Improving the Microstructural Stability of Nextel™ 610 Alumina Fibers Embedded in a Porous Alumina Matrix

TLDR: In this paper, small amounts of silica were added to a CMC starting material with a porous alumina matrix via the sol-gel technique, which effectively suppresses grain growth in fibers and matrix.

Abstract: Nextel™ 610 alumina fibers embedded in ceramic matrix composites (CMCs) with a pure alumina matrix show pronounced grain coarsening above 1350°C, which is not observed for stand-alone fibers under identical conditions. Moreover, a size gradient with small grains in the fiber center and large grains at the fiber periphery occurs. This can be explained in terms of the outward diffusion of silica traces, which normally reduces the fiber grain-boundary mobility. Silica outdiffusion and the related excessive grain growth can be suppressed if small amounts of silica are added to the alumina matrix in the green state. Aluminosilicate green compositions, on the other hand, result in a lower matrix sinterability, thus leading to inferior mechanical properties of the CMC. In this communication, we present a novel method to design a CMC that combines the key benefits of an alumina matrix (superior strength) with that of an aluminosilicate matrix (superior thermal stability). Small amounts of silica were added to a CMC starting material with a porous alumina matrix via the sol–gel technique. The silica post treatment effectively suppresses grain growth in fibers and matrix. Mechanical testing revealed significant strength retention in case of the silica-infiltrated CMC.