Lubricating film thickness measurements with bovine serum

Lubricant film thickness measurements were made for bovine serum solutions under steady state rolling and sliding. The effect of low (30 MPa) and high contact pressures (200 MPa) was examined. In the high pressure rolling tests BS initially formed films 5-50 nm thick over the speed range. However, in subsequent speed sweeps, a relatively speed independent film of 40-50 nm developed. In some cases thick (up to 100 nm) films were formed at low speeds; this behaviour was considered representative of high-viscosity surface layers rather than of solid films. At the end of each test residual boundary films of 9-19 nm were measured under static loading. These are attributed to the multilayer adsorption of protein molecules and will provide surface protection during stance or on initiation of gait. The results at low pressure showed that much thicker films (∼60-80 nm) were formed over the same speed range. Again thicker films were formed at the lower speeds. There was significant scatter in the film thickness results, possibly due to the inherent nature of the fluid, which is an inhomogeneous biological solution. The film thickness/speed behaviour was not representative of a simple Newtonian fluid and this has considerable implications for the development of predictive film thickness models and new designs of artificial hip joints.     

Film Formation by Colloidal Overbased Detergents in Lubricated Contacts

It has been reported in the literature that overbased detergents can possess good antifriction and antiwear properties although the origins of these properties are not fully clear. In practice, over-based detergents are colloidal dispersions and this may be important in determining their properties and mechanism of action.

In the current study, the lubricating properties of commercial, overbased magnesium and calcium sulfonates were measured in thin film, lubricated conditions and compared to a neutral sulfonate additive. A range of techniques was employed to evaluate the tribological performance of solutions of these additives. Film thickness measurements were carried out using optical interferometry and in-contact visualization, while friction and wear measurements complemented the study.

It has been found that, when operating in thin film conditions, overbased detergents deposit solid-like boundary films on the rubbing surfaces. These films form in both rolling and mixed rolling/sliding conditions and, unlike many other colloidally-formed boundary films, are able to survive in high speed, thick film conditions. During formation, the film rapidly reaches a thickness corresponding to one colloid particle diameter, between 10 and 20 nm. After prolonged rubbing, however, the film thickness reaches the equivalent of three particle diameters. No such thick boundary films are observed with the neutral sulfonate.

The boundary films formed by overbased detergents produce a significant reduction in wear. However, for the very smooth surfaces used in this study, they also result in an effective roughening of the very smooth surfaces studied. This leads to an increase in friction in the intermediate speed region by promoting solid-solid contact in thin fluid film conditions.     

The Formation of Viscous Surface Films by Polymer Solutions: Boundary or Elastohydrodynamic Lubrication?

The film-forming properties of a range of polymer solutions have been studied down to very thin film thickness using ultrathin film interferometry.

It has been found that, at very slow rolling speeds, some polymers generate much thicker films than predicted from theory. It appears that these polymers form adsorbed layers between three and 15 nanometers thick on the two solid surfaces. These layers have a viscosity many times higher than that of the bulk solution. Therefore, under slow speed, low film thickness conditions, the contact effectively operates within a viscous boundary layer, generating an elastohydrodynamic-type film much thicker than predicted from the viscosity of the bulk lubricant. As the speed is raised the contact emerges from this boundary layer and reverts to elastohydrodynamic behavior based upon the viscosity of the bulk polymer solution.     

Thin lubricating films behaviour at very high contact pressure

Thin film colorimetric interferometry has been used to examine the behaviour of thin elastohydrodynamic (EHD) lubricant films under very high contact pressures of the order of 0.5–3 GPa. It has been shown that at moderate pressures, the variation of film thickness with speed follows the Hamrock and Dowson prediction down to one nanometer. As the load is increased, however, thin films behave differently from the prediction of the conventional EHD theory. For a certain lubricant and operational conditions, there is a critical rolling speed below which a reduction of film thickness is observed. This behaviour is very similar to that previously predicted computationally by Zhu.     

In Situ Study of Metal Oleate Friction Modifier Additives

There has been debate for many years as to whether long-chain surfactant friction modifier additives reduce friction by forming adsorbed films of monolayer thickness or whether they form films equivalent to several or many multilayers thick. In the work described in this paper, a series of metal oleate friction modifier additives has been synthesized and their film-forming properties compared in rolling-contact conditions by means of ultrathin film interferometry. It has been found that some of these additives form thick boundary films while others do not. It is concluded that thick boundary-film formation results from the formation of insoluble iron(II) oleate on the rubbing surfaces and that, for metal oleates, this occurs only for metals lower than iron in the electrochemical series and is due to a redox reaction involving iron from the steel surface and the metal oleate.     

The Behavior of Polymer Solutions in Concentrated Contacts: Immobile Surface Layer Formation

The film-forming properties of solutions of low-to-medium molecular weight polyisoprenes in synthetic hydrocarbon have been studied using thin film interferometry. It has been shown that the lubricating films formed in rolling, concentrated contacts have two components. At very slow speeds, the polymer forms an immobile film of thickness equivalent to two radii of gyration of the polymer molecules. This film thickness is independent of speed but is gradually squeezed from the contact when motion is halted. The film is likely to represent two monolayers of polymer, one on each surface, and can be regarded as a boundary lubricating film. As the rolling speed is raised, a conventional elastohydrodynamic film is formed which is superimposed on the immobile film.

The immobile films formed are similar to those detected using a force balance apparatus and reported elsewhere in the literature.     

Film Forming and Friction Properties of Overbased Calcium Sulphonate Detergents

The film-forming and friction properties of overbased calcium sulphonate (OBCaSu) detergents in rolling–sliding, thin film, lubricated contact have been investigated. All of the commercial detergents studied form thick, solid-like, calcium carbonate films on the rubbed surfaces, of thickness 100–150 nm. The films have a pad-like structure, interspersed by deep valleys in which practically no film is present. These films have the effect of increasing friction in intermediate speed conditions, an effect which is believed to occur because the pad structure of the film inhibits fluid entrainment and thus postpones the formation of an EHD film to higher entrainment speeds. Large differences were noted between the boundary friction coefficients of the various detergents tested. Two of them gave very low boundary friction coefficients, in the range 0.06–0.08, while the other two gave considerably higher friction. These differences are believed to originate from differences in the structures of the alkyl chains in the sulphonate detergent molecules. The films formed by OBCaSu detergents can be removed very effectively by treatment with EDTA solution and this shows that the films are effective in preventing wear.     

Structure analysis of sputter-coated and ion-beam sputter-coated films: a comparative study

Gold, platinum and tungsten films were deposited by low energy input (7 mA, 450 V), or high deposition rate (80 mA, 1500 V), diode sputter coating and by ion beam sputter coating. Film structures on Formvar coated grids and on the surface of coated erythrocytes, resin embedded, sectioned, and recorded at high magnification in a TEM were compared using computer-assisted measurements and analysis of film thickness and grain size. The average grain size of the thinnest gold and platinum films was relatively independent of the mode or rate of deposition but as the film thickness increased, significant differences in grain size and film structure were observed. Thick platinum or gold films deposited by low energy input sputter coating contained large grain size and electron transparent cracks; however, more even films with narrower cracks but larger grain size were produced at high deposition rates. Ion beam sputter coated gold had relatively large grain size in 10 nm thick films, but beyond this thickness the grains coalesced to form a continuous film. Platinum films deposited by ion beam sputter coating were even and free of electron transparent cracks and had a very small grain size (1–2 nm), which was relatively independent of the film thickness. Tungsten deposition either by low energy input or ion beam sputter coating resulted in fine grained even films which were free of electron transparent cracks. Such films remained granular in substructure and had a grain size of about 1 nm which was relatively independent of film thickness. Tungsten films produced at high deposition rates were of poorer quality. We conclude that thick diode sputter coated platinum and gold films are best deposited at high deposition rates provided the specimens are not heat sensitive, the improvement in film structure being more significant than the slight increase in grain size. Thick diode or ion beam sputter coated gold films should be suitable for low resolution SEM, and thin discontinuous gold films for medium resolution SEM. Diode sputter coated platinum should be suitable for medium resolution SEM and ion beam sputter coated platinum for medium and some high resolution SEM. 1–5 nm thick tungsten films, deposited by low energy input or ion beam sputter coating should be suitable for high resolution SEM, particularly where contrast is of less importance than resolution.     

Boundary Film Formation by Viscosity Index Improvers

Ultrathin film interferometry has been used to measure the film-forming properties of a range of viscosity index improver (VII) solutions in rolling, concentrated contacts. It has been shown that some VIIs form boundary lubricating films of thickness 10 to 30 nm in contacts. These films result from the presence of highly concentrated and thus very viscous layers of polymer solution formed on the two rubbing solid surfaces by polymer adsorption. These boundary films are formed only by some types of VII and can persist up to temperatures in excess of 120°C. The possible implications of this type of boundary lubrication are discussed.     

Elastohydrodynamic properties of water-based fire-resistant hydraulic fluids

The elastohydrodynamic (ehd) lubricating properties of commercial fire-resistant hydraulic fluids in rolling point contacts are described. Earlier work is reviewed and new work presented on water-in-oil and oil-in-water emulsions. It is shown that in general, elastohydrodynamic film thicknesses of water-in-oil emulsions are close to those of their base oils. For very fine particle size emulsions, however, thicker films are observed. Commercial oil-in-water emulsions in fully flooded conditions have not been found to give measurable elastohydrodynamic films. However, when the emulsions are destabilized, a localized pool of oil collects in the contact zone which enables ehd films to form, though these films do not survive very high rolling speeds. Polyglycol solutions give low ehd film thicknesses, approximately one third those of mineral oils of corresponding viscosity. This can be attributed to the low pressure—viscosity coefficient of these solutions.