Oxidation behaviour of SiC-platelets and particulates-reinforced Al2O3/ZrO2 matrix composites

The oxidation behavior of hot-pressed SiC-platelets and particulates-reinforced Al₂O₃/ZrO₂ composites has been studied in an electric furnace at atmospheric pressure at different temperatures. The mass gain as a result of transformation of SiC into SiO₂ is described as a function of oxidation temperature, time and type of SiC. The mass gain up to 1100°C was low, but increased strongly at 1350°C. The oxidation process follows a parabolic rate at all oxidation temperatures. Oxidation of composites containing SiC-particulates is higher than the corresponding one containing SiC-platelets. The activation energy, obtained in the present investigation, was 297–333 kj/mol. Diffusion of oxygen and carbon monoxide through the matrix and oxide products appeared to be the rate controlling process. The reaction products were aluminosilicate glass phase and mullite as indicated by SEM and EDX.     

Effect of the fluoride additives on the oxidation of AlN

The oxidation of AlN powder added by the fluorides at temperatures below 700°C in air was discovered in this study. The obvious onset of oxidation of AlN with cryolite and YF₃ additions is below 700°C with the product of α-Al₂O₃ phase, which usually occurs in single AlN powder above 1100°C. The changes on the weight and the FTIR spectra of the AlN powder fired at temperatures lower than 700°C show that cryolite and YF₃ greatly promote the oxidation of AlN powder at these temperatures. Different from the action of cryolite and YF₃ powder, CaF₂ has no obvious effect on the oxidation of AlN. A possible oxidation process, in part corroborated by FTIR and XRF, was proposed to explain the results in the experiments. The oxidation kinetics of AlN in the presence of cryolite were also discussed at the temperatures ranging from 550 to 700°C from the data of the weight gains in this region. The result shows that the oxidation follows a linear law, which implies a reaction rate-controlled process. The considerably low activation energy of 67 kJ mol⁻¹, which is associated with the quick oxidation and the formation of α-Al₂O₃ at temperatures below 700°C, was determined from the slope of the line fit.     

An internal synthesis method for mullite-chromium carbide composite in a vacuum system

In situ formation of chromium carbide in a mullite matrix through reaction of Cr₂O₃, SiC and A₂O₃ has been studied. Three different chromium compounds, Cr₃Si, Cr₃C₂, Cr₇C₃, and mullite were formed. In a vacuum environment, the Cr₃Si particles formed first and were retained below 1550 °C, while the Cr₇C₃ phase was only dominant above 1600 °C. The Cr₃C₂ phase was the dominant dispersed phase at temperatures of 1450–1500 °C. In an argon environment, the Cr₃C₂ phase was the main product component at temperatures ranging from 1450 to 1550 °C. The mullite phase formed concurrently through the diffusion of the SiO₂ phase into the Al₂O₃. SiO₂ was the product of the reaction between Cr₂O₃ and SiC. The composite hot-pressed at 1450 °C in vacuum gave a flexural strength and fracture toughness of up to 457 MPa and 4.1 MPa m^1/2, respectively.     

Effect of ZrSiO2 and MgO additions on reaction sintering and properties of Al2TiO5-based materials

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 (_20–1000C < 1 × 10^–6 K^–1) and excellent thermal stability were obtained by reaction and sintering of powder mixtures containing both ZrSiO₄ and MgO.     

Preparation of ZrO2-Al2O3 powder by thermal decomposition of gels produced from an aluminium chelate compound and zirconium butoxide

Organic precursors containing Al and Zr atoms were synthesized from an aluminium chelate compound and zirconium n-butoxide. A ZrO₂-Al₂O₃ composite powder was prepared by the thermal decomposition of these precursors. An amorphous phase exists to higher temperatures for this ZrO₂-Al₂O₃ powder than for a comparable powder prepared from aluminium sec-butoxide and zirconium n-butoxide. In addition the tetragonal ZrO₂ phase was stabler in this ZrO₂-Al₂O₃ powder than in a comparison powder. The ZrO₂ grains were 50–500 nm in diameter and were homogeneously dispersed in the Al₂O₃ matrix after heating at 1400 °C.     

Solid state reactions in the ZrO2 · SiO2 - αAl2O3 system

Solid state reactions between ZrO₂· SiO₂ and Al₂O₃ in mixed powders were studied by quantitative X-ray diffraction, density measurements and qualitative EDAX. Data were obtained at temperatures ranging from 1400 to 1600° C for 5 h; the initial molar ratios of the reactants (Al₂O₃/ZrO₂ · SiO₂) varying from 0 to 5. The results indicate that: (1) ZrO₂· SiO₂ and Al₂O₃ react and form ZrO₂, crystalline 3Al₂O₃ · 2SiO₂ and a noncrystalline mullite phase; (2) the non-crystalline mullite phase is an important transitional phase towards equilibrium under subsolidus conditions. In the experimental conditions used the amount of the non-crystalline phase varies by as much as about 15%. This phase is of great importance in the mechanisms of reaction sintering between ZrO₂ · SiO₂ and Al₂O₃.     

Preparation and properties evaluation of zirconia-based/Al2O3 composites as electrolytes for solid oxide fuel cell systems

Yttria-doped zirconia powders containing 3 to 8 mol% Y₂O₃ and 0 to 20 wt% Al₂O₃ were prepared by both mixing commercial oxides and a coprecipitation method, and the mechanical and electrical properties have been examined as a function of the Al₂O₃ content. The bending strength of the composite at room temperature increased with increasing Al₂O₃ content. In the temperature range 500–1000 °C the bending strength increased with Al₂O₃ content up to 10 wt% and then decreased, the measured value at 1000 °C (200 MPa) being higher than those at lower temperatures for cubic zirconia materials. Fracture toughness (K_IC) decreased with increasing Y₂O₃ content in the Al₂O₃-free zirconia materials. Al₂O₃ additions enhanced the fracture toughness and this was maximum (7 MPa m^1/2) for the composite ZrO₂-3 mol% Y₂O₃/10 wt% Al₂O₃. The electrical conductivity of cubic ZrO₂/Al₂O₃ composites decreased monotonically with Al₂O₃ content, but in tetragonal ZrO₂/Al₂O₃ composites hardly varied or apparently increased up to 10 wt% Al₂O₃. At 1000 °C the highest electrical conductivity was 0.30 S cm^–1 for ZrO₂-8 mol% Y₂O₃, and this decreased up to 0.10 S cm^–1 for the composite ZrO₂-8mol% Y₂O₃/20 wt% Al₂O₃.     

Solid-solution range of mullite up to 1800 °C and microstructural development of ceramics

Tetraethoxysilane (TEOS) and Al-sec-butylate (Al-O-Bu) were used for the sol-gel synthesis of mullite ceramics. The starting materials had bulk compositions corresponding to values between 72 and 78 wt% Al₂O₃, and 28 and 22 wt% SiO₂, respectively, and were calcined at 400 °C (A-series) and 1100 °C (B-series). B-series samples, despite their higher green densities, could only be sintered to about 65–70% TD (theoretical density) at 1650 °C, whereas A-series samples achieve values of about 93–98% TD. Ceramics with relatively high amounts of glass phase from large tabular mullite crystals, which are embedded in a finer-grained mullite matrix. As soon as the bulk Al₂O₃ content increases, equiaxed mullite grains appear and the mean grain size becomes smaller, showing a significant difference between the nucleation and crystal growth mechanisms of mullites formed in samples with the lower and higher Al₂O₃-bulk compositions. Depending on the bulk composition of the samples, the temperature-controlled solid-solution of mullite ranges between about 72.7 and 74.3 wt% Al₂O₃ at 1600 °C and 74.1 and 75.4 wt% Al₂O₃ at 1800 °C, indicating that the solid-solution region bends over towards the Al₂O₃-side of the Al₂O₃-SiO₂ phase diagram.     

Multicomponent toughened ceramic materials obtained by reaction sintering

Fully dense zirconia toughened ceramics with a mullite matrix from the basis of information on the quaternary system ZrO₂-Al₂O₃-SiO₂-CaO, in a temperature range as low as 1425 to 1450° C, have been obtained by reaction sintering of zircon/alumina/calcium carbonate mixtures. The shrinkage, advance of reaction, microstructure and mechanical properties of different compositions are reported. The results are explained in terms of a transitory liquid phase that appears at temperatures lower than 1400° C.     

Phase transformations in xerogels of mullite composition

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 (-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.     

Alumina-chromium carbide composite through an internal synthesis method

In situ formation of chromium carbide particles, through a solid state reaction between Cr₂O₃ and SiC, for strengthening AI₂O₃ has been studied. Three kinds of chromium compound, Cr₃Si, Cr₃C₂ and Cr₇C₃ and mullite were formed in the alumina matrix. The reaction behaviour during hot pressing depends on heating parameters such as temperature and atmosphere. In a vacuum environment, the Cr₃Si particles formed first and was the dominant dispersed phase below 1550°C, while the Cr₇C₃ phase was only dominant above 1600°C. The Cr₃C₂ phase emerged briefly then diminished at temperature 1500°C. In an argon environment, however, the Cr₃C₂ phase was the main product component at temperatures ranging from 1450–1550 °C. The mullite phase formed concurrently through the diffusion of SiO₂ phase into the Al₂O₃ matrix, which is a by-product from the reaction between Cr₂O₃ and SiC. Incorporating chromium carbide or suicide particles into the Al₂O₃ matrix induces a strengthening effect. However, only when the content of dispersed phases is low and is mainly of Cr₃C₂ particles, is the strengthening effect significant. For instance, the composite, containing 5 vol% chromium carbide and hot-pressed at 1500°C in argon, gives a flexural strength and fracture toughness up to 600 MPa and 6.1 MPam^1/2, respectively.     

Initial stages of metal-organic chemical-vapor deposition of ZrO2 on a FeCrAl alloy

The initial stages of metal-organic chemical-vapor deposition of ZrO₂ on a model FeCrAl alloy was investigated using synchrotron radiation photoelectron spectroscopy, X-ray absorption spectroscopy, scanning Auger microprobe, and time of flight secondary mass spectrometry. The coatings were grown in ultra-high vacuum at 400 °C and 800 °C using the single source precursor zirconium tetra-tert-butoxide. At 400 °C the coatings mainly consist of tetragonal ZrO₂ and at 800 °C a mixed ZrO₂/Al₂O₃ layer is formed. The Al metal diffuses from the FeCrAl bulk to the metal/coating interface at 400 °C and to the surface of the coating at 800 °C. The result indicates that the reaction mechanism of the growth process is different at the two investigated temperatures.     

Preparation and study of YSTZ-Al2O3 nanocomposites

Yttria stabilized zirconia-alumina (YSTZ-Al₂O₃) nanocomposite system with various Al₂O₃ concentrations has been synthesized by sol-gel route. The experimental techniques XRD, DTA, TGA, FT-Raman, FT-IR, SEM, Vickers hardness measurements, density measurements and Impedance spectroscopy were used to characterize the synthesized specimens. DTA result shows two exothermic reactions: one around 760°C and another around 960°C. XRD results confirm that the specimen starts to crystallize on heating above 750°C. Well resolved XRD reflections corresponding to tetragonal (t) ZrO₂ were obtained after the specimens were heated at 1000°C. FT-Raman results confirmed that the crystallites developed above 750°C was t-ZrO₂. It was observed from the XRD and DTA results that the bulk and grain boundary region crystallize independently in two different temperatures with a difference in temperature of about 200°C. The crystallization temperatures increase with Al₂O₃ contents. At 1300°C, the pure YSTZ and 5 and 10 wt % Al₂O₃ added YSTZ specimens underwent structural transformation from tetragonal to monoclinic ZrO₂. But, the tetragonal symmetry remains stable at 1300°C with an addition of 15 wt % Al₂O₃. The system which retain its tetragonal symmetry at its processing temperature (1300°C) gives high hardness and maximum density values. Almost 100% theoretical density value was obtained at 1300°C with an addition of 15 wt % of Al₂O₃.     

Formation and hot isostatic pressing of ZrO2 solid solution in the system ZrO2-Al2O3

In the system of ZrO₂-Al₂O₃, cubic ZrO₂ solid solutions containing up to 40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminium alkoxides. At higher temperatures, they transform into tetragonal solid solutions. Metastable ZrO₂ solid solution powders containing 25 mol% Al₂O₃ have been sintered at 1000–1150 °C under 196 M Pausing the hot isostatic pressing technique. The solid solution ceramics consisting of homogeneous microstructure with an average grain size of 50 nm exhibited a very high fracture toughness of 23 MN m ^–1.5. They have been characterized by X-ray diffraction and electron probe surface analyses.     

Liquid immiscibility and crystallization in refractory phosphate glasses

The nature and evolution of liquid immiscibility and essentially complete crystallization in an optimized glass composition containing Al₂O₃, SiO₂ and P₂O₅ are examined from the results of electron microscopy, X-ray diffraction and density changes. Phase separation occurs during cooling from the melt; subsequent reheating to 850° C causes crystal nucleation followed by growth of the refractory phases of mullite (nominally 3Al₂O₃-2SiO₂) and AlPO₄ at approximately 1050 and 1160° C, respectively. Additional crystallization of these phases also occurs at 1400° C. Similar materials containing additions of ZrO₂ and Y₂O₃ are also discussed.When this work was conducted, the writers were Graduate Research Assistant and Associate Professor of Materials Engineering, respectively. Mr Lee is currently at Ohio State University.     

Sintering of mullite-based particulate composites containing ZrO2

ZrO₂ and its modified versions containing MgO and Y₂O₃ were selected as particulate reinforcement in order to achieve better mechanical properties of mullite. Particulate composites up to 25 vol % ZrO₂ and its modifications were pressed to 55% relative density at 300 MPa followed by sintering at 1600°C and 1650°C for one hour. Studies were conducted on fracture toughness, transverse rupture strength, hardness, dielectric constant, microstructure and fractography. Composites sintered at 1650°C were found superior in properties than those at 1600°C. The maximum strength of mullite composites was observed at a composition of 10 vol % ZrO₂.     

Crystallization of a coprecipitated mullite precursor during heat treatment

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 -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 -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 - to -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 -Al₂O₃ and SiO₂, and a small exothermic event at 1250° C due to mullite formation.     

Sliding wear of self-mated Al2O3-SiC whiskerreinforced composites at 23–1200°C

Microstructural changes occurring during sliding wear of self-mated Al₂O₃-SiC whiskerreinforced composites were studied using optical, scanning electron microscopy and transmission electron microscopy. Pin-on-disc specimens were slid in air at 2.7 m s^–1 sliding velocity under a 26.5 N load for 1 h. Wear tests were conducted at 23, 600, 800 and 1200°C. Mild wear with a wear factor of 2.4 x 10^–7–1.5 x 10^–6 mm³ N^–1 m^–1 was experienced at all test temperatures. The composite showed evidence of wear by fatigue mechanisms at 800°C and below. Tribochemical reaction (SiC oxidation and reaction of SiO₂ and Al₂O₃) leads to intergranular failure at 1200°C. Distinct microstructural differences existing at each test temperature are reported.Resident Research Associate at NASA Lewis Research Center.     

Glass formation and phase transformations in plasma prepared Al2O3-SiO2 powders

The structure of Al₂O₃-SiO₂ sub-micron powders prepared by oxidation of mixed aluminium-silicon halides in an oxygen-argon high frequency plasma flame has been studied. The powders were completely amorphous up to at least 52 wt % Al₂O₃ and partially amorphous in the range 52 to 88 wt % Al₂O₃. The crystalline phase was mullite up to 75 wt % Al₂O₃ but at higher Al₂O₃ contents a metastable solid solution of SiO₂ in -Al₂O₃ was observed in addition to mullite. Amorphous particles crystallized to mullite on heating to 1000°C, independently of composition. Extension of glass formation towards the high Al₂O₃ end of the Al₂O₃-SiO₂ system as the cooling rate is increased and particle size decreased, may be explained by the effect of viscosity on the nucleation rate of mullite from liquid, for Al₂O₃ contents up to 60 wt %. The viscosity change is relatively small as the Al₂O₃ content is increased beyond 60% and it is suggested that the change in nucleus-liquid interfacial energy with composition is the predominant factor controlling nucleation rate in this range. At Al₂O₃ concentrations greater than approximately 80 wt %, -Al₂O₃ is the phase which nucleates from the melt. A double DTA peak was observed for powders containing more than 80 wt % Al₂O₃. The lower temperature peak is believed to arise from the formation of mullite from a metastable solution of SiO₂ in -Al₂O₃, and the higher temperature peak from crystallization of mullite from the amorphous phase. The presence of SiO₂ in solution in metastable Al₂O₃ increases the temperature of transformation to -Al₂O₃ to greater than 1500° C compared with 1230° C for pure Al₂O₃.     

Use of oxygen isotope to study the transport mechanism during high temperature oxide scale growth

Abstract

The examination of high temperature (HT) oxide scale growth mechanisms was performed using secondary ion mass spectrometry (SIMS) and secondary neutral mass spectrometry (SNMS), in conjunction with ¹⁶O₂/¹⁸O₂ HT oxidation experiments. Cr₂O₃, NiO, ZrO₂ and Al₂O₃ were studied because they constitute excellent representative thermally grown oxide scales: they grow by cationic diffusion (Cr₂O₃, NiO), anionic diffusion (ZrO₂) or mixed anionic-cationic diffusion (Al₂O₃). The oxidation tests were performed first in ¹⁶O₂ and subsequently in ¹⁸O₂ at several temperatures (600–1000°C for NiO, 600°C for ZrO₂, 1000°C for Cr₂O₃ and 1100°C for Al₂O₃). The oxygen isotope distribution observed by SIMS and SNMS profiles are discussed and related with the HT oxidation mechanisms proposed in the literature.