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0. Abstract
1. Introduction
2. Experimental procedures
3. Results and discussion
4. Conclusions
5. References

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Lubricated Rolling-Sliding damage
in Powder Metallurgy alloys

by Giovanni Straffelini, Damiano Martorelli
Department of Materials Engineering, University of Trento, Via Mesiano 77, 38122 TRENTO (Italy)

1.1 Introduction

Powder Metallurgy allows for the obtainment of mechanical parts with complex shapes, high dimensional accuracy and low costs if the production volumes are sufficiently high [1]. A characteristic feature of these materials is the presence of residual porosity in their microstructure. Porosity reduces their mechanical properties, since pores reduce the load-bearing section and induce a strain concentrating effect, which impaires ductility [2].

In the case of wear, however, pores not always play a negative role. In the case of unlubricated sliding, for example, they can trap the wear fragments during sliding, thus reducing wear [3]. But also in the case of lubricated wear they may have a positive effect. Pores, in fact, can be used as a reservoir for oil, which is released during sliding. This is the case of the oil impregnated PM materials, as in sliding bearings. In this case, the higher the porosity content, the higher the amount of oil which can be stored in the materials [4].

Finally, also in the case of lubricated contact fatigue, porosity may have a positive effect. In fact, if porosity content is mainly interconnected (i.e. lower than about 10 %) the oil is allowed to enter the material body through the interconnected pores and the oil hydraulic effect, which help in opening and propagating surface cracks, can be avoided [5]. As an example, Lipp et al. [5, 6] found that, contrarily to all the other mechanical properties, the endurance limit in PM steels decreses if density increases from 7.2 to 7.4 g/cm3.

The design of PM parts is often carried out using empirical relations. However, in order to increase employment of these types of materials (in particular for heavy-duty applications), specific design criteria which properly account for the damage mechanisms have to be developed. In the present investigation the lubricated rolling-sliding surface damage mechanisms in two different (as-sintered as well as surface treated) PM materials were investigated and guidelines for the proper selection of PM materials for lubricated rolling-sliding applications are delineated.

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