## 1 Introduction
Fewer investigations have addressed graphene as nano-coating for macroscale dry sliding contact conditions on copper alloys. To the best of the authors' knowledge, the essential literature on this topic is represented by the papers referred to below. [PERSON] et al. [22] tested graphene transferred to a connecting rod journal bearing and observed that even single-layer graphene could protect the rough substrate surface reducing macroscale wear even under severe conditions despite a relatively high coefficient of friction compared to the bare material. [PERSON] et al. [23] investigated the effect of the growth time in CVD graphene on copper. [PERSON] et al. [24,25] tested SLG and MLG graphene coating on steel substrates prepared by CVD and solution-processing in a variety of dry, liquid, and inert gas humid environments. [PERSON] et al. [26] investigated the combined effect of graphene coatings and textured substrates, claiming that the friction-reducing effect of graphene was enhanced by the surface texturing. [PERSON] et al. [27] investigated the role of vacuum and humidity in reducing the sliding friction of a titanium sphere being slid against MLG deposited on Ni and tested in a pin-on-disk setup. Interestingly, the lowest was measured when the tests were conducted in an air atmosphere with 45% RH, and it was argued that in the presence of hydrogen the defects in the carbon network generated during sliding resulted in the sp\({}^{2}\) to sp\({}^{3}\) transformation and amorphization during of the graphene. The scientific literature also reports several methods to produce graphene sheets for various applications [28], e.g., mechanical delimitation or'scotch-tape' exfoliation [29], chemical and electrochemical exfoliation of graphite [30,31], unrolling/unzipping of carbon nanotubes (CNTs) [32], arc-discharge in various media [33], inkjet printing [34], solution-processing of graphene flakes with drop-casting of graphene films [24], and others. However, few of them are suitable for the production of continuous graphene sheets large enough to cover wide areas of mechanical or electrical components. Among the others, two methods which promise to meet this requirement were considered for this paper:  
* Low-Pressure Chemical Vapor Deposition (LPCVD) on Cu or Ni foils [35,36];
* Self-assembly of graphene nano-platelets (GNPs) via sonication in distilled water [37]. In the previous works of [PERSON] et al. [38; 39; 40], these two deposition techniques were compared for the production of graphene layers deposited onto both metallic and polymeric substrates. These results showed that low-quality MLG produced with cost-effective deposition methods usually outperform high-quality and high-purity SLG by CVD in terms of tribological performance. More recently, [PERSON] et al. investigated solid lubricant particles by combining graphene and iron nanoparticles [41] and the lubrication of graphene for the iron contacts [42]. [PERSON] et al. [43] and [PERSON] et al. [44] investigated, respectively, the performance of copper ions in situ incorporated into graphene oxide nanosheet and copper metal matrix composites reinforced with fluorinated graphene oxide nanosheets as coatings as solid lubricant. [PERSON] et al. [45] analysed wet-transferred solution-processed graphene flakes in solid lubrication. [PERSON] et al. [46] used multi-layer graphene/silicon dioxide coating as solid lubricant, and [PERSON] et al. [47] investigated the tribological behaviour of graphene-based solid lubricants biofunctionalized with hyaluronic acid. In this paper, copper for electrical purposes is used as a substrate. Four different kinds of coating produced according to the above-mentioned methods were tested. The aim of this work is to investigate how graphene nano-coatings synthesized by different processes can modify the macroscale tribological performance of copper.