A. Tonck

The two-layer structure of Zndtp tribofilms: Part I: AES, XPS and XANES analyses

The nature and properties of polyphosphate glasses formed in the antiwear action of zinc dithiophosphate (Zndtp) is investigated. Special attention is paid to the advantage of coupling three surface analytical techniques on the same Zndtp tribofilm: Auger (AES), XPS and XANES spectroscopies. The data show the two-layer structure of the Zndtp film and permit a clear identification of the chemical composition of each layer: a polymer-like zinc long chain polyphosphate overlying a mixed transition metal short chain phosphate. A Chemical Hardness model is found to predict the formation of such a layered tribofilm. Moreover, a tribochemical reaction between the zinc polyphosphate and the iron oxides species is proposed on the basis of the Hard and Soft Acids and Bases (HSAB) principle. This reaction explains the anti-abrasive mechanism of Zndtp and also predicts a depolymerisation of the long chain zinc polyphosphate glass, in very good agreement with AES/XPS/XANES analytical data. The role of residual sulphur in the lubricant is also explained and the model is in agreement with the formation of metal sulphides embedded in the short chain phosphate matrix. This first paper serves as a basis for a detailed study of the mechanical properties of each film.

Improvements in the indentation method with a surface force apparatus

Abstract On the nanometre scale, the actual indenter-material contact area must be carefully determined to obtain reliable values of mechanical properties from an indentation test. On this scale, the contact area is greatly affected by the geometrical tip defect and by the possible formation of plastic pile-up (or sink-in) around the indent. Parameters such as local surface roughness and heterogeneity in surface and in thickness make it di5dt to conduct and to interpret nanoindentation tests. A new method, which couples nanoindentation experiments and imaging procedures, has been developed. Nanoindentation tests and topographic images are performed with a surface force apparatus developed in our laboratory. The important point of our method is the ability of our three-axis device to generate topographic images without having to move the sample. This allows us to determine precisely the actual tip–sample contact area, while performing accurate continuous quantitative quasistatic load measurement and simult...

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

A simple guide to determine elastic properties of films on substrate from nanoindentation experiments

Several models have been developed to extract the intrinsic elastic modulus of thin films from the composite film/substrate modulus value obtained from indentation tests on coated systems. Either analytical, semi-analytical or empirical, they generally propose an expression for the composite modulus as a function of the film and substrate elastic moduli and of the film thickness. When the substrate properties and the film thickness are known, the expression without adjustable parameter contains only the film elastic modulus as an unknown parameter, which can thus be deduced. In this paper, some models from the literature are briefly described and compared with the easy-to-use model we have developed. Based on experimental results obtained with various film/substrate systems, this paper also aims to demonstrate that the error induced by the uncertainty in the knowledge of the substrate Youngs modulus value and in the film thickness value is often as large as the difference resulting from the choice of the model.

Nature of super‐lubricating MoS2 physical vapor deposition coatings

MoS2 is a well‐known solid lubricant used for space applications. Radio frequency (rf) magnetron sputtered MoS2 films are being used increasingly, due to an ultra‐low friction behavior in high vacuum (friction coefficient: ∼0.01). MoS2 coatings with a super‐low friction behavior (friction coefficient: ∼0.001) have been recently synthesized in an ultra‐high vacuum tribometer equipped with a rf magnetron sputtering device. Physicochemical and structural characterizations of the thin films have been carried out using XPS (x‐ray photoelectron spectroscopy) and AES (Auger electron spectroscopy) to determine the extreme surface composition, HRTEM (high‐resolution transmission electron microscopy), GXRD (grazing‐angle x‐ray diffraction) for structural investigations, RBS (Rutherford backscattering spectrometry) for thickness and stoichiometry determination, and NBS (nuclear backscattering spectrometry) for quantitative analysis of oxygen contamination. Indentation tests at nanometric scale have also been perform...

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.

Nanoindentation with a Surface Force Apparatus

This paper is concerned with two different scales, the macroscopic scale (10 nm to 500 nm), and the microscopic or molecular scale (0.1 nm to 10 nm). We report nanoindentation experiments made with a Surface Force Apparatus. The experimental technique is described. Some simple modelling of the nanoindentation tests, linked to the presented experiments, is proposed.

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).