查看更多>>摘要：The boundary lubrication mechanism at the articulating surface of natural synovial joints has been the subject of much discussion in tribology.In this study,to elucidate the lubricating function of the superficial area of articular cartilage and synovial fluid(SF),cartilage specimens were processed with four different treatments:gentle and severe washing with detergent,incubation in NaCl solution,and trypsin digestion to selectively remove certain constituents from the cartilage surface.Subsequently,the frictional characteristics were examined in phosphate-buffered saline(PBS)and SF against glass.Angularly reciprocating sliding tests with a spherical glass probe and square articular cartilage specimens were performed at low contact loads in the mN range to extract the frictional behavior in the superficial area of the cartilage specimens.Meanwhile,the cartilage surface was observed to confirm the effects of treatments on the morphology of the cartilage surface using a fluorescence microscope and water-immersion methods.The coefficient of friction(COF)of the prepared cartilage specimens was varied from 0.05 to over 0.3 in PBS.However,a certain group of cartilage specimens exhibited a low COF of less than 0.1 with limited variation.For the low COF group of specimens,all four treatments increased the COF in PBS to different extents,and fluorescence microscopy revealed that the integrity of the cartilage surface was deteriorated by treatments.This means that the intact cartilage surface had lubricating constituents to maintain low friction,and the removal of such constituents resulted in the loss of the intrinsic boundary lubricity of the cartilage surface.The variation in the COF of the cartilage specimens was suppressed in SF because it had a clear boundary lubrication effect on the cartilage surface.The lubricating effect of SF could be confirmed even after degenerative treatment.
查看更多>>摘要：Carbon fiber reinforced polytetrafluoroethylene(CF/PTFE)composites are known for their exceptional tribological performance when sliding against steel or cast iron in inert gas environments.Compared to experiments in humid air,about an order of magnitude lower wear rate and several times lower coefficient of friction have been reported for tests conducted in dry nitrogen and hydrogen.Moreover,trace moisture has been shown to affect the friction and wear significantly of this tribosystem,although a possible effect of oxygen cannot be ruled out due to uncertainties regarding the oxygen concentrations.While several studies have pointed out the environmental sensitivity of CF/PTFE,the understanding of the underlying mechanisms are very limited.The objective of this research is to investigate the individual and combined effect of oxygen and moisture on the tribological behavior of CF/PTFE sliding against steel.Additionally,this study aims to elucidate the underlying mechanisms that govern the environmental sensitivity of the system.Climate-controlled three-pin-on-disc experiments were conducted in nitrogen atmospheres at various concentrations of oxygen and moisture.The tribological results clearly demonstrate that both moisture and oxygen contribute to increased friction and wear.However,the adverse effect was much more pronounced for oxygen than moisture.A qualitative method was developed to estimate the tribofilm coverage on the CF/PTFE surface.Results showed strong correlation between high coverage of strongly adhered tribofilm and low wear rate.Moreover,a loosely adhered tribofilm was observed on top of the CF/PTFE surface in presence of moisture.FTIR analysis indicated that the loosely adhered tribofilm found in the moisture-enriched environment contained a significant amount of adsorbed water,which may explain the lower coefficient of friction in presence of moisture compared to oxygen.The adsorbed water in the loosely adhered tribofilm could be an indication of moisture-driven lubrication by the non-graphitic carbon in the tribofilm.
查看更多>>摘要：Transient adhesion effects in rolling-sliding contacts influence all aspects of train-track interaction.This is of high importance specifically when these effects result in critically low adhesion,which poses a risk to traction and braking of railway vehicles.This study presents a model that can replicate the transient changes of the coefficient of adhesion with tested water and solid particle mix.The experimental data for the model are measured using a commercial ball-on-disc tribometer.The experimental results showed a liquid reservoir in front of the contact area that slowly reduces in size.This observation was used in the modelling approach to divide the calculation into two stages where the reservoir is present and when it disappears.The model was able to reproduce the occurrence of low adhesion region seen in experimental results with different particle concentrations.
查看更多>>摘要：Lighter and more powerful next generation vehicles and other rotary machinery demand bearings to operate in harsher conditions for higher efficiency,and the continuous development of advanced low-wear and friction materials is thus becoming even more important to meet these requirements.New aluminium composites reinforced with high performance lubricate phases such as graphene nanoplatelets(GNPs)are very promising and have been vigorously investigated.By maintaining a low coefficient of friction(COF)and offering great strength against wear due to their self-lubricating capability,the solid lubricant like GNPs protect the bearing surface from wear damage and prevent change in metallurgical properties during temperature fluctuations.This paper first studies the high-temperature tribological performance of aluminium matrix composites reinforced with GNP,consolidated via powder metallurgy,then elucidates their tribological mechanism.We report that the best tribological performance is achieved by the composite containing 2.0 wt%GNP,with an extraordinarily low COF of 0.09 and a specific wear rate of 3.5×10-2 mm3·N-1·m-1,which represent 75%and 40%reduction respectively,against the plain aluminium consolidated under identical conditions.The in-track and out-of-track Raman analysis have confirmed the role of GNPs in creating a tribofilm on the counterpart surface which contributed to the excellent performance.
查看更多>>摘要：The contact of a rigid body with nominally flat rough surface and an elastic half-space is considered.To solve the contact problem,the Greenwood-Williamson statistical model and the localization principle are used.The developed contact model allows us to investigate the surface approach and the real contact area with taking into account the asperities interaction.It is shown that the mutual influence of asperities changes not only contact characteristics at the macroscale,but also the contact pressure distribution at the microscale.As follows from the results,the inclusion in the contact model of the effect of the mutual influence of asperities is especially significant for studying the real contact area,as well as the contact characteristics at high applied loads.The results calculated according to the proposed approach are in a good agreement with the experimentally observed effects,i.e.,the real contact area saturation and the additional compliance exhaustion.
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