Recrystallization as a controlling process in the wear of some F.C.C. metals

Detailed examination of copper specimens after sliding against 440 C steel in liquid methane at speeds up to 25 m s^?1 and loads of up to 2 kg showed the metal comprising the wear surface to possess a fine cell recrystallized structure. Wear proceeded by the plastic shearing of metal in this near surface region without the occurrence of visible metal transfer. A dynamic balance between the intense shear process at the surface and the nucleation of recrystallized grains was proposed to account for the behavior of the metal at the wear surface. Sliding wear experiments were also conducted on Ag, Cu-10% Al, Cu-10% Sn, Ni and Al. The results were correlated with published hot-working observations and recrystallization kinetics. It was found that low wear and the absence of heavy metal transfer were associated with those metals observed to undergo recrystallization nucleation without prior recovery.     

The wear of wrought aluminium alloys under dry sliding conditions

The sliding wear response of several wrought aluminium alloys (2124, 3004, 5056 and 6092) against a high purity alumina (99.9%) counterface was investigated, at a fixed sliding speed of 1 m/s and a load range of 23–140 N. The counterface was chosen so as to minimise the chemically driven aspects of adhesive wear. Severe wear was observed at all loads, with specific wear rates ranging from 0.37×10⁻⁴ to 2.37×10⁻⁴ mm³/N m. In all cases a mechanically mixed layer (MML) was formed, principally from severely work hardened aluminium alloy, but also including fine alumina particles. The thickness and morphology of the layer depended strongly on alloy composition, but the specific wear rate did not depend on the MML properties in a simple manner. The surface work hardening characteristics differed between alloys, but as with the MML, there was no simple relationship between surface work hardening characteristics and specific wear rate. The main correlation was found between the normalised wear rate and normalised pressure, which implies that the hardness of the starting aluminium alloy is the critical variable.     

Tribological behaviour of duplex treated Ti–6Al–4V: combining nitrogen PSII with a DLC coating

The chemical structure and tribological behaviour of Ti–6Al–4V plasma source ion implanted with nitrogen then DLC-coated in an acetylene plus hydrogen-glow discharge (bias voltage −10 to −30 kV) were investigated. The as-modified samples have a TiN/H:DLC multilayer architecture (coating resistivity 1.6×10⁹ to 2.4×10¹¹ Ω/cm) and exhibit higher hardness, especially at low loads or plastic penetrations in the order of deposition bias voltage −10, −20 and −30 kV. At a lower contact load (1 N) and higher sliding speed (0.05 m/s), frictional properties in most cases improved, as did wear properties. At a higher contact load (5 N) and lower sliding speed (0.04 m/s), friction showed almost no improvement, and wear properties deteriorated. When the material of the counterbody was then changed from AISI 52100 to Ti–6Al–4V modified as the disc (contact load 5 N unchanged, sliding speed decreased), the friction coefficient decreased (but showed no improvement compared with the unmodified sample), while wear properties deteriorated further, and wear was changed from just the disc to both disc and ball, abrasive and adhesive dominated. Transfer films, mainly made up of wear debris transferred from the disc wear surfaces, were formed on the wear scars of the counterbodies. The deterioration of wear properties of the modified samples at the higher contact load is considered to be caused by the “thin ice” effect.     

Chemi-emission of electrons from metal surfaces in the cutting process due to metal/gas interactions

The exo-electron emission behaviour from Ti, W, Mo, Fe, Al, Ni, Cd, Zn and Pb metals was investigated during and after being cut in O₂ and N₂ gases at a gas pressure of 3 × 10⁻² Pa and in a vacuum of 4 × 10⁻⁴ Pa. The results showed that the emission behaviour of electrons during and after cutting depends on the combination of metal and the surrounding gas species. The emission intensity of electrons while cutting various metals in O₂ gas increased sharply with an increase in the negative value of the heat cf formation, ΔH_f of the oxide. The emission intensity in N₂ was also higher at higher negative values of ΔH_f of the metal nitride. Electron emission intensity from the cut metal surface is concluded to be a function of the heat of formation of reaction products of the metal surface with the surrounding gas.     

Nucleation and formation of oxide film with the magnetic field on dry sliding contact of ferromagnetic steel

The nucleation and formation of oxide film in unidirectional dry sliding contact has been studied in a vacuum chamber with and without the application of a magnetic field in the intention of identifying the role of magnetic intensity on the oxidation wear. The wear tests of the steel AISI 1045/steel AISI 1045 couple are investigated on a pin‐disc configuration under three various gas environments: in ambient air, under oxygen at 10⁵ Pa and in vacuum at 5.10⁻⁵ Pa. The formation of oxide layer strongly depends on oxygen partial pressure and magnetic field intensity. These took the form of protective raised ‘islands’ of compacted debris which is developed gradually but rapidly increased as the oxygen partial pressure is increased and which could persist for extended periods during subsequent evacuation. Evidence from various experimental techniques indicates that the compacted debris is a mixture of iron oxides in the form of oxide‐covered particles, although the depth of oxide film has not yet been fully elucidated. Copyright © 2010 John Wiley & Sons, Ltd.     

Super-low friction of MoS2 coatings in various environments

The ultra-low friction coefficient (typically in the 10⁻² range) of MoS₂-based coatings is generally associated with the friction-induced orientation of ‘easy-shear’ planes of the lamellar structure parallel to the sliding direction, particularly in the absence of environmental reactive gases and with moderate normal loads. We used and AES/XPS ultra-high vacuum tribometer coupled to a preparation chamber, thus allowing the deposition of oxygen-free MoS₂ PVD coatings and the performance of friction tests in various controlled atmospheres. Friction of oxygen-free stoichiometric MoS₂ coatings deposited on AISI 52100 steel was studied in ultra-high vacuum (UHV: 5 × 10⁻⁸ Pa), high vacuum (HV: 10⁻³ Pa), dry nitrogen (10⁵ Pa) and ambient air (10⁵ Pa). ‘Super-low’ friction coefficients below 0.004 were recorded in UHV and dry nitrogen, corresponding to a calculated interfacial shear strength in the range of 1 MPa, about ten times lower than for standard coatings. Low friction coefficients of about 0.013–0.015 were recorded in HV, with interfacial shear strength in the range of 5 MPa. Friction in ambient air leads to higher friction coefficients in the range of 0.2. Surface analysis performed inside the wear scars by Auger electron spectroscopy shows no trace of contaminant, except after friction in ambient air where oxygen and carbon contaminants are observed. In the light of already published results, the ‘super-low’ friction behaviour (10⁻³ range) can be attributed to superlubricity, obtained for a particular combination of cystallographic orientation and the absence of contaminants, leading to a considerable decrease in the interfacial shear strength.     

Effect of hydrogenated DLC coating hardness on the tribological properties under water lubrication

Hydrogenated diamond-like carbon (DLC) coatings were deposited using unbalanced magnetron sputtering (UBM) equipment with different hardnesses. Effects of coating hardness on tribological properties were investigated with tribo-tests under water lubrication. Results showed that the wear volume increased rapidly during the initial running-in process, but remained nearly constant after the running-in process. The ball wear rate increased as the hardness of the DLC coating increased when metals (stainless steel and brass) were used as counter parts. In contrast, the UHMWPE ball wear rate was independent of the DLC hardness. TEM analysis and nano-indentation measurements were conducted of the transfer layer on the counter bodies’ contact surfaces. The transfer layer consisted mainly of Fe, O and C. The low friction of DLC coating is attributed to this low hardness transfer layer, which acts as a boundary-lubricating layer with low shear strength.     

Effect of atmospheric pressure on friction and wear of 0.45% C steel in fretting

Oxidative wear is significant in fretting wear when sufficient oxygen is supplied. In vacuum, however, oxide does not form readily. In this paper friction and wear behaviours were studied at various atmospheric pressures in order to clarify the effect of ambient pressure on them.Experiments were conducted with 0.45% C steel at ambient pressures from 1.0 × 10⁵ to 1.3 × 10^?3 Pa. The load was 14 N, the peak-to-peak slip amplitudes were 35 and 110 μm and the frequency was usually 8.3 Hz.Friction behaviours are characterized into three types according to the ambient pressure: 1.0 × 10⁵ ? 10 Pa, 10 ? 10^?1 Pa and below 10^?1 Pa. The coefficient of friction increases with a decrease in ambient pressure below 1 Pa. The critical pressure in fretting is found to be 10 Pa, above which the oxidation rate is independent of the ambient pressure and α-Fe₂O₃ is formed. Wear decreases with ambient pressure below the critical pressure where Fe₃O₄ is formed. Adhesive transfer of metallic debris occurs below 10^?1 Pa.The relationship between the coefficient of friction and oxide thickness is obtained analytically, and the effect of frequency on the oxidation rate is considered.     

Sliding wear behaviour of Zr-ion-implanted D3 tool steel

The wear and friction characteristics of zirconium-ion-implanted AISI D3 tool steel have been investigated using pin-on-disc methods. Ion implantation was carried out using a vacuum-arc-based ion implanter to form multicharged zirconium ion beams at a mean ion energy of 130 keV, and the implantation doses investigated were approximately 3.6×10¹⁶, 5×10¹⁶ and 1×10¹⁷ ions cm⁻². It was found that Zr implantation decreased both the wear and the coefficient of friction. The beneficial effects of Zr implantation in terms of associated Auger electron spectroscopy, Rutherford backscattering spectroscopy and scanning electron microscopy microprobe analyses are described.     

An analysis of the wear features of worm gear tooth flanks

Worm reducers with a double enveloping toroidal surface are a new type of reducer with higher wear resistance and longer service life. A series of contact fatigue tests of worm pairs has been carried out on a microcomputer-controlled test stand of closed power flow type, using an incremental method of increasing the load. During the test, the worm tooth flank completed 2.2 × 10⁷ cycles, and the worm gear tooth flank 1.1 × 10⁶ cycles. Meanwhile, the state of the flank was duplicated by means of a chemical film. This paper discusses the whole process of the occurrence and development of surface cracks as well as the formation of fatigue peeling. It should provide a reliable basis for definition of the law of fatigue damage of the worm gear tooth flank.     

Effect of reinforcement on wear behaviour of aluminium hybrid composites

Abstract

Aluminium metal matrix composites are among the recent developments in engineering applications to meet the present day need of light weight, high strength/weight ratio and good wear properties. In the present study, AlSi10Mg alloy reinforced with 3, 6 and 9 wt-% alumina with constant 3 wt-% graphite particles was produced by stir casting technique. Microstructural investigations as well as evaluation of mechanical properties such as hardness, tensile strength and double shear strength were conducted on composites and unreinforced alloy specimens. Tribological behaviour of hybrid composites was studied using pin on disc test machine. Wornout surfaces were analysed using scanning electron microscopy, and wear debris were analysed using X-ray diffraction. Results revealed that the mechanical properties of hybrid composites were higher than unreinforced alloy. Dry sliding wear test results indicated that the aluminium alloy reinforced with 9 wt-% alumina and 3 wt-% graphite has highest wear resistance compared to unreinforced alloy.     

Hardness,friction and wear studies on hydrogen peroxide treated bovine teeth

A possible problem with peroxide based tooth whitening is the loss of tooth hardness and higher susceptibility to enamel surface wear. This study focussed on the effects of acidic and neutral hydrogen peroxide solutions (6 and 30% w/v) on hardness, friction and wear of bovine enamel. The experiments showed that treatments with neutral peroxide reduced wear and the loss of enamel hardness up to 2–3 times. In addition, further investigation on remineralisation with amorphous calcium phosphate showed an increase in hardness after treatment. Friction coefficients of teeth against steel varied between 0.25 and 0.7, and wear coefficients ranged between ≈10⁻⁶ and 10⁻⁷ mm³/N m. From this study, it is possible to explain the wear behaviour of HP treated enamel with changes in hardness.     

The effect of combined stresses on the transition from mild to severe wear

Using a pin on disc machine experiments were carried out to determine the wear transition stresses for several metals and a simple theory was developed to relate these stresses to the macroscopic hardness of the metals. The results provide strong evidence that the transition from mild to severe wear does depend on both the normal and frictional stresses and that it occurs when the maximum shear stress, in the contacting regions, reaches one-sixth of the material hardness. This criterion however cannot be immediately applied to practical situations as, when the transition starts, the true contact area is still only a fraction of the apparent area.     

Friction and Wear Behavior of Single-Crystal Silicon Carbide in Sliding Contact with Various Metals

Sliding friction experiments were conducted with single-crystal silicon carbide in contact with various metals. Results indicate the coefficient of friction is related to the relative chemical activity of the metals. The more active the metal, the higher the coefficient of friction. All the metals examined transferred to silicon carbide. The chemical activity of the metal and its shear modulus may play important roles in metal-transfer, the form of the wear debris and the surface roughness of the metal wear scar. The more active the metal, and the less resistance to shear, the greater the transfer to silicon carbide and the rougher the wear scar on the surface of the metal. Hexagon-shaped cracking and fracturing formed by cleavage of both prismatic and basal planes is observed on the silicon carbide surface.     

Point contact surface fatigue in zirconia ceramics

The surface deformation and fragmentation behaviour of three zirconia ceramics have been studied by using unlubricated metallic repeated point contact loading at room temperature to investigate the possibilities of cyclic fatigue effects. All tests were conducted on a purpose built computer-controlled apparatus. The zirconias studied were ceria stabilized tetragonal polycrystalline, magnesia partially stabilized, and single crystal calcia stabilized. 120° steel cones were cyclically loaded against the flat, polished zirconia counterfaces, and the damage was observed and analysed as a function of the number of cycles, up to a total of 5 × 10⁵ cycles, for loads of 19.6 ± 9.8 N. The ground tips of the cones plastically deformed during the initial loading cycle to produce a flattened end which conformed with the zirconia counterface. The contact pressures were in the range 4 to 10 GPa. In all cases plastic deformation was observed in the zirconias within, and adjacent to, the contact areas. The degree of plastic deformation increased with increasing number of cycles. After approximately 1 × 10⁴ cycles, localized cracking was induced at the peripheries of the contact zones, which gradually increased in extent until after 5 × 10⁵ cycles there was extensive fragmentation. No material transfer, i.e. metal onto ceramic, or vice versa, was generally observed until after the surface had become rough as a result of the fracturing.     

Sliding wear behaviour of T6 treated A356–TiB2 in-situ composites

Dry sliding wear behaviour of A356–TiB₂ composites in T6 condition was tested using a pin-on-disc wear testing machine. The composites were prepared by the reaction of a mixture of K₂TiF₆ and KBF₄ salts with molten alloy. The wear tests were conducted at normal loads of 19.6–78.4 N and a sliding speed of 1 ms^?1. A detailed SEM study of wear surface and debris was carried out to substantiate the wear results. The results indicate that wear rate of the composites is a strong function of TiB₂ content rather than overall hardness of the composite. The role of Si and TiB₂ particles towards the overall mechanism has been discussed.     

Plate-like wear particle formation in a lubricated ball-on-plate friction pair

The mechanism of formation of plate-like wear particles in a ball-on-plate lubricated friction pair has been examined for wear constants of K < 10⁻¹⁰ (mm³ mm⁻¹ N⁻¹). The plate Vicker's hardness was 2.80–3.00 kN/mm², the sliding speed 1.74 m s⁻¹ and the load 50 N. The following mechanism is suggested: scratching of the surface and formation of ridges at the scratch border, lateral deformation of ridges and formation of thin sheets, and cracking and separation of plate-like particles from these sheets.     

Microstructure and Tribology of Spark Plasma Sintered Fe–Cr–B Metamorphic Alloy Powder

The spark plasma sintering (SPS) process was used to fabricate a bulk Fe–Cr–B alloy (known as Armacor M) from gas-atomized powders. The purpose of this work is to study the microstructure, hardness and tribology of this sintered bulk alloy. Post microstructure and mechanical characterizations were performed to investigate the effects of wear on the microstructure and mechanical properties. Microstructural analysis using X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) showed that SPS successfully produced a fully dense bulk material containing 67 vol.% Cr_1.65Fe_0.35B_0.96 particles dispersed in 33 vol.% solid solution matrix consisting of Fe, Cr and Si. Using nanoindentation, the hardness of the Cr_1.65Fe_0.35B_0.96 particles and the matrix was found to be 24 and 6 GPa, respectively. From microindentation, the bulk hardness of the sintered alloy was 9.7 GPa (991 HV). Dry sliding wear testing under mild conditions (i.e., initial Hertzian mean contact pressure of 280 MPa) was conducted against a stainless steel pin. The steady state coefficient of friction against Armacor M was about 0.82. The wear of Armacor M proceeded primarily by adhesive and mild oxidative wear. The wear volume for Armacor M was 80% less than that of carbon steel and its wear rate was 5.53 × 10⁻⁶ mm³ N⁻¹ m⁻¹.     

Wear of metals at high sliding speeds

High speed sliding wear of AISI 1020 steel, AISI 304 stainless steel and commercially pure titanium (75A) was studied using a pin-on-ring geometry. All the tests were carried out in air without any lubricant. The sliding speed was 0.5–10.0 m s^?1 and the normal force was 49.0 N (5 kgf).The friction coefficient of all the materials tested decreased with the sliding speed; this appears to be a consequence of oxide formation. The wear rate of 304 stainless steel increased monotonically with speed, whereas the wear rate of 1020 steel and titanium first decreased and then increased and again decreased, with a maximum occurring at about 5 m s^?1. The complex variation of the wear rate as a function of speed is explained in terms of the dependence of the friction coefficient, hardness and toughness of the materials on temperature. Microscope examinations of the wear track, the sub-surface of worn specimens and the wear particles indicate that the wear mode was predominantly by subsurface deformation, crack nucleation and growth processes, i.e. the delamination process, similar to the low speed sliding wear of metals. Oxidative and adhesion theories proposed in the past to explain the high speed sliding wear of metals are found to be incompatible with the experimental observations.     

Initial wear of gears

Running-in experiments have been made in a back-to-back gear test rig with hobbed and shaved gears of quenched and tempered steels at different steels at different speeds and loads. Measurements of surface roughness and examinations of surface replicas were used to study the running-in process. The present results show that the running-in period corresponds to less than about 0.3×10⁶ revolutions. The initial wear is rather small but increases with running speed and applied load