Jean-Luc Loubet

Understanding and quantification of elastic and plastic deformation during a scratch test

An understanding of abrasion resistance and the associated surface deformation mechanisms is of primary importance in the materials engineering and design of many important industrial components undergoing wear and abrasion. Instrumented scratch testing has been shown to be a useful tool for characterization of the abrasion resistance of materials. Although most studies on scratch resistance have been limited to the theoretical case of purely plastic materials, experiments on metals and polymers have shown that the contact mechanics and indentation behavior are strongly influenced by the elastic behavior. In this work, the normal and tangential scratch forces, the penetration depth relative to the initial surface and the three-dimensional topographic relief of the scratched surface are measured. These parameters have allowed us to accurately calculate the actual contact area between the indenter and the material, taking into account both elastic deformation and pile-up phenomena. This contact surface was used to estimate the real mean contact pressure during scratch testing. This pressure was compared to the static hardness of the studied materials, as well as to the classical definitions of the scratch hardness. The ratio between the plastic and elastic deformation during a scratch test with a Berkovich indenter was then related to the ratio of the Elastic modulus to the hardness for the tested bulk materials. This scratch experiments were performed on a wide range of materials from polymers to metals and demonstrate the importance of taking both elastic deformation and pile-up into account in order to accurately understand and characterize the scratch resistance of materials.

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.

Surface modifications of LiNbO3 single crystals induced by swift heavy ions

Single crystals of LiNbO3 (Y-cut orientation) have been irradiated at GANIL using different ions (112Sn, 155Gd and 238U) accelerated in the GeV range. All the irradiations were performed at room temperature, with fluences extending from 1010 to 1012 ions cm−2. Rutherford backscattering spectrometry in channeling geometry (RBS-C) was employed to investigate the lattice disorder resulting from the high electronic stopping power (dE/dx)e (between 18 and 40 keV nm−1). Surface swelling of the irradiated samples was evidenced using a profilometer, in conjunction with direct observations in the nanometer scale by means of atomic force microscopy (AFM). According to RBS-C analysis, the damage cross section Ad varies from 4 × 10−13 to 1.4 × 10−12cm2 and depends on both (dE/dx)e and the velocity of the incident ions. A correlation was pointed out between the height of the out of plane step and the (dE/dx)e-induced damage. AFM observations, performed on samples irradiated at the lowest fluences, indicate a significant broadening of the latent track radius at the surface.

Understanding of automotive clearcoats scratch resistance

Abstract Micrometric scratches, mainly due to car wash brushes, decrease the gloss of automotive clearcoats, alter their appearance and prevent them from protecting the car in the best way. Scratch resistance is estimated from the visible damage caused on the clearcoats. As the eye is more sensitive to brittle than to ductile scratches (Microstructures and Microtribology of Polymer surfaces (2000) 428), one way to improve scratch resistance it to decrease the proportion of brittle scratches and to favor healing of ductile scratches. An analysis of the contact during a scratch test shows that the ductile/brittle transition is controlled by both an energetic and a size criterion. The energetic criterion is related to the tensile stress located at the rear of the contact between the indenter and the clearcoat. This stress can enlarge existing cracks, leading to fracture. It depends on the clearcoats viscoelastoplastic properties, the mean deformation imposed by the indenter and the environment (dry tests or tests in water or soapy water). The effect of these parameters on scratch resistance has been studied: indentation and scratch tests with a Nano-indenter XP have been carried out. The healing of ductile scratches has been estimated from scratched samples of clearcoats. The samples have been heated at different temperatures and the remaining scratch depth measured with an atomic force microscope.

Normal and lateral modulation with a scanning force microscope, an analysis: implication in quantitative elastic and friction imaging

Results and an analysis are presented on elastic and friction imaging by indirect force modulation with a scanning force microscope. Two techniques are compared, normal modulation (Z-modulation, perpendicular to the surface of the sample) and lateral modulation of the contact (X-modulation in the plane of the sample, perpendicular to the axis of the cantilever). Theoretical and experimental results show that lateral modulation offers great advantages compared to normal modulation: the images are free of artifacts and can be easily quantified.

Nano-indentation, scratching and atomic force microscopy for evaluating the mar resistance of automotive clearcoats: study of the ductile scratches

Abstract Automotive clearcoats are subjected to different kinds of environmental damage: UV radiation, acid rain, tree sap, birds droppings, stone chipping and marring. Marring, which is mainly due to car wash brushes, decreases the gloss of automotive paint and alters the colour. Optical and atomic force microscope imaging of clearcoats samples brushed by an automated car wash reveals both plastic scratches and fractures. The same type of scratches has been drawn in a controlled manner: the applied load is controlled, the deformation and the strain rate are known. First, the Youngs modulus and the hardness of the clearcoats are calculated from indentation tests at different strain rate values. Next, scratching tests providing greater strain rates allowing us to extend the hardness results from indentation tests. These experiments carried out at room temperature have been supplemented by indentation tests performed at different temperatures (from −10°C to 45°C). Thus, the evolution of the indentation hardness vs. strain rate at different temperatures has been measured. A time–temperature superposition curve has been constructed. This hardness master curve as a function of reduced frequency (reduction temperature of 20°C) allows us to define, for each clearcoat, the activation energy of the viscoplastic process. The found values are close to those of the secondary β transition, usually attributed to local molecular motion, like rotations of side groups or limited motions within the chain backbone. It turns out that mar resistance would largely be dependent of this transition temperature position in comparison with the main mechanical relaxation (α transition) temperature.

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.

Force modulation with a scanning force microscope: an analysis

A magnetic force modulation microscope (FMM) has been employed to measure the dynamic behavior of a contact between a scanning force microscope (SFM) tip and a surface. Our experimental results show the inefficiency of the classical models (two Kelvin-Voigt elements in parallel). A new model which takes into account the normal and tangential stiffness of the contact, and also the geometrical and mechanical properties of the cantilever which hold the tip, is proposed. This model shows that the natural frequency is sensitive to the normal stiffness, only if the ratio of the normal contact stiffness to the cantilever stiffness is between 0.2 and 200. Above this domain, the natural frequency is sensitive to sliding (Mindlin theory).

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.