Plasma-sprayed oxide ceramics on steel substrates

Abstract

The behavior of plasma-sprayed oxide ceramic coatings on several metal substrates was investigated under different mechanical and thermal loading conditions. Metallographic evaluations were carried out to determine the structures of the coatings and interfacial regions. Some of the crystallographic transformations were detected by conducting X-ray diffraction analyses on powders and as-sprayed coatings. In alumina-based powders, most of α -Al 2 O 3 (H) always transformed to γ -Al 2 O 3 (C). In the case of magnesium zirconate powders, monoclinic zirconia, which is present in the initial composition, was completely transformed to cubic zirconia. The spraying of lime-stabilized zirconia resulted in the evaporation of CaO powder (CaCO 3 ) because of its relatively lower evaporation point, and part of the monoclinic zirconia was transformed to tetragonal structure after spraying. It was shown by tensile and three-point bending tests that ceramic coatings have a failure mechanism in which microcrack formations prevail and, as the stress level is increased, macrocrack formations start. Thermal shock and flame tests showed that ceramic coatings are resistant to high temperature gradients and they have good thermal barrier properties. However, relatively long-term heat treatment resulted in oxidation problems at the interfacial region in the case of using AISI 1015 carbon steel substrates. For AISI 304 stainless-steel substrates, the failure mechanisms appeared to be the thermal expansion mismatch and partial destabilization of the coating structure.