Ultrathin Film Interferometry Research Articles

Processes of molecular adsorption and ordering enhanced by mechanical stimuli under high contact pressure

Adsorbed molecular films, referred to as boundary films in tribology, are widely used in various industrial products as a keyway for surface functionalisation, such as lubricity, wettability, and adhesion. Because boundary films are thin nanometre-scale molecular layers and can easily be removed, their formation process cannot be elucidated in detail. In this study, to analyse the growth dynamics of boundary films, the film thickness and molecular orientation of the boundary film of a fatty acid used as an additive in rolling contact as mechanical stimuli were measured in situ. The measurements were performed on simple test lubricants, which were composed of n-hexadecane and stearic acid, at rolling tribological condition between steel and glass (or sapphire) surfaces by ultrathin film interferometry combined with sum-frequency generation spectroscopy according to a unique protocol. The results quantitatively demonstrate shear-induced boundary film formation. The insight gained from these results is anticipated to enable the formulation of high-performance lubricant additives to further reduce friction loss and high-performance glues that can be freely designed for removability.

Process of Boundary Film Formation from Fatty Acid Solution

Ultrathin-film interferometry was used to investigate the developing process of boundary films in a steel-on-silica contact lubricated by a solution of n-hexadecanoic acid dissolved in n-hexadecane. Immersion of the surfaces in the solution is not sufficient to initiate boundary film formation. However, upon a nominally pure rolling contact, boundary film formation is initiated, and, once initiated, the boundary film develops spontaneously. It is believed that the initial monolayer film adsorbed strongly on the solid surface induces subsequent molecular stacking from the solution. The developed boundary film can be partially destroyed by the rolling contact, although the nominally identical rolling contact acts as the trigger of boundary film formation. These findings reveal that the immobile boundary films to reduce friction of solids under high pressure result from the equilibrium of two competing processes: spontaneous development by molecular stacking and mechanical destruction by relative motion of the contact surfaces.

Stribeck Friction Curve in Point EHL Contacts

The ultra thin film interferometry and conventional optical interferometry techniques are used to measure the central film thickness and friction force as a function of the dimensionless speed parameter. Experiments were conducted in point contacts with a mineral bright stock oil and a disk-like chemical compound by changing the temperature, entrainment velocity and surface roughness at a fixed slide-roll ratio. It has been found that the friction coefficient curve, i.e., the relationship between the friction coefficient and the dimensionless speed parameter, changes depending on lubrication regimes, even when the relationship between the central film thickness and the dimensionless speed parameter obeys EHL theory.

A study of lubrication mechanism of oil‐in‐water (O/W) emulsions in steel cold rolling

PurposeOil‐in‐water (O/W) emulsion has been used in industrial rolling mills for many decades, but its lubrication mechanism is still not adequately understood. There is a need to understand the role of chemical ingredients and emulsifier in lubrication and tribological characteristics of rolling oil. With this purpose, the authors selected three commercially available O/W emulsions of different generations and of known industrial performance. The aim is to understand the lubrication mechanism of these rolling oils and to correlate the laboratory findings with that of industrial rolling mills.Design/methodology/approachThe lubrication mechanism has been studied with the help of an ultra thin film interferometry EHD test rig, an advanced experimental rolling mill and a Coulter LS 230 instrument. Film thickness, rolling parameters and droplet size were measured. The coefficient of friction was computed with the help of the measured values of rolling parameters. Emulsion stability and saponification value (SAP) of the selected emulsions were also determined. The results of film thickness, rolling parameters and droplet size have been presented. The lubrication mechanism of the emulsions has been explained on the basis of film thickness, droplet size, emulsion stability, SAP value and coefficient of friction.FindingsResults of the present study reveal that chemistry of O/W emulsions plays an important role in their film forming and tribological behavior. Rolling emulsions of relatively low stability, higher droplet size and high SAP value are found to provide better lubrication and lower coefficient of friction. The results of the present study correlate well with the actual industrial experience except those obtained on EHD test rig.Research limitations/implicationsCoulter LS 230 instrument was available with M/s LUBRIZOL CORP., USA. Only limited study on droplet size was carried. Although the study carried out has given good information but it would have been more practical if the emulsion samples taken from the experimental mill stand would have been studied for droplet size.Practical implicationsFrom understanding point of view of lubrication mechanism of O/W emulsion, it will be useful for oil technologists, tribologists and rolling mill users.Originality/valueThe study is original in nature and gives information on lubrication mechanism of O/W emulsions in steel cold rolling of steel strips.

Study of Zinc Dialkyldithiophosphate Antiwear Film Formation and Removal Processes, Part II: Kinetic Model

Zinc dialkyldithiophosphate (ZDDP) film thickness measurements made using in situ ultrathin-film interferometry and described in Part I of this two-part paper (Fujita, et al. [1]), have been used to develop and test kinetic models of antiwear film formation and removal. The main component of ZDDP film formation involves the gradual coverage of the surfaces by thick, discrete islands of film material. This process can be modeled by combining a simple coverage model in which the rate of film formation is proportional to the fraction of surface not yet covered, with an induction period. The process of film removal can be modeled by assuming that the rate of film loss is proportional to the fourth power of the coverage or film thickness. The combination of these film formation and removal rate equations is able to predict the complex, transient maximum film-forming behavior of secondary ZDDP as well as the process of film formation by primary ZDDP and the removal of antiwear film by dispersant additive. Presented at the STLE Annual Meeting in Toronto, Ontario, Canada May 17-20, 2004 Review led by Elaine Yamaguchi

The Behavior of Diluted Sooted Oils in Lubricated Contacts

An increase in the engine lubricant soot levels has recently been noticed. This increase in soot content of lubricating oils has caused a series of problems in the overall performance of the engine. In a practical context, sooted oils consist of carbonaceous matter suspended in engine oil forming a system commonly known as a colloid sol. The objective of this paper is to better understand the mechanism of action of oil containing soot particles in the lubricated contacts and to identify how the colloidal nature of the sooted oil is related to its performance. This study has shown by means of ultra-thin film interferometry and image analysis techniques that soot colloid particles are entrained in the contact inlet where they can influence the friction and wear characteristics of the base stock. This study shows that soot primary particles are entrained into the contact at slow speeds, affecting the film characteristics of clean engine oils. This entrainment of particles is more pronounced at high temperatures.

超薄膜光干渉法によるナノ摩耗の測定(J23-3 マイクロナノ理工学(3),J23 マイクロナノ理工学)

超薄膜光干渉法によるナノ摩耗の測定(J23-3 マイクロナノ理工学(3),J23 マイクロナノ理工学)

Compression of a Single Transverse Ridge in a Circular Elastohydrodynamic Contact

A detailed experimental study has been made of the behavior of a 100 nm high transversely oriented ridge in an elastohydrodynamic (EHD) contact. Ultra-thin film interferometry has been used to measure film profiles accurately over a very wide range of lubricant film thicknesses, from a few nanometers up to nearly one micron. This enables the recovery of the amplitude of the inlet perturbation geometry with increasing EHD film thickness to be quantified and compared with numerical predictions. In pure rolling under very thin film conditions, corresponding to a smooth surface EHD film thickness of 10 nm, the surfaces near the ridge were squashed down, leading to a constriction in the film of only about 9 percent of the height of the un-deformed ridge. As the EHD film thickness increased, this deformation recovered until the ridge constriction regained about 90 percent of its original height at film thicknesses of about 1 μm. However this relatively rapid recovery only occurred in pure rolling and is attributed to the local perturbation of film convergence which the ridge generates while in the inlet region. This propagates through the contact at the mean speed of the surfaces and—in pure rolling—acts to diminish the effect of local squeeze. When sliding was present, the ridge remained almost fully deformed even when the mean film thickness was as much as twice the height of the original ridge. In this case, the ridge travels through the contact at a different speed from the mean of the two surfaces. The consequent decoupling of the ridge and the convergence perturbation results in a large local pressure due to squeeze which acts to inhibit recovery of the ridge. The general trend of the behavior of the lubricated ridge is shown to be in good agreement with earlier theoretical results.

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. Presented at the 54th Annual Meeting in Las Vegas, Nevada May 23–27, 1999

In situ study of metal oleate friction modifier additives

There has been debate for many years as to whether long-chain surfactant friction modifier additives reduce friction by forming adsorbed films of monolayer thickness or whether they form films equivalent to several or many multilayers thick. In the work described in this paper, a series of metal oleate friction modifier additives has been synthesized and their film-forming properties compared in rolling-contact conditions by means of ultrathin film interferometry. It has been found that some of these additives form thick boundary films while others do not. It is concluded that thick boundary-film formation results from the formation of insoluble iron(II) oleate on the rubbing surfaces and that, for metal oleates, this occurs only for metals lower than iron in the electrochemical series and is due to a redox reaction involving iron from the steel surface and the metal oleate.

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.

316 超薄膜光干渉法用スパッタコーティング膜の開発(機械要素・潤滑I)

316 超薄膜光干渉法用スパッタコーティング膜の開発(機械要素・潤滑I)

Thin films in elastohydrodynamic lubrication: The contribution of experiment

This paper reviews the various experimental techniques that have contributed to an understanding of elastohydrodynamic (EHD) lubrication. After a brief historical overview, attention focuses on the main methods for studying the behaviour of lubricant films in lubro, within EHD contacts, in particular ultra thin film interferometry, film thickness mapping, in-contact infrared and Raman spectroscopy, fluorescence and temperature/shear stress mapping. From this review, the important role of the experimentalist in the study of EHD lubrication is identified and appreciated.

Simple equation for elastohydrodynamic film thickness under acceleration

A simple approximation of EHD film thickness under varying speed conditions is proposed. The equation is based on continuity of flow, by which the film formed at the contact inlet moves downstream within the contact with little subsequent change in its thickness even though the boundary velocities are changing. The approximation is supported by experimental results of non-steady state film thickness measurement using ultra-thin film interferometry. It is also shown by numerical simulation that the approximation holds for film thickness in the rigid piezoviscous regime under line contact so long as the squeeze film effect is insignificant.

Influence of load and lubricants on EHD film thickness and rolling‐contact fatigue lives of AISI 52100 steel balls

AbstractThis work has evaluated the influence of load and type of lubricant on the thickness of the elastohydrodynamic (EHD) film and the rolling‐contact fatigue lives of AISI 52100 steel balls. The lubricants studied have various viscosities and included two mineral oils and five synthetic oils from three families. Firstly, the central film thickness was determined in order to predict the lubrication regime. The stress—number of cycles fatigue curves were then calculated by means of Weibull plots, and the fatigue mechanism was evaluated.The test machine used for the analysis was a Seta‐Shell 1980 four‐ball EP lubricant tester. The 12.7 mm diameter test balls were made from a single batch of carbon‐vacuum‐deoxidised AISI 52100 steel with hardness RC65.Elastohydrodynamic film thickness estimation was carried out using pressure—viscosity coefficients (a). In this study, to calculate (X), a new interferometric technique, ultrathin film interferometry, was employed to measure the film thickness. A practical method was developed for evaluating EHD, mixed film, and solid lubrication processes. Micrographic mapping and energy dispersive spectroscopy (EDS) were used to analyse the rolling track of the test balls.

EHD Film Thickness in Non-Steady State Contacts

This paper describes a study of EHD film thickness in non-steady state contact conditions. A modification of ultrathin film interferometry is employed which is able to measure both central film thickness and film thickness profiles 50 times a second. Film thickness with two perfluoropolyethers and two mineral base oils are investigated in a number of different types of non-steady state motion, including acceleration/deceleration, stop/start and reciprocation. The results demonstrate a range of transient behaviors of EHD film whose thicknesses deviate from those in steady state conditions.

Fractionation of liquid lubricants at solid surfaces

The film-forming properties of lubricant base fluid mixtures in elastohydrodynamic contacts have been studied using ultrathin film interferometry. It has been shown that in binary mixtures where one of the components is more polar than the other, the EHD film thickness formed in the very thin film (< 10 nm region) is controlled by the viscosity of the polar component rather than the viscosity of the blend. This means that a mixture of a highly viscous ester in less viscous hydrocarbon gives thicker than predicted lubricant films in the sub 20 nm region and vice versa. This phenomenon can be ascribed to the fractionation of the lubricant mixtures close to the surface caused by lubricant molecule/surface van der Waals forces.

Measurement and Modelling of Boundary Film Properties of Polymeric Lubricant Additives

Experimental work using ultrathin film interferometry has shown that some polymer solutions in oil form much thicker films at slow speeds in rolling, concentrated contacts than predicted from elastohydrodynamic (EHD) theory. This behaviour can be interpreted as resulting from the polymers forming adsorbed, surface layers of enhanced concentration on the two solid surfaces. Such layers, which are typically 20 nm thick, would be significantly more viscous that the bulk solution and thus produce thicker EHD films. This concept has been supported by modelling the elastohydrodynamic point contact using control volume analysis with a layered surface viscosity. The film thickness behaviour predicted computationally using this technique is quite similar to that found experimentally using polymer solutions.

Boundary Film Formation by Viscosity Index Improvers

Ultrathin film interferometry has been used to measure the film-forming properties of a range of viscosity index improver (VII) solutions in rolling, concentrated contacts. It has been shown that some VIIs form boundary lubricating films of thickness 10 to 30 nm in contacts. These films result from the presence of highly concentrated and thus very viscous layers of polymer solution formed on the two rubbing solid surfaces by polymer adsorption. These boundary films are formed only by some types of VII and can persist up to temperatures in excess of 120°C. The possible implications of this type of boundary lubrication are discussed. Presented as a Society of Tribologists and Lubrication Engineers paper at the STLE/ASME Tribology Conference in Kissimmee, Florida, October 8–11, 1995

The Formation of Viscous Surface Films by Polymer Solutions: Boundary or Elastohydrodynamic Lubrication?

The film-forming properties of a range of polymer solutions have been studied down to very thin film thickness using ultrathin film interferometry. It has been found that, at very slow rolling speeds, some polymers generate much thicker films than predicted from theory. It appears that these polymers form adsorbed layers between three and 15 nanometers thick on the two solid surfaces. These layers have a viscosity many times higher than that of the bulk solution. Therefore, under slow speed, low film thickness conditions, the contact effectively operates within a viscous boundary layer, generating an elastohydrodynamic-type film much thicker than predicted from the viscosity of the bulk lubricant. As the speed is raised the contact emerges from this boundary layer and reverts to elastohydrodynamic behavior based upon the viscosity of the bulk polymer solution.