| The history of tribology can be traced back to long time ago.Engineering experiences and experimental analyses indicate that friction force is dependent on various factors including the physical and chemical properties of the interfaces,surface morphology and working environment etc.Therefore,it is difficult to form a unified friction law which is applicable to all cases.Friction is usually adjusted due to different requirements in mechanical transmission.For example,reducing friction can save energy and improve efficiencies in bearings and gears,while increasing friction can transmit torque and consume energy in friction dampers.Friction exists everywhere.The application of friction would be expended if the friction manipulation can be achieved.The objective of this study is to investigate the effect of interfacial structure on friction force,and to endue friction interfaces with specific friction characteristics through proper design of surface structures.The main research work are as follows:(1)Investigated the effect of structural deformation on friction and tested the friction behaviour between leaves and foam blocks.The experimental results indicate that the thorns on Bothriospermum chinense Bunge and Broussonetia papyrifera leaves deformed as the normal load increased so that the friction force was affected,showing an counterintuitive phenomenon,viz.the friction force decreased with the increase of normal load varying in a certain range.Based on the experiment,a mechanics model was established for a ‘lollipop' structure.The mechanism of negative friction growth was verified by experimental tests on the lollipops fabricated using 3D printing.(2)Based on the theory that friction force is proportional to the real contact area,a unique surface structure consists of several spheres was designed and fabricated.This type of surface structure deforms with the increase of normal load,leading to that the contact region changes from larger spherical surface to smaller spherical surface.According to Hertz's contact theory,the real contact area between the interfaces becomes smaller,which results in the decrease of the friction force.Experimental tests verify that it is feasible manipulate friction force by changing real contact area.In addition,the test results indicate that negative growth of friction can also be achieved based on designed gradient of roughness by distributing substructures on contact surfaces.(3)The characteristics of contact interfaces in relative sliding was analyzed by dynamic modelling.Finite element calculation show that the stiffness of the surface structures affects the deformation in the contact area and thus causes the variation of friction force.Therefore,friction force can be reduced by optimizing the stiffness of surface structures.Numerical simulation indicate that when an object slides on an elastic surface,the friction force,because of energy conservation,may change its direction and pushes the object to accelerate.In this work,the relationship between surface structure and sliding f riction was studied.Experiment,simulation and theoretical analysis verified that the scheme for manipulation of friction using controlling structural deformation,changing real contact area and adjusting structural stiffness etc.is feasible.The researc h conclusion is of guiding significance to friction design. |
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