Influences of sample preparation on the indentation size effect and nanoindentation pop-in on nickel

Careful experiments were performed with a well characterized indenter to systematically investigate the influences of sample preparation on the indentation size effect (ISE) and nanoindentation pop-in behavior in nickel. The Ni (100) surface was prepared by electropolishing and then damaged in a controlled way by polishing with alumina slurry. The damaged layer was systematically removed in steps by colloidal silica polishing (0.02 µm (micrometer)) with the pop-in behavior statistically characterized and the depth dependence of the hardness measured at each step. AFM observation revealed that the electropolished surface is free from scratches, and has the largest roughness. Numerous scratches were observed on the alumina or silica polished specimens, with smaller particle size led to lower roughness. XPS measurements demonstrate that the colloidal silica particles were not embedded in sample surface. Rather, there is an adsorbed soft layer ~1.2 nm (nanometer) thick and a layer of a Ni compound with a thickness of ~ 0.8 nm on the top surface for each polishing step.

With a decreasing thickness of the surface damaged layer, pop-in events start to appear, and the cumulative probability increases until it reaches 100%. The cumulative probability curve shifts to the right with an increase of silica polishing time. Long time silica polishing causes the cumulative probability curve to shift to the right of the electropolished curve. The surface mechanical state for each polishing step can be characterized by the detailed statistics of pop-in behavior.

The displacement cut-off for ISE measurements, since the hardness measurement is complex, was found to be ~50nm. With a decreasing thickness of the damaged layer, the H-h curve gradually moved down from the highest hardnesses for alumina polishing to the lowest hardnesses for the electropolished surface. For each polishing condition, the measured hardness increases with decreasing of indentation depth. However, the hardness increase after electropolishing stage is the smallest, and the hardness increase after alumina polishing is the largest. These observations demonstrate that hardness measurement at small depths is very sensitive to the surface state. The experimental observations are consistent with the mathematical predictions of the Nix-Gao model.