Densification and Crystallisation Behaviour of Barium Magnesium Aluminosilicate Glass Powder Compacts

The densification and crystallisation of barium magnesium aluminosilicate (BMAS) glass powder has been investigated. The aim of the study was to draw conclusions of value for the optimisation of the processing parameters for BMAS matrix ceramic composites. Pressureless sintering and hot-pressing techniques were investigated. The pressureless densification behaviour of cold-uniaxially pressed compacts was determined at isothermal and constant heating rate conditions using a high temperature microscope. The samples could be densified isothermally to full density at 930°C prior to the onset of crystallisation. For compacts sintered at constant heating rates between 800 and 1100°C, it was found that the simultaneous occurrence of crystallisation and densification strongly depends on the heating rate. Using hot-pressing (pressure = 20 MPa) results in full densification in the amorphous state after 1 hour at 925°C. X-ray diffraction analysis was used to characterise the crystallinity of pressureless sintered and hot-pressed samples that were fabricated at temperatures between 850° and 1300°C. The crystallisation behaviour did not change, in qualitative terms, with the pressure applied during hot-pressing. Combination of the densification and crystallisation results demonstrated that the BMAS glass can be densified completely at relatively low temperatures (930°C) in the glassy state. The material can be subsequently crystallised at higher temperatures (between 1100 and 1300°C) yielding a high-temperature-resistant microstructure consisting of Ba-osumilite, celsian and cordierite.