Sandrine Bec

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...

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.

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.

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 charges and tribology of insulating materials

Abstract Electrical charge generation occurring during contact and friction of insulating materials has long been identified. However the contribution of these electrical charges to the frictional behaviour is usually neglected. Based on published results and on our own experimental work on the ability of the dielectric materials to trap charges, we show in this study that the interaction energy during friction depends markedly on these trapped charges. Eventually, we propose the complementary use of the ‘mirror’ method and of surface force measurements to obtain a quantitative evaluation of this contribution.

Wear of a stainless steel in pressurised high temperature water

Abstract The wear behaviour of an austenitic stainless steel is analysed using a tribometer working in pressurised high temperature water (PHTW). The precise contact conditions, low pressure impact and sliding contacts, play an important role on the surface wear in this corrosive medium. The tests which include both impact and sliding are the most severe. The friction films formed during these tests are compared with those obtained with pure oxidation. It is shown that these films participate in the formation of elongated ‘indents’ observed on the surfaces. Nano-indentation measurements consolidate this hypothesis.

Nanoindentation and nanofriction on DLC films

Diamond-like carbon (DLC) coatings have been studied for many years as wear-resistant and low friction materials. Their tribological behaviour depends on the nature of the coating, which is determined by the deposition process. It is also strongly affected by environmental conditions. In this study, the nanomechanical properties and nanofriction behaviour of two a-C:H coatings deposited on a silicon substrate and exhibiting different macroscopic friction behaviours were investigated using a three-axial surface force apparatus equipped with a diamond tip. The hardness, Youngs modulus and Poissons ratio of the two coatings were determined. Viscous recovery of the indents was observed a few days after the indentation tests. Low friction coefficients (around 0.06) were measured for the coating that exhibited macroscopic low friction. For the coating that exhibited a higher friction coefficient, dissipative behaviour measured during nanofriction test was enhanced by sliding, compared to the dissipative behaviour measured during normal indentation, suggesting a link between normal viscous dissipation at nanoscale and high friction.

Structure and mechanical properties of ZDTP films in oil

Abstract For many years, zinc dialkyldithiophosphate (ZDTP) lubricant additives have been extensively used for their anti-oxidant and anti-wear properties in rubbing systems but due to environmental considerations, there is now a need to develop new anti-wear additives with different chemistry but comparable anti-wear efficiency. As the anti-wear action of ZDTP additives is associated to the formation of a protective solid film onto friction surfaces under boundary lubrication conditions, the knowledge of the anti-wear film behaviour is a key point.

Wear damage resulting from sliding impact kinematics in pressurized high temperature water: energetical and statistical approaches

Specific wear of rod cluster control assemblies (RCCA) in pressurized water nuclear reactors (PWR) results from contacts with their guides due to flow-induced vibration. Particular sliding impact contact conditions and the specific environment in PWR coupling temperature and solution chemistry involve original mechanisms of wear. A specific tribometer has been developed during recent years by FRAMATOME-ANP. This simulator operates in experimental conditions close to the sliding impact motion of RCCA assemblies against guides and under a similar environment (temperature, pressure and water chemistry). Numerous and various wear tests were realized in order to study the influence of the sliding impact parameters on wear loss and wear scars. Specific tests with continuous data acquisition were realized. Different wear scars were observed. An analogy with erosion was done. An erosion model was used to determine critical conditions concerning friction coefficient and incidence angle values leading to wear. Considering these critical conditions, it was possible to explain the observed difference of wear loss and facies.