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A technique for producing ceramic preforms with graded porosity via colloidal infiltration of molded polymer sponges has been developed. Polymer sponges have been compression molded into various shapes with continuous and/or stepwise gradients in porosity. Infiltration of the graded sponge with ceramic slurry, followed by pyrolysis of the sponge and sintering of the ceramic, produces a ceramic component with graded porosity. A relationship between compressive strain and the volume fraction of porosity in the sponge has been derived and compared to measured values. The applicability of this technique to the manufacture of planar and axisymmetric composite shapes is demonstrated.

In-situ crack-propagation experiments, in conjunction with thermochemical experiments, have been used to examine the role of discontinuous interphases on the fracture behavior of solid-state diffusion-bonded Cu/α-Al2O3 couples. Clean, interphase-free interfaces exhibit crack extension by brittle decohesion at the crack tip at an initiation fracture energy of 125 J/m2. Crack propagation is characterized by an increase in the fracture energy with increases in the crack length (R-curve behavior). When interfacial chemical reaction products are present, the crack growth is altered, depending on the characteristics of the interphase. The presence of Cu2O needles results in preferential debonding along the Cu2O/Al2O3 interface. On the other hand, finer CuAlo2 needles visibly impede crack propagation and result in a higher interface initiation fracture energy (}190 J/m2) than that of the interphase-free interface. The effects of the Cu2O and CuAlo2 phases on the fracture energy are discussed.

The influence of oxygen on the wetting behavior of copper on single-crystal Al2O3 has been studied. By controlling the oxygen partial pressure (pO2) and oxygen content in the copper simultaneously, contact angle can be varied between 125° and 22°. Evaluation of the Gibbs adsorption equation for the liquid/solid interface at 1300°C suggests that adsorption of a Cu-O complex at that interface plays a key role in promoting wetting. Formation of CuAlO2 and dissolution of Al2O3 in the melt also influence the contact angle, especially in the range of pO2 > 10−5 bar (1 Pa).

The single-edge V-notched-beam (SEVNB) testing geometry was used to measure the crack growth resistance (R-curve) behavior of multilayered alumina-zirconia composites. Fracture mechanics weight function analysis was applied to predict the R-curve behavior of multilayered composites having a stepwise change in composition. These results were then used to differentiate the influence of residual stresses from crack-bridging stresses on the measured R-curve behavior.

Composites containing Ce-ZrO2, Al2O3, and aligned Al2O3 platelets were produced by centrifugal consolidation and pressureless sintering, followed by heat treatments at 1600°C for varied duration. Constituents in the consolidated microstructures were either uniformly distributed throughout or segregated into gradient layers, depending critically on platelet content. Quantitative image analysis was used to examine microstructure development with heat treatment. Changes in the volume fraction, dimensional anisotropy, and gradient of pores and platelets, as well as changes in the phase gradient, were quantified. Microstructure development was strongly dependent on the initial microstructure design attained from suspension processing.

Infiltration of alumina powder compacts by molten copper-oxygen alloys under controlled oxygen partial pressures has been studied. Spontaneous infiltration occurred for copper alloys containing more than approximately 5 at.% oxygen. Despite supersaturation of the copper with oxygen, no Cu2O or interfacial reaction layers of CuAlO2 were resolved by TEM, suggesting the critical wettability for infiltration is not determined by interfacial chemical reaction. CuAlO2 did form, but as discrete inclusions up to 0.5 mm in diameter. Infiltration and microstructure development are discussed in terms of the observed wetting behavior and available data on interfacial oxygen adsorption.

Dilute, dispersed, multicomponent suspensions containing Ce-ZrO{sub 2}, Al{sub 2}O{sub 3}, and varying Al{sub 2}O{sub 3}-platelet contents were centrifugally consolidated. Despite the favorable conditions for segregation, phase and size segregation did not occur in the highest Al{sub 2}O{sub 3}-platelet content suspension. The suspension properties did not consist of agglomeration, high solids fractions, or high suspension viscosities typically attributed with segregation prevention. Thus, a new mechanism is suggested, considering the effects of hydrodynamic particle interactions (interference drag or drafting) on particle arrangement during consolidation. Particle drafting is proposed and supported as a mechanism for preventing segregation, and the importance of hydrodynamic particle interactions in ceramic processing is discussed.

Microstructure development at solid-state diffusion-bonded Cu/α-Al2O3 interfaces has been studied using optical and electron microscopy. High-purity Cu foil was bonded between basal-oriented α-Al2O3 single-crystal plates at 1040°C for 24 h in a vacuum of ∼1.3 × 10−4 Pa (1 × 10−6 torr). Optical microscopy of as-bonded specimens revealed a large Cu grain size, fine pores, and long needles of Cu2O at the interface. Bulk specimens were annealed at 1000°C for various times under controlled oxygen partial pressures in CO/CO2 mixtures. Consistent with a thermochemical analysis, CuAlO2 could be formed at the interfaces. The CuAlO2 was acicular and discontinuous, but occurred in a uniform distribution over the bulk specimen interfaces.

A pseudobinary thermodynamic analysis of the alumina/ copper-oxygen eutectic bonding process yields a value of approximately 1076°C for the invariant temperature of the {Cu(ss), Cu(L1), CuAlO 2 , Al 2 O 3 } equilibrium. This temperature, which is 11°C above the eutectic, corresponds to a critical temperature for stability of the copper aluminate CuAlO 2 in eutectic bonding. Implications for controlling interface microstructure are highlighted.

The details of fluid flow in tape casting of a Newtonian slurry with a parallel blade are analyzed in terms of the relative magnitudes of the driving forces (wall shear and pressure gradient) and the resulting tape thickness. Prior studies are limited to describe the flow under the blade and within the reservoir. Consideration of the flow after the blade exit indicates that the existence of a zero-shear plane might have a negative effect on particle alignment in the tape. The more common case of a beveled blade is also analyzed using the one-dimensional flow model, which is shown to be valid for blade angles up to about 45°. The beveled and parallel blades are compared in terms of productivity, minimum thickness, and potential influence on particle alignment.