Pressureless sintering of $Al_2O_3$-based composites

Abstract

Pressureless sintering has been studied in $Al_2O_3$-TiC particulate and $Al_2O_3$-SiC whisker composites. The current widespread fabrication method for these composites is hot pressing. Sintering and SIN/HIP(sintering followed by hot isostatic pressing) are more desirable methods of fabrication, since complex shapes can be made economically and these processes lend themselves to mass production of near-net-shape material. The densification of $Al_2O_3$-TiC composites is detrimentally affected by chemical reactions between $Al_2O_3$ and TiC. These reactions must be suppressed in order to promote sintering. In this study, the specific reactions occurring in $Al_2O_3$-TiC composites were modeled, using thermodynamic calculations, and verified by experiments. The reaction between $Al_2O_3$ and TiC was suppressed by the use of specially prepared embedding powders allowing pressureless sintering to closed porosity. The $Al_2O_3$-TiC composites were subsequently hot isostatically pressed to $> 99\%$ of theoretical density without encapsulation. Typical flexural strength and fracture toughness of $Al_2O_3$-26 vol% TiC composites were 690 MPa and 4.3 MPa.$m^{1/2}$, respectively. High density compacts, up to 88% of theoretical density, of $Al_2O_3$-SiC whiskers were prepared by pressure casting and impregnation technique. Starting with these green bodies, composites of $Al_2O_3$-20 vol% SiC whiskers were pressureless-sintered to higher than 95% of theoretical density. They were further densified by hot isostatic pressing up to 99% of theoretical density, resulting rupture strength of 680 MPa and fracture toughness of 4. 70 MPa.$m^{1/2}$.