PLASMA-SPRAYED OXIDE CERAMICS ON STEEL SUBSTRATES

The behavior of plasma-sprayed oxide ceramic coatings on several metal substrates was investigated under different mechanical and thermal lo ading conditions. Metallographic evaluations were carried out to deter mine the structures of the coatings and interfacial regions. Some of t he crystallographic transformations were detected by conducting X-ray diffraction analyses on powders and as-sprayed coatings. In alumina-ba sed powders, most of alpha-Al2O3 (H) always transformed to gamma-Al2O3 (C). In the case of magnesium zirconate powders, monoclinic zirconia, which is present in the initial composition, was completely transform ed to cubic zirconia. The spraying of lime-stabilized zirconia resulte d in the evaporation of CaO powder (CaCO3) because of its relatively l ower evaporation point, and part of the monoclinic zirconia was transf ormed to tetragonal structure after spraying. II was shown by tensile and three-point bending tests that ceramic coatings have a failure mec hanism in which microcrack formations prevail and, as the stress level is increased, macrocrack formations start. Thermal shock and flame te sts showed that ceramic coatings are resistant to high temperature gra dients and they have good thermal barrier properties. However, relativ ely long-term heat treatment resulted in oxidation problems at the int erfacial region in the case of using AISI 1015 carbon steel substrates . For AISI 304 stainless-steel substrates, the failure mechanisms appe ared to be the thermal expansion mismatch and partial destabilization of the coating structure. (C) 1997 Elsevier Science S.A.