Improved SiO₂-coatings against high temperature sulphidation by internal stress reduction

Alloys such as AIS1 304 and AIS1 321 stainless steels and Incoloy 800H can be protected against high temperature corrosion by means of amorphous SiO2-coatings deposited by metal organic chemical vapor deposition (MOCVD). The coated alloys are only attacked locally after exposure to a 19% H2, 1% H2S, 1.5% H2O, Ar bal. environment at 823 K (pS2 = 1.2 x 10-9bar and pO2 = 9.3 x 10-29 bar). The crack density observed after sulphidation is identical for silica deposited onto all metallic substrates. Decohesion of amorphous silica coatings from AISI 304 is observed, when the coating thickness exceeds 1.7 µm, while coatings on Incoloy 800H are adherent up to 4 µm. These phenomena can be explained by the fact that the adhesion is a function of the chemical interactions between the metal and the coating, whereas crack formation is only a function of the internal stresses in the coating. Two methods are studied to reduce the coating failure by means of a reduction of internal stresses in the coating, the first is changing the deposition process and the second is reducing the thermal mismatch between the coating and the metal.