Showing papers on "Mullite published in 1990"
01 Mar 1990
TL;DR: The 27Al and 29Si NMR spectra acquired with a 30 s recycle delay of poorly and highly crystalline samples heated at 480 and 500° C, respectively, contain three distinct signals; they discuss their assignment in the light of experiments involving leaching of the samples with aqueous KOH as discussed by the authors.
Abstract: Thermal transformations of kaolinite of different degree of crystallinity have been monitored by 27Al and 29Si high-resolution NMR with magic-angle spinning (MAS NMR), X-ray diffraction, Fourier transform infrared, atomic absorption spectrophotometry and thermogravimetric analysis. NMR shows differences in the dehydroxylation process of kaolinites with different degree of crystallinity and reveals the presence of short-range order in metakaolinite. 29Si NMR spectra acquired with a 30 s recycle delay of poorly and highly crystalline samples heated at 480 and 500° C, respectively, contain three distinct signals; we discuss their assignment in the light of experiments involving leaching of the samples with aqueous KOH. Ca. 40% of Si sites retain their original Q
3 symmetry just above the onset of dehydroxylation and the Q
4 environment is present showing that a small amount of amorphous silica has already segregated. The spectrum of samples treated at 1000° C contains a signal at -110ppm (from Q
4 silicons) and a faint resonance, from mullite, at ca. -87 ppm. 29Si NMR also shows that cristobalite germs are already present at 950–1000° C. The 27Al MAS NMR spectra of metakaolinite reveal the presence of 4-, 5-and 6-coordinated Al. Changes in the three Al populations as a function of temperature have been monitored quantitatively. Below 800° C, 4-and 5-coordinated Al appears at the expense of 6-coordinated Al, but above 800° C the amount of 6-coordinated Al increases again. We suggest a dehydroxylation scheme which accounts for the presence of 4-and 5 coordinated Al. Above 900–950° C the latter signal is no longer present in the 27Al NMR spectra and new 4-and 6-coordinated Al species (mullite and γ-alumina) appear. We propose new ideas for the structure of metakaolinite.
200 citations
TL;DR: The thermal expansion coefficients of transition-metal-free sinter mullite and fused mullite are measured between 25° and 900°C by high-temperature Guinier X-ray diffraction techniques as discussed by the authors.
Abstract: The thermal expansion coefficients of transition-metal-free sinter mullite and fused mullite, and of chromium-doped (11.5 wt% Cr2O3) and iron-doped (10.3 wt% Fe2O3) sinter mullites are measured between 25° and 900°C by high-temperature Guinier X-ray diffraction techniques. Most mullites display low and nonlinear thermal expansions below, but larger and linear expansion above, ∼300°C. Although the temperature-induced c-axis expansion coefficients seem to be less dependent on the compositional state x and on transition-metal incorporation of the Al4+2xSi2–2xO10–x mullite-type phases (α(c) = 5.6 × 10−6/°C to 6.1 × 10−6/°C), thermal a- and b-axis expansion coefficients change more significantly (α(a) = 3.1 × 10−6/°C to 4.1 × 10−6/°C and α(b) = 5.6 × 10−6/°C to 7.0 × 10−6/°C, where the values were calculated between 300° and 900°C). The larger temperature-induced b than c and a expansions probably are caused by intense lengthening of the relatively long and elastic octahedral Al(1)–O(D) bonds in mullite, which form at an angle of about 30° with b, but of about 60° with a. With increasing x value of the transition-metal-free mullites, the volume thermal expansion decreases, while the anisotropy of thermal expansion is reduced simultaneously. We believe that the variation of the thermal expansion coefficients is controlled by the Al* occupancy and by the number of O(C) vacancies in the mullite structure, and also by the disordering distribution of both structural elements. Transition-metal incorporation into mullite has no distinct influence on thermal expansion anisotropy, but does reduce thermal volume expansion. A prestressing of the crystal structure by substitution of Al3+ by the larger Fe3+ and Cr3+ ions may be the main reason for the latter effect.
161 citations
TL;DR: In this paper, a reaction kinetic study of the formation of mullite from single-phase aluminosilicate gels has been performed using dynamic X-ray diffraction.
Abstract: A reaction kinetic study of the formation of mullite from single-phase aluminosilicate gels has been performed using dynamic X-ray diffraction. In situ measurements of conversion and separate infrared spectra data indicate that the amorphous single-phase precursors form crystalline mullite at temperatures as low as 1213 K. All of the data are consistent with a nucleation rate-controlled mechanism, unlike previous work with diffusion-limited powders and diphasic gels. At temperatures below 1400 K, the conversion data are adequately fit with a binary model that can approximate a uniform distribution of activation energies. At higher temperatures, evidence indicates that diffusion mechanisms become the rate-limiting step.
132 citations
TL;DR: In this article, the authors studied the firing transformations undergone by mixtures of illitic-kaolinitic clays containing CaCO 3, free quarts, iron oxides and other impurities.
110 citations
TL;DR: X-ray analysis shows that mullite and silica are the major crystalline phases in fly ash and the weight fractions of these phases are 14.2 and 5.1 wt%, respectively as discussed by the authors.
Abstract: X-ray analysis shows that mullite and silica are the major crystalline phases in fly ash. The “method of known additions” from X-ray diffraction techniques was used to calculate changes in the significant peak intensities of mullite and silica to determine their weight fractions in fly ash. This furthers the efforts of characterizing fly ash, which are being conducted to supplement the search for applications of this abundant material. The weight fractions of crystalline mullite and silica were determined to be 14.2 and 5.1 wt%, respectively. Thermal gravimetric studies as well as SEM and particle size analysis were also conducted on the fly ash.
98 citations
TL;DR: In this paper, a continuum theory for non-Newtonian flow of a two-phase composite containing rigid inclusions is presented, which predicts flow suppression by a factor of (1 - V)q, where V is the volume fraction of the rigid inclusion and q depends on the stress exponent and the inclusion shape.
Abstract: A continuum theory for non-Newtonian flow of a two-phase composite containing rigid inclusions is presented. It predicts flow suppression by a factor of (1 - V)q, where V is the volume fraction of the rigid inclusion and q depends on the stress exponent and the inclusion shape. Stress concentrations in the rigid inclusion have also been evaluated. As the stress exponent increases, flow suppression is more pronounced even though stress concentration is less severe. To test this theory, superplastic flow of zirconia/mullite composites, in which zirconia is a soft, non-Newtonian superplastic matrix and mullite is a rigid phase of various size, shape, and amount, is studied. The continuum theory is found to describe the two-phase superplastic flow reasonably well.
96 citations
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.
90 citations
TL;DR: In this article, a method for synthesizing low density, cellular inorganic materials from sol-gels has been developed, which utilizes viscosity control during gelation to stabilize bubbles generated from freon droplets dispersed in the sol.
Abstract: A method for synthesizing low density, cellular inorganic materials from sol-gels has been developed. The method utilizes viscosity control during gelation to stabilize bubbles generated from freon droplets dispersed in the sol. Cellular silica, mullite and zirconia-toughened mullite have been fabricated with relative densities ranging from 10 to 45% and average cell sizes ranging from 30 to 1000 μm in diameter in dense matrices. This new class of low density, sol-gel-derived materials offers many opportunities in high temperature insulation, low permittivity substrates and catalyst supports, to name a few.
79 citations
TL;DR: In this article, the authors characterized the reaction series in single crystal kaolinite by transmission electron microscopy and attributed the exotherm observed at 980° C to the formation of a spinel phase.
Abstract: The kaolinite-mullite reaction series in single crystal kaolinite has been characterized by transmission electron microscopy. The exotherm observed at 980° C is attributed to the formation of a spinel phase. Mullite crystallites have also been observed with the spinel phase and both phases have a composition near that of 3∶2 mullite. Subsequent heat treatment leads to the growth of mullite crystallites on the original kaolinite plates with the c-axis perpendicular to the plate.
78 citations
TL;DR: In this article, the reaction kinetics for the formation of mullite (3Al2O3 · 2SiO2) from solgel derived precursors were studied using dynamic x-ray diffraction (DXRD) and differential thermal analysis (DTA).
Abstract: The reaction kinetics for the formation of mullite (3Al2O3 · 2SiO2) from sol-gel derived precursors were studied using dynamic x-ray diffraction (DXRD) and differential thermal analysis (DTA). The reaction kinetics of diphasic and single phase gels are compared and different reaction mechanisms are found for each gel. Mullite formation in the diphasic gel exhibits an Avrami type, diffusion-controlled growth mechanism with initial mullite formation temperatures of about 1250 °C and an activation energy on the order 103 kJ/mole. On the other hand, mullite formation from the single phase gel is a nucleation-controlled process with an initial formation temperature of 940 °C and a much lower activation energy of about 300 kJ/mole.
TL;DR: In this article, the XRD patterns of the powders calcined at 1450°C for 12 h showed that mullite and cordierite exist as two different phases, and no additional phases were observed.
Abstract: Mullite/cordierite composite powders containing different proportions of cordierite were prepared by the sol-gel method using boehmite, colloidal silica, and Mg(NO3)2·6H2O. Mullite and cordierite sols were prepared separately and mixed to form the composite sol. Mullitization temperature depends on the cordierite content in the composite. Also, α-cordierite crystallizes at a lower temperature in a mullite-rich (MC20) composite. The XRD patterns of the powders calcined at 1450°C for 12 h showed that mullite and cordierite exist as two different phases, and no additional phases were observed. The IR absorbance spectra of composites showed characteristic peak corresponding to both mullite and cordierite. The sintered density of the powders increases with temperature up to 1450°C and decreases beyound the melting point of cordierite (1455°C). The microstructure of MC30 sintered at 1440°C for 3 h consisted of acicular grains, whereas in MC40 and MC50 equiaxed grain morphology was observed under similar sintering conditions. The flexural strength and Vickers hardness decreases with the increase of cordierite content in the composite. Dielectric constant and thermal expansion showed a similar behavior.
TL;DR: In this article, two sets of Al2TiO5-based composites were prepared by reaction sintering of powder mixtures containing both ZrSiO4 and MgO.
Abstract: Two sets of Al2TiO5-based composites were prepared by reaction sintering of (a) Al2O3/TiO2/ZrSiO4 and (b)Al2O3/TiO2/ZrSiO4/MgO powder mixtures. The influence of the variation of ZrSiO4 content (0 to 10wt%) and the addition of 2 wt% MgO on the reaction-sintering process, microstructure, mechanical and thermal properties, were evaluated. ZrSiO4 addition shifted the Al2TiO5 formation to higher temperatures, whereas MgO accelerated both Al2TiO5 formation and ZrSiO4 decomposition. The presence of ZrSiO4 and an excess of Al2O3 generated a dispersion of ZrO2 and mullite particles in the grain boundaries and enhanced simultaneously the densification process. After sintering in the temperature range 1350 to 1500 ° C, the obtained composites exhibited significantly higher bending strength than the monophasic aluminium titanate (up to 80 M Pa). Al2TiO5 (80wt%)-mullite-ZrO2 composites which combined good mechanical strength (55MPa), low thermal expansion (α20−1000C < 1 × 10−6 K−1) and excellent thermal stability were obtained by reaction and sintering of powder mixtures containing both ZrSiO4 and MgO.
TL;DR: In this paper, the chemical and structural evolutions of sol-gel-derived mullite precursor powders have been characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and infrared spectroscopy (IRS).
Abstract: Monophasic and diphasic xerogels have been prepared as precursors for mullite (3Al2O3-2SiO2). Monophasic xerogel was synthesized from tetraethyl orthosilicate and aluminium nitrate nanohydrate and the diphasic xerogel from colloidal suspension of silica and boehmite. The chemical and structural evolutions, as a function of thermal treatment in these two types of sol-gel-derived mullite precursor powders, have been characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and infrared spectroscopy (IRS). Monophasic xerogel transforms to an aluminium-silicon spinel from an amorphous structure at ∼980 ° C. The spinel then changes into mullite on further heating. Diphasic xerogel forms mullite at ∼1360 ° C. The components of the diphasic powder react independently up to the point of mullite formation. The transformation in the monophasic powder occurs rapidly and yields strongly crystalline mullite with no other phases present. The diphasic powder, however, transforms rather slowly and contains remnants of the starting materials (α-Al2O3, cristobalite) even after heating at high temperatures for long periods (1600 ° C, 6 h). The diphasic powder could be sintered to high density but not the monophasic powder, in spite of its molecular-level homogeneity.
Journal Article•
TL;DR: Several alumina base materials containing about 50 to 90 wt% A12O3 (equivalent oxide content) have been investigated as coatings on silicon carbide (SiC), which were applied as aqueous slurries, which were then dried and abraded to a thickness of about 0.6 mm.
Abstract: Several alumina base materials containing about 50 to 90 wt% A12O3 (equivalent oxide content) have been investigated as coatings on silicon carbide (SiC). Coatings were applied as aqueous slurries, which were then dried and abraded to a thickness of about 0.6 mm. Coated specimens were heated to 1200° C or higher to promote bonding within the coating and to the SiC, then were thermally cycled to 1200° C as a test of integrity and adherence of the coatings. Coatings containing mullite as a major phase performed best in thermal cycle tests. Subsequently, a single-layer coating and two duplex coatings were subjected to corrosion testing at 1200° C under conditions that are very corrosive to SiC. Although cracked and porous, one of the duplex coatings provided significant protection to the SiC.
01 Jan 1990
TL;DR: In this paper, a graphite chamber was used for the reaction between samples of 45 or 55 wt% alumina and a mixture of metallurgical coke and potassium carbonate.
Abstract: A graphite chamber was used for the reaction between samples of 45 or 55 wt% alumina and a mixture of metallurgical coke and potassium carbonate. Thermal treatments were conducted at 1000°C. The results suggest that the potassium attack in silica-alumina bricks is controlled by the following reactions: K2O + SiO2→ K2O → SiO2 in the glassy matrix; 3(K2O · 2SiO2) + 3Al2O3→ 2SiO2· 3(K2O · Al2O3· 2SiO2) + 2SiO2 for short times; and K2O → Al2O3· 2SiO2+ 2SiO2· K2O · Al2O3· 4SiO2 for long times. In 55 wt% alumina bricks containing corundum and tridymite, potassium also attacks those phases forming a glassy phase. The formation of kaliophilite at the matrix/mullite grain interface causes a volumetric expansion of 55.5%, resulting in cracks in the matrix. Because the kaliophilite phase is not in equilibrion with mullite, the former will react with free silica to form leucite that is more thermodynamically stable.
15 Jun 1990-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the fiber-matrix interfacial chemistry in chemical-vapor-infiltrated SiC matrix composites utilizing NICALON SiC and Nextel 400 mullite fibers was investigated.
Abstract: The objective of this program was to investigate the fiber-matrix interfacial chemistry in chemical-vapor-infiltrated SiC matrix composites utilizing NICALON SiC and Nextel 400 mullite fibers and how this interface influences composite properties such as strength, toughness and environmental stability. The SiC matrix was deposited using three different reactants: methyldichlorosilane, methyltrichlorosilane and dimethyldichlorosilane. It was found that by varying the reactant gas flow rates, the ratio of carrier gas to reactant gas, the type of carrier gas (hydrogen or argon), the flushing gas used in the reactor prior to deposition (hydrogen or argon) or the type of silane reactant gas used, the composition of the deposited SiC could be varied from very silicon rich (75 at.%) to carbon rich (60%) to almost pure carbon. Stoichiometric SiC was found to bond very strongly to both NICALON and Nextel fibers, resulting in a weak and brittle composite. A thin carbon interfacial layer deposited either deliberately by the decomposition of methane or inadvertently by the introduction of argon into the reactor prior to silane flow resulted in a weakly bonded fiber-matrix interface and strong and tough composites. However, composites with this type of interface were not oxidatively stable. Preliminary results point to the use of a carbon-rich SiC (mixture of carbon plus SiC) interfacial zone to achieve a relatively weak, crack-deflecting fiber-matrix bond but also exhibiting oxidative stability.
TL;DR: In this paper, the authors show that initial infiltration occurred without complete filling of the porosity by the sol, which led to blockage of the pores in the surface of the preform.
Abstract: Mullite/alumina composites were fabricated by infiltrating porous alumina preforms with a hydrolyzed ethyl silicate sol. Evidence is presented which suggests that initial infiltration occurred without complete filling of the porosity by the sol. Multiple infiltrations were used to increase the amount of SiO2 introduced but led to blockage of the pores in the surface of the preform. Sintered bodies had concentration gradients, SiO2 decreasing from the surface inward. Although mullite limited grain growth in alumina, in partially modified bodies large grains (>1 mm) with a preferred orientation were observed at the interface between the two zones.
TL;DR: In this article, a mullite precursor sol was prepared by mixing boehmite and silica sols, and the mixture of 3.5 wt% F− ion as a 47% solution of HF was gelled.
Abstract: A mullite precursor sol was prepared by mixing boehmite and silica sols. On addition of 3.5 wt% F− ion as a 47% solution of HF, the mullite sol was gelled. The dried gel was ball-milled and calcined at 1400°C for 1 h in an airtight container. Mullite whiskers grew on the (111) plane along the 〈001〉 direction.
TL;DR: In this article, the best conditions for atomic imaging of 3Al2O3-2SiO2 mullite were discussed relative to the atomic resolution microscope at Berkeley, where multibeam images have been produced at a resolution better than 0.19 nm, with a firm transfer of information on the oxygen sublattice.
Abstract: The crystallographic structure of 3Al2O3-2SiO2 mullite has been studied by means of high-resolution electron microscopy. The best conditions for atomic imaging of this compound are discussed relative to the atomic resolution microscope at Berkeley. [001] multibeam images have been produced at a resolution better than 0.19 nm, allowing the cation sublattice to be directly imaged, with a firm transfer of information on the oxygen sublattice. Under optimal conditions of defocus setting and thickness, typical contrasts have been detected and consistently interpreted in terms of the presence of oxygen vacancies along the defective atomic columns. These observations are discussed in relation to the currently accepted model for the average structure of mullite.
TL;DR: In this article, an aqueous heterogeneous redox mixture containing aluminium nitrate, silica fume (soot), and urea in appropriate mole ratio, when rapidly heated to 500°C, boils, foams and ignites with a flame to yield weakly crystalline mullite powder in less than 5 min.
TL;DR: In this paper, the effects of microstructural effects on the mechanical and thermomechancical properties of mullite ceramics have been investigated, including fracture toughness and strength.
Abstract: Microstructural effects on the mechanical and thermomechancical properties of mullite ceramics have been investigated. The mullite ceramics were fabricated by reaction-sintering of mixture of kaolin and Al2O3(M-1) and by pressureless-sintering of a powder prepered from Al2O3 and SiO2 sols (M-2) and Al2O3 sol-ethylsilicate (M-3) system from the sol-gel method. Transmission electron microscopic observation revealed that samples M-1 and M-2 contained foreign phases, but that they were much less in M-2. These showed different temperature dependence in mechanical properties such as hardness, Young's modules, fracture toughness and strength. These differences were explained by the amount of grain boundary impurity phases. Thermal shock fracture behavior was also discussed.
TL;DR: In this paper, a sample of MgO-doped mullite was prepared by the sol-gel method and the three-point bend strength for 0.3 wt% Mg-O dopant at room temperature was 300 MPa and decreased below 200 MPa at 1400° C.
Abstract: Mullite doped with MgO in quantities ranging from 0.01 to 1.5 wt% were prepared by the sol-gel method. The mullitization temperature decreases with increased MgO dopant content. The XRD patterns of the MgO-doped mullite calcined at temperatures up to 1600° C for durations ranging from 1 to 10 h did not show the presence of any other phase except mullite. The IR spectrum shows a broadening of the Al-O absorbance band at 1175cm−1 with MgO content, indicating the solid solution of MgO. Sintering temperature decreases with increased MgO dopant content. The microstructure observed consisted of equiaxed grains. The TEM observation of the microstructure showed the presence of glassy pockets at the triple grain junctions. The thermal coefficient of expansion and dielectric constant were not changed up to 0.75wt% MgO dopant concentration. The three-point bend strength observed for 0.3wt% MgO-doped mullite at room temperature was 300 MPa and decreased below 200 MPa at 1400° C.
TL;DR: In this paper, the evolution of mullite in this precursor powder during heat treatment has been studied using differential thermal analysis, electron microscopy and X-ray diffraction techniques, and it is shown that during calcination below 1100°C the coprecipitate develops γ-Al2O3 and perhaps cristobalite crystallites within the basic grains, whose morphology is otherwise invariant with temperature.
Abstract: Powder of mullite composition (3Al2O3·2SiO2) has been made by a coprecipitation method. The evolution of mullite in this precursor powder during heat treatment has been studied using differential thermal analysis, electron microscopy and X-ray diffraction techniques. It is shown that during calcination below 1100°C the coprecipitate develops γ-Al2O3 and perhaps cristobalite crystallites within the basic grains, whose morphology is otherwise invariant with temperature. Mullite forms above 1100°C by reaction of these γ-Al2O3 and SiO2 crystallites, and the grain morphology changes markedly. Small exothermic events occur at 1000 and 1250 °C. The former is associated with the decomposition of a small content of aluminosilicate or perhaps with the conversion of γ- to θ-Al2O3, and the latter with mullite formation. For comparison, the behaviour of a polymeric mullite precursor during calcination is also examined. This material showed a large exothermic event at 1000°C which could be associated with the decomposition of the (amorphous) aluminosilicate to crystalline γ-Al2O3 and SiO2, and a small exothermic event at 1250° C due to mullite formation.
TL;DR: In this paper, composites of carbon/chemical-vapor-deposited (CVD) Si{sub 3}N{sub 4, carbon/CVD BN, mullite/SiC, and SiC yarn/cVD SiC prepared to determine if there were inherent toughness in these systems.
Abstract: This paper reports composites of carbon/chemical-vapor-deposited (CVD) Si{sub 3}N{sub 4}, carbon/CVD BN, mullite/CVD SiC, and SiC yarn/CVD SiC 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/Si{sub 3}N{sub 4} 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.
TL;DR: In this paper, fine alumina-silica fibres produced by Sumitomo Chemicals have been characterised up to 1200°C and have been found to retain their mechanical properties up to 800°C in both oxidising and inert atmospheres.
TL;DR: In this article, the mechanical properties of ZrSiO4 sintered bodies, fabricated from a synthesized high-purity powder, were studied at various Si/Zr atomic ratios by the observation of microstructures, and the mechanical strength was 320MPa from room temperature to 1400°C.
Abstract: Mechanical properties of ZrSiO4 sintered bodies, fabricated from a synthesized high purity ZrSiO4 powder, were studied at various Si/Zr atomic ratios of ZrSiO4 by the observation of microstructures, The ZrSiO4 powder was prepared by calcining a homogeneous mixture of ZrO2 and SiO2 sols containing ZrSiO4 seed particles, When fired at 1600° to 1700°C, ZrSiO4 sintered bodies had densities of more than 95% of the theoretical density and the grain size was about 2-4μm. The mechanical strength was 320MPa from room temperature to 1400°C. This mechanical property depended on the Si/Zr atomic ratio, and showed the best results at the equimolar composition, Sintered ZrSiO4 bodies had a thermal shock resistance superior to those of mullite sintered bodies or alumina sintered bodies having high stabilities at high temperature.
TL;DR: In this article, the oxidation kinetics of three mullite-SiC composites were studied and the nature of reaction layer formed has been analyzed specifying the oxidation rate constants for each composite.
Abstract: The oxidation kinetics of three mullite-SiC composites were studied. The materials were all processed with the same amount of SiC, 10% by volume; in two of the composites the second phase was added as whiskers, and in the third as powders. The sintered composites were exposed to high temperatures (1200 to 1500 ° C) during variable time periods (maximum 122 h) under the oxidizing furnace atmosphere. The nature of the reaction layer formed has been analysed specifying the oxidation rate constants for each composite. The influence on the bend strength of the composites for one of the isothermal oxidizing treatments has also been measured.