Results of electrocodeposition of gold matrix composite coatings with carbon-based materials are reported, namely ultradispersed diamonds (UDD) and multiwalled carbon nanotubes (MWCNT). Pure gold and gold composite coatings were prepared from a gold sulphite electrolyte with bath loads from 5 to 20 g/l (UDD bath) and from 0.1 to 5 g/l (MWCNT bath). The resulting composites are characterized in terms of carbon content, particle distribution, and their bonding to the matrix, surface morphology, and the influence of particle loading in the electrolyte on matrix microstructure. Vickers hardness, friction, and wear behavior were investigated and are discussed in terms of microstructure characterization. Some notable improvements in the performance of the composites were observed with regard to application as sliding contacts.

1 Introduction

Because of the negligible tendency to tarnish, its low contact resistance, and high conductivity, electrodeposited gold is extensively used for contact surfaces in the electronics industry. For most sliding-contact applications, pure gold is too soft, showing an inherent tendency to cold welding, thus causing crack formation and rupture of the layer. It is known that wear behavior can be improved by micro-alloying the gold deposit, for example by co-deposition of 0.1 to 0.5 percent cobalt or nickel. However, the co-deposition or incorporation of these elements can have a detrimental effect on tarnish resistance and contact resistance [1]. Therefore, a gold-based finish with improved wear resistance for contacts and connectors is needed. This objective is primarily pursued by either modifying existing processes or selecting a suitable design for the contact finish as a multilayer system.

The introduction of submicron particles and nanoparticles has promoted a step forward in the same research direction by giving new perspectives to the electrodeposition of composite coatings with an improved ensemble of key engineering properties [2–5]. Nanocomposite coatings obtained by dispersion of nanosized particles (with dimensions below 100 nm) in the metal matrix could yield the same or even better properties of the matrix metal in spite of lower thicknesses of the layers. In particular, as far as precious metal plating is concerned, there seems to be no future at all for exploiting traditional composite coatings. On the other hand, there are reasonable expectations that a range of applications could be found for thin films and coatings with nanoparticle dispersions. In fact, the most important point with composite electrodeposition is that the nanoscale size of incorporated particles could greatly reduce undesired effects on appearance, surface texture, roughness, and porosity of deposits compared to standard co-deposition processes. For a successful development of nanocomposite electrodeposition, there are few but very demanding requirements to fulfill: the availability of nanoparticles of the desired dispersed phase, the effective dispersion of nanoparticles in an electrolyte, a workaround against nanoparticle agglomeration during deposition, and the feasibility of the devised process according to industry and environmental standards.

The present paper reports on the development and the characterization of gold matrix composite layers with dispersed nanostructured carbon-based materials, namely ultra dispersed diamonds (UDD) and carbon nanotubes (CNT). The choice of nanostructured carbon-based materials as the dispersed phase is motivated basically by the following three reasons. The different allotropes and nanostructures available may be conveniently used to tailor the properties of composites. In line with the mainstream research in electrodeposited nanocomposite layers, nanosized materials such as those used in this work may in principle be effectively dispersed in an electrolyte and therefore co-deposited as fine particles or small aggregates with low impact on surface morphology. As a matter of fact, this third reason is actually the main concern and the most challenging task.

There are very few studies in the literature on electrodeposition of gold composite layers with the dispersion of particles of carbonaceous materials [6, 7]. The authors have reported on the co-deposition of Au/UDD coatings from a sulphite solution, discussing the effect of key process parameters such as pHvalue on co-deposition, sonication treatment, current density, and the mechanism of incorporation [8, 9]. There are no reports on the electrodeposition of Au/CNT coatings. There are but a few studies devoted to the co-deposition of CNT in electrodeposits, for example in aluminum layers [10] and particularly in nickel and nickel alloy layers prepared either by electroless nickel deposition [11] or electrochemical deposition [12, 13]. Recently, a first report on the morphology of Au/UDD and Au/MWCNT coatings was published by the authors [14].

The present work shows that although co-deposition results in a low mass fraction of incorporated carbon, the incorporation of ultra-dispersed diamonds (UDD) and carbon nanotubes (MWCNT) can result in remarkable changes in microstructure and thus also influences microhardness as well as wear behavior.

2 Experimental techniques

Pure gold and gold composite coatings were prepared from gold sulphite electrolyte of the following composition (mol/l): Au(I) 0.06 M; Na₂SO₃ 0.60 M; C₂H₈N₂ (ethylenediamine) 0.06 M; Na₂HPO₄ 0.070; ethylenediaminetetraacetic acid 0.068, pH 7.5 and pH 9.5. As(III) 15 ppm from As₂O₃ and Sb(III) 20 ppm from antimony potassium tartrate K₂[Sb₂(C₄H₄O₆)₂] were used as brighteners in the ultra-dispersed diamond bath and the carbon nanotube bath, respectively. Chemicals of analytical grade and distilled water were used for the bath make-up. Concentration in the electrolyte, i.e., bath load of carbon-based materials, was: 5 to 20 g/l for ultra-dispersed diamonds and 0.1 to 5 g/l for carbon nanotubes.

The electrodeposition cell was a beaker filled with 500 ml of plating bath, mechanically stirred with a two-bladed paddle agitator rotating at 200 rpm. Nickel-phosphorous-coated brass sheets were used as substrate and a platinized titanium mesh as anode. Electroplating and co-electroplating were carried out at a current density of 7 mA cm^–2. Electrolyte temperature was controlled in the range of 55 °C ± 2 K. Baths with carbon-based loads were subjected to ultrasonic treatment (20 kHz) for 20 minutes prior to the deposition processes.

For microstructure studies, a scanning electron microscope (Zeiss NEON40EsB) was used at 25 kV with a scanning transmission electron microscopy (STEM) detector and an EBSD camera (EDAX TSL) for analyses of diffraction patterns of backscattered electrons. For EsB and Inlens SE imaging, voltage was lowered to 1 kV. Cross sections were prepared by diamond grinding with a final argon ion (2.5 kV, 15°) polish. For complementary transmission electron microscopy studies (Hitachi 8100, LaB6 cathode, 200 kV), disks were cut, grinded, and thinned to electron transparency by using argon ions (3 kV, 6°). Carbon content in the composite layers was determined by elemental analysis carried out with an elemental analyzer (Fison EA 1108 CHNS-O). The uncertainty interval, when reported, represents the range defined by the minimum and maximum values measured in elemental analysis of two pieces of the same sample.

Vickers microhardness (HV) data were obtained from penetration depth-load curves using a Fischerscope® H100 microhardness measurement system. Measurement conditions were as follows: 20 mN peak load, 10 seconds of loading and unloading time, and 5 seconds holding time at peak load. Reported values are averages of five measurements taken from three different samples prepared under same conditions in the same bath. Uni-directional sliding-ball wear tests were performed at normal load of 31.5 N and rotation speed of 50 rpm, against a 40 mm steel ball, without lubrication, at 25 °C and in ambient air. Wear test time was limited to either 120 or 180 s, because of the (available) high load compared to the relatively small thickness of the coatings of about 10 μm. Volumetric material loss was subsequently determined by profilometry and optical imaging of the wear tracks (MikroCAD compact) after completion of the tests.

Ultradispersed diamond powder prepared by detonation synthesis was provided by Caspio SA, Switzerland. This material was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Ultra-dispersed diamond crystal structure is that of diamond with a lattice parameter of 0.357 nm. Average nanoparticle size resulting from both TEM observation and XRD measurement according to the Debye-Scherrer formula is 5 nm (Fig. 1). It should be noted that ultra-dispersed diamond powder shows a strong tendency to form clusters with a size in the range of 2 to 14 μm and cluster aggregates with sizes of up to several tens of micrometers.

Multiwalled carbon nanotubes (MWCNT) used in the present study were prepared by catalytic decomposition of acetylene via chemical vapor deposition (CVD) [15] on an iron catalyst placed on an alumina support. The multiwalled carbon nanotubes were purified by a standard acidic treatment, but not milled in order to characterize co-deposition behavior of the material as prepared. Diameters of the multiwalled carbon nanotubes were in the range of 20 to 40 nm, and lengths were several ten micrometers (Fig. 2).

Fig. 1: TEM image of UDD (courtesy of G. Angella, CNRIENI)

Fig. 2: ESEM image of CNT [15]

3 Experimental Results and Discussion

3.1 Au/UDD composites

Electrodeposition tests performed with UDD bath load in the range from 5 to 20 g/l showed two different regimes with regard to both coating properties and co-deposition behavior related to carbon contents observed in the deposits. Here the word regime is used to indicate conditions determining the extent of incorporation and their impact on coating properties. The two regimes can be characterized by the bath load value: a low bath load regime (LBLR) below 10 g/l, and a high bath load regime (HBLR) with UDD concentrations above this value and up to 20 g/l. Electrodeposition tests were also performed at higher bath loads of up to 50 g/l, resulting in severe impact on coating morphology. Therefore, these samples with a bath load of up to 50 g/l were not characterized any further. An overall tendency of increased surface roughness and porosity is noticed for all UDD composites (Fig. 3a–c).

Fig. 3: Surface morphology; a) gold, b) LBLR UDD composite (5 g/l), c) HBLR UDD composite (15 g/l)

The influence of UDD load on carbon content of 10 μm thick composite coatings is summarized in Table 1. For LBLR, carbon content, i.e., the associated UDD incorporation, is marginal. At an ill-defined threshold value of around 10 g/l, a noticeable mass fraction of carbon is incorporated. The first two columns in Table 1 summarize the influence of UDD bath loads on carbon contents of the layers as illustrated in Figure 4.

Tab. 1: Carbon contents and Vickers hardness of UDD composites

Fig. 4: Carbon content of composites versus UDD bath load

Microstructure, grain size, texture development, and particle distribution of the gold matrix composite were characterized by means of complementing electron-microscopic methods and compared to pure gold layers without the influence of UDD bath load.

Bright-field STEM images of pure gold characteristically reveal large grains with series of twin lamellae (Fig. 5a) and interbedded smaller grains. Due to the broad grain size distribution, mean grain size derived from EBSD acquisition averages 130 nm. This is illustrated by the orientation distribution map in Figure 6a. UDD bath load causes a decreasing mean grain size and an incorporation of embedded UDD agglomerates (Fig. 5b and 6b). Grain fining is strikingly enhanced in the case of HBLR (Fig. 5c and 6c), where mean grain size decreases below 100 nm. Mean grain diameters for all investigated composite layers are summarized in Table 1. It should be noted that the distribution of grain diameters is similar to the diameters themselves.

Fig. 5: Microstructure of layers; a) twin lamellae in pure gold layer, b) agglomerated UDD in an LBLR composite (5 g/l), c) UDD agglomerates and grain refinement in an HBLR composite (15 g/l)

EBSD-derived orientation maps of the gold and gold composite layers (Fig. 6) illustrate the grain size distributions as well as the grain orientations by IPF color coding (Fig. 6d). A dominance of distinct fiber textures usually noticed in electroplated layers was not detected. A superposition of weak <110>, <111> and <211> fiber textures was found.

The change in microhardness of the Au/UDD composite coatings is influenced by UDD bath load in a similar way as carbon content is increased and matrix grain size is decreased. An unexpected decrease in microhardness at a bath load of 10 g/l was observed. With further increasing UDD bath load, a maximum value of microhardness was ascertained, which corresponds to an increase of slightly more than 20 percent compared to the pure gold matrix (Table 1, Fig. 7). This increase is clearly associated with the noticeably higher carbon content in the layers and the decrease in mean grain size below 100 nm (Fig. 8).

In summary, the clear influence of UDD bath load on grain size of the gold matrix and an obvious increase in hardness with grain fining is recognizable. However, UDD agglomeration inhibits higher hardening of the composites and influences the wear behavior as reported below.

Fig. 6: Orientation maps of layers; a) gold layer without particles, b) LBLR composite (5 g/l), c) HBLR (15 g/l), d) Color code of orientation maps

As an exception, we unexpectedly observed an example of Au/UDD composite (10 g/l) with extraordinary well-dispersed particles. The resulting very finegrained microstructure shows a mean grain size of 40 nm and numerous monodispersed nanodiamonds, as observed by TEM bright-field and high-resolution imaging (Fig. 9). A clear image of {111} lattice planes with 0.235 nm spacing of the gold matrix is shown at higher magnification (Fig. 9c). Lattice planes partly extend into the diamond particle, but arise from the embedding gold matrix since the thickness of the TEM sample is approximately ten times higher than the diameter of the particles. However, no lattice planes have been detected in the diamond particle.

Fig. 7: Surface Vickers hardness versus carbon weight fraction in UDD composites

Fig. 8: Surface Vickers hardness versus grain size in UDD composites

Wear behavior of the composite coatings did not reveal any significant changes in wear loss and wear track morphology in the presence of co-deposited UDD. This result suggests that the wear mechanism of adhesive failure of the coating is not affected to any significant extent by the observed increase in microhardness. This is possibly due to the high load value used in the wear tests. Further work is needed to develop beneficial effects of UDD incorporation on wear behavior. For instance, composites with high UDD incorporation and enhanced hardness have a promising potential to solve the problem of coldwelding tendency of sliding contacts made from relatively soft pure gold.

Fig. 9: TEM of fine-dispersed UDD in gold; a) bright-field image, b) detail in high-resolution imaging, c) nanodiamond in the matrix, Au {111} lattice planes with 0.235 nm inter-planar distance

3.2 Au/CNT Composites

Electrodeposition tests were performed with an MWCNT bath load changing in the range from 0 to 5 g/l at 7 mA/cm² and a pH-value of 7.5. A significant influence of bath load on surface morphology of the coatings was observed.

As bath load increases, roughening of the surface compared to the fairly featureless surface morphology of the pure Au coating occurs (Fig. 10a). Above a bath load of 2 g/l, a cauliflower-like morphology develops (Fig. 10b). In addition, the relatively distinct orientation contrast significantly decreases. Higher magnifications reveal varying crystal morphology at the surface which suggests grain fining (Fig. 11a, b). With a further increase of bath load up to 5 g/l, surface morphology abruptly changes to a finegrain structure disseminated with large nodules and abnormal growth features, either isolated or bunched. These effects are assumed to be due to the growth disturbance caused by the presence of MWCNT on the surface and their incorporation as these bundles begin to co-deposit, and to the co-deposited impurities, namely residues of the iron catalyst. Overlapping of these two effects may explain the peculiar change in morphology.

Fig. 10: Surface morphology; a) gold layer, b) gold/ MWCNT composite layer (2 g/l)

Fig. 11: Crystal morphology; a) gold layer, b) gold/ MWCNT composite layer (2 g/l)

Carbon content in the deposits increases with MWCNT bath load (Fig. 12). Up to a bath load of 3 g/l, the level of incorporation is low (below 0.1 percent). Not until the bath load is raised to 5 g/l does carbon content increase to a higher value of up to 0.4 percent. Such an abrupt increase in MWCNT co-deposition, along with the observed spread in the incorporation data at 5 g/l, appears to be linked with the possible incorporation of many large bundles of MWCNT.

Microhardness of Au/MWCNT composites changes with carbon content as summarized in Table 2. For carbon contents below 0.1 percent, there is no remarkable change in microhardness compared to the gold matrix. As carbon content increases, microhardness shows a notable increase similar to the values of UDD composites which are inserted in the diagram (Fig. 13) for comparison. Since MWCNT incorporation at a bath load of 5 g/l appears to occur mainly in the manner of rather coarse bundles, the observed change in microhardness with respect to bath load cannot be straightforwardly related to a dispersion hardening effect as in the case of Au/UDD composites. Nevertheless, EBSD investigations (Fig. 14a, b) confirmed a decreased grain size by MWCNT codeposition, accompanied by an enhanced twinning ratio of the gold crystals. The twinning ratio has been estimated to be 20 percent for a 2 g/l MWCNT composite, i.e., the four-fold value of the pure gold layer. This is strikingly illustrated by scanning-electron microscopic imaging using orientation contrast of back-scattered electrons (Fig. 15).

Fig. 12: MWCNT bath load versus carbon content of composites

Fig. 13: Surface Vickers hardness versus carbon weight fraction in MWCNT (full triangles) and UDD composites (open triangles)

Tab. 2: Carbon contents and Vickers hardness of MWCNT composites

An alternative explanation of microhardness enhancement is the possible co-deposition of iron. Iron is known as a hardening alloying element for gold and originates from the MWCNT as an impurity. Therefore, it can hardly be avoided in co-deposition.

Fig. 14: Orientation maps (color coding according to Fig. 6d); a) gold layer, b) gold/MWCNT composite layer (2 g/l)

Fig. 15: Twinning in a gold/MWCNT composite layer

A reasonable increase in wear resistance is observed as a result of the incorporation of MWCNT. Wear volume results for a test time of 180 s are reported in Table 2 and in detail in [12]. It is worth pointing out that even such low carbon content produces a distinct improvement in wear resistance. In addition, even if the co-deposition of iron cannot be neglected for the maximum bath load of 5 g/l, the observed trend has to be attributed mainly to the incorporation of MWCNT.

Friction tests initially showed nearly the same friction coefficients (0.24 to 0.25) for all samples. After 500 seconds, friction coefficients rose discontinuously with carbon contents of the samples (0.29 to 0.34), except for the 0.2 and 0.5 g/l MWCNT with a comparatively smooth surface morphology.

4 Conclusions

Electroplated gold matrix composite coatings with carbon-based nanostructured materials were investigated, characterizing some key properties such as microhardness and wear resistance as well as codeposition behavior in terms of carbon content and morphology changes in the coatings.

Composite coatings were successfully prepared from a sulphite-based gold electrolyte with dispersed UDD or MWCNT. Carbon content increases with bath loads of up to 0.4 percent by weight for both Au/UDD (load up to 20 g/l) and Au/MWCNT composites (load up to 5 g/l). Aggregation of the dispersed material is disadvantageous with regard to an effective incorporation, and therefore, a suitable preparation by both physical and chemical methods has to be improved further.

Gold layers and composites were characterized in terms of microstructure and micromechanical properties. With increasing UDD bath loads, grain refining was observed for Au/UDD composites. Provided high bath load conditions above 10 g/l of UDD, carbon content exceeds a threshold value of about 0.3 percent resulting in a remarkable increase in Vickers hardness. As a result, composites with high UDD incorporation have a promising ability to reduce the tendency to cold welding of sliding gold contacts. This is particularly beneficial when judged in conjunction with the fact that they do not reveal any significant enhancement of embrittlement despite the hardness improvement.

The Vickers hardness of Au/MWCNT composite coatings is similarly affected and is combined with a significantly improved wear performance as compared to the pure gold layer, even at a very low level of co-deposition, resulting in carbon contents in the range below 0.1 percent.

The present work shows that although a low mass fraction of incorporated carbon was reached by co-deposition, the incorporation of either UDD or MWCNT can result in remarkable improvements in microhardness and wear behavior. This is a promising basis for future studies which will be focused on an effective fine-dispersed incorporation with depressed aggregation in the electrolyte. With this objective, chemical modifications of the particle surface as well as physical agitation of the galvanic bath should be considered.

Acknowledgement

The authors wish to thank G. Engelhardt for helpful discussions.

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