| Fluid-lubricated parts play a vital role in the operation of machinery.In lubrication problems,there are high temperature rise and high energy consumption caused by excessive lubricant supply,and friction failure caused by insufficient lubricant supply.In order to obtain the best lubrication performance,the realization of limited lubricant supply and efficient use of lubricant under the premise of meeting the load requirements has become a popular research direction.Surface properties,such as surface wettability and surface microstructure,have an important impact on the friction and lubrication properties of tribo-pairs.In this paper,based on film technology and laser micro-texture technology,nonuniform/gradient wetting of the surface and unidirectional resistance to lubricating oil flow are realized.The influence of the above solid surface modification on the film thickness under limited lubricant supply condition was experimentally studied by using the optical interferometry technique of cylindrical roller-disc contact.At the same time,the effect of discrete lubricant distribution induced by interlace wettability surface is analyzed theoretically.The main research work of this paper is as follows:(1)When the adhesion force between the oil and the solid surface is less than the viscous resistance of the lubricating oil film,the oil can slip on the solid surface,thus reducing the friction of the tribo-pair,and also causing the change of the thickness of the oil film.Based on this concept,the effect of solid-liquid interface adhesion force(interface slip)on line contact hydrodynamic lubrication is studied.Two oleophobic coatings(AF and FAS,respectively)were prepared by coating and vapor deposition.By comparing the thickness of the oil film formed on the original surface and AF and FAS surfaces,it is found that the contact angle and the contact angle hysteresis are correlated with the film thickness.Compared with contact angle,contact angle hysteresis can better reflect the influence of interface wetness on hydrodynamic lubrication.The hydrophobic surface AF prepared by coating method has the characteristics of low potential energy barrier and high bearing capacity at the solidliquid interface.The experimental results validate the theoretical analysis of the local non-monotonic relationship between the film bearing capacity and the adhesion force at the solid-liquid interface proposed by Spikes.(2)In oil lubrication,heterogeneous surface wettability(high surface energy/low surface energy)is used to realize unidirectional transportation of oil and then the oil supply and the load-carrying capacity are improved.Two types of wettability gradient patterns,namely,strip wettability and interlaced wettability,were prepared to prevent the lubricant from running out of the lubrication track.The evolution of the oil reservoir boundary at the inlet was observed.A relative film thickness was defined to quantify the load-carrying capacity loss.The effects of working parameters,various limited lubricant supply volumes and wettability gradient forces on the load-carrying capacity were studied.The experimental results showed that both wettability gradient patterns are capable of improving oil film lubrication in the cylindrical roller-disc contact.The proposed novel interlaced wettability can obtain higher film thickness than the strip wettability.The interlaced wettability structure can obtain higher film thickness than the strip wettability structure.It was observed that the lubricant was mainly redistributed in the form of droplets on the hydrophilic patches of the interlaced wettability surface,that is,the lubricant stays in a two dimensional discontinuous distribution,which is related to the unbalanced interfacial force between the hydrophilic and hydrophobic regions and the transfer of the hydrophobic film during operation.The mechanism of the interlaced wettability pattern to strengthen the lubrication film is that the discrete oil droplets enter the lubricated surface gap earlier than the lubricant in the form of uniform continuous layer,and therefore the tribo-pair can obtain a larger load-carrying region and a higher oil film thickness is induced.(3)The lubrication models of cylindrical roller-disc line contact with uniform oil supply and interlaced oil supply were established,and the problem of transient starved lubrication with cylindrical rollers was solved by multigrid method.The effects on the lubrication film thickness of the interlaced patch width,limited lubricant supply volume and entrainment velocity were studied.It was found that the oil film thickness by the interlaced oil supply is higher than that from the uniform oil supply,and the pressure concentration in the contact area is reduced.With less oil supply,the efficiency of the interlaced oil supply is higher.The width of the interlaced patch is an important parameter that affects the lubrication efficiency,which can be optimized by the calculation.The entrainment velocity can affect the spreading status and effective lubrication time of lubricants.At low velocity,the efficiency of interlaced oil supply can significantly increase.At high velocity,the interlaced oil supply does not work well.(4)Tesla Valve-type micro-textures were prepared and their effects on oil film lubrication were studied under pure rolling reciprocating movement of a cylindrical roller-disk contact.With consideration of the one-way flow resistance of the Tesla Valve and the change of oil film thickness during reciprocating movement,two types of Tesla Valve-type micro-textures were designed: 1)Backward Tesla Valve-type micro-textures,that is,the micro-texture segment with high flow resistance accommodates the acceleration stroke of reciprocating movement and the segment with low flow resistance accommodates the deceleration stroke;2)Forward Tesla Valve-type micro-textures,that is,the micro-texture segment with low flow resistance accommodates the acceleration stroke of reciprocating movement and the segment with high flow resistance accommodates the deceleration stroke.In the direction of high flow resistance,the fluid forms additional local hydrodynamic pressure in the micro-texture,which increases the load-carrying capacity of the oil film.The experimental results show that,compared to the oil film by the original surface,the two types of Tesla Valve microtextures can improve the oil starvation and the loadcarrying capacity of the oil lubrication of the cylindrical roller on the disc.The oil film thickness of backward Tesla Valve-type microtexture is higher than that by the forward Tesla Valve microtexture.In the stroke with the backward Tesla valve micro-texture,the additional hydrodynamic effect always appears during acceleration,which is beneficial to resist the side effect of acceleration on oil film formation.The work in this paper plays a guiding role in realizing limited lubricant supply,improving the utilization rate of lubricant and improving the bearing capacity of oil film,and has reference value in the design and improvement of hydrodynamic bearing.The experimental data obtained,the discrete oil supply lubrication model established,the numerical calculation completed and the conclusions obtained in this paper have a positive effect on the development of the basic theory of lubrication science.At the same time,the completed work has a good reference value for the tribological design of mechanical parts with low energy consumption,the optimization of limited lubricant supply technology,and the improvement of oil film bearing capacity under the starved condition. |
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