Processing and characterisation of three-layer alumina-based composites with enhanced damage tolerance

Abstract

Three-layer alumina-based composites reinforced with iron in the inner layer and with chromium in the outer layers were fabricated by first uniaxially pressing the three-layer assembly, followed by cold isostatic pressing at 300 MPa and sintering in a graphite vacuum furnace at 1500 °C for 1 h. The residual compressive stresses in the outer Cr–Al/Al 2O 3 layers and the residual tensile stresses in the Fe–Al/Al 2O 3 inner layer were predicted as a function of composition and the thickness ratio of the outer and inner layers. Theoretical calculations showed that the compressive stresses in the outer layers increased while the tensile stresses in the inner layer decreased with decreasing outer layer thickness. The existence of compressive stresses was verified by microscopic evidence, which showed that propagation of cracks perpendicular to the interface is suppressed in the outer layer, but promoted in the inner layer. Indentation and subsequent strength testing showed that these layered composites exhibited improved damage tolerance. Three-layer composites showed four-point bend strengths exceeding the bend strength of unindented monolithic Al 2O 3 even after indentation at 300 N.