F. Gutierrez-Mora

Joining of yttria-tetragonal stabilized zirconia polycrystals using nanocrystals

Superplastic flow has been successfully used to join fully dense 3 mol% Y{sub 2}O{sub 3}-tetragonal ZrO{sub 2} polycrystals (Y-TZP) at temperatures as low as 1,350 C, a temperature at which direct diffusional bonding would be unlikely to produce a strong, pore-free joint. The objective of the present work was to determine whether bonding temperatures could be further reduced. To achieve lower bonding temperatures, the authors have investigated the bonding of conventional Y-TZP in which a nanocrystalline Y-TZP with a 20-nm grain size is used as the interlayer between two pieces of 0.3 {micro}m grain sizes Y-TZP. Little is known about the deformation of fully dense nanocrystalline Y-TZP, but recent work indicates that above a threshold stress, the principal deformation mechanism would be grain boundary sliding that results in superplastic flow. Questions on the deformation mechanism in the nanocrystalline Y-TZP are being addressed as part of a larger investigation of high-temperature compressive creep behavior, currently in progress.

Influence of internal stresses in superplastic joining of zirconia toughened alumina

Abstract Joints between various compositions of 3 mol% Y 2 O 3 -stabilized ZrO 2 /Al 2 O 3 composites have been produced by superplastic flow at 1350 °C and strain rates of 1×10 −5 s −1 . The joints are pore free and Vickers indentation induced cracks indicate that the joints are strong. The cracks have been used to measure residual stresses, which can be modified by using an interlayer between the materials to be joined. The results agree well with those obtained by finite-element analysis. Ruby fluorescence has been used to measure the hydrostatic stress.

Creep of nanocrystalline Y-SZP ceramics

Abstract Fully dense nanocrystalline Yttria-Partially Stabilized Zirconia with a grain size of 40 nm has been crept in compression at temperatures ranged between 1100°C and 1200°C and strain rates between 5 × 10 −7 s −1 and 10 −4 s −1 . The creep parameters n = 1.4 and Q = 660 kJ/mol were obtained and the microstructure was observed before and after deformation. The microscopic features and macroscopic parameters showed that these nanocrystalline ceramics probably cannot be deformed superplastically.

Joining alumina/zirconia ceramics

Dense compacts consisting of 40 vol.% Al{sub 2}O{sub 3}-60 vol.% 3 mol% Y{sub 2}O{sub 3}-stabilized ZrO{sub 2} (YSZ) were joined with an interlayer of 50 vol.% Al{sub 2}O{sub 3}-50 vol.% YSZ. A joint was also made at 1200 {sup o}C with an interlayer that was 50 vol.% Al{sub 2}O{sub 3}-50 vol.% ZrO{sub 2}, in which the average particle size of both powders was {<=}20 nm. The joint materials were applied by aerosol spraying, and thus the technique is suitable for commercial application. Superplastic deformation at 1200--1350 {sup o}C required {<=}2 h to form fully dense joints in which minimal grain growth occurred. Virtually perfect joints were also made with various other ratios of Al{sub 2}O{sub 3}-YSZ.

High-temperature mechanical properties of anode-supported bilayers

Abstract Mechanical properties of a porous substrate of 40 vol.% Ni–60 vol.% yttria-stabilized zirconia (YSZ), a Ni–YSZ functional layer, and a YSZ electrolyte layer were measured in compression from 1000 to 1200 °C. The anode-supported bilayer is brittle for temperatures

Erosion and strength degradation of biomorphic SiC

Abstract Solid-particle-erosion studies were conducted on biomorphic SiC based on eucalyptus and pine, reaction-bonded (RB) SiC, and hot-pressed (HP) SiC. The erodents were angular SiC abrasives of average diameter 63, 143, or 390 μm and the impact velocity was 100 m s −1 . Impact occurred at normal incidence. Material loss in all targets occurred by brittle fracture. The biomorphic specimens eroded by formation of both lateral and radial cracks and their erosion rates were higher than both conventional SiCs. The RB SiC eroded as a classic brittle material, by formation and propagation of lateral cracks. The HP SiC, the hardest target, was the most erosion resistant. In erosion of the HP SiC, the abrasive particles, especially the largest ones, fragmented upon impact. The resulting dissipation of energy led to relatively low erosion rates. Flexural strength before and after erosion was measured for the biomorphic eucalyptus, RB SiC, and HP SiC. Erosion damage reduced the flexural strengths of all of the specimens. The relative strength reductions were lowest for the biomorphic eucalyptus and highest for the HP SiC. The hot-pressed SiC responded as predicted by accepted models of impact damage in brittle solids. The responses of the biomorphic and reaction-bonded SiC specimens were modeled as if they consisted of only SiC and porosity. This approximation agreed reasonably well with observed degradations of strength.

Current understanding of superplastic deformation of Y-TZP and its application to joining

A review of the high-temperature, steady-state creep of yttria tetragonal zirconia polycrystals (Y-TZP) as a function of grain size, strain rate, stress, and temperature will be presented in this paper. Data have been analyzed using the standard creep equation incorporating a threshold stress. Microstructural observations of deformed samples indicated that deformation was achieved primarily by grain boundary sliding (GBS). This microscopic characteristic was used to join pieces of these materials. When two pieces of ceramics are compressed in the superplastic regime, GBS will produce a junction between the pieces. Several types of strong junctions have been produced, i.e. between pieces of the same material having different grain sizes, and between pieces of different materials. Scanning electron microscopy of the interfaces showed that each interface was cavity- and crack-free. Vickers indentations and high-temperature mechanical tests indicated that the interfaces were as strong as the matrix. This joining technique allows the construction of a complex shape or a functional gradient material.

Effect of layer interfaces on the high-temperature mechanical properties of alumina/zirconia laminate composites

Abstract High-temperature plastic deformation of laminar composites containing layers of Al 2 O 3 and a mixture of 85 vol.% Al 2 O 3 +15 vol.% 3 mol% Y 2 O 3 -stabilized tetragonal ZrO 2 (ZTA) produced by sequential slip casting is examined in uniaxial compression testing. The mechanical behavior is compared with that of monolithic Al 2 O 3 and ZTA synthesized by the same technique. The layered composites exhibit better creep properties at low strain rate than either of the two constituent materials. The good interfacial adhesion of the layers imparts creep resistance and ductility simultaneously to the laminates.

Synthesis of mullite powders by acrylamide polymerization

Mullite (3Al{sub 2}O{sub 3} {center_dot} 2SiO{sub 2}) ceramics and composites are widely used. Synthesis of mullite powders, especially those that can be readily sintered, remains a focus of much current research. In support of recent efforts to fabricate mullite fibrous monoliths and to use superplastic flow to join ceramics, we have looked to synthesize reactive mullite powders. Recent advances in application of acrylamide polymers to ceramic synthesis offer promise of obtaining large quantities of high-quality powder at relatively low cost. We report here on synthesis from acrylamide monomers of mullite powders of two interesting particle sizes.

Joining particulate and whisker ceramic composites by plastic flow.

Joining of bulk ceramics by plastic flow has been demonstrated for several fine-grained ceramics. We have joined, for example, submicrometer 3 mol% Y2O3 partially stabilized ZrO{sub 2} (YSZ) and YSZ-toughened Al{sub 2}O{sub 3}. The interlayers in these cases consisted of dense, submicrometer-grain-sized pieces of ceramics that have been shown to exhibit superplastic flow. We have extended the joining work to include incorporation of 15 vol.% SiC or 20 vol.% TiC whiskers within the bond layer. Unlike YSZ/Al{sub 2}O{sub 3}, because of the presence of the whiskers, YSZ/Al{sub 2}O{sub 3}/SiC and YSZ/Al{sub 2}O{sub 3}/TiC do not deform superplastically. However, virtually perfect YSZ/Al{sub 2}O{sub 3} joints with and without whiskers could be made by compressing at 1300-1350 C and stresses of <20 MPa.