H. A. Spikes

The Measurement and Study of Very Thin Lubricant Films in Concentrated Contacts

Optical interferometry is now a widely used technique for measuring the separating film thickness in model rolling and sliding elastohydrodynamic contacts. There are two limitations of the method as conventionally employed: first, it cannot easily be used to accurately measure films less than one quarter the wavelength of visible light, i.e. less than about 100 nm. Secondly, only certain, discrete thicknesses, spaced at least 50 nm apart can be determined. This paper describes work aimed at overcoming these limitations so as to make optical interferometry applicable to the study of boundary or very thin film elastohydrodynamic lubrication in rolling contacts. A combination of a solid spacer layer with spectrometric analysis of reflected light from the contact enables very thin lubricant films to be accurately measured. The approach is applied to the study of thin films formed in rolling contacts by low viscosity lubricants. Some anomalies in the relationship between film thickness and speed are found with...

In Lubro Studies of Lubricants in EHD Contacts Using FTIR Absorption Spectroscopy

Research into the physical and chemical processes occurring within lubricated concentrated contacts has traditionally been limited by the necessity to observe events out-of-contact, or under simulated conditions. This paper describes the application of a new technique involving infrared microreflection absorption spectroscopy. The method is applied to the direct determination of the conditions of pressure experienced by lubricants in contacts, to the formation of antiwear films by phosphorus additives and to the study of the alignment of molecules, including viscosity index improvers in contacts. Presented at the 45th Annual Meeting In Denver, Colorado May 7–10, 1990

Elastohydrodynamic Properties of Water-Based Fire-Resistant Hydraulic Fluids

Abstract 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.

A study of parched lubrication

Abstract The elastohydrodynamic regime of lubrication is now quite well understood, to the extent that theoretically derived equations for lubricant film thickness are used routinely in engineering design. The boundary lubrication regime, which occurs at slow rubbing speeds and in which the surfaces are separated only by a chemically formed layer, is less clearly understood but many of the underlying concepts, such as the formation of a monomolecular adsorbed film or a thicker, reacted, glass-like layer, have been both demonstrated and modelled. Between these two regimes there remains, however, a territory about which very little is known. This is the regime where a concentrated contact is heavily starved, permitting no conventional elastohydrodynamic film, and yet there remains between the surfaces a film of lubricant whose presence is governed by its rheology rather than its ability to bond to the rubbing surfaces. Such films exist in applications such as gyroscope bearings and probably in many grease-lubricated systems. This regime has been called “parched lubrication” (E. Kingsbury, Trans. ASME J. Tribol., 107 (1985) 229). Up until the present it has not been possible to do much more than speculate upon the behaviour of this lubricant regime because such very thin films could not be directly measured. It was possible to deposit lubricant films of known thickness onto surfaces and to measure properties such as friction and wear but it was not possible to relate this to the thickness of such films under operating conditions. This paper describes an experimental study of parched lubrication which addresses this problem. A recently developed technique for measuring very thin films in concentrated contacts is employed to monitor the occurrence and persistence of thin lubricant films within highly starved, concentrated contacts under a range of conditions. This is combined with IR reflection-absorption spectroscopy to measure the oil film thickness on the out-of-contact surfaces. The main factors which determine the behaviour and performance of this type of starved film are discussed.

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. Presented at a Society of Tribologists and Lubrication Engineers paper at the STLE/ASME Tribology Conference in New Orlea...

In-Situ Measurement of ZDDP Films in Concentrated Contacts

The film formation behavior of lubricating oils containing zinc dialkyldithiophosphate (ZDDP) additives has been studied in rolling, concentrated contacts using ultra-thin film interferometry. It has been shown that ZDDP-containing oils form chemical films which are additional to the elastohydrodynamic (EHD) films produced by the base oil. ZDDP film formation occurs at elevated temperatures and begins in the range of 130°C to 170°C, depending upon the base oil type. The thickness of the films increases with temperature and test time. The ZDDP films formed are typically 5 to 25 nm thick and appear to be solid-like surface coatings. Presented at the 47th Annual Meeting In Philadelphia, Pennsylvania May 4–7, 1992

EHD Film Formation and Starvation of Oil-in-Water Emulsions

The elastohydrodynamic film-forming properties of oil-in-water emulsions in rolling point contacts have been investigated using a combination of ultrathin film interferomelry and short duration TV microscopy. It has been shown that at slow rolling speeds, all the o/w emulsions tested formed EHD films comparable to those of their water-free components. Above a certain critical rolling speed, which, depended upon the emulsion composition, the film thickness started to decrease. This was due to EHD starvation, and the film thickness/inlet oil meniscus relationships developed for starved oil systems were found to apply to emulsion starvation. At high rolling speeds, the EHD film thickness did not collapse to zero, but instead leveled off at a low value, typically 10 to 30 nm, which persisted to the highest rolling speeds attained. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Diego, California, October 19–21, 1992

Temperature and Scuffing

An important component of most scuffing models is the concept of a critical temperature at which the lubricant film weakens to such an extent that a significant rise in friction coefficient occurs. This then causes a further rise in contact temperature and positive feedback ensues. However, scuffing models differ as to the location of the critical temperature and the mechanism by which it weakens the lubricant film in the contact. In this paper a series of sliding tests have been carried out using steel on sapphire contacts, and the temperature of the contact and its environs has been mapped prior to and during scuffing using infrared emission radiometry. The results are compared with existing scuffing models, and the role of the contact temperature in scuffing is clarified. Presented at the 46th Annual Meeting In Montreal, Quebec, Canada April 29–May 2, 1991

The Influence of Electrochemical Potentials on the Friction and Wear of Iron and Iron Oxides in Aqueous Systems

This paper examines the influence of electrochemical potentials on the friction and wear of iron/iron and iron oxide/alumina oxide rubbing contacts, lubricated with aqueous fluids. The chemical nature of surface films on metals under electrode potential control has been investigated using simultaneous voltammetry, impedance spectroscopy and Fourier transform infrared (FTIR) microspectroscopy. This approach has enabled the friction and wear behavior of rubbing contacts with well-defined and controlled surface compositions to be investigated as a function of electrode potential. It has been shown that electrochemical potentials influence friction coefficient in two separate ways, i.e., by modifying the effective inter-surface normal force via electrochemical double layer effects and by controlling the surface chemistry and then the shear strengths of the films present on the rubbing surfaces. Presented at the 48th Annual Meeting in Calgary, Alberta, Canada May 17-20, 1993

Boundary Lubrication and Boundary Films

Despite many years of research, boundary lubrication remains the least well understood regime in the Tribological pantheon. This is in part because it is a broad church, spanning a wide range of quite different phenomena which are, in practical terms, very difficult to disentangle. Also the processes involved in boundary lubrication are very localised, hidden within the impenetrable walls of a contact, far away from the gaze of the curious scientist. This review paper examines our current understanding of boundary lubrication and boundary films and, in particular, the mechanism and nature of friction-reducing, oiliness films. It shows how modern techniques for looking in-situ at boundary films, either directly within contacts or out of contact but whilst still covered with supernatant oil show great promise in advancing our understanding of such films. Up to the present, these techniques have tended to confirm that there are several different mechanisms by which oiliness films are formed and that each of these may have a role to play in boundary lubrication.