Coprecipitation-derived mullite and mullite-zirconia composites

Precursor powders of mullite-zirconia (0–40 wt% ZrO₂) were prepared by a hydroxide coprecipitation method and their behaviour during calcination between room temperature and 1500 °C was studied using thermal analysis, X-ray diffraction and electron microscopy. The only crystalline phases present in the precalcined powders were bayerite and gibbsite, and these were stable up to 250 °C. Powders containing ZrO₂ were initially amorphous, but on calcination between 250 and 850 °C produced different crystalline phases at temperatures which depended on the amount of zirconia present. Thus in the case of mullite-40 wt% ZrO₂, zirconia crystallized at about 850 °C and was stable up to 1200 °C, when it reacted with free silica to form zircon (ZrSiO₄). Mullite formed above 1250 °C at the expense of zircon and remained stable at higher temperatures. The oxide powders were very homogeneous, and on sintering produced ceramics with a fine-grained uniform microstructure. The powders were very reactive and could be sintered conventionally to near-theoretical density at 1600–1700 °C without sintering aids. The fracture strength of mullite was about 275 MPa, and this could be improved to 350 MPa by hot isostatic pressing the presintered bodies. Addition of zirconia enhanced the sintering kinetics as well as the fracture strength of mullite.     

Alumina-mullite-zirconia composites obtained by reaction sintering: Part I. Microstructure and mechanical behaviour

Zirconia particles can be added to the matrix to overcome the brittleness inherent in ceramic materials, thereby strengthening the material through tetragonal-monoclinic phase transformation of the zirconia. This work focuses on the effect of the percentage of zirconia and mullite in the mechanical and thermomechanical properties of alumina-mullite-zirconia composites that were obtained by reaction sintering of alumina and zircon. Different samples were processed, resulting in composites with an alumina matrix, which was always volumetrically predominant. A percentage of alumina and mullite with maximized mechanical and thermomechanical properties was observed. This maximization is discussed in terms of the microstructure obtained for the composites mentioned above. The toughening mechanisms provided by zirconia and mullite inclusions, based not only on the R-curve behaviour but also on the analysis of the fracture surface, are also discussed in this report. An additional paper will be forthcoming, containing detailed discussions concerning the R-curve behaviour of the same composites.     

Effect of CeO2 on reaction-sintered mullite-ZrO2 ceramics

The effect of ceria on mullite formation and the sintering of zircon and alumina powders was investigated. Quantitative X-ray powder analysis was used to determine the formation of mullite and zirconia of both monoclinic and tetragonal forms. Scanning electron microscopy and electron-probe microanalysis were used for microstructural analysis. It was found that the addition of CeO₂ enhanced the formation of mullite and increased the fraction of tetragonal zirconia. The addition of CeO₂ caused the formation of mullite directly from reaction of zircon with alumina without decomposition of zircon into zirconia and silica. In addition to forming a liquid phase, the ceria essentially formed a solid solution with zirconia. The fracture toughness of the mullite-zirconia composites was about 5.5–6.0 MPa m^1/2.     

Microstructures and mechanical properties of mullite-(yttria, magnesia- and ceria-stabilized) zirconia composites

The microstructures and some mechanical properties of composites containing mullite and each of three different zirconias stabilized with low concentrations of yttria, magnesia, and ceria, have been studied. A sol-gel derived, high-purity, mullite was used as a matrix phase. In the present study, composites were prepared by conventional sintering of mullite and zirconia milled powder mixture. In all the composite materials, large fractions of the tetragonal zirconia (ZrO2) transformed into monoclinic form during cooling from the fabrication temperature. In the use of ceria-stabilized ZrO2, large internal macroscopic voids appeared in the sintered body. The thermal expansion hysteresis associated with the tetragonal/monoclinic transformation was evident only in the mullite/yttria-stabilized ZrO2 composite from which the Ms temperature could be determined. This revised version was published online in November 2006 with corrections to the Cover Date.     

Microstructure and mechanical properties of mullite/ZrO2 composites

The sintering, microstructure and toughness of mullite/ZrO₂ composites with increasing amount of ZrO₂ (0 to 20 vol %) have been studied. A very active premullite powder has been used as matrix. TheK _IC values increase from 2.1 to 3.2 MN m^?3/2 as the volume fraction of zirconia increases from 0 to 0.2. The realtive fraction of tetragonal zirconia decreases as the volume fraction of ZrO₂ increases to reach ～0.1 in the sample with 0.2 volume fraction of ZrO₂. The presence of ZrO₂ enhances the sintering rate and end-point density of the composites. Finally, the increasing toughness in mullite/ZrO₂ composites has been explained by a grain boundary strengthening mechanism produced by a metastable solid solution (～0.5 wt %) of ZrO₂ in mullite.     

Fractographic study of the alumina and zirconia particles embedded in mullite prepared by reaction sintering

Four different mullite-alumina-zirconia composites have been prepared by reaction sintering between alumina and zircon powders using magnesia or spinel (MgAl₂O₄) to increase the sintering and reaction rates. The microstructure of these materials can be described as composed of two parts: the first one is the mullite matrix containing various kinds of zirconia and alumina particles, whereas the second part is an amorphous phase in which alumina submatrices, zirconia and spinel particles are embedded. Examination of fracture surfaces allows one to identify the crack paths and shows that the main differences are related to zirconia inclusions. Analysis of mechanical properties and fracture features leads to the conclusion that crack deflection and microcracking are operative toughening mechanisms for the various materials. Moreover, a crack bowing mechanism is proposed to explain the higher modulus of rupture found for the series of materials prepared with magnesia as a reaction sintering aid. On leave from Instituto de Ceramica Y Vidrio, CSIC, Arganda del Rey, Madrid, Spain.     

硅源对莫来石结合刚玉耐火材料性能的影响

以板状刚玉为骨料,莫来石为结合相,并以高岭土和无定型SiO2微粉作为合成莫来石的硅源,采用反应烧结工艺制备莫来石结合刚玉耐火材料,旨在降低莫来石的烧结成本及获得理想的显微结构.研究表明：以高岭土为硅源,莫来石生成量低于理论量,但生成温度较低,1550℃前已完成莫来石化反应,显微结构较均匀,烧结程度较好,莫来石呈长柱状;...     

Densification and crystallization in crystallizable low temperature co-fired ceramics

This article reports the impact of one critical process parameter, the heating rate during sintering from 530 to 850 °C, on the densification of a crystallized low temperature co-fired ceramics (LTCC). At a low heating rate the densification of LTCC is impeded by the competing crystallization processes, resulting in less shrinkage, lower density and consequently lower dielectric constant. Microstructural evidence is provided to analyze the multiple crystalline phases formed during sintering process. It is concluded that an optimized sintering profile should have a heating rate that allows full densification prior to onset of crystallization, follow by a full crystallization to limit the amount of residual glass for enhanced dielectric properties.     

Sintering and crystallization of mullite powder prepared by sol-gel processing

Mullite powder with the stoichiometric composition (3Al₂O₃.2SiO₂) was synthesized by a sol-gel process, followed by hypercritical drying with CO₂. Within the limits of detection by X-ray diffraction, the powder was amorphous. Crystallization of the powder commenced at 1200 °C and was completed after 1 h at 1350 °C. In situ X-ray analysis showed no intermediate crystalline phases prior to the onset of mullite crystallization and the pattern of the fully crystallized powder was almost identical to that of stoichiometric mullite. The synthesized powder was compacted and sintered to nearly theoretical density below 1250 °C. The microstructure of the sintered sample consisted of nearly equiaxial grains with an average size of 0.2 m. The effect of heating rate (1–15 °C min^–1) on the sintering of the compacted powder was investigated. The sintering rate increased with increasing heating rate, and the maximum in the sintering curve shifted to higher temperatures. The sintering kinetics below 1150 °C can be described by available models for viscous sintering.     

Sintering Characteristics of a Powder Injection Molded Ceria-Stabilized Zirconia–Mullite Composite

Powder injection molding (PIM) technology has the potential for economically manufacturing several complex-shaped zirconia–mullite components in mass production. The sintering behavior of a zirconia–mullite composite fabricated by injection molding was analyzed in this paper. The focus of this study is to assess the dependence on properties and microstructure on PIM processing conditions. The sintered density of the samples displayed a strong dependence on sintering temperature. The hardness of the samples followed a similar trend as sintered density. A maximum fracture toughness of 4.1 ± 0.3 MPa · m^1/2 and strength around 450 ± 60 MPa was observed for samples sintered at 1500°C for 4 h. The properties from this study are significantly higher than the values reported in majority of the prior studies where other technologies like uniaxial and cold isostatic pressing were used to fabricate zirconia–mullite composites. The above results support the suitability of PIM as a manufacturing process for complex-shaped zirconia–mullite components with good mechanical properties.     

Fabrication and characterization of nanocrystalline oxides by crystallization of amorphous precursors

Amorphous zirconia and alumina powders were produced by the electrochemical deposition. Hot-pressing of the zirconia powder at 2.5 GPa and 360°C for 30 min caused simultaneous consolidation of the powder to a dense body and crystallization of a nanocrystalline tetragonal phase. Cold-pressing of the same powder at 2.5 GPa and sintering at 600°C or 800°C for 1 hr resulted in large tetragonal crystallites within the highly porous compacts. Cold-pressing of alumina powder at 2.5 GPa and sintering at 500°C, or 900°C for 2 hrs resulted in partial crystallization of the amorphous phase to a mixture of various polymorphs of alumina. The microstructure was inhomogeneous and composed of both nanometer and submicrometer grains. Microhardness of the hot-pressed partially crystallized nanocrystalline tetragonal zirconia was comparable to that of single crystal monoclinic phase, and decreased after annealing at 450°C for 30 min. Microhardbess of the nanocrystalline alumina was lower by on order of magnitude than that of conventional polycrystalline alumina. The decrease in the microhardness of the sintered specimens was related both to the grain size and the porosity.     

Crystallizability and sinterability of a precursor prepared by oxalate method in ethanol solution

A precursor of yttria-stabilized zirconia (YSZ) powder was prepared by the oxalate method in ethanol solution. The crystallizability and sinterability of the precursor were studied. The crystallizability of the precursor depends on preparation conditions, especially acidity. The final acidity of the solution from which the precursor is produced and the temperature at which the precursor is calcined can strongly influence the crystalline phase composition of the resultant powder, and can, in turn, limit the ability of a sintering compact to reach high density. The YSZ powder which results from calcining the precursor without milling the calcined powder for a long time, may be a less-agglomerated powder with a crystallite size of about 9 nm and a specific surface area of 46.3 m²g^–1. A full tetragonal zirconia polycrystal with 99% theoretical density can be obtained from the highly reactive powder by simple cold-pressing followed by pressureless sintering in air at temperatures as low as 1300 °C.     

Sintering and crystallization of a glass powder in the MgO-Al2O3-SiO2-ZrO2 system

The sintering and crystallization behaviour was studied of a glass powder in the MgO-Al₂O₃-SiO₂-ZrO₂ system in which the main crystal phases to form are clino-enstatite (MgSiO₃) and cubic zirconia (c-ZrO₂). During isothermal, atmospheric sintering of the glass powder, a fine dispersion of c-ZrO₂ particles, 50–100 nm diameter, was observed to form, but this did not appear to inhibit the sintering process. Nucleation of the main crystal phase, clino-enstatite, occurred both within the original glass powder particles and at the former particle surfaces, but the rate of crystallization was greater at the former particle surfaces. The c-ZrO₂ precipitates are thought to act as nucleation sites for the crystallization of the clino-enstatite. Relative densities of up to 98% were attainable during sintering, and were reached at a stage where a significant degree of crystal phase development had already taken place, proving that completion of sintering prior to the commencement of crystallization is not always a pre-requisite for the attainment of high final densities. In the material studied, the large volume contraction ( 11%) on crystallization and the possible release of dissolved gases led to a decrease in relative density as crystallization proceeded. The relative density after complete crystallization was found to be 94%±1%, irrespective of the temperature and duration of the initial sintering stage of heat-treatment, and it appeared that most of the residual porosity was a result of the volume contraction on crystallization rather than poor densification during sintering.     

Phase stability and microstructure of some gel-derived anorthite-based hybrid materials

The technical feasibility for producing five different anorthite-based binary and ternary hybrid ceramics containing zirconia and/or silicon carbide whiskers and/or gel-derivedin situ formed mullite whiskers was examined. The crystallization behaviour of the anorthite and the mullite gels and the phase stability of the hybrid ceramics were studied by X-ray diffractometry. The densification behaviour of the gel-derived materials, including the binary and the ternary composition materials, was examined by measuring densities of the sintered specimens by the immersion method. The microstructures were studied by scanning electron microscopy, supplemented by energy dispersive X-ray spectrometry. The results show the technical feasibility for producing anorthite-based fully dense binary and ternary hybrid ceramics of stable compositions containing zirconia and/or silicon carbide whiskers. However, the compositions containing mullite as a constituent produced hybrid ceramics within situ formed rod-like corundum crystals as the dispersed phase. Discrete monoclinic zirconia was present in all compositions containing this material.     

Microstructural development of woven mullite fibre-reinforced mullite ceramic matrix composites by infiltration processing

Mullite fibre (Nextel 720?)-reinforced mullite ceramic matrix composites (CMCs) with zirconia weak interface were fabricated from heterocoagulated nano-size boehmite/amorphous silica powder particles dispersed in water, using electrophoretic deposition (EPD) and pressure filtration (PF). The nano-size mullite precursor was first prepared and characterised in terms of short-range particle–particle interactions and particle size distribution. Woven Nextel 720 mullite fibres were first desized and then coated with hydrothermally derived zirconia using dip-coating. EPD was performed under constant voltage conditions with varying deposition times, to infiltrate the dispersed powder suspensions into mullite fibre preforms, enabling the parameters necessary for good deposition of stoichiometric mullite to be established. EPD formed bodies were further consolitated using PF. The EPD/PF prepared green body specimens were dried under controlled atmosphere conditions before being sintered at 1200°C for 2 h in air. Mullite fibre mats were fully infiltrated using EPD parameters of 12 V DC applied voltage with 4 min deposition time, then eight EPD infiltrated fibre mats were further consolidated together using PF. The resulting CMC produced contained 35 vol% fibre loading and showed 81% theoretical density aftersintering at 1200°C for 2 h.     

Effect of various oxide additives on sintering of BaO-SiO2 system glass-ceramics

Glass-ceramics can be produced by sintering and crystallization of a pressed glass powder pellet. Crystallization prior to complete densification results in a porous glass-ceramic. A small quantity of various oxides was added to and melted with 35.7 BaO-64.3 SiO₂ (mol %) glass and the sintering characteristic of the glass powder was evaluated in terms of the density relative to the bulk glass density after heating at a constant rate. Some oxide additives, such as Al₂O₃ and ZrO₂, increased the per cent relative density while others, such as Na₂O, decreased it. The achieved per cent relative density was compared with crystallization characteristics determined by differential thermal analysis (DTA) and viscosity determined by the beam-bending method. The per cent relative density showed a good correlation with the viscosity at the crystallization temperature, the higher per cent relative density being achieved for systems with the lower viscosity at the crystallization temperature.     

Sintering of heterogeneous ceramic compacts

The sintering behaviour of alumina powder compacts containing inclusions has been studied. The densification rate is significantly retarded by the addition of coarse, dense, fused alumina particles. The influences of inclusion volume fraction, of inclusion size, of matrix density and of temperature on the reduction in densification rate are reported. A method is proposed for the evaluation of an effective back stress which opposes densification and which is generated by the presence of the inclusions.     

Templated grain growth of textured mullite/zirconia composites

Mullite is an attractive material for advanced ceramic applications, but its low fracture toughness prevents it from widespread industrial applications. Therefore, mullite/zirconia composites were prepared from a reactive mixture of alumina and zircon with additives of TiO₂ and MgO to increase mechanical properties and densification. <001> aluminum borate templates were used to nucleate, and texture mullite in [001]. Mullite/zirconia formation started at 1350 °C and was complete at 1450 °C. Dense mullite/zirconia composites with highly textured mullite were produced after sintering at 1450 °C. A relatively constant tetragonal ZrO₂ content of 11±2 wt.% was retained at room temperature after sintering between 1350 and 1550 °C. A high quality of texture with an orientation parameter of 0.22 and a very narrow distribution of elongated mullite grains within 8.8° around [001] were successfully produced.     

Suspension characteristics and rheological properties of mullite/zirconia powder in methyl isobutyl ketone

Particle size and its distribution, sedimentation bulk density and rheology of mullite, zirconia, and mullite/zirconia mixed suspensions have been studied in terms of oxide loading (20, 30 vol%), and types of additives (dispersant, dispersant/plasticizer, dispersant/plasticizer/binder). Polyester/polyamine, dibutyl phthalate, poly(vinyl butyral), and methyl isobutyl ketone have been used as the dispersant, plasticizer, binder, and liquid medium, respectively. Sedimentation density significantly increased upon adding dispersant; the effect was more pronounced with zirconia suspension most likely due to the fine and hence high specific surface area of zirconia. With further addition of plasticizer and plasticizer/binder, the sedimentation density decreased. The suspension viscosity generally behaved in an opposite manner to the sedimentation density, i.e., low sedimentation density gave high low-shear viscosity, indicative of high order structure formation in the suspended particles. High shear rate rheology showed a shear thinning and its onset began at lower shear rate with higher solid loading. Mullite/zirconia mixed suspension gave intermediate sedimentation and rheological behavior, implying the two types of particles are non-interacting.