The development of new bond coat compositions for thermal barrier coating systems operating under industrial gas turbine conditions

Abstract

Environmental and economic issues are driving the development of increasingly efficient gas turbines. An important step in achieving this is to engineer components which can operate with longer lifetimes and at higher metal temperatures. Inlet temperatures for gas turbines now exceed the melting temperatures of nickel-based superalloys (i.e. 1300–1350°C). The use of advanced air cooling systems coupled with thermal barrier coatings (TBCs) reduces the temperature of the underlying superalloy substrate. The bond coating, an important part of the TBC system, oxidizes to form a slow growing protective oxide layer, while also providing adhesion between the ceramic topcoat and the substrate. NiCoCrAlY overlay coatings are some of the most commonly used bond coatings for industrial gas turbines and extensive research has been undertaken over many years to find the best bond coating composition.This paper reports upon the production of new, model bond coatings with a wide range of different compositions. The focus is on their oxidation behavior at a temperature typically experienced by bond coatings on industrial turbine blades (950°C). A physical vapor deposition technique, magnetron sputtering, has been used to deposit a range of Ni–Co–Cr–Al coatings onto 10mm diameter sapphire substrates. This was achieved through co-sputtering two targets: a Ni–10%Cr, Ni–20%Cr, Ni–50%Cr, Ni–20%Co–40%Cr or Ni–40%Co–20%Cr target and a pure Al target. About a hundred samples with varying compositions were produced by this method. The coatings were then oxidized in air for 500h at 950°C.All samples were assessed by measuring the change in coating thickness, using pre- and post-exposure metrology only, and also the change in specimen weight. This approach has shown that magnetron sputtering successfully deposited 20 to 30μm thick coatings and allowed the calculation of oxide growth rates. Energy dispersive X-ray (EDX) analysis was used to characterize the exact composition of each sample. Additionally, X-ray diffraction (XRD) has been used to identify the major oxides formed during exposure. The selective growth of protective Cr2O3 or Al2O3 or other less protective mixed oxides (depending on the initial coating composition) was observed. This influenced the oxide scale growth rate, indicating which coatings produced more protective oxides and allowing future optimization of the bond coating composition, for service within the turbine section of industrial gas turbines to be planned.