Chrome nitride coatings for applications in plastics processing

Coated moulds, especially cavity surfaces and ejection parts are state of the art in plastics processing. Regarding the plastification unit i.e. screws and barrels coatings are not well established. Wear in processing remains a problem, especially when no abrasion is permitted e.g. in manufacturing optical elements. The surfaces must resist the abrasion of hard glas fibres as well as the corrosive attack of the polymers. Electroplated chrome is one of the answers to these demands, because of its hardness and corrosion resistance. Another solution are thin hard coatings which are not yet well established. Titanium nitride produced by PVD technique was found to show excellent abrasion resistance but often not sufficient corrosion properties. Chrome nitride was found to have great hardness like TiN, corrosion resistance behavior similar to electroplated chrome and in contrast to electroplated coatings no tensile internal stress. The present study deals with the spectrum of characteristic properties of CrxN coatings like hardness, adhesion, internal stress, chemical composition and phases produced by PVD magnetron sputtering. The coatings were deposited on steel, described in characteristics and tested in a model wear test with plasticized moulding material (PA 66-GF50 and others) simulating the tribological situation between screw and barrel of extruders or injection moulds. The tribological behaviour was studied after the complex test procedure. The high temperature of the plasticized material, high abrasion by glas fibres and corrosive influence could only change the coatings properties of CrxN slightly.