Depth-Sensing Indentation on REBa₂Cu₃O_7−δ Single Crystals Obtained from Xenotime Mineral

Abstract

A natural mixture of heavy rare-earth oxides extracted from xenotime mineral have been used to prepare large single crystals of the high-temperature REBa₂Cu₃O_7−δ superconductor, grown using the CuO–BaO self-flux method. Its mechanical properties along the ab-plane were characterized using instrumented indentation. Hardness and elastic moduli were measured by the Oliver and Pharr method, which yielded 7.4 ± 0.2 GPa and the range 135–175 GPa at small depths, respectively. Increased loads promote the nucleation of lateral cracks, which reduce hardness and measured elastic modulus, as indicated by instrumented indentation at higher loads. The indentation fracture toughness, estimated by measuring the radial crack length from cube corner indentations at various loads, was found to be 0.8 ± 0.2 MPa m^1/2. The observed slip systems of REBa₂Cu₃O_7−δ single crystals were [100](001) and [010](001), the same as for YBa₂Cu₃O_7−δ single crystals. The initial stages of deformation and fracture in the indentation process were investigated. The hardness and elastic modulus are not strongly modified by the crystallographic orientation in the ab-plane. This was interpreted in terms of resolved shear stresses in the active slip systems. Evidence of cracking along the {100} and {110} planes on the ab-plane was observed. In conclusion, the mechanical properties of REBa₂Cu₃O_7−δ single crystals prepared from xenotime are equivalent to those of YBa₂Cu₃O_7−δ single crystals from conventional rare-earth oxides.