We conducted Vickers indentation tests on Fe-free (Mg2SiO4) and Fe-bearing (Mg1.8Fe0.2SiO4) olivine single crystals and high-density polycrystalline material with average grain sizes ranging from 170 to 890 nm. The Vickers microhardness (Hv) of the Fe-free polycrystalline material with the finest grain size is ~ 17 GPa at a load of 0.1 N, while that of the Fe-bearing single crystal is ~ 8 GPa at the largest load applied. Overall, Hv decreases with increasing grain size, load (indentation depth), and the presence of Fe. For each grain size, Hv is well characterized by a power law of the form Hv/Hv0∝l-x, where Hv0 is the depth-independent value of Hv, l represents either grain size or indentation depth, and x is 0.09. Despite the small exponent value for each size effect, the nonlinear interaction of the two size effects results in large variations of Hv in our samples. We show that our semi-empirically derived relationship as a function of grain size and indentation depth explains the Hv of both polycrystalline and single-crystal olivine at any indentation conditions. Indentation fracture toughness of the finest-grained material is 0.8 MPam1/2, which increases slightly to 1.1 MPam1/2 with increasing grain size, while the toughness of the single crystals varies from 0.5 to 0.8 MPam1/2 depending on the crystallographic orientation of the fracture planes.
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