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

For many applications of high-temperature-resistant materials it is im portant that there is also a high thermal reflectivity. We have invest igated 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 elec tron beam evaporation respectively. In all cases, the coatings have an amorphous structure. For SiC and Si3N4, we observed a non-stoichiomet ric composition in the coatings and the reflectivity was too low, This is mainly caused by the thinness of the coatings. In the case of Al2O 3, there were some difficulties with the adhesion of the layer. By coa ting at higher substrate temperatures, we observed an amorphous Al2O3 layer with strong adhesion at thicknesses up to 15 mum. The reflectivi ty depends mainly on the coating thickness. There is no effect of the evaporation rate or the oxygen partial pressure.