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Development of Self-lubricating Nanocomposite Coatings impregnated with in-situ formed MoS2 for Tribological Applications

Final Report Summary - NANOCOAT (Development of Self-lubricating Nanocomposite Coatings impregnated with in-situ formed MoS2 for Tribological Applications)

In this project, the following scientific and technological objectives were aimed at:
1) Formation in the electrochemical double layer of MoS2 from aqueous solutions: investigation of the precipitation kinetics or nucleation and growth process of MoS2 compounds in the electrochemical double layer under cathodic polarisation;
2) In-situ formation and electrolytic co-deposition of MoS2 during Co-W alloy deposition from aqueous solutions. Study of electrolytic co-deposition mechanism and double layer phenomena;
3) Investigation of the structural properties of the co-deposited composite layers: annealing behaviour and detailed study (statistical distribution of MoS2 into Co-W, hardness, corrosion, tribology) by advanced test and characterisation techniques (e.g. fretting, TEM, nano-indentation, SEM-FIB, XPS, electrochemical transients);
4) To further strengthen the electrodeposited Co-W-MoS2 composites, incorporation during electrolytic deposition of suspended WC nanoparticles along with MoS2 formed in-situ. Modelling of the distribution of two types of second phase particles in electrocoatings to obtain highest optimised property. Search for a possible correlation between experimentally observed mechanical parameters and the model, understanding of wear mechanism at microscopic scale;
5) Analysis of possible implementation of this technology in industrial practice. Setting up of lab scale coating deposition for the synthesis of test samples relevant to some industrial applications (jointly with French and Belgian SMEs active in surface engineering and more specifically electrodeposition). Analysis of the health and safety aspects for workers and end-users.

This project required a multi-disciplinary approach linking electrochemistry, hydrodynamics, reaction kinetics, co-deposition mechanisms, and material science and engineering. These nanocomposites electrodeposited in a unique way are expected to possess interesting functional properties thanks to the presence of WC on one hand, and an ultra low friction thanks to IF-MoS2 on the other hand. The aim is to demonstrate the potentiality of these nanocomposites to become a suitable alternative to hard chrome, and that their use will contribute to a protection of the environment by suppressing liquid lubricants thanks to their intrinsic self-lubrication properties.

Description of the work and results achieved so far:
Task-1 Nanostructured MoSx thin coatings (0.2-0.4 nm thickness) containing fullerenes, nanotubes and nano-ribbons have been successfully obtained on NiP and CoW substrates by in-situ electrodeposition. MoSx coatings were characterised by SEM, XRD, TEM and Raman Spectroscopy for structural aspects. Thickness was determined by white light interferometry and friction behaviour by fretting tests. The coefficient of friction (0.12) of post-annealed MoSx coatings measured at RT and 50 % relative humidity is comparable or better to that of sputtered MoS2 coatings.

Task-2: Electrodeposition of CoW alloy coatings was done as matrix material for the simultaneous incorporation of WC and MoS2 particles to achieve nanocomposites. Process optimisation for deposition of CoW alloys and CoW-WC nano-composites were carried out by varying plating parameters like current density, rotation speed and additive agents. Both citrate and non-citrate based electrolytes were investigated. However, non-citrate based electrolyte was found to be suitable for incorporation of WC particles and hence chosen for electrodeposition of CoW-WC nanocomposites. Incorporation of WC particles into CoW matrix was found to have profound influence in its wear and corrosion resistance properties with moderate improvement in hardness. The typical measured values of hardness, wear volume for CoW alloy and CoW-WC nanocomposites deposited at 50 were 6.907 ± 0.217 GPa (100 mN applied load) and7.837±0.250 GPa (100 mN applied load), 15789 nm3 and 8725 nm3. The corrosion current (ICorr) density measured in 0.5M NaCl was 21.36 and 9.03 respectively from CoW alloy and CoW-WC nanocomposites. Structural characterisation of these deposited alloys and composites was done by FE-SEM and XRD techniques. The coefficient of friction of CoW-WC nanocomposites was also found be lower (0.41) as compared to similar composition CoW alloys (0.55).

Tribocorrosion properties of these alloys and nano-composites were also investigated in 0.5 M NaCl solution under identical loading conditions. Here to nano-composite materials showed better performance than CoW alloys.

Task-3: In-situ electrodeposition of CoW-MoS2 from sulphate based electrolyte was investigated. EDS analyses confirmed the presence of Co, W, Mo and S in the as-deposited films. It confirms incorporation of MoS2 in the form of molecules. The coating obtained was found to be metallic appearance at low thickness. Suitable heat-treatment under inert atmosphere improves the coating quality. At higher thicknesses, lots of cracks generate and coatings peel-off from the substrate surface.

The present improvement in tribological properties of CoW-WC nanocomposites ensures the initial idea of strengthening the CoW matrix. Not only wear resistance property, improvement in corrosion property was also noticed. Besides these, idea of making lubricated nanostructured MoS2 was also confirmed during the period of incoming phase of the project. In the remaining part of the project, further incorporation of IF-MoS2 inside the CoW matrix will add up the lubricity and hence potentially will reduce the coefficient of friction and wear volume. Thus, the idea of achieving the self-lubricated nanocomposite coatings by electrodeposition will be realised at the end of the project.

So far, CVD and PVD techniques were employed to obtain such types of self-lubricated coatings by simultaneously codepositing self-lubricated particles inside a metal matrix. However, electroplating, being a simple and economical, will definitely get much importance with respect to its commercialisation. Academically, incorporation of two different types of particles with having entirely different properties inside a common matrix will also open up a new area of research for future materials scientists to develop advanced materials.