Research On Tribological Properties Of Bionic Hexagonal Pillar-textured Surfaces

During the study of tribology, the natural rough surfaces have given us lots of inspirations,especially the adhesive skin of tree frog toe pads, which are characterized by pillar-studded hexagonalcells separated by channels. Tree frog and other biological surfaces can be considered as a generalizedsoft material, at the same time, soft materials such as rubber are more and more widely used in today’sindustry and people’s lives. Therefore, fabricating bionic textures on soft materials can be a way toaccomplish surfaces with special tribological functions.In this thesis, bionic hexagonal pillar texture were fabricated on Polydimethylsiloxane(PDMS)surfaces by Lithography&Replica techniques. Friction tests were carried out with a pin-on-disk testerin different lubrication conditions. Both PDMS/PDMS friction pair and the GCr15/PDMS one weretested. The effects and lubrication mechanism of bionic hexagonal pillar texture on PDMS werediscussed. The main conclusions are as follows:1. Under dry sliding, the friction on the interface of PDMS is consisted of adhesive friction,deformation and elastic hysteresis. Compared to the untextured PDMS, the textured ones with higherarea density and smaller diameter shows a better effect of friction reduction, while each friction forceare getting higher with the accelerating of sliding speed.2. Under methyl silicon oil lubrication, both of the friction pairs are performed on the regime ofmixed lubrication at low speed. The appearance of texture brings about higher friction coefficient.However, the friction pairs are performed on the regime of hydrodynamic lubrication at high speedwhile the friction coefficient is mainly effected by the viscosity of lubricant and the texture’s impacton friction is limited.3. Under water lubrication, on one hand, the friction pair of PDMS/PDMS are performed on theregime of boundary lubrication at low speed with a friction reduction by the hexagonal texture, and onthe regime of mixed lubrication at high speed with a significant friction increase, while the channelsaround the pillars perform a fast drainage and an absorptive action. The texture with higher areadensity and bigger diameter has an outstanding effect on increasing the friction coefficient of PDMSsurfaces. This function is quite similar to the one of frog toe pads’ structure. On the other hand, thefriction pair of GCr15/PDMS has a analogous experimental results but a difference at high speed,which has the tendency to turn into the regime of hydrodynamic lubrication. In this friction pair, thehexagonal pillar texture can increase friction force at medium speed.On the basis of experimental phenomena, we can get the tribological properties and regularitiesof hexagonal pillar texture on the above regimes of lubrication. The mechanism of tribology isdiscussed and at the same time we’ve got a proof of the frog toe pads’ structure. The results of thisstudy provide some design basis and methods of friction control for engineering application of softmaterials.