Experimental characterizations of non-seizing solutions for plastic moulding tools

In the plastics moulding industry many parts sustain movement in relation to other pieces: ejectors, slides, dies, etc. Some seizing or micro-welding may appear, especially when lubrication is not used. For many years, different kinds of hard coatings were developed to avoid such problems. In this paper, the investigation analyses these problems related with nitride layers which were obtained thanks to four different processes: salt bath nitriding, salt bath nitriding followed by a post-oxidation, gaseous low pressure nitriding and plasma nitriding. Several properties of the layers were investigated such as hardness, microstructure and constitution of the surface by means of X-ray diffraction (XRD), roughness, and friction coefficient at both room temperature and 100 °C. Some important differences were pointed out between the different layers. The solution provided by a salt bath nitriding followed with an oxidation sequence gave, in terms of tribology, as good results as a physical vapour deposition (PVD) WC-C coating which was used as a reference for this study.