MANUFACTURE AND PROPERTIES OF PLASMA SPRAYED Cr2O3

Low porosity (some %) chromia coatings have been plasma sprayed onto several substrates by atmospherical and vacuum plasma spraying. Different phases appear in the sprayed product; metallic Cr and the metastable tetragonal Cr3O4 phase have been identified. A post annealing treatment completely removes these phases. This behaviour can be correlated with differences between the coefficients of thermal expansion comparing the as-sprayed with the annealed material. Hot Isostatic Pressing improves the interface bonding between coating and substrate and reduces porosity as can be seen from the results of metallographical investigations. If aluminium is present in the substrate alloy, chemical interactions have been observed. First results from measurements of residual stresses in coatings show the influence of coefficients of expansion and substrate temperature during spraying. Gastightness of the sprayed chromia could not yet be established.