Oxidation behaviour of PVD coated tools for metal cutting applications

Sammanfattning

In the 20th century, big innovative steps were made in the cutting tool industry. Two Swedish companies, today Seco Tools AB and Sandvik Coromant, were some of the first companies to use cemented carbide as tool material. In 1969 Sandvik Coromant introduced a layer of titanium carbides on top of a cemented carbide substrate to improve its performance. Coating materials have been continuously developed since, with the aim to support the substrate of the cutting tool. Today, this means that the tool life can be extended by a factor 2 – 3 in certain applications. Coatings are mainly deposited in either of two methods: Chemical Vapor Deposition (CVD) or Physical Vapor Deposition (PVD). High process temperatures (800 – 1000 °C for the CVD method and 400 – 600 °C for the PVD method) leads to tensile stress due to different heat expansion coefficient for the substrate and the coating material. With cathodic arc evaporation, the PVD method used for this master thesis project, compressive stress forms in the coating due to high-energy metal ions bombarding the substrate. Thin PVD coatings have beneficial properties for cutting tools used in finishing operations. Thicker CVD coatings have properties beneficial for cutting tools used in roughing operations. This study compares the performance of four concept coatings with two reference coatings: TiN and TiAlN. The substrate material of the tool is cemented carbide, WC-6 % Co, being coated with the PVD cathodic arc technique. The coatings are heat treated in furnace as well as tested in machining operation, turning, in two different environments: argon respectively air, applied on the tool at high pressure to ensure respectively eliminate oxygen. The workpiece material to be machined was the difficult-to-cut Alloy 718, deployed both at elevated speed to stress the coating thermally for five seconds, and performance tested for a couple of minutes. One concept coating and TiAlN showed great oxidation resistance in furnace at 800 °C, and the other coatings showed medium to low oxidation resistance. The TiAlN outperformed the other coatings in machining experiments. The as-worn tools were studied in scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDX) results showed insufficient data. Hence cross-sectioned tools were further investigated with SEM-EDX, which revealed distribution of elements on top of the coating after machining. SEM studies of cross-sectioned tools used in machining revealed no oxides atop the coating. The conclusion was that the concept coatings and the TiN were not strong enough to machine Alloy 718 to reach conditions when oxidation resistance plays a sufficient role. Results showed that the coatings, with the exception of TiAlN, are most likely mechanically worn, due to adhesion of the workpiece material on the coating which removes the coating mechanically due to the movement of the adhered material. The substrate gets exposed and the contact with Alloy 718 results in its rapid wear, due to the lack of protective coating.