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Significance Statement
Wear of materials is a complicated process that depends on material properties and running conditions. Lubrication is the widely-used method known to reduce wear in mechanical components like gears. The lubricant works to lessen the severity of the superficial shear stress by reducing wear and friction by dissipating heat between any contacting surfaces. Under normal tribological conditions, a thin, compressed elastohydrodynamic oil film separates the contacting surfaces.
However, at times the surfaces operate too close that some surface asperities are in contact and boundary lubrication prevails. When the oil film breaks down, the entire load is assumed to be supported by the asperities leading to mechanical contact. This results in accelerated wear. Scuffing occurs in the start-ups, before full hydrodynamic lubrication occurs, or when the lubricant temperature exceeds a certain value leading to a significant drop in the lubricant viscosity.
Several studies on tribological attributes of austempered ductile iron (ADI) are not certain whether shot peening improves scuffing resistance of austempered ductile iron or not. Therefore, in a collaboration between University of Malta and Clausthal University of Technology in Germany, researchers proposed a method to compare the scuffing resistance of shot peened copper-nickel austempered ductile iron to the as-austempered specimens. The authors evaluated scuffing resistance by testing specimens under boundary lubrication conditions. Their work is published in the journal, Surface & Coatings Technology.
Experimental data of the scuffing of austempered ductile iron, before and after shot peening, was obtained. A cantilever loaded flat ended ADI pin was made to slide on a rotating steel disk. The pin was attached to one end, and to other were attached force transducers. The tribometer was fixed to an on-site computer that tracked and recorded wear and temperature of the pin and disk as well as frictional force.
The authors conducted their experiments under starved lubrication using synthetic base gear oil at ambient temperature. They used a constant loading approach instead of progressive loading, so as to avoid the pin’s behavior from being affected by its speed and loading history. Two conditions, ground as-austempered and shot peened ADI, were tested in this study.
The as-austempered test pieces endured 0.2×104 cycles before failure. The low endurance limit can be pinned to plastic flow of the ausferritic structure and micro-facture of the asperities of the sliding surfaces. However, shot peening lead to a higher scuffing resistance and endured 2.1 x104 cycles before failure. The authors found that, the indentations formed by shot peening served as lubrication pockets. Therefore, the lubricant results in a load-bearing hydrodynamic pressure that supports the load and also results in a low coefficient of friction between the two surfaces. Additionally, the higher hardness and compressive stresses present at the surface of shot peened components contributed to the higher scuffing resistance.
Reference
A. Zammit1, S. Abela1, L. Wagner2, M. Mhaede2, R. Wan2, and M. Grech1. The effect of shot peening on the scuffing resistance of Cu-Ni austempered ductile iron. Surface & Coatings Technology, volume 308 (2016), pages 213-219.
[expand title=”Show Affiliations”]- Department of Metallurgy and Materials Engineering, Faculty of Engineering, University of Malta, Malta
- Institute of Materials Science and Engineering, Clausthal University of Technology, Germany
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