CRITERION FOR DEVELOPMENT OF NOVEL CMAS/VA RESISTANT TBCS BASED ON MORPHOLOGY AND CHEMISTRY VARIATIONS

Abstract

The evolution of CMAS/VA resistant thermal barrier coatings (TBCs) has been the hot topic in the gas turbine industry and the inevitability is growing exponentially with the constantly increasing turbine inlet temperatures (TIT). Numerous research groups have acquired enormous amount of knowledge on the interactions of TBCs with CMAS, damage mechanisms and methods of analysing the CMAS attack. From all the current knowledge, most substantial parameters which define the CMAS/TBC interactions are shown schematically in Fig.1. Chemical composition and microstructure are illustrated as highly influential parameters in case of CMAS/TBC interaction in Fig.1. Variation in the global CMAS/VA composition means difference in their melting points, viscosities, acidic/basic nature. Any developed novel TBC material will react differently to different CMAS/VA compositions and forms numerous reaction products which might bring a completely new effect in the end. In the presentation novel TBCs such as gadolinium zirconate, alumina and yttria rich zirconia coatings are introduced that were applied by means of EB-PVD method and their interactions with synthetic CMAS and natural volcanic ash was studied in detail. The reaction products heavily depend upon the CMAS chemistry and vary with respect to the crystallinity of the CMAS compound. However, the TBC microstructure and the porosity define the kinetics of infiltration, reaction and hence play a vital role in hampering the CMAS infiltration. Two different coating methods electron beam physical vapour deposition (EB-PVD) and atmospheric plasma spray (APS) have entirely different microstructure and porosity network. Moreover, due to the contrastive porous geometries, the sintering of these ceramic coatings differs severely which would change the permeability of the coating. Temperature and time are other two important factors that define the melting process and reaction time for the CMAS and TBC to happen. Two different EB-PVD microstructures 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. Moreover, these results are well supported by FEM simulations using ABAQUS and microstructural modifications where effects of feather arm lengths, inter-columnar gap width and feather inclinations on the infiltration kinetics are evaluated. The experimental infiltration results were compared with theoretical infiltration estimations using a novel mathematical approach proposed in previous studies which assesses the permeability of the coatings with two different methods called ‘concentric pipe and open pipe models’. The infiltration depth was calculated using experimentally measured variables such as contact angle and viscosity. The theoretical and experimental results are in good agreement for CMAS infiltration confirming the validity of the physical model.