| It can be known from the classic lubrication theory that the extremely low viscosity and viscosity-pressure coefficient of pure water make it difficult to form an effective lubricating film at a lower speed,which limits the wide application of water-based lubrication in the industrial field.Inspired by the microstructure and lubrication mechanism of natural synovial fluid joints,scientists have discovered that the friction pair surface of the surface grafted polymer brush can obtain excellent lubrication performance in a water lubricated environment.As a result,since the 1990 s,domestic and foreign scholars have carried out a series of valuable research work on the tribological properties of polymer brushes.Unfortunately,the water lubrication mechanism of polymer brushes has not yet formed a unified conclusion,and most of the research has focused on boundary and mixed lubrication conditions,and very few research work has focused on the water lubrication mechanism under the intervention of hydrodynamic lubrication.Based on this,this paper proposes to use optical interferometry technology to carry out research on the mechanism of polymer brush water lubrication on a ball-disk nanofilm measurement device that is closer to actual working conditions,focusing on the film-forming characteristics of polymer brushes in the contact area.By measuring the thickness and friction coefficient of the water film at different entrainment speeds,and discussing the water lubrication mechanism of the polymer brush under the intervention of hydrodynamic lubrication effect,the main research work is as follows:(1)By surface-initiated polymerization technology(hereinafter referred to as SIATRP),a hydrophilic polymer brush-poly-3-sulfonic acid propyl methacrylate potassium salt(hereinafter referred to as polymer Brush PSPMA)was grafted on the surface of steel ball and glass disc,at a lower entrainment speed(the effect of hydrodynamic lubrication is negligible),the water film formation process of the polymer brush PSPMA in the contact area was observed in situ by using the red and green two-color laser interferometry(0～240 s)It is observed that the shape of the water film changes from the initial round shape to a regular distribution along the direction of entrainment speed over time.It is speculated that the orientation of the free water molecules captured by the polymer chains in the confined space has changed.,That is,the initial isotropy gradually becomes orderly along the direction of the entrainment speed,which proves that the stable lubricating film formed in the contact area is promoted by the hydration effect of the polymer brush;(2)By controlling the continuous change of entrainment speed(1～512 mm/s),the transition from thin film lubrication to elasto-hydrodynamatic lubrication is realized.The results of optical interferometry show that the film thickness is independent of velocity when the entrainment velocity is lower than 16 mm/s,and the film thickness is about 30 ～ 40 nm,which is considered to be in the thin film lubrication state.At this time,hydrodynamic lubrication effect can be ignored,and the hydration effect of the polymer brush is absolutely dominant in the establishment of an effective water film in the contact area;As the speed increases to 32 mm/s,the film thickness and speed increase almost linearly,and the measured value of the film thickness is much higher than the predicted value of the Hook formula.This means that under the intervention of hydrodynamic lubrication effect,the polymer brush exhibits an amazing lubrication enhancement effect,rather than a simple superposition of hydration effect and hydrodynamic lubrication effect;(3)In order to further confirm that the hydration effect of the polymer brush is the core element for the formation of an effective water film,the zirconia ceramic ball and the glass plate are grafted with a hydrophobic polymer brush-polymethacrylate laurel.(hereinafter referred to as polymer Brush PLMA),the experimental results of comparing the polymer brush PSPMA and PLMA show that: because the polymer brush PLMA cannot hydrate and is difficult to support external loads,and it does not have the same excellent lubrication performance as PSPMA under the intervention of hydrodynamic lubrication effect;Even with the increase of the SRR,PLMA still shows a stable and low friction coefficient,that is to say,the anti-friction performance is significantly better than that of PSPMA at a large slip-to-roll ratio.It is inferred that at lower speeds and lower slip-to-roll ratios,the small water droplets in the contact area of the grafted hydrophobic polymer brush PLMA take on the role of the " ball",and this " bearing ball" plays a bearing role,which greatly reduces the friction in the contact area;(4)Based on the characteristics of polymer brush-polymethacryloxyethyl dimethyl ammonium chloride(hereinafter referred to as polymer brush PMETAC)that can respond to external stimuli,sodium dodecylbenzene sulfonate solution is selected The polymer brush PMETAC grafted on the moving surface is modified to transform it from a hydrophilic state to a hydrophobic state.The experimental results show that the friction coefficient and film thickness before modification are higher than those after modification,which preliminarily proves that the lubrication performance can be controlled by the surface modification method.This research work clarifies that the "hydration effect" of the polymer brush is the key element to obtain excellent water lubrication performance;It is found that the polymer brush exhibits a surprising lubrication enhancement effect under the intervention of the hydrodynamic lubrication effect;And the preliminary realization of the surface modification method to change the degree of hydration of the polymer brush to control its lubricating performance,this basic theoretical study can not only achieve high efficiency and energy saving,so as to protect and improve the natural environment for human survival,but also help in-depth understanding The lubrication mechanism of organisms has theoretical and practical significance for guiding the design and development of bionic lubricating materials. |
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