Elastic and Mechanical Properties of Polycrystalline Transparent Yttria as Determined by Indentation Techniques
The Young's modulus (E) and nanoindentation (NI) stress–strain curves of fully dense, transparent yttria were determined using a nano-indenter capable of continually measuring the stiffness (S). Two hemispherical indenters with radii of 1.4 and 5 m were used, in addition to a Berkovich tip. The E values—calculated from the S vs. a (contact radius) curves obtained with both spherical indenters (158–171 GPa) were ≈5% lower than the 176 GPa dynamic modulus measured on the same sample using ultrasound. Depending on range of penetration depths, the Berkovich modulus was ≈1%–5% higher than the dynamic modulus. When the NI load-displacement curves were converted to NI stress–strain curves, the yield point obtained was 7 ± 1 GPa. This value was ≈20% and ≈33% lower than the Vickers and Berkovich hardness values measured on the same sample, respectively. The Vickers microhardness values (8.8 ± 0.2 GPa) and the fracture toughness extracted from the latter (1.5 ± 0.3 MPa·m1/2) are in good agreement with published results on samples with comparable microstructures. The strain hardening rates were almost identical for both tips; no tip size effect was observed. Based on this work, we conclude that S vs. a plots are a powerful, and relatively simple, technique to measure the Young's moduli of polycrystalline ceramics and other hard solids. The fact that one also obtains NI stress–-strain curves is a distinct advantage over the more commonly used Berkovich tip.
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