Thermal Barrier Coatings phase stability and in service degradation

This thesis work is the result of a collaboration between Ansaldo Energia's Materials Laboratory and the Department of Chemistry and Industrial Chemistry of the University of Genoa. The study developed from the industrial need to better understand the degradation processes of the turbine blade thermal barriers coating already used in Ansaldo Energia gas turbines and to test new materials that could be used in the future. The material mostly used as a protective coating is 7YSZ (ZrO2 with 7 wt% of Y2O3) which has excellent characteristics and has been used inside turbines for land use and on aircraft engines since the 1960s. The need to increase the operating temperature of the machines, with a consequent increase in efficiency, makes it necessary to modify this material, as it tends to degrade when it works at temperatures above 1300°C. With cooling from high temperatures, the 7YSZ tends to modify its crystallographic structure until the formation of the monoclinic phase, extremely harmful because it is a source of mechanical fragility and an increase in thermal conductivity. A new material has been proposed by Ansaldo Energia, consisting of a double layer, with the upper layer formed by 14YSZ (ZrO2 with 14% of Y2O3), while the lower one remains 7YSZ for better compatibility with the metal substrate. The new material is crystallographically stable but its properties must be characterized before being used in gas turbines. Starting from their precursor powders and reacting long exposure time at high temperature of sprayed material. In addition, the TBC modifications due to interactions with CMAS (Calcium-Magresium-Alluminium-Silicates) have been investigated. Several tests have been performed for times up to 10000h and temperatures between 1100 and 1400°C. The samples have been examined mainly to evaluate the effect of the interaction on the formation of the monoclinic and other secondary phases, the modification of the microstructure and the variation of physical properties. Finally, to better study the interaction phenomena between oxides, a multicomponent thermodynamic database (Al-Ca-Si-Mg-Si-Y-Zr-O) has been implemented using the CALPHAD method. The thesis work has been divided into an experimental part, carried out at the Ansaldo Energia laboratories and a thermodynamic modeling part, conducted at the Department of Chemistry and Industrial Chemistry. The thermodynamic database for oxides has been also implemented through the experimental study of the MgO-Y2O3-ZrO2 system at the University of Freiberg in Germany, in the group of Prof.Olga Fabrichnaya. The experimental part focused on the analysis of thermal barrier specimens exposed at high temperature and examined with usual materials characterization techniques. For this purpose, quantitative analysis of the detected phases has been carried out using Xray diffraction with quantification using the Rietveld method. The microstructural analyzes have been done using optical microscope and FEG-SEM (Field Emission Gun-Scanning Electron Microscope) equipped with EDS (Energy- dispersive detector) and EBSD (Electron Back Scattered Diffraction) to evaluate microstructure variations, phase compositions, grain shapes and dispersion of the phases within the sample. Furthermore, thermal conductivity tests have been carried out, using thermography technique, thank to the collaboration with the group of Dr. Paolo Bison and Dr. Giovanni Ferrarini of the ITC-CNR of Padua. At the same time, by means of thermodynamic modeling according to the CALPHAD method, the development of a thermodynamic database has been started. It includes the seven main oxides relevant to the CMAS-TBC interaction and our work was mainly focused on the definition of thermodynamic models for the description of the phases present in the system and the assessment of a series of pseudo-binary and pseudo-ternary subsystems . The results obtained allow the definition of the critical conditions of the tested materials in service, to better understand their evolution and the degradation and to define the range of use.