Identification of rolling-sliding damage mechanisms in porous alloys
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I. INTRODUCTION
THE surface damage due to rolling-sliding may be severe in the case of boundary or mixed lubrication regimes,[1] since it is due to cyclic Hertzian stresses as well as surface traction forces. Cyclic Hertzian stresses, in fact, can produce local plasticizations at microstructual inhomogeneities (such as inclusions[2]) or at surface machining marks,[3] which nucleate surface or subsurface fatigue cracks.[4] These cracks can subsequently propagate, mainly parallel to the surface (at the Hertzian depth, where the equivalent stress is highest) and produce, by spalling, debris with the typical flake morphology. The surface traction forces due to friction can, on the other hand, give rise to plastic deformations, which can directly produce a wear fragment by delamination[5] or help in the formation of surface fatigue cracks.[1,6] Ferrous porous alloys, produced by powder metallurgy (PM), are currently used in the production of structural parts, such as gears, which undergo rolling-sliding loading in service. The design of these parts is often carried out using empirical relations, and 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.[7] The interplay between residual porosity and matrix microstructure has thus to be properly considered, since pores act as stress and strain concentrators,[8,9] producing plasticized regions at their edges at very low applied stresses, well below the 0.2 pct offset yield stress.[10] The nucleation of fatigue cracks may thus be quite easy in PM alloys,[11] and fatigue life is mainly determined by their propagation rate.[12] In this respect, it has to be further considered that if porosity G. STRAFFELINI and A. MOLINARI, Associate Professors, and T. MARCU PUSCAS, Researcher, are with the Department of Materials Engineering, University of Trento, 38100 Trento, Italy. Manuscript submitted December 23, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
is mainly interconnected, the propagating crack may find an energetically favorable path following the filmlike network of porosity.[13] In recent years, a number of articles concerning the rolling-sliding resistance of ferrous porous alloys have been presented at PM conferences.[14–17] Most of these articles, however, report contact fatigue data without completely identifying the damage mechanisms. The aim of the present investigation was therefore to study the lubricated rollingsliding damage in a PM Fe-0.6 pct C alloy and in PM iron submitted to a carbonitriding treatment, i.e., a relatively soft and a relatively hard material, containing both closed and interconnected pores. In particular, the damage mechanisms have been identified and modeled using a local approach, which enabled us to properly account for the effect of porosity. II. EXPERIMENTAL PROCEDURE Two materials were produced for the present investigation: an Fe-0.6 wt pct C alloy, using atomized iron powders and natur
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