Oxidation of Silicon Carbide‐Reinforced Oxide‐Matrix Composites at 1375° to 1575°C
TL;DR: In this article, the parabolic rate constants were about 3 orders of magnitude higher than those expected for the oxidation of SiC. Higher rates are caused by higher oxygen permeabilities through the reaction products than through pure silica.
Abstract: Oxidation studies were conducted on Al2O3-SiC and mullite-SiC composites at 1375° to 1575°C in O2 and in Ar-1% O2. The composites were prepared by hot-pressing mixtures of Al2O3 or mullite and SiC powders. The reaction products contained alumina, mullite, an aluminosilicate liquid, and gas bubbles. The parabolic rate constants were about 3 orders of magnitude higher than those expected for the oxidation of SiC. Higher rates are caused by higher oxygen permeabilities through the reaction products than through pure silica. Our results suggest that oxygen permeabilities are comparable in the three condensed phases observed in the reaction products.
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TL;DR: In this paper, a review of crack deflecting coatings for fiber-reinforced ceramic composites is presented, and the implications on the design of coatings and of composite systems using alternative coatings are discussed.
Abstract: Fiber-reinforced ceramic composites achieve high toughness through distributed damage mechanisms. These mechanisms are dependent on matrix cracks deflecting into fiber/matrix interfacial debonding cracks. Oxidation resistance of the fiber coatings often used to enable crack deflection is an important limitation for long-term use in many applications. Research on alternative, mostly oxide, coatings for oxide and non-oxide composites is reviewed. Processing issues, such as fiber coatings and fiber strength degradation, are discussed. Mechanics work related to design of crack deflecting coatings is also reviewed, and implications on the design of coatings and of composite systems using alternative coatings are discussed. Potential topics for further research are identified.
261 citations
TL;DR: In this article, Al2O3 reinforced by SiC whiskers was hot-pressed to investigate the crack-healing behavior of SiC-W. The results showed that SiC made their Al 2O3 tougher, and the surface cracks with length of 2 c = 100 μm could be healed at 1200° or 1300°C for 1 h in air.
Abstract: Al2O3 reinforced by SiC whiskers (Al2O3/SiC-W) was hot-pressed to investigate the crack-healing behavior. Semielliptical surface cracks of 100 μm in surface length were introduced using a Vickers indenter. The specimens containing precracks were crack-healed at temperatures between 1000° and 1300°C for 1 h in air, and their strengths were measured by three-point bending tests at room temperature and elevated temperatures between 400° and 1300°C. The results show that Al2O3/SiC-W possesses considerable crack-healing ability. The surface cracks with length of 2 c= 100 μm could be healed by crack-healing at 1200° or 1300°C for 1 h in air. Fracture toughness of the material was also determined. As expected, the SiC whiskers made their Al2O3 tougher.
103 citations
TL;DR: In this article, a methodology to control the interfacial microstructures in the SiC/Al composite is presented. And the stability of the newly formed interface at elevated temperatures is also made.
98 citations
15 May 2001-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the passive oxidation behavior of SiC particles at elevated temperatures was studied by using scanning electron microscopy (SEM) and X-ray diffraction, and the results showed that nano-MgO formed initially on the surface of the oxidized SiC particle and then turned into nanoMgAl 2 O 4 crystal due to the reaction between the SiO 2 and aluminum alloy containing Mg.
Abstract: The passive oxidation behavior of SiC particles at elevated temperatures has been studied. The evolution of interfacial reaction products in 2014 aluminum alloy composite reinforced with oxidized-SiC particles after extended thermal exposure at elevated temperatures were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction. Although it could prevent the interfacial reaction between SiC particles and aluminum alloy, the preoxidation of SiC particles resulted in the formation of other interfacial reaction products. The results showed that at elevated temperatures nano-MgO formed initially on the surface of the oxidized SiC particles and then turned into nano-MgAl 2 O 4 crystal due to the reaction between the SiO 2 and aluminum alloy containing Mg. TEM observations indicated that the oxidized layer on SiC particles was uniform and had a good bonding with SiC and aluminum alloy.
84 citations
TL;DR: In this article, the isothermal oxidation behavior of Cr2AlC coatings on alumina substrates was investigated in the temperature range of 1230 to 1410°C.
Abstract: The isothermal oxidation behavior of Cr2AlC coatings on alumina substrates was investigated in the temperature range of 1230 to 1410 °C. The structure, surface morphology, microstructure evolution and chemistry of the reaction products have been investigated. In the investigated temperature range, the Cr2AlC films form a dense continuous oxide scale consisting of α-Al2O3 on Cr carbides. The oxidation rates determined by thermo gravimetric analysis (TGA) were parabolic, indicating that diffusion through the scale is the rate limiting mechanism. The activation energy for oxidation was determined to be 348 kJ mol− 1 and the parabolic rate constant at 1230 °C was 7.1 × 10− 10 kg2 m− 4 s− 1. Hence, the oxidation behavior is comparable to NiAl in the temperature range and time intervals investigated. With increasing oxidation time voids form at the interface between oxide and Cr carbides and the amount of Cr7C3 increases at the expense of Cr3C2. Based on our thermodynamic calculations the oxygen partial pressure below the oxide scale increases as Al is depleted and Cr carbides oxidize, resulting in CO gas- and Cr2O3-formation. The formation of gas may together with the depletion of Al and Cr lead to the significant void formation observed in the Cr carbide interlayer. Observation of both Cr carbide precipitates and the formation of (Al,Cr)2O3 solid solution support this notion. For comparison bulk Cr2AlC was oxidized. It is argued that the absence of pores in oxidized bulk Cr2AlC is due to the considerably larger amount of Al available.
82 citations
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TL;DR: The self-diffusion coefficient of oxygen has been determined as a function of temperature in single crystal and polycrystalline aluminum oxide at temperatures up to 1780°C as mentioned in this paper.
Abstract: The self‐diffusion coefficient of oxygen has been determined as a function of temperature in single crystal and polycrystalline aluminum oxide at temperatures up to 1780°C. The rate of exchange between a gas phase and solid particles was measured, utilizing the stable isotope, O18. In single crystals of aluminum oxide intrinsic diffusion occurs in a high temperature region, depending on temperature as D=1.9×103 exp (—152 000/RT). At temperatures below about 1600°C variable results were obtained depending on impurity content and previous heat treatment. For one set of samples experimental results could be represented as D=6.3×10—8 exp (—57 600/RT). The diffusion coefficient of oxygen in polycrystalline samples is about two orders of magnitude larger than that found for the single crystals, and has a somewhat smaller activation energy. With the polycrystalline oxide, variable results were also observed at lower temperatures.
492 citations
Journal Article•
TL;DR: In this paper, powder-whisker mixtures were hot-pressed to obtain densification behavior of the composites, which was influenced by the temperature, pressing pressure, and whisker content.
Abstract: Dense, toughened SiC-whisker-reinforced ceramic matrix (ie Al2O3 and mullite) composites were fabricated by hot-pressing powder-whisker mixtures The densification behavior of the composites was influenced by the temperature, pressing pressure, and whisker content but even at 60 vol pct whisker loading, full density was achieved at 1850 C for the alumina matrix The mechanical performance of these composites was improved In particular, the critical stress intensity factor (equal to about 9 MPa sq rt of m in Al2O3 and equal to about 5 MPa sq rt of m in mullite) exhibited substantial improvements (approximately two-fold increases) over monolithic ceramics In the case of the alumina-20 vol pct SiC whisker composite, the flexural strength was 805 MPa at room temperature and 520 MPa at 1200 C the thermal conductivity was 32 W/mK at room temperature and 10 W/mK at 1000 C 19 references
382 citations
TL;DR: In this article, the oxidation kinetics of several single-crystal and polvcrystalline silicon carbide materials and singlecrystal silicon in dry oxygen over the temperature range 1200° to 1500°C were fitted to the linear-parabolic model of Deal and Grove.
Abstract: The oxidation kinetics of several single-crystal and polvcrystalline silicon carbide materials and single-crystal silicon in dry oxygen over the temperature range 1200° to 1500°C were fitted to the linear-parabolic model of Deal and Grove. The lower oxidation rates of silicon carbide compared to silicon can be rationalized by additional consumption of oxidant in oxidizing carbon to carbon dioxide. The (000J) Si face of the silicon carbide platelets exhibited lower parabolic oxidation rates than the (0001) C face, by a factor of 10 at 1200°C. Apparent activation energies increased from a value of ∼120 kJ/mol below 1400°C to a value of ∼300 kJ/mol above this temperature. The (0001) Si face exhibited this high activation energy over the entire temperature range. The controlled nucleation thermally deposited material exhibited the highest oxidation rates of the polycrystalline materials followed by the hot-pressed and sintered α-silicon carbides. In general, the oxidation rates of the polycrystalline materials were bracketed by the oxidation rates of the basal planes of the single-crystal materials. Higher impurity concentrations and higher density of nucleation sites led to a greater susceptibility to crystallization of the scale which significantly complicated the oxidation behaviors observed. When crystallization of the oxide scale occurred in the form of a layer of spherulitic cristobalite crystals, a retardation of the oxidation rates was observed. An accelerated oxidation behavior was found when this coherent layer was superseded by the formation of fine mullite crystals.
323 citations
TL;DR: This article comporte 20% Cr, 4,5% Al, 0.5% Ti and 0.1% Y 2 O 3, in a cinetique d'oxydation de l'alliage etudie, dans le domaine de temperature 1000-1200°C.
Abstract: L'alliage etudie comporte 20% Cr, 4,5% Al, 0,5% Ti et 0,5% Y 2 O 3 . La cinetique d'oxydation de l'alliage a ete etablie a diverses pressions partielles d'oxygene, dans le domaine de temperature 1000-1200°C
153 citations
TL;DR: Theoretical considerations have been used to evaluate some approaches useful for oxidation protection of carbon/carbon composites as discussed by the authors, including gas phase diffusion, interface reaction, diffusion through liquid B 2 O 3, and diffusion through cracks in solid barrier films.
145 citations