Synthesis and mechanical properties of 40 wt%BAS/Si3N4 ceramic composites

Abstract

Advanced glass-ceramics, such as barium aluminosilicate (BaO · Al2O3 · 2SiO2) have found applications as matrices in composites, as heat exchanger parts and substrate materials [1]. Of the glass-ceramics being considered for high-temperature application, barium aluminosilicate (BAS) is one of the most promising because of its high melting temperature (1760 ◦C) [2], low thermal expansion (2.29 × 10−6 ◦C−1 from 22 ◦C to 1000 ◦C for the monoclinic form) [3], good oxidation resistance and low dielectric constant. However, the monolithic BAS glass-ceramic exhibits relatively poor mechanical properties, which limits its use in many structural applications. Whiskers [4], platelets [5] and continuous fibers [6] have been used to improve the mechanical properties of BAS. While improvements in properties have been demonstrated, ease of production and low cast have been sacrificed. In situ whisker growth can alleviate these problems. Although many candidate whisker materials are difficult to grow in situ, the formation of elongated, whisker-like β-Si3N4 grains has been observed in multiple studies [7–9]. Preparation of dense Si3N4 usually requires the use of additives, which react with the SiO2 present on the Si3N4 surface to form a liquid phase that facilitates densification. Although many oxide additives have been successfully demonstrated to produce dense Si3N4 materials, in most cases, the additives in Si3N4 end up as a grain-boundary glass phase, which softens and degrades the properties at high temperature. Therefore, a good additive system should form the liquid phase at a low temperature (a low liquid eutectic temperature) and later the liquid phase should crystallize completely into a compound with a high melting point. BAS fulfils these requirements. In this study, BAS composites reinforced in situ with Si3N4 whiskers were fabricated to investigate the effects of the BAS additive and sintering condition on