Properties of Polyetheretherketone (PEEK) transferred materials in a PEEK-steel contact

Polyetheretherketone (PEEK) is a high performance polymer that can be an alternative to metal for some moving components in unlubricated conditions. During rubbing, PEEK is transferred to the counterface. The formation and properties of PEEK transfer films on steel and sapphire are studied by in-situ observations of PEEK wear process, contact temperatures and triboemission, as well as FTIR and Raman spectroscopies ex-situ. Our results suggest that frictional heating alone may not be sufficient to generate PEEK degradation observed in the transfer materials. Triboplasma observed during rubbing, together with mechanical shear, may promote generations of radicals and degradation of PEEK, which subsequently influence the properties of PEEK transfer film and performance of polymer-metal tribopair.This work is partly funded by EPSRC (Grant number: EP/L023202/1)

A statistic study on raspberry vesicles: formation and properties

This paper gives a statistic study on the formation of ABC raspberry vesicles under bulk swelling with DPD simulations. All vesicles formed through a disc wrap-up process, i.e. a disc micelle wraps up to form a vesicle. The lifetimes of the disc micelles before they become vesicles can be characterized as short and long (tfast and tslow). Vesicles formed with tfast have a high loading efficiency and a wide size distribution. Most of them have low membrane permeability. They resist structural deformation under shear due to their high bending rigidity. Vesicles formed with tslow have a narrow size distribution. They are small, and have low loading efficiency. A large portion of them have permeable membranes with low bending rigidity and structural defects. Shear could restructure these vesicles, and hence modify their permeability. Adjusting the repulsion between solvophobic polymers and solvents impacts on lifetimes of disc micelles. A reduction in such repulsion favours tslow. The knowledge obtained can be used to design raspberry vesicles of desired size, loading and cargo release properties

Transient aspects of plasma luminescence induced by triboelectrification of polymers

Transient electric gas discharges that occur around sliding interfaces during contact electrification of polymers were studied at millisecond timescales and with micrometre resolution. Deduced vibrational temperatures indicate cold plasma resulting from positive corona discharge. At millisecond timescales, previously unseen rapid discharge events are observed, and modelling suggests that these result from streamer development, triggered by electron emission from the polymer surface. Those which occur over a period of several seconds are shown to be caused by competition between charge generation and the formation of polymer films. The findings explain the interplay between charging and plasma generation and their dependence on wear processes.UK Engineering and Physical Sciences Research Council (EP/J002100/1), Additional equipment funding provided by the Taiho Kogyo Tribology Research Foundation (11A01). Spanish Ministry of Economy, Industry and Competitivity through the grant BIA2016-79582-

Adsorption of organic friction modifier additives

Organic friction modifier additives (OFMs) are surfactant molecules added to engine oils to reduce friction in the boundary lubrication regime. They are thought to work by forming an absorbed layer which provides low friction. This paper studied the relationship between the adsorption of OFMs and their friction and wear reducing properties in a rubbing contact formed by a stationary glass ball and a rotating silicon disk under the boundary lubrication regime. The effect of molecular structure was investigated by using OFMs of various tail saturation and head group chemistry. OFM tested were oleic acid, octadecylamine, oleylamine and glycerol monooleate. The thickness of an OFM adsorbed layer in hexadecane, examined in-situ by spectroscopic ellipsometry and quartz crystal microbalance (QCM), depends on the molecular structure and the concentration of the OFM. As expected, saturated, linear chain gives the thickest film. A critical OFM layer thickness of about 0.6 nm is necessary to achieve low initial and maximum friction. The thicker OFM layers are accompanied by narrower wear tracks, which are rougher than the wider, smoother wear tracks formed with thinner OFM layers. The interplay between the thickness of the OFM layer and wear track surface roughness results in all OFM layers having similar steady friction. This shows that the apparent effect of OFM depends on the stage of rubbing test: initially on friction; and then subsequently on surface damage. Despite OFMs and the base oil having similar refractive indices, ellipsometry was found to be a suitable technique for examining the adsorption of OFM additives from an oil based solution, and showed reasonable correlation with QCM results

Synovial fluid lubrication: The effect of protein interactions on adsorbed and lubricating films

© 2015 Elsevier Ltd. All rights reserved.Synovial fluid lubrication is dependent on protective protein films that form between joint surfaces. Under static conditions surface film formation occurs through adsorption, while under dynamic conditions protein aggregation under shear and load becomes the dominant mechanism. This work examines how the protein content of six model synovial fluids affects film formation under static and rolling conditions and if the changes in properties can be correlated. With an increase in the statically adsorbed mass and the rate of adsorption the film thickness under rolling increased. These increases did not correlate with the total protein content of the fluid, but were dependent on the type of protein. An increase in pH reduced the adsorbed mass, rate of adsorption and film thickness, but was of secondary importance to the type of protein. The rolling film thickness was also correlated with the viscoelastic properties of the films formed under static conditions. In this case thinner rolling films corresponded to the more hydrated, viscoelastic adsorbed films. The strong correlations found between the properties of the adsorbed films and those formed under rolling indicate that the same protein-protein and protein-surface interactions may govern both mechanisms of film formation despite the differences in the film structures