Tensile and Stress-Rupture Behavior of SiC/SiC Minicomposite Containing Chemically Vapor Deposited Zirconia Interphase

The tensile and stress-rupture behavior of SiC/SiC minicomposite containing a chemically vapor deposited (CVD) ZrO₂ interphase was evaluated. Fractographic analyses showed that in situ fiber strength and minicomposite failure loads were strongly dependent on the phase contents and microstructure of the ZrO₂ interphase. When the ZrO₂ interphase structure possessed a weakly bonded interface within the dense ZrO₂ interphase coating layer, the interphase sufficiently protected the fiber surface from processing degradation and promoted matrix crack deflection around the fibers. With this weakly bonded interphase, the stress-rupture properties of SiC/SiC minicomposite at 950° and 1200°C appeared to be controlled by fiber rupture properties, and compared favorably to those previously measured for state-of-the-art BN fiber coatings.     

Structural study of β‐SiC(001) films on Si(001) by laser chemical vapor deposition

β‐SiC thin films have been epitaxially grown on Si(001) substrates by laser chemical vapor deposition. The epitaxial relationship was β‐SiC(001){111}//Si(001){111}, and multiple twins {111} planes were identified. The maximum deposition rate was 23.6 μm/h, which is 5‐200 times higher than that of conventional chemical vapor deposition methods. The density of twins increased with increasing β‐SiC thickness. The cross section of the films exhibited a columnar structure, containing twins at {111} planes that were tilted 15.8° to the surface of substrate. The growth mechanism of the films was discussed.     

Paralinear Oxidation of CVD SiC in Water Vapor

The oxidation kinetics of CVDSiC were monitored by thermogravimetric analysis (TGA) in a 50% H₂O/50% O₂ gas mixture flowing at 1.4 cm/s for temperatures between 1200" and 1400°C. Paralinear weight change kinetics were observed as the water vapor oxidized the SiC and simultaneously volatilized the silica scale. The long-term degradation rate of SiC is determined by the volatility of the silica scale. Rapid SiC surface recession rates were estimated from these data for actual aircraft engine combustor conditions.     

CVD SiC先驱体热分解过程的研究

介绍了各种化学气相沉积（CVD）碳化硅（SiC）先驱体的性质，比较了其沉积过程和沉积产物的差异，并利用HyperChem软件对2，4，6-三甲基-2，4，6-三硅杂庚烷（TMTSH）和甲基三氯硅烷（MTS）2种代表性先驱体的热分解过程进行了计算。结果表明：TMTSH结构中的Si-C键和Si-H键易断裂，分解为小分子自由基或者原子，能够在较低的温度下沉积得到SiC，且沉积速率和沉积效率较高，副产物随时间的变化而有所不同；MTS分子中的G-H键和Si-Cl键的键能都较大，反应所需的活化能较高，只有在较高的温度下才能沉积得到SiC，且沉积的速率和效率并不高。利用HyperChem进行理论计算的结果与实验结果一致。     

Fabrication of SiC/diamond composite coatings by electrophoretic deposition and chemical vapor deposition

In this research, SiC/diamond composite coatings were fabricated by a novel procedure that consisted of the electrophoretic deposition (EPD) of diamond particles onto graphite substrates followed by chemical vapor deposition (CVD) of SiC. Various concentrations of MgCl₂ were employed to increase the deposition rate and uniformity of the deposits during the EPD process by giving a positive charge to diamond particles. The CVD of SiC was found to have a tightly connected diamond‐graphite interface and spherical texture. With higher weight fraction of diamond particles deposits, the wear of steel ball increased, while the wear of SiC coating decreased.     

Fast preparation of (111)‐oriented β‐SiC films without carbon formation by laser chemical vapor deposition from hexamethyldisilane without H2

(111)‐oriented β‐SiC films were prepared by laser chemical vapor deposition using a diode laser (wavelength: 808 nm) from a single liquid precursor of hexamethyldisilane (Si(CH₃)₃–Si(CH₃)₃, HMDS) without H₂. The effects of laser power (P_L), total pressure (P_tot) and deposition temperature (T_dep) on the microstructure, carbon formation and deposition rate (R_dep) were investigated. β‐SiC films with carbon formation and graphite films were prepared at P_L ≥ 170 W and P_to ≥ 1000 Pa, respectively. Carbon formation strongly inhibited the film growth. β‐SiC films without carbon formation were obtained at P_tot = 400‐800 Pa and P_L = 130‐170 W. The maximum R_dep was about 50 μm·h^?1 at P_L = 170 W, P_tot = 600 Pa and T_dep = 1510 K. The investigation of growth mechanism shows that the photolytic of laser played an important role during the depositions.     

CVD法合成的无定型纳米粉体的晶化

以六甲基二硅氨烷（ＨＭＤＳ）为原料,采用ＣＶＤ方法,在不同温度下合成的粉体,经ＸＲＤ测定,确定为无定型．经不同温度晶化处理后,发现１０００℃时合成粉体随着晶化温度的提高,颗粒长大并变长,接近哑铃状．经１５００℃晶化处理后,颗粒呈晶须状,而经１６００℃处理后才完全晶化．１４００℃合成的粉体则随着晶化处理温度的提高,局部的颗粒长大,温度愈高,长大的颗粒愈多．经１５００℃晶化处理后,从ＸＲＤ,ＴＥＭ及ＨＲＥＭ可以看出粉体全部晶化,颗粒的结晶已非常完整,晶格网络清晰可见,颗粒度约为２０—４０ｎｍ,外表面有一层１—２ｎｍ的膜包裹．     

High-Temperature Passive Oxidation of Chemically Vapor Deposited Silicon Carbide

The oxidation behavior of chemically vapor deposited (CVD) SiC at high temperature was investigated using a thermogravimetric technique in the temperatures range of 1823 to 1948 K. The specimens were prepared by chemical vapor deposition using SiCl₄, C₃H₈, and H₂ as source gases. The oxidation behavior of the CVD-SiC indicated "passive" oxidation and a two-step parabolic oxidation kinetics over the entire temperature range. The crystallization of the SiO₂ film formed may have caused this two-step parabolic behavior. The parabolic oxidation rate constant ( K _p) varied with the square root of the oxygen partial pressure ( P ^1/2_O2). The activation energy for the oxidation was determined to be 345 and 387 kJ · mol⁻¹. These values suggest that the diffusion process of the oxygen ion which passes through the SiO₂ film is rate-controlling.     

低温化学气相沉积SiC涂层显微结构及晶体结构研究

在CH_3SiCl_3-H_2体系中,采用化学气相沉积法(CVD)在1000～1300℃制备了SiC涂层。研究了SiC涂层的沉积速率和温度之间的关系,发现低温化学气相沉积SiC为动力学控制过程,反应的表观活化能为85～156 kJ/mol。SiC涂层的外观颜色及涂层表面的显微结构随沉积温度变化而呈现规律的变化:当沉积温度<1150℃时,SiC涂层的外观颜色为银白色,涂层表面致密、光滑;当温度≥1150℃时,SiC涂层外观颜色逐渐变暗,涂层表面变得疏松、粗糙。利用XRD分析了不同沉积温度下SiC涂层的晶体结构,随着温度的升高,SiC涂层的结晶由不完整趋向于完整;当沉积温度≥1150℃,SiC涂层的XRD谱图中除了β-SiC外还出现了少量α-SiC。     

High-Temperature Oxidation of Chemically Vapor-Deposited Silicon Carbide in Wet Oxygen at 1823 to 1923 K

The oxidation of chemically vapor-deposited SiC in wet O₂ (water vapor partial pressure = 0.01 MPa, total pressure = 0.1 MPa) was examined using a thermogravimetric technique in the temperature range of 1823 to 1923 K. The oxidation kinetics follow a linear-parabolic relationship over the entire temperature range. The activation energies of linear and parabolic rate constants were 428 and 397 kJ · mol⁻¹, respectively. The results suggested that the rate-controlling step is a chemical reaction at an SiC/SiO₂ interface in the linear oxidation regime, and the rate-controlling step is an oxygen diffusion process through the oxide film (cristobalite) in the parabolic oxidation regime.     

Surface Pretreatments of Silicon Nitride for CVD Diamond Deposition

The efficiency of different surface pretreatments (four standard chemical etchings and four diamond powder abrasive procedures) on silicon nitride (Si₃N₄) substrates for chemical vapor deposition (CVD) of diamond has been systematically investigated. Blank Si₃N₄ samples were polished with colloidal silica (∼0.25 μm). Diamond nucleation and growth runs were conducted in a microwave plasma chemical vapor deposition apparatus for 10 min and 6 h, respectively. Superior results concerning nucleation density ( N _d∼ 10¹⁰ cm⁻² after 10 min), film uniformity, and grain size (below 2 μm after 6 h) were obtained for the mechanically microflawed samples, revealing that chemical etchings (hot and cold strong acids, molten base or CF₄ plasma) are not crucial for good CVD diamond quality on Si₃N₄.     

稀释气体对化学气相沉积SiC涂层生长行为的影响

以CH3SiCl3-H2为反应气体,采用Ar和H2作为稀释气体。在1100℃、负压条件下,由化学气相沉积制备了SiC涂层,研究了稀释气体对涂层沉积速率、形貌以及晶体结构的影响。以Ar为稀释气体时,随着稀释气体流量的增加沉积速率迅速减小;用Ar作稀释气体制备的SiC涂层相对粗糙,随着Ar流量的增加,晶粒簇之间的空隙较大,涂层变得疏松。XRD分析表明：当稀释气体Ar流量超过200ml／min时,涂层中除了β-SiC外,还逐渐出现了少量的α-SiC。以H2为稀释气体时,当H2流量增加到400ml／min时,涂层的沉积速率迅速增大;以H2为稀释气体制备的SiC涂层致密、光滑,沉积的SiC涂层全部是β-SiC,且具有非常强的(111)晶面取向,涂层中无α-SiC出现。     

Preparation of Fine Grained SiC Layer by Fluidized Bed Chemical Vapor Deposition with Pulsed Propylene

A novel strategy named pulsed propylene method was developed to prepare fine grained SiC layer. The SiC layer was uniformly coated on ceramic microspheres by fluidized bed chemical vapor deposition. Propylene was introduced into the deposition system in pulsed form at proper time intervals. By adjusting the propylene concentration and the pulse interval, the grain size of SiC can be controlled from micrometer scale to nanometer scale and the microstructure of the SiC layer changed from large columnar grains to fine equiaxed grains. The SiC layer with average grain size less than 300 nm was obtained with the pulsed propylene under a volume concentration of 5% at an injection time interval of 2 min. The as‐prepared fine grained SiC layer exhibited higher hardness and higher oxidation resistance than that of columnar grained SiC.     

High-Temperature Active Oxidation of Chemically Vapor-Deposited Silicon Carbide in an Ar─O2 Atmosphere

Active oxidation behavior of chemically vapor-deposited silicon carbide in an Ar─O₂ atmosphere at 0.1 MPa was examined in the temperature range between 1840 and 1923 K. The transition from active oxidation (mass loss) to passive oxidation (mass gain) was observed at certain distinct oxygen partial pressures ( P _O2^t). The values of P _O2^t increased with increasing temperature and with decreasing total gas flow rates. This behavior was well explained by Wagner's model and thermodynamic calculations. Active oxidation rates ( k _a) increased with increasing O₂ partial pressures and total gas flow rates. The rate-controlling step of the active oxidation was concluded to be O₂ diffusion through the gaseous boundary layer.     

Elastic-Property Measurements on CVI SiC/SiC Minicomposites by Acoustic Microscopy

In situ elastic properties of the fiber and matrix in composites obtained via chemical vapor infiltration/chemical vapor deposition (CVI-CVD) have been determined using acoustic microscopy. An unusually high frequency was used to attain the resolution that was required by the structure and small size of the constituents in these nonhomogeneous materials. A coupling liquid was prepared with acoustic properties that were chosen with consideration of the high frequency and the material characteristics. Local measurements on the fibers and matrix were achieved by studying the acoustic signature that was processed on very small areas. The elastic modulus of the fibers and matrices was deduced from the velocity measurements.     

Modeling of Laser-Induced Chemical Vapor Deposition of Silicon Carbide Rods from Tetramethylsilane

Finite-difference fluid-dynamics modeling has been used to predict deposition rates, fractional amounts of phases, and deposition morphology for the codeposition of silicon carbide and pyrolitic carbon from tetramethylsilane via laser-induced chemical vapor deposition (LCVD). Calculated results agree fairly well with rod deposition experiments. The morphologic features of rods that have been grown using LCVD are examined and explained using the results of the finite-difference calculations.     

Coal Slag Protection of Silicon Carbide with Chemically Vapor Deposited Mullite Coatings

The corrosion characteristics of bulk alumina, SiC, mullite, and CVD mullite coatings on SiC in contact with coal slag were investigated. Uncoated SiC corroded in the presence of coal slag, forming mixed FeSi phases and carbon. Bulk Al₂O₃ and slag formed a diffusional phase believed to be the spinel hercynite (Fe,Mg)O·(Al,Fe)₂O₃. After exposure to coal slag, a compositional difference was observed at bulk mullite/coal slag interfaces, yet this diffusional phase did not appreciably degrade the mullite samples and no cracking was observed. CVD mullite coatings offered protection to SiC in a simulated coal gasification atmosphere with corrosion protection dependent on the uniformity of the coating. Microprobe analysis of the CVD mullite coating/slag interface showed the formation of a Fe(Mg)Al.     

化学气相反应法制备SiC涂层

采用化学气相反应法,以3种不同工艺在C／C复合材料表面制备了SiC涂层,并检测了其抗氧化性能．以工业用Si和辅助剂SiO2为原料,在高温、惰性环境中反应产生SiO蒸气,将其引入反应室与C／C复合材料在不同温度下进行气相反应,在试样表面生成一层致密的SiC涂层。X射线衍射分析表明：涂层是由β-SiC组成。从试样截面的扫描电镜可知：不同工艺制得的SiC涂层界面过渡带颗粒的微观形貌各异。经最优工艺制备的涂层过渡带很窄,有β-SiC纳米晶须生成,且其抗氧化性能最佳。     

CVI法快速制备C/SiC复合材料

为缩短CVI法制备C／SiC复合材料的工艺周期并降低成本，研究了CVI工艺过程中沉积温度、MTS（CH3SiC3)摩尔分数和气体流量对SiC沉积速率和MTS有效利用率的影响，实验结果表明：提高沉积温度，常压下1100℃时增大MTS摩尔分数（11％→19％），都有利于提高SiC沉积速率；提高沉积温度和降低反应物气体流量，能提高MTS有效利用率，在优化的工艺条件下，预制体的微观孔隙内沉积了致密的SiC基体，沉积速率达到142μm/h左右，并有效消除了基体中裂纹的形成，MTS的有效利用率为11％－27％。     

Microstructure and mechanical property of high growth rate SiC via continuous hot-wire CVD

An average growth rate of SiC at 3.4-28.5 μm/min can be achieved via continuous hot-wire CVD method with input powers of 300-380 W. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) analysis, and X-ray diffraction (XRD) method, combined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were applied to investigate the microstructure of deposits. At 300 W, amorphous SiC and Si were main products in deposit, which were rapidly replaced by crystallite SiC containing high density stacking faults at 340 W and above. Moreover, with the grain size of SiC increasing from ~25 to ~420 nm, the stacking faults probability decreasing from 0.179 to 0.125, as well as the surface morphology changed from loose-packed granules to a well-defined faceted structure with strong (111) texture. Structural changes led to the increase of deposit's Young's modulus from 266.3 to 341.5 GPa, and the mean tensile strength of SiC filament from 1.57 to 3.03 GPa. The successive growth of W/SiC interfacial layer above 360 W resulted in the reduction in mean tensile strength and Weibull moduls of SiC monofilaments, which agrees with the prediction from critical interfacial layer thickness theory.