Friction coefficient and wear mode transition in micro-scale abrasion tests

The purpose of this work was to conduct ball-cratering wear tests to monitor both normal and tangential forces. Balls of 52100 steel, a specimen of H10 tool steel and an abrasive slurry prepared with silicon carbide particles and distilled water were used. Optical microscopy analysis of the worn craters revealed the presence of only grooving abrasion. However, a more detailed analysis conducted by SEM has indicated that different degrees of rolling abrasion have also occurred along the grooves. The results also showed that the normal force plays an important role in the scattering of the values of the friction coefficient.     

Friction and wear behaviour of sintered steels submitted to sliding and abrasion tests

Fe–C–Mo and Fe–C–Cr steels were sintered by PM processes carried out using different values of temperature and pressure, leading to different microstructures and density values. Flat specimens were submitted to tribological tests in order to evaluate their behaviour under both dry sliding and abrasive wear conditions. A flat-on-cylinder tribometer was used for the sliding tests, while a micro-scale ball cratering device was used for the abrasion tests. The dry sliding wear resistance of the PM steels was mainly influenced by the composition and sintering conditions. In this regard, the best behavior was observed for the more hardenable Fe–C–Mo steels with higher Mo content, sintered under conditions giving rise to bainitic microstructures. A determining role was also played by the porosity content and pore shape: reduction in porosity (obtained by increasing the sintering temperature and the compacting pressure), as well as an increase in pore roundness, led to a significant improvement in the resistance to sliding wear. A mild oxidative wear regime were observed for all the sintered steels under relatively low values of the applied load, while an increase of the applied load led to a delamination wear regime. The resistance to abrasive wear was low for all the tested steels, irrespective of composition and sintering cycle.     

Friction and wear tests for elastomers

A common use of elastomers in business machines is as rollers, controlling the motion of such items as checks, paper forms and printer ribbons. In these applications both the friction and the wear resistance characteristics of the elastomer are of major significance. In this paper, two tests, developed to perform such characterizations against paper and ribbon surfaces, are described. Two case histories in which these test methods were effectively employed in resolving design problems are discussed. In addition, data for several elastomers are presented and compared.     

Mechanical modelling of micro-scale abrasion in superfinish belt grinding

In synchronized transmission, the shift lever force and corresponding shiftability are the most important design considerations. The present paper introduces an engineering attempt to rigorously model a synchronizing functional surface (cone surface of idler gear) according to its finish specifications. The virtual input surface is generated by an original fractal function, which reproduces the surface “signature” due to the wheel grinding process. To model the subsequent superfinishing operation by belt finishing process, which uses a soft-coated belt as a tool, an algorithm simulating the abrasive polishing conditions is especially developed and applied to rework the initial fractal surface. The basic idea of this model is that the higher the height of a peak of the profile, the lower its probability of resistance during an abrasion cycle. The belt finishing process is hence modeled by five parameters: two parameters that characterize the initial surface (fractal dimension and range amplitude) and three parameters describing the abrasion polishing process (probability of resistance, wear volume and the number of abrasion cycles). In order to ascertain that the component will be manufactured to the required specifications, the model's parameters have to be determined. For this goal, a functional model with an optimization scheme is created. This simulation provides the morphology of the initial surface and how to cope with the superfinishing process to obtain the functionality of the surface. An elevated initial roughness is required from which slow erosion is proceeded to erode peaks and conserve some valleys of the initial profile (lubricant tanks). Finally, it is shown that automotive designers impose morphological specifications obtained by the belt grinding process to prevent scuffing of the motor parts.     

Friction and wear bench tests of different engine liner surface finishes

Six different bore finishes of combustion engines were bench-tested against PVD-coated rings. Ring and liner specimens were removed from actual Heavy-Duty Diesel engine. The test was conducted in a CETR UMT-2 reciprocating tester. Current bore honings, e.g. slide honing, as well as recently developed structured laser were tested.For friction bench tests, the specimens were tested at different reciprocating speeds and loads. When lubricant condition approached the boundary regime (speed/load tending to zero), most of the finishes showed the usual boundary friction coefficient 0.11. When the conditions tended to the hydrodynamic regime, the specimen removed from the non-laser region of the laser-structured finish showed a remarkably low friction coefficient, followed by the laser-structured ones.For wear bench tests, the lubricant was doped with hard particles, and higher load and speed were used to accelerate the wear rate. Wear tests ran for 4 h and, as expected, friction coefficient reduced during the wear test due to break-in. In general, friction during the wear test followed the friction test ranking. The remarkable exception was the UV laser finish, which initially presented one of the highest coefficient and after 0.5 h of testing turned to be one of the lowest. Liner and ring wear was measured post-test by profilemeter. In general, the smoother surfaces presented lower ring and bore wear.Greenwood asperity parameters and Patir and Cheng flow factors were calculated from two measured surfaces. A computer model was applied to calculate the oil film thickness and friction for the different speed and load conditions. The calculated friction results showed good adherence to the measured data.     

Friction machine for accelerated wear tests of frictional rolling elements

The construction of a friction machine for accelerated tests of friction rolling elements for frictional torque and wear. A study of the influence of rolling resistance force, rotational speed, and external load on the wear of samples has been carried out. A regression mathematical model that makes it possible to predict the amount of wear on the components of the friction pair under loading conditions has been obtained.     

Friction and wear of ceramics

The adhesion, friction, wear and lubricated behaviors of both oxide and non-oxide ceramics are reviewed. Ceramics are examined in contact with themselves, other harder materials and metals. Elastic, plastic and fracture behavior of ceramics in solid state contact is discussed. The contact load necessary to initiate fracture in ceramics is shown to be appreciably reduced with tangential motion. Both friction and wear of ceramics are anisotropic and relate to crystal structure as with metals. Grit size effects in two- and three-body abrasive wear are observed for ceramics. Both free energy of oxide formation and the d valence bond character of metals are related to the friction and wear characteristics for metals in contact with ceramics. Surface contaminants affect friction and adhesive wear. For example, carbon on silicon carbide and chlorine on aluminum oxide reduce friction while oxygen on metal surfaces in contact with ceramics increases friction. Lubrication increases the critical load necessary to initiate fracture of ceramics both in indentation and with sliding or rubbing.     

Further look at correlation between ASTM G65 rubber wheel abrasion and pin-on-disc wear tests for data conversion

Abstract

It is desirable that wear testing results obtained using different techniques can be mutually converted. In this study, wear rates of two groups of materials, i.e. cast iron and steel, were evaluated using the ASTM G65 dry sand rubber wheel abrasion and pin-on-disc wear testers respectively. The conversion between results obtained using the two different methods was investigated. It was shown that the two sets of wear data can be mutually converted, following a linear relation. The slope and position of the line that fits the data are, however, affected by Young’s modulus of the materials and the applied load. Such effects are attributed to the fact that the wear rate of the materials measured using the G65 method was influenced by not only hardness but also elastic modulus, while for the pin-on-disc tests, the wear rate was more dominated by hardness of the target materials especially under larger applied loads. Relevant mechanisms are discussed with a further look at the Archard’s wear equation.     

Friction and wear processes in piston rings

It has been recognised that a large part of the top piston ring wear of an ic engine takes place in boundary lubrication around top dead centre (tdc) position. A quantitative assessment of the friction behaviour using actual piston ring and cylinder liner under conditions close to tdc has been made. The factors responsible for wear under these conditions have been identified as surface temperature, peak combustion pressure, total energy on the wearing surfaces and other physical properties of the material under sliding     

The role of abrasion and corrosion in grinding media wear

A range of ferrous grinding media compositions was subjected to laboratory marked ball wear tests in an ongoing research program at the Mineral Resources Research Center which was aimed at elucidating the responsible wear mechanisms during the grinding of minerals. Most of the wear data in this paper were obtained using a model quartz-pyrrhotite mineral slurry but these have been supplemented, where appropriate, with a small amount of data from wear tests on taconite ore and a Cu-Ni gabbroic ore. The role of abrasive and/or corrosive wear was determined by comparing the wear rates which result under a range of grinding atmospheres, i.e. O₂, N₂ or compressed air, flushing through a laboratory mill of 8 in diameter coupled with examination of ball surfaces after grinding using scanning electron microscopy.     

Influence of the normal force,abrasive slurry concentration and abrasive wear modes on the coefficient of friction in ball-cratering wear tests

The purpose of this work is to study the influence of the normal force (N), abrasive slurry concentration (C) and abrasive wear modes on the coefficient of friction in ball-cratering wear tests. Experiments were conducted with balls of AISI 52100 steel, an AISI H10 tool-steel specimen and abrasive slurries prepared with black silicon carbide (SiC) particles+distilled water. The tangential (T) and normal loads were monitored throughout the tests and the results have shown that: (i) the coefficient of friction behavior was independent of the normal force and (ii) both the concentrations of abrasive slurries and the subsequent action of the abrasive wear modes, generally, did not affect the behavior or magnitude of the coefficient of friction.     

Friction coefficient and wear rate of sputter-deposited thin films and plasma-sprayed thick coatings at high temperatures in room air

Silver, calcium fluoride (CaF_x with x = 1.85) and chromium-carbon (Cr₃C₂) thin films were deposited onto various tribological test specimens by sputtering. The friction properties of sputter-deposited Ag and CaF_x single layers as well as Ag/CaF_x multilayer films were determined by ball-on-disk tribological tests conducted in room air under various experimental conditions. The tribological properties (friction coefficient and wear rate) of sputter-deposited CaF_x films were also determined at 500°C by pin-on-disk tribological tests performed with pin specimens made of cobalt-based alloy (alacrite). Chromium-carbon films sputter-deposited onto alacrite disk and counterfaces were found to be of interest for reducing the formation of alacrite wear debris in the wear tracks; thus reduced friction coefficient and wear rate values were obtained. The friction behavior of sputter-deposited CaF_x/Cr₃C₂ thin bilayer structures and plasma-sprayed (PS) chromium carbide/Ag/BaF₂-CaF₂ eutectic composite coatings (PS-212 type coatings) was investigated by plane-on-plane tribological tests conducted in room air at 500°C and 700°C. The friction performance of solid lubricant thin bilayer films was compared with that of thick PS-212 type coatings similar to coatings developed by NASA.     

A statistical model describing wear traces in three-body abrasion

Cutting wear is a mechanism by which material is removed through three-body abrasion. In the present paper, a statistical model describing the wear traces on a worn surface is proposed for three-body abrasion, to estimate cutting wear as a proportion of the total wear of the test materials. Using this model, a statistical analysis of the traces on the worn surface was made after short-travel, three-body abrasion tests. The results showed that cutting was not a great proportion of the total wear of the test materials under the conditions of three-body abrasion.     

Friction and wear of highly-elastic solids

It is shown that the normal forces set up by large shear deformations make sliding impossible when a critical compressive deformation, of the order of 10%, is imposed on a sliding block of highly-elastic material. When the compressive load is maintained constant, rather than the deformation, the way in which the compressive stress is distributed leads to sliding only by means of slip bands which originate at the leading edge of the block and then pass through the contact zone from front to back, as found by Schallamach¹. Thus, a discontinuity is predicted in frictional sliding as the compressive stress is increased from zero, and this occurs at a critical value which depends upon the coefficient of friction, the shear modulus of the material, and the detailed shape of the contact zone. Corresponding changes in wear behavior are inferred.     

Friction and wear of sintered iron

Using a wear test rig of the disc and pin type the influence of the sliding velocity V and contact pressure P on the friction and wear of sintered iron under unlubricated (dry) conditions was studied. The results showed that for a fixed test duration the coefficient of friction and wear varied according to the law KP^aV^b.     

Friction and wear of aluminium-silicon alloys

The friction and wear of aluminium-silicon alloys have been studied using a pin-on-disk machine together with metallographic investigations, microhardness testing and scanning electron microscopy. Silicon content does not appear to be a dominant factor in wear resistance. The eutectic alloy was the most wear resistant of the alloys studied. The predominant wear mode appears to be by separation of all or part of the work-hardened layers from random patches on the wear track. Nucleation sites for wear debris are probably created below the sliding surface as a result of Hertzian stressing.     

Friction and wear of Cu-Ni alloys

The friction and wear of Cu-Ni alloy blocks sliding against 440C and coated steel rings have been studied. Sliding speed was 50 mm s^?1 and test atmospheres were argon and dry air. A wide range of experimental techniques was used to characterize the worn specimen and the debris particles. X-ray diffraction, scanning electron microscopy, and energy-dispersive analysis of X-rays have been particularly helpful. Transfer of ring material was observed in most cases, especially when the debris particles were ferromagnetic. The wear rates of the Cu-Ni blocks are largely determined by the amount of transferred ring material. The wear rates of the Cu-Ni alloys decrease with increasing transfer of ring material. However, the wear rate of the ring increases as transfer continues. This process often leads to a transition involving severe damage to the ring. Brushes which remove most of the loose debris significantly reduce transfer and postpone the transition.