ABSTRACT: Coatings are frequently applied on gas turbine components in order to restrict surface degradation such as corrosion and oxidation of the structural material or to thermally insulate the structural material against the hot environment, thereby increasing the efficiency of the turbine. However, in order to obtain accurate lifetime expectancies and performance of the coatings system it is necessary to have a reliable understanding of the mechanical properties and failure mechanisms of the coatings.
In this thesis, mechanical and fracture behaviour have been studied for a NiAl coating applied by a pack cementation process, an air-plasma sprayed NiCoCrAlY bondcoat, a vacuum plasma-sprayed NiCrAlY bondcoat and an air plasma-sprayed ZrO2 + 6-8 % Y2O3 topcoat. The mechanical tests were carried out at a temperature interval between room temperature and 860oC. Small punch tests and spherical indentation were the test methods applied for this purpose, in which existing bending and indentation theory were adopted for interpretation of the test results. Efforts were made to validate the test methods to ensure their relevance for coating property measurements. It was found that the combination of these two methods gives capability to predict the temperature dependence of several relevant mechanical properties of gas turbine coatings, for example the hardness, elastic modulus, yield strength, fracture strength, flow stress-strain behaviour and ductility. Furthermore, the plasma-sprayed coatings were tested in both as-coated and heat-treated condition, which revealed significant difference in properties. Microstructural examination of the bondcoats showed that oxidation with loss of aluminium plays an important role in the coating degradation and for the property changes in the coatings.

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SOURCE: M. Eskner, “Mechanical Behavior of Gas Turbine Coatings.” Doctoral thesis. Materials Science and Engineering. Royal Institute of Technoloyg. Stockholm. 2004.