Strength Improvement in Transformation‐Toughened Alumina by Selective Phase Transformation

A1 2 O 3 ‐15 vol% ZrO 2 bar‐shaped ceramic specimens were fabricated in the green state in such a way that the near surface regions consisted of A1 2 O 3 and unstabilized ZrO 2 while the bulk consisted of A1 2 O 3 and partially stabilized ZrO 2 . After sintering, specimens had macroscopic residual compressive surface stresses and balancing interior tensile stresses due to the tetragonal‐to‐monoclinic phase transformation in the outer layers which occurs during cooling. The depth of the surface region was controlled during green forming. Residual surface compressive stresses at room temperature varied between 100 and 400 MPa 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 of tetragonal zirconia into monoclinic polymorph which occurred upon cooling from the sintering temperature. Specimens with residual surface compressive stresses were 200 MPa stronger at 750°C than monolithic specimens, demonstrating the viability of this approach for improving elevated‐temperature mechanical properties.