Surface roughness evolutions in sliding wear process

Wear debris analysis is a technique for machine condition monitoring and fault diagnosis. One key issue that affects the application of wear debris analysis for machine condition monitoring is whether the morphology of the wear particles accurately depicts their original states and the surface morphology of the components from which the particles separate. This study aimed to investigate the evolution of the surface morphology of wear debris in relation to change in the surface morphology of wear components in sliding wear process. Sliding wear tests were conducted using a ball-on-disc tester under proper lubrication and improper lubrication conditions. The study of the particle size distribution and the surfaces of both the wear debris and the tested samples in relation to the wear condition and the wear rates of the wear components were carried out in this study. The evolutions of the surface topographies of both the wear debris and the wear components as wear progressed were investigated. This study has provided insight to the progress of material degradation through the study of wear debris. The results of this research have clearly demonstrated that: (a) there is a good correlation of the surface morphology of wear debris and that of the wear components, and (b) the surface morphology of wear debris contains valuable information for machine condition monitoring.     

Chemical effects in sliding wear of aluminum

The mechanism of the sliding wear of metals in corrosive media was investigated. In particular, the role of chemical heterogeneities on chemical interactions between the sliding surface and its environment was studied using 2024 aluminum alloy and sodium chloride solutions of varying pH and NaCl concentration. Sliding wear tests with a cylinder-on-cylinder geometry were performed at a sliding speed of 1 m min^?1 and a normal load of 9.8 N (1 kgf). The results show that at pH = 0 and pH = 14, the wear rate is dominated by the dissolution of aluminum into the NaCl solution. In the intermediate pH range, the wear rate is shown to be controlled by the conjoint actions of corrosion and delamination wear. Microscope examination of the worn specimens by means of a scanning electron microscope further confirms that the mode of corrosion is of a localized nature.     

Development of a wear equation for polymer-metal sliding in terms of the fatigue and topography of the sliding surfaces

A wear equation has been derived using the concept of fatigue failure due to asperity interactions in the contact region between sliding bodies. One of the three principal stresses that arise in the contact zone under the effect of a normal as well as a tangential load is of tensile nature. It is this principal stress that has been considered to be responsible for the initiation and propagation of fatigue cracks. It is assumed that the deformation in the contact zone is of elastic nature and that both the contacting surfaces are covered with asperities that have spherical tips. The wear equation involves the asperity height distribution φ(z). The particular distribution for a sliding situation is determined from experimental studies of the topography of sliding surfaces. The wear equation indicates that the wear rate depends upon the fatigue properties of the weaker material, normal load, sliding speed, coefficient of friction, moduli of elasticity of the contacting materials, asperity density, asperity radius of curvature and the distribution and standard deviation of asperity heights. The variation of wear with these parameters as indicated by the wear equation is in agreement with the experimental studies already reported in the literature.     

The influence of topography on the specific dissipated friction power in ultra-mild sliding wear: Experiment and simulation

Current political, economic and ecological guidelines demand the increase of power densities of nearly all machinery parts. In order to further lower the wear rate towards the ultra-mild sliding wear regime, an integral approach is needed, which has to regard contact conditions, surface topography, surface chemistry, as well as sub-surface properties. Still, there are no simple parameters to classify the performance of a tribosystem. In this study the area affected by tribocontacts is calculated by means of a three dimensional elastic–ideal plastic contact model. The surfaces are prepared by means of conventional machining procedures and characterized by scanning white light interferometry. The further input data as to normal and friction forces are derived by reciprocating sliding wear tests under boundary lubrication conditions of carburized steel against carburized steel and 52100 steel against case-hardened spheroidal cast iron. This contribution will depict the distinct influence of the topography on friction and ultra-mild sliding wear of common Fe-base materials and point out the marked importance of highly localized effects, which govern the acting mechanisms.     

Residual stresses and sliding wear

The residual stresses that develop during the wear of AISI-SAE 1018 and 4340 steels have been examined. The entire three-dimensional stress tensor was obtained. A normal stress perpendicular to the surface, predicted by theory, has been found, but its magnitude is too small to affect the wear rate. There are also significant shear stresses. The wear process rapidly alters any initial stress distribution produced by heat treatment or peening to such a degree that the wear rate is not affected by these stresses, unless they are initially larger than those that can be produced by the wear parameters.     

Surface morphology in engineering applications: Influence of roughness on sliding and wear in dry fretting

Influence of initial surface roughness on friction and wear processes under fretting conditions was investigated experimentally. Rough surfaces (Ra=0.15-2.52 μm) were prepared on two materials: carbon alloy (AISI 1034) and titanium alloy (Ti-6Al-4V). Strong influence of initial surface roughness on friction and wear processes is reported for both tested materials. Lower coefficient of friction and increase in wear rate was observed for rough surfaces. Wear activation energy is increasing for smoother surfaces. Lower initial roughness of surface subjected to gross slip fretting can delay activation of wear process and reduce wear rate; however, it can slightly increase the coefficient of friction.     

The effects of retained austenite on dry sliding wear behavior of carburized steels

Ring-on-square tests on two kinds of low-alloy carburized steel which were AISI 8620 and 4140 were carried out to study the dry sliding wear behavior. The influence of different retained austenite level of 6% to 40% was evaluated while trying to eliminate other factors. Test results show that the effects of grain size and carburized steel species are negligible in dry sliding wear behavior. While the influence of retained austenite is negligible at 20 kg load condition, wear resistance is decreased at 40 kg load condition as the retained austenite level is increased from 6% to 30%. However, wear resistance is again increased above about 30% of retained austenite level at 40 kg load condition.     

Comments on the sliding wear of metals

The author reviews selected experimental results which have contributed to improved understanding of sliding wear processes. The emphasis is on the chemical and structural changes which occur at and near the surface of metallic materials during sliding in different environments. The importance of plastic deformation, fracture, transfer, mechanical mixing, phase transformations and oxidation is discussed. Examples of transitions are described, and interesting correlations noted. In selecting the content of this paper, the author includes controversial results and conclusions and raises questions about the development of wear equations, interpretations of the wear coefficient, the importance of adhesion, the roles of hardness, the causes of transitions and the location of debris-producing cracks.     

High-temperature sliding wear of metals

Temperature can have a considerable effect on the extent of wear damage to metallic components. During reciprocating sliding, under conditions where frictional heating has little impact on surface temperatures, there is generally a transition from severe wear to mild wear after a time of sliding that decreases with increase in ambient temperature. This is due to the generation and retention of oxide and partially-oxidized metal debris particles on the contacting load-bearing surfaces; these are compacted and agglomerated by the sliding action, giving protective layers on such surfaces. At low temperatures, from 20 to 200°C, the layers generally consist of loosely-compacted particles; at higher temperatures, there is an increase in the rates of generation and retention of particles while compaction, sintering and oxidation of the particles in the layers are facilitated, leading to development of hard, very protective oxide ‘glaze’ surfaces. This paper reviews some of the main findings of extensive research programmes into the development of such wear-protective layers, including a model that accounts closely for the observed effects of temperature on wear rates during like-on-like sliding.     

Silicon segregation in sliding wear

Friction experiments were conducted on a couple consisting of an Fe-Ni pin sliding against a tool steel disk. The Fe-Ni pin contained a small amount of silicon (0.18%). In a mild vacuum environment (0.1 mmHg at 10% relative humidity) at loads below a critical value (13.0 N, at a sliding speed of 0.22 m s^?1), silicon segregation to the pin surface took place, resulting in the formation of a glassy film. Under these experimental conditions the friction coefficient and wear values were very low (0.28 and 5 × 10^?3mm³km^?1 respectively). At loads higher than the critical value (in the same mild vacuum environment) as well as under atmospheric conditions, no silicon segregation could be detected. The corresponding values of the friction coefficient and the wear rate were much higher (0.54 and about 0.4 mm³ km^?1 respectively). It is suggested that the beneficial glassy layer can build up only when the rate of diffusion of silicon to the surface is higher than the rate of material removal (wear).     

The effects of alumina fibres on the sliding wear of a cast aluminium alloy

Dry sliding wear performance of a squeeze cast aluminium alloy-alumina fibre composite has been examined in this investigation using a pin-on-disc machine. A composite in the form of a pin was evaluated against a rotating EN 25 steel disc. The wear response of the base alloy was also studied to assess the influence of a reinforcing phase over a range of applied pressures until the onset of seizure. Incorporation of alumina fibres resulted in superior wear performance of the base alloy, i.e. reduced wear loss, improved seizure pressure and reduced rise in temperature near contact surfaces. Onset of seizure in general caused significantly higher wear loss and temperature rise and large adhesion of the specimen material to the disc surface. A longitudinal cross-section of worn samples suggested nominal wear-induced microstructural changes and deformation in the subsurface regions. The wear surfaces revealed smooth and continuous grooves with less damaged regions prior to onset of seizure, while severe surface damage was observed thereafter. Similarly, debris particles generated during onset of seizure were coarser. The presence of deeper grooves on the wear surfaces and iron mass in the debris particles indicated abrasion to be one of the wear mechanisms in addition to adhesion. That the debris particles were mainly of flake type consisting of microcracks indicated that material removal occurred mainly by delamination.     

Study on transition between fretting and reciprocating sliding wear

An experimental investigation was conducted to find the associated changes in characteristics of wear before and after the transition between fretting and reciprocating sliding wear. A set of experiments were carried out using a AISI 52100 steel ball rubbing against a plate specimen made from the same steel under dry condition. Wear coefficient, wear volume, coefficient of friction, profile of the scars and wear debris were analyzed. The results displayed that there were significant differences in wear coefficient, wear volume, profile of the wear scars and wear debris before and after the transition. Wear coefficient and wear volume at a constant sliding distance were found to be the most appropriate for identifying the transition amplitude between fretting and reciprocating sliding wear.     

Friction and wear at high sliding speeds

The wear of rotating band materials and projectile steel at very high sliding speeds is of great importance in the development of improved cannon. This is especially true for large caliber, high muzzle velocity weapons where excess wear on the bore near the muzzle can limit their useful lives. An extensive experimental study was supported by the U. S. Army from about 1946 to 1956 and a great deal of data was collected at sliding speeds up to 1800 ft s^?1 using a sophisticated high speed pin-on-disk test device. These data indicate that the mechanism of wear at high sliding speeds is surface melting followed by subsequent removal of a portion of the melted surface layer. This means that a rotating band material must be high melting if it is to have good wear resistance at high sliding speeds. It does not mean, however, that compatibility, crystal structure, hardness etc. have no effect because, for a short distance down the bore, the surface of the rotating bands is not completely melted.     

Dry sliding wear of diamond materials

In pin-on-disc tests, diamond composites, consisting of diamonds imbedded in a silicon matrix, were run against themselves in air at a sliding speed of 125 cm s^?1 and for loads up to 3.6 kgf. In addition, a few experiments involving sintered diamond compacts rubbing against a rotating metal ring in a ring-and-block configuration were conducted. For the diamond composite wear tests, wear was found to be proportional to load and sliding distance for P ? 3.0 kgf. For both the diamond composite and the diamond compact, the wear rates were very low and similar to those previously observed for single-crystal diamonds rubbed by diamond and metal.     

Friction and dry sliding wear behaviour of cermets

The friction and wear behaviour of cermets/steel rubbing pairs were investigated. Friction and wear tests were carried out using three different crèmets on the base of tungsten, titanium and chromium carbides under dry sliding conditions against steel disk (0.45% C). Sliding wear tests were carried out using modified block-on-ring equipment at a sliding speed of 2.2 m/s and normal load 40 N.It is shown that wear resistance and coefficient of friction depend on the type and chemical composition of the cermets. The WC–Co cermets have the highest wear resistance. The wear rate of WC–Co and TiC–NiMo cermets increased with increasing binder content in the cermets. The wear of Cr₃C₂–Ni cermets is more complicated and depends on the composition of cermets. The wear of WC–Co cermets is caused mainly by preferential removal of the cobalt binder, followed by fracture of the intergranular boundaries and fragmentation of the carbide grains. The main wear mechanism in the TiC–NiMo cermets is polishing (micro-abrasion) and adhesion, resulting in a low wear rate. The main wear mechanism of Cr₃C₂–Ni cermets involves thermal cracking and fatigue-related crushing of large carbide grains and carbide framework and also adhesion.     

An investigation on sliding wear behavior of PVD coatings

Ti-6Al-4V alloy rubbing against aluminum-bronze 630 was evaluated in this work. High velocity oxygen fuel (HVOF) WC-10%Co-4%Cr thermal sprayed and TiN, CrN and DLC physical vapor deposition (PVD) coatings were applied to increase titanium substrate wear resistance. Pin-on-disk tests were performed with a normal force of 5 N and at a speed of 0.5 m/s, with a quantitative comparison between the five conditions studied. Results showed higher wear resistance for Ti-6Al-4V alloy DLC coated and aluminum-bronze 630 tribological pair and that the presence of graphite carbon structure acting as solid lubricant was the main wear preventing mechanism.     

Surface potential effects on friction and abrasion of sliding contacts lubricated by aqueous solutions

This study has investigated the relationship between applied interfacial electrical potentials and friction and abrasion for steel/steel contacts in alkaline aqueous-based solutions. The potential at a steel-aqueous lubricated surface is important since it determines a number of important surface properties which influence the overall friction and resistance to abrasion. The experimental approach used a pin-on-disc rig incorporating potentiostatic control of the disc between −1.0 and +1.0 V overpotential.Tests employed a load of 50 N at a sliding speed of 0.03 m s⁻¹. EN 24 grade steel, equivalent to AISI 4340, was used as the material for the pin and disc. Tests were conducted in electrolyte that contained electroactive species, namely octanoate ions, which could be “switched on” to the surface. Coefficient of friction measurements were carried out throughout testing and later linked to potential or current density behaviour to assess the mechanical and electrochemical interactions and its effect on wear and friction. The effects of lubricity of the adsorbed layers are discussed and used to explain the performance.