Wear resistance of PVD coatings in plastics processing

In view of the increasing filler content, processing temperatures and throughput in plastics, wear in processing will remain a problem. This is particularly true of processes where no surface damage is permitted because of the subsequent change of dimensions or the intoxicating effects of the loosened particles, as is the case, for instance, in the manufacture of polymeric optical glass fibres. In particular, raw materials that exhibit abrasiveness as well as corrosiveness create a severe complex tribological load. The surface of processing equipment must be hard enough to withstand the mechanical attack of the hard glass fibres and should also be corrosion resistant. The potential of PVD coatings for wear protection in plastic processing has been studied when exposed to solid as well as molten plastic raw material. Wear tests have been carried out for commercially available glass fibre-reinforced polyamide (PA) and glass fibre-reinforced polyphenylene sulphide (PPS). The methods used were the platelet apparatus for the molten materials and the DKI universal wear and friction tester for the solid raw material. It was found that PPS (compared with PA) created higher abrasion in the solid zone and lower abrasion in the plasticized stage, although it may have higher corrosiveness. Among the PVD coatings tested the best results were achieved with CrN. In comparison with standard metals used for plastics machining the results show that, in general, PVD coatings offer a good opportunity for wear protection: not so much in the solids conveying zone but more so in the melt zone. As yet little is known about the scope of multi-layered PVD coatings, in which by combining the individual layers a combined wear resistance against complex loads may be achieved.