Investigation of the high-temperature performance of thermal barrier coating systems for steam turbine applications

In load-following power plants it is necessary to adjust the electrical output to the demand within adequate time. When a steam turbine is used in a load-following power plant the availability of the plant is mainly limited by the thermal cycling behaviour of the steam turbine. Thermal barrier coatings (TBCs) offer the potential to increase the operational flexibility of steam turbines. When applied to a turbine, TBCs may alter the temperature regime of components in a way that enables to shorten the start-up / shut-down procedure. Such an improvement would make load-following steam power plants more flexible in terms of electricity output. In the present study the suitability of atmospheric plasma sprayed thermal barrier coating systems for steam turbine applications was investigated. Coating systems consisted of a metallic bondcoat for oxidation protection, a ceramic TBC for thermal insulation and a metallic topcoat for erosion protection. The substrates on which the coatings were applied were ferritic / martensitic 1%Cr and 10%Cr-steels and the Ni-base alloy Nimonic 80A. The investigated bondcoats were Ni20Cr, Fe22Cr and a CoNiCrAlY coating. The TBCs consisted of ZrO2/7wt%Y2O3. The topcoat was Ni20Cr. The coating systems were tested at ambient pressure in Ar-50%H2O at 550 to 700°C for up to 10000h. TBCs were found to be stable under the investigated conditions. The bondcoat microstructure strongly depended on the plasma spraying parameters and particle size of the spraying powder. On the ferritic / martensitic steels CoNiCrAlY coatings showed the best performance during high temperature exposure. No cracks or delamination were observed. Submicrometer thin, protective Al2O3 scales formed, which were found to prevent interdiffusion between the bondcoat and the substrate material. Ni20Cr and Fe22Cr bondcoats showed more oxide formation than CoNiCrAlY coatings. Also, interdiffusion with the substrate material occurred. On the Nimonic 80A substrate only Ni20Cr and Fe22Cr bondcoats were investigated. The Ni20Cr bondcoat showed best oxidation resistance and almost totally suppressed substrate oxidation. No TBC damage or coating delamination was observed on Ni20Cr. The Ni20Cr topcoats formed cracks propagating in the underlying ceramic thus causing significant TBC damage. The reason for the crack formation are the mechanical stresses which develop when the topcoats cool down to room temperature after plasma spraying