MoSi2-再结晶SiC复合材料的高温抗氧化性能及氧化机理

在SiC粉中添加MoSi2粉,采用模压成型、无压烧成方法制备MoSi2-再结晶SiC（RSiC）复合材料。利用扫描电子显微镜、X射线衍射和等温氧化法研究复合材料的高温抗氧化性能及氧化机理。结果表明,所得复合材料中SiC为6H型,部分MoSi2转变为六方结构M04.8Si3Co6,添加MoSi2前后样品的氧化产物均为方石英,样品表面生成的氧化膜形貌相似。氧化过程中样品质量变化与时间关系遵循抛物线规律,随MoSi2添加量增加,复合材料的抗氧化性能显著提高,其中,添加20％（质量分数）MoSi2所得复合材料在1500℃循环氧化100h后质量增加量仅为未添加MoSh样品的37％。当MoSi2添加量为10％时,复合材料的抗氧化性能随样品烧成温度的升高先提高后降低,2300℃烧成所得材料有较好的高温抗氧化性能,其氧化速率常数为0．99mg^2／（cm^4·h）。在氧化初始阶段,M04.8Si3C06和MoSi2首先发生氧化反应,随氧化时间增加,M04.8Si3Co6和MoSi2消耗殆尽,此后的氧化则主要为M05Si3和SiC的氧化。Si02膜的致密性和膜厚度与膜中M05Si3的含量有关。     

Fabrication of New Cemented Carbide Containing Diamond Coated with Nanometer-Sized SiC Particles

A simple process for depositing a coating of silicon carbide (SiC) crystallites ∼10 nm in size onto diamond particles has been developed. SiO powders react with diamond in a vacuum at 1350°C to form a uniform β-SiC polycrystalline layer ∼60 nm thick. The SiC coating improves the oxidation resistance of the diamond. A cemented carbide material containing 20-vol%-SiC-coated diamond particles was sintered to a relative density of 99.5% by pulsed-electric-current sintering. A Vickers hardness and indentation fracture toughness of 15 GPa and 16.3 MPa·m^1/2, respectively, were obtained. This toughness is two times higher than that of cemented carbide containing no particles. The higher toughness is attributed to deflection and blockage of crack propagation by the diamond particles.     

Oxidation Resistance of Multiwalled Carbon Nanotubes Coated with Silicon Carbide

Multiwalled carbon nanotubes (MWCNTs) were coated with a SiC layer using SiO vapor. The growth mechanism of SiC and the oxidation resistance of the SiC-coated MWCNTs were studied. The growth of the SiC layer was controlled by adjusting the partial pressure of CO₂ using carbon felt placed in a crucible. The nanometer-sized SiC particles were deposited onto the tubes by the reaction between SiO( g ) and CO( g ). On the other hand, the thin surface of the MWCNTs was converted to the SiC layer when the carbon felt was not used. The oxidation durability of MWCNTs was improved by the SiC coating. MWCNTs were oxidized completely in air at 650°C for 60 min. However, about 90 mass% of the SiC-coated MWCNTs remained after the same oxidation test.     

Improvement in the Oxidation Resistance of Oxide-Matrix Silicon Carbide-Particulate Composites by Mullite Infiltration

A SiC–∼50 vol% mullite-particulate composite fabricated by melt infiltration was found to exhibit excellent oxidation resistance at temperatures >1500°C in air. Cristobalite was found on the surface of samples after 100 h oxidation at temperatures of 1515°, 1620°, and 1650°C. The oxidation rate constant at 1515°C was almost comparable to hot-pressed bulk SiC, and at least 1 order of magnitude lower than the lowest value for the oxide-matrix SiC-particulate composites made by conventional processes, as reported in the literature and made by melt infiltration in the present study.     

HIP-Sintered Composites of C (Diamond)/SiC

Diamond (carbon) and silicon powders were mixed and HIPed under temperatures of 1300°–1500°C and pressure at 50 MPa for 30 min. When heated at >1300°C, the products were >90% sintered compacts. Density and bending strength were measured. The highest values of 3.3 g/cm³ and 750 MPa were obtained when the starting material was a mixture of fine and coarse-grained diamond and silicon powder. The photomicrograph of polished surface of the product revealed that it consisted primarily of two types of substances with few pores. XRD showed the coexistence of diamond and SiC. No trace of conversion reaction from diamond to graphite was seen, although the sample was treated under conditions in which diamond was thermodynamically metastable. The summarized results suggest that the HIP process can be a useful way to synthesize diamond/SiC composites.     

Passive-Oxidation Kinetics of SiC Microparticles

We investigated the oxidation kinetics of SiC materials in the form of powders (average dimension 4 μm) in the temperature range 1100°–1500°C in dry air. The oxidation process was monitored through the relative mass gain in a thermobalance. As the specific surface area of the particles was measured, the recorded mass gain could be converted into the corresponding oxide thickness. The oxidation isotherms were fitted to a linear-parabolic equation, and the parabolic rate constant was evaluated. Up to 1400°C, temperature dependence can be described by a single activation energy of 179 kJ/mol, which increases in the 1400°–1500°C temperature range. These results are compared with the oxidation behavior of sintered polycrystalline and monocrystalline SiC materials.     

Oxidation Resistance Evaluation of Silicon Carbide Ceramics with Various Additives

Five silicon carbide ceramics with various additives were evaluated for oxidation resistance at 1300°C in flowing dry and wet air. In the dry atmosphere, the oxidation of the five samples was diffusion-controlled, and in wet atmosphere they exhibited a linear relation beween weight gain by oxidation and water vapor content. Water vapor in the atmosphere strongly accelerated oxidation. The influence of oxidation on room-temperature strength was complex, but the samples were not as affected by oxidation.     

Oxidation of Silicon and Silicon Carbide in Ozone-Containing Atmospheres at 973 K

The oxidation behavior of a silicon wafer, chemically vapor-deposited SiC, and single-crystal SiC was investigated in an oxygen—2%–7% ozone gas mixture at 973 K. The thickness of the oxide film that formed during oxidation was measured by ellipsometry. The oxidation rates in the ozone-containing atmosphere were much higher than those in a pure oxygen atmosphere. The parabolic oxidation kinetics were observed for both silicon and SiC. The parabolic rate constants varied linearly with the ozone-gas partial pressure. Inward diffusion of atomic oxygen formed by the dissociation of ozone gas through the SiO₂ film apparently was the rate-controlling process.     

Intermediate Temperature Internal Oxidation of a SiC/SiCN Composite with a Polymer‐Derived Matrix

SiC‐based composites exhibit oxidative embrittlement at intermediate temperatures. Although the mechanisms of internal oxidation in composites with initially hermetic matrices have been studied extensively, comparable studies on composites with semipermeable matrices, such as those produced by polymer infiltration and pyrolysis, have not been reported. The present article focuses on the latter class of composites, specifically a SiC_f /SiCN_m with a dual BN/Si₃N₄ fiber coating. It describes detailed SEM and TEM analyses of the microstructure before and after oxidation in dry air or water vapor at 800°C. The results show that internal oxidation is more aggressive in water vapor and occurs appreciably even in the absence of an applied stress. The sequence of oxidation of the constituent phases appears to be consistent with the underlying thermodynamic hierarchy for the respective oxidation reactions. Notably, contrary to existing models based on preferential oxidation of BN coatings, oxidation occurs first on the SiC fiber surfaces and the Si₃N₄ overcoat; crystalline BN remains even after significant fiber and matrix oxidation has occurred. The results are discussed in terms of rate‐controlling kinetic processes, the effect of oxidant type, and applied stress.     

Micro Four‐Layer SiC Coating on Matrix Graphite Spheres of HTR Fuel Elements by Two‐Step Pack Cementation

A micro four‐layer SiC coating, which includes inner transition layer, fine‐grained layer, dense bulk layer, and outer loose layer, was fabricated on the matrix graphite spheres of high‐temperature gas‐cooled reactor fuel elements to improve the oxidation‐resistant property by a two‐step pack cementation process. According to the experiment results, the micro four‐layer can be differentiated by SiC grain size and microstructure variation. The oxidation tests at 1773 K for 200 h reveal that the coating structure could effectively improve the oxidation resistance of matrix graphite spheres with a weight gain of 0.52 wt%, and the fine‐grained and dense bulk layers are evidenced as two main antioxidation layers. Although the thermal expansion coefficients of SiC and matrix graphite do not match each other so well, no obvious stress cracking was observed after thermal shocking tests from 1773 K to room temperature for 100 times.     

Microstructure and Oxidation Behavior of Silicon Carbide Fibers Derived from Polycarbosilane

Polycarbosilane-derived SiC fibers (CG Nicalon, Hi-Nicalon, and Hi-Nicalon type S) were exposed for 1–100 h at 1273–1673 K in air. Oxide layer growth and changes in tensile strength for these fibers were examined after exposure. The three types of SiC fibers decreased in strength as the oxide layer thickness increased. Fracture origins were located near the oxide layer–fiber interface. The Hi-Nicalon type S showed better oxidation resistance than the other polycarbosilane-derived SiC fibers after exposure in air at 1673 K for 10 h. This result was attributed to the nature of the silicon oxide layer on the surface of the SiC fibers.     

In situ Growth of SiC Whisker in Pyrolyzed Monolithic Mixture of AHPCS and SiC

In situ whisker growth was observed during heat treatment of allylhydridopolycarbosilane (AHPCS) and SiC powder in the temperature range of 1250°–1350°C. TEM equipped with an electron energy loss spectrometer (EELS) verified that the banded whiskers are twinned single-grained β-SiC. Convergent beam electron diffraction patterns (CBED) of the whisker tips were consistent with formation by a vapor–solid (VS) mechanism. The effects of process variables on whisker growth were addressed. The mechanism of whisker growth was discussed and attributed to the reaction between the gaseous products of the polymers AHPCS and polycarbosilane (PCS). Thermal decomposition behavior of the polymers was followed to relate gas evolution to whisker formation.     

Some New Perspectives on Oxidation of Silicon Carbide and Silicon Nitride

This study provides new perspectives on why the oxidation rates of silicon carbide and silicon nitride are lower than those of silicon and on the conditions under which gas bubbles can form on them. The effects on oxidation of various rate-limiting steps are evaluated by considering the partial pressure gradients of various species, such as O₂, CO, and N₂. Also calculated are the parabolic rate constants for the situations when the rates are controlled by oxygen and/or carbon monoxide (or nitrogen) diffusion. These considerations indicate that the oxidation of silicon carbide and silicon nitride should be mixed controlled, influenced both by an interface reaction and diffusion.     

Paralinear Oxidation of CVD SiC in Simulated Fuel-Rich Combustion

The oxidation kinetics of CVD SiC were measured by thermogravimetric analysis (TGA) in a 4H₂·12H₂O·10CO·7CO₂·67N₂ gas mixture flowing at 0.44 cm/s at temperatures between 1300° and 1450°C in fused quartz furnace tubes at 1 atm total pressure. The SiC was oxidized to form solid SiO₂. At ≥1350°C, the SiO₂ was in turn volatilized. Volatilization kinetics were consistent with the thermodynamic predictions based on SiO formation. These two simultaneous reactions resulted in overall paralinear kinetics. A curve fitting technique was used to determine the linear and parabolic rate constants from the paralinear kinetic data. Volatilization of the protective SiO₂ scale resulted in accelerated consumption of SiC. Recession rates under conditions more representative of actual combustors were estimated from the furnace data.     

Oxidation kinetics strength of Hi‐NicalonTM‐S SiC fiber after oxidation in dry and wet air

Hi‐Nicalon?‐S SiC fiber strengths and Weibull moduli were measured after oxidation for up to 100 hours between 700°C and 1400°C in wet and dry air. SiO₂ scale thickness and crystallization extent were measured by TEM. The effect of furnace environment on trace element levels in the SiO₂ scales was characterized by secondary ion mass spectroscopy. Crystallization kinetics and Deal‐Grove oxidation kinetics for glass and crystalline scale, and the transition between them, were modeled and determined. Crystallization retards oxidation kinetics, and scale that formed in the crystalline state was heavily deformed by the growth stress accompanying SiC oxidation volume expansion. Glass scales formed in dry air slightly increased fiber strength. Glass scales formed in wet air did not increase strength, and in some cases significantly decreased strength. Scales more than 200 nm thick were usually partially or completely crystallized, which degraded fiber strength. Contamination of scales by trace impurities such as Al and Ca during heat treatment inhibited crystallization. The oxidation kinetics and the strengths of oxidized Hi‐Nicalon?‐S fibers are compared with previous studies on SiC fibers, bulk SiC, and single‐crystal SiC. Empirical relationships between oxidation temperature, time, scale thickness, and strength are determined and discussed.     

Si3N4／纳米SiC复相陶瓷的研究

采用纳米ＳｉＣ粉体制备了Ｓｉ３Ｎ４／纳米ＳｉＣｐ复相陶瓷。研究了制备工艺、纳米ＳｉＣ含量对材料性能及显微结构的影响,并对材料显微结构特点与强韧化机制进行了分析 。结果表明：添加２０ｖｏ％〈１００ｎｍ的ＳｉＣ粉体时,复相陶瓷的室温抗弯强度达８５６ＭＰａ,当添加１０ｖｏ％上述ＳｉＣ粉体时,复相陶瓷的增韧效果最佳,断裂韧性达８．２７ＭＰａｍ＾１／２,比基体材料提高了２３％。     

ZrB2–SiC多层陶瓷的抗氧化性

用传统陶瓷的流延工艺制备ZrB2–SiC多层陶瓷。用Archimedes法测定ZrB2–SiC多层陶瓷的相对密度。用扫描电子显微镜观察其显微结构,并进行循环抗氧化性能评价。结果表明：ZrB2–SiC多层陶瓷在1 950℃烧结的致密度达到99.7%,材料的抗氧化过程主要可分为两个阶段：第一阶段低熔点相的挥发,出现质量损失;第二阶段氧化层的形成,降低进一步氧化速率。抗氧化性能较ZrB2–SiC复相陶瓷有很大提高。     

Oxidation and Volatilization of Silica Formers in Water Vapor

At high temperatures, SiC and Si₃N₄ react with water vapor to form a SiO₂ scale. SiO₂ scales also react with water vapor to form a volatile Si(OH)₄ species. These simultaneous reactions, one forming SiO₂ and the other removing SiO₂, are described by paralinear kinetics. A steady state, in which these reactions occur at the same rate, is eventually achieved. After steady state is achieved, the oxide found on the surface is a constant thickness, and recession of the underlying material occurs at a linear rate. The steady-state oxide thickness, the time to achieve steady state, and the steady-state recession rate can be described in terms of the rate constants for the oxidation and volatilization reactions. In addition, the oxide thickness, the time to achieve steady state, and the recession rate also can be determined from parameters that describe a water-vapor-containing environment. Accordingly, maps have been developed to show these steady-state conditions as a function of reaction rate constants, pressure, and gas velocity. These maps can be used to predict the behavior of SiO₂ formers in water-vapor-containing environments, such as combustion environments. Finally, these maps are used to explore the limits of the paralinear oxidation model for SiC and Si₃N₄.     

Observations of Accelerated Silicon Carbide Recession by Oxidation at High Water-Vapor Pressures

A study of the exposure of SiC at 1200°C and high water-vapor pressures (1.5 atm) has shown SiC recession rates that exceed what is predicted based on parabolic oxidation at water-vapor pressures of less than or equal to ∼1 atm. After exposure to these conditions, distinct silica-scale structures are observed; thick, porous, nonprotective cristobalite scales form above a thin, dense silica layer. The porous cristobalite thickens with exposure time, while the thickness of the underlying dense layer remains constant. These observations suggest a moving-boundary phenomenon that is controlled by the rapid conversion of dense vitreous silica to a porous, nonprotective crystalline SiO₂.     

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.