Atomic-scale insights into CMAS-induced grain boundary embrittlement in thermal barrier ceramics

We report a study on grain boundary (GB) embrittlement in yttria stabilised zirconia (YSZ) ceramics induced by calcia-magnesia-alumino-silicate (CMAS). Model ceramics composed of tetragonal YSZ grains and intergranular CMAS were fabricated through sintering of YSZ-CMAS powder mixtures with incremental CMAS additions up to 1 wt%. Microstructural characterisations reveal that, although glassy CMAS films are present at triple junctions and between YSZ grains, most GBs are free of glassy films but enriched with CMAS-derived impurities. As the CMAS content increases the fracture toughness drops significantly, and the fracture mode shifts increasingly towards intergranular. Fractographic analysis at the atomic scale shows that fractures occur within the cores of glass-free GBs, whereas in GBs containing glassy CMAS films, fractures occur at the CMAS/YSZ interface, where a 2 nm layer of monoclinic YSZ is present. The CMAS-induced grain boundary embrittlement is explained from the perspective of fracture energy, with contributions from GB energy, ionic bonding, oxygen vacancies and ionic size mismatch taken into consideration.