Investigation into Nanoindentation and Nanoscratch Behavior in Oxide Ceramics

Nanoindentation and nanoscratch behavior has been systematically investigated in ultrafine-grained alumina ceramics, single crystal sapphire wafers and nanocrystalline ceramics, including yttria, calcium-doped-yttria, magnesia, yttria-stablized-zirconia and magnesia-yttria ceramic nanocomposites by using a Berkovich nanoindenter XPS system in indentation and scratch modes under ramp loading conditions, respectively. The dense ceramic samples were fabricated by conventional spark plasma sintering and high-pressure spark plasma sintering.;For alumina ceramics, the nanoindentation and nanoscratch results suggest that grain refinement to the ultrafine grain size regime can improve both the mechanical behavior such as hardness, modulus and dynamic surface mechanical properties, and plastic deformation capability at the nano- and micro-scales.;The evolution of plastic deformation in the M-plane sapphire sample during the nanoscratch process includes lattice disorder, dislocation loops, stacking faults on the basal plane, dislocation glide and finally basal twin formation from TEM characterization. Microcracks, suggesting brittle fracture, increased in frequency at the edge of the scratch groove in the high-load region. Crack deflection is observed due to the transformation of Mode II dominant fracture to Mode I dominant fracture. Elastic and residual stress field solutions have been utilized to explain the observed microstructural features.;Intriguing plastic deformation features in terms of strain flow maps surrounding nanoindentation imprints and nanoscratch grooves have been observed in a dense, high purity nanocrystalline yttria ceramic sample. Investigation into the strain rate sensitivity reveals that the plastic deformation is primarily driven by dislocation activity. Grain size dependence on nanoscratch behavior in the nanocrystalline yttria, calcium-doped-yttria and yttria-stablized zirconia ceramic samples indicates that the samples with finer grain size show the higher potential for plastic deformation despite different crystal structures.