Thermal barrier coatings for gas turbines
Thermal barrier coatings are an integral part of both stationary and aircraft gas turbines, since they are the only coatings that can withstand the high fuel gas temperature needed for efficient operations. The standard that has been established worldwide for thermal barrier coatings is yttria-stablized zirconia (YSZ) that has been partially stabilized with 7-8 weight % Y2O3.
YSZ has a number of outstanding properties for this application, including low thermal conductivity, high expansion coefficients for reducing thermal tensions in the composite with the metallic substrate, and good fracture toughness.
At present, two manufacturing processes are used in industrial series production of thermal barrier coatings: electron beam physical vapour deposition (EB-PVD) and atmospheric plasma spraying (APS). For coating stationary gas turbines, the more affordable APS process is used nearly exclusively. We will be focusing on this process in this section. With this method, powder particles from the coating material are introduced into the hot gas torch of a plasma burner via a carrier gas stream. In the hot gas stream, the particles are accelerated and heated until they melt. Figure 1 shows a triplex plasma burner in operation. These burners have three cathodes for producing the plasma and are characterized by their high application rates and good process stability. The hot, liquid, and accelerated particles then hit the substrate. In so doing they are shaped and cool rapidly. This yields a lamellar microstructure that typically displays pores and microcracks. The figure shows this type of microstructure.
This microstructure is important for the efficiency of the coating systems in operation, since it results in a low elasticity module with correspondingly low tensions and allows good strain tolerance achieved by the cracks opening and individual spraying lamella falling off. Good strain tolerance is necessary, because the difference in the thermal expansion between the substrate and the thermal barrier coating during heating and cooling leads to thermal tensions. In addition, the numerous microcracks also reduce the thermal conductivity and in so doing, improve the thermal insulation effect of the coatings.
In addition to the ceramic thermal barrier coating, a thermal barrier coating system also consists of a metallic interlayer, referred to as the bond coat. As the name suggests, this layer improves the bonding between the metallic substrate and the ceramic top coat. It also protects the substrates from oxidation and corrosion due to the hot fuel gases. This is necessary, because the top coats are porous and gas permeable. In the case of atmospheric plasma-sprayed thermal barrier coatings, MCrAlY alloys are generally used as the bond coat, whereby M stands for Ni or Co. The coatings are fabricated by various thermal spraying techniques, such as vacuum plasma spraying or high-velocity oxy-fuel spraying. During operation, the coatings develop a dense top coat of aluminium oxide that significantly retards further oxidation of the material. A high level of roughness with Ra – values above approx. 8 µm is important for good bonding of the thermal barrier coating to the bond coat, because the bonding primarily occurs via mechanical bonding.
In addition to the segmented structure for the ceramic top coat, other approaches are also used to boost the efficiency of thermal barrier coatings. For example, with special production conditions, relatively dense layers can be deposited that display a large number of segmentation cracks. If this type of coating is subjected to tensile stress, which is the case during heating in the turbine, the segmentation cracks can open.
This can prevent the development of high tensions in the coating and in turn, prevent damage. In addition to this refinement of the APS technology, new processes are also currently undergoing testing. Here, suspension plasma spraying is a particularly interesting technique. With this process, instead of powder source materials, suspensions are introduced in the plasma torch. This process allows the adjustment of high segmentation crack densities of over 10/mm with simultaneous low heat conductivity. Another process currently under development is the LPPS-PVD process. For this process, powder source materials are evaporated in a plasma burner, leading to the deposition of columnar crystalline films, which demonstrate outstanding performance in cycling trials.
Apart from the development of extremely high-performance coatings made of YSZ, a further focus of the studies at Forschungszentrum Jülich's IEK-1 is on the development of new thermal barrier coating materials. This is necessary due to the limited temperature stability of YSZ. Long-term use is limited to temperatures of approx. 1200 °C, since at higher temperatures, phase transformations and increased density of the coatings occur. Pyrochlores such as Gd2Zr2O7, perovskites such as SrZrO3 and (hexa-) aluminates are of particular interest in this area.
Owing to the relatively low fracture toughness of the new thermal barrier coating materials, the double-layer systems developed in Jülich are usually used. For these systems, first a coat made of the tough YSZ is deposited on the bond coat and then the new ceramic is deposited on top of it. See figure.
Especially the coatings formed from pyrochlores demonstrate excellent behaviour when subjected to cyclic thermal strain, for instance, in a gas burner test stand.
Coating of turbine components
In today's fossil power plants, electric power is generated by means of turbines. These turbines convert the heat into kinetic energy (rotation of the turbine wheel) when the volume of a gas expands when heated. From the energy of the rotation, electrical energy is generated. Due to very fundamental physical laws, a high temperature of the gas is a crucial factor for high efficiency of the energy conversion. At temperatures over approx. 600 °C, the internal components of the turbine–generally metals–must be protected from the temperature, because they cannot withstand these conditions for long periods. This protection can be provided by coating the turbine components with ceramics.
At the Institute of Energy and Climate Research (IEK-1), research is conducted on thermal spraying as an efficient and economical coating technique for power plant components. In addition to the coating technique, investigating and developing improved materials for coating is a central research topic.