Effect of second phase on mechanical properties and toughening of Al 2O 3 based ceramic composites

Abstract

This work considers the fabrication of Al 2O 3 - based composites with three different types of microstructure: nano composites with the nano-dispersed second phase, hybrid composites with both micro- and nano-sized dispersed second phase, and elongated composites with needle-like in situ dispersed second phases. Methods for improving the mechanical properties of Al 2O 3 ceramics were investigated using Al 2O 3/5 vol% SiC composites fabricated by hot-pressing. Very fine SiC particles were dispersed uniformly in an Al 2O 3 matrix. However, larger SiC particulates existed in grain-boundaries of alumina. The flexural strength was inversely proportional to the square root of the matrix grain size. TEM observation indicated that propagating cracks were deflected by the dispersed SiC particulates. High density Al 2O 3/SiC/YAG hybrid composites having an equiaxed second phase were fabricated in the temperature range from 1000 to 1800°C using SiC and Y 2O 3 powders as additives. YAG (yttrium aluminum garnet, Y 3Al 5O 12) was formed as the second phase from the reaction between Al 2O 3 and Y 2O 3 above 1400°C. Also, Al 2O 3/LaAl 11O 18 (lanthanum-β-alumina) composites, having an elongated second phase, were successfully fabricated using La 2O 3 powder as additives. Microstructural observation of the hot-pressed samples were done by SEM + TEM; the planes were analyzed by XRD. Mechanical properties such as the flexural strength and the fracture toughness of the composites were investigated and exceeded the mechanical properties of the monolithic Al 2O 3. Additionally, the composites having elongated grains showed higher toughness, due to grain bridging, than the composites having an equiaxed second phase.