TU Wien Investigates Structure of Ceramic Coatings

TU Wien Researches Ceramic Coating Structure Extremely thin ceramic coatings can completely alter the properties of technical components. Coatings can be used to increase metals' resistance to heat or corrosion. Coating processes also play a role in large turbine blades as well as in highly stressed tools in production technology. TU Wien (Vienna) investigated what factors determine the stability of such coatings. Some results obtained at the DESY synchrotron in Hamburg proved quite surprising: Ceramic layers break down in a completely different way than metals. Loss of material durability played almost no role in this case, while the decisive factor was found to be the intensity of stress peak concentrations (stress intensity factor). This finding will change the method used in the future to measure and further improve the resistance of thin films. The study was published in Acta Materialia. Prof. Helmut Riedl, head of the Applied Surface and Coating Technology research group at the Institute of Materials Science and Technology at TU Wien, states: "In many applications, periodic loads are a major problem. If you repeatedly expose metal components to a certain force, changes occur at the microscopic scale." Some atoms can shift position, creating layers that can slide over one another, small cracks can develop, and ultimately lead to the fracture of the entire component. Such material fatigue effects are seen everywhere in engineering and are well studied. However, what happens to thin coatings under stress is less clear. Lukas Zauner, a doctoral researcher working at the Applied Surface and Coating Technology Research Group, explains: "Ceramic coatings are typically only a few nanometers to 10 μm thick, and their behavior is completely different from that of a solid ceramic piece." To best understand this behavior, entirely new measurement methods were developed at TU Wien: Rather than testing metal and ceramic coating together as is usually done, the research team left the metal out, produced extremely thin samples from various ceramic materials typically used in thin film technology, and repeatedly exposed them—tens of millions of times—to various loads in a precisely defined manner. To fully determine whether the ceramic's atomic structure changed as a result, the team took the experimental setup to Hamburg: there, at DESY's synchrotron, extremely well-focused X-rays are available that can be used to examine various points of the sample during the loading experiment. Even small changes in crystal structure or in the distance between neighboring atoms should be detectable this way. However, these measurements surprisingly showed the following: The ceramic practically does not change. Even millions of load cycles do not cause material fatigue. "Standard ceramics fatigue according to certain patterns similar to the type of fatigue we know from metals. However, these extremely thin layers do not exhibit this behavior," says Helmut Riedl. "Their microstructure at the end is the same as it was at the beginning." This means that the durability of thin layers is determined only by fracture toughness: If you exceed a characteristic load limit of the material, the layer suddenly and irreversibly disappears. However, all loads below this threshold are not a problem; they do not age the ceramic layer and have practically no effect. Helmut Riedl states: "Of course, this also changes the strategy for designing research projects for new, improved ceramic coating materials. You don't have to conduct long-term tests; it is sufficient to find out which material breaks under which force through a simple load test. There is no need to worry about how to mitigate fatigue effects in the material; you only need to find materials with the highest possible fracture toughness—which is not a simple task in itself." The team has already succeeded in finding a very promising candidate for this: a certain form of chromium diboride proved to be surprisingly resistant in tests. This opens the way for research that will gain significant success in the future. Academic Reference: L. Zauner et al, Assessing the fracture and fatigue resistance of nanostructured thin films, Acta Materialia (2022). DOI: 10.1016/j.actamat.2022.118260 Source: https://phys.org/news/2022-11-nanostructured-ceramic-coatings-fatigue.html