EFFECT OF OXIDATION OF SiC ON AGING CHARACTERISTICS OF SiC PARTICLES AND SHORT MULLITE FIBERS REINFORCED 2024Al COMPOSITE

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Mechanical properties and surface characteristics of SiC fibers irradiated by swift heavy ions

SiC fibers were irradiated by 414.4-MeV^(112)Sn^(27.3+)ions to different fluences(5.0×10^(12),6.0×10^(13),1.6×10^(14),and 1.92×10^(15)ions/cm^(2)).^(112)Sn^(27.3+)deposited its energy mainly via electron energy loss and passed through the SiC fiber.Then,the mechanical properties and surface characteristics of fibers were studied using a specific single filament tensile test and field emission scanning electron microscopy.Results revealed that the carbon concentration on the fiber surface increased while the silicon concentration decreased.Moreover,the addition of oxygen was found to correlate with an increase in ion fluence.Meanwhile,the fiber surface morphology of the least fluence(5.0×10^(12)ions/cm^(2))irradiated specimen displayed no obvious changes and its diameter was slightly reduced.With successive increases of ion fluence,large grains/bubbles on the fiber surface first appeared and then disappeared,and the diameter of fibers evidently increased.Moreover,at the highest fluence(1.92×10^(15)Sn ions/cm^(2))irradiated specimen,some fibers were brittle fractured.As a result,the mean tensile strength and the average elastic modulus of the fibers generally decreased with respect to the ion fluence.The degradation mechanisms of mechanical properties of SiC fibers under irradiation are discussed in detail.

Effects of Ni catalyzer on growth velocity and morphology of SiC nano-fibers

Composite felts reinforced by both SiC nano-fibers(SiC-NFs)and carbon fibers were prepared at 1 273 K using Ni granules as catalyzers with different deposition time.SiC-NFs were deposited on the surface of the carbon fibers in situ by catalytic chemical vapor deposition(CCVD).The phase,microstructure and morphology of the fibers after electroplating and deposition were characterized by XRD,SEM and TEM.The results show that the SiC-NFs produced by CCVD are composed of single crystal of β-SiC.It is found that smaller nano-granules are more active as catalyzers.The resulting SiC-NFs appear more spindle-like and have a more homogeneous dispersion.The mass change of the samples before and after deposition shows that using more Ni granules results in a faster growth velocity of SiC-NFs.With the same electroplating time,the growth velocity of the SiC-NFs first increases and then decreases.At around 4 h,it reaches the maximum growth velocity,and it becomes nearly constant at around 8 h.After 8 h, the stable growth velocity of the electroplated Ni samples is faster than that of the conventional sample produced without catalyzers, because the SiC-NFs can improve the specific surface area and the activity of the surface.

Effect of properties of SiC fibers on longitudinal tensile behavior of SiC_f/Ti-6Al-4V composites

Three types of SiC fibers with different tensile strength were employed to prepare unidirectional titanium matrix composites. The strengths of the original SiC fibers and extracted fibers from the composites were measured. The results show that the mechanical properties of fibers are greatly damaged by the consolidation processing of the composite. The strength data of the extracted fibers are used to predict the strength of the composites according to two theoretic models. The Globe Load-Sharing(GLS) model overestimates the strength of the composites. If the Local Load-Sharing(LLS) model assumes that failure occurs after the formation of a cluster with three broken fibers, the model can predict the strength of the composites exactly.

Effect of oxygen content on tensile strength of polymer-derived SiC fibers

Air-curing is usually applied to the polymer-derived SiC fibers and, as a result, oxygen is embedded to the material. An effective relationship between oxygen content of the SiC fibers and mass gain of their precursor fibers was established. Results also showed that oxygen content has a great influence on the mechanical properties and excellent tensile strength is usually obtained at the oxygen content of 12%～13%, similar to the density of SiC fibers. Oxygen content has a positive effect on the ceramic yield, and thus, is good to the density and tensile strength; while, oxygen content is also negative to volume content of SiC phase and crystallization of the SiC fibers, and thus, detrimental to the density and tensile strength. Both of the two effects result in the peak behavior of the tensile strength of SiC fibers.

Effect of High-temperature Annealing on Mechanical Performance and Microstructures of Different Oxygen SiC Fibers

In order to explore the effect of high-temperature annealing on the mechanical performances and microstructures of different oxygen SiC fibers, two types of silicon carbide（SiC）-based fibers, specified as XD-SiC fibers（low oxygen） and Nicalon-201 fibers（high oxygen）, were annealed in Ar for 1 h at 800 ℃, 1 000 and 1 200 ℃, respectively. Mechanical properties of these fibers were characterized via a monofilament tensile method, with observation of the damaged monofilament by SEM. Also, the effects of annealing on the microstructure and chemical compositions of the fibers were studied. The experimental results indicated that the tensile strength decreased with the increase of annealing temperatures,after annealing-treatment at 1200℃, XD-SiC fibers remained 84% of its original strength, while Nicalon-201 fibers remained only 58% of its original strength. Crystallization and chemical composition of the fibers are the dominating factors for their mechanical performance at high temperatures. The microstructure changes of XD-SiC fibers are mainly composed of the growth of β-SiC, for Nicalon-201 fibers, evaporation of gases is the main change for microstructure.

Microstructural Changes of T1-6A1-4V Matrix by the Incorporation of Continuous SiC Fibers

In SiC（f）/Ti-6Al-4V composites, the microstructure of the matrix close to the fiber was different from that of the fiber-less material. Microstructure observations show that a layer of fine grains was located adjacent to the fiber, and more dislocations and faults were found in this region. Higher recrystallization nucleation rate due to the undeformed SiC fiber and thermal residual stress induced during cooling from the fabrication temperature caused the microstructural changes of the matrix. Hardness measurement indicates that the matrix in the fiber neighborhood was strengthened, and the strengthening effect decreased with distance away from the fiber.

Hi-Nicalon SiC纤维高温热处理后的断裂机理研究

测定了Hi-Nicalon SiC纤维在惰性气氛下高温热处理后的断裂强度,并对其强度进行了Weibull分布统计分析,通过SEM研究了温度变化对Hi-Nicalon SiC纤维表面形貌和力学性能的影响,着重分析了Hi-Nicalon SiC纤维的裂纹扩展机制及断裂机理。研究表明,在惰性气氛保护下其强度随退火温度的升高而下降,且经过1800℃长时间(10h)退火后的Hi-Nicalon SiC纤维基体转为晶态,断裂模式也就从非晶态的脆性断裂转变为晶态材料的解理断裂。经统计分析,纤维表面部分活性氧化造成的结构缺陷可忽略,断裂源主要集中在纤维内部,惰性气氛下纤维强度下降主要是由解理断裂作用的结果。

Microstructure and interface thermal stability of C/Mo double-coated SiC fiber reinforced γ-TiAl matrix composites

C/Mo duplex coating interfacially modified SiC fiber-reinforced γ-TiAl matrix composite （SiCf/C/Mo/γ-TiA1） was prepared by foil-fiber-foil method to investigate its interfacial modification effect. SiCf/C/TiAl composites were also prepared under the same processing condition for comparision. Both kinds of the composites were thermally exposed in vacuum at 800 and 900℃ for different durations in order to study thermal stability of the interfacial zone. With the aids of scanning electron microscope （SEM） and energy dispersive spectrometer （EDS）, the interracial microstructures of the composites were investigated. The results reveal that, although adding the Mo coating, the interfacial reaction product of the SiCf/C/Mo/TiAl composite is the same with that of the SiCf/C/TiA1 composite, which is TiC/Ti2AlC between the coating and the matrix. However, C/Mo duplex coating is more efficient in hindering interfacial reaction than C single coating at 900 ℃ and below. In addition, a new layer of interfacial reaction product was found between Ti2AlC and the matrix after 900 ℃, 200 h thermal exposure, which is rich in V and close to the chemical composition of B2 phase.

单向Hi-Nicalon/SiC复合材料的工艺与性能

对比了采用先驱体转化法及热压法制备的单向Hi Nicalon纤维增强SiC基复合材料的性能差异 ,结果表明 :制备工艺对复合材料的微观结构和性能有极大的影响。采用先驱体转化制备的Hi Nicalon/SiC复合材料具有较好的性能 ,弯曲强度为 70 3.6MPa ,断裂韧性为 2 3.1MPa·m1/2 ;两种工艺制备的碳化硅基复合材料性能产生差别的主要原因是高温下Hi

SiC_(f)/Ti65复合材料的拉伸行为研究

采用磁控溅射先驱丝法结合热等静压工艺制备SiC_(f)/Ti65复合材料,研究了SiC_(f)/Ti65复合材料室温、高温拉伸行为,揭示了SiC_(f)/Ti65复合材料拉伸断裂机制。研究结果表明:SiC_(f)/Ti65复合材料的室温、高温抗拉强度相比于Ti65合金的抗拉强度分别提升29%和164%,验证了复合材料的增强效果。SiC_(f)/Ti65复合材料的室温拉伸断裂机制为反应层断裂、纤维/基体界面脱粘、基体脆性断裂、纤维断裂、包套韧性断裂;高温断裂机制为反应层断裂、纤维/基体界面脱粘、纤维断裂、W/SiC界面脱粘、基体和包套韧性断裂。

国产Hi-Nicalon型SiC纤维热处理后微观结构及性能演化

为考察国产Hi-Nicalon型SiC纤维在高温下的结构-性能演化规律,对国产Hi-Nicalon型SiC纤维分别在空气和氩气环境下进行了不同温度热处理,并对纤维的微观结构及纤维束丝力学性能演化进行了表征与测试。结果表明,在空气环境下,当超过1 100℃时,国产Hi-Nicalon型SiC纤维束丝强度开始下降,伴随着纤维表面生成SiO_2氧化膜,当温度超过1 200℃时,纤维表面会形成SiO_2氧化膜鼓泡。在氩气环境下1 100～1 500℃时,纤维束丝强度开始发生缓慢劣化。当热处理温度超过1 500℃时纤维束丝强度开始加速劣化,伴随着纤维开始发生β-SiC到α-SiC相变以及SiC晶粒尺寸增大。

Effects of Nitridation on Properties of SiC Fiber and Interface of Ti Matrix Composite

Prenitridation of the TiBx coating surface of the Sigma SM1240 SiC fiber can form more stable compounds at the surface and obstruct the release of boron atoms into the Ti-based alloy matrix. The effect of nitridation on the tensile strength of the fiber was investigated in this work. Nitridation could degrade the tensile strength of the SiC fiber if the treating temperature and time are not optimized. The chemical reaction between the W core and SiC and the modification of fiber microstructure during the nitridation are responsible for the degradation in strength. The strength can be maintained by further optimization of the treating temperature and time. Therefore, stabilizing the surface of TiBx coating and hence the interface of the SiCf/Ti composite by the nitridation of the SiC fiber is a feasible technique for practical applications.

Microstructure Characterization of Long W Core SiC Fiber

Microstructure of SiC fiber manufactured by chemical vapor deposition （CVD） onto tungsten （W） wire core was investigated by analytical electron microscopy （AEM）. The results reveal that the fiber consists of W core, SiC sheath and C-coating. SiC sheath could be subdivided into two parts according to whether containing C rich stripe, or not. An emphasis was put on W/SiC interfacial reaction products and the transition zone between sub-layers in SiC sheath. The W/SiC interface consists of three layers of reaction production, namely, W2C, W5Si3 and WC. And there are amounts of facet faults existing in （100） face of WC crystalline and two classes of stack faults in WC have been revealed. The formation essence of different sublayers in SiC sheath was also discussed.

Conversion of polyaluminocarbosilane(PACS) to Si-Al-C-(O) fibers:evolutions and effect of oxygen

The evolvement of oxygen from polyaluminocarbosilane(PACS) to Si-Al-C-(O) fibers and its effect on properties were investigated by element analysis, solid-state 27Al nuclear magnetic resonance(NMR), Fourier transform infrared spectroscopy(FT-IR), thermo-gravimetric analyses(TGA), scanning electron microscope(SEM) and X-ray diffraction(XRD). Element analysis of PACS precursor polymer gives an empirical formula of SiC2.1H11.1O0.12Al0.024. 27Al NMR spectra mass gain shows that the oxygen of cured PACS fibers comes from aluminum aletylacetanate (Al(AcAc)3) and the curing process. Oxygen content can be regarded as a constant mass during the pyrolysis process. During the sintering process of Si-Al-C-O fibers into Si-Al-C fibers, oxygen and carbon decreases with the release of a small amount of CO and/or SiO. Oxygen has a positive effect on the ceramic yield while has a negative effect on the crystallization of Si-Al-C-O fibers. It has great influence on mechanical properties of Si-Al-C-O and excellent tensile strength is usually obtained at the oxygen content of 8%-10%. The Si-Al-C-(O) fibers have excellent thermal stability and creep resistance.

Kinetics and mechanism of interfacial reaction in SCS-6 SiC continuous fiber-reinforced Ti-Al intermetallic matrix composites

SCS-6 SiC continuous fiber-reinforced Ti-Al intermetallics-matrix composites were fabricated by HIP method and then heat-treated in vacuum under different conditions. The interfacial reaction kinetics and mechanism were studied by using SEM, EDS and XRD. The results show that the content fluctuation of reactive elements such as C, Ti and Si appears in interfacial reaction layers, and multi-layer interfacial reaction compounds form. Alloying element Nb in matrix remarkably diffuses into interfacial reaction zone and changes the activation energy for the interfacial reaction layer growth following a role of parabolic rate. The activation energy (Qk) and (k0) of SCS-6 SiC/super α2 and SCS-6 SiC/Ti2AlNb are 317.664 kJ/mol, 175.709 kJ/mol and 5.4438×10-2 m/s1/2, 1.44×10-5 m/s1/2; respectively, and the diffusion coefficient (DC) is about 10-18—10-20 m2/s. It is confirmed that the SCS-6 SiC/Ti-Al intermetallic composites have higher interface compatibility and stability. Furthermore, compared with SCS-6 SiC/super α2, the interface compatibility and stability of SCS-6 SiC/Ti2AlNb are even higher.

Thermal stability and curing kinetics of polycarbosilane fibers

Thermal stability and curing kinetics of polycarbosilane (PCS) fibers were studied by thermogravimetry (TG), Fourier transform infrared spectroscopy(FT-IR). Curing is an essential step in the preparation of SiC fibers and the properties of SiC fibers are affected greatly by curing conditions. TG measurement performed in air shows that mass gain starts at approximately 200℃ and PCS fibers are sensitive to oxygen. Curing with oxygen, which results in crosslinking on the surface, enabled PCS fibers to retain its shape during high-temperature pyrolysis. The curing of PCS fibers is oxidation of Si─H and Si─CH3, then Si─O─Si and Si─O─C bonds are formed. This is a first order reaction, with activation energy of 79.27 kJ/mol, and the pre-exponential factor is calculated as 3.07×106. The kinetics model was obtained and the experimental data of PCS fibers show good agreement with the kinetics model.

SiC纤维预制体结构对SiC_(f)/BN/SiC复合材料致密化和力学性能的影响

为了研究碳化硅(SiC)纤维预制体的结构对SiC基体的化学气相渗透(CVI)工艺致密化过程的影响,利用模拟计算方法分析了2D叠层、2.5D和3D正交三种编织结构SiC预制体在SiC致密化过程中的孔隙结构演变规律,并采用三氯化硼-氨气-氢气-氮气(BCl_(3)-NH_(3)-H_(2)-N_(2))混合气体和三氯甲基硅烷-氢气-氮气(MTS-H_(2)-N_(2))混合气体作为前驱体,利用CVI致密化工艺制备出三种预制体结构的SiC_(f)/BN/SiC复合材料。结果表明,SiC纤维预制体结构不同,SiC_(f)/BN/SiC复合材料的致密化效率不同,其中2.5D纤维预制体的致密化速率最快,3D正交预制体的致密化速率最慢。致密化120 h后,三种预制体结构的复合材料密度均达到2.3 g/cm~3以上,开孔率为10%左右。三种结构的SiC_(f)/BN/SiC复合材料拉伸强度为130~170 MPa。在层间剪切过程中,2D结构SiC_(f)/BN/SiC复合材料与2.5D结构SiC_(f)/BN/SiC复合材料样品都出现了XY面的纤维剥离现象,而3D结构SiC_(f)/BN/SiC复合材料样品发生了正交Z向纤维的断裂。由此可知,SiC纤维预制体结构影响了所形成复合材料的致密化过程和力学性能。

Preparation of continuous Si-Fe-C-O functional ceramic fibers

A new polymer named polyferrocarbosilane(PFCS) was prepared from polydimethylsilane and ferrocene. The spinnability of this polymer can be tailored by controlling the content of ferrocene in the polymer. The prepared polymer was spun into a continuous polymer fiber that was subsequently cured in air and heat-treated finally in N2 up to 1 350 ℃ for conversion into Si-Fe-C-O fibers. The resulted Si-Fe-C-O fibers display low specific resistance and magnetic property due to the existence of Fe, which also reduces the specific resistance significantly to 10-2Ω·cm at room temperature when the amount of ferrocene in feed is as low as 3.0% (mass fraction). The resulted Si-Fe-C-O fibers, with C/Si molar ratio of about 1.3 and the maximum Fe content of about 2.0% (mole fraction), are composed of β-SiC and small amount of Fe3Si-like crystalline and have an average tensile strength of about 2.0 GPa.

连续SiC纤维表面铂涂层的制备及抗氧化性研究

采用金属有机化合物化学气相沉积法(MOCVD)在连续SiC纤维表面沉积了约0.8μm厚的Pt涂层,用扫描电镜对涂层的形貌进行观察,用EDS能谱和XRD对涂层的成分进行了分析,并对沉积涂层后的纤维在不同温度和时间下的抗氧化性能进行了测试。结果表明,制备得到的铂涂层光滑致密,与纤维结合牢固,有效弥补了纤维表面缺陷;对沉积涂层后的纤维进行力学表征和抗氧化测试,发现铂涂层不仅增加了SiC纤维的抗拉强度,而且经700~1400℃氧化1h后纤维的抗氧化性能大幅提高。