Abstract
The microstructural and stress evolution of thick (25μm) alumina films on dense alumina substrates sintered at temperatures from 1300°C to 1600°C has been investigated. In this study the constraint on sintering was monitored in the absence of significant differences in thermal expansion between the film and the substrate. For comparison purposes unconstrained alumina pellets sintered at 1300°C–1600°C were also examined. Overall, the constrained alumina densified less than the free alumina, as expected, although at intermediate temperatures densification rates were comparable. Sintering in the direction perpendicular to the substrate was enhanced with respect to that parallel to the substrate as a means of stress relaxation. Using fluorescence spectroscopy the residual stresses of the films parallel to the substrates were measured; residual tensile stresses as high as 450±40MPa were exhibited by the films. The considerable stress development resulted in cracking and delamination of the film from the substrate, subsequently film constraint was reduced and densification was not impeded.
Highlights
Free sintering is a well-known method for the manufacture of monolithic ceramic bodies from a porous powder compact
The density and grain size both increase with increasing sintering temperature
When sintered at 1400 1C and above, the films initially exhibit greater increases in density compared to the pellets before showing smaller incremental changes in density following sintering at 1500 1C and 1600 1C, with the density parallel to the substrate being affected to a greater degree
Summary
Free sintering is a well-known method for the manufacture of monolithic ceramic bodies from a porous powder compact. As a consequence the shrinkage of the ceramic during sintering in the directions parallel to the substrate is fully constrained and internal tensile stresses develop as a result, but the direction perpendicular to the substrate is considered free from the constraint [5,6]. The viscous drag, which is more pronounced closer to the substrate, results in the loss of contact between certain particles in the sintering ceramic as the substrate drag forces resist normal densifying sintering forces.
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