Gordon D. Lamb

On the Increase in Boundary Friction with Sliding Speed

During the last sixty years there have been consistent reports in the literature that in the boundary lubrication regime, some, but not all, solutions of organic friction modifiers give extremely low friction at very low sliding speed, that then increases linearly with the logarithm of sliding speed. This article first reviews some previous studies that show this phenomenon and describes the main mechanisms proposed to explain it. New friction-sliding speed data are then presented, which show that an increase in friction with sliding speed occurs with saturated alkyl chain organic friction modifiers but not with unsaturated chain, oleyl-based ones, at the concentrations studied. It is, however, shown that elaidic acid, the trans-isomer of oleic acid gives friction that increases with sliding speed. A key difference between these two compounds is that the cis arrangement of carbon–carbon bonds around the double bond of oleic acid means that the molecule cannot easily adopt a linear configuration, while elaidic acid can. This suggests that the ability of an organic friction modifier to produce friction that increases with sliding speed originates from its ability to form close-packed layers on steel surfaces. Importantly, even though oleyl derivatives do not show friction that increases with sliding speed, they still reduce friction quite significantly over the sliding speed range studied, although to a lesser extent than their saturated analogues, especially at low sliding speeds.

Pyrolysis-GC and MS applied to study oligomer formation in the degradation of polystyrene and styrene copolymers

Abstract Polymer pyrolysis measurements can have quantitative significance only if the method of pyrolysis and the method of analysis have been carefully chosen to be appropriate for the problem being studied. Factors relevant to these choices are briefly discussed, with special reference to the technique of pyrolysis-gas chromatography. General methods of obtaining mechanisms of pyrolysis by this technique are reviewed. Recent results on the application of the method to study oligomer formation in the degradation of polystyrene and some styrene copolymers are presented in more detail. The overall conclusion from this work is that the intramolecular transfer mechanism, commonly assumed to be responsible for oligomer formation, is not compatible with the results obtained from polystyrene and its copolymers in certain situations. There is some evidence to suggest that dimers and trimers are possibly formed by monomer recombination, occurring both within the molten polymer and in the vapour phase above it.

Durability of ZDDP Tribofilms Formed in DLC/DLC Contacts

The tribological properties of diamond-like carbon (DLC) coatings have drawn much attention of OEMs and lubricant manufacturers in recent years. It is important to know whether conventional friction modifier and antiwear additives can form durable films and work as effectively with DLCs as they normally do on steel surfaces. In this study, the film-forming and friction properties of the antiwear additive ZDDP and the strength of tribofilms formed by this additive on five widely used DLC types, namely a-C:H, a-C:H:W, a-C:H:WC, Si-DLC and ta-C, have been investigated. It is found that ZDDP-derived tribofilms form on all the DLCs but exhibit different friction characteristics based on DLC type. With all DLCs, the amount of tribofilm elements measured after durability tests was less than that measured initially. Over 90xa0% of thiophosphate and 70xa0% of sulphide/sulphate were lost during durability tests. ZDDP tribofilms were found to be strongly adhered on Si-DLC and a-C:H compared with the other DLCs. The ZDDP tribofilms formed in DLC/DLC contact appear to be similar in structure to those formed in steel/steel contact but not to exhibit the antiwear performance seen in steel/steel contacts.

Polymer degradation mechanisms: New approaches

Abstract In studying the thermal degradation of polymers, experimental problems such as that of taking the samples rapidly to chosen degradation temperatures can be solved by appropriate filament design and the use of sufficiently small samples. The pyrolysis-GC technique can then provide reliable quantitative data which can be used in the following ways to obtain mechanistic information about degradation processes. (I) Dependence of rate constants for volatiles evolution on the initial molecular weight of the sample, and on the pyrolysis temperature, can give specific information about the mechanisms of initiation and temination in chain degradation processes. (II) Direct comparisons of observed rate constants for volatiles evolution can indicate whether all observed products are primary products. (III) Anomalous dependence of product yields with decreasing sample thickness can give information about diffusion control of degradation rates. (IV) Anomalous dependence of product yields and ratios which increases with increasing sample thickness can distinguish primary, secondary, and even tertiary products, and can moreover lead to the interpretation of the overall pyrolysis mechanisms in terms of a reaction scheme involving parallel and consecutive reactions. All of the above are illustrated with examples from studies by the authors on a number of polymeric systems.

Sample size dependence in pyrolysis: An embarrassment, or a utility?

Abstract Product ratios, fractional conversions, and rates of pyrolysis may show confusing variations with sample size or thickness. There are several reasons for this, amongst which heat transfer and temperature gradients across the sample are important. To obtain meaningful results, a suitable approach is to reduce sample size to find a sample thickness range over which the reaction parameters are independent of the size. The objective in this approach is thereby to eliminate any effects of sample size. Recent work however has shown that investigations of the dependence of reaction parameters on sample size may reveal mechanistic features of the pyrolysis process itself. Two such features are: (a) secondary reactions which occur during the residence time of primary products in the melt, and (b) diffusion-restriction of the interaction of long-chain radicals, when the thickness of the degrading film is comparable with the diameter of the hydrodynamic volume of the polymer molecules. Both types of study have been performed on a range of polystyrene and PMMA samples.

Spurious Mild Wear Measurement Using White Light Interference Microscopy in the Presence of Antiwear Films

Commercial scanning white light interference microscopes are able to map surface topography to very high resolution and can thus measure very small amounts of wear on rubbed surfaces. These instruments are therefore being increasingly used to study aspects of mild wear. It is shown in this Technical Note that, when used on surfaces that have been rubbed in lubricants containing the antiwear additive zinc dialkyldithiophosphate (ZDDP), some white light interference microscopes can produce an apparent but spurious wear measurement. The transparent and relatively thick antiwear film generated by ZDDP on rubbed surfaces produces an optical path difference that is incorrectly interpreted as wear. It is thus suggested that surfaces that have been rubbed in lubricants containing ZDDP or other thick film-forming additives should be treated to remove tribofilms prior to wear measurement using scanning interference microscopy.

In Situ Study of Model Organic Friction Modifiers Using Liquid Cell AFM; Saturated and Mono-unsaturated Carboxylic Acids

Fatty acids and their derivatives have been used as model organic friction modifiers for almost a century, but there is still debate as to the nature of the boundary films that they form on rubbed surfaces. In this study, in situ liquid cell atomic force microscopy (AFM) is used to monitor the self-assembly of boundary films from solutions of fatty acids in alkanes on to mica surfaces. Because the mica surfaces are wholly immersed in solution, it is possible to study directly changes in the morphology and friction of these films over time and during heating and cooling. It has been found that stearic acid and elaidic acid, which are able to adopt linear molecular configurations, form irregular islands on mica that are tens to hundreds of microns in diameter and typically 1.6xa0nm thick, corresponding to domains of tilted single monolayers. At a relatively high concentration of 0.01xa0M, stearic acid in hexadecane forms an almost complete monolayer, but at lower concentrations, in dodecane solution and for elaidic acid solutions, these films remain incomplete after prolonged immersion of more than a day. The films formed by fatty acids on mica are displaced by repeated scanning in contact mode AFM but can be imaged without damage using tapping mode AFM. Rubbed quartz surfaces from a sliding ball-on-disc test were also scanned ex situ using AFM, and these showed that stearic acid forms similar monolayer island films on quartz in macro-scale friction experiments as are found on mica. Oleic acid solutions behave quite differently from stearic acid and elaidic acid, forming irregular globular films on both mica and rubbed quartz surfaces. This is believed to be because its cis-double bond geometry means that, unlike its trans-isomer elaidic acid or saturated stearic acid, it is unable to adopt a linear molecular configuration and so is less able to form close-packed monolayers.

In Situ Study of Model Organic Friction Modifiers Using Liquid Cell AFM: Self-Assembly of Octadecylamine

AbstractnLiquid cell AFM has been applied to study in situ the formation and properties of self-assembled films formed on mica surfaces by octadecylamine from alkane solution. Mica surfaces immersed in hexadecane or dodecane at room temperature show no identifiable surface films. However, when octadecylamine solution is injected into the cell, a boundary film forms almost immediately. This film takes the form of irregular islands of mean diameter approximately 1–3xa0µm and thickness typically 1.5xa0nm when measured in contact mode. These islands are believed to correspond to patches of vertically oriented but tilted octadecylamine or ammonium salt held together primarily by van der Waals forces between adjacent alkyl chains. These films are quite labile in that during scanning of the tip in both tapping mode and contact mode changes to the shape of the islands take place, including consolidation of the island density in the scanned region and depletion from around this area. In situ experiments in which the temperature of the cell is varied over time show that the initially formed islands disappear at a temperature of ca. 35xa0°C but are reformed when the cell is re-cooled. Similar tests on samples that remain immersed in solution for extended periods show more stable films, with islands being lost only above about 50–60xa0°C. The work shows that liquid cell AFM has great promise for studying the formation and properties of the boundary films formed by organic friction modifiers.

New Method of Measuring Permanent Viscosity Loss of Polymer-Containing Lubricants

An ultrahigh shear rate viscometer (USV) was used to measure the viscosity of polymer solutions. It was found that some polymer solutions in base oil, including those used as engine oil viscosity modifiers, show permanent viscosity loss when subjected to very high shear rates above 106 s−1. The USV was modified to automatically carry out a series of viscosity measurements on the same test lubricant sample. This enabled the accumulation of permanent viscosity loss to be measured over successive strain cycles. As expected, permanent viscosity loss increased with both strain rate and molecular weight. When carried out at 5 × 106 s−1 and 100°C, the test was more severe than the Kurt Orbahn test because samples of lubricants subjected to the latter underwent further shear thinning in the USV. The USV test appears to be a rapid and convenient way to quantify the permanent viscosity loss of polymer-containing lubricants for engine use, and a protocol to assess permanent viscosity loss (PVL) and permanent shear stability index (PSSI) based on viscosity measurements at 106 s−1 before and after shear thinning is outlined. The study also shows that it is important to take into account possible permanent viscosity loss when measuring the viscosity of polymer solutions in very high shear rate viscometers such as the USV. This can be done by minimizing the amount of shear to which the lubricant is subjected or by taking successive measurements and subtracting the permanent viscosity loss taking place in each of the first few strain rate cycles.

Effect of oil rheology and chemistry on journal-bearing friction and wear

Legislation and market pressures are calling for increased engine power, reduced engine size, and improved fuel consumption. The use of low-viscosity lubricants is considered as a means to enhance fuel economy by reducing viscous friction, particularly in engine bearings. Journal bearings mostly operate under hydrodynamic lubrication with a thin film of oil separating the journal and bearing shell. There are, however, certain conditions, especially under high load or low speed, when the film thickness will be low enough to allow boundary lubrication to occur. In this study, the effect of lubricants with different viscosities, different types of viscosity modifiers, different additives, different types of dispersants, and different lubricant formulations have been studied under hydrodynamic and boundary lubrication regimes. For hydrodynamic conditions, a high-temperature high-shear viscometer, meeting the requirements of ASTM D4741 was used to measure viscosity at 106 s−1. In addition, a new ultra high-shear viscometer, from PCS Instruments, was used to measure viscosity at shear rates near to 107 s−1. Bearing weight loss and load bearing capacity were measured on a rig developed in-house using a specially designed half-bearing shell loaded against a rotating journal. A PCS journal-bearing rig was used to measure the bearing friction under transient load.