Transformation toughening of machineable mica glass ceramics

A new two-step heating process was recently developed for the strengthening of mica glass ceramics. Two mechanisms of subsequent toughening were applied by the second heat treatment: (i) the crystallization of a second silicate phase and (ii) the transformation toughening by addition of ZrO2. Transmission-electron-microscopy (TEM) studies were performed to investigate in detail the microstructure development in this two-step heating process. In principle, the crystallization mechanisms were found to be similar for all samples investigated. However, the resulting microstructures deffer significantly even after small compositional changes. In particular, nanocrystalline ZrO2 is precipitated along (001)-boundaries within mica aggregates or as larger particles within the other phases. Only the latter ones are able to grow up to sizes appopriate for transformation tougening. The results indicate routes for optimizing the properties of this type of glass ceramics.