Abstract
Nanoindentation morphologies of crystalline copper have been probed at the grain scale. Experimental tests have been conducted on nanocrystalline (NC), ultrafine-grained (UFG), and coarse-grained (CG) copper samples with a new Berkvoich indenter at the strain rate of 0.04/s without holding time at an indentation depth of 2000 nm at room temperature. As the grain size increases, the height of the pile-up around the residual indentation increases and then exhibits a slightly decrease in the CG Cu. The maximum of the pile-up in the CG Cu obviously deviates from the center of the indenter sides. Our analysis has revealed that the dislocation motion and GB activities in the NC Cu, some cross- and multiple-slip dislocations inside the larger grain in the UFG Cu, and forest dislocations from the intragranular Frank-Read sources in the CG Cu would directly induce this distinct pile-up effect.
Highlights
The nanoindentation technique has been widely used to characterize the mechanical properties of bulk- and thin-film materials on nano- and microscopic scales, such as elastic modulus (E) and hardness (H)
Square-shaped nanoindentation specimens of NC/UFG/CG Cu with a gauge size of 30 × 30 × 2 mm3 were cut, and mechanically grinded with SiC papers, and polished with a microcloth using a slurry of 0.5 μm alumina
All of the samples were loaded at strain rates of 0.4/s, 0.04/s, and 0.004/s to the indentation depth of 2000 nm without holding time under the procedure of the continuous stiffness measurement (CSM)
Summary
The nanoindentation technique has been widely used to characterize the mechanical properties of bulk- and thin-film materials on nano- and microscopic scales, such as elastic modulus (E) and hardness (H). In some cases there exists an apparent upward extrusion at the edge of the contact with the indenter in some metals, known as pile-up, which means that the actual contact area is larger than the value calculated by the Oliver-Pharr method [1]. Some studies have reported that the true contact area is 60% larger than the measured value as serious pile-up occurs, leading to the overestimation of the E and H [2,3,4,5,6,7]. There is no convince evidence to suggest that any new modified method considering pile-up at a larger indentation depth could bring more accurate measurement results, and the Oliver-Pharr method still is considered to provide relatively reliable E and H values.
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