Low elastic modulus and hardness, as well as anomalous indentation behavior, have been observed during indentation of xenotime rare-earth orthophosphate ceramics (REPO4s) with compositions near the monazite/xenotime phase boundary. Pressure-induced phase transformation has been identified as a potential cause for both observations. This study comprehensively characterizes the mechanical properties and indentation behavior of four elemental REPO4 materials (EuPO4, GdPO4, TbPO4, and DyPO4) that span the monazite/xenotime phase boundary using ex situ nanoindentation for a range of loading rates and indentation depths. In situ nanoindentation within a SEM was used to correlate discrete load-depth behavior to the development of surface features. Anomalous, elbow-type behavior was not restricted to xenotimes, but occurred in all four materials; thus we concluded that the presence of an elbow in the indentation data was not a unique identifier of phase transformation in rare-earth orthophosphates. Furthermore, it was shown that the elastic modulus of each of these compositions approached the value predicted by simulations and hardness was consistently above 5 GPa, provided that the samples were processed to nearly full density.
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The researchers wish to acknowledge Aaron Miller for help in processing the hot pressed TbPO4 pellet and the contributions of Zachary McMullen in the synthesis, processing, and sample preparation of the EuPO4, GdPO4, and DyPO4. The researchers also wish to acknowledge the Virginia Ferguson Research Group at the University of Colorado, Boulder for use of their high load indentation cell. In situ nanoindentation were completed through an awarded user proposal: U2015B0008 at the Center for Integrated Nanotechnology at Los Alamos National Laboratory. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396. This research was funded by the National Science Foundation (NSF) under Award No.: DMR-1352499.
- Mechanical characterization
- Phase transformation