[1] Department of Ceramics and Refractory Materials, RWTH Aachen University, Mauerstrasse 5, 52064 Aachen, Germany
[2] Department of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52074 Aachen, Germany
[3] Central Facility for Electron Microscopy, University RWTH Aachen University, Ahornstrasse 55, 52074 Aachen, Germany
[4] Department of Prosthodontics, Clinic of the Ludwig Maximilian University München, Goethestrasse 70, 80336 München, Germany