NanoStructured Materials, Vol. 12, pp. 1035-1040, 1999 Elsevier Science Ltd 8 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 09659773/99/$-see front matter
PRACTICAL APPLICATIONS FOR ELECTRODEPOSITED NANOCRYSTALLINE MATERIALS A. Robertson, U. Erb’ and G. Palumbo Materials Department, Ontario Hydro Technologies, 800 Kipling Avenue, Toronto, Canada M8Z 5S4 of Metallurgy and Materials Science, University of Toronto, Toronto, Canada M5S 3E4
Micro Engineered ‘Department
Abstract - Electrodeposition provides a cost-effective means of producing fully dense nanocrystaltine (lOnm-1OOnmavg. grain size) metals, alloys, and metal-matrix composites as coatings or free-standing forms cfoil, sheet, wire, complex shapes). The electrosynthesis approach is highly adaptable to conventional industrial material processes, yielding significant material and process improvements from relatively small capital equipment and process modifications. In this paper, several examples of current and emerging commercial applications for this technology are presented. ~1999 Acta Metallurgica Inc.
INTRODUCTION From pioneering efforts in the late 1980’s [ 11, the electrodeposition of nanostructures has advanced rapidly to commercial application as a result of (1) an established industrial infrastructure (i.e., electroplating and electroforming industries), (2) a relatively low cost of application whereby nanomaterials can be produced by simple modification of bath chemistries and electrical parameters used in current plating and electroforming operations, (3) the capability in a single-step process to produce metals, alloys, and metal-matrix composites in various forms (i.e., coatings, free-standing complex shapes), and most importantly (4) the ability to produce fullv dense nanostructures free of extraneous porosity. The importance of the latter cannot be overemphasized with regard to industrial application :since many of the extraordinary properties previously attributed to nanostructures in ceramics, reduction in saturation (e.g., reduction in elastic modulus, superplasticity magnetization etc.), have since been demonstrated to be an artifact of residual porosity in these materials. From the outset, the fully dense nanomaterials produced by electrodeposition have displalyed predictable material properties based upon their increased content of intercrystalline defects. This ‘predictability’ in ultimate material performance has accelerated the adoption of nanomaterials by industry, whereby, such extreme grain refinement simply represents another metallurgical tool for microstructural optimization. In this paper, an overview of some current and emerging practical applications for electrodeposited nanocrystalline materials are presented and discussed in light of the importance of property-specific grain size ‘optimization’ rather than grain miniaturization for its own sake. 1035
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MECHANICAL APPLICATIONS As would be expected from Hall-Petch considerations, numerous practical applications for nanocrystalline materials are based upon opportunities for high strength coatings and free-standing structural components. The mechanical properties of nanocrystalline nickel (99.99%) with grain size of 1Onm and lOOnm, in comparison to conventional polycrystalline material, are shown in Table 1. In addition to remarkable increases in hardness, yield strength and ultimate tensile strength with decreasing grain size, it is interesting to note that the work hardening coefficient decreases with decreasing grain size to virtually zero at a grain size of 1Onm . The ductility of the material decreases with decreasing grain size from 50% elongation to failure in tension for conventional material to 15% at 1OOnmgrain size and approximately 1% at 1Onm grain size. As is also shown in this table, the fatigue performance of the IOOnm material is fully consistent with that of the conventional Ni. Compared to conventional polycrystalline Ni, nanocrystalline Ni electrodeposits exhibit drastically reduced wear rates and lower coefficient of friction as determined in dry air pin-on-disc tests . However, contrary to earlier measurements on nanocrystalline materials prepared by consolidation of powders [5-71 nanocrystalline nickel electrodeposits do not show significant reduction in Young’s modulus. This result provides further support for earlier findings [8,9], which demonstrated that the previously reported reductions in modulus with nanomaterials produced by powder consolidation, were likely the result of high residual porosity. The superior mechanical properties of these electrodeposited nanostructures have led to one of their first large scale industrial applications- the ElectrosleeveTM process for in-situ repair of nuclear steam generator tubing [IO,1 11. In this application, nanocrystalline Ni (1OOnm) is electroformed on the inside surface of steam generator tubes to effect a complete structural repair at sites where the structural integrity of the original tube has been compromised (e.g., corrosion, stress corrosion cracking etc.). Figure 1 shows a cut-away view of an installed Electrosleevem. The high strength and good ductility of this IOOnm grain size material permits the use of a thin ‘sleeve’ (OS-lmm) which minimizes the impact on fluid flow and heat transfer in the steam generator. TABLE 1 Summary of the Mechanical Properties of C‘onventional and Nanocrvstallinc : Nickel Property 1 Ni lOpm 1 Ni lOOurn 1 Ni 1Onm 10-3 fi9n I >9nn Yield Strength, MPa 12SW
Elongation in Bending, % (25’C) Modulus of Elasticity. GPa (2S’C) 2 Vickers Hardness, kghllm __.._. Work Hardening Coefficient Fatigue Strength, MPa (10’ cycles/air/25”C) Wear Rate (dry air pin on disc), p.m3/pm Coefficient of Friction (dry air pin on disc) 7
lA0 - .V
0.4 241 1330 0.9
3n4 __ .
?iul IV” 0.15 275
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Figure 1. Cut-away view of an installed nano-Ni ElectrosleeveTM on a host Alloy 600 nuclear steam generator tube. Recent geometric models and experimental findings [13,14] have shown that nanostructured materials can also possess a high resistance to intergranular cracking Several emerging applications for nano-materials processes, including creep cracking. possessing high intergranular cracking resistance include, high cycle-life lead-acid battery (positive) grids, and shaped charge liners (Cu, Pb, Ni) for military and industrial applications (e.g., demolition, oil well penetrators etc.) (see Fig.2). CATALYTIC AND ENERGY STORAGE APPLICATIONS The high density of intercrystalline defects present within the bulk, and intersecting the free surface of nanostructured materials provides considerable opportunity in catalytic and hydrogen storage applications. Hydrogen transport rates and storage capacity in nanocrystalline Ni have been previously shown to be significantly enhanced over that observed in conventional material [15,16]; six to ten-fold increases in hydrogen diffusivity, and 60-fold increases in hydrogen storage capacity have been determined at room temperature. In addition, a higher electrocatalytic behaviour has been noted with regard to the hydrogen evolution reaction (HER) for alkaline water electrolysis at room temperature [ 161. Several applications are being developed for the use of these materials in Nickel Metal Hydride battery systems, and as alkaline fuel cell electrodes. MAGNETIC AND ELECTRICAL APPLICATIONS Some of the most promising industrial applications for nanostructured materials are in the area of soft magnets for high efficiency transformers, motors etc. Anticipated reductions in magnetocrystalline anisotropy and coercivity as grain size is reduced below the mean thickness of a magnetic domain wall in conventional materials, have generated considerable development activity in this area. However, many previous studies reported a large reduction in saturation magnetization (up to 40%) with decreasing grain size in nanomateria!ls produced by powder consolidation [7, 17,181. In contrast, this detrimental effect has not been observed in nanocrystalline materials produced by electrodeposition [ 191.
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Figure 2. Nanocrystalline -Cu (left), and -Ni (middle) shaped charge liners, electroformed on a Ti mandrel (right). Theoretical investigation and modeling  have shown that grain size should have little effect on magnetic moment; chemical disorder introduced by alloying or impurity elements being a more significant contributor to reductions in saturation magnetization. These electrodeposited nanocrystals can thus provide for soft magnets possessing a low coercivity without compromise of saturation magnetization. As depicted in Figure 3, the industrial use of these high performance ferromagnetic materials in motor, transformer and shielding applications has been accelerated by the recent development of a drum plating process for cost-effectively producing large quantities of sheet, foil, and wire in nanocrystalline form.
Figure 3. Prototype drum-plater for producing nanocrystalline sheet, foil and wire products.
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Another major application for drum-plated nanocrystalline material (as in Fig. 3) is in the production of copper foil for printed circuit boards, where enhanced etching rates and reduced line spacing/pitch can be achieved by reducing grain size. Figure 4 shows a crosssectional field emission scanning electron micrograph of nanocrystalline Cu foil produced for this application. Grain size has been optimized on the basis of calculated electrical resistivity for nanocrysmlline Cu  as summarized in Figure 5. A 50nm to IOOnm grain size provides optimum etchability while maintaining good electrical conductivity.
4. F’ield emission scanning electron micrograph of electrodeposited nanocrystalline Cu foil having an average grain size of 50nm.
Grain Size ([email protected]
Figure 5. Calculated room temperature electrical resistivity of pure Cu as a function of grain size .
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SUMMARY Several current and emerging applications for electrodeposited nanocrystalline materials have been presented. Property-specific ootimization of grain size promises to yield new high performance structural and functional materials for a variety of products and industrial applications. ACKNOWLEDGMENTS Financial support from Ontario Hydro and the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
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