSITE POWDER AND NANO SiC POWDER AT HIGH FREQUENCIES AND MICROSTRUCTURE CHARACTERIZATION

The dielectric properties of nano Si/C/N composite powder and nano SiC powder at high frequencies have been studied. The nano Si/C/N composite powder and nano SiC powder were synthesized from hexamethyldisilazane ((Me 3Si) 2NH) (Me:CH 3) and SiH 4 C 2H 2 respectively by a laser induced gas phase reaction. The complex permittivities of the nano Si/C/N composite powder and nano SiC powder were measured between 8 2GHz and 12 4GHz. The real and imaginary parts of the complex permittivities of nano Si/C/N composite powder are much higher than those of nano SiC powder. The SiC microcrystalline in the nano Si/C/N composite powder dissolved a great deal of nitrogen. The local structure around Si atoms changed by introducing N into SiC. Carbon atoms around Si were substituted by N atoms. So charged defects and quasi free electrons moved in response to the electric field, diffusion or polarization current resulted from the field propagation. The high ε″and loss factor tgδ(ε″/ε′) of Si/C/N composite powder were due to the dielectric relaxation.

Preparation and properties of C/C-SiC brake composites fabricated by warm compacted-in situ reaction

Carbon fibre reinforced carbon and silicon carbide dual matrix composites（C/C-SiC） were fabricated by the warm compacted-in situ reaction.The microstructure,mechanical properties,tribological properties,and wear mechanism of C/C-SiC composites at different brake speeds were investigated.The results indicate that the composites are composed of 58wt%C,37wt%SiC,and 5wt%Si.The density and open porosity are 2.0 g.cm^（-3） and 10%,respectively.The C/C-SiC brake composites exhibit good mechanical properties.The flexural strength can reach up to 160 MPa,and the impact strength can reach 2.5 kJ.m^（-2）.The C/C-SiC brake composites show excellent tribological performances.The friction coefficient is between 0.57 and 0.67 at the brake speeds from 8 to 24 m·s^（-1）.The brake is stable,and the wear rate is less than 2.02×10^（-6） cm^3·J^（-1）.These results show that the C/C-SiC brake composites are the promising candidates for advanced brake and clutch systems.

Analytical Model of Elastic Modulus and Coefficient of Thermal Expansion for 2.5D C/SiC Composite

To make better use of 2.5D C/SiC composites in industry, it is necessary to understand the mechanical properties. A finite element model＇of 2.5D composites is established, by considering the fiber undulation and the porosity in 2.5D C/SiC composites. The fiber direction of warp is defined by cosine function to simulate the undulation of warp, and based on uniform strain assumption, analytical model of the elastic modulus and coefficient of thermal expansion （CTE） for 2.5D C/SiC composites were established by using dual- scale model. The result is found to correlate reasonably well with the predicted results and experimental results. The parametric study also demonstrates the effects of the fiber volume fraction, distance of warp yarn, and porosity in micro-scale on the mechanical properties and the coefficients of thermal expansion.

Monotonic tensile behavior analysis of three-dimensional needle-punched woven C/SiC composites by acoustic emission

High toughness and reliable three-dimensional needled C/SiC composites were fabricated by chemical vapor infiltration （CVI）. An approach to analyze the tensile behaviors at room temperature and the damage accumulation of the composites by means of acoustic emission was researched. Also the fracture morphology was examined by S-4700 SEM after tensile tests to prove the damage mechanism. The results indicate that the cumulative energy of acoustic emission （AE） signals can be used to monitor and evaluate the damage evolution in ceramic-matrix composites. The initiation of room-temperature tensile damage in C/SiC composites occurred with the growth of micro-cracks in the matrix at the stress level about 40% of the ultimate fracture stress. The level 70% of the fracture stress could be defined as the critical damage strength.

Preparation and Anti-oxidation Mechanism of Mullite/Yttrium Silicate Coatings on C/SiC Composites

In order to enhance the oxidation resistance of C/Si C composites, mullite/yttrium silicate coatings were fabricated on C/Si C composites through dip-coating route. Al_2O_3-SiO_2 sol with high solid content was selected as the raw material for mullite and ＂silicone resin ＋ Y_2O_3 powder＂ slurry was used to synthesize yttrium silicate. The microstructure and phase composition of coatings were characterized, and the investigation on oxidation resistance and anti-oxidation mechanism was emphasized. The as-fabricated coatings consisting of SiO_2-rich mullite phase and Y_2Si_2O_7 phase show high density and favorable bonding to C/Si C composites. After oxidized at 1 400 ℃ and 1 500 ℃ for 30 min in static air, the coating-containing C/Si C composites possess 91.9% and 102.4% of the original flexural strength, respectively. The desirable thermal stability of coatings and the further densification of coatings due to viscous flow of rich SiO_2 and Y-Si-Al-O glass are responsible for the excellent oxidation resistance. In addition, the coating-containing composites retain 99.0% of the original flexural strength and the coatings exhibit no cracking and desquamation after 12 times of thermal shock from 1 400 ℃ to room temperature, which are ascribed to the combination of anti-oxidation mechanism and preferable physical and chemical compatibility among C/Si C composites, mullite and Y_2Si_2O_7. The carbothermal reaction at 1 600 ℃ between free carbon in C/Si C substrate and rich SiO_2 in mullite results in severe frothing and desquamation of coatings and obvious degradation in oxidation resistance.

Tribological characteristics of C/C-SiC braking composites under dry and wet conditions

C/C-SiC braking composites,based on reinforcement of carbon fibers and matrices of carbon and silicon carbide,were fabricated by warm compaction and in situ reaction process.The tribological characteristics of C/C-SiC braking composites under dry and wet conditions were investigated by means of MM-1000 type of friction testing machine.The influence of dry and wet conditions on the tribological characteristics of the C/C-SiC composites was ascertained.Under dry condition,C/C-SiC braking composites show superior tribological characteristics,including high coefficient of friction (0.38),good abrasive resistance (thickness loss is 1.10 μm per cycle) and steady breaking.The main wear mechanism is plastic deformation and abrasion caused by plough.Under wet condition,frictional films form on the worn surface.The coefficient of friction (0.35) could maintain mostly,and the thickness loss (0.70 μm per cycle) reduces to a certain extent.Furthermore,braking curves are steady and adhesion and oxidation are the main wear mechanisms.

Oxidation behavior of oxidation protective coatings for C/C-SiC composites at 1500 ℃

Porous carbon/carbon preforms were infiltrated with melted silicon to form C/C-SiC composites. Three-layer Si-Mo coating prepared by slurry painting and SiC/Si-Mo multilayer coating prepared by chemical vapor deposition(CVD) alternated with slurry painting were applied on C/C-SiC composites, respectively. The oxidation of three samples at 1 500 ℃ was compared. The results show that the C/C-SiC substrate is distorted quickly. Three-layer Si-Mo coating is out of service soon due to the formation of many bubbles on surface. The mass loss of coated sample is 0.76% after 1 h oxidation. The sample with SiC/Si-Mo multilayer coating gains mass even after 105 h oxidation. SiC/Si-Mo multilayer coating can provide longtime protection for C/C-SiC composites and has excellent thermal shock resistance. This is attributed to the combination of dense SiC layer and porous Si-Mo layer. Dense SiC layer plays the dual role of physical and chemical barrier, and resists the oxidation of porous Si-Mo layer. Porous Si-Mo layer improves the thermal shock resistance of the coating.

Failure criteria for C/SiC composites under plane stress state

The applicability and limitation of several quadratic strength theories were investigated with respect to 2D-C/SiC and 2.5D-C/SiC composites. A kind of damage-based failure criterion, referred to as D-criterion, is proposed for non- linear ceramic composites. Meanwhile, the newly developed criterion is prelim- inarily validated under tension-shear combined loadings. The prediction shows that the failure envelope given by D-criterion is lower than that from Tsai-Hill and Hoffman criteria. This reveals that the damage-based criterion is reasonable for evaluation of damage-dominated failure strength.

Synthesis of ZrO_2-SiC composite powder and effect of its addition on properties of Al_2O_3-C refractories

ZrO2-SiC composite powder was synthesized by carbothermal reduction of zircon in argon atmosphere, and it was used as the additive to prepare Al2O3-C refractories. The effects of heating temperature on the synthesis process and the addition of the synthesized composite powder on the properties of the Al2O3-C refractories were investigated. The results show that the synthesized composite powder can be easily obtained by heating the mixture of zircon and carbon black at 1 873 K for 4 h in argon atmosphere, and the relative contents of ZrO2 and SiC in sample reach about 83.7% and 16.3%, respectively. The bulk density, crushing strength and thermal shock resistance of the Al2O3-C refractories can be improved obviously by the addition of the synthesized ZrO2-SiC composite powder.

Microstructure and mechanical properties of 3D fine-woven punctured C/C composites with P_yC/SiC/TaC interphases

The 3D fine-woven punctured C/C-(PyC/SiC/TaC)composites,composed of PyC/SiC/TaC interphases and pyrocarbon (PyC)matrix,were synthesized by isothermal chemical vapor infiltration(ICVI)methods.The alternating layers and the structure of these composites were examined by polarized light microscopy(PLM),X-ray diffractometry(XRD)and scanning electron microscopy(SEM).It is found that the PyC matrix has rough laminar(RL)structure,the TaC layer has NaCl-type cubic structure,and the SiC layer has few wurtzite type 10H-SiC besidesβ-SiC structure.The effects of fiber coating and the bulk density on the tensile and flexural properties of composites along X or Y and Z direction were investigated.It is shown that fiber coated 3D woven punctured C/C composites have good tensile and flexural strength,and the maximum of flexural strength is 375 MPa in X or Y direction at density of 1.89 g/cm 3 ,which is about three times higher than that of samples without TaC/SiC fiber coating.The flexural strength and bending strength increase with increasing the density of the composites.The analysis of fracture surfaces reveals that fibers and fiber bundles are pulled out in composites,indicating that the composite exhibits a non-linear failure behavior through propagation and deflection of the cracks.