Why Teflon is so slippery while other polymers are not

Polytetrafluoroethylene [PTFE (Teflon)] is a uniquely slippery polymer, with a coefficient of friction that is an order of magnitude lower than that of other polymers. Though known as nonsticky, PTFE leaves a layer of material behind on the substrate while sliding. Here, we use contact-sensitive fluorescent probes to image the sliding contact in situ: We show that slip happens at an internal PTFE-PTFE interface that has an unusually low shear strength of 0.8 MPa. This weak internal interface directly leads to low friction and enables transfer of the PTFE film to the substrate even in the absence of strong adhesion.

Figure 1

Fluorescent probes emit strongly when confined, which reveals the area of contact between a polymer sphere and a DCDHF-coated glass coverslip [(a) and (b) “contact”]. After the contact is broken, fluorescence emission uncovers the presence of a residual transfer film [(b) “after”], which only forms in a sliding PTFE contact, not in a purely normal contact, nor when PMMA is the contacting polymer. Normal force is $\approx 1$ mN for contact images shown in (a), and 85 mN in (b). The scale bar indicates 10 $\mu \mathrm{m}$. In (b), the intensities of the “after” images are adjusted to correct for background fluorescence and inhomogeneous lighting. The contact plane is immersed in a refractive-index-matched liquid (formamide or DMSO) during measurements.

Figure 2

Friction force, normal force, and contact area are measured for PTFE and PMMA spheres contacting DCDHF-coated glass coverslips. (a) The kinetic friction force is proportional to the contact area, frictional shear strength ${\tau }_{\text{frict}}=F_f/A=0.8$ MPa for PTFE and 11 MPa for PMMA. Inset: Kinetic friction force is the measured steady state force after the static friction peak. (b) The contact area evolves sublinearly respective to the normal force, indicative of elastoplastic strain hardening contact mechanics [18, 21]. (c) The kinetic friction force is approximately proportional to normal force, ${\mu }_{\text{PTFE}}=F{}_f/F{}_n\approx 0.02$, whereas ${\mu }_{\text{PMMA}}\approx 0.12$.

Figure 3

Force-displacement curves for PTFE and PMMA spheres ($D=1.59$ mm) approaching (dashed line) and retracting from (solid line) the substrate (a DCDHF-coated glass coverslip). Crosses show PTFE retracting from the coverslip immersed in refractive-index-matched liquid (formamide). Negative displacement indicates separation between surfaces. The approach and retraction velocity is 10 nm ${\mathrm{s}}^{-1}$.

Figure 4

Schematic image of relevant interfaces in the transfer process. Slip happens at the weaker, PTFE-PTFE interface (purple). The adhesive shear strength at the PTFE-substrate interface (green) is more than an order of magnitude larger than the frictional shear strength, thus more than enough to keep the film stuck during sliding.

Figure 5

PTFE spheres sliding on a dry or a liquid-immersed DCDHF-coated glass coverslip, or on a plasma-cleaned silicon wafer all show the same low friction, ${\mu }_{\text{PTFE}}\approx 0.02$. The dashed line is a guide to the eye.

Figure 6

XPS spectrum (Pass energy (PE) $=300$ eV) of the F $1s$ region of the Si/PTFE substrate (magenta) and the Si control substrate (blue). The distinct peak at 690 eV for the Si/PTFE substrate is consistent with PTFE transfer. The inset shows the F $1s$ spectrum of the pristine PTFE sphere prior to sliding and serves as a reference. The PTFE reference spectrum has been shifted to correct for surface charging, while the spectra on Si have been aligned to the Si $2p$ position. The offset in the F $1s$ position shows that the transferred layer also shows surface charging, indicating an insulating character that is consistent with PTFE.

Figure 7

Ellipsometric angles $\mathrm{\Delta }$ and $\mathrm{\Psi }$ as a function of angle of incidence $\varphi$ for a Si wafer with a native oxide layer (Si ref) and a Si wafer after PTFE has slid over it (Si/PTFE). A two-layer fit to the reference Si wafer data reveals a thickness of the silicon oxide layer of around 2 nm. A three-layer fit to the Si/PTFE data shows that on top of the silicon oxide layer, 3 nm of PTFE are present.