Surface Crystallization of a MgO/YO/SiO/AlO/ZrO Glass: Growth of an Oriented β-YSiO Layer and Epitaxial ZrO.

The crystallization behavior of a glass with the composition 54.7 SiO·10.9 AlO·15.0 MgO·3.4 ZrO·16.0 YO is studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD) and (scanning) transmission electron microscopy [(S)TEM] including energy-dispersive X-ray spectrometry (EDXS). This glass shows the sole surface crystallization of four different yttrium silicates of the composition YSiO (YS). The almost simultaneous but independent nucleation of α-, β-, δ-, and ε-YS at the surface is followed by growth into the bulk, where ε-YS quickly dominates a first crystallized layer. An accumulation of Mg at the growth front probably triggers a secondary nucleation of β-YS, which forms a thin compact layer before fragmenting into a highly oriented layer of fine grained crystals occupying the remaining bulk. The residual glass between the YS growth structures allows the crystallization of indialite, yttrium stabilized ZrO (Y-ZrO) and very probably μ-cordierite during cooling. Hence, this glass basically shows the inverted order of crystallization observed in other magnesium yttrium alumosilicate glasses containing less YO. An epitaxial relationship between Y-ZrO and ε-YS is proven and multiple twinning relationships occur in the YS phases.