PVD COATINGS WITH HIGH IR EMISSIVITY FOR HIGH-TEMPERATURE APPLICATIONS OF CO-BASED ALLOYS

For many applications of high-temperature-resistant materials it is important that there is also a high thermal reflectivity. We have investigated for such cases different coatings of SiC, Si3N, and Al2O3 on a high-temperature-resistant alloy prepared by reactive physical vapour deposition with a hollow cathode, arc discharge evaporator and by electron beam evaporation respectively. In all cases, the coatings have an amorphous structure. For SiC and Si3N4, we observed a non-stoichiometric composition in the coatings and the reflectivity was too low, This is mainly caused by the thinness of the coatings. In the case of Al2O3, there were some difficulties with the adhesion of the layer. By coating at higher substrate temperatures, we observed an amorphous Al2O3 layer with strong adhesion at thicknesses up to 15 mum. The reflectivity depends mainly on the coating thickness. There is no effect of the evaporation rate or the oxygen partial pressure.