Vickers indentation tests on olivine: size effects

Abstract

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 ($H_{{\text{v}}}$) 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, $H_{{\text{v}}}$ decreases with increasing grain size, load (indentation depth), and the presence of Fe. For each grain size, $H_{{\text{v}}}$ is well characterized by a power law of the form $H_{{\text{v}}} /H_{{\text{v}}}^{0} \propto l^{ - x}$, where $H_{{\text{v}}}^{0}$ is the depth-independent value of $H_{{\text{v}}}$, $l$ 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 $H_{{\text{v}}}$ in our samples. We show that our semi-empirically derived relationship as a function of grain size and indentation depth explains the $H_{{\text{v}}}$ of both polycrystalline and single-crystal olivine at any indentation conditions. Indentation fracture toughness of the finest-grained material is 0.8 ${\text{MPa}}\;{\text{m}}^{1/2}$, which increases slightly to 1.1 ${\text{MPa}}\;{\text{m}}^{1/2}$ with increasing grain size, while the toughness of the single crystals varies from 0.5 to 0.8 ${\text{MPa}}\;{\text{m}}^{1/2}$ depending on the crystallographic orientation of the fracture planes.