J.J. Wert

The influence of stacking fault energy and adhesion on the wear of copper and aluminum bronze

Abstract The subsurface deformation induced in pure copper and a series of Cu-Al alloys has been characterized using scanning electron microscopy after prolonged dry sliding contact against an Al 2 O 3 cylinder. Scanning electron microscopy was found to be a very useful technique for examining the deformation in depth. The strength of the adhesive bond between copper and Al 2 O 3 was found to be less than the cohesive strength of either material. Therefore true transfer layers were not formed. Owing to dynamic recrystallization occurring near the surface, extremely large strains ( i.e. greater than 12) were sustained and resulted in very low wear rates in agreement with the results of Bill and Wisander. A mechanically mixed layer, which is continuous with the substrate and incorporates oxide particles, was observed. Two mechanisms were identified for the production of wear debris. The layered structure proposed by Heilmann to explain the random misorientation of subgrains was confirmed. The adhesive strength of the Cu-Al alloys examined with Al 2 O 3 was greater than the cohesive strengths of the alloys. Therefore metal transfer to the slider occurred. The subsurface deformation behavior of aluminum bronze was found to be very dependent on both the stacking fault energy (SFE) and the orientation of individual grains and is related to the wear rate. In the case of aluminum bronze sliding against A1 2 O 3 , adhesion effects overshadow the influence of SFE. However, in other systems, it is expected that SFE effects will play a dominant role in determining wear response.

Erosion resistance of diamond coatings

Abstract Preliminary solid particle erosion testing of 10 μm thick diamond films on silicon substrates showed excellent erosion resistance. The application of diamond coatings to materials that are commonly used in erosive environments should result in significant improvement over other erosion-resistant coatings. Diamond films were applied to silicon, tungsten and Ti-6Al-4V using a high-pressure microwave-assisted plasma deposition system and were determined to have a high degree of sp 3 bonding using Raman spectroscopy. Erosion tests were performed on diamond-coated silicon, tungsten and Ti-6Al-4V, as well as on uncoated substrates and alternative coatings. The erosion resistances of the target materials were compared using volume loss versus erodent mass plots and steady-state erosion-rate data. Diamond-coated silicon showed the highest erosion resistance. Diamond-coated Ti-6Al-4V also showed high erosion resistance when compared with uncoated materials. Erosion mechanisms were identified by examination of eroded surfaces using scanning electron microscopy. Diamond coatings on Ti-6Al-4V substrates fail by delamination of the coating from the substrate. This is believed to be a result of high residual stresses and stress concentrations at the coating-substrate interface. Diamond coatings on silicon and tungsten substrates are gradually removed by a brittle erosion process.

An x-ray diffraction study of the effect of stacking fault energy on the wear behavior of Cu-Al alloys

Abstract This study details the effects of stacking fault energy (SFE) in the wear process of several Cu-Al alloys. An X-ray line broadening analysis was made of the wear-induced deformation caused by a reciprocating sapphire slider in a dry argon atmosphere and also of cold-worked filings of the same compositions. The X-ray line profiles were analyzed by both the WarrenAverbach multiple-order method and the Rothman-Cohen method. The resulting stacking fault probabilities, the r.m.s. strains and the effective particle sizes were correlated with the wear rate. The analysis has confirmed several general trends that accompany the Cu-Al alloys with decreasing SFE: increasing r.m.s. strain, decreasing effective particle size, increasing stacking fault probability and increasing wear rate. The deformation substructure associated with cold-worked filings is very similar to that observed in the transfer layer of this series of alloys.

The influence of surface finish and strain hardening on near-surface residual stress and the friction and wear behavior of A2, D2 and CPM-10V tool steels

Abstract The effect of surface finishing method on near-surface residual stress, relative cold work and sliding friction and wear behavior of commercially available air hardening A2, D2, and CPM-10V tool steels was investigated. Microstructural characterization was accomplished using X-ray diffraction and scanning electron and optical microscopy combined with macrohardness measurements. The wear tests were performed using low load-slow speed reciprocating sliding conditions with tool steel flats in unlubricated contact with a 52100 steel spherical slider. The coefficient of friction and mass loss were measured during the course of wear testing and residual stress measurements were made before and after wear testing. Surface grinding and polishing conditions had a significant effect on surface residual stress but had little effect on tool flat sliding wear rates. Residual stress measurements performed after wear testing showed a decrease in compressive residual stresses as wear rate increased. Tool steel wear rates were a linear function of sliding distance except for the initial run-in period. The low wear rates, the run-in friction behavior, and the constancy of near-surface mechanical properties indicate that during run-in surface finish was wear rate controlling and that sample microstructure and composition controlled long-term sliding friction and wear behavior. The low wear rates and the post wear residual stress measurements also indicate that oxidative wear was the primary mechanism responsible for tool flat material attrition. CPM-10V steel flats exhibited the lowest coefficient of friction and the highest resistance to wear which can be attributed to the high volume fraction and homogeneity of V-Cr-C carbides.

The role of oxidation in the friction and wear behavior of solid solution Cu-Al alloys in reciprocating sliding contact with sapphire and D2 tool steel

Abstract The tribological behavior of dilute solid solution Cu-Al alloys in sliding contact with sapphire and D2 steel was investigated. The worn and unworn contacting surfaces and wear debris were characterized using an appropriate combination of X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, optical microscopy, optical stereomicroscopy, hardness and X-ray photoelectron spectroscopy (XPS). Wear testing was performed with a slow speed reciprocating wear tester with cylinder-on-flat geometry. The coefficient of friction (COF) was recorded for each wear cycle during run-in and periodically recorded during the steady state wear regime. Cumulative wear rates were measured for both the Cu-Al alloy flats and slider cylinders. The wear rate of the solid solution Cu-Al alloys was found to increase with increasing aluminum content. Worn alloy surfaces exhibited increasing mechanical damage and decreasing copper oxide coverage as aluminum content increased. High aluminum contents were found to promote planar slip, adhesive wear and the formation of metallic wear debris. Increased adhesion and metallic transfer resulted in a parallel increase in the COF. Surface segregation of aluminum was observed in the wear track areas for all alloy compositions. Angle-resolve XPS (ARXPS) analysis showed significant aluminum enrichment of the surface for as-polished and partially sputter-cleaned Cu-6wt.%Al alloys. For the tribosystem described, the friction and wear behavior of solid solution Cu-Al alloys is dependent on the adherence of the wear-induced copper oxide to the underlying substrate. Neither increased alloy hardness nor the ability to strain harden increased wear resistance. Instead, as aluminum content increased, a substantial increase in the adhesive wear of the Cu-Al alloy flats was observed during the run-in period. Surface segregation of aluminum resulted in the formation of an aluminum oxide layer. The incompatibility of A1 2 O 3 and the metallic substrate creates an interface susceptible to disruption by surface shear forces during the wear process. The increased interfacial adhesion results in alloy transfer to the opposing slider and an increase in wear.

The role of stacking fault energy and induced residual stresses on the sliding wear of aluminum bronze

Abstract An analysis was made of the residual stresses induced during sliding contact of a series of Cu-Al alloys of various stacking fault energies (SFEs) using the conventional two-exposure X-ray technique. Compressive residual stresses were found in all alloys which increased with decreasing SFE in a manner analogous to the strain-hardening behavior. The macroscopic wear rate was found to be linearly related to the compressive stresses induced during the wear process. The increase in wear rate associated with increasing hardness and residual compressive stresses is attributed to the presence of a residual tensile stress normal to the surface.

A microscopic study of the behavior of selected Al-Cu alloys in unlubricated sliding wear☆

Abstract A study of the effects of a second phase on sliding wear resistance has been conducted under several dry sliding conditions. This investigation employed Cu-11.2Al and the four distinct microstructures available from Al-4.5Cu with proper heat treatment. Additionally, single-phase reference materials were included for the simulation of matrix phase response to wear testing for comparative purposes. Specimens were run in pin-on-disk, spiraling pin-on-disk, and scratch-type wear tests. Wear-induced damage was evaluated by optical and electron microscopy and correlated with the results of mechanical tests. Results suggested that for cases mainly involving abrasion, the principal effect of the presence of a second phase on wear resistance is the resultant increase in bulk hardness. For predominantly adhesive wear events, involving extensive plastic shearing of counterfacial surfaces and transfer, the principal effect of a second phase is microstructural — refining the average size of transfer particles and wear debris. The nature of the second phase, with respect to both morphology and mechanical compatibility with the matrix, will determine the magnitude of the effect.

The effects of gaseous environments on the wear of commercial purity titanium

Abstract The effect of several gaseous environments on the wear of commercial purity titanium has been investigated with the aid of the scanning electron microscope. Hydrogen, nitrogen and laboratory air have been used in both the extremely dry and the humid conditions. Changes in wear susceptibility by variations in environment change the basic mechanisms of wear asperity deterioration (particle formation). Increased wear rate in dry hydrogen is due to an accelerated surface fatigue mechanism. The three gases produce essentially the same wear characteristics when sufficient water vapor is present. This is attributed to the adsorption characteristics of the wear couple-environment system resulting in severe wear by surface deterioration due to enhanced adhesion.

A model for adhesive wear

From a fundamental viewpoint, adhesive wear must be related to the cohesive strengths of the contacting materials and the adhesive strength of the interface. These quantities are related to the electronic structures of the specific atomic species involved. Using an energy balance approach, an expression has been derived which permits calculation of the adhesion energy of two contacting materials from fundamental quantities. A model based on the ratio of the energy of adhesion to the cohesive energy of an asperity junction has been developed to predict the relative wear rates of a variety of metal/metal couples. Using high-purity Cu cylinders sliding against pure Fe, Ni, Nb, Mo, Ti, W, V, and Zr, the validity of the model was experimentally confirmed using a reciprocating right cylinder-on-flat tribometer.

Ion-beam-enhanced adhesion of iron films to sapphire substrates

Abstract The effect of implantation of different ion species on the adhesion of iron films to sapphire substrates was investigated. The implantation energies were adjusted to ensure that the ion concentration profiles, damage profiles, and recoil distributions were the same for each species. For all implantations, the peak ion concentration was at the film-substrate interface. The adhesion of the films was measured by a pull test and a scratch test. For a fluence of 1 × 10 15 ions cm -2 , implantation of chromium (300 keV) and iron (320 keV) increased the bond strength whereas implantation of nickel (340 keV) did not. The effect is proposed to be due to changes in the interfacial energy resulting from the presence of the ion species at the interface. Only a narrow zone is affected; the mixing at the interface is less than 10 nm.