Plasma nitride layer design of tool steels for optimized load-bearing capacity and adherence of hard coatings
Duplex surface layers produced by means of a suitable combination of plasma nitriding/nitrocarburizing and hard coating are characterized by a high resistance against complex tribological and/or corrosive stresses enhancing the strength and lifetime of tool steels. A central challenge of this kind of duplex treatment is the design of nitride layers to meet load-specific as well as coating-specific requirements.
The current study focuses on the influence of plasma nitriding parameters, e.g. treatment duration and gas composition of the N2-H2 plasma, and initial surface condition on the resulting nitride layer structure and properties of tool steels (AISI D2, AISI M2).
The characterization of nitride layers was performed by means of cross sectional thickness measurement determining the compound layer thickness (CLT), Vickers hardness measurement, glow discharge optical emission spectroscopy (GDOES), and X-ray diffraction (XRD). Surface roughness was evaluated using optical profilometry and scanning electron microscopy (SEM). The difference in tribological performance was verified by means of unlubricated linear-reciprocating sliding ball-on-flat tests against tungsten carbide.
Plasma nitriding process parameters are correlated to resulting nitride layer characteristics in order to provide a basic knowledge for the subsequent generation of a hard coating considering further surface conditioning steps.