Initial Wear Rate Research Articles

The fretting of mild steel in air

A series of fretting experiments has been undertaken on mild steel in air, at room temperature using flat contact areas 15 mm square with direct measurement of specimen relative movement and a high speed hydraulic drive system. Loads of 1000 and 200 N were employed with peak-to-peak amplitudes between 5 and 115 μm at frequencies generally around 190 Hz. At 1000 N there are two main types of fretting behaviour. Above 20 μm, the wear behaviour appears to be unrelated to the amplitude over a wide range of fretting distances. This type of wear produces copious quantities of loose oxide debris, pitting and peak formation. Although the relationship between specimen weight loss and fretting distance may be conveniently represented by two straight lines, a fast early rate and a slower rate following on after about 4 × 10 6 cycles, there are good reasons to believe that this presentation is misleading. It suggests a transition in behaviour at around a large number of cycles. However, there is no external evidence to support the idea of a change in mechanism here. The concept that, in fretting, wear is being thwarted by the products of its own making provides a better fit to the data and is more consistent with other observations. At amplitudes below 10 μm there is far less wear and only minor pitting. There is little evidence for a fast initial wear rate and the oxide formed is more coherent. Even in long-term experiments, the oxide only covers a small part of the surfaces and the wear rate is much lower; it may even approach zero. Between 10 and 20 μm, the behaviour is transitional with some evidence of surface roughening and rapid initial material loss. Fairly coherent oxide forms in the longer term, when wear rates may be similar to those found below 10 μm. However, in this region there is far more variability. The results, and the appearance of fretted specimens, suggest that the amount of wear will be a function of specimen geometry and surface texture. It is concluded that the function of the debris is so central as to undermine the concept of a specific wear rate which may be defined in advance of testing under representative conditions. Thus, to a large extent, the variability in results between different workers represents an underlying truth of the fretting process.

Comparison of direct and indirect methods for analyzing wear of posterior composite restorations

The vast majority of recent clinical research involving wear analyses of posterior composite resin restorations have used either the direct evaluation method (USPHS) or the indirect cast comparison (Leinfelder) method. However, there has never been any established correlation of the two wear scales. The objective of this study was to determine the amount of wear for the USPHS alfa-bravo transition on the basis of cast comparison data. Clinical wear data were collected over five years by both the direct method and the indirect method. Three materials were used involving a total of 221 restorations in 61 patients. Each restoration was evaluated at baseline, six months, one, two, three, and five years by each method. Then, for those restorations which underwent a transition from an alfa to a bravo clinical rating, the absolute wear at the transition was determined by averaging the indirect ratings just before and just after the transition. The mean wear corresponding to the alfa-bravo transition was 192 ± 60 μm. Because of the large width of the alfa category up to the alfa-bravo transition, extensive early changes and high initial wear rates cannot be detected by the USPHS system.

Tribological Characteristics of Nitrogen (N+) Implanted Iron

The effect of implantation of nitrogen ions (1.5 MeV) on the friction and wear characteristics of pure ion sliding against M-50 steel (unimplanted) was studied in a pin-on-disk sliding friction apparatus. Test conditions included room temperature (25 C), a dry air atmosphere, a load of 1/2 kg (4.9 N), sliding velocities of 0.043 to 0.078 m/sec (15 to 25 rpm), a pure hydrocarbon lubricant (n-hexadecane), or a U.S.P. mineral oil and nitrogen ion implantation doses of 5x10 to the 15th power and 5x10 to the 17th power ions/sq cm. No differences in wear rates were observed in the low dose experiments. In the high dose experiments, small reductions in initial (40 percent) and steady state (20 percent) wear rates were observed for nitrogen implanted iron riders as compared with unimplanted controls. No differences in average friction coefficients were noted for either dose. Auger electron spectroscopy combined with argon ion bombardment revealed a subsurface Gaussian nitrogen distribution with a maximum concentration of 6 atomic percent at a depth of 0.8 microns. Similar analysis within the wear scar of an implanted rider after 20 microns of wear yielded only background nitrogen concentration. No inward migration of nitrogen ions was observed.

Influence of chemicals on wear of dental composites.

The wear resistance of a commercial dental restorative composite exposed to different chemicals was measured with a pin-disc apparatus. The initial wear rate increased dramatically once the composite had been immersed in chemicals known to soften the cross-linked matrix material, which is a copolymer derived from BIS-GMA and diluent monomer. The increase of wear rate in a chemically softened composite was found to persist over a depth beyond 160 μm for specimens immersed over two wk prior to wear testing.

The effect of surface topography on the wear of steel

The pattern of roughness and wear has been obtained by varying the speed, the number of passes and the pre-roughness of the samples in both dry and lubricated sliding conditions. Three-dimensional graphs of wear versus speed versus sliding time and roughness of the worn surfaces were produced and a linear relation between the variation of the roughness and the amount of wear at a high number of passes has been determined. The initial dry wear rate is influenced by the prior surface roughness. In the case of lubricated conditions the influence of prior roughness is not pronounced. In dry sliding the smooth surfaces in the running-in stage roughen and the rough surfaces become smoother, while in lubricated conditions both surfaces become smoother in the first few passes. Sliding perpendicular to the lay produces rougher surfaces and more wear than is the case in the parallel direction.

Initial wear rates of soft metallic films

A theoretical expression was derived for the initial wear rates of hard conical asperities ploughing a soft metallic film. The theory yields a wear equation similar to the well-known existing empirical formula, although the new wear coefficient K appears to be dominated by the surface topography of the system rather than by the material properties. The initial wear rates of the film appear to be determined by the system rather than by the material. The general characteristics of the model are confirmed by experimental results for the initial wear rates of ion-plated tin and lead films.

Hydrodynamic Lubrication and Wear in Wavy Contacting Face Seals

A model of face seal lubrication is proposed and developed. Hydrodynamic lubrication for rough surfaces, surface waviness, asperity load support, elastic deflection, and wear are considered in the model. Predictions of the ratio of hydrodynamic load support to asperity load support are made for a face seal sealing a low viscosity liquid where some contact does occur and surface roughness is important. The hydrodynamic lubrication is caused by circumferential surface waviness on the seal faces. Waviness is caused by initial out of flatness or any of the various distortions that occur on seal ring faces in operation. The equilibrium solution to the problem yields one dimensional hydrodynamic and asperity pressure distributions, mean film thickness, elastic deflection, and friction for a given load on the seal faces. The solution is found numerically. It is shown that the fraction of hydrodynamic load support depends on many parameters including the waviness amplitude, number of waves around the seal, face width, ring stiffness, and most importantly, surface roughness. For the particular seal examined the fraction of load support would be small for the amount of waviness expected in this seal. However, if the surface roughness were lower, almost complete lift-off is possible. The results of the analysis show why the initial friction and wear rates in mechanical face seals may vary widely; the fraction of hydrodynamic load support depends on the roughness and waviness which are not necessarily controlled. Finally, it is shown how such initial waviness effects disappear as the surface profile is altered by wear. This may take a long or short time, depending on the initial amount of hydrodynamic load support, but unless complete liftoff is achieved under all operating conditions, the effects of initial waviness will vanish in time for steady state conditions. Practical implications are drawn for selecting some seal parameters to enhance initial hydrodynamic load support without causing significant leakage.

Surface topography and integrity effects on sliding wear

The effect of surface roughness and integrity on the sliding wear of metals was investigated experimentally. The results are consistent with the delamination theory of wear. The initial wear rate was influenced by the surface roughness and the applied load but the steady state wear rate was independent of the initial roughness. Under low applied loads delamination of smooth surfaces commences soon after sliding is initiated, whereas the delamination of rough surfaces is delayed until the original asperities are worn. Consequently, under low loads the initial wear rate of a smooth surface is higher than that of a rougher surface. The opposite is found under high loads since original asperities are removed immediately. It is also shown that machining damage to the surface or the subsurface (in the form of deformation, voids and cracks) accelerates the initial wear rate of the machined surface.

The Tribological Behavior of Polycrystalline Cobalt as Related to Crystallographic Texture and Structure

The friction and wear behavior of polycrystalline hexagonal cobalt was determined under normal atmospheric conditions, using heavy loads and sliding speeds ranging from 0.92 cm/sec to 36.8 cm/sec. Very uniform wear was observed for most conditions together with a low coefficient of friction. X-ray diffraction measurements showed that this desirable behavior is caused by the formation of a preferred orientation during sliding. Under high-load high-speed conditions a different mode of wear was observed, best described as brittle disintegration. When cobalt was alloyed with 8 percent iron and thus rendered face-centered cubic, a deeply torn wear track, an increased initial wear rate and a high coefficient of friction were obtained. Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE International Lubrication Conference held in New York City, October 9–12, 1972