Investigation of Ni-MoS₂ self-lubricating composite coating via electrodeposition as a potential alternative for Ni-PTFE coating

Abstract

Nickel-molybdenum disulphide (Ni-MoS₂) composite coating is studied in this work via electrodeposition from a Watts nickel bath as an alternative to the widely applied Nickel-Polytetrafluoroethylene (Ni-PTFE) coating for self-lubrication surface treatment. The toxicity of fluorinated surfactants involved in PTFE synthesis raises problems of severe long term environmental pollution and debilitating health issues resulting in adverse publicity and tougher industrial regulation. This study considers MoS₂ as an alternative solid lubricant, which is a naturally occurring material produced in more eco-friendly processes. Compared to inert PTFE particles, the hydrophobic and electrically conductive MoS₂ particles are found to induce the early onset of non-uniform coating growth which could result in extensive porous and rough surface features rather than a compact coating structure. This study is focused on the effects of electroplating process parameters on the composition, texture and the tribological performances of Ni-MoS₂ composite coatings. Due to varying tribology test conditions in reported COF values, the findings are compared against a Ni-PTFE coating produced under similar conditions in order to draw both quantitative and qualitative analysis on the self-lubrication performances of the Ni-MoS₂ coating. A dry COF as low as 0.07 is found for Ni-MoS₂ coating against steel in this work, which is superior to a value of 0.1 for the Ni-PTFE coating comparison and significantly lower than values of 0.2 – 0.4 reported in literature. The low COF of the Ni-MoS₂ coating against steel is attributed to following points: 1. Highly effective particle dispersion prior to the electroplating process via high-shear mixing resulting in narrow particle size distribution with mean diameters of 1 micron from the original 10 micron in magnetic stirring. 2. The addition of wetting surfactant CTAB to stabilise particle suspension before and during electroplating, 3. Low particle concentration in bath (1 – 2 g L^-1) and constant mechanical agitation during electroplating to avoid particle agglomeration on the coating surface, 4. The addition of the brightener Saccharin reduced surface roughness of the coating via grain refinement and levelling effect to produce a more compact composite coating structure. Furthermore, a concept of gradient layer composite coating with increasing particle content towards the surface is investigated due to concern that a high solid lubricant content in the composite coating could compromise its mechanical property e.g., hardness, and adhesion, hence limiting the tribological performance. The Ni-MoS₂ composite coating is produced via stepped bath agitation speeds during one-hour electroplating and show enhanced load bearing capability with lower COF compared to a single layer coating with high particle content. The facile one-pot process with agitation control could be adapted for manual or automated industrial plating. Although it show promise, the Ni-MoS₂ composite electrodeposition process in this study still provided challenges in achieving a fully compact and smooth structure while maintaining high particle content for low COF solid lubrication. Future R & D is proposed, based on findings in this work and the literature.

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