S. K. Roy Chowdhury

Adhesion and adhesional friction at the contact between solids

Abstract The paper describes a theoretical study of adhesion and adhesional friction between solids with small-scale surface asperities using an elastic-plastic model of contact deformation. The well-established elastic and plastic adhesion indices are used to consider the different conditions that arise as a result of varying load and material parameters. From the predicted loading and unloading behaviour of solids, transitional values of the adhesion indices at which the influence of surface forces become insignificant are obtained. The results agree well with the existing experimental observations. The study also shows that, when the elastic adhesion index is low, the frictional force is almost proportional to the applied load. This may apply to solids with low elastic moduli. However, if the index is high the coefficient of friction depends on the applied load in the lower ranges and approaches a constant low value at higher loads.

Wear characteristic and biocompatibility of some polymer composite acetabular cups

Abstract UHMWPE and HDPE have been successfully used as acetabular cup materials over several decades but recently there is concern over their biosafety. Composites of these and other polymers offer comparable mechanical and tribological properties with improved biocompatibility. Very little attention, however, has been paid towards the development of hip and knee prostheses using such polymer composites. In the present work new polymer composites of HDPE reinforced with different percentages of kevlar and carbon fibre have been developed and acetabular cups prepared by compression moulding. A walk simulator, developed in-house was used for testing the tribological performance of these cups. The results are encouraging and some of the new composites may be considered to be potential future materials for acetabular cups. Biocompatibility tests with these materials give haemolysis counts well within the acceptable range and further more our results indicate a significant improvement in the biocompatibility of the polymer composites over their parent polymers.

A fractal analysis of adhesive wear at the contact between rough solids

This paper describes a theoretical study of adhesive wear at the contact between surfaces with nanometric level asperities and at low loads in order to arrive at predictive formulations that also take into account the effect of scale-dependent surface topography. The analysis of adhesive wear is closely associated with that of adhesion. Adhesive bonds may occur at the peaks of nanometric size asperities even though the microscopic roughness would inhibit the surfaces from approaching close separation. In order to account for the effect of asperities ranging from nanometer to micrometer level a fractal approach is used in the present analysis. The results broadly confirm the dependence of wear volume on normal load and also on adhesion arising out of surface forces. It also seems that the variation of fractal dimension and fractal adhesion indices may produce extreme conditions like very high wear even under tensile load or near zero wear which is certainly advantageous in practical situations.

Experimental investigation into the effect of 3D surface roughness parameters on flash temperature

Purpose – Although dependence of contact surface temperatures between rough sliding bodies on surface topography is more explicitly described in terms of three‐dimensional (3D) topographic parameters, no work has yet been reported on this aspect. The paper seeks to carry out experiments to systematically correlate the 3D surface parameters to the contact temperature rise.Design/methodology/approach – The surface temperatures at the contact between a relatively smooth zinc sulphide pin held against a rotating mild steel disc of varying surface topography were measured using an infrared thermal imaging system under different load and sliding velocity conditions. The main objective was to study the effect of 3D surface roughness parameters on the contact temperature rise.Findings – The results indicate a rise in maximum contact temperature with the increase in a number of 3D parameters, such as, average surface roughness Sa, ten‐point height parameter Sz, skewness of the surface height distribution Ssk, mean...

Prediction of Flash Temperature at the Contact Between Sliding Bodies With Nanoscale Surface Roughness

In view of the difficulty in measurement of flash temperature rise at the contact between rough sliding bodies a good deal of work has been carried out in the last few decades to predict flash temperatures theoretically. However, as surfaces become smoother and loading decreases in applications such as MEMS, NEMS and magnetic storage devices measurement of flash temperature becomes increasingly more difficult due to the nanometer scale asperity interactions. Consequently measurement of flash temperature at the nanoscale asperity contact has not yet been possible. The analysis of flash temperature rise under these circumstances is no less challenging since it must consider not only the small-scale asperity height distributions but also the surface forces those may operate at very small surface separations. The paper attempts to predict the flash temperature rise analytically using a fractal approach to describe the nanoscale asperity interactions at low loads and also taking into account the influence of relevant parameters including the surface forces. The important observation here is that in addition to the dependence on load, speed, and material parameters the flash temperature steadily rises with surface adhesion but falls with the fractal dimension D until a critical value of around 1.5, and then rises again. The flash temperature also falls with Fourier number. Under certain combinations of load, speed, and material parameters, extremely high flash temperature is predicted while under certain other parametric combinations extremely low flash temperature may occur. The later parametric combination is certainly of much practical importance.

A feed back control system for plain bearings using film thickness measurement

Abstract A method for on-line monitoring and control of hydrodynamic journal bearings using film thickness measurement is proposed such that an adequate film thickness is maintained at all times. Such systems are considered to be useful in industrial applications, particularly in large installations such as power plants, rolling mills etc. The method uses the journal speed as the controlling parameter purely for demonstration purposes. Other alternatives such as use of an external lubrication pump to provide hybrid type of bearing operation may be envisaged. An analysis of the control system to predict the stability and steady state error is also included. The initial experimental results show that the proposed system works well.

An Analysis of Surface Temperature Rise at the Contact between Sliding Bodies with Small-Scale Surface Roughness

A theoretical model for analyzing surface temperature rise between extremely smooth sliding bodies considering the effect of adhesion forces on the contact conditions is developed. The important observation is that in addition to the dependence on load, speed, and material parameters the contact temperature steadily increases with surface adhesion. In the nanometric range of asperity heights contact temperature rise drops to some extent with the increase in roughness whereas in the micro- or macroscale surface roughness, the temperature rise steadily increases with surface roughness. Under certain combinations of load, speed, and material parameters extremely high contact temperature rise is predicted, whereas under certain other parametric combinations extremely low temperature rise may occur. The latter parametric combination is of significant practical importance.

Prediction of Polymer Wear—An Analytical Model and Experimental Validation

Because of its industrial relevance, wear of engineering polymers has been studied extensively both experimentally and theoretically. The wear mechanism of polymers is complex due to the influence of numerous parameters and it has long been realized that a predicting tool for wear of polymers in dry and lubricated sliding is of practical importance. Polymer wear models hitherto have been largely done by fitting the experimental data to an empirical equation and some major contributions have been made by a number of authors in this area. However, a more fundamental approach would be to analyze the experimental evidence collectively on the basis of the variables involved and the mechanisms leading to particle detachment. Although some progress has been made in this direction, a need exists to consolidate the major experimental findings in order to develop a comprehensive analytical wear model. The present work attempts to develop such a wear model and validate the proposed model experimentally. The wear equations are presented in two groups, one representing primarily abrasive wear and the other the fatigue mechanism, since the two mechanisms operate in distinct roughness ranges. Each group consists of four equations representing different contact speed and temperature ranges. The results indicate that the surface forces dominate in the low roughness range while at the higher roughness ranges abrasive wear is predominant. Among other observations the results also indicate that a unique value of the ratio of equivalent elastic modulus to hardness (E//H) exists where wear may either be insignificant or very large depending on the parametric combination. These predictions are likely to be of practical importance.

Prediction of contact surface temperature between rough sliding bodies – numerical analysis and experiments

Purpose – The papers aim is to predict numerically the contact temperatures between two rough sliding bodies and to compare with the experimental results.Design/methodology/approach – An elastic contact algorithm is used to analyze the normal contact between two nominally smooth surfaces. The algorithm evaluates real contact area using digitized roughness data and the corresponding contact pressure distribution. Using finite element method a steady state 3D temperature distribution at the interface between the sliding bodies is obtained. Using infrared (IR) imaging technique, experiments were carried out to measure the contact temperature distribution between rough rubbing bodies with a systematic variation of surface roughness and operating variables.Findings – Contact temperature distributions over a wide range of normal load, sliding velocity and surface roughness have been obtained. It was seen that the maximum contact temperature expectedly increases with surface roughness (Sa values), normal load a...

A Fractal Analysis of Flash Temperature at the Contact Between Sliding Bodies With Small Scale Surface Roughness

In view of the difficulty in measurement of temperature rise at the contact between sliding bodies with engineering scale roughness a good deal of theoretical work has been carried out in the last few decades. However, as surfaces become smoother and loading decreases in applications such as MEMS, NEMS and magnetic storage devices measurement of flash temperature becomes increasingly more difficult due to the nanometer scale asperity interactions. Consequently measurement of flash temperature at the nano-scale asperity contact has not yet been possible. The analysis of flash temperature rise under these circumstances is no less challenging since it must consider not only the small-scale asperity height distributions but also the surface forces those may operate at very small surface separations. The paper attempts to predict the flash temperature rise analytically using a fractal approach to describe the nano-scale asperity interactions at low loads and also taking into account the influence of relevant parameters including the surface forces. The results show in general that the contact surface temperature steadily increases with load, nano-scale roughness and surface forces. Interestingly, the fractal analysis presents a wide spectrum of solutions. While under certain combinations of fractal and material parameters extremely high contact temperature rise is predicted, under certain other parametric combinations extremely low temperature rise can be seen. The later parametric combination is certainly of much practical use.Copyright