By Tim Miser, Associate Editor
|Thermal barrier coatings can be applied using different processes. They can be wet-sprayed or applied using an air plasma spray gun, as pictured here. Photo courtesy: GE Power & Water|
The quest for higher efficiencies in modern combined-cycle power plants is relentless. State-of-the-art facilities now operate at efficiency rates surpassing 60 percent, and the industry doesn’t seem inclined to rest on this accomplishment. Even now, engineers at major and minor OEMs alike are working tirelessly to improve these numbers still further. To do this, they are increasing firing temperatures within gas turbines and, in the process, creating operating conditions that would try even the most resilient components. Manufacturing components that can survive these conditions means making significant advances in material science, and increasingly these advances have been realized not in improvements to the metallic substrates of the machinery itself, but to the coatings that shield this machinery from the severe abuse of hot gas paths.
Temperatures within a turbine are brutal. Metal components can easily reach 800-900°C, though they can withstand these extremes and continue to function normally. Gas temperatures, however, can climb to greater than 1500°C, which is beyond the point at which metallic components can survive intact. To maintain the integrity of these components then, it is essential to have coatings in place between them and the hot gas paths to which they are exposed.
“Coatings provide a thermal barrier that maintains the metal at a cooler temperature,” says Ramesh Subramanian, Principal Expert in Gas Turbine Engineering at Siemens Energy. “The industry relies largely on ceramic materials which are applied to the metallic substrate via a spray process, much the way you would apply stucco to drywall, except that ceramic materials actually melt and then solidify.”
Research and Development
“Relative to other technologies, the impact of coatings is very high,” Subramanian says. “This is because coatings represent the first line of defense against hot gases in a gas turbine. The largest gains in engine efficiency are leveraged in the turbine. Siemens is always pushing for higher efficiencies in this area, especially in its H-class turbines. Materials have been asked to cope with increasingly hot temperatures over the decades. Because of this, the most fertile area of research, development, and applied science exists in the materials and coatings for the hot section of the gas turbine. To improve coatings, it is necessary to understand the material science. What are the atoms doing? What is the chemistry doing? How can we change particular atoms to achieve a desired outcome?
Jon Schaeffer confirms the value of research and development in material science as it applies to gas turbine coatings. As Senior Manager for Materials and Processes Engineering at GE Power & Water, he’s well positioned to know. “We must continually iterate our patented chemistry to perform better in our machines,” he says. “Using the information we get back from the field, we are able to adjust the chemistry to achieve better results from our coatings. Within the combustor we are very interested in making sure the base metals don’t melt, especially in our high-efficiency HA gas turbines that fire at such high temperatures. Here we use ceramic coatings as part of our Thermal Barrier Coating (TBC) system. “
“One of the key properties of coating materials is thermal conductivity,” Subramanian adds. “The goal is always to reduce the thermal conductivity as much as possible, and the only way to do this is by playing games with the periodic table and changing various elements in and out to discover what works and what doesn’t work. This is where university research is so valuable. The industry can then take this new knowledge from academia and figure out how to manufacture it for practical use.”
Subramanian continues: “Materials last longer at lower temperatures. That means it’s important to insulate them with effective coatings. As hot gases become still hotter in the quest for greater efficiency, these coatings must become either thicker or better, and good research helps us make this happen. As part of our Siemens Berlin gas turbine manufacturing plant, which has one of the most modern and high-capacity test beds in the world, we have commissioned a new burner test center for gas turbines in Ludwigsfelde near Berlin. Here we conduct tests to ensure that advanced coatings maintain high temperature durability in combination with advanced cooling technologies, under the very demanding combustion conditions of our H-class engines. We test technology in realistic engine conditions to enable faster iteration and implementation of our products.”
Applying the Coating
The engineering challenges don’t end with the discovery of dependable coating materials, however. Just as crucial are the processes used to apply these coatings to metal components. Not unexpectedly, different components within a gas turbine experience different degrees of heat and mechanical stresses during operation. These variable conditions mean that different parts of a turbine require different types of coatings, and these coatings require different application processes to render them ready for duty.
|Only by combining an adhesive layer with a thermal-insulating layer have Siemens researchers succeeded in building a protective coating for turbine blades that can withstand high operating temperatures. Siemens’ coated blades have a service life six times longer than that of unprotected blades. Photo courtesy: Siemens Energy|
“Our compressor, combustor, and turbine coatings are sprayed using various processes,” says Schaeffer. “Compressor components are wet-sprayed using a process that most people are familiar with. Combustor and turbine components are coated using a different process in which very high-temperatures are used to melt coating materials. We use this process for both metal and ceramic coatings. We use a similar system within the turbine.”
“Applying material is a lot like baking a cake,” Subramanian says. “Even if you have the right ingredients, things can go wrong if you don’t process them correctly. You can end up with a hard cake instead of a light, fluffy one. Materials in engines are really no different. You need to know how to heat them in a way that will give them the right textures so they can work for a very long time. When blades experience coating spallation, they become damaged very quickly. In order to ensure that utilities don’t experience down time then, it’s important to make sure coatings do not fall off so that components can continue to work reliably.”
“One challenge we face in coating components is brought about by larger machines like GE’s 7HA and 9HA gas turbines,” says Schaeffer. “Machines of this size mean it takes longer to process components. Because of this, we need to ensure that our processing can be completed with limited amounts of cycles. The longer things stay hot in processing, the more undesirable things can happen. This makes materials with lower thermal conductivity very advantageous. For example, if we would normally need to apply 20 mils of a coating system, but we can actually apply only 10 mils of a superior coating that has the same heat-handling capabilities, we can do this faster and create all kinds of bonus processing synergies.”
Because the metallic components of turbines have different expansion coefficients than the ceramic coatings that are applied to them, it is essential to build into any coating system the ability to expand and contract freely. In this way, coatings can maintain their integrity rather than cracking and falling off, even as the metal beneath them expands at a different rate than they do. This expansion is facilitated by the deliberate introduction of cracks into the ceramic.
“We introduce expansion cracks to the coatings which work very much the way an expansion crack in concrete would,” says Schaeffer. “The metal has a higher expansion coefficient than the overlying ceramic materials. To handle this differential, cracks allow the TBC to expand and contract during service. GE’s Dense Vertically Cracked (DVC) ceramic coatings make all this possible, and as a bonus they also impart a lot of erosion resistance to our coatings. Many companies apply porous ceramic coatings which deteriorate very quickly in service. The density of our coatings allows them to handle the debris that comes through a machine.”
Schaeffer continues: “We can also place additives in the coatings to create desirable qualities. For instance, calcium magnesium aluminum silicate (CMAS) can form on components during service. These accumulations come from sand and dust in the environment which are sucked into a turbine and deposited on the TBC. Once there they can actually glue together the vertical cracks such that they can no longer expand as designed. To combat this, we add layers of materials that mitigate against this issue. Stability is a big issue in coatings. We’re interested in coating systems that are fully stabilized at higher temperatures, so that they don’t come apart in service.”
Due to the unrelenting pressures of high-efficiency firing temperatures, even well-coated components wear out eventually. “Owners and operators would love to have machines that last forever,” Schaeffer says, “but this simply isn’t possible given the duty cycles that we must work with. Because of this, it is important that turbines are repairable at the component level. Most of these repairs take place in-house at GE. It’s very challenging to find ways to sufficiently manage the material science such that repairs can be made in the field. We’ve had some success, but there’s a lot a progress still to be made when it comes to restoring OEM capability at the local level.”
“GE is very fortunate to have our research center,” adds Schaeffer. “This acts as a synergistic champion across the business, bringing in a knowledge storehouse from the company’s many areas of expertise including healthcare, oil and gas, aviation, power, etc. For instance, we have benefitted from GE’s expertise in CT imaging that comes out of the healthcare field. In addition to imaging the human body, this technology also allows us to inspect the internal components of large machinery. Just as a doctor might need to find a lesion or abnormality within a patient without injuring them, our engineers also need to find lesions or abnormalities in their components without destroying them. This allows us to investigate problems without sacrificing assets.”