Mechanical properties of sinter-forged Al2O3-ZrO2 ceramics

Two kinds of composites, Al₂O₃-25 wt% ZrO₂(2 mol% Y₂O₃) (Y-ZTA), Al₂O₃-25 wt% ZrO₂(8 mol% CeO₂) (Ce-ZTA) were produced by the sinter-forging process. The effect of presintering temperature on the mechanical properties of the composites was examined. The sinter-forging process increased the room-temperature bending strength in comparison with pressureless sintering, owing to the smaller grain size in sinter-forged bodies than in pressureless sintered ones. It was found necessary to keep the presintering temperature considerably lower than sinter-forging temperature in order to improve the room-temperature strength. The strength of sinter-forged Ce-ZTA was higher than that of sinter-forged Y-ZTA. The residual surface compressive stress induced by the phase transition during grinding in Ce-ZTA was found to be effective to further improve the strength and fracture toughness.     

Synthesis and sintering behaviour in CeO2-ZrO2 ceramics

The co-precipitation method has been employed to prepare CeO₂-ZrO₂ ceramics. The application of a wet chemical method is expected to yield highly sinterable material at lower sintering temperatures. The characteristics of the synthesized powders are evaluated with respect to the particle size distribution, calcination step, and the degree of agglomeration. The sintering behaviour of the prepared powder is studied at various temperatures to obtain different phase distributions and grain sizes. The amount of the monoclinic phase in the as-sintered specimen is decreased with increasing CeO₂ contents in CeO₂-ZrO₂. 13.7 mol% CeO₂ is sufficient to achieve a tetragonal phase in the CeO₂-ZrO₂ system. In addition, Y₂O₃ and MgO dopants in CeO₂-ZrO₃ reduce the grain size and result in a fully tetragonal phase for the 10 mol% CeO₂ matrix.     

Electrical properties of ceramics in the system Li2O-ZnO

The electrical properties of poly-phase ceramics in the system Li₂O-ZnO were studied. The specimens obtained were confirmed to be composed of Li₂ZnO₂, ZnO and some Li₄ZnO₃, by X-ray diffraction. Li₂ZnO₂ was assumed to possess a hexagonal structure with cell parametersa=0.812 nm andc=0.673 nm. Electrical conductivity increased with increasing temperature up to 420° C, and decreased abruptly above 420° C. The temperature dependence of the ionic transference number showed that ionic conduction was dominating above 300° C, while mixed conduction occurred below 300° C. The dependence of electrical resistivity on relative humidity was also measured at room temperature. Electrical resistivity exponentially decreased five orders of magnitude with increasing relative humidity from 5 to 95%. The decrease in electrical resistivity with increasing relative humidity might be caused by LiOH formed on the surface of the specimens.     

CaO-TiO_2烧结助剂对SiC基复相材料烧结性能的影响

以质量分数30%Al2O3-70%SiC为基,外加2%~10%CaO-TiO2复合烧结助剂,制备了一系列SiC基复相材料。研究了不同的烧成温度和CaOTiO2含量对该复相材料烧结性能的影响。研究结果表明,1 400~1 500℃,随着烧结温度升高,烧结致密性明显提高。烧结温度为1 450℃时,试样基本致密。当烧结助剂达到10%(质量分数)时,显气孔率降至0.15%,陶瓷烧结致密。烧结助剂CaO-TiO2,α-Al2O3和方石英反应,在1 200℃已经生成CaAl2Si2O8,在1 300℃以后才开始生成CaTiSiO5,促进6H-SiC进一步氧化。烧结过程中,CaAl2Si2O8转化为玻璃相,在促进莫来石生成的同时也使试样致密。在1 500℃,复合材料的主晶相为6H-SiC和莫来石。     

Mechanical properties of alkoxy-derived cordierite ceramics

Mechanical properties of cordierite ceramics prepared by the controlled hydrolysis and following polycondensation of aluminium, magnesium and silicon alkoxides were investigated in great detail. Flexural strengths of- and-cordierite ceramics are about 60 and 100 MPa, respectively. The flexural strengths of these ceramics are mainly influenced by cracks arising from thermal mismatch between - and -cordierite precipitated during sintering. High fracture toughness of-cordierite ceramics prepared by this method is ascribed to the fine microstructure of the ceramics. The high-temperature flexural strength of-cordierite ceramics is little reduced below 1000° C because of high purity of the ceramics.[/p]     

Mechanical properties and fracture behaviour of ZrO2-Y2O3 ceramics

Stabilized ZrO₂-Y₂O₃ ceramics have been prepared with varying grain sizes and microstructures with the help of different preparation techniques. Bi₂O₃ has been added as a sinter aid to some of the samples. This results in a certain amount of a zirconia-rich second phase. For Bi₂O₃-free samples the fracture toughness (K _lc), and therefore the fracture energy, increases with decreasing grain size. A linear relation with the inverse square root of the average grain size is found. The highest value ofK _lc amounts to 4.1 MPa m^1/2. Fracture toughness values of 1.9±0.2 MPa m^1/2 are measured for Bi₂O₃ containing materials. The fracture surfaces are intergrannular for Bi₂O₃-containing and transgranular for Bi₂O₃-free samples, respectively.     

Mechanical properties of ultra-fine grained zirconia ceramics

The mechanical properties of tetragonal zirconia (TZP) materials doped with Y, Ce or Ti were studied as a function of temperature and grain size. Fine grained Y-TZP (grain size < 0.3 m) shows values for fracture toughness and strength at room temperature, which are comparable with the coarse grained transformation toughened materials, despite lacking transformation toughening. The morphology of the fracture surface points to crack deflection as the most important toughening mechanism. At 800 °C fracture toughness and strength are higher than in coarse grained Y-TZP materials. Doping Y-TZP with Ce or Ti results in a similar trend in mechanical properties, for fine grained material, as for the Y-TZP materials.     

Mechanical properties and microstructures of co-precipitation derived tetragonal Y2O3-ZrO2-Al2O3 composites

Y-TZP Al₂O₃ specimens (2.5 mol% Y₂O₃-ZrO₂ and 5 to 30 wt% Al₂ 03) were prepared from coprecipitated powders and their mechanical properties were studied. The addition of alumina to Y-TZP improves the attainable density of the materials after sintering at 1500° C and reduces the degradation of their densities due to porosity formation when the materials are sintered above 1500° C. Near theoretical density could be achieved for most of the samples after HIPing at 1500° C for 1/2 h at 200 M Pa pressure. The fracture strength of the HIPed specimens was in the range 2.0 to 2.4 GPa and the stress intensity factor was in the range 3.5 to 6.0 MPa m^1/2. The mechanical strength of the materials was not degraded seriously after autoclaving in water at 175° C for 24 h. The surface layer of transformed monoclinic zirconia was less than 70 m thick even after autoclaving at 175° C for 5 days.     

ZrO_2增韧Al_2O_3陶瓷的力学性能和增韧机制

对通过热压烧结法制备的3种陶瓷99.5vol%Al2O3(AD995)、ZrO2(15vol%)/Al2O3和ZrO2(25vol%)/Al2O3的力学性能和增韧机制进行了实验和理论研究。基于复合材料细观力学理论并考虑ZrO2的相变特性,建立了描述ZrO2/Al2O3陶瓷力学性能的本构模型。结果表明:ZrO2的加入细化了基体Al2O3晶粒,ZrO2/Al2O3陶瓷的致密性得到提高;3种陶瓷试件的破坏呈现小变形到脆性破坏的特点,压缩加载下试件应力-应变曲线近似为线性关系;AD995陶瓷的断裂韧性为5.65 MPa·m1/2,ZrO2(25vol%)/Al2O3陶瓷的断裂韧性为8.42 MPa·m1/2,提高了近50%;随ZrO2增韧相含量的增加,ZrO2/Al2O3陶瓷的弹性模量降低而断裂韧性增加,这一变化趋势与实验结果有良好的一致性。     

Mechanical properties and fracture behaviour of Al2O3-TiC-Co advanced ceramics

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Crystallization of gels in the SiO2-Al2O3-ZrO2 system

The preparation of glasses and ceramics from gels in the ternary system of SiO₂-Al₂O₃-ZrO₂ has been investigated. The phase relations in this system suggest the existence of a stable joint between mullite and ZrO₂ in addition to the joint between mullite and zircon. The gel technique allows metastable ZrO₂ particles to be readily dispersed in alumino-silicate matrices.     

Crystallization of gels in the SiO2-ZrO2-B2O3 system

The preparation of glasses from gels in the ternary system of SiO₂-ZrO₂-B₂O₃ has been investigated. The crystallization character of ZrO₂ particles and high-temperature structure stability in the SiO₂-ZrO₂-B₂O₃ gels were analyzed. The effect of B₂O₃ as the inhibitor of crystallization was illustrated. Finally, the result that the purified DD3 single crystal superalloy melt can retain high undercooling in the SiO₂-ZrO₂-B₂O₃ coating mold indicates that the coating has an ideal non-catalytic nucleation property.     

Kinetics of non-isothermal precipitation process of the perovskite phase in CaO-TiO2-SiO2-Al2O3-MgO system

Kinetics of non-isothermal precipitate process and crystal growth of perovskite (CaO·TiO₂) phase in CaO-TiO₂-SiO₂-Al₂O₃-MgO system were studied. The experimental results show that the relative volume fraction can be described by the equation given by Matusita et al. and the experiential expression of average crystal radius was obtained. The particle coarsening in non-isothermal process has important effects on the crystal growth of perovskite phase.