A critical durability issue of thermal barrier coatings (TBCs) can include their resistance to degradation due to molten deposits arising from fuel impurities and air-ingested CMAS (calcium-magnesium alumino silicate) sand deposits. Experimentally, free standing YSZ and CoNiCrAlY coatings, both 300 μm in thickness, were processed by individually depositing 8YSZ and CoNiCrAlY powders on graphite substrates by air plasma spraying (APS) process. High temperature degradation of these coatings in contact with fuel impurities such as V₂O₅, P₂O₅, NaVO₃, Na₂SO₄ and CaSO₄, and a model CMAS deposit, were investigated at temperatures up to 1400°C. Phase transformations, degradation reactions and microstructural development were examined by using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Density-functional theory calculations were use to understand fundamental aspects of structure and reactions at the surface of fluorite-structured oxides. It was found that oxygen-vacancy clustering is critical for surface reactivity. Mitigation of degradation in YSZ due to infiltration, reaction and phase transformation will be discussed.