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In the present paper the applicability of state and rate dependent friction laws in describing the phenomena of high speed slip at metal-on-metal interfaces is investigated. For the purpose of model validation, results of plate-impact pressure-shear friction experiments were conducted by Irfan in 1998 and Irfan and Prakash in 2000 using a Ti6AI4V and Carpenter Hampden tool-steel tribo pair are employed. In these experiments high normal pressures (1-3 GPa) and slip speeds of approximately 50 m/s were attained during the high-speed slip event. Moreover, these experiments were designed to investigate the evolution of friction stress in response to step changes in normal pressure and also in the applied shear stress during the high-speed slip event. A step drop in normal pressure is observed to result in an exponential decay of the friction stress to a new steady-state characteristic of the current normal pressure and the current slip velocity. A step drop in applied shear stress is observed to lead to an initial drop in friction stress, which later increases toward a new steady-state friction stress level. In response to the step drop in applied shear stress the slip velocity initially increases and then decreases to a new steady-state level consistent with the new friction stress level. A modified rate and state dependent friction model that employs both velocity and normal stress dependent state variables is used to simulate the experimental results. A good correlation is found between the experimental results and the predictions of the proposed state and rate dependent friction model. [DOI: 10.1115/1.2401217] Keywords: mathematical modeling, high speed frictional slip, plate impact friction experiments, state and rate dependent friction

Scuffing or scoring is an important form of damage leading to component replacements in lubricated mechanical systems such as power drives, gears, bearings, cams and followers, piston rings etc. Since scuffing necessarily involves localized welding of asperities, suitable surface modifications or coating can impart a good resistance to scuffing. A new class of low temperature salt bath nitriding process provides good resistance to scuffing. The current work is on evaluation of the scuffing resistance of AISI 4340 steels imparted with two such treatments. Experimental investigations were conducted on Sursulf and Arcor treated AISI 4340 steel specimen using a pin on ring test system under boundary lubrication condition. Continuous monitoring of friction records and (near surface) bulk temperature were done under step loading. Limiting loads and load velocity relations were evaluated and using the data generated and a thermo-mechanical wear model, performance is indexed. Off line studies on surface finish, hardness variations and surface transformations were also accomplished. Some salient aspects of the investigation and data generated are presented here. Onset of scuffing was observed to be far delayed with pin specimen imparted with these treatments compared to hardening. [DOI: 10.1115/1.1504089]

Flattening of random rough surfaces on a workpiece undergoing bulk deformation has been analyzed using a model of the surface consisting of just two wavelength components. Asperities are flattened at a rate which depends on the ratio of the initial r.m.s, amplitudes of the long and short wavelength components. The flattening behavior of the long wavelength asperities only becomes important when the amplitude of the long wavelength asperities is much greater than that of the shorter wavelength asperities. The surface modification was investigated experimentally by cold rolling of aluminum strips. The power spectral density of the roughness was used to extract appropriate amplitudes for the short and long wavelength components of roughness. The change in roughness amplitudes showed excellent agreement with theory.

The influence of harmonic normal loads on sliding friction is investigated through analysis of contacts consisting of conical and spherical sliders of hard materials on softer metal surfaces. Friction for such contacts is assumed to result from a plowing component and a shearing component. Calculations and experiments show that the coefficient of friction is essentially independent of normal load for contacts with conical sliders. However, for spherical sliders the relation between the coefficient of friction and normal load is highly nonlinear. In the presence of harmonic variations in normal load, this non-linearity causes a shift in the average coefficient of friction. For ideal lubricated contacts, the shearing component of friction is very small and for this case, it is shown that the maximum average reduction in the coefficient of friction is ten percent. When the shearing component is more significant, as with dry contacts, the shift is less. For example, when the shear strength is one-sixth the hardness of the softer material, the maximum average reduction in the coefficient of friction is five percent.

The velocity, normal pressure, and slider size dependence of the coefficient of dry friction of metals in the range of high sliding velocities (V [greater than or equal to] 1 m/s) is investigated theoretically. Failure of the adhesive junctions by adiabatic shear banding is considered as the underlying process. The concept of asperity shearing by the adiabatic shear banding mechanism represents a new approach to unlubricated high velocity friction. Analytical solutions of a coupled thermomechanical problem are given for two constitutive relations. Numerical solutions for steel-on-steel friction showing a decrease of the coefficient of friction with the sliding velocity for different normal pressures are presented. The model is considered to be adequate in the velocity range of 1-10 m/s where friction enhanced oxidation or surface melting are believed not to interfere with the asperity shearing process.

Fretting is the tribological phenomenon observed in nominally-clamped components which experience vibratory loads or oscillations. Associated with fretting contacts are regions of small-amplitude relative motion or microslip that occurs at the edges of contact. A newly-available infrared technology capable of resolving temperatures fields finely, both spatially and temporally, is used to characterize the near-surface conditions associated with fretting contact between an aluminum alloy cylinder and fiat. Both frictional heating due to interfacial slip and the coupled-thermoelastic effect arising from strains in the material induce these temperatures. The experimental results provide insight into not only the magnitude and distribution of near-surface temperatures, but also the nature of the contact stress field and the mechanics of partial slip fretting contacts. Comparisons of the measured temperature fields are made with those predicted by considering both conduction of the frictional heat flux and coupled-thermoelastic theory.