Interfacial Glass Structure Affecting Micromechanism Of Fracture in a Fluorine-Doped Si3N4-SiC Composite

Abstract

In the field of structural ceramics, ceramic-ceramic composites such as Si3N4-SiC components have gained wide interest owing to their potential application at elevated temperatures. The incoporation of ceramic reinforcements, e.g., SiC whiskers or platelets, into a ceramic matrix can improve mechanical properties, however, densification is rendered difficult. Due to the high covalent bonding character of Si3N4, liquid-assisted sintering is required for complete densification even of monolithic materials [1,2]. The addition of metal oxides or transition metal oxides, which react with the SiC2 present on the Si3N4-particle surface to form a silica-rich liquid at high sintering temperatures, enables liquid-phase sintering. The remains of this liquid are commonly present at triple-grain junctions and along grain boundaries as a secondary glass. Post-densification heat treatment can partially crystallize these glass pockets [3]. One of the aims to further improve high-temperature performance of ceramic composites is to drastically reduce the amount of secondary phases. A high volume fraction of an amorphous phase strongly decreases the mechanical properties, because the glass softens at relatively low service temperatures, i.e., about 900°C. Therefore, materials were prepared without the addition of further sintering aids [4,5]. Hereby, liquid-assisted densification is achieved by the SiO2 present in the Si3N4 starting powder. Owing to the high melting temperature of pure silica glass, hot-isostatic pressing (HIPing) was utilized as the densification technique. These materials, formed by HIPing without the addition of sintering aids, could be fully densified and, moreover, showed superior creep behavior, when compared with commercial materials [61]. It should be emphasized that such Si3N4 materials with pure silica present at triple pockets and along grain boundaries, revealed a rather low fracture resistance, which is due to the predominantly transgranular mode of fracture [7,8].