T.G. Mathia

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.

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.

Dynamics of contact line motion during the wetting of rough surfaces and correlation with topographical surface parameters.

Dynamics of contact line motion and wettability is essential in many industrial applications such as liquid coating, lubrication, printing, painting, condensation, etc. However, the wettability of surfaces depends not only on liquid-solid chemical properties but also can be strongly affected by surface roughness. As a practical application of controlled wettability, we can mention the self-cleaning surfaces, protective clothing, microfluidics devices, electro wetting, etc. In this article, we experimentally investigate the spreading of droplets deposited onto rough surfaces. Anisotropic surfaces were prepared by abrasive polishing on the following materials: aluminium alloy AA7064, titanium alloy Ti-6Al-4V, steel AISI 8630, copper alloy UNS C17000, machinable glass ceramic, and poly-methylmethacrylate. Topographical 2D parameters were calculated according to the following standards, defining Geometrical Product Specifications (GPS): ISO 4287, ISO 12085, ISO 13565, ISO 12780, and ISO 12181. The influence of topographical parameters on wettability and spreading phenomenon has been evaluated by statistical covariance analysis. The following parameters have strong influence on fluid spreading on rough surfaces: R(mr) is the relative material ratio of the roughness profile, T(rc) is the microgeometric material ratio, P(mr) is the relative material ratio of the raw profile, K(r) is the mean slope of the roughness motifs, RON(t) is the peak to valley roundness deviation, and P(sk) is the Skewness of the raw profile. The physical meaning of selected parameters is discussed, and K(r) (the mean slope of the roughness motifs) is selected as the most important and physically meaningful parameter. It has been found that for all tested materials, fluid spreading shows increasing tendency when mean slope of the roughness motifs (K(r) ) increases.

The morphological tree transform of surface motifs. Incidence in tribology

In this paper we show two new developments in the three-dimensional characterisation of rough surfaces. The basic idea is to decompose the three-dimensional roughness as a combination of 3D motifs defined by amplitude, wave length, local and overall direction of various components. The multi-scale approach developed in this work is very useful in tribology, the summit of the morphological tree permits the characterisation of the waviness of the summits envelope, each summit can be characterised by the height and the radius of curvature in x and y direction necessary for the computing of Hertzian pressure. The local slopes of the summits envelope play an important role in Coulombic friction, adherence and leakage. The fluid reservoir can be characterised by the multi-scale aspect of the 3D motifs size and their orientation.

Effect of lateral resolution on topographical images and three-dimensional functional parameters

The influence of the analysis scale on the characterisation of surface roughness is set out in this work. First the effect of lateral resolution on the local morphology of a random engineered surface is shown. The effect of the geometry of a tactile profilometer tip is studied by the variation of the tip radius of curvature. The incidence of the scale observation is analysed from micro- to nano-scale with three microscopes: a tactile, laser and atomic force microscope. The lateral resolution influences not only the roughness amplitude parameters, but also spectral range, slopes, local radius of curvature of the summits, bearing area, developed surface, the void or material volume. In the second half of this work the contribution of fractal geometry is examined. This is because, through its roughness index, called the Holder index, it may be independent of the measurement. By using the Weierstrass-Mandelbrot function as a three-dimensional multi-scale model of the topography, the evolution of the bearing area, the developed surface and the volume as a function of the Holder roughness index can be shown.

Sclerometric characterization of nearly brittle materials

Abstract The abrasion of nearly brittle materials can be considered as a multipoint scratching and indenting process. Knowledge of flow morphologies around the abrasive protuberances, the determination of the energy dissipated during single-contact scratching experiments and the estimation of the forces acting on the abrasive particles are therefore essential for a better understanding of the wear and finishing processes based on abrasion phenomena. Experiments using linear and pendular sclerometers were conducted on various materials such as glass, ceramics, metals and polymers. The results described in the present paper show the effects of the geometry, the speed and the rheology on the abrasive process. The forces induced and the morphologies of the scratches are studied. The behaviour of the abrasive contact is in particular very nicely described by a critical transition from ductile to brittle abrasion which is analysed in detail for soda-lime glass. This transition is itself correlated with the dynamic hardness and the dynamic toughness of the tested materials. The results have a bearing on the prediction of the behaviour of nearly brittle materials when used as bulk materials or surface coatings in tribological applications dealing with abrasive wear.

The influence of surface oxidation on the wear resistance of cast iron

This paper reports a study of the influence of surface oxidation on the wear resistance of ductile iron, grey iron and vermicular iron during dry sliding friction. The mechanisms of wear are also reported. This study shows that the effect of surface oxidation (formed under normal atmospheric conditions) on the wear rate depends on the complex functions of graphite morphology and matrix structure. Generally the presence of surface oxidation decreases the wear rate of grey iron but increases the wear rate of ductile iron and vermicular iron when the cast iron has high hardness. This trend is reversed for low hardness cast iron.

Surface morphology and wettability of sandblasted PEEK and its composites

PolyEtherEtherKetone (PEEK) is an advanced high-performance thermoplastic polymer, and its composites are used extensively in the aeronautical industry. This paper presents an experimental approach to determine the role of sandblasting treatment on surface morphology modifications of PEEK and its composites, with the aim of developing a topographic characterization in order to propose pertinent parameters that correlate with contact angles from wettability measurement. Sandblasting (fine abrasive particle projection) was selected as the surface treatment, in order to obtain various morphologically quasi-isotropic surfaces. Two surface metrological approaches to topographical characterization were used to correlate the wettability behavior with the surface roughness parameters, the first based on 2D profile analysis and the second on 3D topography analysis. Two different unreinforced grades of PEEK and four composites: discontinuous carbon fiber or glass fiber-reinforced, oriented, and unoriented, were studied. The experimental results indicated the sandblasting process duration necessary to reach a morphological steady state. It was stated that one of the pertinent parameters is the mean slope of roughness motif in 2D profile characterization, as confirmed by previous findings for anisotropic morphologies. However, for all cases, a new topographic parameter Sr , combining the surface amplitude and the summit density distribution, is proposed as a factor well-correlated with wettability characteristics.

A sclerometric study of unidirectionally solidified Cr-Mo white cast irons

Abstract Low speed scratch experiments were carried out on a unidirectionally solidified bar of a eutectic Cr-Mo white iron. By means of a piezoelectric sensor the normal and tangential forces acting on the indenter faces were measured. For each phase the “scratch hardness”, the “scratch toughness” and the “specific grooving energy” were calculated. These values make it possible to draw conclusions about the contribution of each phase to the abrasive wear process. The effect of imposed conditions such as the normal force (0.15–2.0 N) and the scratch speed (125–520 μm s−1) was analysed. The influence of some metallurgical variables such as the intercarbide spacing on the abrasion resistance was indicated.

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.