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。     

液相烧结SiC陶瓷

采用Al2O3、Y2O3为助烧剂,热压烧结获得了致密的α-SiC和β-SiC陶瓷,研究了起始粉末的性能对烧结体的物相组成和显微结构的影响。实验结果表明,Al2O3、Y2O3原位形成了YAG,材料以液相烧结机制致密化,并通过溶解和再析出机制,促进晶体生长。物相分析表明,β-SiC陶瓷粉末在烧结过程中发生了β→α的相变。显微结构观察显示,β-SiC陶瓷中生成了长柱状晶粒。     

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%.     

富氧烧成陶瓷的物相结构与性能研究

利用不同O2浓度的模拟富氧气氛烧成陶瓷试样,采用TG/DSC、XRD、SEM、三点弯曲及阿基米德原理进行了烧成气氛中O2浓度及组分-SiO2/Al2O3变化对陶瓷试样的结构及机械强度影响研究.结果表明:富氧气氛中O2浓度的增加及组分中SiO2/Al2O3的增加对于陶瓷试样的结构改善、机械强度提高均具有促进作用.与普通空气烧成相比,富氧技术的应用能适当的缩短烧成周期,给陶瓷工业带来一定能源节约.     

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.     

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.     

不同烧结温度制备的双相磷酸钙生物陶瓷及其力学性能研究

以煅烧缺钙磷灰石制备的双相磷酸钙(BCP)粉体为原料,采用热压烧结的方法制备了一种可用于骨替代、力学性能优良的生物陶瓷材料.研究了烧结温度对BCP陶瓷力学性能、物相组成和微观形貌的影响.当烧结温度为1100℃和1150℃时,BCP陶瓷的物相组成为HA和β-TCP,断裂方式以穿晶断裂为主;当烧结温度为1200℃时,BCP陶瓷的物相组成为HA和α-TCP,断裂方式为穿晶和沿晶混合断裂模式.当烧结温度为1150℃时,BCP陶瓷的弯曲强度和断裂韧性最高,分别为98 MPa和0.99 MPa·m1/2.     

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 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.     

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。