How CMAS infiltration degrades grain boundaries in thermal barrier ceramics: dual pathways to embrittlement

Thermal barrier coatings used in aeroengines are vulnerable to calcia-magnesia-alumino-silicate (CMAS) attack, which infiltrates through pores and grain boundaries (GBs), accelerating coating degradation. However, the influence of CMAS on GB fracture resistance remains insufficiently understood. Here, we combine high-resolution characterization, micromechanical testing, and density functional theory (DFT) calculations to elucidate how infiltrated CMAS degrades fracture toughness and alters fracture behaviour in yttria-stabilized zirconia (YSZ), a model thermal barrier ceramic. Two distinct GB types are identified in CMAS-infiltrated layers: (i) GBs containing nanoscale glassy CMAS films near the surface, and (ii) GBs enriched with CMAS-derived cations (Al³⁺ and Si⁴⁺) but without discernible glassy phases at greater depths. Micromechanical tests reveal a fracture toughness gradient that increases from the surface inward, accompanied by a transition from intergranular to transgranular fracture. When GBs are filled with glassy CMAS films, cracks preferentially propagate through the intergranular CMAS layers due to its low intrinsic toughness, leading to pronounced GB embrittlement. However, an unexpected yet important finding is that GBs doped with Al³⁺ and Si⁴⁺ also exhibit significant embrittlement even in the absence of continuous glassy CMAS films. DFT calculations show that the incorporation of Al³⁺ and Si⁴⁺ into GBs weakens Zr-O bonds along potential fracture paths through site-specific substitution or interstitial occupation. This bond-weakening effect-independent of whether Al³⁺ or Si⁴⁺ acts alone or together-increases GB energy while reducing separation energy, thereby lowering fracture toughness with increasing impurity concentration. The study provides direct atomistic insight into CMAS-induced GB embrittlement mechanisms in thermal barrier ceramics.