The friction coefficient on polycarbonate as a function of the contact pressure and nanoscale roughness

Abstract

During dry friction the rheological behavior at the interface between a polymer surface and a rigid indenter is linked to the local pressure and the roughness of the surface of the indenter. This study on polycarbonate reports experimental results obtained at ambient temperature using rigid, spherical glass indenters of various radii. In a first step, a plot of the experimental friction coefficient l versus the mean contact pressure p mean showed that below the yield stress (p mean < 100 MPa) the intrinsic friction coefficient follows a master curve for smooth indenters. In a second step, roughened indenters were prepared by chemical etching, which allowed the monitoring of nanoscale roughness parameters. From 5.5 to 140 nm Rrms, the friction coefficient l progressively fell to a plastic-like constant value, indicating that the nanoroughness mediates the friction. These results form the basis for a study of the rheology of confined polymer layers. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 580-588, 2012

Analysis of the effect of nano-roughness on the friction of a vitreous polymer

“…6b plots the experimental mean contact pressure p mean against the ratio a/R or tan(b) for several indenters. Previous work showed that these data follow the classical laws of contact mechanics [19]. It was concluded that the PC surface is in its elastic regime for p mean o30 MPa, in its elasto-plastic regime for 30 MPao p mean o150 MPa and in its fully plastic regime for p mean 4150 MPa.…”

“…Sliding friction experiments are performed in our experimental set-up using an original in situ display fully described elsewhere [19]. The motion of the tip and normal load F n are monitored, while the tangential force F t is recorded and the normal contact area S of a transparent material is visualized and analyzed with a built-in imaging system (Fig.…”

“…We used the average value of the measured tangential load F t to calculate m. Fig. 2 shows that the local friction coefficient decreases as the rms roughness of the indenters increases (þRrms: 5.5 nm, JRrms: 8.5 nm) until plastic deformation occurs (.Rrms: 140 nm) [19]. This experimental intrinsic friction coefficient can be directly related to the mean interfacial scission t mean (t mean ¼m local n p mean ): the interfacial rheological behavior depends on the nanoroughnness of the tip.…”

“…We measured the apparent friction coefficient m (friction force normalized by the normal load). We deduced from in situ observations the mean contact pressure p mean and local friction coefficient m local thanks to our original experimental device described in detail elsewhere [18,19]. To determine the effect of nano-roughness on the friction, controlled random textures on the nano-scale were created on spherical borosilicate glass indenters using chemical etching (hydrofluoric acid) and characterized by AFM measurements in the contact mode ( E1 nm spatial resolution and a few nms lateral resolution).…”

“…The effect of these nanoshapes on the contact characteristics (real pressure distribution, real contact areay) and internal stresses in the subsurface layers may then be compared to the macro-shape contact characteristics. This paper employs previously published experimental measurements of the sliding friction of nano-roughened borosilicate indenters on a rejuvenated polycarbonate surface [19]. It aims at introducing some physical aspects in the contact from interfacial scission.…”