Ceramics are the durable, high temperature material of choice for many technical applications, such as coatings for turbine blades in jet engines, but water vapor can unexpectedly change the properties of a ceramic. For example, at high temperatures when a ceramic can soften, it can be even easier to deform when high pressure water vapor is present. Different elements in a ceramic may segregate more rapidly when water vapor is introduced at high temperatures, creating a non-uniform structure and weak spots where failure can occur. The mechanism for these changes is not currently understood. This research identifies which ceramic materials are most susceptible to changing properties when exposed to high temperature water vapor, and studies the conditions for which water vapor can improve or degrade the ceramic. Underrepresented community college students and returning U.S. veterans are also participating on this project.
TECHNICAL DETAILS: This project studies whether grain boundary diffusion in oxides is significantly affected by exposure to water vapor at high temperatures by studying the magnitude of this phenomenon. This exploratory research is important for understanding the consequences of exposure of solid oxides to water vapor at high temperatures, as ceramics are pushed to be used in harsher and higher temperature environments and designed with smaller and smaller dimensions. Studies measuring changes in sintering, creep and segregation in the presence of water vapor are used to determine if grain boundary diffusion is enhanced. The role of incorporated protons on grain boundary diffusion is also evaluated as a possible mechanism. This research determines if the development of mitigation measures is necessary for the use of oxide ceramics in water vapor at high temperatures and if water vapor enhanced sintering as a low energy route is feasible. Students are trained to learn advanced analytical techniques, included cutting-edge electron microscopy and spectroscopy techniques.
Ceramics are used in many engineering applications, from coatings in jet engines to nuclear reactor components to catalysts that make fuel. Ceramics are usually very stable materials but this can change when they are exposed to high temperature steam. This research project studied how exposure to steam can affect the properties of ceramics, an important consideration since many of the engineering applications for technical ceramics include not only exposure to high temperatures but also atmospheric conditions that can include high humidity or direct steam exposure. This study used Al2O3, an aluminum oxide ceramic, for evaluation, since it is an important technological ceramic widely used in industry. This research measured the changes that occur when steam passes over the surface of a ceramic material with small inclusions of nickel metal particles. Water in steam reacts with oxygen on the surface of oxide ceramics and form a hydroxyl (OH)-. The hydroxyl ion (OH) - is smaller and lower charge than oxygen ions, potentially allowing for faster diffusion (movement) into a ceramic than simple oxygen. Inside the in Al2O3, Nickel (Ni) metal particles turn into NiAl2O4 when oxygen from the surface reaches the Ni particles. The presence of NiAl2O4 served as a marker as to how far the oxygen moved in (diffused in) as it could be easily identified by scanning electron microscopy. Results after exposure to dry air were compared to results in high temperature water vapor for a range of temperatures and times. It was found that that there was significantly faster (over 100% faster at 1250°C) in-diffusion when oxygen was present as (OH)- from absorbed water in the atmosphere. These results show that oxygen enters the ceramic and moves through it more quickly in humid air than in dry air. These results identify a major concern that unwanted effects such as more rapid corrosion and accelerated deformation will occur at surfaces of oxide ceramics in high temperature water vapor. If the component dimension is small, as is the case in coatings, fibers, and nanostructures, it may not perform as expected when exposed to water vapor at high temperatures. This research points out the need to design materials to incorporate limitations on use if applications for such components include high temperature environments with high humidity or steam. A second thrust of this work was to study sintering (firing) of ceramics in water vapor. These results showed that there was a modest effect in water vapor helping make oxide ceramics dense at lower temperatures, but only for certain types of ceramics. Another important aspect of this research project was the training of the next generation of scientists and engineers. One Materials Science and Engineering Ph.D. student completed his degree with support from this project, four undergraduate engineering students were mentored in research projects with this grant, and two high school students from the San Juan Capistrano Breakthrough program had opportunities to conduct a summer of research in the lab working on this project.
