A Contribution to explain the Mechanisms of Adhesive Wear in Plastics Processing by example of Polycarbonate

In the plastics industry, adhesive wear resulting from hot melt flow is one of the main damage mechanisms affecting extrusion tools. Such damages strongly influence the economic efficiency and the product quality. Due to their beneficial properties, Cr-based nitride hard coatings deposited by physical vapor deposition (PVD) are applied as protective coatings. These coatings can prevent the formation of iron oxides, which have an influence on the wetting of plastic melt on the tool surface. In the present work, four different CrAl-based nitride and oxynitride monolayer coatings were synthesized onto the tool steel substrate AISI 420 (X42Cr13, 1.2083) by means of a hybrid direct current and a high power pulsed magnetron sputtering (dcMS/HPPMS) process. Various Cr/Al metal ratios led to different chemical compositions of the nitride and oxynitride coatings. All coatings as well as one uncoated steel substrate were analyzed prior to and after long-term heat treatment as well as before contact angle measurements and after shear tests in order to investigate the influence of the respective processes on the chemical composition of the native passive film at the surface; analyses were run using X-ray photoelectron spectroscopy (XPS). Furthermore, the influence on the wetting behavior of polycarbonate melt was studied by means of high temperature contact angle measurements; the influence on the shear energy necessary for removing the solidified plastic droplet was investigated using shear tests. It could be shown that the heat treatment leads to a significant increase of polycarbonate wetting on the uncoated steel compared to the coated samples. Additionally, the coatings with an increased Al content exhibit a lower wetting rate compared to the other coatings and the uncoated steel. A beneficial effect of an oxidative post-treatment after deposition has been demonstrated.