Advanced protective coatings for gas turbine blading

摘要

The new gas turbines now being marketed are characterized by outputs and efficiencies which were unthinkable just a few years ago [1]. One such highly developed gas turbine was recently introduced by Siemens following successful prototype testing (Figure 1) [2]. The machine has one of the highest measured efficiencies of the existing large machines in combined cycle (GuD) application (Figure 2). Akey factor for achieving such efficiency is the highest possible turbine inlet temperature [3], currently approx. 1400 deg C. In such a machine, it is the turbine blades which are subjected to the greatest thermal and mechanical stresses. They are also subjected to extreme chemical stress in the form of oxidation, which in the following is understood as the corrosive action due almost exclusively to the temperature of the turbine blade surface and (to a much lesser degree) the pressure and oxygen content of the hot gas. In many cases, this is compounded by hot corrosion, which results in accelerated oxidation due to impurities in the fuel and air. In terms of physics, this demanding challenge requires the use of cooling techniques which push the envelope of feasibility. In terms of materials engineering, an innovative multifaceted solution is called for In more concrete therms, this means a combination of convection, impingement and film cooling of blades made of the strongest high-temperature alloy materials and coated with one or possibly multiple coatings. The base material ensures the blade's mechanical integrity while the coating(s) provide(s) protection against the oxidizing and corrosive attack, as well as the thermal stresses which cannot be sufficiently mitigated by cooling [4]. Table 1 shows a typical service loading combination for a Stage 1 moving blade. Figure 3 shows one such blade with film cooling adjacent to a stationary blade which is likewise film cooled. This design takes particularly good advantage of the available cooling air. Furthermore, the moving blade is manufactured using single-crystal casting technology to enhance its high-temperature strength and improve fatigue strength. The graphic in Figure 4 illustrates the superiority of single crystal materials over polycrystalline or directionally solidified nickel-base superalloys. The coating is a third-generation NiCoCrAlY VPS (vacuum plasma spray) coating. In the following, we will discuss the current status of coating developments for large, stationary gas turbines and present solutions for achieving important development objectives.