Thermal cycling effect on the phase stability and fracture resistance of synthetic barium/magnesium aluminosilicate systems

Abstract

This paper focuses on the high-temperature solid-state synthesis of barium-magnesium-aluminosilicate (BMAS) system and its thermal cycling stability between the room temperature and 1200 °C. The BMAS powder was synthesized by adding 0.8 mol of MgO for 1 mol of BaAl2Si2O8 (BAS system) to get a stabilized monoclinic phase (celsian-BMAS). The undoped BAS system with a metastable hexagonal phase (hexacelsian-BAS) was also produced for comparison. Both celsian-BMAS and hexacelsian-BAS powders were sintered by spark plasma sintering (SPS) technique at 1300 °C and 50 MPa. In-situ X-ray diffraction and thermogravimetric analyses together with hardness and indentation fracture resistance measurements proved the thermal stability of the celsian-BMAS system. In contrast, the hexacelsian-BAS compound suffered typical phase transformations during the thermal cycling test, causing its failure in less than 25 cycles. The presence of magnesium in the BMAS system led to the formation of secondary phases, such as Ba/Mg-rich micas and/or barium silicides, which did not affect the thermal integrity of the stable celsian phase. The celsian-BMAS system exhibited a linear thermal expansion in all unit cell directions, giving better thermal reliability to this material over the hexacelsian-BAS system. Besides, this work emphasizes the phase transformations in the studied systems that have been ignored in previous research, and it also compares several common equations for calculation of the indentation fracture resistance.