Tribological behaviour of DLC and Si-DLC films deposited on nitrile rubber for handpump piston seals

摘要

Large numbers of rural populations in the developing world rely on the handpump for access to drinking water from ground sources. The functional sustainability of handpumps is affected by the low availability of handpumps at any given period of time. The wear of the nitrile rubber piston seals contribute significantly towards this low availability accounting for a significant percentage of maintenance interventions.ududIn this study a novel approach is used to improve the wear resistance of the nitrile rubber piston seals. Hybrid coatings of diamond-like carbon (DLC) and silicon doped diamond-like carbon (Si-DLC), with and without Si-C interlayers, were deposited onto both sample nitrile rubber substrates and actual piston seals. The deposition was done using a combination of closed field unbalanced magnetron sputtering ion plating (CFUBMSIP) and plasma enhanced chemical vapour deposition (PECVD) in Ar/C4H10 plasma.ududFilm characterisation was carried out using visible (488 nm) and UV (325 nm) Raman spectroscopy , X-ray Photoelectron Spectroscopy (XPS), Energy Dispersive X-ray (EDX) analysis, Scanning electron microscopy (SEM), surface profilometry, hardness measurements, hydrophobicity and surface energy analysis, adhesion and flexibility analysis, and tribological investigations under dry and wet conditions for normal loads of 1N and 5N. A piston seal wear test rig was used to test the wear resistance of the coated piston seals.ududExperimental results show a dense, non-columnar microstructure with a dendritic morphology. This dendritic crack-like structure promotes film flexibility. The G peak position for the coatings was centred approximately around 1580 cm-1. The intensity ratio (ID/IG) generally tended to increase for coatings with Si-C interlayer. Full width at half maximum, FWHM(G), was observed to reduce for coatings with Si-C interlayers. Calculated hydrogen values for all of the films were between 24% and 31%. The internal compressive residual stress in the coatings was reduced. The contact angle of water droplets showed that the coatings were hydrophobic. The deposited coatings showed excellent adherence with an adhesion rating of 4A for films with a Si-C interlayer. The composite micro-hardness was highest for DLC coatings at 15.5 GPa for indentation load of 147.1 mN using a Vickers micro-hardness tester. Nano-indentation results show that all of the coatings tested had the same order of nano-mechanical properties with hardness values of approximately 3.6GPa and Elastic modulus values of 35GPa. XPS survey scans showed the contributions from C 1s (~285 eV), O 1s (~531 eV), Si 2p (~100 eV) and Si 2s (~151 eV). C-Si (C 1s) and Si-C (Si 2p) configurations corresponding to bonding between carbon and silicon atoms in SiC were observed. The sp3/sp2 ratio increased for the Si-DLC and DLC with Si-C interlayer coating compared to the DLC coating. All of the coatings showed excellent tribological results with an over 50% reduction in the coefficient of friction compared to uncoated nitrile rubber. There was no penetration of the coatings for normal loads of 1N and 5N under wet sliding and 1 N normal load under dry sliding, which is important for potential application onto piston seals.