J.M. Georges

Mechanism of boundary lubrication with zinc dithiophosphate

Abstract Experiments were carried out with pure zinc dithiophosphate in paraffin as a lubricant. Different friction tests using smooth and rough surfaces were studied by surface analysis (Auger electron spectroscopy and secondary ion mass spectroscopy) and lubricant analysis (thin layer chromatography). Four types of surface transformation were detected: soft track, cutting track, delamination and adherent film. The wear rate was minimal when an adherent film was formed on steel and cast iron surfaces. The role of the additive in the formation of the film is outlined. Surface products play an important part in the production of a paste in the contact zone. This paste, which is compressed in the interface, is the dominant factor affecting wear behaviour.

Measurements of intermolecular forces and the rheology of dodecane between alumina surfaces

Abstract The principles of a new surface force instrument are presented. Its features allow the intermolecular forces and rheology between a sphere and a plane to be measured continuously and simultaneously. A “transfer function” gives both the viscous and the elastic behavior of the interface. The study of the interface alumina-n-dodecane-alumina shows that, if the layer thickness is greater than 5 nm, the dodecane behaves like a Newtonian liquid with its bulk viscosity. For distances less than 5 nm, deviations are attributed to steric effects and the surface roughness of the solids. At the same time, during the inward and outward approaches a hysteresis in the interaction force occurs. A comparison between experimental curves and the theoretical attractive van der Waals force shows best agreement in the unloading period, and when the roughness is very slight.

Synergistic effects of MoDTC and ZDTP on frictional behaviour of tribofilms at the nanometer scale

The layered structure and the rheological properties of anti-wear films, generated in a rolling/sliding contact from lubricants containing zinc dialkyldithiophosphate (ZDTP) and/or molybdenum dialkyldithiocarbamate (MoDTC) additives, have been studied by dynamic nanoindentation experiments coupled with a simple modelling of the stiffness measurements. Local nano-friction experiments were conducted with the same device in order to determine the evolution of the friction coefficient as a function of the applied pressure for the different lubricant formulations. For the MoDTC film, the applied pressure in the friction test remains low (<0.5 GPa) and the apparent friction coefficient is high (μ > 0.4). For the tribofilms containing MoDTC together with ZDTP, which permits the applied pressure to increase up to a few GPa through some accommodation process, a very low friction domain appears (0.01 < μ < 0.05), located a few nanometers below the surface of the tribofilm. This low friction coefficient is attributed to the presence of MoS2 planes sliding over each other in a favourable configuration obtained when the pressure is sufficiently high, which is made possible by the presence of ZDTP.

Boundary lubrication with anti-wear additives: study of interface film formation by electrical contact resistance

Abstract It has been shown that a rapid measuring technique for electrical contact resistance (ecr) has potential applications in lubricant additives studies. In boundary lubrication of a metallic interface with zinc dialkyl dithiophosphate (DTPZn) or other anti-wear additives blended in the lubricant, anti-wear efficiency relies on interface film formation which acts also as an insulating barrier for electrical current. Investigations in the 1ω to 10 Mω range of resistance, with subsequent and appropriate ecr signal treatment and analysis, indicate that systematic use of ecr permits visualisation and study of interface film formation. This paper shows that two interface films can be detected, with two different levels of average ecr, 0–1 kω and 100k–10Mω. Discussion is based on an interface model and attempts to find what kind of further information on film formation may be obtained from ecr measurements

Interfacial friction of wetted monolayers

Abstract A “molecular tribometer” has been constructed [1]that measures directly, with great accuracy, the forces and the displacements that act between metallic surfaces bearing organic layers in a liquid medium as they slide past each other. Its originality is due to its low compliances (2×10−7m N−1 for the normal direction, 2×10−6 m N−1 for the two other directions), which permit mechanical control of the thin layer. In this work, it is shown that an anisotropic thin monolayer of stearic acid adsorbed an each cobalt surface is more compliant than the contact of the cobalt surfaces and hence reduces the apparent shear moduli of the contact zone and controls the shear process. The “interfacial” friction depends on the sliding speed and is very sensitive to the very small variations of the film interface thickness. Two competitive and opposing processes occur during the sliding: crushing and lifting of the monolayer. It is suggested that the crushing appears at very low speed, when the transit time is close to that of the molecules monolayer. Our findings have direct applications for the properties of boundary lubricants and for the rheological behaviour of both surfactant and polymer molecules.

High-pressure lubrication with lamellar structures in aqueous lubricant

The lubrication mechanisms of water-based fluids are not yet well understood, especially when extreme load conditions are applied. The example of lamellar crystallite particles in water, provided by an emulsion-like technology, is described in this paper. It is shown, for the first time, that the presence of such nanostructures in the sliding interface can provide extreme-pressure lubrication. This water-based lubricant is particularly efficient when one of the two solid surfaces is made of brass. In this case, a multilayer is created in the contact. It is formed by a brass layer transferred to the counter face, the brass surface itself and a lamellar film adherent to the former layers. This lamellar system is able to sustain pressures up to a few GPa and to maintain a low friction coefficient.

Rheology of Olefinic Copolymer Layers Adsorbed on Solid Surfaces

The rheology of two adsorbed layers of olefinic copolymers (OCPs) between two cobalt surfaces was investigated using a surface force apparatus (SFA). Each polymer layer is obtained by adsorption in a dilute solution of OCP and dispersant OCP (DOCP) in a 175 neutral solvent basestock at 24°C. The aim of these experiments is the mechanical characterization of the compressed polymer films. The hydrodynamic thickness RH detected is compared with the thickness of the polymer layers L and the hydrodynamic radius RH of the polymer coil in solution. All these distances, strongly related to the chemical structure of the polymer, indicate that the layers are not always homogeneous. For OCP, 0.55L ≈ LH ≈ 2RH, and for DOCP L ≈ LH ≈ 2.7RH. Finally the two layers present different inelastic behaviors. The DOCP layer presents a mesh more compact and more uniform than the OCP layer as indicated by the thickness measurements. Presented as a Society of Tribologists and Lubrication Engineers paper at the STLE/ASME Tribology...

Electrical and mechanical contact between rough gold surfaces in air

An original coupling of electrical and mechanical measurements is reported; it concerns the phenomena arising from the interaction between two very close macroscopic electrodes, one a sphere and the other a plane, studied in air. The current-distance and current-voltage curves clearly attest to a tunnelling transfer mechanism, and prove to be in very good agreement with theoretical predictions taking into account the roughness of surfaces. Both electrical and mechanical interpretations coherently indicate the existence of an intermediate substance in the interface, plausibly condensed water with the addition of organic contaminant(s).

Film Thickness and Mechanical Properties of Adsorbed Neutral and Basic Zinc Diisobutyl Dithiophosphates

A surface force apparatus (SFA) has been used to determine film thickness and film properties of adsorbed neutral and basic diisobutyl zinc dithiophosphates (ZnDTPs) on cobalt surfaces Sliding experiments were also carried out and film characteristics determined after sliding. Dodecane was used as the solvent. Experiments with dodecane only show dodecane molecules form an immobile monolayer on cobalt surfaces. When subjected to sliding, this monolayer is displaced, and attractive forces between the cobalt surfaces are observed. Neither the films formed initially upon adsorption of the ZnDTPs nor those films after sliding are displaced by the tangential motion. After 80 nm of sliding the coefficient of friction was 1.2 in dodecane and 0.4 in the ZnDTP solutions. Elastic compressive and shear moduli for the physisorbed ZnDTP films were ∼ 0.1 GPa and 0.01-1 GPa, respectively. The distance of first repulsion between the approaching surfaces corresponds roughly to two monolayers of ZnDTP on each surface. Under compression, ZnDTP molecules that are not firmly bonded to the surface are expelled from the contact. At 15-hour adsorption times for both ZnDTPs, the thickness of the confined layer under load corresponds to one monomolecular layer separating the two surfaces. For the neutral ZnDTP, a monomolecular layer thickness is I nm and for basic ZnDTP it is 1.6 nm. A possible explanation for a single molecular-layer separation is that the adsorbed ZnDTP molecules are sparsely distributed on the surfaces, so sparsely that when the surfaces approach the adsorbed molecules interleave, forming a layer of molecules of monomolecular thickness, of which some are attached to one surface, and of which the remainder are attached to the other surface. The SFA affords not only mechanical properties of thin films but also some information on their molecular structure.

A study of the durability of boundary films

Abstract The frictional properties of solid surfaces are modified by the adsorption of organic molecular films. The lifetime of a monomolecular film of a fatty acid which was deposited on a surface to form a solid/monolayer/air interface has been investigated. The continual presence of an agglomerate has been observed during frictional contact under boundary lubrication conditions. This agglomerate is composed of several substances resulting from the deterioration and recombination of fatty acid molecules with each other and/or the surrounding media.