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.     

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.     

Mullite/Alumina Particulate Composites by Infiltration Processing: IV, Residual Stress Profiles

Residual stress profiles through mullite/alumina bodies prepared by an infiltration process were determined. In one approach, residual stress profiles were predicted by obtaining concentration profiles and treating the bodies as laminates. In a second approach, a strain gage technique in which part of the sample was ground away while recording the resulting strain was used. The results showed that residual compressive stresses existed in the surface region of all composite samples. The strain gage approach indicated values of surface compression in the range of 176 to 291 MPa, which were higher than those predicted using the laminate approach. The results were compared with changes in the mechanical properties and found to be consistent with the increases in the strength and indentation fracture toughness which resulted when mullite was added to alumina by the infiltration technique.     

Mechanical Properties of Alumina/Silicon Carbide Whisker Composites

The improvement of mechanical properties of Al₂O₃/SiC whisker composites has been studied with emphasis on the effects of the whisker content and of the hot-pressing temperature. Mechanical properties such as fracture toughness and fracture strength increased with increasing whisker content up to 40 wt%. In the case of the high SiC whisker content of 40 wt%, fracture toughness of the sample hot-pressed at 1900° decreased significantly, in spite of densification, compared with one hot-pressed at 1850°. Fracture toughness strongly depended on the microstructure, especially the distribution of SiC whiskers rather than the grain size of the Al₂O₃ matrix.     

Effect of Processing on Mechanical Properties of Platelet-Reinforced Mullite Composites

SiC- and Al₂O₃-platelet-reinforced mullite-matrix composites were fabricated by both conventional powder processing and a pressure filtration route with constant filtration rate. Consolidation during the pressure filtration experiment was monitored as a function of time with respect to actual pressing stages. The packing density of green bodies was strongly affected by the different processing methods. Either fracture toughness (3.2 MPa · m^1/2) or bending strength (344 MPa) increased, depending on surface conditioning of the SiC platelets. Pressure filtration was shown to decrease processing flaws in the platelet-reinforced bodies leading to substantial improvement of the mechanical properties.     

Enhanced Mechanical Properties of Alumina by Dispersed Titanium Diboride Particulate Inclusions

The mechanical properties of composite ceramics composed of 0 to 20 vol% of titanium diboride particles dispersed in an α-alumina matrix were investigated. The alumina–titanium diboride composite powder was hot-pressed at 1470°C for 20 min to achieve over 98.8% of the theoretical composite density. The strength and fracture toughness of the twophase, hot-pressed composite were both significantly improved compared to the single-phase alumina. Results from different methods of measuring the stress intensity factor, ( K _{I c} ) are compared and discussed.     

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.     

Al2O3对ZTM陶瓷结构与性能的影响

本文研究了氧化铝对ZTM陶瓷结构和性能的影响.发现在烧结过程中,氧化铝可固溶于莫来石颗粒形成富铝型柱状莫来石,产生的体积膨胀增强了基质对氧化锆颗粒的约束,使材料中的四方氧化锆相对含量增加,其强韧化效果进一步发挥,明显改善了材料的力学性能.     

Mullite/SiAlON/Alumina Composites by Infiltration Processing

The formation of mullite/SiAlON/alumina composites was studied by infiltrating a SiAlON/alumina-base composite with two different solutions, followed by thermal treatment. The base composite was prepared from a mixture of tabular Al₂O₃ grains, fume SiO₂, and aluminum powders. The mixture was pressed into test bars and nitrided in a nitrogen-gas (N₂) atmosphere at 1480°C. The infiltrants were prehydrolyzed ethyl polysilicate solution and ethyl polysilicatealuminum nitrate solution. The composites were infiltrated under vacuum, cured at 100°C, and precalcined in air at 700°C. This infiltration process was repeated several times to produce bars that had been subjected to multiple infiltrations, then the bars were calcined in a N₂ atmosphere at 1480°C to obtain mullite/SiAlON/alumina composites. The infiltration process increased the percentage of nitrogenous crystalline and mullite phases in the matrix; therefore, a decrease of the composite microporosity was observed. The infiltration increased the mechanical strength of the composites. Of the two composites, the one produced using prehydrolyzed ethyl polysilicate as the infiltrant had a higher mechanical strength, before and after being subjected to a severe thermal shock.     

工艺制备对钙长石/莫来石复合材料力学性能的影响

利用钙长石和自制莫来石晶须为主要原料,通过固相法制备莫来石/钙长石复合材料。研究了工艺制备方法对钙长石/莫来石复合材料性能的影响。实验结果表明:合适的保温点(1000℃,保温1h)在1400℃烧结对复合材料的力学性能有至关重要的影响,二次重烧结法比一次烧成所制备的复合材料力学性能有所提高。XRD和SEM分析表明:由于钙长石相和莫来石相通过玻璃相紧密结合,提高了材料的力学性能。     

莫来石晶种对反应烧结ZrO2/莫来石复合材料力学性能的影响

采用反应烧结法制备了ZrO2/莫来石复合陶瓷材料,研究了添加莫来石晶种对材料力学性能的影响.结果表明,在完全莫来石化之前,莫来石过早形成致使不含晶种试样的密度低于含晶种试样,抗弯强度低于含晶种试样.而随着反应的进行,至反应完全以后,由于晶种的添加使气孔多沿晶界分布,晶界上的气孔在之后的烧结过程中易于排出,使含晶种试样获得较高的致密度.此外,添加晶种还有效减小了莫来石的晶粒尺寸.因此,反应完全后含晶种试样的强度高于不含晶种试样.2种试样的抗弯强度在800℃的测试温度下达到最大值,当温度进一步升高时,含晶种试样比不含晶种试样的强度下降得快.     

Effect of Dynamic Compaction on the Retention of Tetragonal Zirconia and Mechanical Properties of Alumina/Zirconia Composites

Dynamic consolidation techniques were employed to investigate the retention of tetragonal zirconia and degree of consolidation in alumina/zirconia powder compacts. Heating the specimens prior to explosive shock compaction increased the tetragonal-phase retention significantly. Low shock pressures yielded no macrocracking, although final densities were low (60% to 70% of the theoretical density). Heat treatment following dynamic consolidation enhanced the retention of the tetragonal zirconia polymorph regardless of the shock pressure employed. Compact densities were increased to over 90% of theoretical at relatively low sintering temperatures (1300°C). Hardness, toughness, and Young's modulus of the compacts were comparable to those achieved in composites that were synthesized using more conventional techniques. Dynamic compaction offers an alternative method for the fabrication of zirconia-toughened alumina ceramics.     

Alumina Agglomerate Effects on Toughness-Curve Behavior of Alumina–Mullite Composites

Toughness-curve ( T -curve) behavior of composites of spherical, polycrystalline, coarse-grained, alumina agglomerates dispersed throughout a constant-toughness, fine-grained, 50–50 vol% alumina–mullite matrix has been evaluated as a function of agglomerate content for the range 15 to 45 vol%. T -curve behavior was evaluated using the indentation-strength method. Increasing alumina agglomerate content resulted in a progressive increase of large indentation load strengths with negligible change of plateau strength levels at small indentation loads. This behavior is consistent with underlying T -curves that rise to greater values and are shifted toward longer crack lengths with increasing agglomerate content, suggesting that both bridge spacing and bridge potency increase with increasing agglomerate content over the range tested. The proposed relationships between bridge spacing and agglomerate content, and bridge potency and agglomerate content, are rationalized in terms of residual stress considerations. The indentation-strength data also demonstrated that the composite containing the greatest alumina agglomerate content, 45 vol%, exhibited the greatest flaw tolerance.     

Mullite/Alumina Mixtures for Use as Porous Matrices in Oxide Fiber Composites

Weakly bonded particle mixtures of mullite and alumina are assessed as candidate matrixes for use in porous matrix ceramic composites. Conditions for the deflection of a matrix crack at a fiber-matrix interface are used to identify the combinations of modulus and toughness of the fibers and the matrix for which damage-tolerant behavior is expected to occur in the composite. Accordingly, the present study focuses on the modulus and toughness of the particle mixtures, as well as the changes in these properties following aging at elevated temperature comparable to the targeted upper-use temperature for oxide composites. Models based on bonded particle aggregates are presented, assessed, and calibrated. The experimental and modeling results are combined to predict the critical aging times at which damage tolerance is lost because of sintering at the particle junctions and the associated changes in mechanical properties. For an aging temperature of 1200°C, the critical time exceeds 10 000 h for the mullite-rich mixtures.     

Mullite—Alumina Composites by Extrusion

The rheological properties of a paste containing chopped alumina fiber and particulate silica suspended in a gelled boehmite liquid phase have been evaluated using a physically based extrusion model. When sintered, the paste formed a mullite-alumina fiber composite. Extrudates with fiber volumes up to 30% in the sintered product were prepared. During extrusion, the pressure drop was largely independent of extrudate velocity, fiber length, and the fiber concentration. All pastes showed significant yield behavior leading to good postextrusion shape retention. For any given fiber length, it was shown that there exists a critical volume fraction above which fiber-fiber interactions are so great that both yield and wall shear stresses increase. At these high concentrations of fiber, inhomogeneities also increase. Up to the critical volume fraction, dispersed wet fibers produced lower extrusion parameters than when dry fibers were used as the starting material. The observed behavior is explained in terms of low viscosity liquid formation above the yield point of the boehmite gel.     

Microstructure and Mechanical Properties of Three-Dimensional Textile Hi-Nicalon SiC/SiC Composites by Chemical Vapor Infiltration

Three-dimensional textile Hi-Nicalon SiC-fiber-reinforced SiC composites were fabricated using chemical vapor infiltration. The microstructure and mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.5 g·cm⁻³ after the three-dimensional SiC perform was infiltrated for 30 h. The values of flexural strength were 860 MPa at room temperature and 1010 MPa at 1300°C under vacuum. Above the infiltration temperature, the failure behavior of the composites became brittle because of the strong interfacial bonding and the mismatch of thermal expansion coefficients between fiber and matrix. The fracture toughness was 30.2 MPa·m^1/2. The obtained value of shear strength was 67.5 MPa. The composites exhibited excellent impact resistance, and the dynamic fracture toughness of 36.0 kJ·m⁻² was measured using Charpy impact tests.     

Microstructures and Properties of Three Composites of Alumina, Mullite, and Monoclinic SrAl2Si2O8

Three composites that were 96% alumina were mixed and uniaxially dry-pressed into bars and pellets; all had monoclinic SrAl₂Si₂O₈ as an intergranular phase. The diffraction patterns, microstructure, density, dielectric properties, strength, and toughness were measured. The first composition, which contained crystalline SrCO₃, Al₂O₃, and SiO₂, in a 1:1:2 molar ratio, as the 4% component, densified but was generally inferior to the second and third compositions, which contained strontium aluminosilicate (SrAl _x Si _y O _z , SAS) glass as the 4% component, in terms of mechanical properties, defects, and monoclinic SrAl₂Si₂O₈ transformation. The second composition, which lacked B₂O₃, was very tough and was comparable to commercial alumina, in terms of the dielectric constant. The third, which contained 0.068% of B₂O₃ that was dissolved in the SAS glass as a sintering aid, had high strength and toughness and no macroscopically visible defects. Mullite formed, in addition to monoclinic SrAl₂Si₂O₈ in all three composites. Alumina–monoclinic SrAl₂Si₂O₈ composites of the third composition had room-temperature properties that were comparable to commercial aluminas that contained 96% alumina and also had potential for mechanical and refractory applications.     

Mechanical Properties of Textured Alumina Made by High-Temperature Deformation

The mechanical properties of a textured alumina made by high-temperature deformation of normal-purity sintered alumina have been investigated. The textured alumina shows very high bending strength and extremely high fracture toughness. Fracture toughness of more than 10 MPa·m^1/2 was measured by the single-edge precracked beam method, and even using the single-edge V-notched beam method, toughness of over 8 MPa·m^1/2 was obtained. This high fracture toughness was attributed to a large number of aligned small platelike grains of the textured structure enhancing the grain bridging effect.     

Microstructure and Mechanical Properties of Mullite–Silicon Carbide Composites

Mullite-SiC-whisker composites were prepared by powder processing using two commercial SiC whiskers. These composites were prepared by sintering rather than hot-pressing. A mulliteSlC-powder composite and a base line mallite material were also prepared for comparison with the two whisker composite materials. Fracture toughness measurements showed significant enhancement in only one of the whisker composite materials. The microstructure of the four materials was examined by scanning electron microscopy and transmission electron microscopy to assist in the explanation of the mechanical behavior of these composites. The examinations suggested that most of the toughening results from second-phase particles, with only limited toughening from effects associated with whiskers per se. In one case, higher toughness was partially associated with the formation of sialon phase by reaction with the whiskers and the furnace environment.