On the mechanism of ZDDP antiwear film formation

Zinc dialkyldithiophosphate additives are used to control wear and inhibit oxidation in almost all engine oils as well as many other types of lubricant. They limit wear primarily by forming a thick, protective, phosphate glass-based tribofilm on rubbing surfaces. This film formation can occur at low temperatures and is relatively indifferent to the chemical nature of the substrate. There has been considerable debate as to what drives ZDDP tribofilm formation, why it occurs only on surfaces that experience sliding and whether film formation is controlled primarily by temperature, pressure, triboemission or some other factor. This paper describes a novel approach to the problem by studying the formation of ZDDP films in full film EHD conditions from two lubricants having very different EHD friction properties. This shows that ZDDP film formation does not require solid-solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model. The shear stress present in a high pressure contact can reduce the thermal activation energy for ZDDP by at least half, greatly increasing the reaction rate. This mechanism explains the origins of many practically important features of ZDDP films; their topography, their thickness and the conditions under which they form. The insights that this study provides should prove valuable both in optimising ZDDP structure and in modelling ZDDP antiwear behaviour. The findings also highlight the importance of mechanochemistry to the behaviour of lubricant additives in general

Stress-augmented thermal activation: Tribology feels the force

In stress-augmented thermal activation, the activation energy barrier that controls the rate of atomic and molecular processes is reduced by the application of stress, with the result that the rate of these processes increases exponentially with applied stress. This concept has particular relevance to Tribology, and since its development in the early twentieth century, it has been applied to develop important models of plastic flow, sliding friction, rheology, wear, and tribochemistry. This paper reviews the development of stress-augmented thermal activation and its application to all of these areas of Tribology. The strengths and limitations of the approach are then discussed and future directions considered. From the scientific point of view, the concept of stress-augmented thermal activation is important since it enables the development of models that describe macroscale tribological performance, such as friction coefficient or tribofilm formation, in terms of the structure and behaviour of individual atoms and molecules. This both helps us understand these processes at a fundamental level and also provides tools for the informed design of lubricants and surfaces

The influence of slide–roll ratio on ZDDP tribofilm formation

The anti-wear performance and action mecha- nisms of zinc dithiophosphate (ZDDP) have been investi- gated under various test conditions. The Mini Traction Machine–Space Layer Imaging (MTM–SLIM) is a widely used and useful method for monitoring tribofilm formation by ZDDPs. However, tests are generally carried out in mixed sliding–rolling conditions, typically between 50 % SRR (slide–roll ratio) and 100 % SRR. In this paper, the authors describe an investigation of ZDDP film formation at SRRs much higher than 100 % SRR, including pure sliding conditions using a novel MTM–SLIM technique. At high SRRs, ZDDP tribofilms form without damaging the ball surfaces so long as both surfaces move above a threshold speed with respect to the contact, regardless of whether the two surfaces move in the same or opposing directions. In pure sliding conditions, although the worn area expands with time under pure sliding conditions showing that wear takes place, tribofilms are still built up throughout a test and the ZDDP has a beneficial effect on wear rate. The very early stages of film formation are studied to show that a tribofilm with a high concentration of S is formed initially and then replaced with a film having a high concentration of Zn and P

Triboelectrochemistry: influence of applied electrical potentials on friction and wear of lubricated contacts

Research on the effects of applied electrical potential on friction and wear, a topic sometimes termed “Triboelectrochemistry”, has been reviewed. Historically, most such research has focussed on aqueous lubricants, whose relatively high electrical conductivities enable use of three-electrode electrochemical kinetic techniques, in which the electrode potential at a single electrode | fluid interface is controlled relative to a suitable reference electrode. This has led to identification of several different mechanisms by which applied electrode potentials can influence friction and wear. Of these, the most practically important are: (i) promotion of adsorption/desorption of polar additives on tribological surfaces by controlling the latters’ surface charges; (ii) stimulation or suppression of redox reactions involving either oxygen or lubricant additives at tribological surfaces. In recent years, there has been growing interest in the effects of applied electrical potentials on rubbing contacts lubricated by non-aqueous lubricants, such as ester- and hydrocarbon-based oils. Two different approaches have been used to study this. In one, a DC potential difference in the mV to V range is applied directly across a thin film, lubricated contact to form a pair of electrode | fluid interfaces. This has been found to promote some additive reactions and to influence friction and wear. However, little systematic exploration has been reported of the underlying processes and generally the electrode potentials at the interfaces have not been well defined. The second approach is to increase the conductivity of non-aqueous lubricants by adding secondary electrolytes and/or using micro/nanoscale electrodes, to enable the use of three-electrode electrochemical methods at single metal | fluid interfaces, with reference and counter electrodes. A recent development has been the introduction of ionic liquids as both base fluids and lubricant additives. These have relatively high electrical conductivities, allowing control of applied electrode potentials of individual metal | fluid interfaces, again with reference and counter electrodes. The broadening use of “green”, aqueous-based lubricants also enlarges the possible future scope of applied electrode potentials in tribology. From research to date, there would appear to be considerable opportunities for using applied electrical potentials both to promote desirable and to supress unwanted lubricant interactions with rubbing surfaces, thereby improving the tribological performance of lubricated machine components

Particulate lubricants in cosmetic applications

Polymer powders are commonly added to cosmetic formulations to improve product performance and skin feel. This study investigates the effect of particle concentration and size on the lubricating properties of powder suspensions. Results are reported for various particle sizes and concentrations. When the tribological contact was fully immersed the addition of particles had no effect. However different behaviour was observed when the contact was only partially lubricated. In this case, a three-stage friction coefficient curve was observed. By varying the particle size and concentration it was shown that the duration and magnitude of each stage can be controlled