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Al/sub 2/O/sub 3/-ZrO/sub 2/ three layered composites were fabricated in such a way that the outer layers of the bar-shaped specimens consisted of Al/sub 2/O/sub 3/ and unstabilized ZrO/sub 2/, and the inner layer consisted of Al/sub 2/O/sub 3/ and partially-stabilized ZrO/sub 2/. Upon cooling from the sintering temperature, part of the ZrO/sub 2/ in the outer layers transformed from tetragonal (t) to monoclinic (m), while the inside layer ZrO/sub 2/ remained tetragonal. The transformation created compressive stresses in the outer layers and tensile stresses in the bulk. The depth of the outer layers (and therefore the magnitude of the residual stresses) was controlled during green forming. Residual stresses at room temperature varied depending on the outer layer thickness. The increased strengths of the three layer specimens were obtained in the as-fired unground condition, demonstrating that the stresses introduced are the result of transformation-induced stresses during cooling, and did not from transformation produced by grinding. Specimens with residual surface compressive stresses were 200 MPa stronger at 750/degree/C than monolithic specimens, demonstrating the viability of this approach for improving elevated temperature mechanical properties. Strain gauge, x-ray diffraction, and indentation/strength measurements were used to verify the existence of the macroscopic residual stresses.more » Slip casting and tape casting were investigated as alternative green forming routes to powder pressing. The substitution of HfO/sub 2/ for ZrO/sub 2/ in order to increase the transformation temperature was investigated. 28 refs., 25 figs., 7 tabs.« less |
