Influence of core/shell nanoparticles on the fretting behavior of gold-coated electrical contacts

In this paper, a method to fabricate a novel gold coating co-deposited with nanoparticles with a core/shell structure —Ag (shell) @ Al2O3 (core) — is introduced. The influence of the nanoparticle concentration in the electrolyte and the agitation of the galvanic bath on the lifetime of the nanoparticles co-deposited gold coating is investigated. The gold coatings made of the favored nanoparticle concentration in the electrolyte show a 2.2-fold of the average lifetime of the pure gold coating in the wear and fretting corrosion test. By employing a higher bath agitation in the electroplating process, a smoother nanoparticles co-deposited gold coating with fewer pores and finer grains is produced, which improves the average lifetime of the coatings to 3.7-fold of the co-deposited coatings produced under a lower bath agitation, while the standard deviation of the lifetime of the coatings in the wear and fretting corrosion test is decreased. To reveal the mechanisms of the lifetime improvement, several analysis methods are applied, such as wear scar analysis and focused ion beam scanning electron microscopy (SEM+FIB). Fig. 1 Comparison of the lifetime of different coatings. Background A gold coating is recommended for applications functioning under very severe operation conditions, such as highly corrosive environments, or where a stable current should be reliably guaranteed even at very low voltage. However, the high price of gold material limits the use of gold coatings. Thus, a method to reduce gold consumption — for example, to lower the required gold coating thickness without decreasing the lifetime and performance of the gold coating — is of great interest. In our previous study, it was proved that the Ag (shell) @ Al2O3 (core) nanoparticles can significantly improve the lifetime of silver-coated electrical contacts because of the increased hardness by the nanoparticle core and the good electrical conductivity of the functionalizing silver shell. In this paper, the capability in the lifetime improvement by co-depositing the Ag (shell) @ Al2O3 (core) nanoparticles into the gold coating for electrical contacts is studied. Furthermore, the mechanism to enhance the lifetime by co-deposition of nanoparticles into gold coating is investigated. Methods The Ag@Al2O3 nanoparticles are produced by a self-developed device in the laboratory and then galvanic co-deposited into the gold coating. Three different nanoparticle concentrations in the electrolyte for the gold coating co-deposition are used. Two agitations are applied in the galvanic process with 50 mg/L nanoparticle concentration in the gold electrolyte. After the fabrication of the gold coating co-deposited with Ag@Al2O3 nanoparticles, the lifetime of the gold-coated electrical contacts are measured in wear and fretting corrosion tests. After the test, several analysis methods, such as wear scar analysis and FIB+SEM, are applied. The details of the materials, the fabrication process and the tests are provided in the full paper. Fig. 2 Microstructure of the gold coatings produced with low bath agitation and (a) 25 mg/L, (b) 50 mg/L and (c) 75 mg/L Ag@Al2O3 nanoparticle concentration in the gold electrolyte. Results and discussion By co-depositing the Ag@Al2O3 nanoparticles, the lifetime is greatly improved compared to the pure gold coating, and a favored nanoparticle concentration in the electrolyte is found as 50 mg/L. By increasing the gold bath agitation, a significant improvement in the lifetime and reduced spread of the lifetime of the electrical contacts are observed. A comparison between the bare nanoparticles and the functionalized Ag@Al2O3 nanoparticles with regard to the capability to improve the lifetime of the gold coating is shown in Fig. 1. The average lifetime of the Ag@Al2O3 nanoparticles co-deposited gold coatings in this study is increased to 3.4-fold of the lifetime of the pure gold coatings. This value was 2.8-fold in the former study with the bare Al2O3 nanoparticles. In the wear scar analysis, in comparison with the sample which has a longer lifetime, the edge of the wear scar is raised on the sample with a lower lifetime. This reveals that apart from the typical failure mechanism fretting wear, the peeling-off of the gold coating can also be responsible for the failure of the electrical contacts, depending on the quality of the gold coating. With the co-deposition of the nanoparticles into the gold coating, the bond between the gold layer and the nickel underlayer can be weakened.Flat samples, produced with different Ag@Al2O3 nanoparticle concentrations in the gold electrolyte and low bath agitation, are analyzed by SEM+FIB (Fig. 2). Grain refinement is observed near the boundary of the gold coating and the nickel underlayer. Due to the grain refinement, the hardness of the plating can be improved. Increasing the Ag@Al2O3 nanoparticle concentration in the gold electrolyte promotes the formation of a thicker grain-refined region. On the other hand, more pores are observed in the coating produced with the higher nanoparticle concentration in electrolyte, which can cause a weak bond of the gold coating to the nickel underlayer. As a result, the gold coating can be easily peeled off in the wear and fretting corrosion test and the lifetime of the gold coating can be reduced. A sample produced with high bath agitation is cut using FIB and analyzed by SEM to study the microstructure of the gold coating (Fig. 3). The gold coating thickness is more uniform under high bath agitation. Moreover, the overall grain size of the whole gold coating produced with high bath agitation is smaller than that produced with low bath agitation. Fewer pores are observed in the gold coating produced with high bath agitation. Consequently, the high bath agitation promotes the improvement of the coating quality and thus the lifetime of the gold coating as well as the spread of the lifetime. Conclusions By co-depositing the core/shell nanoparticles, the lifetime of the gold coatings is greatly improved. Compared to the former study with bare or discontinuous shell nanoparticles, the average lifetime of the gold coating in the wear and fretting corrosion test is significantly extended. Fig. 3 Microstructure of the gold coating produced with 50 mg/L Ag@Al2O3 nanoparticle concentration in the gold electrolyteand high bath agitation. By co-depositing the nanoparticles, grain refinement is observed in the gold coating. The co-deposited nanoparticles can act as nucleation sites, which leads to the grain refinement of the gold coating and helps to improve the microhardness. The thickness of the grain refinement area increases with a higher Ag@Al2O3 nanoparticle concentration in the gold electrolyte. However, more pores in the gold coating near the gold/nickel boundary are found when produced with a higher Ag@Al2O3 nanoparticle concentration in the gold electrolyte. These pores weaken the bond between the gold layer and the nickel underlayer. The favored Ag@Al2O3 nanoparticle concentration in the gold electrolyte combines the advantage of grain refinement and few pores in the gold coating near the gold/nickel boundary so that a long lifetime can be attained. When deposited with a higher bath agitation, the lifetime of the gold-coated electrical contacts is greatly enhanced. The reasons for the lifetime extension of the gold coating produced under a higher bath agitation are the more uniform coating thickness of the gold layer, fewer pores in the gold coating and a smaller overall grain size in the gold coating. Funding This work has been partly supported by the European Union via the European Regional Development Fund as part of the Ziel 2-Programme 2007-2013 (EFRE), Funding No. 28 03 321 02. References A list of all references is available in the full paper.

Influence of core/shell nanoparticles on the fretting behavior of gold-coated electrical contacts

Authored by: Haomiao Yuan and Jian Song

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