S.C. Lim

Overview no. 55 Wear-Mechanism maps

The potential of Wear-Mechanism Diagrams is explored. Diagrams which show the rate and the regime of dominance of each of a number of mechanisms of dry wear (delamination, mild and severe oxidation, melting, seizure, etc.) are constructed empirically (that is, from experimental data alone) and by modelling (by theoretical analysis calibrated to experiment). The method is applied to steels, and has wider application as a way of classifying and ordering wear data, and of showing the relationships between competing wear mechanisms.

Wear-rate transitions and their relationship to wear mechanisms

Abstract This paper is concerned with ways of classifying wear and presenting the information on a wear map. The unlubricated sliding wear of steel is classified into seven elementary mechanisms of material removal. The two principal modes of wear, namely mild and severe wear, provide an alternative but related classification. These two approaches are combined and the results are displayed on a wear-mode map. The map shows the modes of wear and the dominant wear-mechanisms on a diagram with axes of contact pressure and velocity representing the sliding conditions. The mode map also shows the extent of the transition regions between modes. Three types of transition are identified, load, velocity and distancedependent. The transitions responsible for the change in mode are displayed on a wear-transition map.

The effects of sliding conditions on the dry friction of metals

Abstract A survey of the variations of μ in the dry sliding of steel on steel over a wide range of sliding conditions is presented in the form of a friction-regime map. At slow sliding speeds (v 1 m/s), the initial surface condition is quickly replaced by one characteristic of the wear process, and the coefficient of friction becomes, increasingly, dependent on the pressure at the surface and the sliding velocity, not the surface state. At very high loads and velocities, a layer of molten metal forms between the sliding surfaces, reducing μ to very low values.

Recent development in the fabrication of metal matrix-particulate composites using powder metallurgy techniques

It is advantageous to fabricate metal matrix-particulate composites (MMPCs) using powder metallurgy (PM) because the fabricated composites possess a higher dislocation density, a small sub-grain size and limited segregation of particles, which, when combined, result in superior mechanical properties. The various PM-related processes currently in use in the fabrication of MMPCs, are reviewed, outlining the common problems encountered in each of these fabrication processes. The more recently developed PM techniques to fabricate MMPCs are also discussed.

Recent developments in wear- mechanism maps

This paper presents a summary of the authors personal view of the development of wear-mechanism maps, culminating in the presentation of some recently proposed maps. These maps, which present wear data in a graphical manner, are able to provide a more global picture of how materials in relative motions behave when different sliding conditions are encountered; they also provide the relationships between various dominant mechanisms of wear that are observed to occur under different sliding conditions as well as the anticipated rates of wear. Some thoughts on future directions for research in this area are also presented.

The relevance of wear-mechanism maps to mild-oxidational wear

The presence of oxygen in the environment in which a steel sliding system operates will promote a mild form of wear with wear debris consisting mainly of iron oxides. Of the oxidation-dominated mechanisms, mild-oxidational wear (the prefix describes the extent of oxidation and not the wear rate) has been most extensively investigated. In this paper, examples will be used to show that the wear-mechanism map for the unlubricated sliding of steels can adequately predict the occurrence of mild-oxidational wear and the trend of wear rates as well as describe the resultant features on the worn surfaces. It is also shown that this map is relevant to delamination wear and to test geometries other than the pin-on-disk configuration. It is suggested that the more-recently constructed wear maps for aluminium and magnesium alloys could similarly be used to predict the wear characteristics of these alloys during sliding.

The unlubricated wear of sintered iron

Abstract The unlubricated wear of sintered pure iron has been investigated under different sliding conditions using a dynamic wear rig operating in the scanning electron microscope and a simple pin-on-disk apparatus operating in air. Delamination and ploughing mechanisms were found to be responsible for wear during sliding under the different conditions. Using normalized variables, the measured wear rates agreed well with the predictions of Archards law and correlated with wear data from other sources. The effect of porosity in the iron has also been studied. Open pores on sliding surfaces were found to be important in generating and trapping wear debris. When the trapping mechanism was bypassed by cleaning, or rendered ineffective by pore closure, the wear behaviour of high porosity sintered iron approached that of the lower porosity specimens.

Wear of a spray-deposited hypereutectic aluminium-silicon alloy

Abstract This paper compares the wear characteristics of hypereutectic aluminium-silicon alloys processed using two different routes: the more recent spray-deposition technique and the conventional casting process. Unlubricated pin-on-disk wear tests revealed that the spray-deposited alloy has a better resistance to wear than its conventionally-cast counterpart. It is suggested that the increased hardness and the uniform distribution of the fine second phase in the spray-deposited alloy endowed it with a better wear resistance compared to the conventionally-cast alloy over the range of sliding conditions investigated presently.

Annealing of plasma-sprayed WC-Co coating

Abstract The effects of vacuum annealing on the physical soundness and resultant tensile bond strength and wear properties of plasma-sprayed WC-17%Co coatings on a ductile cast iron substrate have been investigated. The as-sprayed specimens were annealed at temperatures from 500 to 1000 °C for up to 12 h. The results showed that, due to differential sintering within the coating, vertical cracks were formed at the interface between darker cobalt-rich and lighter cobalt-deficient splats. The average lenght and density of cracks increased with increasing temperature at intermediate annealing temperatures from 500 to 700 °C, but decreased with increasing annealing temperature and time at higher annealing temperatures due to crack sintering and ovulation effects. Significant precipitation of WC crystallites and interdiffusion at the coating-substrate interface also occurred at the higher temperatures. The annealing-induced cracks produced a detrimental effect on the bond strength of plasma-sprayed WC-17%Co coatings. The decrease in tensile bond strength was especially pronounced for coatings annealed at intermediate temperatures from 500 to 700 °C. For coatings annealed at 800 °C and above, the tensile bond strength improved steadily over that of coatings annealed at the lower temperatures, their magnitudes increasing with increasing annealing temperature and time. Despite the above, bond strengths of the as-sprayed coating and those annealed at and above 900 °C were undetermined because failure occurred in the adhesive. Cylinder-on-cylinder wear tests showed that the wear behaviours of the as-sprayed coating and of coatings annealed under various conditions were comparable. Two wear mechanisms were identified: plastic deformation of the γ-Co matrix and spallation of splats. The wear test results were consistent with spallation being the dominant wear mechanism, both in as-sprayed and annealed conditions.

Processing, microstructure, and properties of Mg–SiC composites synthesised using fluxless casting process

Abstract In the present study, elemental magnesium and magnesium–silicon carbide composites were synthesised using the methodology of fluxless casting followed by hot extrusion. Microstructural characterisation studies revealed low porosity and a completely recrystallised matrix in every material. The average size of the recrystallised grains was found to decrease with an increasing presence of SiC particulates. For the reinforced magnesium, fairly uniform distribution of SiC particulates and good SiC–Mg interfacial integrity was realised. The results of X-ray diffraction studies indicated the absence of oxide phases and no evidence of interfacial reaction products except in the case of Mg–26.0 wt-%SiC sample. Results of physical and mechanical properties characterisation revealed that an increase in the amount of SiC particulates incorporated leads to an increase in macrohardness and elastic modulus, which does not affect the 0.2% yield strength and reduces the ultimate tensile strength, ductility, and coefficient of thermal expansion. The weight percentage of SiC particulates when plotted against hardness and 0.2% yield strength revealed a linear correlationship. An attempt is made to investigate the effect of increasing amount of SiC particulates on the microstructural features, and physical and mechanical properties of the magnesium matrix.