Vickers indentation tests on olivine: size effects

We conducted Vickers indentation tests on Fe-free (Mg₂SiO₄) and Fe-bearing (Mg_1.8Fe_0.2SiO₄) olivine single crystals and high-density polycrystalline material with average grain sizes ranging from 170 to 890 nm. The Vickers microhardness (H_v) 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, H_v decreases with increasing grain size, load (indentation depth), and the presence of Fe. For each grain size, H_v is well characterized by a power law of the form Hv/Hv0∝l-x, where Hv0 is the depth-independent value of H_v, 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 H_v in our samples. We show that our semi-empirically derived relationship as a function of grain size and indentation depth explains the H_v 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.