Effect of tungsten 0-8Â wt.% on the oxidation of Co-Cr alloys

The nature and the morphology of the formed oxide layers at the alloys' surface have great importance on the quality of the ceramic/metal bond. This study shows the effect of the tungsten addition on the formation and the growth of the oxide layers on the Co-Cr alloys in preoxidation temperature range 950-1050 °C, thus including the temperature range of the ceramisation process that these alloys will undergo later. Their characterization, by microprobe analysis, scanning electron microscopy (SEM), spectrography with luminescent discharge (SLD) and low angle X-ray diffraction, reveals the formation of two distinct layers: the first one, from the external surface, is formed only of pure CoO, while the second one is a mixture of Cr2O3 particles and cobalt-tungsten oxide dispersed in a CoCr2O4 matrix. The tungsten addition up to 8 wt.% modifies considerably the thickness of the formed layers as a consequence of its accumulating in the second layer, thus blocking the cobalt diffusion towards the alloy surface.