PTFE Transfer Research Articles

Characterization of PTFE on Silicon Wafer Tribological Transfer Films by XPS, Imaging XPS and AFM

Atomic force microscopy (AFM) shows that PTFE-on-silicon wafer tribological transfer films consist of narrow ribbons of PTFE, <1 μm wide and <10 nm high, aligned with the film draw direction. Within the ribbons the PTFE molecular chains are also aligned with the draw direction. Such materials are of interest as substrates for the epitaxial crystallization of polymers and small molecules. X-ray photoelectron spectroscopy demonstrates that the amount of PTFE put down increases with deposition temperature and pressure. The XPS azimuthal angle dependence at low electron take-off angle is consistent with the aligned ribbon morphology. Azimuthal and polar angle dependence and charging studies show that hydrocarbon contamination observed in the XPS spectrum is located on top of the silicon wafer substrate, in the channels between the PTFE ribbons. Imaging XPS confirms the pressure dependence of PTFE coverage, and at low deposition pressure reveals tracks of PTFE ∼100 μm wide, aligned with the film draw direction. Together with the AFM data this leads to a description of PTFE transfer films as fractal materials.

Tribological performance of bronze-filled PTFE facings for machine tool slideways

The friction and wear behaviour of bronze-filled PTFE facings against cast iron has been investigated under dry conditions in the laboratory. The work shows that the surface of the PTFE composite becomes enriched with bronze particles during sliding, owing to the preferential wear of the PTFE matrix. This contributes to the improved wear resistance of the composite, since the particles block the slip of the PTFE lamellae. The results also indicate that the onset of equilibrium wear after running-in is due to the development of a continuous PTFE transfer film on the cast iron counterface which shields the cast iron from further chemical adhesion and presents a much less aggressive surface to the PTFE counterface. A load-dependent transition from mild to severe wear was found at 54 N and occurred whether the PTFE was used as the flat plate or the pin. Failure of the PTFE facing occurred catastrophically owing to fracture in the adhesive with the failure thickness and time both decreasing with increasing load.

The friction and wear properties of electroless Ni-polytetrafluoroethylene composite coating

The electroless Ni-polytetrafluoroethylene (PTFE) composite coatings were prepared by using CuSO4 as the stabilizing agent and cation or non-ionic fluorcarbon compounds as the surfactants in the plating bath. The effects of heat treatment temperature on the hardness, microstructure and tribiological properties of the coatings were investigated. On the basis of observing and analysing the coatings using optical microscopy, scanning electron microscopy, X-ray diffraction and electron probe microanalysis the mechanisms of friction and wear of the composite coatings were proposed. The results show that heat treatment at 200 °C has only a slight effect on the friction and wear properties of the coating, whereas heat treatment at 400 °C gives the highest hardness and the lowest wear rate. Moreover, the friction and wear properties of the coatings may be attributed mainly to scuffing and the formation of PTFE transfer film.

XPS Analysis of the Effect of Fillers on PTFE Transfer Film Development in Sliding Contacts

The development of transfer films atop steel counterfaces in contact with unfilled and bronze-filled PTFE has been studied using X-ray photoelectron spectroscopy. The sliding apparatus was contained within the vacuum of the analytical system, so the effects of the native oxide, hydrocarbon, and adsorbed gaseous surface layers of the steel upon the PTFE transfer behavior could be studied in-situ. For both the filled and the unfilled PTFE, cleaner surfaces promoted greater amounts of transfer. Metal fluorides, which formed at the transfer film/counterface interface, were found solely in cases where the native oxide had been removed to expose the metallic surface prior to sliding. These fluorides also were found at clean metal/PTFE interfaces formed in the absence of frictional contact. A fraction of these fluorides resulted from irradiation damage inherent in XPS analysis. PTFE transfer films were found to build up with repeated sliding passes, by a process in which strands of transfer filled in the remaini...

Tribo-Characteristics of Self-Lubricating Ball Bearings for the LE-7 Liquid Hydrogen Rocket-Turbopump

The tribo characteristics of self-lubricating 40-mm-bore ball bearings with a retainer of glass cloth-polytetrafluoroelhylene (PTFE) laminate, which has elliptical pockets with a large pocket clearance, were tested under thrust loads at speeds up to 50000 rpm, 2 million DN, in liquid hydrogen (LH2) and in liquid nitrogen (LN2). During testing, the bearing torque, outer-race temperature, and electric resistance between the inner and outer races were monitored to verify the formation and rupture of a PTFE transfer film. Testing showed that the bearings having the elliptical retainer pockets were superior to the conventional bearings with circular pockets. It was determined that at the maximum inner race spinning velocity of about 5 mls a PTFE transfer film could sustain the maximum Hertz stress, up to about 2000 N/mm2, in LH2, without severe film rupture resulting in bearing seizure. In LN2, the critical load capacity of PTFE transfer film with bearing seizure was about 2700 N/mm2.

On PTFE transfer and thermoactivation mechanism of wear

PTFE transferred layers were found to decrease or increase the friction force depending on variations in adhesional interaction on the contact area when the layer is being formed. The molecular component of adhesion is of great importance. After a layer of transferred PTFE has formed on the contacting surface, PTFE begins to wear by a thermoactivation mechanism with an activation energy of 40–60 kJ mol -1. These low values of activation energy indicate PTFE wear to be associated most of all with the breaking of weak intermolecular bonds and with interplanar shear and slipping of crystalline aggregates formed in the band structure of the material.

Molecule structure variations in friction of stainless steel/PTFE and its composite

The friction and wear characteristics of PTFE and one of its composites, JS material rubbing against stainless steel, were determined with a pin-disk tester in this study. The JS material is a multilayer composite composed of PTFE layer containing metal oxide and others, porous bronze layer, copper-plating layer and steel back. The submicroscopic features of frictional surfaces of stainless steel and JS materials were observed with an electron probe microanalyzer (EPM). By analysis of fractional surfaces of stainless steel pins in various operating stages with X-ray photoelectron spectroscopy (XPS), chemical shifts of Cls, Ols, and Fls peaks, F-ion gathering in surface transfer films, and generation of a metal fluoride and an unknown compound containing oxygen were found. The determination of JS material wear debris with electron spin resonance spectroscopy (ESR) showed that polymeric radicals different in structure and stable in air existed. The authors consider that these PTFE molecule structure variations might be of benefit to the adhesion of PTFE transfer film to the rubbed stainless steel surface, which is important to improve the friction and wear performance of PTFE.