Marc Rubat du Merac*, Martin Bram, Jürgen Malzbender, Mirko Ziegner, Marcin Rasinski, Olivier Guillon**

Forschungszentrum Jülich GmbH, Jülich, Germany
CeramTec GmbH, Plochingen, Germany
JARA-Energy, Aachen, Jülich, Germany

A simple but effective concept has been developed for toughening transparent ceramics. For the first time, unprecedented surface compression of a polycrystalline transparent ceramic was demonstrated by applying functional, low thermal-expansion coatings. For proof of concept, a thin, transparent yttria coating was deposited on a transparent yttria-stabilized zirconia substrate by physical vapor deposition. Details of processing substrate and coating are summarized. After cooling down from processing temperature, lower coefficient of thermal expansion (CTE) of yttria generated residual compressive stresses on the sample surface, which in turn, increased the fracture toughness. Toughness could be further improved by thermal post-treatment due to cation counter-diffusion at the substrate-coating interface. In sum, compressive stresses of up to 750 MPa - same order of magnitude as for ion-exchange strengthened glasses – resulted in fracture toughness increase by a factor 2 – 3 depending on the processing conditions. As side effect, transmittance can be increased when selecting a coating material with a lower refractive index than substrate due to reduced reflection. Our transmittance measurements revealed this correlation. It is expected that transmittance increase can be even tailored to specific wavelengths by adjusting the coating thickness. The novel technique does not require post-polishing and can be applied as an after-treatment to finished products. If choosing the right material combinations, the concept can be transferred to other transparent polycrystalline ceramics as well.