RE4Hf3O12 hafnates developed for multifunctional thermal/environmental barrier coating applications

Rare-earth hafnates RE4Hf3O12 exhibit tremendous potential as candidates for exterior thermal barrier coatings (TBCs) in the manufacture of multilayer thermal/environmental barrier coating (T/EBC) architectures, providing enhanced protection for SiCf/SiC ceramic matrix composites (CMCs). Three RE4Hf3O12 (RE = Y, Er and Yb) ceramics were fabricated, and their phase structures were characterized by x-ray diffractometry and Raman spectroscopy. It has been revealed that the crystal structure of RE4Hf3O12 shows great dependence on the ionic radius ratio r(RE)/r(Hf). The three RE4Hf3O12 compounds not only demonstrate excellent phase stability up to 1500 degrees C, but also exhibit exceptional chemical inertness to Yb-based monosilicate, a leading EBC material. Furthermore, the intermediate linear thermal expansion coefficient (8-9 x 10(-6) K-1), low thermal conductivity (1.30-1.86 W/m<middle dot>K, 1000 degrees C), high hardness (similar to 10.0 GPa) and fracture toughness (1.93-2.02 MPa m(1/2)) jointly highlight the potential T/EBC application of RE4Hf3O12 compounds, especially Yb4Hf3O12.