Tribology - Materials,Surfaces & Interfaces

This study examines abrasive and adhesive wear behaviour of austenitic stainless steel and its TiC alloyed composite produced through powder metallurgy technique. Abrasive wear tests have been carried out using a pin on disc wear tester under loads of 10, 20 and 30 N. For adhesive wear tests, a block on ring wear tester has been used under loads of 20, 40, 60 and 80 N. A possible correlation between the hardness, microstructure and wear behaviour of the samples has been investigated. The abrasive wear tests have revealed that the highest rate of mass loss occurred in the austenitic matrix stainless steel sample; also, mass losses decreased with an increased rate of reinforcing material in the composite. In adhesive wear tests, interparticle spacing developed from severe wear and extreme plastic deformation under heavy loads; however, at low loads, oxidation type wear was shown to be dominant.

The mechanisms of friction and wear in boundary lubrication are complex with influences from the surface roughness and hardness of surfaces, the lubricant and the wear products. Introduction of a texture on either surface can influence several important parameters. Wear particles can be collected or produced by the surface texture. A lubricating film can suffer or gain and the lubrication regime might change. This paper presents an overview of the tribological effects and important parameters of textured surfaces in sliding boundary lubricated contact, based on the experience of the authors and on published results. Examples of successful and less successful textured contacts are given and some recommendations regarding size, orientation and textured area fraction are presented.

In the present study, the wear behaviour of cross ply (0/90°) C–C composite with 60 vol.-% fibres has been studied with sliding distance, applied load and sliding velocities. The measurement of specimen temperature has been carried out to study the effect of frictional heating. Furthermore, wear debris and wear track observations are correlated to understand the wear mechanism. The bulk wear increases linearly with distance after an initial running-in period. The temperature studies reveal that frictional heating is more with increase in load or sliding velocity under dry conditions, however, presence of lubrication reduces frictional heating, because exposure of surface for direct contact is reduced, and hence wear rate in all studies with lubrication is less than that under dry condition. The wear track studies show graphite powder, peeling of fibres and dislodging of the surface. At low loads, smearing of graphite powder keeps the wear rate low, but as the load increases; dislodging, delamination of surface and breaking of fibres dominate, and wear rate sharply increases, however, sliding velocity initially enhances the graphite formation reducing the wear, but as the velocity reached an optimum value, there is extensive breakage of fibres, dislodging and delamination of surface, and the wear rate increases sharply.

This work deals with the investigation on the friction and wear behaviour of as cast hypereutectic Al–Si alloy reinforced with copper coated short carbon fibres both before and after heat treatment. The composites are fabricated using liquid metallurgy route. Friction and wear tests were conducted using a pin on disc machine under dry conditions. The loads (contact pressure) and sliding velocities have been varied from 10 to 50 N (contact pressure of 0˙12–0˙60 MPa) and 0˙3 to 1˙2 m s^–1 respectively. Results reveal that coefficient of friction and wear rate of the composite are lower than that of the matrix alloy in both heat treated and unheat treated conditions. The coefficient of friction of the matrix alloy and its composite decreased with increased load up to 30 N and increased beyond this load. The wear rates of both the matrix alloy and its composite increased with the increasing load. However, at all the loads and sliding velocities studied, the developed composite exhibited a lower coefficient of friction and wear rates when compared with the matrix alloy. Energy dispersive X-ray analysis of the worn surfaces of Al–Si–2 wt-%C_f have revealed greater proportions of iron pick up from the counter steel disc when compared with the matrix alloy.

The polishing phenomenon of road pavements under the vehicle traffic constitutes the main mechanism inherent to the loss of skid resistance over time. A better understanding of this phenomenon would allow an improvement of road safety. This study comprises a review of laboratory test and a model simulating the polishing of road surfaces. The laboratory test uses a polishing machine so called ‘Wehner–Schulze’ which can reproduce the evolution of the road texture from specimens taken directly from road or made in laboratory. The model is based on contact mechanics considerations and considers that the local polishing rate is controlled by the pressure distribution between the car tyre and the road surface. The model evaluates the pressure distribution by taking into account the surface roughness, the applied load and the mechanical properties of the bodies in contact. From the pressure distribution and a proposed wear law, the local roughness evolution of the surface pavement can be readily evaluated. The removal material is proportional to the contact pressure and to the aggregate properties used in roads mix design. The results show that the predicted road profile evolution is consistent with those given by experiments.

A plasma enhanced chemical vapour deposition (PECVD) amorphous carbon coating was deposited onto 100Cr6 steel substrates having varying degrees of surface roughness. The samples were subsequently evaluated to determine the correlation between substrate roughness and coating performance. The steel substrates were prepared before coating deposition to attain five different levels of roughness: (a) ground; (b) superfinished (SF); (c) polished to 1000 grit; (d) polished to 220 grit and (e) polished to a 1 μm diamond finish. The aim of the investigation was to determine the degree of finish required for good tribological performance and coating adhesion. The mechanical and tribological properties of the samples were assessed by nanoindentation, ramped load scratch testing, and pin on disk wear testing. Nanoindentation testing was used to determine the hardness of the samples and the relative contributions to the system hardness from the substrate and coating were separated using the model of Korsunsky et al. Nanoindentation testing showed that the coating hardness (when separated from the system hardness) was lower for the samples with the SF substrate than the others: the reasons for this are discussed in the light of Raman measurements on the fractions of diamond-like and graphite-like bonding in the coatings. Ramped load scratch testing was used to determine coating adhesion and the scratch test failure mode. With the exception of the samples with the ground substrate finish, studies of the friction coefficient plots during scratch testing showed little variation between the samples, and SEM imaging revealed a common failure mode of severe spallation at the scratch track border. The samples with the ground substrate showed differences in response between scratches parallel and perpendicular to the grinding direction, with scratches parallel to the grinding direction showing more severe spallation. The average critical load to failure, as determined by the point of first failure in the scanning electron microscope, was lower for the coatings on the SF substrate than the coatings on the 220 grit, 1000 grit and 1 micron finished substrates. The critical load to failure for the samples with ground substrates was lower than the other substrate surface finishes. Pin on disk wear testing of the samples against a steel ball revealed that the major effect of the varying substrate roughness was on the wear of the counterface, with rougher substrate finishes generally resulting in higher wear rates of the counterface, although the smoothest substrate finish, the micrometre finish, also resulted in higher wear. The sample whose substrate was superfinished gave least wear of the counterface and this was therefore the optimum finish for the samples when considering their performance in a tribological couple.

This paper describes the theoretical analysis of the effect of micropolar fluid on the lubrication characteristics of porous inclined stepped composite bearing. The lubricant with additives in the film region and also in the porous region is modelled as Eringen's micropolar fluid, which is characterised by the presence of suspended rigid particles with microstructures. The generalised Reynolds type equation is derived for the most general porous bearing configuration (porous composite bearings) lubricated with micropolar fluid. The closed form expressions are obtained for the fluid film pressure, load carrying capacity, frictional force and coefficient of friction. These expressions can be utilised to obtain the performance characteristics of four different bearing systems, namely, porous plane inclined slider, porous composite tapered land bearing, porous stepped bearing and composite porous tapered concave bearing. It is observed that the micropolar fluid lubricants provide an increased load carrying capacity and decreased coefficient of friction as compared to the corresponding Newtonian case.