Tribological Investigation Of A Self-Assembled Graphene Coating On Titanium Substrates

Titanium and its alloys are promising choices for Micro/Nano-electromechanical-systems(MEMS/NEMS) because of their high strength-to-weight ratio, good conductivity, excellent corrosion and heat resistance, and outstanding biocompatibility. Friction and stiction are the important factors which affect the performance and reliability of MEMS/NEMS, since the gap between parts in micro-machine is as low as micro- or even nano-scale. The improvement of the surface tribological properties of titanium parts would dramatically enlarge the application of Ti material in MEMS/NEMS.By taking full advantage of the outstanding mechanical and tribological properties of graphene material, a graphene/silane self-assembled coating have been prepared onto the surface of titanium plates in this study, to improve the surface tribological and wear-resistant properties of titanium. The prepared coating is as thick as under 5nm, so that it hardly harms the dimension precision for parts in MEMS/NEMS. Compared with traditional linear-shaped organic coatings, the coating in our study holds better load-carrying capacity, and thus can obviously increase the surface tribological properties of titanium parts.There are two key points for surface coating research: the bonding strength between the substrate and the coating, and the surface characteristics of the coating.Firstly, in order to prepare graphene coating on titanium surface, the bonding strength between the substrate and graphene sheets should be well considered. Compared with silicon substrates, it is harder to prepare well oriented and homogeneously distributed self-assembled coatings on titanium substrates; on the other hand, graphene is chemically inert, so that it is very difficult to prepare graphene coatings via self-assembly method. In this study, γ-Aminopropyltriethoxysilane(APS) has been used as a coupling agent to obtain APS coating on titanium surface by way of self-assembly technique. The APS coating is well oriented and homogeneously distributed, with amino groups located at the top of the coating. Moreover, graphene oxide(GO) has been used as a precursor for graphene to be coated onto the surface of APS coating through the chemical bonding between amino groups of APS and oxygenous groups of GO. Through the above treatment, the obtained GO-APS has a strong bonding strength.Secondly, the surface energy of GO-APS coatings should be lowered down to obtain excellent tribological properties. Although GO is a wear-resistant material, GO coating has great amount of oxygenous groups on its surface so that severe adhesions can be easily found during contact. In this study, heat or hydrothermal reduction processes have been designed to remove the oxygenous groups of GO sheets, by which graphene coating have been successfully prepared. The graphene coating has obviously improved the tribological properties of titanium substrates by reducing surface energy of the coating.Based upon those ideas, this study is the first effort to prepare graphene-APS coating on titanium substrates using self-assembly technique. The main research achievements are:1. Advanced investigation and testing methods have been utilized to reveal the preparing mechanisms for the prepared coatings. Coordinated with macrotribological tests, we have obained the optimized self-assembly and reduction process of the graphene-APS coating. APS coating has been obtained with well-order and good bonding strength with the substrate. Oxygenous groups of GO can chemically react with amino groups of APS, so that a well-distributed GO-APScoating has been prepared with strong interlayer bonding strength. The surface energy of the coating has been efficienctly reduced by heat and hydrothermal processes which have successfully removed oxygenous groups of GO coatings. Hydrothermal reduction method has shown better effects. The obtained graphene-APS coatings are hydrophobic under the thickness of 5nm which makes it very suitable for surface lubracating treatment of parts in extreme clearance environment.2. Macro-tribological properties of those prepared coatings have been systematically studied. The influences of varied loads, sliding speeds, and bio-corrosive conditions on the tribological performances of reduced GO coatings have been tested. Graphene sheets have excellent wear resistance and low coefficient of friction. As a surface planar coating, graphene coating passes friction energy to a larger area than the real contact area during friction tests. Moreover, the interlayer bonding strength of graphene-APS coating can strengthen the wear resistant properties. The coating has performed good bio-tribological property owing to the anti-corrosion property of graphene and the APS coating.3. Nano-tribological properties of those prepared coatings have been systematically studied. The influences of varied loads, sliding speeds, temperatures, and relative humidities on the tribological performances of the prepared coatings have been tested. Results show that graphene coatings have excellent and stable tribological properties under all tested conditions, and thus have improved the nanotribological properties of titanium substrates. Hydrothermal reduced GO(HRGO) coatings have the best tribological performance.4. Nano-wear behavior of HRGO-APS coating has been investigated. With great anti-friction properties, coordinated with low adhesive and friction forces, HRGO-APS coating has well protected the titanium substrates during nano-wear tests. The anti-wear mechanism has been explained in respect of the space structure of the HRGO sheets in the coating.The achieved research results can boost the application of the assembly technique for graphene coatings in the field of surface engineering. This study has provided a new approach for lubrication study for devices with extremely thin clearance in MEMS, and is promising for enriching the theories in the field of surface engineering and tribology for metallic materials. Moreover, this study provides testing data and research methods for the application of graphene nano coatings in the field of MEMS/NEMS.