Processing and Microstructure of Mullite-Zirconia Composites Prepared from Sol-Gel Powders

A high-purity stoichiometric mullite precursor was obtained by hydrolysis of the alkoxides Al(OC₃H₇)₃ and Si(OC₂H₂)₄. Fully sintered mullite ceramics can be prepared from sol-gel powders by sintering them at 1600°C for 4 h in air with the addition of 15 to 20 Vol% ZrO₂ or 1 to 3 mol% Y₂O₃ or both. Introduction of 1 to 3 mol% Y₂O₃ aids the retention of tetragonal ZrO₂; the volume fraction of t -ZrO₂ retained increases with increasing Y₂O₃ content. The maximum t -ZrO₂ retained reaches 34% in a matrix of synthetic mullite with 3 mol% Y₂O₃, but most of this t -ZrO₂ does not undergo stress-induced transformation during grinding.     

Preparation of Mullite-Zirconia Composites from Glass Powder

Glass powders with the composition mullite/5 wt% ZrO₂, prepared by rapid solidification, were used to prepare a poly-crystalline ceramic by hot-pressing to 1040°C. The as-prepared structure consisted of a fine-grain-sized (∼0.1 μm) solid solution of ZrO₂ in a tetragonal form of mullite. Heat treatment between 1300° and 1660°C resulted in a range of microstructures consisting of tetragonal ZrO₂ particles dispersed in mullite. Transformable tetragonal ZrO₂ was observed only after heat treatment at 1600°C.     

多晶氧化铝纤维对莫来石—氧化锆陶瓷材料的增强研究

以电熔莫来石、半稳定氧化锆和α-氧化铝微粉为主要原料,制备莫来石—氧化锆陶瓷材料。在材料中分别外加了（V%）0、5%、10%、15%和20%的多晶氧化铝纤维,并将试样在1500℃保温3h烧成,制备出氧化铝纤维增韧的莫来石—氧化锆复相陶瓷。研究了氧化铝纤维对试样的加热线收缩率、常温耐压强度、常温抗折强度和热震稳定性的影响。结果表明：加入适量的多晶氧化铝纤维能够显著降低莫来石—氧化锆复相陶瓷的加热线收缩率,大幅度提高其常温抗折强度和热震稳定性,而常温耐压强度只有轻微下降。     

氟化钙对常压烧结Si3N4/BN复相陶瓷结构与性能的影响

以微米级Si3N4和h-BN粉末为原料,CaF2–Al2O3–Y2O3为烧结助剂,采用常压烧结工艺制备了BN体积含量为25%的Si3N4/BN复相陶瓷。研究了CaF2添加量对Si3N4/BN复相陶瓷材料力学性能的影响,并通过X射线衍射和场发射扫描电镜分析了复相陶瓷的物相组成和显微组织。结果表明：随着CaF2添加量增加,制备的Si3N4/BN复相陶瓷材料气孔率逐渐增大,收缩率变小,相对密度减小。添加量为2%（质量分数）时,Si3N4/BN复相陶瓷的室温抗弯强度达145.5MPa。添加适量的CaF2可在Si3N4/BN复相陶瓷材料常压烧结过程中较大程度地破坏h-BN的卡片房式结构,将微米级的h-BN颗粒变成纳米级颗粒。     

Seasonal Variation of Microstructure and Sintered Strength of Dry-Pressed Alumina

An origin was investigated for the variation of the density and the fracture strength of sintered alumina with the manufacturing season. Direct observation using immersion microscopy was utilized to examine the microstructures of granules, green bodies, and sintered samples for two specific cases: samples made in summer and others made in winter. This method revealed a seasonal difference in the pore structure of both green and sintered bodies. The variation of the density and the fracture strength with the manufacturing season was ascribed to the different concentrations of large pore defects in sintered bodies, which were developed from the green body structures. Formation of large pore defects resulted from void spaces at the center and at the boundary of granules in the green bodies. High temperature and humidity contributed to an increase in the deformability of granules, reducing defect sizes in summer and thus improving fracture strength.     

Microstructure Refinement of Sintered Alumina by a Two-Step Sintering Technique

For a few oxide ceramics, the use of an initial precoarsening step prior to densification (referred to as two-step sintering) has been observed to produce an improvement in the microstructural homogeneity during subsequent sintering. In the present work, the effect of a precoarsening step (50 h at 800°C) on the subsequent densification and microstructural evolution of high-quality alumina (Al2O3) powder compacts during constant-heating-rate sintering (4°C/min to 1450°C) was characterized in detail. The data were compared with those for similar compacts that were sintered conventionally (without the heat treatment step) and used to explore the mechanism of microstructural improvement during two-step sintering. After the precoarsening step, the average pore size was larger, but the distribution in pore sizes was narrower, than those for similar compacts that were sintered conventionally to 800°C. In subsequent sintering, the microstructure of the precoarsened compact evolved in a more homogeneous manner and, at the same density, the amount of closed porosity was lower for the compacts that were sintered by the two-step technique, in comparison to the conventional heating schedule. Furthermore, a measurably higher final density, a smaller average grain size, and a narrower distribution in grain sizes were achieved with the two-step technique. The microstructural refinement that was produced by the two-step sintering technique is explained in terms of a reduction in the effects of differential densification and the resulting delay of the pore channel pinch-off to higher density.     

Mechanical Properties and Microstructure of Alumina-Glass Composites

Alumina blanks were prepared via isostatic pressing and sintering at 1400°C for 2 h, whereas alumina-glass composites were prepared by infiltrating the molten glass into the partially sintered alumina compacts. The samples had a high bending strength (340 MPa) and high fracture toughness (3.91 MPa·m^1/2) and were free of shrinkage. Concurrently, zirconia (5 wt%) was used as an additive to alumina to improve the mechanical properties of both partially sintered alumina and alumina-glass composites. Results show that zirconia notably improves the mechanical properties of the partially sintered alumina but increases that of the composites by no more than 10%.     

Microstructure and Compressive Strength of SiC-Platelet-Reinforced Borosilicate Composites

The microstructure and compressive strength of SiC-platelet-reinforced borosilicate composites have been examined in this study. During sintering, following cold compaction, borosilicate glass crystallized into cristobalite, and the thermal expansion mismatch between the parent glass and the crystallized phase led to extensive microcracking of the matrix. Cristobalite growth (hence cracking) could be suppressed to some extent by opting for a rapid hot-pressing cycle. Composites fabricated with various volume fractions of SiC platelets were tested in compression. A maximum compressive strength of 510 MPa was observed at 40 vol % of platelets. Further, the compressive flow behavior of these composites has been explored in the vicinity of the glass transition temperature. At temperatures above 625°C, borosilicate glass and its composites exhibited Newtonian viscous flow characteristics. Their flow stress at a given strain rate is, however, seen to increase with increasing volume fraction of SiC platelets.     

Effect of Heat Treatment on Fracture Toughness of Alumina-Diamond Composites Sintered at High Pressures

Alumina composites containing dispersed diamond particles were fabricated under very high pressure at a temperature where diamond is stable. Subsequent heat treatment in vacuum improves the toughness of the 5 vol% composite but not that of the 10 vol% composite. The heat treatment time was varied for both composites, and the influence of graphitization and morphology of the second-phase particles on toughness were studied.     

Microstructure of Alumina Composites Containing Niobium and Niobium Aluminides

The microstructures of niobium-based alumina composites prepared by pressureless sintering of compacts of attrition milled Al₂O₃, Nb, and Al powder mixtures were studied. The addition of a small amount of Al is assumed to assist in rapid sintering. X-ray diffraction analyses show that Al₂O₃, Nb, NbO, and the intermetallics AlNb₂ and AlNb₃ are present in the composites. Electron microscopy studies confirm the existence of these phases and reveal dense, fine-grained (≤500 nm) composites. Al₂O₃ and Nb grains form the matrix. NbO occurs as grains and additionally as small particles within Al₂O₃ grains and at Al₂O₃/Al₂O₃ grain boundaries. The intermetallic AlNb₂ and AlNb₃ phases do not exceed 300 nm in size if they occur at grain boundaries, and possess even smaller dimensions when occluded within Al₂O₃ grains or located at Al₂O₃ triple junctions. While the niobium intermetallics are expected to form during the heating cycle before reaching the sintering temperature, the NbO is assumed to form during the cooling cycle due to precipitation of oxygen dissolved in the niobium.     

三维编织复合材料的细观结构与力学性能

归纳、梳理了三维编织复合材料细观结构表征方面较有代表性的单胞模型,分析、比较各结构模型的优缺点,从理论分析与试验测试两方面总结三维编织复合材料刚度和强度性能的研究成果与进展,探讨了细观结构表征与力学性能预报中存在的主要问题,并展望今后的研究重点与发展方向。     

Strength and Microstructure of Brazed Alumina-Silicon Carbide Ceramic-Matrix Composites

Alumina-silicon carbide whisker (Al₂O₃-SiC _w ) composite pieces were brazed under high vacuum with Ag-Cu-based active braze alloys. Joined specimens exhibit mean fracture strengths of 480 MPa (4.5 wt% titanium in braze) and 591 MPa (2.0 wt% titanium in braze) in four-point bend tests; these strengths are 78%−96% of the measured strength of the composite. The interfacial phases have been identified, by using transmission electron microscopy and electron probe microanalysis, as being Ti₃(Cu,Al,Si)₃O, Ti₅Si₃, TiC, and γ-TiO, and the relative locations of the interfacial phases are described. The high strengths are believed to result from both good wetting behavior under high vacuum and the composite nature of the reaction product morphology.     

Microstructure Development of Oxycarbide Composites during Active-Filler-Controlled Polymer Pyrolysis

The microstructure development of a ceramic composite material fabricated by active-filler-controlled polymer pyrolysis (AFCOP) was investigated. During heating of a polysiloxane precursor mixed with titanium powder in argon atmosphere up to 1400°C, thermally induced decomposition of the polymer phase is combined with simultaneous carburization of the transition metal filler. Precipitation of nanocrystalline titanium carbide at the filler particle surface starts above 400°C, and larger, faceted carbide particles have growth above 800°C. A skeleton of turbostratic carbon is formed above 800°C in the polymer-derived silicon oxycarbide matrix from which b-silicon carbide and cristobalite crystallize above 1000°C. A pronounced reduction in linear shrinkage involved in polymer–ceramic conversion is observed. The shrinkage reduction ranges from more than 25% for the filler-free precursor to less than 10% in the presence of 30 vol% of the titanium filler. Thus, active-filler-controlled pyrolysis offers the possibility of controlling shrinkage and porosity formation during polymer–ceramic conversion in order to fabricate bulk components from organometallic polymer precursor systems.     

Microstructure and Mechanical Properties of alpha-Silicon Carbide Sintered with Yttrium-Aluminum Garnet and Silica

The effect of glassy-phase chemistry, using Y₃Al₅O₁₂ (YAG) and SiO₂ as sintering additives, on the microstructure and mechanical properties of liquid-phase-sintered, and subsequently annealed, α-SiC materials was investigated. The microstructural development of annealed materials was insensitive to changes in glass chemistry. The mechanical properties vs SiO₂/YAG ratio curve had a maximum; i.e., there was a small glass composition range at which optimum mechanical properties were realized. The best results were obtained when the ratio was ∼0.5. The flexural strength and fracture toughness of the material were >450 MPa and >6 MPa·m^1/2, respectively.     

Microstructure Control of Sintered Porous Yttria-Stabilized Zirconia as a Durable Thermal Shielding Material

The microstructure of a thermal shielding material affects its thermal conductivity and mechanical property. In this study, the effects of the sintering temperature and the polymethyl methacrylate powder as a pore-former on the microstructure of a sintered porous yttria-stabilized zirconia (YSZ), which is used as a durable thermal shielding material, were investigated. It became clear that the microstructure of the sintered YSZ could be controlled by the particle size and the amount of the pore-former and the sintering temperature. The effect of the yttria amount in the YSZ on the microstructure was also clarified.     

Cu-BTC改性水泥基复合材料的微观结构和力学性能

以金属-有机骨架化合物(Cu-BTC)为改性剂,对水泥基复合材料进行了改性,获得了具有规整蛛网状结构的改性水泥基复合材料,并运用FT-IR、XRD、SEM、EDs等对其结构及性能进行了分析.实验结果表明:引入Cu-BTC并不会改变水化产物的构成,而Cu-BTC含有的活性—COOH能够诱导水化产物形成生长位点,水化产物后...