Effect of PDMS Material Properties and Sliding Velocity on Friction Peak in Tacky Regime

When the contact interface of elastomers changes from wet to dry, a transient high friction coefficient always appears and has been termed as tacky regime. The maximum friction coefficient is named as friction peak, and the increase of the friction peak comparing with the dry one is called the relative increase percentage of friction peak. This paper investigated this friction peak for different elastomers under different velocities, so as to provide a theoretical basis for the adjustment of the friction peak. In the present study, a cross-linked polymer, polydimethylsiloxane (PDMS) film was used as the soft-material surface. The mass ratio of the curing agent to the base material alters the elastic modulus of PDMS. Four mass ratios were used including 1:5, 1:10, 1:15 and 1:20. The dynamic frictional behavior of the contact interface between silicon nitride sphere and PDMS film during the wetting transition was studied by a custom-built optical in-situ tribometer, and the effects of elastic modulus and sliding velocity on the relative increase percentage of friction peak were analyzed. Optical interference based methods were used to observe the deformation of PDMS film. In dry condition, the friction coefficient increased with the elastic modulus of PDMS film decreased, which should be related to the material properties of elastomers. With the decrease of curing agent content, the elastic modulus of PDMS decreased and the PDMS film became more prone to be deformed. The increase resistance provided by the deformation of PDMS film should be the main reason for the increase of dry friction coefficient. The result of frictional experimented when the surface transitioned from wet to dry showed that the relative increase percentage of friction peak decreased or even disappeared with the decrease of elastic modulus of elastomers. For different cross-linked PDMS film, the wetting contact angles were approximate, which excluded the influence of wettability on the friction peak. In order to explore the influence of elastic modulus on the friction peak in tacky regime, the relationship between the relative increase percentage of friction peak and the elastic modulus of PDMS film was fitted. The fitting result showed that the relative increase percentage of friction peak was proportional to 1.45 power of elastic modulus. When the elastic modulus was small, the relative growth percentage of friction peak increased slowly; however, when the elastic modulus was large, the relative growth percentage of friction peak increased greatly. The result of contact morphology illustrated that with the decrease of elastic modulus of elastomers, Schallamach wave began to appear in the contact interface, more air was carried and the larger distance was propagated gradually. We hypothesize that for the softer PDMS film, the decrease of the friction peak may be related to the appearance of the Schallamach wave; Maybe for the harder PDMS film, the material properties of elastomer have little effect on the friction peak and the influence of the residual water film entrapped in the contact interface on the friction peak is dominant. In addition, with the increase of sliding speed, the relative increase percentage of the friction peak decreased gradually. The result of contact morphology illustrated that the propagation frequency of Schallamach waves increased with the increase of sliding velocity, which supports the conclusion that the reduction of friction peak was accompanied by Schallamach wave. This study provided theoretical guidance for adjusting friction peaks of viscoelastic bodies by elastic modulus and sliding velocity.