Surface Crystallization of a MgO/Y2O3/SiO2/Al2O3/ZrO2 Glass: Growth of an Oriented beta-Y2Si2O7 Layer and Epitaxial ZrO2

The crystallization behavior of a glass with the composition 54.7 SiO2 center dot 10.9 Al2O3 center dot 15.0MgO center dot 3.4ZrO(2)center dot 16.0 Y2O3 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 Y2Si2O7 (YS). The almost simultaneous but independent nucleation of alpha-, beta-, gamma-, and e-YS at the surface is followed by growth into the bulk, where e-YS quickly dominates a first crystallized layer. An accumulation of Mg at the growth front probably triggers a secondary nucleation of beta-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 ZrO2 (Y-ZrO2) and very probably mu-cordierite during cooling. Hence, this glass basically shows the inverted order of crystallization observed in other magnesium yttrium alumosilicate glasses containing less Y2O3. An epitaxial relationship between Y-ZrO2 and epsilon-YS is proven and multiple twinning relationships occur in the YS phases.