Hall–Petch breakdown manifested in abrasive wear resistance of nanocrystalline nickel
Introduction
The very high strength and hardness of nanocrystalline metals suggest various potential structural applications, and have provided an impetus for the development and mechanical characterization of many nanostructured metals, alloys, and composites (for recent reviews see, e.g., Refs. [1], [2]). Although there are numerous studies on the mechanical behavior of nanocrystalline metals through standard hardness, compression or tension testing, the mechanics of abrasion and wear have received little attention in the submicrocrystalline range.
Most experimental investigations on the wear behavior of nanostructured materials have focused on dual-phase alloys and composites, and show that nanostructured materials exhibit improved wear resistance compared with their coarse-structured counterparts [3], [4], [5], [6], [7]. Systematic studies of wear in pure nanocrystalline metals are less common. Farhat et al. [8] have studied pure nanocrystalline aluminum prepared by a magnetron sputtering technique, and Jeong et al. [9] examined the effect of grain size on the wear of nanocrystalline nickel (n-Ni) electrodeposits. Both of these studies found the abrasive wear resistance proportional to hardness, though most of the data from those studies focused on grain sizes where the Hall–Petch strengthening relationship is valid, and decreasing grain size leads to higher hardness. Although Jeong et al. [9] noted a change in the Hall–Petch slope at their finest grain sizes, neither of these works observed an inverse Hall–Petch effect at the finest grain sizes, where further grain refinement leads to a decrease in strength [10].
In the present work, we use instrumented nanoindentation and the nanoscratch technique, to explore the relationship between hardness and abrasive scratch resistance in the regime of the cross-over between Hall–Petch and inverse Hall–Petch behavior.
Section snippets
Experimental procedures
Foils of n-Ni were fabricated by direct current (DC) electrodeposition, in a bath composed of NiSO4–6H2O (300 g/l), NiCl2–6H2O (45 g/l), H3BO3 (45 g/l), with additions of saccharine (5 g/l) and sodium lauryl sulfonate (0.25 g/l). The temperature of the plating bath was maintained at 65 °C, and the pH was constant at a value in the range 2.04–5.50, selected for each individual bath by adding diluted sulfuric acid (to decrease pH) or NaOH (to increase pH). The current density was 0.05 A/cm2 for
Results and discussion
Microstructural and property details of the n-Ni specimens are summarized in Table 1, along with data for a coarse-grained specimen of high purity annealed nickel. The average grain size, d, of each n-Ni specimen was found to be identical to within ±10% when determined from broadening of either the (2 0 0) or (2 2 0) reflection, giving agreement to better than ±0.2 nm at the finest grain size of 12 nm. The X-ray measurements were also validated by TEM observations, as shown for the specimen
Conclusions
Instrumented indentation and nanoscratch experiments have been conducted on bulk n-Ni specimens with several grain sizes, fabricated by DC electrodeposition. As reported by other investigators using DC electrodeposited specimens, we find that nickel exhibits Hall–Petch strengthening to grain sizes near ∼14 nm. Additionally, we find that at a finer grain size of 12 nm, the hardness of n-Ni decreases in an apparent breakdown of the Hall–Petch relationship. The breakdown of Hall–Petch
Acknowledgements
This work was performed under the auspices of the US Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract no. W-7405-Eng-48. The transmission electron microscopy of L.M. Hsiung and M. Wall (both at LLNL) is gratefully acknowledged.
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