Electrodeposited and other Coatings Solid-Solid Interfaces-Electrodeposited and Dynamic Coatings BY J. P. G. FARR AND G. W. ROWE Department of Industrial Metallurgy, University of Birmingham, P.O. Box 363, Birmingham B15 2TT Received 26th June, 1912 Observation of the first stages in the electrodeposition of oxide and of metals on to metallic substrates is considered. The paper is concerned with applications of electron and ion microscopy and with the possibility of correlating microscopic observations with electrochemical experiments. Mechanical effects of the electrochemical treatment are mentioned briefly. The solid-solid interface in electrodeposition is rather inaccessible, and most recent research has concentrated on the metal/solution interface. Even these studies leave poorly understood the structure of the electrical double layer at solid-electrode/ electrolyte surfaces, through which both the initial and subsequent growth processes occur.Furthermore, despite a number of careful kinetic experiments on metal/metal ion exchange, there are undecided matters relevant to the initial stages in the electro- deposition of one metal on to another. A model of electrocrystallization that has gained general acceptance involves electron transfer to give " adatoms " or " adions ", followed by surface diffusion of adspecies to the various points of incorporation in the There is another possibility, that direct deposition occurs at growth sites involving diffusion zones in the electrolyteY4* and in practice mechanisms may be mixed. Bewick and Thirsk have re-emphasized that to extract kinetic constants for actual crystal growth requires information on the topography of the growing surface ; the growth site (step-line) spacing must be measured independently.If surface diffusion contributes significantly, some estimate of adion concentrations and mobilities should be possible. Adion concentrations ri at the reversible potential have been found to range over two orders of magnitude on a variety of metals; agreement between workers is not precise, the derived concentrations involve estimates of the " true " atomic surface area compared with the " apparent " macroscopic areas and on other electrode parameters.6 Surface self-diffusivities D, could be obtained from ri and the adatom flux Vi, knowing the growth site spacing I, since D, = VgZ2/rgb.In fact, I has normally been estimated rather than measured. Typical values have been cmY1 > 10-5*5 cm.2 Recent electron microscopy suggests that on copper, lead and nickel electrodes I may be less than 50 A (see fig. 1). We may conclude that kinetic experiments do not give unequivocal evidence for adspecies, but that in some electrocrystallization systems they probably are intermediates although they are not yet well characterized. The position is more obscure for the first stages of deposition of a new phase on to a foreign substrate. It is convenient to consider anodic and cathodic processes separately, and to deal first with anodic processes, for it may be necessary to remove anodic artefacts before a substrate is in an appropriate condition for cathodic deposition experiments.177178 SOLID-SOLID INTERFACES ANODIC PROCESSES Kinetics and electron microscopy are well reviewed elsewhere. Attention is drawn to a brief survey by Redhead and Armstrong of techniques for analysis of the structure and composition of adsorbed layers, which includes electron impact desorp- tion, applied by Roberts (this Discussion) to the study of polymer formation on metal substrates. Ellipsometry is capable of detecting the formation of a fraction of a monolayer equivalent of surface film in electrochemical experiments,lO* but it may not be possible to distinguish between very thin optically uniaxial films and isotropic films l 2 (see also P a ~ h l e y ~ ~ ) . of the iridium surface with examination of a fine tip of this metal in the field-ion microscope.l 4 The distinctions between layer formation and localized growth, and between phase formation (e.g., formation of phase oxide) and chemisorption can be observed directly. In particular, for iridium there is close similarity between surfaces allowed to oxidize in the air and those anodized in dilute sulphuric acid to a potential less than that required for oxygen evolution. Hoare has described the general features of the anodic oxidation of iridium. Bold and Breiter l6 performed potential sweep experi- ments which our own experiments confirm. There is good agreement with earlier work l7 that oxygen coverage is highly reversible and that an IrO, layer does not thicken appreciably because this oxide is conductive, thus allowing oxygen evolution to occur on its surface.Fortes and Ralph l8 first achieved the imaging of phase IrO, in the field ion microscope; they describe the limitations and requirements of the technique for this type of application; and they describe the structure of the interface between iridium and the oxide formed at temperatures above 500°C. Schubert and Ralph l 9 have extended this work to the examination of anodic films on iridium. They report a relatively uniform, disordered film less than five monolayers thick formed below 1.0 V with respect to hydrogen. Around 1.5 V, deep pits formed in the { 100) regions. Tyson, Southworth and Farr applying triangular sweeps, limited by the onset of hydrogen evolution cathodically and of oxygen evolution anodically, find a similar thin uniform disordered film, but after as many as 20 sweeps (730 mV/s) corrosion of { 100) regions is apparent (see fig.2). Nanis and Javet 2o also found that treating an iridium tip with 2 N H2S04 for 10 min followed by 10 min in air produced a surface perturbation extending to only 3-atom layers. A similar observation was made by Rendulic and Miiller 21 on a previously characterized tip exposed to the atmosphere for 15 min. There is therefore good agreement in the results from anodizing experiments and from ion microscopy on the oxidation of the iridium surface. It is unfortunate that few metals form satisfactory specimens for ion microscopy. Schubert and Ralph l9 have studied platinum specimens, again finding only " amorphous " anodic films up to 2.25 V (against hydrogen). The air-formed film was indistinguishable from the anodic film even within the range 0.95-1.2 V where Pt (OH), has been postulated 22 or partial Pt-0 coverage may occur.23 A slight thickening of this film occurred between 1.2 and 2.25 V but there was no localized pitting as with iridium.Phase oxide has therefore not been imaged on anodized specimens so far, even though in its anodic irreversibility platinum is very different from iridium (cf. Stonehart, Kozlowska and Conway 24). We have found promising a combination of a potentiodynamic treatment CATHODIC PROCESSES FIELD ION MICROSCOPY The ready reduction of the thin oxygen coverage suggests that the interface between an iridium substrate and a metallic deposit may be accessible in the ion-(6) FIG.1 .-Electron microscope replica images of the fine structure on copper, lead and nickel electrodes (J. P. G. Farr, G. W. Greene, A. Loong and A. McNeil). (a) Electropolished surface of copper (near (100)). x 200,000. Size of fine features is about 50 A. (b) Cu electrodeposited on to copper [To face page 178 (near (loo)), for 30 s at 10 mA/cm from acidified copper sulphate solution. x 200,000.(4 FIG. 1 .-(c) Electropolished surface of Ni (between (100) and (310)). x 150,000 (cf. electropolished Ni (lll), ref. (48)); (d) the surface of a lead electrode prepared for exchange measurements by electropolishing, followed by lightly etching in 10 % HN03. x 150,000 (1.5 mm = lOOA).(6) FIG. 2.-Field ion microscope images of an iridium tip.(J. P. G. Farr, H. N. Southworth and A. Tyson.) (a) Field-evaporated iridium tip, before electrolytic treatment. A { loo} region lies centrally, SO that along the equator there is a progression from (002) through (113) to the (111) poles. (b) The Same tip, after subjection to 50 potential sweeps from the onset of hydrogen evolution to that of oxygm in 1 MH,SO,. Between 2 and 3 monolayers werz field-evaporated from the (1 11) poles in the microscope before recording this image.(4 FIG. 2.-(c) The same tip, after the field evaporation of another 2-3 monolayers from the (1 11) poles. The (002) image is now restored, showing that the result of electrolytic treatment was a pit, some 5-6 atomic planes deep, in this region.J . P .G . FARR A N D G. W. ROWE 179 microscope. Nanis and Javet 2o performed the simplest experiment, comparing their acid treated and aerially oxidized iridium tip with tips immersed in 2N H2S04 solutions containing M iridium ions. They found much deeper perturbation than in experiments without iridium ions in the electrolyte. For comparatively short immersion times (less than 5 min) an order of relative exchange currents was suggested io(l 11) < io(lOO) < i0(210). For times longer than 5 min the treatment produced features protruding some 30 atomic layers, centred on [3 101 directions. These features were interpreted as spikes of electrodeposited iridium and although their electrochemical conditions were not well defined, Nanis and Javet were able to esti- mate an upper limit, (Tyson, South- worth and Farr have observed similar spikes in multiple potential sweep experiments in solutions containing trace amounts of iridium).Probably, local cells are estab- lished, the cathodic spikes supporting oxidation elsewhere ; there was no evidence that the localization of growth was due to the emergence of screw dislocations. More control was exercized by Rendulic and Muller 21 in electrodepositing platin- um from hot H2Pt(N02)2S04 on to iridium and tungsten ion microscope tips. Platinum deposits some 500 A thick on tungsten were not epitaxial ; they were poly- crystalline with dimensions 50-500 A and showed heavy lattice distortion. The deposition on iridium was more interesting. Epitaxy was not obtained at high current densities which resulted in [l 1 11 oriented crystals ; at the highest current densities the deposit consisted of small heavily deformed crystals.However, at lower current densities the expected epitaxy was found. The first nucleation of platinum on iridium occurred near (012) and (135) areas, where islands of about 40 atoms of platinum repeated the iridium structure. Only part of the surface grew epitaxially, the remaind- er being covered by [l 1 11 oriented crystals. Within the epitaxial layer there were cracks some 20-lOOA long at the interface and it was suggested that these were a result of lattice misfit (2.18 %). Lattice defects in the substrate sometimes continued in the epitaxial coating. Rendulic and Muller obtained poycrystalline deposits of copper or iridium and tungsten. These few experiments suggest that the ion microscope may be useful in the study of nucleation and epitaxy on characterized substrates.It is possible that electroplating may be a useful preparative technique whereby metals that would rupture under the field stress in the microscope may be supported on adequately strong substrates.21 A cm-2 for the exchange current density. TRANSMISSION ELECTRON MICROSCOPY Advances in the study of vapour-phase thin-film epitaxy have been reviewed by Pashley 2 5 and by Matthews.26 The nucleation of electrodeposited gold films on (1 11) silver substrates was studied in detail by Dickson, Jacobs and P a ~ h l e y . ~ ~ They found considerable similarity with films formed by evaporation on to similar substrates and concluded that the mechanism of growth is similar.Significant difference was observed in the influence of substrate surface irregularities on nucleation and growth, and it was suggested that this was due to local electric field effects in electrodeposition that did not apply to evaporated deposits. These authors found that islands of gold were nucleated randomly on smooth silver substrates and that the islands subsequently coalesced in a “liquid-like” way. Double positioning (i.e., two twin related orienta- tions) can occur in the (1 l l) silver substrate which therefore contains non-coherent twin boundaries perpendicular to the plane of the film. The double-positioning structure is continued in the electrodeposited gold, the boundaries remaining per- pendicular to the plane of the film until the deposit becomes a complete film.At this stage, overlapping of one orientation by the other tends to occur, giving stepped180 SOLID-SOLID INTERFACES boundaries through the gold deposit. Dislocation structures result near the double- positioning boundaries. Probably, the growth of gold on gold occurs by the lateral spread of layers no more than a few atoms thick, a process deduced for the electro- crystallization of other metals from light and electron microscopy.2* 28 There appears to be evidence for the surface migration of gold over the substrate and over the gold islands. It is particularly interesting that Dickson, Jacobs and Pashley agree with Bassett and Pashley 29 that the first lOA of deposit make no contribution to island growth.They suggest that some alloying occurs between gold and silver during the initial stages of deposition. The misfit between gold and silver is only 0.18 % and even if alloying should increase the strain, Matthews 30 has demonstrated by the absence of misfit dislocations in the early stages of growth that pseudomorphism occurs in this system. Pseudomorphism is, however, no longer regarded as necessary for epitaxy. There have been some other, less detailed, studies of electrodeposited films by transmission electron microscopy (see, e.g., ref. (31-33) and work cited in ref. (27)). However, the experiments surveyed here show how the microstructure of electro- deposits may change markedly during the post-nucleation stage, by coalescence. Caution seems appropriate in deducing the nature of the initial stages of growth from observations of the structure and topography of thickened deposits.Even in the type of experiment made by Dickson et al., care was required to avoid producing artefacts when stripping the gold by dissolving away the silver substrate, and possibly some electrochemical control would have been desirable. The substrates used by Dickson 27 and by Ives et aZ.33 were better characterized than those in many other studies on the electrocrystallization of metals. Using bulk substrates, nucleation may be governed by segregated impurity 34-36 ; the presence of substructure in single crystal substrates and the technique of annealing adopted may affect the perfection of epitaxy 36 ; slight misorientation from a low-index substrate orientation may outweigh the influence of emergent screw dislocations on the initial 38 All these effects require examination in greater detail at the earliest stages of growth and at high resolution.It is suggested that the observations of Verma and Wilman (this Discussion) using the powerful technique of grazing incidence high-energy electron diffraction 9* 2 5 on the nickel + copper system are compatible with the transmission electron microscopy described and with the transmission electron diffraction of Sullivan and Oxley (this Discussion) for the equivalent substrate orientations. There is also agreement with the findings of Keen and Farr 28 (Zu, Cu), Farr and Cliffe 39 (Co, Ni) and Browns- word and Farr 35 (Cu, Ni) concerned with later stages of epitaxial growth.ELECTROCHEMICAL EXPERIMENTS A variety of electrochemical experiments on the deposition of small amounts of metal on inert substrates is relevant to our preoccupation with the interface structure at the commencement of the formation of a new phase. It has been shown that sub- monolayer amounts of silver will deposit on gold and platinum at appreciable under- voltages. Sandoz and co-workers 40 survey a number of voltage sweep experiments for these and other metals; their own results agree with data obtained by Rogers et d 4 1 using a tracer technique. Rogers et al. have presented evidence for the deposi- tion of copper at electrode potentials anodic to the equilibrium potential on a number of Others have demonstrated this for copper on smooth 4 3 9 45 and platinized platinum 44 and for thallium on smooth platinum.46 In potential-sweep experiments, there is fair agreement that the first current peaksJ .P . G . FARR AND G . W. ROWE 181 in the cathodic excursion are due to monolayer and sub-monolayer phenomena; subsequent peaks correspond to crystallization of the new phase. (In cases of gross lattice misfit, only one peak may OCCU~.~') The first peaks are ascribed to adsorptive effects, the local bonds between, e.g., Cu and Pt, being stronger than those between Cu and Cu. These results, again, are qualitatively not incompatible with the type of observations that have been made in both the field ion microscope and by electron microscopy although equivalent systems have not yet been examined, nor have the techniques been combined.Clearly, electrochemical experiments of this type would usefully supplement experiments such as those by Menzies and Stirling (this Discussion) on alloy deposition. ELECTROCHEMISTRY AND MACHINING APPLIED TO KINETIC FRICTION STUDIES We consider dynamic coating primarily in the context of tribology and suggest that a profitable inter-relationship with electrochemistry can be established. Electro- chemical techniques permit accurate kinetic investigations, and friction is a sensitive mechanical parameter indicating surface changes. As shown by Tabor (this Discussion), intimate contact between clean surfaces -t 2 0 0 - I00 I I 1 I I I I - 2 0 4 I V (against N.H.E.) FIG. 3.-The friction of molybdenum sliders in M/10 H&04 containing M/10 thioacetamide : (a) coefficient of friction ; (6) current flowing.182 SOLID-SOLID INTERFACES results in cohesion across an interface that is comparable with that across a grain boundary.Any adsorbed surface films greatly reduce the surface forces and the friction. Probably the most important adsorbate is oxygen, either from the air or from gaseous solution in a lubricant. One way of studying such effects is by metal cutting experiments, for cutting provides large quantities of nascent surface without the necessity of high-vacuum cleaning. The time constants of adsorptive processes have been estimated from capacitance measurements after cutting aluminium in an aqueous chloride An effect attributed to ionization of the metal occurs in & .d .I c, 0 3- 0 - 4 0.6 0 .2 Oa8:! 9 - - 0 . 8 - 0 . 7 -0.6 - 0 . 5 V (against Ptd Pt) FIG. 4.-The friction of an iron-chromium alloy in molten KClfLiCI eutectic at 400°C. The friction falls as the potential is made less negative, but remains at a low value for all potentials once the film is formed. (a) Coefficient of friction ; (b) current flowing. about s, and finally molecular film formation taking several seconds. Similar experiments, involving scraping various metal surfaces at frequencies up to 500 times/s, have enabled Anderson and co- workers to distinguish between double-layer charging and faradaic currents.50 An alternative technique of scouring metal surfaces has been used, e.g., by Tomashov and Ver~hinina,~~ to ascertain the presence of oxide and the extent of adsorption.s, followed by hydrogen discharge over ELECTROCHEMICAL LUBRICATION Early studies of platinum in H2S04 showed a strong dependence of friction upon electrode potential 5 2 * 53 but this was easily obscured by small traces of sulphideJ . P . G. FARR A N D G . W . ROWE 183 impurity. Fig. 3 shows that controlled low friction can be obtained by formation of a film on molybdenum from a suitable S-containing ele~trolyte.~~ At a potential of +0.3 V, relative to a normal hydrogen electrode, the friction falls steadily over several minutes and a visible film is formed. Electron diffraction confirms the pres- ence of MoS, in this film, together with some oxides of molybdenum. The same principle can be applied for high-temperature lubrication from a molten- salt bath 5 5 (fig.4). The kinetics of the film formation can be studied by applying the galvanostatic pulse technique, following the potential developed as a function of time.55 EXAMINATION OF THE REHBINDER EFFECT Metal cutting has been used by Barlow 56 to examine the Rehbinder effect,57 i.e., the change in mechanical properties due to a surface active agent. A very small electrode was traversed along the surface of a workpiece just ahead of a cutting tool so that the stock material was anodically etched away. When the etching region coincided exactly with the zone of intense shearing at the root of the swarf, the cutting force was appreciably reduced. This was explained 5 8 as the removal of the oxide film which otherwise imposed a barrier to the emergence of dislocations. A similar effect was obtained by the use of reactive lubricants containing chlorine or iodine, which formed surface layers that were weaker than the oxide. A series of papers by Westwood 5 9 has shown the important connections between surface layers, surface potential and dislocation movement, especially in semi- conductor material.We are grateful to (the late) Prof. E. C. Rollason for the provision of laboratory facilities and for his interest; we thank Messrs Joseph Lucas Limited, Climax Molybdenum Co. Ltd. and W. Canning & Co. Ltd., for financial support. R. S. Perkins and T. N. Anderson, in Modern Aspects of Electrochemistry (Butterworths, London, 1969), vol. 5, p. 203. N. A. Hampson, in Electrochemistry IZ (Spec. 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