Reactive wetting of mullite Al sub(2)[Al sub(2 + 2)xSi sub(2 - 2)x O sub(10 - )xsingle crystals by yttrium-aluminosilicate and borosilicate glasses

Mullite, Al sub(2)[Al sub(2 + 2)xSi sub(2 - 2)xO sub(10 - )x is introduced as a suitable reactive wetting model system for oxide ceramic compounds. Apparent contact angles on single crystal mullite substrates have been measured from sessile drop experiments involving (i) a highly reactive yttrium aluminosilicate (YAS) glass and (ii) a less reactive borosilicate (BS) glass. The apparent contact angles decrease with crystallographic orientation in the following order: (010) > (100) > (001) independent of glass composition. The surface energy, g sub(SV), has been identified as the dominant term controlling reactive wetting with g sub(SV) (010) < g sub(SV)(100) < g sub(SV)(001). This order of surface energy values is rationalized in terms of the high anisotropy of the crystal structure and elastic properties of mullite. The YAS glass reacts stronger with the polycrystalline 3/2 mullite substrate due to the grain boundaries acting as fast diffusion paths. In the YAS/mullite system, analytical electron microscopy shows that for the single crystal 2/1 mullite substrate, a corundum + Y sub(2)Si sub(2)O sub(7)-rich crystalline phase assemblage results upon devitrification while in the case of the polycrystalline 3/2 mullite substrate a 2/1 mullite + Y sub(2)Si sub(2)O sub(7)-rich crystalline assemblage is formed instead. In the less reactive borosilicate system secondary 2/1 mullite microcrystals precipitate at the S/L interface with (i) random orientations established on polycrystalline substrates and (ii) characteristic preferred orientations on the single crystal substrate. A thin Al-rich interdiffusion zone (3 km) right at the S/L interface is revealed for both glass systems.