Fabrication and Characteristic of Nextel 720 Fiber‐Reinforced Silicon Nitride Matrix Composites by Chemical Vapor Infiltration Process

Most current processes for fiber‐reinforced silicon nitride composites are conducted at very high temperature, which is not possible to use oxide fiber as reinforcement. Here, low‐temperature process of chemical vapor infiltration (CVI) was utilized to fabricate Nextel 720 oxide fiber tow‐reinforced silicon nitride matrix composite with PyC as interphase. The tensile strength was analyzed by Weibull distribution. The microstructure showed that there were two types of interface bonding. The strong interface bonding determined the unexpected low strength of the composites. This indicated that the suitable interface design is the urgent issue for oxide fiber‐reinforced silicon nitride composite by CVI.     

先进陶瓷基复合材料制备技术-CVI法现状及进展

化学气相渗透法(CVI)是制备先进陶瓷基复合材料最赋潜力的技术.本文概要阐述了CVI法的原理与动力学机制,论述了CVI先进陶瓷基复合材料中纤维、基体、界面的研究现状,对不同类型的CVI工艺及目前的CVI模拟技术作了一定的评价,提出了CVI技术的发展方向和研究课题.     

Three-Dimensional Carbon/Silicon Carbide Composites Prepared by Chemical Vapor Infiltration

Continuous-carbon-fiber-reinforced silicon carbide composites (C/SiC) were prepared by chemical vapor infiltration in which the preforms were fabricated with the three-dimensional braid method. The mechanical properties and microstructures were investigated. For the composites with no interfacial layer, flexural strength and fracture toughness increased with density of the composites, and the maximum values were 520 MPa and 16.5 MPa·m^1/2, respectively. The fracture behavior was dependent on the interfacial bonding between fiber/matrix and fiber bundle/bundle which was determined by the density of the composites. Heat treatment had a significant influence on the mechanical properties and fracture behavior. The composites with pyrolysis interfacial layers exhibited characteristic fracture and relatively low strength (300 MPa).     

Silicon Carbide/Silicon and Silicon Carbide/Silicon Carbide Composites Produced by Chemical Vapor Infiltration

Composites of SiC/Si and SiC/SiC were prepared from single yarns of SiC. The use of carbon coatings on SiC yarn prevented the degradation normally observed when chemically vapor deposited Si is applied to SiC yarn. The strength, however, was not retained when the composite was heated at elevated temperatures in air. In contrast, the strength of a SiC/C/SiC composite was not reduced after this composite was heated at elevated temperatures, even when the fiber ends were exposed.     

Chemical-Vapor-Infiltrated Silicon Nitride, Boron Nitride, and Silicon Carbide Matrix Composites

Composites of carbon/chemical-vapor-deposited (CVD) Si₃N₄, carbon/CVD BN, mullite/CVD SiC, and SiC yarn/CVD SiC were prepared to determine if there were inherent toughness in these systems. The matrices were deposited at high enough temperatures to ensure that they were crystalline, which should make them more stable at high temperatures. The fiber-matrix bonding in the C/Si₃N₄ composite appeared to be too strong; the layers of BN in the matrix of the C/BN were too weakly bonded; and the mullite/SiC composite was not as tough as the SiC/SiC composites. Only the SiC yarn/CVD SiC composite exhibited both strength and toughness.     

Fabrication of Carbon/Silicon Carbide Composites by Isothermal Chemical Vapor Infiltration,Using the In Situ Whisker-Growing and Matrix-Filling Process

C/SiC composites were prepared via isothermal chemical vapor infiltration (ICVI). A novel process of in situ whisker growing and matrix filling during ICVI was devised to reduce the porosity of the C/SiC composites, by alternating the dilute-gas species. C/SiC composites with increased density were prepared successfully using this novel process, in comparison with those obtained from the conventional ICVI process. The whiskers seem to have grown into the large pores and modified the pore structure that is filled by the SiC matrix.     

Fabrication of Cf/SiC Composites by Vapor Silicon Infiltration

Carbon fiber reinforced silicon carbide matrix composites were fabricated by the vapor silicon infiltration process. The density and the open porosity of the composite infiltrated at 1973 K were 2.25 g/cm³ and ∼6%, respectively. The flexural strength of the composite at ambient conditions was 300 MPa. When the infiltration temperature decreased, the density and flexural strength of the composite also decreased. However, the resulting composite materials exhibited non-brittle fracture behavior.     

Fabrication of Silicon Carbide Whisker/Alumina Composite by Thermal-Gradient Chemical Vapor Infiltration

SiC( w )/Al₂O₃ composites were made from an AlCl₃-H₂-CO₂ mixture by a thermal-gradient chemical vapor infiltration (CVI) method. Al₂O₃ was deposited from the reaction of AlCl₃ and H₂O, which was produced from the oxidation of H₂ by CO₂. The densification rate was measured at various reactant compositions and total pressures. When the reaction rate or total pressure increased, the rate-controling step shifted from H₂O production to AlCl₃ diffusion, which led to premature pore closing. To obtain dense composites in a short infiltration time, the diffusion rate of AlCl₃ had to be increased by decreasing the total pressure.     

Analytical Modeling of Chemical Vapor Infiltration in Fabrication of Ceramic Composites

A model for chemical vapor infiltration is applied to the study of the growth of alumina from the chemical reaction among AlCl₃, H₂, and CO₂ within a SiC-fiber bundle which is situated in an isothermal hot-wall reactor. The pore space between the fibers is simulated by cylindrical capillary tubes. The model considers binary diffusion of CO₂ and H₂, chemical reaction on the inner surface of the tube, and deposition film growth. Furthermore, diffusion-controiled and chemical-reaction-controlled processes are taken into account to determine the dominating process in chemical vapor infiltration. Both molecular diffusion and Knudsen diffusion are considered sequentially in this model during the infiltration process. Based upon this model, the optimum processing conditions required for chemical vapor infiltration to form a SiC/Al₂O₃ composite can be predicted for different fiber preform systems.     

Fabrication and Characterization of Three-Dimensional Carbon Fiber Reinforced Silicon Carbide and Silicon Nitride Composites

Three-dimensional (3D) carbon fiber reinforced SiC and Si₃N₄ composites have been fabricated using repeated infiltration of an organosilicon slurry under vacuum and pressure. Open porosity of the infiltrated body was reduced from 40% after the first infiltration to approximately 8% after the seventh cycle. Further reduction of open porosity to less than 3% was accomplished by hot-press densification. The maximum values of flexural strength and fracture toughness were, respectively, 260 MPa and 7.3 MPa·m^1/2for C/Si₃N₄ composites, and 185 MPa and 6 MPa·m^1/2 for C/SiC composite.     

Nanofiber Formation in the Fabrication of Carbon/Silicon Carbide Ceramic Matrix Nanocomposites by Slurry Impregnation and Pulse Chemical Vapor Infiltration

The objectives of this work were to investigate the fabrication of carbon-fiber-reinforced SiC ceramic nanocomposites using the slurry impregnation process and the pulse chemical vapor infiltration (PCVI) process and to study the influences of processing parameters of the PCVI process on the microstructure variation of the nanocomposites. In this work, SiC nanosized powder was added to the matrix precursor (silicon powder mixed with phenolic resin), followed by the impregnation of the slurry into the preform. In the PCVI process, to densify the nanocomposites, tetramethylsilane (TMS) vapor mixed with hydrogen was used as the vapor precursor for matrix deposition. Fabrication parameters, such as reactant concentrations, pulse number, and holding time, were studied. Morphologies obtained from various processes were compared.     

Silicon Carbide Monofilament-Reinforced Silicon Nitride or Silicon Carbide Matrix Composites

SiC-monofilament-reinforced SiC or Si₃N₄ matrix composites were fabricated by hot-pressing, and their mechanical properties and effects of filaments and filament coating layers were studied. Relationships between frictional stress of filament/matrix interface and fracture toughness of SiC monofilament/Si₃N₄ matrix composites were also investigated. As a result, it was confirmed experimentally that in the case of composites fractured with filament pullout, the fracture toughness increased as the frictional stress increased. On the other hand, when frictional stress was too large (>about 80 MPa) for the filament to be pulled out, fracture toughnesses of the composites were almost the same and not so much improved over that of Si₃N₄ monolithic ceramics. The filament coating layers were found to have a significant effect on the frictional stress of the SiC monofilament/Si₃N₄ matrix interface and consequently the fracture toughness of the composites. Also the crack propagation behavior in the SiC monofilament/Si₃N₄ matrix composites was observed during flexural loading and cyclic loading tests by an in situ observation apparatus consisting of an SEM and a bending machine. The filament effect which obstructed crack propagation was clearly observed. Fatigue crack growth was not detected after 300 cyclic load applications.     

氮化硅复合材料的研究进展

简述了氮化硅的结构、性质和粉体制备,综述了近年来氮化硅基复合材料的研究进展。     

Low-Temperature Metal-Organic Chemical Vapor Deposition of Silicon Nitride

Amorphous silicon nitride films have been deposited on single-crystal silicon from the gas mixture of methylsilazane and ammonia at 873 to 1073 K. The films have been characterized by ellipsometry, Fourier transform infrared spectroscopy, and Auger electron spectroscopy. The Si-C, Si-H, and C-H bonds in methylsilazane can be effectively cleaved and the associated C and H species removed. The structure and composition of the films do not show any apparent dependence on the deposition temperature.     

Determination of Fracture Toughness in 2-D Woven SiC Matrix Composites Made by Chemical Vapor Infiltration

An approach to the R -curve behavior of 2-D woven ceramic matrix composites is proposed. This approach takes into account the presence of a process zone at the crack tip. The strain energy release rate is determined using an equivalent system of two specimens free of process zones. The process zone size is determined independently by assimilating a notch tip region to a beam of bimaterial. Compact tension specimens having different dimensions and single-edge notched beam specimens were used for the analysis. Important trends in toughening of ceramic matrix composites were anticipated.     

Chemical Vapor Deposited Sic Matrix Composites

Composites of Sic yarnlchemical vapor deposited Sic matrix and multitei chemical vapor deposited Sic matrix were prepared using methyldichloro-silane and applying a thermal gradient in a low-pressure reactor environment. The preparation parameters were selected from an X-ray study where the pressure and temperature were varied. The Sic-reinforced composite had a higher strength (450 versus 120 MPu) and exhibited more fiber pullout than the mullite-reinforced composite.     

Optimization of Chemical Vapor Deposition Parameters for Fabrication of Oxidation-Resistant Mullite Coatings on Silicon Nitride

Fabrication of mullite (3Al₂O₃·2SiO₂) coatings by chemical vapor deposition (CVD) using AlCl₃–SiCl₄–H₂–CO₂ gas mixtures was studied. The resultant CVD mullite coating microstructures were sensitive to gas-phase composition and deposition temperature. Chemical thermodynamic calculations performed on the AlCl₃–SiCl₄–H₂–CO₂ system were used to predict an equilibrium CVD phase diagram. Results from the thermodynamic analysis, process optimization, and effects of various process parameters on coating morphology are discussed. Dense, adherent crystalline CVD mullite coatings ∼2 μm thick were successfully grown on Si₃N₄ substrates at 1000°C and 10.7 kPa total pressure. The resultant coatings were 001 textured and contained well-faceted grains ∼0.3–0.5 μm in size.     

Kinetics of Silicon Nitride Chemical Vapor Deposition from Silicon Tetrafluoride and Ammonia

Rate laws for the chemical vapor deposition of Si₃N₄ from SiF₄and NH₃ are obtained by fitting the results of pareametric reactor experiments with a one-dimensional steady-state model for the reactor. The model includes axial mass transport by both convection and multicomponent diffusion, and allowance is made for the use of an expressions with adjustable constants are used to account for deposition on both crystalline and amorphous surfaces, as well as the heterogeneous decomposition of NH₃. In addition, there are mechanisms that determine the actual degree of surface crystallinity at any location. Optimum values for the rate constants are found by searching for the best overall fit to the experimental deposition rate and crystallinity data. It is shown that the model, with simple second-order expressions for the deposition rates, is quite successful in reproducing the experimentally observed effects of temperature, flow rate, reactant mole ratio, and axial position.     

Silicon Carbide Whisker Stability During Processing of Silicon Nitride Matrix Composites

The effects of two different sources of SiC whiskers on the chemistry and microstructure of the SiC-whisker—Si₃N₄ composites were evaluated using scanning transmission electron microscopy. Analyses were performed after presintering in N₂ and after encapsulated hot isostatic pressing. Significant differences in the porosity, α- to β-Si₃N₄ conversion, and whisker degradation were observed after presintering. It was also noted that whiskers containing surface iron impurities were converted to Si₃N₄ during processing. Whiskers from the source having low surface iron exhibited little reaction. After hot isostatic pressing, some oxidation of the cleaner whiskers was observed.     

Fabrication and Microstructure of C/SiC Composites Using a Novel Heaterless Chemical Vapor Infiltration Technique

Fabrication of C/SiC composites by using the heaterless chemical vapor infiltration (HCVI) technique, which is an improved technology based on the conventional chemical vapor infiltration, is reported for the first time in this paper. In the HCVI process, a gradient temperature field formed in the fiber preform overcomes the problems of slow diffusion and restricted permeability of gaseous reactant species to some extent, and the electro-deposition is necessary to accelerate the SiC deposition rates. The highest linear deposition rates of SiC matrix within inter-fiber pores are 0.33 μm/h. Microstructures of the C/SiC composites are uniform, and the inter-fiber and inter-ply pores can be well infiltrated. The longitudinal and circumferential microcracks are found in the composites.