Surface Mo-Enrichment and Plasma Carburizing on Sintered Iron: Microstructure and Tribological Properties

In this investigation, the effect of plasma treatments (molybdenum surface enrichment + plasma carburizing) on microstructural and mechanical properties of sintered iron was investigated. The plasma surface enrichment was performed using a molybdenum cathode. Subsequently, the specimens were plasma carburized. To assess the tribological behavior, reciprocating dry sliding wear tests was performed using a ball-on-flat configuration. There was little variation in the surface morphology of the plasma-carburized sintered iron when compared to the plain sintered iron, whereas for Mo-enriched specimens, in particular FeMoC, large β-Mo₂C and ξ-Fe₂MoC particles were identified. The surface topography was modified accordingly. Indeed, average quadratic roughness varied order of magnitude, and the skewness moved to the positive value (Ssk > 0; predominance of picks) for the FeMoC samples. There was a close correlation between the specimen’s wear rate and subsurface hardness. In fact, the harder samples (FeMoC) that presented surface hardness about 4 times greater than the softer ones (Fe) also showed a reduction in the wear rate around 4 times as well as inducing more considerable wear of the counterbodies. The friction, in turn, was little affected by the presence of surface layers, presenting slightly lower values for the cemented layers. The tribolayers developed in all tested samples were characterized as transformed structures consisting of repeatedly deformed, comminuted, oxidized, agglomerated, and compacted wear particles. The subsurface hardness was the determining property to obtain the lowest wear rate, regardless of the initial roughness of the surface.