R.B. Waterhouse

Fretting debris and the delamination theory of wear

Abstract A SEM study of fretted surfaces of 0.7 carbon steel, commercially pure titanium, and Al-Zn-Mg alloy shows that loose wear particles are produced by the propagation of sub-surface cracks in a similar manner to that postulated in the delamination theory of unidirectional wear. This leads to the detachment of oxide coated plates of metal ranging in thickness from 1.3 to 3.5 μm. The continuing fretting action grinds the initial wear particles down to particles of smaller size with a higher oxide content.

The role of adhesion and delamination in the fretting wear of metallic materials

Abstract Fretting corrosion is a mild form of wear which is distinguished from unidirectional sliding wear by the small amplitude of movement and the low relative velocity of the surfaces. Macroscopic adhesion occurs in the early stages of the process, and there is evidence of the making and breaking of local welds and a roughening of the surface. Eventually adhesion falls off, the surfaces become smoother and removal of material from the surfaces occurs by delamination. The change-over from adhesion to delamination is a function of the particular material and the nature of the environment. The process is investigated in a number of materials of commercial interest such as mild steel, stainless steel and titanium alloys. The adhesion stage is more pronounced the nobler the metal and the more inert the environment. In systems exhibiting adhesion the number of fretting cycles to develop maximum adhesion corresponds to the number of cycles to initiate a propagating fatigue crack. However, it is thought that fall-off in adhesion is the significant factor.

The initiation of fatigue cracks in A 0.7% carbon steel by fretting

Abstract The origin of fatigue cracks in fretting fatigue is the boundary between the slip and non-slip areas in the contact region. They arise at this boundary because of the high stress concentration. When a crack is formed it relieves the stress concentration at this point, and the boundary between the slip and non-slip areas moves inwards, resulting in the initiation of another crack which propagates more rapidly than the first crack because of the higher stress concentration. Crack formation can be prevented or retarded by ensuring that slip occurs over the entire contact region.

Lubricated fretting wear of a high-strength eutectoid steel rope wire

Abstract The fretting wear behaviour of heavily work-hardened eutectoid steel wire is an important issue in the construction and usage of locked coil steel ropes. These ropes, which are used for high duty applications in aggressive environments, such as those encountered in the mining industries, can exhibit a variable service life. Earlier papers examined the design features of this class of rope and aspects of the operating regime, which may contribute to this variable life. The as-drawn surface of the wire was determined to play a significant role in the fretting behaviour, initially suppressing wear and friction. In this paper, the influence of low viscosity oils, with and without graphite additions, on the fretting behaviour of the as-drawn wire are examined. Oil bath lubrication suppresses effectively wear and friction throughout the tests. Of greater significance, however, a smear of oil, more typical of service conditions, is also effective, at least during the early stages of testing. The addition of graphite to this smear of oil aids its retention, and its ability to lubricate and form a shield around the fretting interface. The presence of oil inhibits the ingress of oxygen and consequently, at higher normal forces, when significant breakdown of the oil film occurs, micro-welding and tearing of the clean metal surfaces follows. Even so, wear and friction are still suppressed effectively.

Fretting in aqueous media, particularly of roping steels in seawater☆

Abstract A model is described to account for the wear rates, coefficients of friction and surface features observed when fretting a 0.64% C high tensile roping steel in seawater. The work includes a comparison of the fretting behaviour, both in air and in selected aqueous environments, with that of an austenitic stainless steel. When fretting in air, mechanical and oxidational components are suggested, although in aqueous solutions an additional electrochemical corrosion component becomes important. Although a liquid can help to reduce friction by acting as a lubricant, the effects of which are most noticeable in the early stages of fretting, under most circumstances the liquid will give rise to a corrosion process which accounts for high wear rates and ultimately to increasing coefficients of friction through the accumulation of trapped corrosion debris. Only in solutions of low ionic conductivity and/or very low dissolved oxygen concentration can the corrosion rate be reduced, but only by the application of a suitable level of cathodic protection can it be eliminated altogether to achieve desirable friction and wear behaviour. A number of experiments were carried out in artificial seawater to give some indication of the fretting corrosion behaviour of roping steels used in offshore environments.

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

Abstract 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 10 6 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.

Fretting at high temperatures

Abstract The fretting damage to an austenitic stainless steel, type 321, in CO 2 is much reduced at temperatures above 400°C by the formation of a glaze type oxide. Increasing the normal pressure from 2 to 6.9 MN m −2 at 650°C greatly increased the extent and quality of the glaze. The nickel-based alloy, Inconel 718, developed glaze oxide when fretted at 540°C in air, as indicated by a low coefficient of friction and wear rate. At 280°C, the glaze was only found at greater amplitudes of slip. Although the titanium alloy Ti-6Al-4V in air at 200 to 400°C developed a surface oxide which had some of the superficial features of a glaze, it nevertheless did not reduce the coefficient of friction to values characteristic of glaze. The common feature of high-temperature alloys which develop protective glaze oxides is that they are capable under conditions of sliding and fretting of forming a spinel type oxide which, however, must be adequately supported by a creep-resistant substrate at the operating temperature

Adhesion of metal surfaces under fretting conditions I. Like metals in contact

Abstract The adhesion developed between metal surfaces in fretting in a protective atmosphere initially increases linearly with amplitude of slip, passes through a maximum, and eventually may fall to zero. In air the maximum is much lower and the adhesion falls to zero at a much lower amplitude. Adhesion is inversely proportional to the log of the intrinsic equilibrium hardness of the materials. Alloying additions reduce the adhesion by increasing the intrinsic hardness and by modifying the surface oxide films.

On the behaviour of an oil lubricated fretting contact

Abstract Although many engineering situations involving fretting damage are lubricated, comparatively little has been reported on this aspect of fretting wear. The viscosity of the lubricating oil and its boundary layer performance are expected to influence fretting behaviour, in addition to the normal fretting parameters, such as stroke and contact force. This paper examines the effect of lubrication regime, oil viscosity and stroke on the behaviour of a ball-against-flat specimen arrangement. Ball and flat specimens were both manufactured from a bearing steel (SUJ2). Polybutane oils, without additives, covering a range of viscosities from 1 to 10 000 cSt, and fretting strokes up to 35 μm were investigated. The lubricating oil was added to the fretting interface after 0, 3 and 2000 fretting cycles had been completed. Lubrication regime, oil viscosity and stroke were all found to affect fretting behaviour in terms of both coefficient of friction (or traction coefficient) and wear. For strokes less than 9 μm, i.e. for conditions approaching almost complete ‘stick’, coefficient of friction values under oil lubrication were well in excess of double those observed without it. These high values suggest that the oil was unable to penetrate into the fretting contact region, but did maintain a shield around it, so that metal-on-metal contact was maintained under oxygen deprived conditions. The lowest values of steady state coefficient of friction (≈ 0.2) were observed when oil lubrication was applied after 2000 cycles had been completed, indicating that surface roughening and the presence of oxide films and oxidised debris assisted penetration of the lubricant into the fretting contact zone.

Fretting damage in locked coil steel ropes

Abstract Locked coil steel ropes are used for high duty applications in aggressive environments, such as those encountered in the mining industry. The service life of this class of rope can be variable. This paper examines the design features of the rope, and the operating regime, which may contribute to this variability in service life. These ropes are constructed from cold drawn eutectoid and hypo-eutectoid steel wires. A rope is examined after a period in service close to its design life. Of the degradation mechanisms considered, fretting is found to produce significant wear at points of contact between the layers of the rope, near its outer surface. Initiation of fretting wear is encouraged by the breakdown of the added lubricant and the magnitude of the contact stresses. Two further papers will examine the fretting behaviour of a typical eutectoid steel wire used in this class of rope. In the first of these papers the residual lubricant coating from the wire manufacturing process is identified as playing an important role in the fretting behaviour. The second paper covers the influence of low viscosity oils, and solid additives, on the fretting behaviour.