High-temperature tribological characterization of commercial TiAlN coatings

This study was performed with the aim of evaluating the relative tribological behaviour at high temperature of (Ti(1-x)Al(x))N coatings commercially deposited on WC inserts. The (Ti(1-x)Al(x))N multilayered, nanostructured and single-layer coatings, which contained different Ti/Al atomic ratios varying from 7/3 to 2/3 respectively, were deposited by employing a commercial PVD cathodic arc process. The absolute hardness value for each coating is also reported and has been calculated from the Vickers microhardness measurements by using one of the models published in the literature. Standard ball-on-disc testing was conducted in order to determine friction coefficients and wear rates for these systems against a 6 mm alumina ball. These tests have been carried out in conditions that are not common in industrial use, e.g. metal cutting tools inasmuch as alumina is not a representative workpiece material. The sliding tests were performed out at 25, 500 and 700 °C with 5 N normal loads. At 25 °C, a wear volume, V, of approximately 10(-2) mm(3) was obtained for all the tested coatings. When the test temperature increased to 500 °C, the single-layered coatings showed a wear volume of the same order of magnitude as those tested at room temperature. The multilayered coated samples decreased their wear volume by one order of magnitude, whereas the nanostructured samples showed almost no wear. At 700 °C, the wear volume values reported for all samples were similar and of the same order of magnitude as those tested at room temperature. The wear mechanism is discussed together with the morphological and compositional characteristics, determined by SEM coupled with EDX analysis.