Keijiro Hiraga

Superplasticity in alumina enhanced by co-dispersion of 10% zirconia and 10% spinel particles

Abstract Superplastic deformation behavior is examined for Al 2 O 3 -based ceramics dispersed with 10 vol% ZrO 2 and 10 vol% spinel (MgO·1.3 Al 2 O 3 ) particles. The multiple-phase dispersion considerably decreases the rate of grain growth during deformation, leading to enhanced superplasticity (larger tensile elongation and higher strain rate). Maximum tensile elongation reaches 850% at a strain rate of 5.0×10 −4 s −1 and at 1500°C. Grain growth during deformation is found to follow a theoretical model based on a grain boundary diffusion mechanism. The creep parameters corrected for concurrent grain growth are 2.2 as the stress exponent, 3.2 as the grain size exponent and 751 kJ/mol as the activation energy. Spherical ZrO 2 particles embedded in elongated Al 2 O 3 grains in deformed specimens suggest that the deformation mechanism of the present material is strongly related to grain boundary diffusion. Being different from other superplastic aluminas, cavities in the present material tended to grow in the direction parallel to the stress axis.

Superplastic Tensile Ductility Enhanced by Grain Size Refinement in a Zirconia-Dispersed Alumina

High-temperature tensile ductility in fine-grained pure alumina is limited to {approximately}20% in engineering strain owing to rapid dynamic grain growth accompanied by large strain hardening and resultant severe cavitation. Accordingly, many trials have been made to suppress the dynamic grain growth by use of an additive such as MgO or ZrO{sub 2}. The dynamic grain growth of MgO-doped alumina, however, is still active to limit the tensile ductility to {approximately}80% at 1,623--1,773 K. Although some additional improvement is possible by the codoping of CuO or NiO, the maximum tensile elongation has remained 140%. On the other hand, ZrO{sub 2}-particle dispersion is much more effective in suppressing grain growth and hence strain hardening, whereas the resultant tensile ductility stays up to 110% or less at 1,723--1,773 K. It has been attributed to the increment of flow stress caused inherently by ZrO{sub 2}-dispersion through the suppression of grain boundary sliding. Regardless of these preceding studies, the approach by ZrO{sub 2}-particle pinning has not been completed, because the initial grain sizes of alumina reached about 1 {micro}m already in earlier experiments under tension. A possibility remains to enhance the tensile ductility of ZrO{sub 2}-dispersed alumina by such a reduction in grain size asmorexa0» can compensate the increment of flow stress due to ZrO{sub 2}-addition. From this point of view, the present study examined the high-temperature tensile properties of a fine-grained, 10-vol%-ZrO{sub 2}-dispersed alumina prepared by colloidal processing. The results will demonstrate that large tensile elongation exceeding 500% can be obtained when the initial grain size is maintained below 0.5 {micro}m.«xa0less

Pressure filtration and sintering of fine zirconia powder

Abstract Pressure filtration is applied to the consolidation of 3 mol% yttria-doped zirconia fine powder with average particle size of 60 nm over an applied pressure range of 2.5 to 10.0 MPa. Two kinds of aqueous suspension with different rheological behavior, well-dispersed and insufficiently dispersed, respectively, are prepared by changing the amount of dispersant. The kinetics and mechanics of the consolidation of particles are discussed from the dehydration and consolidation rates of the suspensions based on Darcys law and the Kozeny-Carman equation. The consolidated layer prepared from the well-dispersed suspension shows a good sinterability. Cold isostatic pressing at 400 MPa increases the sintered density of the consolidated layer prepared from the insufficiently dispersed suspension.

The tensile creep behavior of superplastic tetragonal zirconia doped with small amounts of SiO2

Abstract The constant stress tensile creep behavior is reported of a tetragonal zirconia (3Y-TZP) with a grain size of 0.27 μm as a function of SiO 2 addition up to 2.5 wt%. The effects of SiO 2 addition are classed into two definite regimes. In the first regime, up to 0.3 wt% additions, both stress exponent, n , and apparent activation energy, Q , decrease steeply and strain rate, g3, increases also steeply as the amount of SiO 2 increases. In the second regime, with additions more than 0.3 wt%, n and Q remain almost constant while g3 increases gradually as the amount of SiO 2 increases. The appearance of the respective regimes is closely correlated with grain boundary microstructures modified by the SiO 2 addition.

Cavitation damage during high temperature tensile deformation in fine-grained alumina doped with magnesia or zirconia

The high temperature tensile ductility of alumina doped with MgO or ZrO{sub 2} is limited around 70--110% for initial grain sizes of about 1 {micro}m. Such limitation may be correlated with strain hardening due to insufficiently suppressed grain growth in MgO-doped alumina or the level of flow stress heightened owing to second phase pinning and/or the intergranular segregation of Zr{sup 4+} ions in ZrO{sub 2}-doped one. This is because higher flow stresses can be assumed to accelerate damage process and thereby to limit tensile ductility. In comparison between these materials, however, such an approach based simply on flow rather similar tensile ductilities as above, irrespective of noticeable differences both in strain hardening behavior and in the level of flow stress between them. Although information on intergranular cavitation damage will give an additional basis for explanation, there has been little quantitative work on cavitation in there martials. The present study, therefore, examined the evolution of cavitation damage in a 0.2-wt%-MgO-doped alumina and a 10-vol%-ZrO{sub 2}-doped alumina with the same initial grain size of 1.0 {micro}m. An emphasis was placed on characterizing the difference in cavitation behavior between the materials.

Preparation of fine-grained monoclinic zirconia ceramics by colloidal processing

Fine-grained monoclinic zirconia ceramic was made from well-dispersed zirconia sol prepared by the hydrolysis of zirconium chloride oxide octahydrate. Dechlorinated and concentrated zirconia sol was consolidated by pressure filtration. The relative green density of the compact was improved by the following cold isostatic pressing treatment at 400 MPa. The compact was densified by pressureless sintering to >98% of theoretical density in air at 1100 °C, which is lower than that of monoclinic to tetragonal transformation of pure zirconia. The average grain size of the sintered monoclinic zirconia ceramics was 92 nm.

High Verdet constant of Ti-doped terbium aluminum garnet (TAG) ceramics

The temperature dependence of the Verdet constant of a 0.8 at % Ti-doped terbium aluminum garnet (TAG) ceramics was investigated using lasers with wavelengths of 632.8 and 1064 nm. A high value of the Verdet constant was obtained at 296 K – namely, 184 and 53 rad/Tm for 632.8 and 1064 nm, respectively. The Verdet constant of the Ti:TAG ceramics at 1064 nm is about 1.5 times higher than that of the terbium gallium garnet (TGG) ceramics. The transmittance of this sample was about 75% at the wavelength of 1 μm. This material represents a possible candidate for next-generation devices that utilize the magneto-optic effect.

Effect of Silica Doping on the Electrical Conductivity of 3 mol % Yttria-Stabilized Tetragonal Zirconia Prepared by Colloidal Processing

Silica-doped (SiO2 = 0 - 1.0 wt%) 3Y-TZP (3 mol % yttria-doped tetragonal zirconia polycrystal) ceramics are prepared from hetero-coagulated aqueous suspension by colloidal processing. Consolidation of the suspension was carried out by pressure filtration at 10 MPa followed by cold isostatic pressing (CIP) at 400 MPa. Consolidated compacts are densified to a relative density over 99% by sintering at 1573 K for 2 h. The formation of glass pockets at grain boundary multiple junctions was observed by SEM for ≥0.5 wt % silica-doped samples. Electrical conductivity measurements were performed to evaluate the modification of grain-boundaries by silica. The apparent grain boundary conductivity decreased with an increase in silica content and became nearly constant above 0.3 wt % of silica, while the bulk conductivity was constant with silica content.

Thermo-optic properties of ceramic YAG at high temperatures

The thermo-optic coefficient dn/dT at 632.8 nm and thermal expansion coefficient α of transparent ceramic yttrium aluminum garnet (YAG) were measured between room temperature and 600 K. The data showed that dn/dT increases with temperature and α is in good agreement with that of single-crystal YAG. To the best of our knowledge, these are the first experimental data of the thermo-optic properties of highly transparent ceramic YAG above room temperature. We also present, using previously reported values measured below room temperature, fitting parameters for dn/dT that are valid over a wide temperature range (70–600 K) with an average error of 2.0%.

Preparation and sintering of silica-doped zirconia by colloidal processing

Silica-doped (SiO{sub 2} = 0--1.0 mass%) zirconia (3 mol% Y{sub 2}O{sub 3}-doped tetragonal ZrO{sub 2}) compacts are prepared from hetero-coagulated and well-dispersed suspensions by colloidal processing. The suspensions are consolidated by a pressure filtration technique. The green density of the compacts consolidated from the well-dispersed suspensions is higher than that from the hetero-coagulated suspensions. The lower density of the latter compacts is improved by a subsequent cold isostatic pressing (CIP) at 400 MPa. The sinterability of the compacts at 1,200 C is greatly affected by the amount of doped silica. The densification and grain growth are hindered by silica doping above 0.3 wt% at 1,200 C. All the compacts are densificated to a relative density of above 99% by sintering at 1,300 C for 2 h.