The CMAS associated degradation of 7YSZ TBC layers is one of the serious problems in the aero engines that operate in dusty environments. CMAS infiltrates into TBC at high temperatures and stiffens the TBC which ultimately loses its strain tolerance and gets delaminated. The EB-PVD technique is used to coat TBCs exhibiting a columnar microstructure on parts such as blades and on vanes. By varying the EB-PVD process parameters, columnar morphology and porosity of the 7YSZ coating is changed and its effect on the CMAS infiltration behaviour is studied in detail. Two different TBC pore geometries were created and infiltration experiments were carried out at 1250 • C and 1225 • C for different time intervals. The 7YSZ coating with more 'feathery' features has resulted in higher CMAS resistance by at least by a factor of 2 than its less 'feathery' counterpart. These results are explained on the basis of a proposed physical model.

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Infiltration and deposition of CaSO 4 in thermal barrier coatings (TBC) in addition to the CMAS deposits was found in many occasions on real aviation engines. The source and role of CaSO 4 on the degradation of TBC is not well understood. CaSO 4 containing CMAS was synthesized and a systematic study of its role on the CMAS infiltration behavior in EB-PVD 7YSZ is presented in this work. Its influence on the melting and crystallization behavior of CMAS was studied with the help of differential scanning calorimetry. The decomposition of CaSO 4 into CaO and SO 3 was observed at 1050°C in laboratory air under the presence of CMAS using mass spectroscopy and in situ high-temperature XRD. The same amount of CaO is brought into the CMAS system by means of adding CaCO 3 , which will eventually decompose into CaO and CO 2 at 700°C. CMAS infiltration tests were carried out at different temperatures with and without CaSO 4 /CaCO 3 and the results demonstrate that the sulfur has no direct effect on the aggressiveness of the anhydrite containing CMAS with regard to its infiltration behavior in EB-PVD 7YSZ at high temperatures. The extra amount of calcia added to CMAS that is introduced by the evaporating species is responsible for enhanced infiltration of the deposits into the TBC.