Abstract
This article reports a comparative study of the nanoscale and macroscale tribological attributes of alumina and stainless steel surfaces immersed in aqueous suspensions of positively (hydroxylated) or negatively (carboxylated) charged nanodiamonds (ND). Immersion in −ND suspensions resulted in a decrease in the macroscopic friction coefficients to values in the range 0.05–0.1 for both stainless steel and alumina, while +ND suspensions yielded an increase in friction for stainless steel contacts but little to no increase for alumina contacts. Quartz crystal microbalance (QCM), atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements were employed to assess nanoparticle uptake, surface polishing, and resistance to solid–liquid interfacial shear motion. The QCM studies revealed abrupt changes to the surfaces of both alumina and stainless steel upon injection of –ND into the surrounding water environment that are consistent with strong attachment of NDs and/or chemical changes to the surfaces. AFM images of the surfaces indicated slight increases in the surface roughness upon an exposure to both +ND and −ND suspensions. A suggested mechanism for these observations is that carboxylated −NDs from aqueous suspensions are forming robust lubricious deposits on stainless and alumina surfaces that enable gliding of the surfaces through the −ND suspensions with relatively low resistance to shear. In contrast, +ND suspensions are failing to improve tribological performance for either of the surfaces and may have abraded existing protective boundary layers in the case of stainless steel contacts. This study therefore reveals atomic scale details associated with systems that exhibit starkly different macroscale tribological properties, enabling future efforts to predict and design complex lubricant interfaces.
Highlights
Interest in nanoparticles as eco-friendly lubricant additives has grown tremendously in recent years [1,2]
Substantial increases in μk were observed for the stainless steel surfaces exposed to the +ND suspensions while alumina surfaces showed only modest to negligible increases in μk
The data do not appear to correlate with the σs or fractal dimension D of the samples [42], which were measured by atomic force microscopy (AFM) to respectively be (6 nm, 2.4); (9 nm, 2.2); and (50 nm, 2.1) for the polished AISI 52100 stainless steel disk, 304 stainless steel penny washer, and alumina disk samples
Summary
Interest in nanoparticles as eco-friendly lubricant additives has grown tremendously in recent years [1,2]. The field is driven in a large part by a pressing need to replace hazardous additive materials in present-day oil-based lubrication technologies and to eliminate the serious environmental risks associated with oil leakage and disposal [3,4,5]. Water-based lubricant systems are a attractive target for nanoparticulate additives since conventional oil additives generally fail to improve tribological performance in aqueous environments. Numerous studies of nanoparticulate additives to oil-based systems have been reported in the literature, with many displaying significant improvements in macroscopic friction and wear rates [6]. The low shear strength of water is beneficial in the hydrodynamic regime of lubrication, under normal loads it enables contact between opposing surfaces. Nanoparticles exhibit a great potential for replacement of the centuries-old oil-based lubricating technologies
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