2.5D C/SiC复合材料超声振动辅助铣削数值模拟研究

为了揭示2.5D C/SiC陶瓷基复合材料在铣削加工过程中的去除机理及表面损伤特性,开展了其传统铣削和超声振动辅助铣削实验,建立了两类铣削工况下的三维数值模型。结果表明,相对于传统铣削,超声振动辅助铣削在其高频的交变载荷下,降低材料的微切削厚度,减小材料的变形挠度从而使径向和切向的切削力分别都降低了68%、72%;可有效改善2.5D C/SiC复合材料加工表面损伤,如倾斜断裂面、粗糙断裂面;其加工表面残余压应力明显大于传统铣削;经过实验验证,所建立的数值模型可以有效地模拟材料去除过程和0°、90°纤维的断裂形貌;为2.5D C/SiC复合材料的高效低损伤加工提供了理论依据和指导。

Compositional and Hollow Engineering of Silicon Carbide/Carbon Microspheres as High-Performance Microwave Absorbing Materials with Good Environmental Tolerance

Microwave absorbing materials(MAMs)characterized by high absorption efficiency and good environmental tolerance are highly desirable in practical applications.Both silicon carbide and carbon are considered as stable MAMs under some rigorous conditions,while their composites still fail to produce satisfactory microwave absorption performance regardless of the improvements as compared with the individuals.Herein,we have successfully implemented compositional and structural engineering to fabricate hollow Si C/C microspheres with controllable composition.The simultaneous modulation on dielectric properties and impedance matching can be easily achieved as the change in the composition of these composites.The formation of hollow structure not only favors lightweight feature,but also generates considerable contribution to microwave attenuation capacity.With the synergistic effect of composition and structure,the optimized SiC/C composite exhibits excellent performance,whose the strongest reflection loss intensity and broadest effective absorption reach-60.8 dB and 5.1 GHz,respectively,and its microwave absorption properties are actually superior to those of most SiC/C composites in previous studies.In addition,the stability tests of microwave absorption capacity after exposure to harsh conditions and Radar Cross Section simulation data demonstrate that hollow SiC/C microspheres from compositional and structural optimization have a bright prospect in practical applications.

2.5D-C/SiC复合材料超声振动辅助磨削力研究

为探究2.5D-C/SiC复合材料超声振动辅助磨削去除机理和磨削力影响规律,首先通过试验分析了2.5D-C/SiC复合材料基体、纬向纤维、经向纤维和钉扎部分的去除机理;然后通过正交试验分析了磨削工艺参数(主轴转速n s、进给速度v_(w)、磨削深度a_(p)和超声振幅A)对法向和切向磨削力的影响主次;最后通过单因素试验分析了磨削工艺参数对法向和切向磨削力的影响规律。试验结果表明,材料的去除是基体碎裂、纤维断裂、纤维拨出、界面脱粘等因素的综合作用;法向磨削力的影响主次为:a_(p)>v_(w)>n_(s)>A;切向磨削力的影响主次为:v_(w)>a_(p)>n_(s)>A;磨削力与n_(s)和A成反比,与a_(p)和v_(w)成正比。

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.

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.

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.

Wet friction performance of C/C-SiC composites prepared by new processing route

C/C-SiC composites with SiC island distribution were prepared via a new processing route. The fabrication process mainly included silicon infiltration by ultrasonic vibration, chemical vapor deposition (CVD), siliconizing, liquid phase impregnation and carbonization. The wear and friction properties were tested by an MM-1000 wet friction machine. The results show that SiC phases are mainly distributed between carbon fibers and pyrocarbons as well as among the pryocarbons. The dynamic friction coefficient of the composites decreases gradually from 0.126 to 0.088 with the increase of the surface pressure from 0.5 to 2.5 MPa at the same rotary speed. Furthermore, under the constant surface pressure, the dynamic friction coefficient increases from 0.114 to 0.126 with the increase of the rotary speed from 1 500 to 2 500 r/min. However, the coefficient decreases to 0.104 when the rotary speed exceeds 4 500 r/min. During the friction process, the friction coefficient of C/C-SiC composite is between 0.088 and 0.126,and the wear value is zero after 300 times brake testing.

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.

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.

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.

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.

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.

Elastic Modulus Prediction of Three-dimension-4 Directional Braided C_f/SiC Composite Based on Double-scale Model

Double-scale model for three-dimension-4 directional（3D-4d） braided C/SiC composites has been proposed to investigate its elastic properties. The double-scale model involves micro-scale that takes fiber/ matrix/porosity in fibers tows into consideration with unit cell which considers the 3D-4d braiding structure. Micro-optical photographs of composites have been taken to study the braided structure. Then a parameterized finite element model that reflects the structure of 3D-4d braided composites is proposed. Double-scale elastic modulus prediction model is developed to predict the elastic properties of 3D-4d braided C/SiC composites. Stiffness and eompliance-averaging method and energy method are adopted to predict the elastic properties of composites. Static-tension experiments have been conducted to investigate the elastic modulus of 3D-4d braided C/SiC composites. Finally, the effect of micro-porosity in fibers tows on the elastic modulus of 3D-4d braided C/SiC composites has been studied. According to the conclusion of this thesis, elastic modulus predicted by energy method and stiffness-averaging method both find good agreement with the experimental values, when taking the micro-porosity in fibers tows into consideration. Differences between the theoretical and experimental values become smaller.

Microstructure Characterization of 3D C/SiC Composites in Combustion Gas Environments

C/SiC composites prepared by chemical vapor infiltration（CVI） were subjected to a stationary loading of 160 MPa in a combustion gas environment with flame temperature of 1300 ℃.Lifetime of C/SiC composites in such environment was measured.Microstructures of the composites after the testing were also characterized by SEM.The experimental results indicate the lifetime of C/SiC composites is average 2.3 hours in combustion gas environments.The combustion gas flow accelerates the damage of carbon fibers and the failure of the composites by speeding up the diffusion of gas reactants and products,destroying the layer of SiO2 on the surface of SiC coating and bringing fused SiO2 inside the composites.The fracture face of C/SiC is uneven,i e,a flat area close to the windward side and a pulling-out of long fibers near the leeward side,which results from the directionality effect of the combustion gas flow.

FRACTURE MODE AND PROCESS OF SiCw+B4Cp/MB15 MAGNESIUM MATRIX COMPOSITE

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浆料浸渍辅助PIP工艺制备C/HfC-SiC复合材料的微观结构及性能研究

针对高速飞行器对于防热/承载一体化超高温陶瓷基复合材料的迫切需求,以及现有反应型HfC先驱体存在的成本高、效率低和致密效果差等不足,本研究将HfC亚微米粉体配制成稳定的陶瓷浆料,利用浆料加压浸渍辅助先驱体浸渍裂解(PIP)工艺制备了HfC基体均匀分布的C/HfC-SiC复合材料,探讨了HfC含量对于复合材料微观结构、力学与烧蚀性能的影响。结果表明,当HfC实际体积分数为13.1%~20.3%时,复合材料密度为2.20~2.58 g·cm^(-3),开孔率约为5%。通过单层碳布加压浸渍陶瓷浆料,HfC颗粒能够分散到纤维束内部,且在复合材料中分布比较均匀。提高HfC含量会降低复合材料纤维含量,其力学性能也呈现出降低趋势。当HfC体积分数为20.3%时,复合材料的密度、拉伸强度和断裂韧性分别为2.58 g·cm^(-3)、147 MPa和9.3 MPa·m^(1/2);经氧乙炔焰烧蚀60 s后,复合材料的线烧蚀率和质量烧蚀率分别为0.0062 mm/s和0.005 g/s,烧蚀过程中形成的熔融相Hf_(x)Si_(y)O_(z)能覆盖在材料表面,起到良好的保护作用。