A collaboration of scientists led by researchers with the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) has uncovered clues at the atomic level that could lead to a new generation of much tougher advanced ceramics to be used in applications like gas turbine engines.
"Our findings are a prime factor in understanding the origin of the mechanical properties in advanced ceramics and should make it possible to do the precise tailoring in the future that will critically improve the performances of these materials over a wide range of applications," said Robert Ritchie, a materials scientist who has a joint appointment with Berkeley Lab's Materials Sciences Division and the University of California at Berkeley's Department of Materials Science and Engineering.
Ritchie and Alexander Ziegler, a member of Ritchie's research group, were the principal authors of a paper by the collaboration which appears in the Dec. 3 issue of the journal Science. The other co-authors were Christian Kisielowski and Nigel Browning of Berkeley Lab, Juan Idrobo of UC Davis and Michael Cinibulk of the Air Force Research Laboratory in Ohio.
Ceramics are probably the oldest construction materials known, their use dating back thousands of years, when they were made from wet clay and baked at high temperatures until hard. Today's advanced ceramics are made from powders of complex chemical compounds and their production requires careful control at every stage of the process.
Said Ziegler, "To enhance the toughness of a silicon nitride ceramic, it is often necessary to engineer a thin (nanoscale) film in the ceramic's grain boundaries, which cracks when the ceramic begins to fracture. This promotes the formation of grain bridges which span across the crack, making it more difficult for the crack to propagate."
Understanding the nature and properties of these nano-sized intergranular films is crucial to enhancing ceramic toughness, according to Ritchie and Ziegler. However, critical information about the chemical composition, atomic structure and bonding characteristics of such films has long been missing.
"The problem was the nanometer dimensions of the intergranular films," Ziegler says. "To gain information on the local atomic structure and bonding characteristics requires characterization at Angstrom (single-atom) to sub-Angstrom scales. Until recently, no microscopes or chemical analysis probes have been able to resolve such information at these length scales."
NCEM, however, houses a scanning transmission electron microscope (STEM) optimized for materials applications that require the highest resolutions for both imaging and spectroscopy. With the help of NCEM staff members Kisielowski and Browning, Ritchie, Ziegler and the collaboration used this microscope, in combination with an imaging technique called "high-angle annular dark-field STEM," and a chemical analysis technique called electron-energy-loss-spectroscopy (EELS), to examine a silicon nitride ceramic doped with several different rare-earth elements. They specifically looked at how the atomic bonding configuration of the intergranular phase changed with a change in the rare earth sintering additive.
"We were able to determine the exact location of each rare-earth atom and to see how these atoms specifically bonded to the interface between the intergranular phase and the matrix grains of the ceramic," Ritchie said. "We saw that each rare-earth element attaches to the interface differently, depending on its atomic size, electronic configuration, and the presence of oxygen atoms along the interface. This information can be related to the fracture toughness of the ceramic, which means we should be able to atomistically tailor the grain boundaries in future ceramics to give optimum mechanical properties."
The collaborators say their results with the silicon nitride ceramic and the rare earth glassy films should be applicable to other types of advanced ceramics as well.
Said Ritchie, "It's interesting, but intergranular glassy films used to be thought of as an undesirable feature in ceramics, much like inclusions in steels. We now realize they are the key feature that promotes ceramic toughness."
Berkeley Lab is a U.S. Department of Energy national laboratory. It conducts unclassified scientific research and is managed by the University of California.
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