Influence of nitridation on surface microstructure and properties of graded cemented carbides with Co and Ni binders

The influence of a nitridation treatment on the microstructure and the properties of (W,Ti)C-based cemented carbides with Co and Ni binders is investigated. Nitridation results in the formation of a very fine-grained (Ti,Ta,Nb)(C,N) phase (γN-phase) with inclusions of binder and WC clusters in the near-surface region of the cemented carbides. Rietveld refinements of X-ray diffractograms reveal differences in the domain size and the microstrains of the γN-phase for cemented carbides with Co-binder and Ni-binder. Nitridation reduces the domain size and increases microstrains in the γ-phase. The chemical composition, the morphology and defects of the binder were investigated by TEM/EDX before and after the nitridation treatment. The results reveal that a high nitrogen activity leads to the preferred dissolution of the core-rim structure of the mixed cubic carbide grains during the formation of the γN-phase. The nitridation heat treatment improves the wear and corrosion resistance of cemented carbides with both Co-binder and Ni-binder.