TRIBOLOGICAL BEHAVIOR AND CORROSION PERFORMANCE OF TI-B-N HARD COATINGS UNDER PLASTIC MANUFACTURING CONDITIONS

During plastics processing, an increasing content of abrasive fillers and corrosive components in combination with high working temperatures up to 450-degrees-C produce extreme wear conditions. As shown in previous work, TiN coatings are wear resistant against hot molten plastic material. In this paper we deal with thin hard single-phase and multiphase coatings within the extended coating system Ti-B-N. The films were deposited onto steel substrates using an r.f. sputter process with r.f. substrate bias. In order to cover a broad spectrum of Ti-B-N coatings, the substrate bias, nitrogen gas flow and total deposition gas pressure were all varied systematically. The coatings were tested with regard to their fundamental mechanical properties such as hardness and adhesion. Additionally, the coating structure was investigated by scanning electron microscopy. Corrosion tests were also done to see how the coatings performed under this important condition. Following these tests the wear behaviour of the coating systems under plastics processing conditions was investigated in a model wear test. Corresponding to the melting zone of screw barrel systems, the test pieces were subjected to abrasive-corrosive wear under elevated temperatures. It is shown that the combination of high hardness, dense structure, good adhesion and-most important-excellent corrosion resistance leads to superior wear resistance of r.f. bias sputtered Ti-B-N coatings. The wear behaviour can be further improved if a dense covalent Al2O3 surface layer is deposited on top of the Ti-B-N film.