Emissivity of plasma coatings of the Si-Ti-Cr system on niobium substrate

Calorimetry study on the emissivity of heat-resistant plasma coatings of the Si-Ti-Cr system on nonalloyed niobium and the Nb-4OTi-5Al alloy has been carried out in 1x10-4-5x10-5 mm Hg vacuum within the temperature range from 700 deg to 1350 deg C. It is found that integrated hemispherical blackness of plasma silicide coatings depends on the chemical composition and strOture of the silicide phases; and for the Si-Ti-Cr coatings it is in 0.6-07 range up to 1300 deg C. High emissivity of the coatings is related to the rigid type of the interatomic bond (covalent or ion-covalent) of the main structure components of the plasma coatings and to the presence of some chromium, in the oxide film of amorphous silica on the surface of the formed coating the highest chromium oxide has a strong integrated blackness (approximately 0.8-0.9). Chromium oxide sublimation in 1x10-4-1x10-3 mm Hg vacuum at a temperature higher than 1350 deg C results in irreversible decrease of coating emissivity