Terms Of Wear Mechanisms Research Articles

Revisiting polymer tribology for heavy duty application

Polymers are frequently considered as a candidate material for heavy duty tribological applications. Standard tribological testing for such applications may result in unrealistic tribological properties in terms of wear mechanism, transfer film formation and friction coefficient etc. Thus the present research aims to explore the tribological behaviour of currently available neat polymers under heavy duty loading condition. In this background, nine different polymers (PAI, PEI, PC, PPSU, PA6, PET, PPS, PVDF, and UHMWPE) of amorphous and semi-crystalline groups were chosen from engineering and high performance grades. Large scale reciprocating sliding wear tests with 50×50mm contact area were used. Considering heavy duty loading condition, the operational parameter were idealised at 10kN normal force and 50mm/s sliding speed. From the test results the polymers are classified into three groups based on the wear mechanisms and the transfer layer characteristics observed from the counterface. Group 1 did not persist any transfer layer (PC and PEI), whereas Group 2 materials (PET, PPS, PPSU and PAI) has lumpy discontinuous transfer layer and finally Group 3 materials evidenced uniform thin transfer layer. Clear signs of three body abrasion were evidenced in Group 1 material which is due to the absence of transfer layer and also from the large loose debris. Poor adhesion of inhomogeneous lumpy transfer layer in Group 2 showed an intermediate wear rate. Based on the wear and friction performance, the Group 3 materials (PVDF, UHMWPE and PA6) are proposed as the candidate materials for heavy duty tribological application. In regards to the transfer layer thickness and wear mechanisms at large scale testing, the present observations showed incomparable results with the literature. This research concludes that the revisit to polymer tribology at high load condition has indicated the difference in wear mechanism and the corresponding tribological characteristics in large scale testing. As a future work multi-scale testing is an absolute must for the complete understanding of tribological characteristics w.r.t a specific material.

Wear Particle Analysis and the Evolution of the Plastically Deformed Layer on Australian Rail Steel

Particle analysis methodology is presented, together with the morphology of the wear debris formed during rolling contact fatigue. Wear particles are characterised by their surface topography and in terms of wear mechanism. Rail-wheel materials are subjected to severe plastic deformation as the contact loading progresses, which contributes to a mechanism of major damage in head-hardened rail steel. Most of the current methodologies involve sectioning of the rail-wheel discs to trace material damage phenomena such as crack propagation and plastic strain accumulation. This paper proposes methodology to analyse the development of the plastically deformed layer by sectioning wear particles using the focussed ion beam (FIB) milling method. Moreover, it highlights the processes of oxidation and rail surface delamination during unlubricated rolling contact fatigue.

Effect of the metallic matrix on the sliding wear of plasma assisted debinded and sintered MIM self-lubricating steel

A new processing route to obtain a homogeneous dispersion of discrete particles of solid lubricant in the volume of sintered steels produced by metal injection moulding (MIM) was recently presented. In this technique, both thermal debinding and sintering are carried out in a single thermal cycle using a Plasma Assisted Debinding and Sintering (PADS) process. The homogeneous dispersion of discrete particles of solid lubricant was achieved by in situ formation of graphite nodules due to the dissociation of a precursor (SiC particles) mixed with the metallic matrix powders during the feedstock preparation. Nodules of graphite (size≤20μm) presenting a nanostructured stacking of graphite foils with thickness of a few nanometres were obtained. In this work, the effect of the metallic matrix composition on the sliding wear behaviour (dry reciprocating sliding tests; normal load 7N; frequency 2Hz; stroke 10mm; 5mm diameter 52100 steel ball) of 3% SiC plasma assisted debinded and sintered (T=1150°C, 1h) self-lubricating steel is presented and discussed. Three different matrix compositions (Fe+0.6% C; Fe+0.6% C+4.0% Ni; Fe+0.6% C+4.0% Ni+1.0% Mo) were analysed. The mechanical properties were greatly increased by the addition of Nickel and further improved (2x) by the presence of Molybdenum. Increasing the hardness of the composites induced a strong decrease of the average friction coefficient, which reached values as low as 0.04. The counter body wear rate was greatly affected by the matrix composition (4x). Similar behaviour was observed for the wear rate of the specimen. The results were discussed in terms of wear mechanisms, which were characterised using SEM and micro Raman spectroscopy.

Performance of carbide and ceramic tools in the milling of compact graphite iron - CGI

Grey cast iron (GCI) is the most common material used in diesel engine blocks. However, to increase the pressures in the combustion chamber when this kind of alloy is used, it is necessary to increase the block wall thickness, what raises its weight and, consequently, does not fulfill the requirements. Thus, the compacted graphite iron (CGI) appears as an alternative for such application. It has characteristics of heat conductivity and damping similar to the GCI, but with superior mechanical properties, making possible the manufacturing of lighter engines with better performance. However, the use of CGI presents as disadvantage its worse machinability, when compared with GCI, stimulating development of machining techniques and cutting tool materials. The goal of this work is to analyze the performance of two tool materials (carbide and ceramic Si3N4) in the finishing milling of the fire face of the engine block made of CGI. To reach this goal these two materials were compared in terms of wear mechanisms and tool life in different cutting speeds. The main conclusion was that, for conditions similar to those used in this work, carbide is better than ceramic in terms of tool life in the milling CGI.

Susceptibility to scratch damage of high density polyethylene coating

The scratch behaviour of a polymeric coating, based on high density polyethylene (HDPE), used for pipeline application was investigated in this work. Instrumented scratch testing was used for the characterization of the friction and wear behaviour of such materials. Static contact of a diamond conical indenter against the polyethylene (PE) coating was experimentally studied. Normal and tangential forces are measured during the scratch experiments using various scratch parameters (such as sliding velocity, attack angle, applied normal load). All the experiments were performed under unlubricated conditions and at room temperature. Moreover, some hygrothermally aged coating samples were considered in order to analyze the effect of immersion aging on their scratch behaviour. Before being subjected to scratch experiments, these samples were immersed in synthetic sea water at a set temperature of 70 °C and for 6 months. The scratch behaviour of the polymeric coating was discussed in terms of friction coefficient, wear resistance and wear mechanisms. Thus, scratch tracks were analyzed qualitatively and quantitatively. To characterize the scratch process, three-dimensional topography of the scratched surfaces was measured. In fact, wear morphologies of the PE coating were observed after scratching by tactile profilometer and laser interferometer. Friction response was correlated to wear mechanisms. Wear volume can be considered relevant responses to describe the material's scratch resistance. It was found that friction and wear behaviours are significantly affected by scratch parameters. Hygrothermal aging considerably affects the wear resistance of the studied coating. In terms of wear mechanisms, the ploughing controls the wear of such PE coating whole the retained test conditions.

Wear Characteristics of Particulate Reinforced Metal Matrix Composites Fabricated by a Pressureless Metal Infiltration Process

The effect of size and volume fraction of ceramic particles with sliding speed on the wear properties were investigated for metal matrix composites fabricated by a pressureless metal infiltration process. The particulate metal matrix composites exhibited about 5.5 - 6 times greater wear resistance compared with AC8A alloys at high sliding speed, and by increasing the particle size and decreasing the volume fraction the wear resistance improved. The wear resistance of the metal matrix composites and AC8A alloy represented different aspects: the wear loss of the AC8A alloy increased with sliding speed linearly, whereas, the metal matrix composites displayed more wear loss than the AC8A alloy in the slow-speed region. However, a transition point of wear loss was found in the middle-speed region, which shows the minimum wear loss. Furthermore, wear loss in the high-speed region exhibited almost the same value as the slow-speed region. In terms of wear mechanism, the metal matrix composites showed abrasive wear at a slow to high sliding speed generally. However, the AC8A alloy showed abrasive wear at low sliding speed and adhesive and melt wear at a high sliding speed.

Erosive wear of thermally sprayed tungsten coatings

A detailed study has been made of the erosion behaviour of air plasma sprayed (APS) and vacuum plasma sprayed (VPS) tungsten coatings. The erosion rates observed under a variety of test conditions are discussed in terms of wear mechanisms, coating microstructure and mechanical properties. Both coating systems exhibited similar trends in wear behaviour as a function of impingement angle, particle velocity and size. However, the steady-state erosion rates of APS coatings were an order of magnitude greater than those of either VPS coatings or bulk material tested under similar conditions. For APS tungsten coatings, the higher erosion rates are attributed to the loss of individual splats by brittle boundary fracture which is caused by the presence of an oxide layer at the intersplat boundaries. In contrast, VPS tungsten coatings were well bonded and oxide free and the material removal mechanisms involved plastic deformation, i.e. ploughing of the surface of splats. Scratch adhesion and tensile test measurements were performed to quantify the differences in the mechanical properties of the coatings in terms of bulk modulus and intersplat cohesion.

High temperature plastics bearing compositions

The chemistry and technology of high temperature plastics is outlined and the development of filled compositions for use as bearing materials is discussed with reference to their friction and wear characteristics, physical properties and fabrication problems. The development of a journal bearing lined with a polyphenylene sulphide-based composition is described and a comparison made with a commercially available epoxy-based material in tests covering a wide range of temperatures, loads and sliding speeds. The results, which are discussed in terms of wear mechanisms, indicate that high temperature plastics could offer a significant improvement over conventional polymers under conditions where bearing surface temperatures in excess of 120°C are likely to be encountered under dry or poor lubrication conditions.