High temperature mechanical properties of reaction-sintered mullite/zirconia and mullite/alumina/zirconia composites

Strength and fracture toughness of reaction-sintered mullite/zirconia composites (RSMZ) and reaction-sintered mullite/alumina/zirconia composites have been investigated as a function of temperature. Thermal shock resistance has also been determined. It was found that dispersion of zirconia particles and the particular microstructure of mullite obtained by means of anin situ reaction process leads to improved properties, with a room temperature fracture toughness of about 5.25 MPa m^1/2. Up to 1000° C fracture strength and toughness values are quite high, which make these materials potential candidates for high temperature applications.     

Alumina–mullite–zirconia composites: Part II Microstructural development and toughening

Densification characteristics and fracture toughness of the alumina–mullite–zirconia (AMZ) composites fabricated by the colloidal mixing route were examined. The densification of boehmite–silica–zirconia precursor compacts was characterized by three distinct stages. Stage I was represented by a rapid shrinkage between 1100 and 1300°C and was caused by the viscous flow sintering of the SiO2 component. Stage II was characterized by a temporary cessation of the shrinkage caused by the mullitization. Stage III was represented by a restoration of the shrinkage for temperatures above 1450 °C. The α-Al2O3 seeding facilitated the formation of elongated grains in the AMZ composites, and these elongated grains correspond to α-Al2O3. The toughening caused by the microcrack nucleation was comparable to that by the t → m martensitic transformation and increased with increasing total content of zirconia.     

Fracture toughness,strength and Vickers hardness of yttria-ceria-doped tetragonal zirconia/alumina composites fabricated by hot isostatic pressing

Yttria-ceria-doped tetragonal zirconia ((Y, Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing (HIP) at 1400–1600 °C and 147 MPa for 30 min in Ar gas using fine powders prepared by hydrolysis of ZrOCl₂ solution. The mechanical properties of these ceramic composites were evaluated. The fracture toughness and bending strength of the composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5-4 mol% CeO₂-ZrO₂, 2.5 mol% YO_1.5-4 mol% CeO₂-ZrO₂ and 2.5 mol% YO_1.5-5.5 mol% CeO₂-ZrO₂ fabricated by HIP at 1400 °C were 6–7 MPa m^1/2 and 1700–1800 MPa. Fracture toughness, strength and hardness of (Y, Ce)-TZP/Al₂O₃ composites were strongly dependent on HIP temperature. The fracture strength and hardness were increased, and grain growth of zirconia grains and phase transformation from the tetragonal to the monoclinic structure of (Y, Ce)-TZP during HIP in Ar at high temperature (1600 °C) were suppressed by the dispersion of Al₂O₃ into (Y, Ce)-TZP.     

Dielectric and electromechanical properties of barium titanate single crystals grown by templated grain growth

Single crystals of BaTiO(3) were grown by templated grain growth (TGG). TGG involves contacting a single crystal "template" to a sintered polycrystalline matrix, then heating the assemblage to a temperature that promotes the migration of the single crystal boundary through the matrix. In this investigation the properties of millimeter-sized, plate-shaped and bar-shaped, single crystals of BaTiO(3) grown by TGG were examined in order to compare the results to single crystals grown by conventional methods. A dielectric constant, epsilon(33)(T)/epsilon(0), of 260, polarizations of P(R)~16 muC/cm(2) and P(sat)~21.5 muC/cm(2), electromechanical coupling coefficients of k(33)~0.51 and k(31)~0.18, and a piezoelectric coefficient of d(33)~140 pC/N were achieved. Entrapped porosity in the crystals made fully poling the crystals difficult.     

模板晶粒生长技术制备SrBi2Nb2O9织构陶瓷

以熔盐法制备的片状SrBi2Nb2O9晶体为模板剂,采用模板晶粒生长技术和流延法制备了Sr-Bi2Nb2O9织构陶瓷,研究了模板含量对SrBi2Nb2O9织构陶瓷烧结行为、织构度、显微结构的影响。结果表明:模板含量为10wt%时,1200℃保温2h烧结可获得体积密度最大的SrBi2Nb2O9织构陶瓷,模板含量继续增加,体积密度降低;织构陶瓷的晶粒尺寸随模板含量的增加而逐渐增大,且晶粒取向性生长趋于明显,当模板含量为10wt%时,织构化SrBi2Nb2O9陶瓷的晶粒取向率f达到最大值,为0.81;织构化陶瓷的压电常数d33达到13pC/N,高于固相法制备的陶瓷的压电常数。     

Microstructure and mechanical properties of mullite/ZrO2 composites

The sintering, microstructure and toughness of mullite/ZrO₂ composites with increasing amount of ZrO₂ (0 to 20 vol %) have been studied. A very active premullite powder has been used as matrix. TheK _IC values increase from 2.1 to 3.2 MN m^?3/2 as the volume fraction of zirconia increases from 0 to 0.2. The realtive fraction of tetragonal zirconia decreases as the volume fraction of ZrO₂ increases to reach ～0.1 in the sample with 0.2 volume fraction of ZrO₂. The presence of ZrO₂ enhances the sintering rate and end-point density of the composites. Finally, the increasing toughness in mullite/ZrO₂ composites has been explained by a grain boundary strengthening mechanism produced by a metastable solid solution (～0.5 wt %) of ZrO₂ in mullite.     

Suspension characteristics and rheological properties of mullite/zirconia powder in methyl isobutyl ketone

Particle size and its distribution, sedimentation bulk density and rheology of mullite, zirconia, and mullite/zirconia mixed suspensions have been studied in terms of oxide loading (20, 30 vol%), and types of additives (dispersant, dispersant/plasticizer, dispersant/plasticizer/binder). Polyester/polyamine, dibutyl phthalate, poly(vinyl butyral), and methyl isobutyl ketone have been used as the dispersant, plasticizer, binder, and liquid medium, respectively. Sedimentation density significantly increased upon adding dispersant; the effect was more pronounced with zirconia suspension most likely due to the fine and hence high specific surface area of zirconia. With further addition of plasticizer and plasticizer/binder, the sedimentation density decreased. The suspension viscosity generally behaved in an opposite manner to the sedimentation density, i.e., low sedimentation density gave high low-shear viscosity, indicative of high order structure formation in the suspended particles. High shear rate rheology showed a shear thinning and its onset began at lower shear rate with higher solid loading. Mullite/zirconia mixed suspension gave intermediate sedimentation and rheological behavior, implying the two types of particles are non-interacting.     

Microstructure and texture of copper/niobium composites processed by ECAE

The structure refinement is a challenge for conductors used for the winding of resistive coils producing non-destructive pulsed magnetic fields over 80T. These nanocomposite conductors composed of a conducting multiscale Cu matrix embedding Nb reinforcing nanofilaments are usually manufactured by using a Severe Plastic Deformation (SPD) process based on hot extrusion, accumulative cold drawing and bundling (ADB) steps [Thilly et al. Philos Mag A 82:925, 2002]. Equal-channel angular extrusion (ECAE) is investigated here as an alternative route since it may provide faster refinement to obtain the ultrafine structure needed for optimized electrical and mechanical properties of the conductors. Therefore, copper-niobium specimens obtained by hot extrusion were processed by ECAE at room temperature. The specific die of the ECAE tool used here is constituted by a round channel with three angles corresponding to a total equivalent strain of about 2.5. Deformed samples were examined by optical microscopy and characterized by hardness profiles and x-ray diffraction (texture pole figures). After one ECAE-pass, the shape of the samples is modified but no trace of damage appeared at the Cu-Nb interfaces. An increase of the hardness values localized in the copper matrix is revealed whereas the hardness of the niobium remains unchanged. Prior to ECAE, the hot extrusion process induced a fibre texture for both copper and niobium. Two fibre texture components were observed for copper: <111> and <200>. A single <110> fibre texture component is evidenced for the niobium. After ECAE a significant variation of the texture is observed in relation with the strong shear induced by this process.     

Templated grain growth of SrBi2Ta2O9 ceramics: Mechanism of texture development

Textured SrBi₂Ta₂O₉ (SBT) ceramics were fabricated via templated grain growth (TGG) technique using platelet-like SBT single crystal templates. The templates (5 wt%) were embedded in a fine-grain SBT powder matrix containing 3 wt% of Bi₂O₃ excess that were subjected to uniaxial pressing and sintering at 1000–1250 °C for up to 24 h. Microstructural characterization by SEM was performed to establish the effect of sintering parameters on the grain growth and texture development. It was found that the ceramics developed a bimodal microstructure with notable concentration of large (longer than 90 μm) aligned grains with c-axis oriented parallel to the pressing direction. The mechanism controlling the texture development and grain growth in SBT ceramics is discussed.     

Evaluation of Weibull master curves of zirconia ceramics and zirconia/alumina composites

Abstracts are not published in this journal     

Evaluation of Weibull master curves of zirconia ceramics and zirconia/alumina composites

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

Microstructure and properties of BN/Ni-Cu composites fabricated by powder technology

Microsize Powders of Ni and Cu were prepared by water atomization technique to fabricate metal matrix composites containing various percentages of nanosized boron nitride particles (1, 2, 3, 4, 5 wt. % of BN in a matrix containing (20 wt. %Ni and 80 wt. %Cu). The prepared mixtures were cold compacted under 400 MPa, and sintered for 2 h at 1000 °C in a controlled atmosphere of 3:2 N₂/H₂ gas mixtures. The microstructure and the chemical composition of the prepared powders as well as the consolidated composites were investigated by X-ray diffraction as well as field emission scanning electron microscope (FESEM) equipped with an energy dispersive spectrometer (EDS). The produced Cu and Ni powders have spheroid shape of size less than 100 microns, but the investigated BN has an equiaxed particle shape and particle size of ～ 500 nm. It has been also observed that BN and Ni particles were homogeneously distributed in the Cu matrix of the present BN/Ni-Cu composites. The density, electrical resistivity, saturation magnetization and hardness of the composites were measured. It was observed that, by increasing BN content, the relative density was decreased, while the saturation magnetization, electrical resistivity and hardness were increased.     

Phase formation during sintering of nanocrystalline zirconia/stainless steel functionally graded composite layers

Microstructural development and phase formation at the interface of yttria stabilized zirconia (3Y-TZP)/430L stainless steel composite layers produced by co-sintering method were studied by SEM, HRTEM, micro-focus XRD, and EPMA. Formation of a rich chromium boundary layer at the interface was noticed, which revealed Cr aggregation at the interface at the elevated temperatures. Misfit dislocations were also observed at the joint interface to tackle the mismatch crystallographic orientations between the ceramic and metal layer. The results of the micro-focus XRD showed formation of no new phases at the boundary zone. Microstructural studies also revealed a retarded grain growth in the nanostructured zirconia due to the pinning effect of ultra-fine pores located at the grain-boundary junctions. The shear strength of the ceramic/metal joint was found to depend on the sintering atmosphere and varied in the range of 47-66 MPa.     

高压浸渍-炭化制备炭/炭复合材料的组织结构

为研究高压浸渍-炭化制备的炭/炭复合材料的组织结构,以1 K PAN基高强度炭纤维为增强体,以调制中温煤沥青为基体前驱体,采用超高压浸渍-炭化工艺制备出2.5D沥青基炭/炭复合材料.采用偏光显微镜及SEM电镜对材料内部的组织形态进行了观察.研究表明:以中温沥青为基体前驱体所制备的炭/炭复合材料,在纤维束内,由于纤维之间的孔隙较小,形成的基体组织主要为镶嵌组织;而在纤维束之间,由于空间较大,出现的基体组织既有镶嵌型组织,也有域型组织.在沥青基炭基体中,有孔洞、裂纹、沟槽等缺陷.     

Sintering and grain growth of 3 mol% yttria zirconia in a microwave field

A comparative study of the sintering and grain growth of 3 mol% yttria zirconia using conventional and microwave heating was performed. Extensive measurements of grain size were performed at various stages of densification, and following isothermal ageing at 1500 °C for 1, 5, 10 and 15 h. Microwave heating was found to enhance densification processes during constant rate heating. The grain size/density relationship for the microwave-sintered samples was shifted in the direction of increased density for density values less than 96% of the theoretical value when compared to conventionally heated samples. This suggests that there may be a difference in the predominant diffusion mechanisms operating during the initial and intermediate stages of sintering. Results of the ageing experiments showed that once densification was near completion, grain growth was accelerated in the microwave field, and exaggerated grain growth occurred.     

Low temperature needle like mullite grain formation in sol-gel precursors coated on SiC porous substrates

Dense mullite coating having thickness in the range of 3 to 5 μm was produced from sol-gel mullite precursor coated on SiC porous substrates at heat treatment temperatures as low as 1300 °C. Mullite formed in the coating layer was characterised by X-ray diffraction. The precursors have an average particle size of 170 nm and the mullite formed in the coating in situ has a grain size of 3-5 μm. Mullite grains formed on the SiC have needle like morphology. The mullite formation has been explained on the basis of reaction between the silica-alumina nano precursor and the needle like morphology has been similar to that formed from a liquid phase. The gas permeation analysis shows that there is considerable difference between gas pressure while using SiC substrate before and after coating and hence clearly indicated reduction in pore size. This particular approach is good since usual mullite formation is at high temperature and is difficult to attain small grain size. Further, in situ formed mullite, in this investigation covers the SiC surface protecting the SiC from oxidation at high temperature.