Research on Boundary Adsorption Behaviors in Low-viscosity Media with Active Agent and Surface Modification

Author:Wu Bo

Supervisor:gu le


Degree Year:2019





Lowering the viscosity of lubricating media contributes to reducing viscous dissipation under full-film lubrication,but increases the risk of breakdown of hydrodynamic lubricating films.Under the lubrication of low-viscosity media,it is very likely that more asperities will come into direct contact and the contact strength will become higher,thereby resulting in severer boundary wear.Lubricating media and/or additives usually adsorb onto friction surfaces to form a solid-like film different from the bulk phases.The boundary adsorption layers possess certain load-carrying capacity and can help reduce asperity-to-asperity contact strength,even prevent local contact.The load-carrying capacity of a boundary adsorption layer is intimately dependent on its structural properties(e.g.,thickness,compressibility and adsorption/assembly strength).In this dissertation,three industrially important tribological materials,poly(naphthalene sulfonate)(PNS),poly(carboxylate ether)(PCE)and JP-10(C10H16),were employed and their boundary adsorption and boundary tribological behaviors were investigated in the conditions of low-viscosity media with active agent and interface modification.To reveal the effects of ionic valence(monovalent K+vs divalent Ca2+)and Ca2+ion concentration on structural properties and interations between adsorption layers,PNS’s boundary adsorption behaviors on mica were comparatively studied in0.1 M K2SO4 and 0.1 M Ca(ON3)2 as well as 5 mM Ca(ON3)2 solutions.It was found that divalent Ca2+ions possessed stronger capacity of assembling PNS molecules to develop multi-layer adsorption than monovalent K+ions and that Ca2+ions can bridge the opposing PNS films while K+ions cannot.Furthermore,increasing Ca2+ion concentration can effectively promote more PNS molecules to assemble a thicker adsorption layer,but cannot strengthen the bridging.To reveal the effect of molecular structure on structural properties of PCE adsorption layers and experimentally show the Ca2+ion bridging mechnism,boundary adsorption behaviors of various PCEs on mica were comparatively studied in 0.1 M K2SO4 and 0.1 M K2SO4 with saturated Ca(OH)2.These PCEs shared a similar molecular configuration,but had different specific molecular structural parameters.To more closely mimic actual colloidal suspension system conditions,silica was also used as the substrate surfaces in the experiments carried out in Ca(OH)2-saturated 0.1 M K2SO4 solution with and without PCE.The studies revealed that specific molecular structural parameters can modify PCE’s adsorption and that PCE did not adsorb onto negatively charged mica or silica in high-calcium aqueous conditions.Adsorption mechanism and conformation of JP-10 on metal surfaces were analyzed according to molecular structure properties.To show the effect of JP-10adsorption on friction,boundary tribological behaviors of M50 bearing steel sliding against a Si3N4 ball were comparatively investigated under dry friction,in deionized water,in JP-10 and in professional aviation engine lubricating oil(4050 oil)at a load of 1.5 GPa/3.8 N and a sliding speed of 100 mm/s for 7,200 s at room temperature in ambient atmosphere.To show the load-carrying properties of JP-10boundary adsorption layer,friction tests were further perfomed in deionized water,JP-10 and 4050 oil under a variety of load conditions of 0.7–2.3 GPa/0.39–13.8 N at the constant sliding speed 100 mm/s,respectively.To evaluate the hydrodynamic lubrication capacity of JP-10,friction tests were also conducted in deionized water,JP-10 and 4050 oil at a wide range of sliding speeds from 10 mm/s up to 2,500mm/s under the constant load 1.5 GPa/3.8 N,respectively.The studies indicated that JP-10 had decent boundary and hydrodynamic lubrication performance,but the lubricity was significantly weaker than that of professional lubricating oil.To explore tribological suface modification methods for metal-ceramic tribo-pairs lubricated by JP-10,polished M50 steel samples were treated with silver film deposition,silver film alloying,nitrogen ion implantation,TiAlN coating deposition and Ta coating alloying,respectively,and boundary tribological performance of these surface-modified and untreated M50 steel samples were studied while sliding in JP-10 against a Si3N4 ball in a series of high load conditions at room temperature in ambient atmosphere.It was found that surface-modified,hard samples exhibited significantly improved boundary friction and wear properties compared to original M50 steel.The present work in this dissertation reveals the interactions of PNS,PCE and JP-10 with specific surfaces in low-viscosity medium conditions and their boundary adsorption layer structural properties as well as the interactions between the layers,respectively.The research provides a reliable guidance to their molecular structure design and optimization as well as corresponding tribological surface design,and also inspires subnanometer-resolution characterization methods for boundary adsorption layers/films on tailed tribological surfaces.