Secondary Phases in Hot‐Pressed Aluminum‐Boron‐Carbon–Silicon Carbide

Abstract

Silicon carbide hot‐pressed with aluminum, boron, and carbon as sintering aids (ABC–SiC), was studied by transmission electron microscopy. Both grain‐boundary films and inclusions were prevalent in this material. The present study characterized the inclusions located in triple‐junctions, grain boundaries, and the interior of the SiC grains, with emphases on phases not scrutinized before. These inclusions were crystalline, in contrast to the amorphous grain‐boundary films. Two dominant types of boron‐free triple‐junction phases containing Al(Si)‐O‐C‐(S) and Al(Si)‐O were identified, where sulfur was an unexpected contaminant, and silicon came from SiO2 or from dissolution of SiC. Boron‐containing inclusions with a composition Al‐O‐B‐C were frequently observed inside SiC grains. Although the boron‐free aluminum‐rich phases wet the grain boundaries completely and are, therefore, effective sintering additives, the boron‐containing Al(Si)‐O‐B‐C did not wet the grain boundaries. The structure and chemical composition of these boron‐containing intragranular inclusions were determined, and their mechanism of formation is discussed.