Identification of rolling-sliding damage mechanisms in porous alloys

Lubricated rolling-sliding damage in a relatively soft Fe-0.6 pet C alloy a nd a relatively hard carbonitrided iron, both produced by powder metallurgy , has been investigated. Damage mechanisms were controlled by large-scale a s well as small-scale plastic deformations. A large-scale, bulk plastic def ormation process produced surface densification in the Fe-0.6 pet C alloy. Formation of surface cracks by asperity-scale plastic shearing was also obs erved in both materials. Small-scale plastic deformation processes, restric ted to the pore edges, gave rise to the formation of fatigue microcracks at the boundary between the densified and undensified region in the Fe-0.6 pe t C alloy. A similar effect was found at a depth of between 550 and 1000 mu m in the carbonitrided material. Moreover, in the Fe-0.6 pet C alloy, these plastic deformations also triggered the formation and propagation of macro cracks, which produced macroscopic damage by spalling. The damage mechanism s due to small-scale plastic deformations were explained on the basis of a local approach model, able to account for the influence of pores on the mec hanical behavior of the materials. However, this approach could not explain the microcracks, which were found at the surface pores in the carbonitride d material. Their formation was ascribed to the interplay between the surfa ce tensile (friction) stresses and the low matrix toughness of the material near the surface.