A theoretical study has been undertaken of the phenomenon of non-steady state lubrication of concentrated point and line contacts. This has been based upon a study of the effect of squeeze-film action in both hydrodynamically and elastohydrodynamically lubricated contacts. The work described in this thesis is in two main sections. The first is concerned with squeeze-film lubrication of lightly loaded point contacts where the surfaces are taken to be rigid. A complete analytical solution is developed considering both piezoviscous and isoviscous fluids. Formulae representing the relationships between the controlling parameters of hydrodynamic lubrication of point contacts with a pure squeeze action became available. In the second section general numerical solutions to the non-steady state hydrodynamic and elastohydrodynamic lubrication problems for line contacts are developed. The finite difference approximation method has been used to solve simultaneously the Reynolds, elasticity and load equations at successive time steps. To avoid convergence difficulties and reduce computing effort a tri-diagonal matrix algorithm has been incorporated. A wide range of line contact lubrication problems is considered. These include the hydrodynamic and elastohydrodynamic lubrication of cylinders with either pure squeeze-film action or under combined 'entraining' and 'squeeze-film' action. Graphical representations of pressure distributions and film shapes are presented in time sequence as the gap between the two cylinders changes. The variations of the dimensionless minimum film thickness, velocity of approach and peak pressure under various loading conditions are also included. The formation and development of the elastic indentation under constant load with squeeze-film action and with the combined effect of squeeze and entraining action has been ascertained. It has been established that the squeeze-film action plays an effective role in the enhancement of the smallest value of the minimum film thickness occurring under oscillating loading conditions. This effect is more pronounced in elastohydrodynamic than in hydrodynamic lubrication. This general analysis of time dependent lubrication problems for both hydrodynamic and elastohydrodynamic conditions permits specific studies to be undertaken of realistic machine components operating under non-steady state conditions.

Single-phase (neat oil) and two-phase (oil-in-water emulsions) lubricants are widely used in metal forming processes, where speeds as high as 20 m s-1 are reached. Most of the previous work done on both neat oil and on oil-in-water emulsions has focused on low speed behaviour (below 5 m s-1) and, as a result, the low speed behaviour of oil-in-water emulsions is well understood. Under these conditions, the lubricating oil film is composed predominantly of oil and the thickness of the film is similar to that for neat oil. However, the behaviour at high speed is entirely different. No experimental film thickness and friction results at speeds above 5 m s-1 are available for neat oil and only one study (Zhu et al., 1994) has reported the film thickness behaviour of oil-in- water emulsions above this speed whereas no friction measurements at speeds above 3.5 m s-1 have been carried out using oil-in-water emulsions. Consequently, to date, the behaviour of neat oil and the relation of emulsion composition to film forming ability at high rolling speeds could not be described. This project is aimed at investigating the mechanism of film formation and the film forming and friction properties of single-phase and two-phase lubricants in high speed rolling/sliding contacts. An EHD test rig was modified to measure film thickness and friction of oil-inwater emulsions in very high speed, rolling/sliding conditions (up to a mean rolling speed of 20 m s-1). Ultrathin film interferometry was used to investigate film thickness while infrared temperature mapping of the contact was used to obtain maps showing the rate of heat input into the surface, from which shear stresses and friction could be calculated. Light induced fluorescence was also employed using a water-soluble and an oil-soluble dye to allow visualization of the contact (at low speeds) and help in investigating the composition of the entrained lubricant at these high speeds. Results showed that, for neat oils, the major factor affecting the film formed at high speed is shear heating. For dilute emulsions at the highest speeds, the film thickness and friction are close to those obtained with pure water. More concentrated emulsions give slightly higher film thicknesses. A comparison of the results with earlier theoretical predictions was carried out. Together with the fluorescence results, this suggested that high speed leads to the entrainment of a micro-emulsion.

One of the most important requirements of an adhesive joint is the ability to retain a significant proportion of its load-bearing capability for the long periods under the wide variety of environmental conditions encountered during its service-life. There exists a need to improve the understanding of the mechanics and mechanisms associated with the durability of adhesive joints in hostile environments, such as one of the most potentially damaging and frequently encountered, water, to further their future extensive use in engineering applications. In the present research, four important aspects in relation to the durability of adhesive joints have been investigated. These were: (a) developing sound short-term accelerated test methodologies to assess the durability of adhesive joints; (b) understanding the mechanisms of environmental attack on different types of surface pretreated adhesively bonded aluminium alloy substrates; (c) developing adhesive/primer/pretreatment systems which possess excellent long-term durability; and (d) investigating the potential of environmentally-friendly organosilanes as primers to enhance the intrinsic adhesion of adhesive joints. The results from these studies showed that: (a) constant displacement rate and cyclicfatigue tests provide excellent quantitative durability test methodologies; (b) fracture mechanics and advanced surface analysis of adhesive joints have proven surface pretreatments using phosphoric acid anodising (PAA) are far superior to those employing a grit blasting and degreasing (GBD) pretreatment which can be attributed to the increased surface area and excellent bonding morphology of the anodised oxide surface, allowing deep penetration of the viscous adhesive and impeding water ingress at the adhesive/substrate interface; (c) the durability performance of PAA pretreated adhesive joints employing a primer (PAAP) are superior to those without a primer specifically in water, and acid-based surface pretreatments are significantly advanced compared to simple GBD; and (d) the self assembling long carbon-chain silanes enhance the durability of adhesive joints via the formation of covalent bonds between the adhesive and the activated silane monolayer deposited on the substrate.

Diesel engines are becoming increasingly popular in both passenger and commercial vehicles because they offer better fuel efficiency than their gasoline counterparts. However, a disadvantage of this type of engine is the high volumes of soot it produces, which can contribute to an increase in wear of the engine components. A number of wear mechanisms have been proposed to explain wear by soot, of which the abrasive mechanism is still the most widely accepted. Lubricant additives such as zinc dialkyldithiophosphate (ZDDP) are frequently used as antiwear (AW) agents but the combined effect of soot and additives such as ZDDP on wear is not well understood. The aim of the work described in this thesis is to explore the impact of soot on wear both in the absence and presence of lubricant additives, of particular interest is the additive ZDDP. A unidirectional sliding/rolling test is used to explore the impact of film-thickness on wear while a reciprocating wear tester enables accurate wear measurements for individual test lubricants. The AW additives studied, most especially ZDDP, show excellent antiwear behaviour in the absence of carbon black (CB) – used as a substitute for soot. However, once CB is combined with AW additives, an unusual wear pattern emerges and an increase in wear is observed. A new mechanism of wear by soot is therefore suggested and discussed to explain this effect.

Polymer additives, named Viscosity Modifiers (VMs), have been added to motor engine lubricants for more than 50 years, where they are used to increase the viscosity index of their blends. It is well known that solutions of VMs exhibit shear thinning under high strain rate conditions as are present in engine journal bearings. In the past, this shear thinning was regarded as undesirable since it might reduce hydrodynamic film thickness. However it has been reported that multigrade oils containing VMs provide lower engine friction than polymer-free oils of comparable viscosity and the most likely origin of this is a reduction of hydrodynamic friction due to temporary shear thinning of the polymer solutions. In this thesis, the role of polymer shear thinning in reducing hydrodynamic friction is clarified by the experimental work carried out on a range of different polymer types. This study includes the viscosity measurements over a wide range of shear rate and also friction and film thickness analyses performed in a soft elastohydrodynamic lubrication contact in order to avoid piezoviscous effects. A very recent technique based on laser induced fluorescence technology is used to evaluate the film thickness. These experimental results are compared with theoretical calculations, which allows based on the rheological data of each polymer solution, the impact of polymer shear thinning on hydrodynamic friction to be quantified. The studied polymer solutions show a decrease in film thickness and in hydrodynamic friction compared to Newtonian fluids of similar viscosity. This reduction is ascribed to the temporary shear thinning of polymer blends. The proposed theoretical models show good agreement with the experimental values. They appear to be useful tools to quantify the impact of polymer shear thinning on film thickness and friction. In addition to this main work, the polymer solutions are analysed under high contact pressure and the shear stability of some commercials viscosity modifiers are studied. A new method is developed to quantify the permanent viscosity loss of each polymer solution and this is compared to the industrial Kurt-Orbahn test.

Wet clutches are used in automatic transmissions to bring about gear changes and also to reduce energy loss in the torque converter. These friction devices are susceptible to stick-slip effects, which result in the vehicle giving an unsteady ride. Stick-slip effects can be avoided by ensuring the wet clutch and lubricant combination produces a friction coefficient that increases with sliding speed. This friction characteristic is achieved by using a specific material on one of the two clutch surfaces and by using certain surface active chemicals, which are added to the lubricant. Although wet clutches have been studied throughout the industry for many decades, the mechanism of the generated friction is still not fully understood. In this thesis the friction mechanisms are clarified by the experimental study of the wet clutch in terms of its real area of contact, its flash temperatures and the friction characteristics, which are measured over a broad range of conditions. These results are used along with theoretical calculations to first clarify the lubrication regime, which is found to be predominantly boundary due to the roughness of the friction material and the small size of the contact units formed. The generated friction is then attributed to surface active additives, which form solid-like films on the clutch surfaces. These friction characteristics can be modified by varying the nature of the solid-like film, and when a close-packed film is formed, this displays the friction increasing with speed characteristic due to an activated shearing mechanism, which is linked to the speed of molecular rearrangement at the surface.

The study of Tribology between soft or compliant surfaces is not well understood despite its importance to many biological and engineering applications, ranging from synovial joints to rubber o-ring seals. It has also been shown that the science of Tribology and lubrication in compliant contacts is an important factor in the sensory perception and functionality of skin, hair and the oral cavity, and so has an immediate application of the design of consumer products such as skin creams, hair conditioners and foodstuffs. This thesis aims to improve our understanding of thin film lubrication between soft, deformable surfaces under light loading and low-pressure conditions. The primary focus of the thesis is the development of techniques by which to measure the film thickness between compliant surfaces, from the nano- to the micro-scale. Several experimental techniques currently exist for measuring film thickness in hard, metallic contacts and these are widely employed in Tribology research of engineering systems. However they require considerable modification to be applicable to compliant contacts. This thesis describes the development of two such techniques; · a optical interferometric technique; for measuring nano-scale thicknesses in compliant contacts; · a laser induced fluorescence technique; developed to enable measurement of lubricant thickness of relatively thick films in compliant contacts.

The thesis concerns an integrated approach to investigating running−in of metallic surfaces under rolling/sliding conditions in the presence of the ZDDP anti−wear additive. Running−in occurs in the initial period of the contact process and involves rapid mechanical and chemical changes of rubbing surfaces, bulk material and lubricant, leading to reaching a steady state, in which an increased load−carrying capacity of the surfaces is achieved. Previous research suggested that anti−wear additives, such as ZDDPs, can prevent or postpone the effective running−in, preserving the relative surface roughness and maintaining the severity of asperity interactions. The friction, surface topography and the ZDDP tribofilm growth were measured in a number of rolling/sliding experiments carried out with an MTM tribotester. The specimens were varied according to their surface finish and hardness. To assess the influence of running−in on the change of lubrication regime, Stribeck curves were obtained at certain intervals during the test. Additionally, chosen worn surfaces and wear debris were analysed by a range of electron microscopy techniques. A novel method of wear calculation of rough surfaces was employed, based on a comparison of the 3D topography scans of the initial and worn surfaces. ZDDP films were chemically removed from the specimen surfaces using ethylenediaminetetraacetic acid (EDTA) sodium salt solution to ensure the correct results of the optical topography measurements. Studying running−in required the tests to be periodically interrupted, e.g. for topography measurements, and such interruptions were found to affect the wear, roughness and friction results; however, the main trends remained unchanged. A combination of AISI 1013 carbon steel and AISI 52100 bearing steel specimens was tested first, and the former were found unsuitable for use in this study, possibly due to the influence of the silicon carbide particles embedded in their surface on the wear mechanisms involved. As a result, the specimens made of AISI 52100 steel, but of different hardness, were used in the further testing. A transition from boundary to mixed lubrication regime as a result of running−in, evident in the friction and electrical contact resistance results, was found only in the tests carried out without the additive. In the presence of ZDDP, the friction was higher than in the corresponding base oil tests, and the highest increase was observed in the mixed regime. The friction changes with time did not correlate with the increase of the tribofilm thickness, which suggested that the friction depended on the surface coverage with the tribofilm rather than its thickness. However, the possible influence of the film roughness on the surface topography was not investigated. While the rate of the film formation could be related to the value of the lambda ratio, the tribofilm removal was found to depend on the direction of the roughness lay. The asperity−level surface conformity was assessed using a cross−correlation analysis technique and it was found to be influenced by the orientation of the surface finish. Additionally, the EHL analysis of the profiles of a rough surface taken at different times during the test was used to observed the evolution of the contact pressure distribution with running−in. The optical phenomena involved in the spurious wear effect, previously found in the results of WLI topography measurements of surfaces with ZDDP films, were studied with a WLI scanning microscope and a confocal system. However, this effect was not observed in the results from the tempered AISI 52100 steel specimens, possibly because of the presence of the temper oxide on the surface. The reflectivity of the top layer of the film was low compared to the surface under the film. Under certain measurement conditions, the observed effect was assumed to be caused predominantly by a difference of the refractive index between the tribofilm and air. Thus, on the basis of the height difference between the surface with a film and after the film removal, and corrected for the difference in refractive index, the estimated thickness of the tribofilm was found for every measurement point. The resultant film thickness was within the range expected of a typical ZDDP tribofilm, and for a rough surface it was higher on the tops of asperities. A closer examination of the film properties and optical system design could be a foundation of a new film thickness measurement method, which would require accurate information to be obtained from both top and bottom surfaces of the film.

Micropitting was studied using a three-contact disc machine having a central roller in contact with three harder, annular counter-discs (“rings”) of precisely controlled roughness. Roughness, running conditions, base stock and additive concentration were varied. The response of the same lubricants in a reciprocating sliding wear test operating in the boundary regime was also studied. It was reported in previous work that when a common antiwear additive, secondary zinc dialkyl-dithio-phosphate (ZDDP) is used in solution into a mineral oil, it provides enough protection to the rough surfaces leading to micropitting. In this work, different type of roughness finish, effect of lambda ratio on micropitting, effect of the antiwear concentration, as well as the effect of a friction modifier were studied. Results of experimental studies of the rolling contact behaviour of carburised steel rollers are reported. All the tests with the additive present led to micropitting. This being true for both longitudinal and transverse finish. However, transverse led to most severe case of micropitting wear. For different film thickness, micropitting was observed for various lambda ratio, however, micropitting wear was only observed for a calculated film thickness below 12% of the centre-line average roughness of the rings. It was found that there was an approximately inverse correlation between the micropitting damage in the disc machine test and the mild wear in the reciprocating sliding test. The surface profiles were measured at given intervals during each test and the result confirmed that the nature of the antiwear additive slow-down the run-in of rough surfaces contributing to the onset of micropitting. Indeed, antiwear additives suppress the gradual smoothing of the rough surfaces, which takes place when a pure base stock is used under mild conditions. For cases closer to industry, it is common to use a combination of additives for various reasons. Indeed, antiwear and friction modifier additives are often used in formulated oils, a combination which leads to reduced boundary friction. The effect of the reduction of the friction at the asperity level was studied. A common friction modifier agent, commonly known as molybdenum dithio-carbamate (MoDTC), was added in solution to a mineral oil containing the antiwear additive. It was found that for the oil containing both ZDDP blended with MoDTC the surface of the tested roller exhibited micropitting wear. However, this micropitting wear gradually disappeared with continued running; this was not the case when the mineral oil was tested with ZDDP alone. The optical micrographs of the damaged surfaces showed that the subsurface cracks disappeared on the roller tested with ZDDP and MoDTC. After the test, further surface analysis of the counter-discs showed the presence of MoS2 deposits on the asperity crests. The oil containing the friction modifier was tested using an MTM tribotester and this test confirmed the reduction in friction for a regime close to boundary lubrication. It is speculated that the improved micropitting behaviour resulted from the effect of a reduction in local tensile stress due to reduced asperity friction. This may have reduced the opening of the surface cracks and inhibited their extension.

ZDPs (zinc dialkylphosphates) are sulphur-free analogues of the conventional antiwear additives ZDDPs (zinc dialkyldithiophosphates). Recently, the application of ZDPs in practical use has been investigated as an alternative of ZDDPs. The key benefit of ZDPs is that they do not contain sulphur which may damage some catalysts in after-treatment exhaust systems. Utilizing ZDPs instead of ZDDPs contributes to decrease in sulphur content in engine oils. In addition, ZDP-formulated oils show better base number retention than ZDDPs, which could contribute to prolongation of engine oil life. This is because ZDPs do not generate sulphuric acid in their decomposition process, while ZDDPs generate sulphuric acid which consumes detergents. However there is still little known about the mechanism of action of ZDPs as antiwear additives. Therefore, the purpose of this study is to clarify how ZDP antiwear additives work by investigating the film forming and friction properties of ZDPs. The main technique employed was MTM-SLIM (mini traction machine - spacer layer interferometry). This machine can measure friction properties under various rubbing conditions and monitor film formation by additives during rubbing tests. This study has shown that ZDPs form stable reaction films by rubbing, which protect metal surfaces from wear. This is fundamentally similar to the mechanism for antiwear performance of ZDDPs. However, some differences between ZDPs and ZDDPs are seen in their filmforming properties, which may affect their antiwear performances. For example, the filmforming rates of ZDPs are slower than those of ZDDPs at the initial stage. Stabilized films formed by ZDPs are thinner than films formed by ZDDPs. By contrast, ZDPs show similar friction properties to ZDDPs. In the boundary and mixed lubrication regions, there is no noticeable difference in friction between stabilized films formed by ZDPs and ZDDPs. In addition, these properties are largely affected by various factors such as rubbing conditions, concentrations, and alkyl structures of the additives. For example, ZDPs having linear alkyl chains show slower film formation and lower boundary friction than ZDPs having branched alkyl chains. Moreover, other additives such as dispersants and detergents largely affect the properties of ZDPs. Generally, highly polar additives appear to retard the film formation of ZDPs. The mechanisms for the above features are also discussed in this thesis.