Topographical analysis by 3-D profilometry in the study of the oxidation of Cr–N coated hard metal tools

Conventionally oxidation studies involve thermogravimetric analysis, in which the weight gain is related to the oxide formation. In this work, the oxidation of 4 μm and 7 μm Cr–N coated hard metal tools was monitored using the 3-D profilometry, in conjunction with XRD, EDS and SEM analyses. For oxidation tests at 850 °C in air, a higher weight gain was observed for 4 μm Cr–N system. After oxidation, XRD detected chromium and tungsten oxides for the thinner film system, whereas only chromium oxide was observed for the other. Nevertheless, SEM and EDS results indicated that Cr and W oxides exist on both systems. During oxidation process, besides the outward diffusion of Cr and a probably outward diffusion of substrate elements, such as W, the inward diffusion of oxygen also occurred in some degree through the films, with O2 reaching the substrate and forming tungsten oxide. This process generated a volumetric expansion, which resulted in local adhesion loss and film rupture. These results indicated that, on coated systems oxidation studies, the weight gain could be not only related with film oxidation, but also with film and substrate oxidation.