Plasma nitrided compound layers in sintered parts: Microstructures and wear mechanisms

The shape and dimensions of the component play an essential role in plasma nitriding processes, a surface treatment crucial for the powder metallurgy industry since salt bath and gas nitriding poorly interact with porous structures. When the geometry of the components contains cavities, holes, or even parallel flat surfaces, the hollow cathode effect (HCE) can be activated, and, as a consequence, microstructural transformations that develop columnar nitride compound layers and the formation of a N-rich eutectoid, the braunite, may occur. The tribological properties of nitride compound layers with microstructural characteristics induced by the hollow cathode effect were evaluated in this work. Hollow cylinders of sintered iron were plasma nitrided and the tribological behaviour was assessed via reciprocating dry sliding tests. Unlike the friction coefficient, which is high (~0.4) regardless of the surface treatment, the wear rate is strongly increased by the presence of columnar morphology (2.4x) and braunite layer (2.1x), which are at the origin of brittle mechanisms such as crack propagation and interaction, resulting in the detachment of the compound layer. •Hollow cylinders of sintered pure iron were plasma nitrided.•The component geometry influences the microstructure of the plasma nitride layers.•The wear rate is affected by the microstructural characteristics induced by HCE.•Columnar structure and braunite layer reduce the wear resistance of nitride layers.•Microstructural features induced by HCE do not influence the friction coefficient.