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.

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.

Friction and Wear Behaviors of C/C-SiC Composites under Water Lubricated Conditions

C/C-SiC composites have the potentiality to be applied in shield pumps of nuclear reactors as the bearing material because of their low density,good mechanical properties and excellent tribological properties.The C/C-SiC composites are fabricated via reactive melt infiltration(RMI)using silicon liquid infiltrated in C/C matrix composites.Friction and wear behaviors of C/C-SiC composites under water lubricated conditions are investigated using the block-on-ring test at room temperature,and compared with those of the resin graphite which is used as the bearing material in shield pumps at present.In addition,friction and wear mechanisms of C/C-SiC composites under water lubricated conditions have been discussed.Results show that tensile strengths of C/C-SiC composites are 150210 MPa,and compressive strengths are 403536 MPa.Friction and wear behaviors of C/C-SiC composites are closely related to the load and the speed.The time to reach a stable friction status decreases with the increase of the speed.Though the friction coefficient of C/C-SiC composites under water lubricated conditions is slightly higher than that of graphite,the wear rate of C/C-SiC composites is much lower,which suggests that the C/C-SiC composites can sustain a longer life during operation.

The influence of ablation products on the ablation resistance of C/C-SiC composites and the growth mechanism of SiO_2 nanowires

Ablation under oxyacetylene torch with heat flux of 4186.8(10%kW/m2 for 20 s was performed to evaluate the ablation resistance of C/C-SiC composites fabricated by chemical vapor infiltration(CVI) combined with liquid silicon infiltration(LSI) process.The results indicated that C/C-SiC composites present a better ablation resistance than C/C composites without doped SiC.The doped SiC and the ablation products SiO_2 derived from it play key roles in ablation process.Bulk quantities of SiO_2 nanowires with diameter of 80 nm-150 nm and length of tens microns were observed on the surface of specimens after ablation.The growth mechanism of the SiO_2 nanowires was interpreted with a developed vapor-liquid-solid(VLS) driven by the temperature gradient.

Controlling the Si/C ratio in SiC matrix based on the modified polymethysilane for C/C-SiC composites with enhanced mechanical properties

Fabricating SiC matrix with controllable Si/C ratio for C/C-SiC composites via precursor infiltration and pyrolysis is difficult to realize due to the absence of suitable precursors.Here,SiC precursors with Si-rich(Si/C=1.23),nearstoichiometric(Si/C=1.01),and C-rich(Si/C=0.88)pyrolyzed ceramics at 1200℃ were successfully synthesized by the novel modification of polymethysilane.The structure and ceramization of synthesized SiC precursors were studied,and C/C-SiC composites with similar Si/C ratio in SiC matrix were also fabricated.The results revealed that the defects and Young’s modulus of SiC matrix and the thermal residual stress on pyrolytic carbon coated carbon fiber were determined by the Si/C ratio.As a result,the Si/C ratio exerted significant effects on the crack deflection behavior and the mechanical properties of the composites.The composites with near-stoichiometric SiC matrix showed excellent mechanical properties because of the effective crack deflection and the moderate Young’s modulus of their SiC matrix.The flexural strength,flexural modulus,and compressive strength were 423±27 MPa,33.41±4.52 GPa,and 393±8 MPa,respectively.Meanwhile,after a heat treatment at 1600℃,the role of carbon fiber toughening was improved and accompanied by a decrease in mechanical properties for all composites,which was attributed to the increased defects in SiC matrix and the damaged interfaces by the thermal residual stress.

Effect of Heat Treatment on Microstructure and Mechanical Properties of Multiscale SiC_p Hybrid Reinforced 6061 Aluminum Matrix Composites

The performance of solid solution aging treatment on aluminum matrix composites prepared by powder metallurgy and reinforced with 6061 aluminum alloy powder as matrix;meanwhile, nano silicon carbide particles(nm Si Cp), submicron silicon carbide particles(1 μm Si Cp) and Ti particles were studied. The Al/Si Cp composite powder was prepared by high-energy ball milling, and then cold-pressed, sintered, hotextruded, and then heat-treated with different solution temperatures and aging times for the extruded composites. Optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy(EDS), X-ray diffractometer(XRD) and extrusion testing were used to analyze and test the microstructure and mechanical properties of aluminum matrix composites. The results show that after the multi-stage solid solution at 530 ℃×2 h+535 ℃×2 h+540 ℃×2 h, the particles are mainly equiaxed grains and uniformly distributed. There is no reinforcement agglomeration, and the surface is dense and the insoluble phase is basically dissolved. In the matrix, the strengthening effect is good, and the hardness and compressive strength are 179.43 HV and 680.42 MPa, respectively. Under this solution process, when the aluminum matrix composites are aged at 170 ℃ for 10 h, the hardness and compressive strength can reach their peaks and increase to 195.82 HV and 721.48 MPa, respectively.

沉积热解炭与树脂炭比例对C/C-SiC复合材料弯曲强度的影响

以针刺整体炭毡为坯体,采用树脂浸渍和化学气相沉积混合的方法制备得到相同密度的C/C多孔体,采用熔硅浸渗(RMI)制得C/C-SiC复合材料。研究了沉积热解炭与树脂炭比例对基体显微结构及弯曲性能的影响。结果表明:随着沉积得到的沉积热解炭含量增加,C/C-SiC复合材料基体内部孔隙率先降后升,而密度、弯曲强度先增后减。当沉积热解炭与树脂炭的比例为1∶1时,C/C-SiC复合材料密度与弯曲强度最高,分别达到2.03 g/cm^(3)和240 MPa。

Experimental Investigation of the Anisotropic Thermal Conductivity of C/SiC Composite Thin Slab

Fiber-reinforced composites possess anisotropic mechanical and heat transfer properties due to their anisotropic fibers and structure distribution.In C/Si C composites,the out-of-plane thermal conductivity has mainly been studied,whereas the in-plane thermal conductivity has received less attention due to their limited thickness.

原位生长SiC晶须对C/C-SiC复合材料摩擦磨损性能影响

采用聚合物浸渍热解(PIP)技术向C/C-SiC复合材料纤维束内引入了SiC晶须(SiC_(W)),然后结合化学气相渗透(CVI)和液硅渗透(LSI)法制备了C/SiCW-C-SiC复合材料,系统研究了原位生长SiCW的物相组成及典型微结构特征,在不同制动初速度下测试了C/SiC_(W)-C-SiC复合材料的摩擦磨损性能,结果表明:引入SiC_(W)后,对平均摩擦系数和瞬时摩擦系数影响较小,摩擦性能未发生明显变化,中高速制动时,C/SiC_(W)-C-SiC的磨损率相比于C/C-SiC可降低20%~60%。

Microwave absorption properties of Ni/C@SiC composites prepared by precursor impregnation and pyrolysis processes

In the present study,the unique three-dimensional graphene coated nickel(Ni/C)foam reinforced silicon carbide(Ni/C@SiC)composites were first obtained via the precursor impregnation and pyrolysis(PIP)processes.The microstructure images indicated that the SiC fillers were successfully prepared in the skeleton pores of the Ni/C foam.The influence of the PIP cycles on the microwave absorption performances was researched,and the results indicated that after the primary PIP process,Ni/C@SiC-I possessed the optimal microwave absorbing performance with a minimum reflection loss(RL)of-25.87 d B at 5.28 GHz and 5.00 mm.Besides,the RL values could be below-10.00 dB from 5.88 GHz to 7.74 GHz when the corresponding matching thickness was 3.85 mm.However,the microwave absorption properties of Ni/C@SiC-II and Ni/C@SiC-Ⅲwere tremendously degraded as the PIP times increased.At last,the electromagnetic parameter,dielectric loss,attenuation constant as well as impedance matching coefficient were further investigated to analyze the absorbing mechanism,which opened a new path for the certain scientific evaluation of the absorbing materials and had extremely important to the defence technology.

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.

Effect of SiC Location on the Ablation of C/C-SiC Composites in Two Heat Fluxes

How layer-segregated distribution of SiC affects the ablation of C/C-SiC composites was studied in the present work.A certain amount of SiC particles was deposited at the non-woven(C/C—SiC-1) and web(C/C-SiC-2) layer of 2D needle-punched carbon fibre fabric reinforced pyrocarbon composites,respectively.Ablation under oxyacetylene torch demonstrated that the two composites have similar ablation rates in heat flux of 2.38 MW/m^2 whereas ablation rates of C/C-SiC-2 were much higher than those of C/C—SiC-1 when heat flux was 4.18 MW/m^2.SiO_2 covered partially the defective surface of both composites in the lower heat flux.The different SiC locations induced distinct defects and then led to the two composites' dissimilar ablation rates in the higher heat flux.

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.

A Multi-interlayer LMAS Joint of C/C-SiC Composites and LAS Glass Ceramics

Porous C/C-SiC composites were prepared through a two-step chemical vapor infiltration process,and a multi-interlayer joint of Li20-MgO-Al_2O_3-SiO_2(LMAS) was applied to join C/C-SiC composites and lithium aluminum silicate(LAS) glass ceramics by means of a vacuum hot-pressing technique.Plenty of SiC whiskers were generated in the pores of low-density C/C composites during chemical vapor deposition process,which is essentia! to form a zigzag interface structure between C/C-SiC substrate and the LMAS interlayer.The average shear strength of the LMAS joint was improved from 12.17 to 19.91 MPa after changing the composites from high-density C/C composites(1.75 g/cm^3) with a CVD-SiC coating to the C/C-SiC composites with a low density(1.48 g/cm^3).The improvement of the joint strength is mainly attributed to the formation of the inlay structure at the SiC-C/C and SiC-LMAS interfaces.

Preparation,Properties and Application of C/C-SiC Composites Fabricated by Warm Compacted-in situ Reaction

Carbon fibre reinforced carbon and SiC dual matrices composites (C/C-SiC) show superior tribological properties,high thermal shock resistance and good abrasive resistance,and they are promising candidates for advanced brake and clutch systems.The microstructure,mechanical properties,friction and wear properties,and application of the C/C-SiC composites fabricated by warm compacted-in situ reaction were introduced.The results indicated that the composites were composed of 50-60 wt pct carbon,2-10 wt pct residual silicon and 30-40 wt pct silicon carbide.The C/C-SiC brake composites exhibited good mechanical properties.The value of flexural strength and compressive strength could reach 160 and 112 MPa,respectively.The impact strength was about 2.5 kJ·m-2.The C/C-SiC brake composites showed excellent tribological performance,including high coefficient of friction (0.38),good abrasive resistance (1.10 μm/cycle) and brake steadily on dry condition.The tribological properties on wet condition could be mostly maintained.The silicon carbide matrix in C/C-SiC brake composites improved the wear resistance,and the graphite played the lubrication function,and right volume content of graphite was helpful to forming friction film to reduce the wear rate.These results showed that C/C-SiC composites fabricated by warm compacted-in situ reaction had excellent properties for use as brake materials.

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

碳纤维增强了碳和硅碳化物双矩阵 compositesC/C-SiC 被温暖的压缩在里面的 situ reaction.The 微观结构制作，机械性质， tribological 性质，并且在速度是的不同刹车穿 C/C-SiC composites 的机制 investigated.The 结果显示 composites 由 58wt%C,37wt%SiC 组成，并且 5wt%Si .The 密度和开的孔是 2.0 g.cm^-3 并且10%， respectively.The C/C-SiC 刹车 composites 展览好机械 properties.The

Oxidation mechanisms of C-SiC-TaC-C composites prepared by chemical vapor infiltration

To improve the oxidation-resistance properties,SiC and TaC species were introduced in C/C composites by chemical vapor infiltration(CVI) methods. The oxidation-resistance properties of C-SiC-TaC-C composites were studied by X-Ray diffractometry(XRD),JEOL-6360LV scanning electronic microscopy(SEM) and AdventurerTM electronic balance with precision of 0.1 mg. The results show that,1) the oxidation rate of the composites increases continuously with time at all experimental temperatures;2) The oxidation rate increases with temperature within 700-1 100 ℃,slowly in 700-800 ℃,acutely in 800-1 100 ℃;it reaches a maximum value at 1 100 ℃,then decreases within 1 100-1 400 ℃;3) The relationship curve of oxidation rate with temperature can be divided into three regions. The oxidation rate is controlled by reactivity in region Ⅰ,the mixed effects of reactivity and gas diffusion in region Ⅱ,gas phase diffusion in region Ⅲ;4) The composites exhibit a higher oxidation onset temperature in low temperature region and a lower oxidation rate at high temperature due to the oxidation of TaC to(Ta,O) and the formation of the dense SiO2-Ta2O5 oxide layer respectively. With the addition of SiC/TaC species,the oxidation-resistant properties of C/C composites can be improved effectively.

TEM STUDY OF C-B_4C-SiC COMPOSITES WITH SILICON ADDITIVE

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C/C-SiC摩擦材料研究及应用进展

C/C-SiC复合材料是一种新型高性能陶瓷基摩擦材料,具有密度低、抗氧化性好、摩擦性能高且性能稳定等一系列优点,在汽车、高速列车、飞机等高能载制动领域具有广泛的应用前景。文章从制备工艺、摩擦机理、应用前景等方面,对C/C-SiC摩擦材料展开了系统论述,同时对存在的问题提出了建议。