Aero-engines operating in sand-laden (CMAS/CaO–MgO–Al ₂O ₃–SiO ₂) environments often encounter severe problems with thermal barrier coatings (TBCs) due to CMAS infiltration and erosion damage. This study focuses on a deeper understanding of the erosion behavior of CMAS-infiltrated EB-PVD Gd ₂Zr ₂O ₇ TBCs. The study includes isothermal infiltration of different CMAS and subsequent erosion tests at room temperature. In addition to the erosion behavior of the entire coating, the influence of different reaction products within the reaction layer on erosion failure was investigated by measuring the hardness and Young's modulus of the individual phases using in-situ REM-Nanoindentation. It was found that a garnet layer above the reaction layer and spinel inclusions within a thick apatite/fluorite reaction layer, can improve the erosion resistance of this reaction layer by 30–40%. Furthermore, a correlation between the erosion behavior and the hardness vs. Young's modulus relation, obtained from nanoindentation over the entire coating, was observed for a consistent microstructure.