Cooperation with Siemens
The use of thermal barrier coatings (TBCs) makes it possible to use higher temperatures – sometimes as high as 1,600 degrees Celsius – in the hot sections of a gas turbine engine. This gives higher efficiency and, in the long term, a lower impact on the environment. The materials commonly used for TBCs, however, exhibit certain limitations with respect to reaching the highest possible temperatures.
During the use of the coatings, they experience stress in the turbine and eventually fail: two important processes that contribute to this are oxidation and corrosion. This is particularly the case if the turbines are often started and stopped, as is becoming evermore common in order to compensate for cyclical variations in solar energy and wind power, and to balance the electrical supply to the grid.
Manufacturing of a turbine at Siemens in Finspång. Photo credit: Siemens
In his thesis, Performance of MCrAlX coatings: Oxidation, Hot Corrosion and Interdiffusion, Pimin Zhang investigates the properties of coatings known as “MCrAlX” coatings, and how layers of aluminium oxide can protect components. The research has been carried out in collaboration with Siemens Industrial Turbomachinery in Finspång.
What is critical for effective oxidation protection is the formation of a stable, dense and slow-growing aluminium oxide layer as the outermost surface of the coating. This layer is added during the manufacture of the gas turbine and subsequently develops as oxidation occurs during machine operation.
“Our alloys can contain more substances than the previously used ones. One reason that this is important is that it makes the surface coating more stable”, says Pimin Zhang.
The ability to form and retain the protective surface layer depends on the chemical composition and microstructure of the surface coating. It is also affected by variations in the application process during manufacture, and modification of the surfaces before coating.
Pimin Zhang shows in the thesis how the layer of aluminium oxide develops during the oxidation process, which is important to improve the strength of MCrAlX coatings.
How much better are the new coatings than the old ones?
“It’s difficult to estimate. But a patent has been applied for to cover one of them, and I believe that they can have a huge practical significance”, says Pimin Zhang.
More exact maintenance
The second part of the doctoral thesis describes the development of a criterion for the formation of an aluminium oxide layer, which can in turn be used to calculate the lifetime of a particular surface coating. The criterion can be of major practical benefit and can be used, for example, to decide the service interval for a gas turbine.
Pimin Zhang in the laboratory.
The criterion has been validated in long-term oxidation tests of five surface coatings. Some of these tests have had a duration of up to 1 year.
“Several factors affect the need for maintenance, including, for example, the surrounding environment and how the turbine is used. But my work at least means that maintenance can adapted more exactly to the need than was previously possible.”
For Pimin Zhang, the fact that the research can be used for practical application in industry is important. Siemens has been an important part of the project: one of the supervisors is from the company. At the same time, the theoretical results are also important and increase our knowledge about how different materials react in different conditions.
“I am happy when both practice and theory combine. Now I’m hoping to stay at LiU for a while and tie up some loose ends. There are still some results that I haven’t quite had the time to process”, he says.