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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.), 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.

Keywords: Zinc Dithiophosphate, Antiwear, ZDDP, ZnDTP, Tribofilm, Interferometry, Wear, Boundary Lubrication, Model

The effectiveness of various combinations of borate-, sulfur-, and phosphorus-containing additives blended in gear oil to form antiwear tribofilms on steel surfaces sliding in the boundary lubrication regime was investigated experimentally. The formation of protective tribofilms and their tribochemical activity in the temperature range of 32–100°C were analyzed in terms of coefficient of friction and contact voltage results. The antiwear performance obtained with each blend was evaluated by wear volume calculations based on surface profilometry measurements. Scanning electron microscopy studies provided insight into the dominant wear mechanisms at various temperatures. The tribological performance of the tribofilms and associated wear mechanisms were found to depend strongly on the type of additive(s), tribofilm composition, and temperature. This study has revealed that sulfide-dominated tribofilms produce lower friction, whereas borate- and phosphate-dominated tribofilms are more effective in increasing the wear resistance of the steel surfaces.

Keywords: Additives, Boundary Lubrication, Friction, Antiwear Tribofilms, Wear Mechanisms

Electrical contact resistance (ECR) studies, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and X-ray absorption near-edge structure spectroscopy (XANES) were carried out on specimens run with oils containing 0.05% phosphorus as either primary zinc dialkyldithio-phosphate (ZnDTP) or secondary ZnDTP in Group II base oil. A series of progressively longer ECR experiments were run on each ZnDTP. At the end of each run in the series, the ball was removed and preserved for surface analysis. The surface analyses were designed to observe chemical species deposited on the surface and within the deposited films. The observation of surface phenomena at different intervening times during the ECR experiment, allowing for characterization of the maturing antiwear film, was the distinct feature of these experiments. In general, short ECR experiments gave poorer films than long ECR experiments. Atomic concentrations versus depth were determined from AES. Quite strikingly, the antiwear films formed after only 10 min of the ECR experiment showed that both primary and secondary ZnDTPs form a thin film (∼70 Å) very rapidly. Those films are rich in Zn, P, and S. Auger and XANES analyses of the same specimens were not as revealing, most likely due to the small wear scar on the balls and the unfortunately relatively large beam cross section. ECR, XPS, and AES were then performed on oils containing nonphosphorus antiwear agents in American Petroleum Institute Group II base oil. Several nonphosphorus supplemental antiwear inhibitors were evaluated. These experiments showed separation in apparent performance among the various components.

Keywords: Lubricants, Friction, Wear, Antiwear Agents, Bench Testing, Electrical Contact Resistance, Surface Analysis, XPS, XANES

A progressive reduction in the permitted level of phosphorus in lubricating oils, coupled with concern to maintain engine and transmission durability, means that it is becoming increasingly important to understand the detailed mechanism of antiwear additive behavior.

This paper describes a new experimental technique, which is able to measure both the thickness and distribution of antiwear additive films in rolling/sliding contacts. This enables the kinetics of antiwear film build-up to be investigated and the influence of the reaction film on friction and wear to be monitored. In the current paper, this technique is used to compare the film-forming behavior of ash-containing and ashless antiwear additives.

Keywords: Lubricating oils, wear, transmissions, logistics, durability

A method has been developed for monitoring the film-forming properties of antiwear additives in rolling-sliding, lubricated contacts. This makes it possible to study both the kinetics of reaction film growth and also the evolution of the film morphology as a function of rubbing time. The technique has been applied to investigate the behavior of a zinc dialkyl-dithiophosphate (ZDDP) additive solution and to correlate this with simultaneous friction and wear measurements.

The results show that ZDDP forms a thick, solid-like, reaction film in the rubbing tracks, with negligible film growth outside of the track. This film is extremely effective in preventing metal-metal contact. However the film is unevenly-distributed, with its roughness oriented in the direction of sliding. This directional roughness inhibits the entrainment of fluid film in the mixed lubrication regime, increases the proportion of load supported by solid-solid contact and consequently results in the high friction often associated with the use of ZDDP additives.

Keywords: Antiwear Additives, Zinc Dialkyldithiophosphate, Friction, Wear, Optical Interferometry, Spacer Layer

Since 1940s the principal source of an antiwear additive in crankcase applications has been due to a family of additives known as zincdialkyithiophosphate (ZDDP). In this study, we have applied a novel technique, the mini traction with spacer layer image mapping (MTM SLIM) to study film formation characteristics of monoblend oils containing only basestock and ZDDP and also fully formulated oils containing a boron antiwear additive. The purpose of this study was initially to understand the build up of ZDDP film and then to establish whether boron compounds alone could you provide significant antiwear films under mixed rolling and sliding contact.

Keywords: Zinc dialkydithiophosphate, Mini traction machine with spacer layer image mapping

The stringent and often competing requirements of high fuel economy and low emissions are placing increasing emphasis on the selection of appropriate base oils for modem engine lubricants. Two properties now recognized as important in engine oil design are the elastohydrodynamic (EHD) traction coefficient and the pressure-viscosity coefficient. The former determines the level of friction in high pressure contacts such as cams, while the latter plays a major role in determining the EHD film thickness. Unfortunately, for many fluids there is a broad correlation between the two properties so that a low traction coefficient implies a low pressure viscosity coefficient and thus film thickness. This paper measures the traction and film-forming properties of a wide range of base oil types at realistic engine oil temperatures in order to both explore the extent of this correlation and to determine the dependence of EHD lubricant properties on base oil compositions.

Keywords: Lubricants, Elastohydrodynamic Lubrication, Friction

Cosmetic powders are regularly employed in skin creams and cosmetic formulations to improve performance and enhance skin feel. A previous study investigated the effect of particle concentration and size on the lubricating properties of powder suspensions in smooth, compliant contacts [Timm et al., Tribol. Int. (2011)]. In this paper the tribological properties of cosmetic powder suspensions are investigated in compliant contacts having model fingerprintlike surface topography. Friction coefficients were measured for a series of powder suspensions with varying particle size and concentration in apolydimethylsiloxane (PDMS)/PDMS contact. A commercial tribometer (MTM, PCS Instruments) was employed to measure friction as a function of rubbing time (20 min), under pure sliding (50 mm/s) and low load (0.5 N) conditions. Compared to results using smooth surfaces, it was clear that surface topographyhas a pronounced affect on the timedependent tribological behavior of the cosmetic powder suspensionsstudied. A two-stage friction coefficient versus time curve was observed. By varying the particle size and concentration it was shown that the duration and magnitude of each stage can be controlled.

Keywords: Cosmetics, Friction, Humans, Lubricants, Particle Size, Powders, Surface Properties,  Suspensions

Friction is investigated in a rolling–sliding, lubricated, steel ball on elastomer flat contact. Two different types of friction are identified: rolling friction, which results from the movement of the surfaces relative to the contact, and sliding or interfacial friction, which arises from relative motion of the two contacting surfaces. A novel experimental technique is
described to measure these two types of friction simultaneously in a single test. This enables separate rolling and interfacial ‘Stribeck-type’ friction curves to be produced for Newtonian lubricants. These curves are compared with theoretical predictions of friction. The results show that rolling friction originates primarily from two sources: Poiseuille flow of lubricant in the contact and elastic hysteresis. There are also two main types of interfacial friction; due to Couette flow and solid surface adhesion. For compliant elastomer-on-steel contacts, rolling friction forms a significant proportion of the total friction even at quite high slide–roll ratios.

Keywords: Soft-elastohydrodynamic, Isoviscous-elastic, Friction, Rolling friction, Sliding friction, Interfacial friction, Couette friction, Poiseuille friction, Hysteresis