Langmuir-Blodgett films in high-pressure rolling contacts

Ultrathin film interferometry has been used to measure the film thickness of deposited Langmuir-Blodgett layers of stearic acid in a rolling, high pressure, ball on flat contact.Deposited multilayers up to five monolayers thick have been found to remain in a vertically-oriented configuration in static contact up to Hertzian pressures of at least 0.7 GPa. It has been shown that one deposited monolayer is extremely durable in rolling conditions and continues to show a thickness of at least 2 nm (the vertically-oriented monolayer) even at high speeds and in the presence of supernatant hydrocarbon solvent. Multilayers showed variable behavior. Two monolayers rapidly reduced to one, while three initial monolayers stabilized after rolling at a film thickness corresponding to between one and two monolayers. A four monolayer initial film reduced only slightly in thickness during rolling, to reach a level corresponding to three monolayers.The results have been interpreted in terms of the transfer of one or more deposited molecular layers from the disc to the ball surface<br/

Optimizing film formation by oil-in-water emulsions

Oil-in-water (O/W) emulsions are widely used in metal rolling and cutting and also as fire-resistant hydraulic fluids. For both of these applications they require a reasonable degree of lubricating ability—in metal rolling to protect the surfaces against wear and seizure and to provide a controlled value of friction, and in hydraulics to limit the extent of wear, scuffing and fatigue of rubbing pump components.The lubricating properties of O/W emulsions are significantly poorer than those of most water-free oils and this deficiency can create service problems, such as excessive wear or rolling contact fatigue of hydraulic pumps or the uneven rolling or heat streaking of metal strip. Considerable work has gone into the design of O/W emulsions which have acceptable lubricating properties combined with satisfactory emulsion stability and corrosion resistance. Much of this work has been empirical, although there have also been a number of more fundamental studies of the film-forming and friction properties of emulsions. The latter have shown that the ability of O/W emulsions to form films in rubbing contacts is very dependent on the rolling speed of the contact and also the composition of the emulsion.This paper describes a study of the relationship between the composition and properties of O/W emulsions and the extent to which they form elastohydrodynamic films in contacts over a range of speeds. By applying surface chemical principles to maximize the tendency of the oil in O/W emulsions to wet the surfaces, it is shown how the composition of an O/W emulsion can be optimized to yield satisfactory film formation and, thus, low friction and wear over a wide range of speed conditions<br/

The Influence of Soot and Dispersant on ZDDP Film Thickness and Friction

The effect of dispersed soot in engine oils is an increasingly important issue in terms of both engine durability and fuel efficiency. Using carbon black as a soot analogue, a study has been carried out to investigate the main factors that determine the impact of soot on friction and ZDDP film formation in formulated oils. It has been found that dispersed carbon black can rapidly remove ZDDP reaction films by abrasion. However, this removal can be prevented or limited by the choice of an optimal dispersant additiv

Mechanisms of oiliness additives

Ultrathin film interferometry has been used to measure the boundary film-forming behaviour of long chain, carboxylic acid oiliness additives. It has been shown that in dry conditions, these acids form very thin films of around 2–3 nm thickness. However when water is present, some acids form significantly thicker films, around 10 nm in thickness. The behaviour of these films is very similar to that previously seen with metal carboxylate additive films, including thick film collapse at high rolling speeds followed by film reformation at slow speeds. It is suggested thick films formed by long chain carboxylic acid additives result from reaction of the acids at the rolling solid surfaces in the presence of water to form deposits of insoluble iron carboxylate

Film thickness and friction properties of oiliness additives

The boundary lubricating properties of long chain carboxylic acid additives in solution have been studied using a combination of optical interferometry, friction measurement and electrochemistry. This shows that thin, monolayer-scale films are formed under reducing conditions. These films are very effective at limiting friction and wear. In oxidising conditions much thicker films formed, which reduce friction in the mixed regime but not in true boundary lubricatio

Hydrodynamic friction reduction in a MAC-hexadecane lubricated MEMS contact

10.1007/s11249-012-0056-zTribology Letters491217-22

Friction reduction in low-load hydrodynamic lubrication with a hydrophobic surface

A novel tribometer capable of measuring low friction forces and low loads at high speeds has been employed to measure the friction coefficient in a pure sliding, ball-on-flat contact in hydrodynamic lubrication conditions. The tribometer was custom-built for measuring friction at low loads, to allow the authors to investigate the feasibility of using the liquid-slip phenomenon for the lubrication of high-sliding MEMS. The theory behind lubrication with liquid slip and its effect on friction is briefly discussed. Contacting surfaces were treated to create hydrophobic/hydrophilic or hydrophilic/hydrophilic pairs. Hydrophobic surfaces were made by coating mica with a self-assembled silane monolayer while the hydrophilic surfaces used were freshly cleaved mica and plasma-cleaned steel. Experiments were conducted at sliding speeds of up to 2 m/s and loads below 0.2 N. An aqueous glycerol solution was used as lubricant. Results obtained with hydrophilic/hydrophilic surfaces were in accord with hydrodynamic lubrication theory. Tests with hydrophobic/hydrophilic surfaces revealed a reduction in friction, which may be attributed to lubricant slip against the hydrophobic surface.<br/

The effect of emulsifier concentration on the lubricating properties of oil-in-water emulsions

Although the use of oil-in-water (O/W) emulsions as metalworking fluids is widespread, the mechanisms of emulsion lubrication are not yet well understood. Several theories have been proposed but there is not a clear agreement about the effect of different operating conditions and emulsion properties on the lubricating performance of O/W emulsions. In the present study, the film forming ability of O/W emulsions as a function of emulsifier concentration is studied. The emulsifier content exerts a strong influence on all the emulsion properties, such as stability, droplet size distribution, surface and interfacial tension, wetting ability, etc., as well as on the lubricating behaviour, so it has been used to ascertain the relationship between all the properties involved. Three different emulsifiers—anionic, nonionic and cationic—were used at different concentrations in the design of lubricant O/W emulsions. Experimental results show that the work of adhesion of oil droplets on the metal surface is a valuable parameter to predict the ability of emulsions to form thick films in elastohydrodynamic (EHD) contacts. The influence of pH value of O/W emulsions on their lubricating behaviour is also verified. The overall conclusion is that the interactions between metal and oil droplets rule the mechanism of lubrication and that this interaction is primarily controlled by emulsifier concentration