Strain‐Induced Deformation in Magnesia–Alumina Layered Composites

Abstract

Ceramic beams are induced in situ to form complex shapes at elevated temperature without the application of an external stress. This process has been demonstrated for thin alumina substrates coated with a layer of magnesia. The internal strain causing the substrates to deform at elevated temperature arises as a consequence of strain mismatch accompanying the penetration of the coating into the substrate. The magnitude of the deformation depends on the amount of coating applied, on the thickness of the substrate, on the density of the substrate, and on the temperature. During exposure of the beams to elevated temperature, the magnesia coating reacts with the alumina substrate to form the spinel phase; the resulting volume change accompanying the phase transformation is likely the predominant driving force for deformation.

Effect of Processing on the Microstructure and Induced‐Strain Mismatch in Magnesia–Alumina‐Layered Composites

“…The longer the hold time, the deeper into the sample Mg is found and the lower the surface concentration. Although this nominal overall composition corresponds to the equilibrium single-phase field of spinel, 31 X-ray diffraction results 5,8 indicate the presence of alumina along with spinel in the h 1 region. Figure 6 shows a SEM image of the sample held at 16001C for 1 h, where the sharp nature of the interface between regions with and without Mg is again evident.…”

“…[1][2][3][4][5] Upon heating, the substrates, which were initially flat, deformed into the shape of an arc. [1][2][3][4][5] Upon heating, the substrates, which were initially flat, deformed into the shape of an arc.…”

“…[3][4][5] This modified approach has several advantages. [3][4][5] This modified approach has several advantages.…”

“…In earlier work, 3,5 we presented the effect of processing variables on the curvature of alumina substrates coated with magnesia. The curvature was then used with bilayer mechanics models 4 to obtain the overall strain mismatch.…”

“…The curvature was then used with bilayer mechanics models 4 to obtain the overall strain mismatch. 5,8 In this work, we present detailed microstructural analysis of the substrates, which indicates to what depth the magnesium penetrates into the sample. 5,8 In this work, we present detailed microstructural analysis of the substrates, which indicates to what depth the magnesium penetrates into the sample.…”