Electrodeposited Nanocrystalline Materials

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5

ElectrodepositedNanocrystallineMaterials

Uwe Erb, Karl T. Aust, and Gino Palumbo

1.0 INTRODUCTION

Over the past decade, the synthesis of nanostructured materials byelectrodeposition has been advanced from a laboratory scale phenomenon toa practical industrial materials technology. This chapter addresses the synthe-sis of nanocrystalline materials by electrodeposition methods as well asstructure-property relationships for a variety of pure metals and alloys.Comparison with structure-property relationships observed for materialsproduced by other synthesis methods are given wherever possible. Someemerging industrial applications are also presented.

2.0 SYNTHESIS OF NANOSTRUCTUREDMATERIALS BY ELECTRODEPOSITION

From the synthesis point of view, electrodeposition is one of theoldest methods used to produce nanostructured materials for many years,

180 Chapter 5 - Electrodeposited Nanocrystalline Materials

probably inadvertently in most cases. Consequently, there are numerousearly reports in the literature describing electrodeposits with ultrafinestructures; many examples are given in Ref. 1. However, no systematicstudies were published before the late 1980s[2][3] on the synthesis ofnanocrystalline materials by electrodeposition in an attempt to optimizecertain properties by deliberately controlling the volume fractions of grainboundaries and triple junctions in the material. In fact, the synthesis ofnanostructured materials with grain size control during the electrodeposi-tion process can be considered a distinct form of grain boundary engineer-ing in which the grain boundary content (types and quantities of grainboundaries) of a material are controlled during material processing toachieve certain physical, chemical and mechanical properties.[4]–[6] The finalresult is thus a bulk interfacial material, as originally defined by Gleiter,[7]

which does not require any further processing of precursor powder material.In this respect, electrodeposited nanocrystals are quite different from othernanostructures which are based on consolidated particles.

Potentially there are a very large number of pure metals, alloys,composites, and ceramics which can be electrodeposited with grain sizesless than 100 nm. For example, the literature contains numerous examplesgiving electrochemical processing windows for the synthesis of nanoc-rystalline pure metals (e.g., Ni,[8]–[10] Co,[11] Pd,[12] and Cu[11]), binaryalloys (e.g., Ni-P,[2][3] Ni-Fe,[13][14] Zn-Ni,[15][16] Pd-Fe,[17] and Co-W[18]),and ternary alloys (e.g., Ni-Fe-Cr[19]–[21]). Even multilayered structures orcompositionally modulated alloys (e.g., Cu-Pb,[22] Cu-Ni,[23]–[25] Ag-Pd,[26]

Ni-P[27]), metal matrix composites (e.g., Ni-Si C[9]), ceramics (e.g., ZrO2[28]),

and ceramic nanocomposites (e.g., TlaPbbOc[29]) have been successfully

produced by electrodeposition methods. However, the latter are not consid-ered in this chapter; this review is limited to equiaxed pure metals andalloys with grain sizes less than 100 nm, without considering grainshape modifications.[30]

Crystalline metal electrodeposits exhibit several types of growthforms including layers, blocks, pyramids, ridges, spiral growth forms,dendrites, powders, and whiskers.[31] These morphologies have been stud-ied extensively and various models have been advanced to correlate specificgrowth forms with electrodeposition parameters and substrate microstruc-ture.[31][32] Electrodeposition parameters are bath composition, pH, tempera-ture, overpotential, bath additives, etc., while important microstructuralfeatures of the substrate include grain size, crystallographic texture, disloca-tion density, internal stress, and the like.[31][32]

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Electrocrystallization (Fig. 1) occurs either by the build up of existingcrystals or the formation of new ones.[33] These two processes are incompetition with each other and are influenced by different factors. The twokey mechanisms which have been identified as the major rate-determiningsteps for nanocrystal formation are charge transfer at the electrode surfaceand surface diffusion of adions on the crystal surface.[34] Earlier, Fischerpresented a classification of microstructures typically observed in electrode-posits.[35] One of the key factors in the microstructural evolution of electrode-posits in terms of grain size and shape is inhibition, for example, resulting fromreduced surface diffusion of adions by adsorption of foreign species (such asgrain refiners) on the growing surface. With increasing inhibition, the depositstructure changes from basis oriented and reproduction type (BR) to twintransition types (TT), to field oriented type (FT), and finally to unorienteddispersion type (UD).[36] A large number of grain refiners have been describedin the literature (see for example, Ref. 37); their effectiveness depends uponsurface adsorption characteristics, compatibility with the electrolyte, tem-perature stability, etc. For example, saccharin,[38] coumarin,[39] thiorea,[39]

and HCOOH[40] have all been successfully applied to achieve grain refine-ment down to the nanocrystalline range for nickel electrodeposits.

Figure 1. Two stages of electrocrystallization according to Bockris, et al.[34]

Section 2.0 - Synthesis by Electrodeposition


The second important factor in nanocrystal formation duringelectrocrystallization is overpotential.[33][34] Grain growth is favored at lowoverpotential and high surface diffusion rates. On the other hand, highoverpotential and low diffusion rates promote the formation of new nuclei.These conditions can be experimentally achieved when using pulse plating(Fig. 2), where the peak current density can be considerably higher than thelimiting current density attained for the same electrolyte during direct currentplating.

Figure 2. Generalized pulse current waveform. T is the period of the waveform, in arecurrent densities and tn are pulse durations.[33]

While many of the processes associated with the crystallizationstage (Fig. 1) are still poorly understood, the previous work has shown thatelectrodeposition will result in nanostructured materials when electrodepo-sition variables (e.g., bath composition, pH, temperature, current density,etc.) are chosen such that electrocrystallization results in massivenucleation and reduced grain growth. Under these conditions the effect ofthe substrate on the resulting bulk electrodeposit often becomes negligible(for example, see Ref. 41).

Electrodeposition of nanocrystalline materials is not limited toapplications as coating in-production or in-situ on structures and compo-nents. As discussed in more detail in Sec. 5, this method also provides forcost-effective production of freestanding forms such as ultrathin foil, wire,sheet, and plate, as well as complex shapes.

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3.0 STRUCTURE OF NANOCRYSTALLINEMETAL ELECTRODEPOSITS

This chapter deals mainly with equiaxed nanostructured electrode-posits, although layered or grain-shape modified structures can also besynthesized by electrodeposition.[30] Figure 3 shows bright field, dark field,diffraction pattern, and grain size distribution of a nanocrystalline Nispecimen produced by direct current plating from a modified Wattsbath.[14] Electrodeposition of nanocrystals typically operates far fromequilibrium conditions. Consequently, the material obtained is a non-equilibrium structure which is primarily manifested in the small grain sizeand the associated large volume fraction of grain boundaries and triplejunctions. In addition, alloys produced by this method can show consider-able extensions of the solid solubility range similar to what is observed inmaterials produced by other non-equilibrium processing routes, such asrapid solidification. For example, the room temperature solid solubility forP in Ni is negligible.[42] On the other hand, electrodeposited Ni-P can formsolid solutions containing phosphorus levels of 10 wt% or more.[2][3]

Similarly, extended solubility ranges were also observed in other alloys,such as Co-W,[18] Zn-Ni,[16] and Ni-Mo.[43]

Figure 3. TEM micrographs for electrodeposited nanocrystalline Ni (a) bright field, (b) darkfield, (c) electron diffraction pattern, and (d) grain size distribution.[14]

(a) (b)

(c) (d)

Section 3.0 - Structure of Nanocrystalline Metal Electrodeposits


Depending on the electrodeposition parameters, the grain sizedistribution of electrodeposited nanocrystals is relatively narrow as shown,for example, for Ni in Fig. 3. The crystallographic texture depends stronglyon the electroplating parameters as demonstrated in Fig. 4 for pulse platednickel nanocrystals.[38] In this example, the crystallographic texture changesfrom a strong (200) fiber texture to a (111) (200) double fiber texture withincreasing saccharin content in the plating bath. The series of x-raydiffraction scans presented in Fig. 4 also show that the saccharin concen-tration in the plating bath has a strong effect on the grain size of the material.This is evident from the increasing line broadening with increasing saccha-rin concentration.

Figure 4. X-ray diffraction patterns showing the influence of saccharin concentration inthe electrolyte on the preferred orientation of nickel electrodeposits produced by pulseplating.[38]

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High-resolution electron microscopy has revealed that the grainboundary structure in electrodeposited nanocrystals is similar to the struc-ture found in conventional polycrystalline materials.[44] This finding isin agreement with previous results by Thomas, et al.,[45] but in contrast toearlier work by Wunderlich, et al.,[46] who observed extended grainboundary structures in nanocrystalline palladium produced by inert gascondensation.

Using position annihilation spectroscopy, Würschum, et al.,[12]

recently reported relatively large free volumes in electrodeposited nanocrys-talline Pd which they described as nanopores (4 missing atoms) or nanovoids(10–15 missing atoms) containing light impurity atoms. However, such largefree volumes may be a particular microstructural feature of electrodepositedPd. For other electrodeposited nanocrystals, porosity is usually negligible asrecently demonstrated by detailed density measurements[47] and positronannihilation spectroscopy.[48]

Porosity-free electrodeposited nanocrystals are a distinct form ofgrain boundary engineered materials. If the intercrystalline region of amaterial is considered to consist of distinct grain boundary and triplejunction defects, the influence of these defects on the bulk properties ofnanocrystalline materials will depend upon their relative volume fractions.A three-dimensional treatment involving tetrakaidecahedral grains, wheregrain boundaries are represented by the faces of the polyhedron, and triplejunctions by the edges, has been applied,[4] and more recently generalizedto any grain shape.[49] Figure 5 shows calculated volume fractions for thegrain boundary, triple junction, and total intercrystalline component in thegrain size range from 2 nm to 1000 nm, calculated for a boundary thicknessof 1 nm. The intercrystalline volume fraction increases from a value of~0.3% at 1000 nm to ≥50% at grain sizes smaller than 5 nm. In the rangeof 100 nm to 2 nm, the triple junction volume fraction increases by threeorders of magnitude, while the grain boundary volume fraction increases bya little over one order of magnitude. The grain boundary volume fractionalso shows a plateau at a grain size of ~3 nm, while the triple junctionvolume fraction continues to increase and becomes equivalent to the grainboundary volume fraction at a grain size of ~2 nm.

The plateau in grain boundary volume fraction coincides withvalues of grain size below which a transition to the noncrystalline state isusually observed. Figure 6 schematically illustrates a limiting case for thisphenomenon.[50] When the mean grain size becomes very small, individualcrystals can be better represented as spherical clusters of atoms. Underthese conditions, the grain boundary can be represented by the point of

Section 3.0 - Structure of Nanocrystalline Metal Electrodeposits


contact between adjacent spherical clusters, while the triple junction regionassumes a relatively large volume. Thus, it was postulated[50] that thetransition to the noncrystalline state can be defined as the crystal (cluster)size where the ratio of triple junctions to grain boundary volume fractionbegins to approach infinite values.

Figure 5. The effect of grain size (d ) on calculated volume fractions for intercrystallineregions, grain boundaries, and triple junctions, assuming a grain boundary thicknessof 1 nm.[4]

Figure 6. Schematic representation of a postulated limiting grain size for crystallinitywhere the triple junction to grain boundary volume fraction ratio approaches infinitevalues.[50]

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4.0 PROPERTIES

A critical assessment of the properties measured to date on electrode-posited nanocrystals shows that these can be classified into two basiccategories. The first group of properties are strongly dependent on grain size.These include strength, ductility and hardness,[14][21][50]–[61] wear resistanceand coefficient of friction,[62] electrical resistivity,[10][11][63] coercivity,[64] solidsolubility,[2][3][16][18][43] hydrogen solubility and diffusivity,[65][66] resistance tolocalized corrosion and intergranular stress corrosion cracking,[59][60][67]–[70]

and thermal stability.[44][48][50][56][59][60][71]–[78] On the other hand, the secondgroup of properties including bulk density,[47] thermal expansion,[48][79]

Young’s modulus,[53][57][60][80]–[83] resistance to salt spray environment,[41]

and saturation magnetization[39][54][64][80][84]–[89] are little affected by grainsize. In the following sections some of these properties are discussed in moredetail and comparisons with properties observed in nanostructured materialsproduced by other methods are made.

4.1 Mechanical Properties

As expected, the plastic deformation behavior of electrodepositednanocrystalline materials is strongly dependent on grain size. Much of theearly work was concerned with room temperature microhardness mea-surements on free-standing sheet samples (typical thickness 0.1–0.5 mm)which were initially electrodeposited onto a Ti substrate and thenremoved from the Ti for hardness measurements. Figure 7a shows theresults obtained by Palumbo, et al.,[51] for room temperature Vickershardness measurements of Ni-P electrodeposits. Also shown are the resultsby Chokshi, et al.,[91] on nanocrystalline Pd and Cu produced by the inertgas condensation technique. Initial increases, followed by significantdecreases in hardness are noted with decreasing grain size (d) in thenanocrystal range, i.e., d ≤ 20 nm. The observed decreases in hardnessare contrary to Hall-Petch behavior and consistent with results reportedelsewhere[92][93] for nanocrystalline materials. Others, e.g., Ref. 94, haveonly reported a reduction in the Hall-Petch slope in the nanometer range.Recently, a study of room temperature tensile strength of nanocrystallineNi[61] showed a behavior consistent with that of the hardness studies(Fig. 8).

Section 4.0 - Properties


(a)

(b)

Figure 7. (a) Vickers hardness measurements for nanocrystalline Ni-P,[51] Pd,[91] andCu.[91] (b) Corresponding intercrystalline volume fractions.[4]

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Chokshi, et al.,[91] interpreted their results in terms of room tempera-ture Coble creep, arising from the disorder associated with large intercrystallinevolume fractions. However, in one study[95] it appeared that grain boundarydiffusional creep is not an appreciable factor in determining the roomtemperature mechanical behavior of nanocrystalline Cu and Pd. The onsetof decreasing hardness, i.e., deviation from Hall-Petch behavior, in thesesystems occurs at grain sizes where triple lines begin to comprise asignificant fraction of the bulk specimen volume (see Fig. 7b). The observedphenomena are in general agreement with the triple line softening effectsfirst reported by Rabukhin,[96] who investigated the effect of triple junctionson the room temperature tensile properties of conventional polycrystallinewires (Al, Cu, W) having various grain sizes. By electrochemical thinning ofthe wires to a diameter less than the average grain size, triple junctions couldbe eliminated from the microstructure. In all cases, an increase in strengthand decrease in ductility was noted on such a transition from an equiaxed to

Figure 8. The result of fitting the yielding strength of nanocrystalline nickel electrodepositsto a composite model incorporating strength contributions from grain boundaries (σgb),triple junctions (σtj), and quadruple nodes (σqn).[61]



bamboo grain structure. The grain size dependence of the proof stress wasfound to obey the Hall-Petch relationship; however, at constant grain size,lower values were always obtained with the equiaxed geometry. Morerecently, using a similar experimental approach, Lehockey and co-work-ers[97] also confirmed triple line softening effects in Ni.

Modified dislocation pile-up theories involving small numbers ofdislocations[98][99] can be used to explain the deviation behavior of the Hall-Petch relationship but not the negative slopes shown in Figs. 7 and 8. Asignificant reduction in the Hall-Petch slope value was predicted by Smith,et al.,[100] to occur for the extreme case of only one dislocation loop beingexpanded against the grain boundary obstacle stress. Wang, et al.,[55]

concluded that the dislocation pile-up mechanism no longer applies tonanocrystalline materials below a critical grain size, e.g., about 10 nm forfcc metals. A composite model based upon geometric considerations interms of the volume fraction of crystalline and intercrystalline componentswas proposed by Wang, et al.,[55][61] to evaluate the strength of nanocrystal-line materials. It was shown that this model can be used for interpreting thevarious observations involving deviation from the Hall-Petch relationshipand a negative Hall-Petch slope.

In addition to grain boundaries and triple junctions, this analysisalso included quadruple nodes where triple lines (usually four) are linkedup.[49] The strength contributions for grain boundaries (σgb), triple junc-tions (σtl) and quadruple modes (σqn) was shown to have the followingsequence:[61] σgb > σtl > σqn.

Wang, et al.,[61] also derived an analytical expression for assessingthe creep rate of nanocrystalline materials by a diffusion mechanism,including triple line diffusion. The overall creep rate is the sum of the creeprate due to lattice diffusion, grain boundary diffusion, and triple linediffusion. It was predicted that the creep rate due to triple line diffusion willexhibit a stronger grain size dependence than that due to grain boundarydiffusion. For example, the contribution of triple line diffusion to steady-statecreep rate appears to be the inverse of d4 (d = grain size), which is one orderhigher than grain boundary diffusion and two orders higher than latticediffusion in terms of grain size dependence. In addition, the secondary creeprate is still linearly proportional to the applied tensile stress, compared tothe dislocation mechanism in which the exponent of the applied stress isusually greater than three.

The upshot of the work by Wang, et al.,[61] is that, at high stresslevels, grain boundary sliding is the major room temperature deformationmechanism in nanocrystalline pure Ni electrodeposits. However, the

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contribution from creep mechanisms through intercrystalline regions can besignificant for smaller grain size. A negative Hall-Petch slope was observedwhen the grain size was below 10 nm. It was suggested that the deviationfrom the Hall-Petch relationship can be attributed to a dynamic creepprocess due to diffusion mechanisms.

Recently, a more complete study on mechanical properties ofnanocrystalline materials was performed in conjunction with the develop-ment of the first large scale industrial application of electrodepositednanocrystalline materials, the ElectrosleeveTM technology (ElectrosleeveTM

is a registered Trademark of Ontario Hydro, Canada; see Sec. 5 for details).The results of various mechanical properties of nanocrystalline nickel withgrain size of 10 nm and 100 nm in comparison to conventional polycrystal-line material are shown in Table 1. In addition to the remarkable increasesin hardness, yield strength, and ultimate tensile strength with decreasinggrain size, it is interesting to note that the work hardening coefficientdecreases with decreasing grain size to virtually zero at a grain size of 10nm. The ductility of the material decreases with decreasing grain size from50% elongation to failure in tension for conventional material to 15% at 100nm grain size and about 1% at 10 nm grain size. Generally somewhat greaterductility was observed in bending. A slight recovery in ductility wasobserved for grain sizes less than 10 nm.[61]

Compared to conventional polycrystalline Ni, nanocrystalline Nielectrodeposits exhibited drastically reduced wear rates and lower coeffi-cients of friction as determined in dry air pin-on-disc tests.[62]

Contrary to earlier measurements on nanocrystalline materialsprepared by consolidation of precursor powder particles,[95][101][102] nanoc-rystalline nickel electrodeposits do not show a significant reduction inYoung’s modulus. This result provides further support for earlier findingsof Krstic, et al.,[81] and Zugic, et al.,[83] which demonstrated that thepreviously reported reductions in modulus with nanoprocessing were likelythe result of high residual porosity.

With respect to the hardness curve for Ni-P shown in Fig. 7a, itshould be noted that the grain size for the smallest grain sizes (<3 nm) wasderived from x-ray line broadening measurements. However, the x-raydiffraction scans for these particular samples resembled those typicallyobtained for amorphous structures. These electrodeposits contained in-creasing P content with decreasing grain size and it has been previouslyshown[2][3] that there is a smooth, but not fully characterized, transition fromthe nanocrystalline to the amorphous structure. The smooth decrease inhardness through the nanocrystalline to the amorphous transition in Fig. 7a


192Table 1. Mechanical Properties of Conventional and Nanocrystalline Nickel

Property Conventional a Nano-Ni 100nm Nano-Ni 10nm

Yield Strength, MPa (25oC) 103 690 >900

Yield Strength, MPa (350oC) — 620 —

Ultimate Tensile Strength, MPa (25oC) 403 1100 >2000

Ultimate Tensile Strength, MPa (350oC) — 760 —

Tensile Elongation, % (25oC) 50 >15 1

Elongation in Bending, % (25oC) — >40 —

Modulus of Elasticity, GPa (25oC) 207 214 204

Vickers Hardness, kg/mm2 140 300 650

Work Hardening Coefficient 0.4 0.15 0.0

Fatigue Strength, MPa (108 cycles/air/25oC) 241 275 —

Wear Rate (dry air pin on disc), µm3/ µm 1330 — 7.9

Coefficient of Friction (dry air pin on disc) 0.9 — 0.5

a: ASM Metals Handbook, ASM International, Metals Park, OH, 2:437 (1993)

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indicates that a common structural element may be responsible for ductilizationin both the nanocrystalline and amorphous states. This transition coincideswith the region following the plateau in the grain boundary volume fraction(discussed in Sec. 3.0) below which the triple junction volume fractionsassume relatively large values. It was speculated that the common structuralelement which could be responsible for the ductilization is the disclination.[51]

4.2 Corrosion Properties

In general, the corrosion resistance of nanocrystalline materials inaqueous solutions is of great importance in assessing a wide range ofpotential future applications. To date, research in this area is still scarce andrelatively few studies have addressed this issue. For the case of thecorrosion behavior of nanocrystalline materials produced by crystalliza-tion of amorphous precursor materials (e.g., Refs. 103–108), both benefi-cial and detrimental effects of the nanostructure formation on the corrosionperformance were observed. The conflicting results are, to a large extent,due to the poorly characterized microstructures of the crystallized amor-phous materials. On the other hand, for nanostructured materials producedby electrodeposition, considerable advances in the understanding of micro-structure on the corrosion properties have been made in recentyears.[59][60][67]–[70]

In previous studies,[67][68] potentiodynamic and potentiostatic po-larizations in de-areated 2N H2SO4 (pH = 0) were conducted on bulk (2 cmsquare coupons, 0.2 mm thick) nanocrystalline pure Ni at grain sizes of 32,50, and 500 nanometers and compared with polycrystalline pure Ni (grainsize of 100 µm). Figure 9 shows the potentiodynamic anodic polarizationcurves of these specimens. The nanocrystalline specimens exhibit the sameactive-passive-transpassive behavior typical of conventional Ni. However,differences are evident in the passive current density and the open circuitpotential. The nanocrystalline specimens show a higher current density inthe passive region resulting in higher corrosion rates. These higher currentdensities were attributed to the higher grain boundary and triple junctioncontent in the nanocrystalline specimens, which provide sites for electro-chemical activity. However, this difference in current density diminishes athigher potentials (1100 mV SCE) at which the overall dissolution rateoverwhelms the structure-controlled dissolution rate observed at lowerpotentials.



Another notable difference in the potentiodynamic response ofnanocrystalline and polycrystalline specimens is the open circuit potential.The positive shift of the open circuit potential for the nanocrystallinespecimens is thought to be the result of the catalysis of the hydrogenevolution reaction.[67]

Figure 9. Potentiodynamic polarization curves for nanocrystalline and polycrystalline Niin 2N H2SO4 at ambient temperature.[67]

Figure 10 shows scanning electron micrographs of nickel with a) 32nm and b) 100 µm grain size, held potentiostatically at 1200 mV (SCE) in 2NH2SO4 for 2000 seconds.[67] Both specimens exhibit extensive corrosion butthe nanocrystalline Ni is more uniformly corroded while the specimen with100 µm grain size shows extensive localized attack along the grain boundariesand triple junctions. X-ray photoelectron spectroscopy of the specimenspolarized in the passive region showed that the passive film formed on thenanostructured specimen is more defective than that formed on the polycrys-talline specimen, while the thickness of the passive layer was the same on bothspecimens.[109] This higher defective film on the nanocrystalline specimen

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allows for a more uniform breakdown of the passive film, which in turn leadsto a more uniform corrosion. In contrast, as has been previously shown in Ref.110, in coarse-grained Ni the breakdown of the passive film occurs first at thegrain boundaries and triple junctions rather than the crystal surface, leadingto preferential attack at these defects.

Figure 10. SEM micrographs of Ni with (a) 100 µm and (b) 32nm grain size heldpotentiostatically at 1200 mV (SCE) in 2N H2SO4 for 2000 seconds.[67][68]

(a)

(b)



Similar observations were made for the corrosion behavior ofnanocrystalline 304 stainless steel (grain size 25 nm) in HC1 produced bysputtering.[111] The reduced susceptibility to localized corrosion was attrib-uted to the fine-grained microstructure, which allowed for a uniformdistribution of C1- ions.

More recently, the corrosion behavior of nanocrystalline Ni wasalso studied in 30 wt% KOH solution[70] and pH neutral solution containing3 wt% sodium chloride.[43] The results were similar to the corrosionbehavior observed in sulfuric acid. The general corrosion was somewhatenhanced compared to conventional polycrystalline Ni; however, thenanostructured materials were much more immune to localized attackwhich often can lead to catastrophic failures.

Using the ASTM B-117 salt spray test, it was found that themicrostructure of Ni has little effect on the overall corrosion performanceunder these electrochemical conditions.[41] Both conventional polycrystal-line and nanostructured coatings gave the same corrosion protection to mildsteel substrates.

Further corrosion testing was performed on nanocrystalline Niunder conditions required for steam generator alloy application as part ofthe ElectrosleeveTM development program.[59] Tests ASTM G28 (suscep-tibility to intergranular attack), ASTM G48 (susceptibility to pitting andcrevice corrosion), ASTM G35, G36, and G44 (susceptibility to stresscorrosion cracking in polythionic acids, magnesium chloride, and alternateimmersion in sodium chloride, respectively) were performed. The resultsshowed that electrodeposited nanostructured Ni with a grain size of 100 nmis intrinsically resistant to intergranular processes such as intergranularattack and intergranular stress corrosion cracking. The material was foundto be resistant to pitting attack and only slightly susceptible to crevicecorrosion.

A second series of tests focused on specific environments that areknown to be detrimental to steam generator materials.[59] Environmentsincluded alkaline, acidic, and a combination of oxidizing and reducingspecies. The tests revealed excellent resistance of the nanocrystallinenickel to alkaline environments and reducing acidic environments. Thecorrosion resistance to oxidizing and acidic environments was found to belimited.

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4.3 Hydrogen Transport and Activity

The transport behavior of hydrogen in electrodeposited nanocrys-talline Ni foil (average grain size of 17 nm) at 293 K was determined usingan electrochemical double cell.[65] Figure 11 shows a typical hydrogenpermeation curve, where the anodic exit current (I ) is plotted as a functionof cathodic charging time (t). Three distinct breakthrough events are clearlyevident, as indicated by the arrows in Fig. 11. On the basis of determineddiffusivities, permeation flux values, and area (volume) fraction consider-ations,[65] these breakthrough events were considered to be due to hydrogentransport through distinct triple junction, grain boundary, and lattice paths,respectively. The triple junction diffusivity was determined to be approxi-mately three times faster than grain boundary diffusivity, and 70 timesfaster than lattice diffusion. Other studies[112] have also shown that diffu-sive transport occurs at a considerably faster rate through the triplejunctions than along the adjoining grain boundaries. These results providesupport for the defect character of triple junctions. Furthermore, theexistence of a “measurable” triple junction diffusivity in nanocrystalline Niindicates the importance of triple junction defects in the bulk properties ofnanocrystalline materials.

Figure 11. Anodic exit current density (I) as a function of cathodic charging time (t) at 0.1mA/cm2 for nanocrystalline Ni (17 nm grain size) foil of 0.017 cm thickness.[65]



As shown in Fig. 12, nanocrystalline Ni having an average grain sizeof 20 nm is also observed to display significantly higher electrocatalyticbehavior when compared to 1) cold worked, 2) fine-grained, and 3) fullyannealed reference structures with regard to the hydrogen evolution reac-tion (HER) for alkaline water electrolysis at room temperature.[66]

Figure 12. Room temperature Tafel plots of HER for electropolished nanocrystalline,80% cold worked, fine grained (1 µm) and fully annealed Ni in 0.1N NaOH.[66]

The enhanced HER kinetics observed here are considered to be thedirect result of the high area fraction of grain boundaries (and to someextent, triple junctions) intersecting the free surface of the electrode. In amore recent study,[43] it was shown that the HER kinetics can be furtherenhanced by alloying nanocrystalline Ni with molybdenum.

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An additional study[66] into the transport behavior of hydrogen innickel as determined by an electrolytic charging technique revealed thatsubstantial increases in hydrogen diffusivity and capacity are obtainedwhen Ni is in nanocrystalline form. Figure 13 illustrates three representa-tive permeation transients corresponding to hydrogen transport throughnanocrystalline (20 nm), fine grained (1 µm), and single crystalline Nifoils of 140 µm thickness. Detection of permeated hydrogen in the Ni bi-electrodes of identical thickness is observed in the following order: 1)nanocrystalline, 2) fine grained, 3) single crystal structures.

Figure 13. Hydrogen permeation transients showing anodic exit current density (flux) vs.time for nanocrystalline (20 nm), fine grained (1 µm), and single crystal Ni foils.[66]

In addition, the apparent concentration of hydrogen in the 20 nmsample is found to be approximately 60 times greater than that of the singlecrystal structure with regard to the permeation transients shown in Fig. 13.The increased hydrogen diffusivity and capacity are attributed to highintercrystalline content, which provides 1) a high density of short circuitdiffusion paths and 2) large free volumes to which increased segregation ofhydrogen can occur.



Recently, permeation experiments were conducted in a doublechamber ultra-high vacuum system separated by a test nickel specimen.[113]

Hydrogen permeabilities and diffusivities through microcrystalline andnanocrystalline Ni were measured in the temperature range of 30°C to200°C. Steady-state permeability measurements indicate that nanocrystal-line Ni (average grain size of 78 nm) displays enhanced permeability below50°C (e.g., a factor of six at 30°C), as compared to the microcrystalline Ni(average grain size of 3 µm). Also, diffusivity measurements in combina-tion with hydrogen trapping site density measurements suggest that thereare more intercrystalline hydrogen trapping sites in the nanocrystalline Ni.

4.4 Magnetic Properties

Conflicting results have been reported regarding the dependence ofcertain magnetic properties on the grain size of the material. While theunderstanding of the magnetic structure of nanostructured materials is stillfar from complete, a clear picture is now emerging regarding the saturationmagnetization, Ms, where the early contradictory results can be explainedin terms of the chemical and physical microstructure of the nanocrystallinematerials.

Initially many studies reported that, for nanocrystalline materials,there is a large reduction in saturation magnetization with decreasing grainsize.[101][114]–[117] Gleiter first reported a 40% decrease in saturation mag-netization compared to bulk α-iron for nanocrystalline iron with 6 nm grainsize which was produced by consolidating nanocrystalline particles pro-duced by the inert gas condensation technique.[101]

This behavior was attributed to differences in the magnetic micro-structure between nanocrystalline and conventional polycrystalline iron.Similarly, strong effects of particle size on saturation magnetization havebeen observed in the study of uncompacted ultrafine particles produced bythe gas evaporation method.[114][115] In the case of ultrafine particles (10–50nm) of Ni, Co, and Fe, Gong, et al.,[114] observed a rapid decrease insaturation magnetization with decreasing grain size which they attributed toantiferromagnetic oxide layers on the ultrafine metal particles. In anotherstudy on ultrafine particles it was found that the normalized magnetizationratio decreases with decreasing particle diameter.[115] The reduction insaturation magnetization was linked to surface effects, which were consid-ered more important in the case of smaller particles. Schaefer, et al.,[116] also

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noted a decrease in Ms in consolidated nanocrystalline nickel powderproduced by gas evaporation which they explained in terms of structuraldisorder of the interfaces. The magnetic moment of the interfacial atomswas calculated to be nearly half that of the atoms in the bulk material.Furthermore, Yao, et al.,[117] also found that the saturation magnetization ofultrafine Ni particles decreases drastically with decreasing grain size. Krill,et al.,[118] reported that the spontaneous magnetization of nanocrystallineGd samples produced by gas condensation and subsequent compaction wasapproximately 75% of the value for polycrystalline Gd. It should be notedthat all of the previous materials were produced using the gas condensationmethod which yields materials with high internal porosity that can providelarge surface areas for oxide formation after exposing the samples to air.

In contrast, Aus, et al.,[64] reported, for the first time, that thesaturation magnetization of nanocrystalline Ni was not strongly dependenton the grain size. In this study, the grain sizes of Ni varied from 100 µm to10 nm, and for the Ni samples with the smallest grain size, the observed Ms

was only 10% less than for conventional polycrystalline Ni. These resultswere obtained on bulk nanocrystalline Ni produced by electrodepositionand were explained in terms of the negligible porosity/oxide formation inthis material.

The finding by Aus, et al.,[64] agrees well with results of recentcalculations which assessed the effect of structural disorder, introducedby grain boundaries, on the magnetic properties of nanocrystalline met-als.[86]–[88] In these studies, grain boundary configurations representingvarious degrees of disorder were generated using molecular dynamicssimulations with embedded-atom potentials. They ranged from Σ3 bound-aries with minimum structural disorder through Σ5 and Σ13 special grainboundaries of intermediate structural disorder to random amorphous grainboundaries with maximum disorder. Electronic structure calculations wereperformed using the tight-binding linear muffin-tin orbital atomic-sphere-approximation method. These calculations have shown that the magneticmoment is rather insensitive to the degree of structural disorder associatedwith grain boundaries. Even when the entire material was amorphous, theaverage moment was found to be reduced by only 15%. It was concludedthat, for the case of nanocrystalline Ni with a grain size of 10 nm at whichthe grain boundary atoms comprise about 30% of the volume, the overalleffect of structural disorder on the average moment is very small, in goodagreement with the experimental data reported by Aus, et al.,[64] forelectrodeposited nickel.



More recently, there have been other reports confirming the earlyresults by Aus, et al.,[64] for nanocrystalline Ni. For example, Daroczi, et,al.,[119][120] reported for nanocrystalline nickel prepared by ball milling, thatthere is no observable difference in Ms for materials with 7 nm and 50 µmgrain sizes. Bakonyi, et al.,[11] observed the same trend for nanocrystallineNi, also prepared by electrodeposition. Weissmüller, et al.,[89] confirmedthe earlier measurements by Aus, et al.,[64] reporting only small changes inMs for electrodeposited nanocrystalline Ni with 18 nm grain size. Kisker,et al.,[121] presented new results for gas condensed Ni which, in contrast totheir earlier work,[116] now showed the saturation magnetization to beindependent of grain size as long as the gas condensed material was notexposed to air. However, after exposure to air, Ms decreased to about80% of its original value.

Aus, et al.,[122] and Szpunar, et al.,[90] have recently presentedfurther experimental evidence and detailed calculations for nanocrystallineNi-Fe, Ni-P, Co, and Co-W which further support their earlier findings thatstructural disorder introduced by grain boundaries and triple junction hasan insignificant effect on saturation magnetization compared with chemi-cal disorder introduced by alloying additions.

4.5 Thermal Stability

The thermal stability of nanocrystalline materials is of consider-able importance for applications at elevated temperatures. For electrode-posited nanocrystals, the thermal stability has been assessed by in-situtransmission electron microscopy,[44][71][73] conventional annealing fol-lowed by TEM analysis,[74]–[76] and differential scanning calorimetry(DSC) experiments from which activation energies for grain growth weredetermined using the Kissinger[123] analysis.[48] Additional indirect experi-ments on the thermal stability involved hardness measurements as afunction of annealing time.[59][60]

Figure 14 shows the grain growth kinetics for a nanocrystalline Ni-1.2 wt% P alloy as evaluated from in-situ electron microscopy studies.[71]

At 473 K, no grain growth was observed and the material was stable as asolid solution. At 673 K, substantial grain growth was observed withinthe first few minutes of annealing, resulting in a microcrystalline twophase (Ni + Ni3 P) structure. However, at 573 K and 623 K, the grain sizeinitially increased rapidly by a factor of 2–3 and then became essentiallyindependent of annealing time. Similar behavior was observed for a Ni-Salloy at 573 K.[73]

203

Grain growth kinetics leading to a constant characteristic grain sizeis common for systems subjected to large grain boundary dragging forces.The most obvious dragging mechanism for these alloy systems is precipi-tate-induced Zener type drag.[124] However, considering the extremelylarge driving forces for grain growth expected in these materials (e.g., about200 J/cm3 at a grain size of 20 nm[72]), the observed thermal stabilizationmay not be attributed solely to such a mechanism. In nanocrystallinematerials, an additional dragging force may be due to triple junctions.[71] Ithas been shown that grain growth in fine-grained polycrystalline materialsmay be controlled by the intrinsic mobility of triple junctions.[125] A furthercontribution of triple junctions to the thermal stability of nanostructuredmaterials is the result of preferential solute segregation to these sites.[126]

Such solute enrichment at triple junctions in annealed nanostructuredNi-0.12 wt% S was recently observed by scanning transmissionmicroscopy.[75]

Klement, et al.,[75] investigated the thermal stability of 10 nm and20 nm Ni using DSC (heating rate 10 K/min) and TEM. The temperature atwhich the material tends to become unstable was found to be as low as353 K. This instability was attributed to “nucleation” and abnormal graingrowth producing a dual-sized microstructure after annealing in the range

Figure 14. Grain size as a function of annealing time for electrodeposited nanocrystallineNi-1.2 wt % P.[71]



of about 400 to 550 K. The origin of the abnormal grain growth may be the“clusters” of subgrains observed in the nanocrystalline nickel deposits, asindicated by Moiré patterns in TEM studies[75][76] and high resolutionmicroscopy.[44] In order to nucleate the grains subsequently observed inabnormal grain growth, these nanometer-sized subgrains have only to rotateslightly towards each other to form a larger grain. This mechanism isanalogous to the subgrain coalescence model of primary recrystalliza-tion.[127][128] In fact, changes of grain orientation caused by rigid bodyrotations have been observed directly during the annealing of nanocrystal-line thin films of gold; the observed rates of grain rotation were consistentwith a mechanism based upon diffusion-limited grain boundary sliding inresponse to the variation of grain boundary energy with misorientation.[129]

Gertsman and Birringer[130] have suggested that inhomogeneity of grainboundary structure and non-uniform interface segregation contribute toabnormal grain growth observed at ambient temperature in nanocrystallinecopper. The results obtained by Klement, et al.,[75] for abnormal graingrowth and S segregation at grain boundaries and triple lines in annealednanocrystalline Ni provide support for this interpretation.

For nanocrystalline Ni (starting grain sizes ranging from 15–30nm), the activation energies for grain growth as determined by Kissingeranalysis[123] from differential scanning calorimetry studies[48] were in therange of 1.2–1.4 eV which corresponds to the activation energy of grainboundary diffusion in Ni.[131] Considerably higher activation energies weremeasured for nanocrystalline Ni alloys containing P alloying additions. Forexample, for nanocrystalline Ni-1.2% P (starting grain size 10 nm), theactivation energy was 2.25 eV[44] which was likely due to additional solute,Zener and triple junction drag.

The beneficial effect of microalloying on the thermal stability hasrecently been further demonstrated for nanocrystalline nickel (approxi-mately 100 nm grain size) developed for the ElectrosleeveTM applica-tion.[59][60] In this case, the thermal stability was assessed indirectly bymeasuring the hardness of pure nanocrystalline Ni and nanocrystalline Ni-P (<3000 ppm P) annealed at 616 K as a function of annealing time. For purenanocrystalline Ni, the hardness decreased rapidly from about 420 VHN to150 VHN within the first 100 minutes of annealing. However, for nanoc-rystalline Ni-P the hardness remained unchanged at 420 VHN for annealingtimes of in excess of 106 minutes. In this case, the thermal stability ofmicroalloyed Ni was attributed mainly to solute drag and possible Zenerdrag by micro-precipitates.

205

A recent theoretical and experimental study[132] on the effect of graingrowth on resultant grain boundary character distributions indicates thatthe thermal stability of nanocrystalline materials may be further enhancedby the tendency for these ultrafine grained materials to form “special” low-energy grain boundaries during the early stages of grain growth.

4.6 Thermal Expansion and Heat Capacity

Thermal expansion, αL, and specific heat, CP, measurements onnanocrystalline electrodeposits[48][79] showed quite different results fromthose reported earlier on nanostructures produced by inert gas condensa-tion[133] or crystallization of amorphous precursors.[134] Rupp andBirringer[133] found αL for gas condensed nanocrystalline copper of 8 nmgrain size to be nearly twice that of regular polycrystalline copper. How-ever, the nanocrystalline copper was only 90% dense. Heat capacities ofnanocrystalline copper and palladium were also found to be increased by10% and 40%, respectively.[133] Thermodynamic models based onquasiharmonic approximations used to predict theoretically the thermody-namic properties as a function of grain boundary free volume have pro-duced similar increases in αL (70–85%) and CP (10–25%) for nanocrystal-line palladium with respect to conventional polycrystalline palladium.[135]

Fecht’s calculations[136] also predict increases by a factor of two in bothvolumetric thermal expansion and heat capacity. Lu, et al.,[134] observed anincrease in αL of about 60% and 12% in CP for Ni-P alloys crystallized fromamorphous precursors. However, these materials consisted of two phases(fcc Ni and bct Ni3P) and cannot be compared directly with single-phasematerials.

More recent results by Gleiter[137] showed that the differences forαL between nanocrystalline and conventional polycrystalline copper de-pend strongly on the applied pressure during consolidation of gas con-densed powder. The general trend was that this difference decreased withincreasing compaction pressure. Some compacted nanocrystals even showeda slight reduction of αL from the value of polycrystalline material.

In contrast, for electrodeposited nanocrystals, neither thermalexpansion nor specific heat showed significant differences when comparedto their polycrystalline counterparts.[48][79] Figure 15 shows the temperaturedependence of the linear thermal expansion coefficient, αL, for both the as-plated nanocrystalline nickel (20 nm grain size) and conventional nickel(100 µm grain size) between 140 and 500 K. Between 140 and 205 K, αL is



slightly higher for the nanocrystalline nickel. However, between 205 and 500K, αL of nanocrystalline nickel is reduced from the value observed forpolycrystalline nickel; the maximum reduction is 2.6% at 500 K. Overall,the effect of grain size on αL for pure, fully-dense Ni is rather small. Figure16 shows the heat capacity, CP, as a function of temperature for nanocrys-talline (20 nm grain size) and normal polycrystalline (100 µm grain size)nickel. Over the entire temperature range, the heat capacity of nanocrystal-line nickel is marginally increased above CP for polycrystalline nickel by2.5–5%.

Figure 15. Temperature dependent coefficient of linear thermal expansion of nanocrystal-line (20 nm grain size) and conventional polycrystalline (100 µm grain size) nickel.[79]

207

As for the case of Young’s modulus and saturation magnetization,these results clearly show that certain properties measured on nanostruc-tured materials are not necessarily the result of increased volume fractionsof interfaces in the materials. Rather than attributing major propertychanges to the presence of large densities of grain boundaries and triplejunctions, other microstructural defects such as porosity, impurities, andthe like must be considered.

4.7 Electrical Properties

A comparison of results of electrical property measurements per-formed on nanostructured materials produced by different synthesis routes(e.g., gas condensation,[101] electrodeposition[10][11][63]) show very similartrends. In most cases, the electrical resistivity was observed to increase with

Figure 16. Temperature dependent isobaric heat capacity of nanocrystalline (20 nm grainsize) and polycrystalline (100 µm grain size) nickel.[79]



decreasing grain size. For example, the room temperature resistivity for Niwas increased from about 6 µΩ cm at 100 µm grain size in fully-annealedmaterial, to about 22 µΩ cm at 11 nm grain size in electrodepositedmaterial.[63] This can be attributed to electron scattering at defects, such asgrain boundaries and triple junctions. In fact, a linear relationship betweenexcess resistivity—defined as the total resistivity of the nanocrystallinematerial minus the resistivity of conventional polycrystalline material (100µm grain size) with negligible intercrystalline volume fraction—wasobserved for nanocrystalline Ni of varying grain size.[63]

There is also good agreement as far as the temperature coefficientof resistivity is concerned; both nanocrystalline materials produced by inertgas condensation,[101] electrodeposited Ni,[11][63] and Co[11] show decreas-ing values with decreasing grain size.

For electrodeposited Cu, Bakonyi, et al.,[11] found no effect of grainsize on electrical transport properties which they attributed to the negligibleeffect of atomic disorder on the density of states around the Fermi level ofcopper.

5.0 APPLICATIONS

Electrodeposited nanostructures have advanced rapidly to com-mercial applications as a result of:

1. An established industrial infrastructure (i.e., electroplat-ing and electroforming industries).

2. A relatively low cost of application, wherebynanomaterials can be produced by simple modification ofbath chemistries and electrical parameters used in currentplating and electroforming operations.

3. The capability in a single-step process of producingmetals, alloys, and metal-matrix composites in variousforms (i.e., coatings, free-standing complex shapes).

4. The ability to produce fully dense nanostructures free ofextraneous porosity.

209

The importance of the latter cannot be overemphasized with regardto industrial applications since, as has been outlined in previous sections,many of the extraordinary properties initially attributed to nanostructureshave since been demonstrated to be an artifact of residual porosity in thesematerials. From the outset, the fully dense nanomaterials produced byelectrodeposition have displayed predictable material properties basedupon their increased content of intercrystalline defects. This “predictabil-ity” in ultimate material performance has accelerated the adoption ofnanomaterials by industry, whereby such extreme grain refinement simplyrepresents another metallurgical tool for microstructural optimization. Inthis section, an overview of some current and emerging practical applica-tions for electrodeposited nanocrystalline materials are presented anddiscussed in light of the importance of property-specific grain size “optimi-zation” rather than grain miniaturization for its own sake.

5.1. Structural Applications

As would be expected from Hall-Petch considerations, numerouspractical applications for nanocrystalline materials are based upon oppor-tunities for high-strength coatings and free-standing structural compo-nents. The superior mechanical properties of these electrodepositednanostructures have led to one of their first large scale industrial applica-tions—the Electrosleeve™ process for in-situ repair of nuclear steamgenerator tubing.[60] This proprietary process[138] has been successfullyimplemented in both Canadian CANDU and U.S. Pressurized WaterReactors, and has been incorporated as a standard procedure for pressuretubing repair.[139] In this application, nanocrystalline Ni (100 nm) iselectroformed on the inside surface of steam generator tubes to effect acomplete structural repair at sites where the structural integrity of theoriginal tube has been compromised (e.g., corrosion, stress corrosioncracking, etc.). Figure 17 shows a cut-away view of an installedElectrosleeve™. The high strength and good ductility of this 100 nm grainsize material permits the use of a thin “sleeve” (0.5–1 mm) which mini-mizes the impact on fluid flow and heat transfer in the steam generator.

Recent geometric models and experimental findings[140][141] haveshown that nanostructured materials can also possess a high resistance tointergranular cracking processes, including creep cracking. Several emerg-ing applications for nanocrystalline materials possessing high intergranu-lar cracking resistance include lead-acid battery (positive) grids, and

Section 5.0 - Applications


Figure 17. Cut-away view of an installed nanocrystalline Ni Electrosleeve™ on a hostAlloy 600 nuclear steam generator tube.

Figure 18. Nanocrystalline-Cu (left), and -Ni (middle) shaped charge liners (81 mm),electroformed on a Ti mandrel (right).

211

shaped charge liners (Cu, Pb, Ni) for military and industrial applications (e.g.,demolition, oil well penetrators, etc.) (see Fig. 18); applications in whichdurability and performance are frequently compromised by prematureintergranular failure.

5.2 Functional Applications

Some of the most promising industrial applications for nanostruc-tured materials are in the area of soft magnets for high-efficiency trans-formers, motors, etc. Anticipated reductions in magnetocrystalline anisot-ropy and coercivity, as grain size is reduced below the mean thickness ofa magnetic domain wall in conventional materials, have generated consid-erable development activity in this area. Figure 19 summarizes the potentialopportunity for nanocrystalline soft magnets, whereby these electrodepos-ited nanocrystals can possess a low coercivity without compromise ofsaturation magnetization.

Figure 19. Typical coercivity and saturation magnetization ranges for several ferromag-netic materials.



As depicted in Fig. 20, the industrial use of these high-performanceferromagnetic materials in motor, transformer, and shielding applicationshave been accelerated by the recent development of a drum plating processfor cost-effectively producing large quantities of sheet, foil, and wire innanocrystalline form.

Figure 20. Prototype drum-plater for producing nanocrystalline sheet, foil and wireproducts.

Another major application for drum-plated nanocrystalline material(as in Fig. 20) is in the production of copper foil for printed circuit boards,where enhanced etching rates and reduced line spacing/pitch can beachieved by reducing grain size. Figure 21 shows a cross-sectional fieldemission scanning electron micrograph of nanocrystalline Cu foil producedfor this application. Grain size has been optimized on the basis of calculatedelectrical resistivity for nanocrystalline Cu[143] as summarized in Fig. 22. A50 nm to 100 nm grain size provides optimum etchability while maintaininggood electrical conductivity.

As previously discussed in Ref. 66, the high density ofintercrystalline defects present within the bulk, and intersecting the freesurface of nanostructured materials, provides considerable opportunity incatalytic and hydrogen storage applications. Several applications are being

213

developed for the use of these materials, either as an electrodeposited coatingor electroformed free-standing component in Nickel Metal Hydride batterysystems, and as alkaline fuel cell electrodes (see Fig. 23).

Figure 21. Cross-sectional field emission scanning electron micrograph of electrodepos-ited nanocrystalline Cu foil having an average grain size of 50 nm.

Figure 22. Calculated room temperature electrical resistivity of Cu as a function of grainsize.[142]



5.3 Coating Applications

The improved hardness, wear resistance, and corrosion resistance,coupled with undiminished saturation magnetization and predictable ther-mal expansion, elastic properties, and electrical resistivity, make nanocrys-talline coatings ideal candidates for protective and functional coatings(e.g., as used in hard facing on softer, less wear resistant coatings, recordingheads, electronic connectors, replacement coatings for chromium andcadmium in automotive and aerospace applications). For applications asthin coatings (on the order of a few nanometers thick), the microstruc-tural evolution of the deposit with increasing coating thickness can bea major concern. Many previous studies on electrodeposited metals, notnecessarily in nanocrystalline form, have shown that the grain size usuallyincreases considerably with increasing coating thickness. (See, forexample, Ref. 143.) For the nanocrystalline Ni electrodeposits studied byBakonyi, et al.,[40] it was reported that the deposit initially was amorphousright at the substrate interface. This was followed by the transition tonanocrystalline structure, and then a gradual increase in grain size wasobserved. In contrast, nanocrystalline Ni electrodeposits produced follow-ing the procedures given by Erb, et al.,[8][39] showed that in most cases thenanostructure was fully established right at the interface with the substrateand that the grain size was essentially independent of coating thickness

Figure 23. Alkaline fuel cell electrodes coated with a nanocrystalline Ni-Mo alloy.

215

(Fig. 24). For certain electrochemical and substrate conditions, a thintransition layer of larger grains was observed in which the initialstructure was influenced by the larger grains of the substrate, presum-ably by an epitaxy mechanism. However, even in this case, a constant,thickness-independent structure was fully established within the first200 nm from the interface.

Figure 24. TEM bright field micrograph of cross section showing nanocrystalline nickelcoating (right) deposited onto polycrystalline bronze electronic connector substrate (left).Cross section prepared by ultramicrotomy.

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Electrodeposited Nanocrystalline Materials