Pressureless Densification of Alumina - Titanium Diboride Ceramic Matrix Composites

by Hunt, Michael Patrick

The research focus was to determine diffusion mechanisms responsible for densification behavior of SHS produced Al2O3/TiB2 Ceramic Matrix Composites (CMCs). Previous research has shown SHS produced Al2O3/TiB2 composites exhibited unique microstructural properties that contributed to high strength, fracture toughness, and hardness properties. Pressureless densification of SHS produced Al2O3/TiB2 composites would provide a cost savings because the equipment for pressureless densification is less expensive and less complicated than equipment required for densification with pressure. Models for sintering of CMCs and calculation of Sintering Time Constants (STC) were used to predict the densification behavior of the SHS produced Al2O3/TiB2 composite. The Levin, Dirnfeld, Shwam equation was used to determine the Rate Controlling Diffusion Mechanism (RCDM) and activation energy for sintering. X-Ray Diffraction (XRD) analysis of the as-milled reaction product powder revealed the presence of an aluminum borate (Al18B4O33) as a third phase, as well as, in pressureless heat treated samples. Based on experimental results and analysis, it seemed possible the Al18B4O33 compound may have formed by reaction of Al2O3 with TiB2 along their interfaces. Aluminum borates have been observed to form Al18B4O33 (s) + B2O3 (l) at temperatures above 1000°C. The RCDM for densification of SHS produced Al2O3/TiB2 was found to be liquid phase diffusion with volume diffusion also likely being active during densification. In addition, Al18B4O33 seemed to be the preferred compound formed during oxidation. Further research should be performed to control formation of Al18B4O33; as well as, on the oxidation behavior of the SHS produced Al2O3/TiB2.