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Development of thin ceramic coatings for the protection against temperature and stress induced rumpling of the metal surface of turbine blades

Final Activity Report Summary - RUMPLING PROTECTION (Development of thin ceramic coatings for the protection against temperature and stress induced rumpling of the metal surface of turbine blades)

Surfaces of turbine blades may suffer under rumpling due to the high temperatures and mechanical loads. The objective of the project was to prepare ceramic protective coatings for turbine blades in gas turbines for protection against this surface rumpling.

Electrophoretic deposition (EPD) allowed for the fabrication of ceramic coatings at lower cost and higher speed than most other deposition techniques. The processing consisted of powder deposition from a suspension under the influence of an electric field and subsequent consolidation of the coating by sintering. The basis of the powder was zirconia with 5 wt % yttria. Two different types of suspensions with zirconia powder were investigated, namely methyl-ethyl-ketone and ethanol based suspensions. Methyl-ethyl-ketone gave a very stable suspension, but was not environmentally harmless. Ethanol was favoured as suspension, because it was both environmentally friendly and cheaper.

The standard sintering temperature was 1 200 degrees Celsius, which could easily damage or change the substrate and also meant high production costs. In order to reduce the sintering temperature, suspensions with the addition of ZrN were investigated. Due to reaction bonding, the heat treatment of coatings from this mixture in air at a remarkable low temperature of 1 000 degrees Celsius resulted in a consolidated coating. Adherent coatings with coating thicknesses up to 0.1 mm with a high porosity were obtained. The ceramic surfaces were smooth and the microstructure was homogeneous.

The elastic modulus of the EPD coatings was derived from impulse excitation experiments and the thermal conductivity from laser flash analysis. The elastic modulus was about 22 GPa and the thermal conductivity between 0.4 and 0.6 W/mK at room temperature, both decreasing slightly with temperature. Heat treatment in air at 1 100 degrees Celsius for 100 h significantly increased the thermal conductivity, however compared with commercially available coatings the EPD coatings had much lower thermal conductivities. The exceptionally low thermal conductivity made EPD coatings a promising candidate for thermal barrier coatings.

The temperature conditions in turbines were simulated by exposing the specimens to temperature cycles between room temperature and 1 100 degrees Celsius. It was observed that for a limited number of cycles (between 25 and 50, depending on the type of suspension) the ceramic coatings successfully suppressed rumpling. For higher cycle numbers however the rumpling force seemed to be too strong and rumpling related spallation was observed.

Within the project, successful EPD ceramic coatings with low elastic modulus, very low thermal conductivity and rumpling protection capabilities were prepared. Nevertheless, for long term applications the adherence should be improved.