Fabrication of Mullite and Mullite-Matrix Composites by Transient Viscous Sintering of Composite Powders

Mullite was fabricated by a process referred to as transient viscous sintering (TVS). Composite particles which consisted of inner cores of α-alumina and outer coatings of amorphous silica were used. Powder compacts prepared with these particles were viscously sintered to almost full density at relatively low temperatures (∼1300°C). Compacts were subsequently converted to dense, fine-grained mullite at higher temperatures (∼1500°C) by reaction between the alumina and silica. The TVS process was also used to fabricate mullite/zirconia/alumina, mullite/silicon carbide particle, and mullite/silicon carbide whisker composites. Densification was enhanced compared with other recent studies of sintering of mullite-based composites. This was attributed to three factors: viscous flow of the amorphous silica coating on the particles, avoidance of mullite formation until higher temperatures, and increased threshold concentration for the development of percolation networks.     

Mullite/Alumina Particulate Composites by Infiltration Processing

A method has been developed for incorporating mullite into alumina by infiltrating an alumina preform with a prehydrolyzed ethyl silicate solution, followed by heating to decompose the infiltrant, form mullite, and densify the mullite/alumina composite. It has been found that the major portion of the weight loss of gelled ethyl silicate sols occurs in the 250° to 350°C range. Mullite formed in infiltrated bodies at ∼1475°C and specimens containing 12 to 15 vol% mullite reached 98.5% of the theoretical density after heating for 2 h at 1650°C. The mullite was found to be well dispersed within the alumina matrix and its presence decreased grain growth in the alumina by more than an order of magnitude.     

Preparation of Mullite Cordierite Composite Powders by the Sol-Gel Method: Its Characteristics and Sintering

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.     

Mullite Alumina Particulate Composites by Infiltration Processing: II, Infiltration and Characterization

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.     

Colloidal Processing and Mechanical Properties of Whisker-Reinforced Mullite Matrix Composites

Aqueous colloidal suspensions in the two systems of CVD-processed ultrafine mullite powder (<0.1 μm), -Si₃N₄ whisker and -mullite whisker, were prepared near the isoelectric point of mullite (pH 7.0) to prevent cracking during drying of wet green compacts consolidated by filtration. The freeze-dried porous green compacts were hot-pressed with a carbon die at 1500°C for 1 h at a pressure of 39 MPa in N₂ atmosphere. The relative densities of the mullite matrix composites with whiskers of 0 to 10 vol% were in the range of 95.2% to 99.8%. Increasing the fraction of Si₃N₄ whisker increased the density, flexural strength, and fracture toughness of the hot-pressed composites. On the other hand, addition of the mullite whisker increased the fracture toughness but decreased the density and strength of the composites.     

颗粒涂层法制备莫来石瓷的研究

把吸附水分的α-Al_2O_3湿粉分散到含表面活性剂硬脂胺、Span-60的80Vol％环已烷-20Vol％正硅酸乙脂(TEOS)疏水溶剂中,经长时间的密封搅拌处理,制得了α-Al_2O_3颗粒表面有无定形SiO_2涂层的复合粉料。用透射电镜观察和ζ-电位测量相结合的方法证明在α-Al_2O_3颗粒表面得到了SiO_2涂层。对复合粉料进行了热分析和物相分析。复合粉料成型后,研究了坯体的烧结过程和烧结中的物相变化情况。试样在1600℃烧结得到接近致密、晶粒尺寸均匀、细小的莫来石瓷。根据实验现象对颗粒涂层过程和坯体的致密化过程进行了详细探讨。     

Mullite/Alumina Particulate Composites by Infiltration Processing: III, Mechanical Properties

The effect of incorporating mullite into alumina by an infiltration process on the mechanical properties was investigated. Data for Young's modulus, strength, and fracture toughness for various composite compositions were compared with those for the unreinforced matrix (alumina). Measurements of Young's modulus by a resonance technique showed that the addition of mullite decreased Young's modulus. Up to 14 vol%, these changes were close to those expected, but above this mullite content, the decrease was more dramatic and indicated specimen damage during processing. The addition of mullite led, in some cases, to increases of more than 60% in both the strength (biaxial flexure) and indentation fracture toughness. These increases have been attributed to the method of introducing mullite and the resulting residual compressive surface stresses. The strength of the indented composite bodies deviated from the ideal behavior, indicating the probability of R -curve behavior in these materials.     

Hybrid Gels for Homoepitactic Nucleation of Mullite

Hybrid gels, defined as gels from mixtures of polymerically and colloidally derived sols, offer many opportunities for crystalline microstructure development upon heating. In this study, hybrid mullite gels are formed by mixing a colloidal boehmite—silica sol with a polymeric aluminum nitrate—tetraethoxysilane-derived sol. The polymeric gel crystallizes in situ to form mullite that acts as seed crystals for homoepitactic nucleation during the subsequent transformation of the colloidal component of the hybrid gel. Compared with the entirely colloidal gel, the introduction of a 30 wt% polymeric gel fraction results in an increase in apparent nucleation frequency from ∼5x10¹¹ to ∼1x10¹⁴ nuclei / cm³ at 1375°C, a reduction in high-temperature grain size from 1.4 to 0.4 μm at 1550°C, and an increase in the degree of microstructural homogeneity, as evidenced by intragranular pore removal.     

Mullite Whiskers from Precursor Gel Powders

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.     

Upper Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS

Current state-of-the-art environmental barrier coatings (EBCs) for Si-based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al₂O₃·2SiO₂) or mullite + BSAS ((1− x )BaO· x SrO·Al₂O₃·2SiO₂, 0 ≤ x ≤ 1) layer, and a BSAS top coat. Areas of concern for long-term durability are environmental durability, chemical compatibility, volatility, phase stability, and thermal conductivity. Variants of this family of EBC were applied onto monolithic SiC and melt-infiltrated SiC/SiC composites. Reaction between BSAS and silica results in a low-melting (∼1300°C) glass, which can cause the spallation of the EBC. At temperatures greater than ∼1400°C BSAS suffers significant recession via volatilization in water-vapor-containing atmospheres. Both reactions can be EBC life-limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. Initial rapid increase in thermal conductivity at temperatures as low as 1300°C indicates the sintering of EBC.     

Fabrication and Properties of Low-Shrinkage Reaction-Bonded Mullite

Mullite ceramics were fabricated according to the recently developed reaction-bonded Al₂O₃ (RBAO) technology. Green compacts consisting of mechanically alloyed Al, SiC, and Al₂O₃ were heat-treated in two steps. During the first hold at 1200°C, Al and SiC were oxidized to form Al₂O₃ and SiO₂. On further heating, mullite was formed which then sintered during the second hold at 1550°C. All reactions involved in the process were associated with volume expansions that almost compensated for the shrinkage on sintering. Processing details and microstructure development are discussed. Reaction-bonded mullite ceramics exhibit high fracture strength, e.g., 290 MPa at a density of 97% of theoretical density.     

Optimization of Chemical Vapor Deposition Parameters for Fabrication of Oxidation-Resistant Mullite Coatings on Silicon Nitride

Fabrication of mullite (3Al₂O₃·2SiO₂) coatings by chemical vapor deposition (CVD) using AlCl₃–SiCl₄–H₂–CO₂ gas mixtures was studied. The resultant CVD mullite coating microstructures were sensitive to gas-phase composition and deposition temperature. Chemical thermodynamic calculations performed on the AlCl₃–SiCl₄–H₂–CO₂ system were used to predict an equilibrium CVD phase diagram. Results from the thermodynamic analysis, process optimization, and effects of various process parameters on coating morphology are discussed. Dense, adherent crystalline CVD mullite coatings ∼2 μm thick were successfully grown on Si₃N₄ substrates at 1000°C and 10.7 kPa total pressure. The resultant coatings were 001 textured and contained well-faceted grains ∼0.3–0.5 μm in size.     

Desiliconization of Mullite Felt

High-temperature evaporation from 80% porous, rigid mullite (3Al₂O₃·2SiO₂) whisker felt was studied under vacuum and at various helium pressures using gravimetry, X-ray diffractometry, SEM, and EDS. Heat treatments at 1350° to 1550°C resulted in evaporation of SiO₂ from mullite with a rate strongly dependent on temperature and pressure. A concentration gradient of SiO₂ was observed through the cross section of samples after heating under vacuum, indicating that SiO₂ preferentially evaporated from whiskers on the periphery of the samples. The SiO₂ concentration gradient was accompanied by sharp microstructural changes across the specimens. The gradient was decreased by raising the ambient helium pressure during heat treatment. A mathematical model was developed to predict the SiO₂ concentration profile. The model agrees well with experimental results and demonstrates that the diffusivity of SiO₂ in the vapor phase controls the gradient of SiO₂ through the cross section of mullite felt.     

用Sol－Gel法合成莫来石超细粉机理及过程的研究

以正硅酸乙酯（ＴＥＯＳ）和硝酸铝为原料，用Ｓｏｌ－Ｇｅｌ法合成了莫来石超细粉。探讨了ＴＥＯＳ在硝酸铝存在下的水解缩聚机理。并用差热一逸气、Ｘ一射线衍射、红外光谱及透射电镜等对莫来石的形成过程、结构及颗粒大小进行了研究。结果表明，参与了Ｓｏｌ－Ｇｅｌ过程，并起酸的催化作用，Ａｌ￣（３＋）也参与了网络结构。莫来石相是经过铝硅尖晶石转化成的，在１１５０℃明显生成。１２００℃尖晶石相消失，全部为莫来石，平均粒径为０．０５μｍ。     

Infiltration and Pyrolysis of Polytitanocarbosilane in an Si-Ti-C-O Fabric/Mullite Porous Composite

Incorporating Si-Ti-C-O fabric into a mullite matrix is expected to increase the fracture energy of mullite ceramics. The present paper describes the processing of an Si-Ti-C-O fabric/mullite/polytitanocarbosilane composite. A polytitanocarbosilane (a precursor of Si-Ti-C-O fiber)/xylene solution was infiltrated into a laminated porous mullite composite with 35–37 vol% fabric and thermally decomposed to an amorphous solid at 1000°C, in an argon atmosphere, to decrease the porosity and residual stress induced by the difference in thermal and mechanical properties between the Si-Ti-C-O fabric and the mullite. The decrease in porosity of the composite with pyrolysis of the precursor polymer was analyzed theoretically, and those results were used to control the effective experimental parameters. The infiltration/pyrolysis process was repeated eight times to produce a composite of 90.4% theoretical density. The composite exhibited significant pseudoductility, with a fracture energy of 11.4 kJ/m² and a flexural strength of 290 MPa.     

Preparation of Highly Pure Fine Mullite Powder

The preparation of high-purity mullite powders (3Al₂O₃· 2SiO₂) was examined using aluminum salts or boehmite and fumed silica. The combination of aluminum sulfate and fumed silica was found to have the highest quality for mullite powder. A slurry consisting of an aqueous solution of aluminum sulfate and fumed silica was dried and heated at 1350°C and was attrition-milled for 2 h. The mullite powder obtained was composed of submicrometer, uniform, and equiaxial particles with narrow size distribution. It was hot-pressed at 1600°C for 1 h under 10 MPa or was sintered at 1650°C for 3 h. The bulk densities of the products made by both processes were 3.16 and 3.09 g/cm³.     

Cyclic Infiltration of Porous Zirconia Preforms with a Liquid Solution of Mullite Precursor

Composites of mullite and zirconia were fabricated via the cyclic infiltration of porous zirconia-based preforms with a liquid mullite precursor. The maximal amount of mullite precursor that could be infiltrated was dependent primarily on the initial open porosity of the preforms. When a zirconia preform with an initial open porosity of ∼58% was cyclically infiltrated to saturation, the open porosity was reduced to ∼43%, with a median pore diameter of 15 nm. After sintering at a temperature of 1500°C for 2 h, the saturation-infiltrated zirconia preforms could be densified to ∼98% of the theoretical density. In zirconia samples, infiltrated mullite had a tendency to coalesce into large, elongated grains as the sintering temperature was increased. The presence of infiltrated mullite did not have a significant effect on the zirconia grain structure. The distribution of mullite in the samples was nonuniform, and the distribution profiles varied as the number of infiltration cycles varied. Although the sintered density and hardness showed small improvements after saturation infiltration, the fracture toughness did not increase.     

莫来石晶粒柱状生长对莫来石基陶瓷力学性能的影响

将AlF3加入莫来石(莫来石/氧化锆)先质中,生坯试样经密闭处理及常压烧结,可获得莫来石柱状晶粒生长.实验结果表明:莫来石柱状生长使莫来石陶瓷在其强度不下降的前提下,韧性提高,且随柱状晶粒粒径增大而增加有类似架桥作用.ZTM(含AlF3)材料韧性的提高是由于莫来石柱状晶粒生长使t-ZrO2形态发生了变化,具有较大长径比或有尖角的不规则形态的t-ZrO2晶粒容易发生应力诱导相变,使材料力学性能得到明显提高.     

Epitactic Nucleation of Spinel in Aluminosilicate Gels and Its Effect on Mullite Crystallization

Control over the structure of hybrid (colloidal + molecular) aluminosilicate gels was utilized to demonstrate that precursor chemistry has a direct and controllable effect on the ∼1000°C crystallization of spinel and mullite in molecular precursor systems. Synthesis or preparation conditions leading to the development of a cubic, transition alumina result in the epitactic nucleation of spinel at ∼1000°C in gels that otherwise crystallize directly to mullite at ∼1000°C. Thus, the preference for spinel nucleation in gels derived from solution precursor systems whose chemistries promote formation of transition alumina readily explains the reported inability to obtain substantial mullite yields at ∼1000°C. Isothermal transformation kinetics of colloidal and hybrid gels show that in the absence of direct mullite formation at ∼1000°C, the release of alumina from the spinel-type crystal structure becomes the rate-controlling step in the transformation. This necessitates higher temperatures for mullite formation and limits the kinetic enhancement possible with extrinsic increases in mullite nucleation frequency.