Mechanical behavior of rare‐earth orthophosphates near the monazite/xenotime boundary characterized by nanoindentation

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.