4.1 Conclusions

In the present investigation the lubricated rolling-sliding damage mechanisms in two as sintered and carbonitrided PM materials were investigated. Four types of damage mechanisms were identified:

1. Macroscopic spalling (in the as sintered materials only);
2. Formation of surface microcracks by plastic shearing;
3. Surface densification (in the as sintered materials only);
4. Formation of numerous microcracks at the pore edges located in the subsurface regions or at the surface pores.

In the last case they were observed in the carbonitrided materials only. The formation of subsurface microcracks was explained by making reference to a local approach based on the comparison between the equivalent local stress and the matrix yield strength. By means of this approach, however, the presence of microcracks at the edges of surface pores in the carbonitrided materials could not be explained. In fact, these cracks were produced by asperity fatigue due to repetitive tensile (friction) stresses.

A way to increase the contact fatigue resistance of PM alloys is, clearly, the increase in the matrix yield strength. Care, however, has to be paid not the excessively decrease matrix fracture toughness, since the formation of surface microcracks could impair the surface integrity of the materials.