Strength Improvement in Transformation Toughened Ceramics using Compressive Residual Surface Stresses

Abstract

ABSTRACTA1 2 03–15 vol. % ZrO2 bar shaped composite specimens were fabricated by pressing three layers. The two outer layers consisted of Al2O3 and unstabilized ZrO2 (primarily in the monoclinic polymorph), and the inner layer consisted of Al2O3 and partially stabilized zirconia in the tetragonal polymorph. The transformation of ZrO2 from tetragonal to monoclinic, upon cooling from sintering temperature, led to the establishment of residual compressive stresses in the outer layers. Flexural tests at room temperature showed that residual stresses contributed to strength increasing from 450 to 825 MPa. The existence of these stresses was verified by measuring apparent fracture toughness, as well as using strain gages. Strength and toughness data were obtained at 500, 750, and 1000°C. X-ray diffraction was used to explain the elevated temperature data by monitoring the monoclinic to tetragonal transformation upon heating to 1000°C.