The Accelerating Effect of CaSO4 Within CMAS (CaO-MgO-Al2O3-SiO2) and Its Effect on the Infiltration Behavior in EB-PVD 7YSZ

Abstract

Infiltration and deposition of CaSO4 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 CaSO4 on the degradation of TBC is not well understood. CaSO4 containing CMAS was synthesized and a systematic study of its role on the CMAS infiltration behaviour in EB-PVD 7YSZ is presented in this work. Its influence on the melting and crystallisation behaviour of CMAS was studied with the help of differential scanning calorimetry. The decomposition of CaSO4 into CaO and SO3 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 CaCO3 which will eventually decompose into CaO and CO2 at 700 °C. CMAS infiltration tests were carried out at different temperatures with and without CaSO4/CaCO3 and the results demonstrate that the sulphur has no direct effect on the aggressiveness of the anhydrite containing CMAS with regard to its infiltration behaviour 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.