The low temperature hexagonal to orthorhombic transformation in Si 3 N 4 reinforced BAS matrix composites
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W.D. Porter and O.B. Cavin High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6064 (Received 28 October 1994; accepted 25 January 1995)
The amount of Si3N4 in a Si 3 N 4 -BaO • A12O3 • 2SiO2 (BAS) composite and the time of sintering was examined in the context of the low temperature hexagonal to orthorhombic transformation in BAS. It was found that with increasing amounts of Si3N4 in the composite, the temperature of the hexagonal to orthorhombic transformation was decreased. As the sintering time was increased for a given composite composition, a drop in the temperature of the hexagonal to orthorhombic transformation was observed, which was linked to an increase in the /3-Si 3 N 4 content in the composite. In addition, as the Si 3 N 4 content in the composite was increased, a resultant drop in the coefficient of thermal expansion of the composite occurred. The extent of the linear dimensional change of the composite during the BAS hexagonal to orthorhombic transformation is reported.
I. INTRODUCTION Barium aluminosilicate (BaO • A12O3 • 2SiO 2 , or commonly referred to as BAS) glass ceramics are potential matrix materials for advanced ceramic matrix composites. Celsian, one of the naturally stable phases of BAS, was first discovered in 1895 by Sjogern in the manganese mines of Jakobsberg, Sweden.1 It was considered to be a triclinic phase. Strandmark2 established its monoclinic symmetry in 1903. Thermodynamically, celsian is the low temperature stable phase of BAS and transforms reversibly to the hexacelsian form at 1590 °C.3 Hexacelsian has a hexagonal structure and thermodynamically is the high temperature stable phase of BAS. However, hexacelsian exists in a metastable form at temperatures below 1590 °C, as the hexcelsian to celsian transformation is very sluggish.4 In addition, Yoshiki and Matsumoto5 reported that at 300 °C the metastable hexacelsian undergoes a reversible phase transformation to an orthorhombic structure, which is associated with a significant volume change. Takeuchi6 determined that the body-centered orthorhombic phase has a structure that is similar to the hexacelsian form. The change in structure is confined to a slight shift of the light atoms and, as a result, the x-ray diffraction pattern for both the hexacelsian and orthorhombic phases appear to be identical. This phase transformation may introduce residual stresses when BAS is used as a matrix material in composites. When BAS-forming materials are mixed with predominantly a - S i 3 N 4 powders and sintered, whiskers of /3-Si 3 N 4 are formed, producing a whisker-reinforced 1256
J. Mater. Res., Vol. 10, No. 5, May 1995
composite. The role played by the Si3N4 on transformations within the BAS is little understood. In this study, the compositions and sintering times were systematically varied to produce several composites. Room temperature x-ray diffraction (RTXRD), high temperature x-ray diffraction (HTXRD), differential scanning calorimetry (DSC), and dilatometric analysis were carried
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