EFFECT OF 2ND-PHASE ON MECHANICAL-PROPERTIES AND TOUGHENING OF AL2O3 BASED CERAMIC COMPOSITES

This work considers the fabrication of Al2O3- 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 Al2O3 ceramics were investigated using Al2O3/5 vol% SiC composites fabricated by hot-pressing. Very fine SiC particles were dispersed uniformly in an Al2O3 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 Al2O3/SiC/YAG hybrid composites having an equiaxed second phase were fabricated in the temperature range from 1000 to 1800 degrees C using SiC and Y2O3 powders as additives. YAG (yttrium aluminum garnet, Y3Al5O12) was formed as the second phase from the reaction between Al2O3 and Y2O3 above 1400 degrees C. Also, Al2O3/LaAl11O18 (lanthanum-beta-alumina) composites, having an elongated second phase, were successfully fabricated using La2O3 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 Al2O3 Additionally, the composites having elongated grains showed higher toughness, due to grain bridging, than the composites having an equiaxed second phase.