The Influence Of Surface Topography And Mechanical Properties Of The Material On Mixed Lubrication

Most machine elements work under mixed lubrication condition, which makes mixedlubrication become an important subject in tribology. Both hydrodynamic lubrication andcontacting asperities co-exist under mixed lubrication condition. Therefore, the research onmixed lubrication is difficult. In this dissertation, a deterministic mathematic model formixed lubrication was built. The influence of surface topography and mechanical propertiesof the material was calculated with this model and experiments. The main contributions ofthis dissertation include:1. A reconstruction method of measuring surfaces was proposed. The measuring surfacewas reconstructed according to surface parameters. Through the analysis on the experimentdata and its amplitude-frequency distribution in frequency domain, a method was proposedto determine the information of reconstructed surface. Through the comparison betweenmeasuring surfaces and reconstructed surfaces, the reconstructed surfaces effectivelyreflected the basic characteristics of measuring surfaces, meanwhile, the random noise wasfiltered out by this method.2. Considering the characteristic of mixed lubrication, A deterministic mathematicmodel for mixed lubrication was built. The model included the influence of pressure onviscosity and density of lubricants and the rheology effect of lubricant. For the differentshear stress formed by hydrodynamic lubrication and contacting asperities, a frictioncoefficient model was built based on the Bair and Winer rheology model and Rabinowiczcontacting equation.3. The influence of roughness amplitude, roughness density and roughness directions onmixed lubrication was analyzed. Mixed lubrication parameters and friction coefficient wereused to calculate the effect of surface topography. The tribology pairs with larger amplitudecould form a larger friction coefficient. The pressure and lubricant film changed severely with the density of roughness density, the friction coefficient increased with the roughnessdensity. The roughness direction had effect on lubrication performance. When the roughnessdirection paralleled to the direction of lubricant flow, the hindering effect of roughness onlubricant flow was weekend and the friction coefficient decreased.4. The influence of elastic mechanical properties of the material on mixed lubricationwas analyzed. The tribology performance of four kinds of materials was calculated underdifferent load condition. Mixed lubrication parameters were used to character the change oftribology performance. For smooth surface, the elastic module had impact on the amplitudeand position of maximum pressure as well as the second pressure spike. The greater theelastic module was, the greater the maximum pressure formed. As the entrainment velocityincreased, the position of maximum pressure transfered from the center of contact region toits outlet. The material with larger elastic module corresponded to the smaller critical speedof position conversion of maximum pressure. For rough surfaces, the material with a largerelastic module demanded a larger critical velocity to work in hydrodynamic lubricationregion.5. The influence of plastic deformation on mixed lubrication was investigated. Theplastic deformation will occur when the external load exceeds the yielding limit of thematerial. Stainless steel was used as the material of the tribology pair and the relationship ofstress and strain was measured by tensile testing machine. A stress tensor calculation methodof the node in contacting body was applied to calculate the stress tensor under lubricationcondition. Then, the plastic deformation was calculated based on the reciprocal theorem. Atlast, the plastic deformation was coupled into mixed lubrication model to calculate pressureand film distribution. The results showed that when considering plastic deformation, thenominal contact region extended, the pressure was flatten, the lubricant film decreased andthe friction coefficient increased.6. Ploydimethylsioxane (PDMS) was cited as sample to study the tribology preferenceof soft material. The mixed lubrication model and friction coefficient model weremodificated according to the characteristic of soft material. The effective of these modelswas validated by the experiment data. Based on these two models, the characteristic of lubricant film formed by soft tribology was discussed, the effect of hydrodynamiclubrication and asperities contacting on friction coefficient was analyzed. Overpressuretheory was used to analyze the reason for the change of friction coefficient.