The influence of oxygen and water vapour on the friction and wear of an aluminium alloy under fretting conditions

An investigation was conducted to determine the effect of oxygen and water vapour on the friction and wear behaviour of an Al-Zn-Mg alloy under fretting conditions. Fretting wear experiments were carried out in wet air, dry air and in dry argon. In this case the peak-to-peak relative slip amplitude was varied from 20 to 260 μm to determine the critical slip amplitude of fretting wear in these environments.The experimental results indicated that the wear rates in dry air and in dry argon under macroslip conditions were almost the same and quite lower than the wear rate in wet air. This revealed that the effect of oxygen on fretting wear was not large but that water vapour accelerated the fretting wear of the aluminium alloy. The cyclically softened material due to overaging was observed below the contact surface during fretting in wet air. The mechanism involved rapid fretting wear in wet air which caused the removal of a heavily work-hardened layer as it was formed but the softened material below it was not removed.     

Elevated Temperature Evaluation of Fretting and Metal Transfer between Coated Titanium Components

During fretting, small amplitude displacements and high normal surface loads combined with abrasive oxide particles cause surface damage that acts as initiation sites for fatigue cracks. Since these conditions are prevalent within the titanium dovetail joints of jet engines a wear mode analysis was performed on extended service jet engine disks and compressor blades. The results of the wear mode analysis indicated that titanium from the uncoated disk was transferred to the softer copper-nickel-indium coated dovetail surface of the blades. This transfer created titanium on titanium contact and eventually fretting wear. In order to simulate these conditions, a moderate displacement (125 μm), low cycle phase followed by a small displacement (25 μm), high cycle fretting phase utilizing a cylinder on flat configuration was developed. The analysis and test procedure developed during this study will ultimately aid in the selection and evaluation of a new coating capable of preventing fretting.     

Influence of Deposition Positions on Fretting Behaviors of DLC Coating on Ti-6Al-4V

Abstract

The influence of diamond-like carbon (DLC) coating positions—coated flat, coated cylinder, and self-mated coated surface tribopairs—on the fretting behaviors of Ti-6Al-4V were investigated using a fretting wear test rig with a cylinder-on-flat contact. The results indicated that, for tests without coating (Ti-6Al-4V–Ti-6Al-4V contact), the friction (Q_max/P) was high (0.8–1.2), wear volumes were large (0.08–0.1?mm³) under a large displacement amplitude of ±40 µm and small (close to 0) under a small displacement amplitude of ±20 µm, and the wear debris was composed of Ti-6Al-4V flakes and oxidized particles. For tests with the DLC coating, under low load conditions, the DLC coating was not removed or was only partially removed, Q_max/P was low (≤0.2), and the wear volumes were small. Under high load conditions, the coating was entirely removed, Q_max/P was high (0.6–0.8), and the wear volumes were similar to those in tests without coating. The wear debris was composed of DLC particles, Ti-6Al-4V flakes, and oxidized particles. The DLC coating was damaged more severely when deposited on a flat surface than when deposited on a cylindrical surface. The DLC coating was damaged more severely when sliding against a DLC-coated countersurface than when sliding against the Ti-6Al-4V alloy.     

A Study of Surface Damage at Low-Amplitude Slip

The effect of extremely small slip amplitudes (0 to 5 μm) on transitions in the fretting process, such as initiation of surface damage, development of severe surface damage, microcrack initiation and the development of mild wear, was investigated. For SAE 52100 against SAE 52100 steel, the minimum slip amplitude associated with the onset of mild oxidation or surface staining was approximately 0.06 μm. Studies at higher amplitudes of motion indicated a transition from minimal surface damage to severe or significant damage at 2.8 μm. A further slight increase in amplitude to approximately 3.0 μm resulted in a transition into a regime characterized by fatigue crack formation. These transformations were found to be influenced to some extent by material composition and hardness. The onset of severe surface damage occurred at 1.1 μm for SAE 52100 against SAE 1018 and at 0.5 μm for a nickel chrome Hastelloy B alloy against SAE 1018 steel. In general, the amplitude of microslip characterizing the transition from extremely mild to severe surface damage was found to increase with increasing material hardness.     

The fretting wear of Ti-6Al-4v and aged inconel 718 at elevated temperatures

The fretting wear of Ti-6Al-4V and Inconel 718 was investigated with a sphere-on-flat configuration. The spherical surface was 100 mm in radius and in all tests was made of the same material as the flat. The normal load was 2.75 N and the frequency of the tangential movement was 50 Hz. Two amplitudes of slip were used, 10 and 40 μm. Tests were conducted in air at temperatures up to 600 °C for the titanium alloy and up to 540 °C for the nickel alloy. High temperature strain gauges enabled a continuous record of the tangential stress to be made and subsequent calculation of the coefficient of friction. Wear was assessed from measurement of the scar volume. At 280 and 540 °C at an amplitude of 40 μm the coefficient of friction and wear rate decreased to a low value on the nickel alloy. This only occurred at 540 °C for the lower amplitude of slip. Low friction and wear are associated with the formation of a “glaze” oxide, which requires a larger slip amplitude at lower temperatures for its formation.The titanium alloy generally exhibited higher coefficients of friction which continued to increase at 10⁶ cycles, although wear rates at 200 °C and above were comparable with those on the nickel alloy. “Glaze” oxide begins to form at 200 °C and is well developed at 400 °C. At 600 °C breakdown occurs owing to local creep of the substrate.     

Surface Hardness and Abrasive Wear Resistance of Ion-Implanted Steels

The hardnesses of nitrogen-implanted steel surfaces have been measured with an abrasive wear technique capable of characterizing surface layers as thin as 25 nm. Treated steel disks and reference disks were abraded with 1–5 μm diamond, and relative wear resistances were calculated from the mass losses. Surface hardness was obtained from a relationship between wear resistance and hardness.

The surface of a hardened and tempered carbon steel implanted with nitrogen ions (10¹⁷/cm²) was significantly harder than with other treatments including quench hardening and nitriding. The hardness decreased to the bulk value over a depth corresponding to the initial implantation depth.

Nitorgen-implanted stainless-steel surfaces wore faster than un-implanted ones, possibly due to interference with transformation hardening which normally occurs during wearing. This “softening” effect persisted to depths several times the depth of implantation, and may help to explain the reduction of sliding wear produced by the implantation of stainless steels. Analyses by Auger electron spectroscopy indicated nitrogen migrated toward the bulk during wear.

Titanium implanted in stainless steel (4.6 × 10¹⁷ ions/cm²) produced a very hard surface with more than 10 times the abrasive wear resistance of the bulk metal.     

The influence of electrochemical potential on wear

Electrochemical potentials between ?1.4 and +1.2 V (saturated calomel electrode) applied to Ni(200) in the nominally inert electrolyte 1 M NaClO₄ resulted in an observed order of magnitude variation in the wear rate of the nickel under fretting and sliding conditions. The pin-on-disc experiments were carried out with a fused Al₂O₃ pin loaded at 9.8 and 19.6 MPa sliding on discs at 10 m min^?1, and fretting wear was simulated by a high frequency (10⁵–10⁷cm^?2s^?1) low amplitude impact of a vibrating Al₂O₃ pin on the disc. These results suggest that it is possible to control wear in ionically conducting fluids through the application of appropriate electrochemical potentials.     

The influence of experimental conditions on the wear of the metal surface during fretting of steel on polycarbonate

The effects of experimental conditions on the amount of wear of the metal surface during fretting of steel on polycarbonate in laboratory air have been studied within the following limits: amplitude 2–20 μm, frequency 10–120 Hz and normal load 130–830 g. The influence of water vapour on the wear has also been investigated.The polycarbonate induces fretting damage of the steel, with α-Fe₂O₃ particles being transferred from the steel to the polymer surface. After an incubation period during which wear does not take place a running in period occurs during which the rate of wear decreases with the number of cycles, followed by a steady state period, during which the rate of wear remains fairly constant. The length of the incubation period generally increases with decreasing amplitude of slip and with increasing frequency of vibration, while the amount of subsequent wear generally increases with increasing amplitude of slip, with decreasing frequency of vibration and with decreasing applied load within the range studied. It is found that water vapour content has the most significant effect on the amount of wear. In moist oxygen, moist argon and moist nitrogen (relative humidity about 85%) the amount of wear is greater than in laboratory air (relative humidity about 50%), while in dry gases virtually no wear of the metal is observed.     

High speed tribological properties of graphite fiber/ Cu-Sn matrix composites

High speed dry friction experiments of graphite fiber/Cu-Sn matrix composites against steel were conducted at sliding velocities up to 235 m s^?1. The composite samples were prepared by the method of liquid metal infiltration. It has been determined that the friction coefficient and the wear rate depend on the amount of tin in the matrix, orientation of fibers relative to the sliding surface, the sliding velocity, the graphite grain size and the degree of liquid metal infiltration within the fibers. The increase in tin content leads to a decrease in both friction and wear due to an increase in matrix hardness. Specimens tested with the fibers oriented perpendicular to the sliding surface exhibit better frictional behavior than those with fibers parallel to the sliding surface. Both friction coefficient and wear rate reach a minimum value at a velocity between about 120 and 180 m s^?1. Large graphite grain size and poor liquid metal infiltration within the fibers have a detrimental effect on wear.     

Fretting Wear Behavior of Medium Carbon Steel Modified by Low Temperature Gas Multi-component Thermo-chemical Treatment

The introduction of surface engineering is expected to be an effective strategy against fretting damage. A large number of studies show that the low gas multi-component (such as carbon, nitrogen, sulphur and oxygen, etc) thermo-chemical treatment(LTGMTT) can overcome the brittleness of nitriding process, and upgrade the surface hardness and improve the wear resistance and fatigue properties of the work-pieces significantly. However, there are few reports on the anti-fretting properties of the LTGMTT modified layer up to now, which limits the applications of fretting. So this paper discusses the fretting wear behavior of modified layer on the surface of LZ50 (0.48%C) steel prepared by low temperature gas multi-component thermo-chemical treatment (LTGMTT) technology. The fretting wear tests of the modified layer flat specimens and its substrate (LZ50 steel) against 52100 steel balls with diameter of 40 mm are carried out under normal load of 150 N and displacement amplitudes varied from 2 μm to 40 μm. Characterization of the modified layer and dynamic analyses in combination with microscopic examinations were performed through the means of scanning electron microscope(SEM), optical microscope(OM), X-ray diffraction(XRD) and surface profilometer. The experimental results showed that the modified layer with a total thickness of 60 μm was consisted of three parts, i.e., loose layer, compound layer and diffusion layer. Compared with the substrate, the range of the mixed fretting regime(MFR) of the LTGMTT modified layer diminished, and the slip regime(SR) of the modified layer shifted to the direction of smaller displacement amplitude. The coefficient of friction(COF) of the modified layer was lower than that of the substrate in the initial stage. For the modified layer, the damage in partial slip regime(PSR) was very slight. The fretting wear mechanism of the modified layer both in MFR and SR was abrasive wear and delamination. The modified layer presented better wear resistance than the substrate in PSR and MFR; however, in SR, the wear resistance of the modified layer decreased with the increase of the displacement amplitudes. The experimental results can provide some experimental bases for promoting industrial application of LTGMTT modified layer in anti-fretting wear.     

Effects of Contact Pressure and Sliding Speed on the Unlubricated Friction and Wear Properties of Zn-15Al-3Cu-1Si Alloy

The unlubricated friction and wear properties of Zn-15Al-3Cu-1Si alloy were studied over a range of contact pressure (1–5 MPa) and sliding speed (0.5–2.5 ms^?1) for a sliding distance of 2,500 m using a block-on-disc type test machine. It was observed that as the contact pressure increased, the friction coefficient of the alloy decreased but its working temperature, surface roughness, and wear volume increased. Sliding speed had no significant effect on the friction coefficient of the alloy but increased its working temperature, surface roughness, and wear volume. It was also observed that the formation of a hard and brittle surface layer had a great influence on the wear behavior of the experimental alloy. The hardness and thickness of this layer increased with increasing contact pressure and sliding speed. However, contact pressure was found to be much more effective on the hardness of the surface layer of this alloy. Both adhesion and abrasion were observed to be the dominant wear mechanisms for the alloy under the given sliding conditions. The results obtained from the friction and wear tests are discussed in terms of the test conditions and microstructural changes that take place during sliding.     

Ti-6Al-4V燕尾榫结构微动疲劳裂纹萌生及扩展行为研究

针对Ti-6Al-4V钛合金燕尾榫连接结构在不同载荷下的微动疲劳现象,采用榫形微动疲劳试验进行研究,并对裂纹萌生扩展、微动磨损及断口进行分析。结果表明,微动疲劳使构件疲劳寿命显著降低约70%;疲劳载荷对微动裂纹扩展的影响比对裂纹萌生的影响更大;微动疲劳裂纹起始于接触面边缘,与接触表面约成45°角,裂纹扩展到60~150μm后转向与接触表面垂直;微动疲劳断口形貌表面在微动磨损区具有多个裂纹源点,但只有一个主裂纹形成。     

The effect of slip amplitude on fretting

The effect of slip amplitude on the mechanism of fretting was investigated. Measurements of wear volume, frictional coefficient and of electrical contact resistance were carried out to clarify the wear mechanism. X-ray microdiffraction was used to observe the difference of wear behaviour, and scanning electron microscopic observations were made.At small slip amplitudes wear damage was small compared with that at larger amplitudes the transition being in the region of 70 μ.At smaller slip amplitudes fretting oxidation, a mild type of wear occurs. At larger slip amplitudes, adhesion and abrasion together with oxidation cause fretting wear. At much larger slip amplitudes, wear similar to reciprocating sliding wear occurs.     

Amplitude-controlled transitions in fretting: the comparative behaviour of six materials

Fretting tests have been carried out on six materials: EN3, EN56 and EN58 steels, copper, titanium and aluminium bronze. Each was tested at 1000 N normal load, for a total fretting distance of 2 km at peak-to-peak amplitudes of 6.5 and 65 μm. With the exception of the last material, the appearance of the fretted specimens differed at the two amplitudes and the difference in appearance was directly related to the amount of wear experienced. At the lower amplitude wear was always slight with very small, flat, smooth oxide beds, a few microscopic pits and very little material loss. At the higher amplitude wear tended to be far more severe with large amounts of pitting, surface roughening and the formation of more extensive, and generally looser, oxide debris. The results suggest that amplitude-based transitions in fretting behaviour appear to be real and widespread. Thus the phenomena associated with fretting are closely allied to the mechanics of the fretting process and are not purely material properties.     

Oil-Lubricated Fretting Behaviors of 304 Stainless Steels under Different Fretting Strokes and Normal Loads

The influence of oil lubrication on the fretting wear behaviors of 304 stainless steel flat specimens under different fretting strokes and normal loads has been investigated. The results proved that fretting regimes and fretting wear behaviors of 304 stainless steels were closely related to the fretting conditions. In general, the increase in normal load could increase wear damage during sliding wear. However, according to the results, a significant reduction in wear volume and increase in friction coefficient was observed when the normal load was increased to critical values of 40 and 50 N at a fretting stroke of 50 μm due to the transformation of the fretting regime from a gross slip regime to partial slip regime. Only when the fretting stroke further increased to a higher value of 70 μm at 50 N, fretting could enter the gross slip regime. There was low wear volume and a high friction coefficient when fretting was in the partial slip regime, because oil penetration was poor. The wear mechanisms were fatigue damage and plastic deformation. There was high wear volume and low friction coefficient when fretting was in the gross slip regime, because the oil could penetrate into the contact surfaces. Unlike the wear mechanisms in the partial slip regime, fretting damage of 304 stainless steels was mainly caused by abrasive wear in the gross slip regime.     

Friction and wear behaviour of aluminium alloy coconut shell char particulate composites

The friction and wear characteristics of Al-11.8%Si alloys containing 10–25 vol.% (3–8 wt.%) dispersions of coconut shell char particles (average size, 125 μm) were evaluated under dry conditions with a pin-on-disc machine. At the lower sliding speed of 0.56 m s^?1, the wear rates and friction coefficients of the composites decreased with increasing volume per cent of dispersed char particles in the aluminium alloy matrix. Scanning electron microscopy observations have revealed the presence of adhering shell char fragments on the worn-out surface of the composites and the average roughness R_a for the worn-out surface of the composite (Al-11.8%Si-8%char) was much less (1.9 μm) than for the worn-out surface of the matrix (3.2 μm). At the higher sliding speed of 5.38 m s^?1, the wear rates increased with increasing volume per cent of dispersed char particles in the matrix and the R_a value for the composite (Al-11.8%Si-8%char) was higher (5.2 μm) than for the matrix (4.6 μm). The worn-out surface of the composites did not show the presence of adhering shell char fragments. The reduction in wear rates and friction coefficients of composites at the lower sliding speed of 0.56 m s^?1 with respect to the matrix alloy wear was attributed to the presence of adhered fragmented bits of shell char on the wearing composite surface.     

Results of high resolution slip measurements in a fretting experiment

An optical method of slip measurement in a fretting experiment has been developed which allows the measurement of slip amplitude to ±0.95 μm. Average sliding velocities of fretting interfaces during each 1.9 μm of relative motion can be calculated from the slip measurement data. Results on 4130 steel indicate that previously reported temperature variations of the fretting interface closely follow the pattern of the instantaneous sliding velocity of the interface, as is expected for frictional heating. The pattern of sliding velocity versus time is very regular from one fretting cycle to the next. It is concluded that any welding or adhesion which occurs at the interface is either on such a small scale that it does not disrupt the regular motion of sliding parts or is so infrequent in occurrence that it was not observed in any of the present experiments.     

Different aspects of the role of wear debris in fretting wear

Two different aspects of the role of oxide wear debris in fretting wear are studied by allowing them to escape from the interface during sliding. This is accomplished by laser surface texturing that forms regular micro-pores topography on the friction surfaces which enables this escape. It is found that the role of oxide wear debris depends on the dominant fretting wear mechanism. Their presence in the interface protects the friction surfaces when the dominant wear mechanism is adhesive and harms the friction surfaces when this mechanism is abrasive. The escape of oxide wear debris into the micro-pores results in up to 84% reduction in the electrical contact resistance of the textured fretting surfaces.     

Influence of high-power diode laser heat treatment on wear resistance of a mold steel

The effect of hardness on wear loss and wear behavior during fretting was studied. A high-power diode laser was used to achieve the surface hardening of a mold steel (AISI P20-improved) at temperatures of 1000 and 1200 °C. A hardness increment was detected in laser heat-treated specimens, which may be attributed to phase transformation from ferrite to martensite, influencing wear loss and fretting wear behavior. In the fretting test results, smaller wear scars and less wear loss were observed for laser heat-treated specimens in comparison to those of base metal. Moreover, relatively more stable friction coefficient profiles were obtained for the laser heat-treated specimens due to uniform contact characteristics at two contacting surfaces. The effectiveness of the proposed technique was verified by the morphology of the wear scars of the treated specimens, which had a smooth appearance and minor abrasion grooves.

     

On the fretting wear mechanism of Zr-alloys

Zirconium alloys are highly desirable in nuclear applications due to their transparency to thermal energy neutrons and for their high corrosion resistance. The main objective of this study is to investigate the fretting wear mechanism of Zr–2.5%Nb alloy. The experimental work was carried out in air at 265 °C, using a specially designed fretting wear tribometer. The transfer of material, the change in the wear volume and the maximum wear depth with the number of cycles were measured through 3D mapping of the topography of the fretted surface. SEM and Fourier Transform Infrared Interferometry methods were used to examine the microspall pits and to measure the distribution of the thickness of oxide layer in the fretting region. For relatively small slip amplitude, the results showed that the fretting wear mechanism is initially dominated by adhesion and abrasion actions and then by delamination and surface fatigue. The time variation of the wear losses was shown to be cyclic until a steady state value is reached. At high slip amplitudes, however, abrasion and delamination are the only dominant wear mechanisms. The volumetric wear losses were found to decrease monotonically with the number of cycles. A novel approach was introduced, whereby the thermal and electrical contact resistances of the fretting interface are simultaneously measured. The results demonstrated the potential use of this non-intrusive approach for real-time monitoring of the fretting wear mechanism.