High-temperature corrosion protection of gas turbine blades with micro-coatings and nano-coatings : simulation and experiments
Material degradation at high temperature is a serious problem in gas turbines in aircraft. In these systems, the expansion blades experience high temperatures ranging from 850 Celsius to in excess of 1000 Celsius. High temperature corrosion is very significant therefore in such systems and either limits their use or reduces their life, considerably affecting the efficiency and the performance of the devices. One of the main methods for protecting blades from corrosion damage is coating. Oxidation is the most important high-temperature corrosion reaction. In this PhD therefore, the focus has been to explore both experimentally and numerically the performance of micro-coatings and nano-coatings for high temperature corrosion protection of gas turbine blade specimens. Titanium and silicon carbide (ceramic) materials were used for the micro-coating (double layer coating). An aluminium oxide and titanium oxide mix was deployed for the nano-coating (single layer coating). TGA (thermo-gravimetric analysis) and SEM (scanning electron microscopy) tests of both flame-sprayed micro- and nano-coated specimens have been conducted. Corrosion rates (linked to oxidation) were obtained with the parabolic model for both types of coatings tested in TGA. Morphology was studied using SEM. Numerical simulations have been conducted (thermal stress finite element and also CFD simulations) for micro-coatings and stress analysis and thermal stress analysis simulations have been performed for nano-coatings. ANSYS mechanical software was used for all stress simulations with the ANSYS FLUENT discrete phase solver employed for CFD modelling of high temperature gas effects and erosion on the micro-coating coating models. For the micro-coatings both single (titanium) and double (titanium and silicon carbide) systems were studied whereas for the nano-coatings, a single layer was found to be sufficient. While the micro-coatings did offer good protection, double layers (titanium and silicon carbide) were needed and still some high stress zones were observed in the core section of the specimen face, although better performance was achieved than with a single titanium coating layer. Erosion was also more effectively reduced with the double layer micro-coating on the specimen. Overall, the nano-coatings were found to achieve significantly superior performance to micro-coatings both in the experiments and simulations. Greater than 99% reduction in corrosion rate was achieved in the experiments for the micro-coatings compared with the uncoated bare steel (AISI304) sample, and 45.4% reduction in corrosion rate wasIIIachieved in the experiments for the nano-coatings compared with the micro-coated sample. For the nano-coating, a single layer of aluminium oxide (60%)-titanium oxide (40%) significantly reduced high stress zones, minimized cracking, and suppressed surface degradation under high temperature conditions, thereby producing the best protection of all coatings studied. The present PhD has made important novel contributions to modern practical high temperature gas turbine protection, and has shown, in particular, that a significant decrease in corrosion rate and much lower thermal stresses and lower surface damage are produced with nano-coatings compared with micro-coatings. A number of publications have also been produced, which appear in the Appendix.
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