Crack formation,exfoliation, and ridge formation in 500 °C annealed sol–gel silica coatings on stainless steel SUS304: Part I. Microscopic observations and elemental analyses

Uniformly compact silica coatings of thicknesses >400 nm on Cr- and Ni-containing steels are practically desirable but difficult to obtain. A non-aqueous sol–gel silica coating on SUS304 was studied. After the coated substrates were treated at 500 °C in air, coatings with thicknesses of ～350 to ～1000 nm were obtained that were largely uniform, non-porous, adherent, and with minimal residual organic content, although some cracks and exfoliated sites existed. TEM/EDX analysis indicated that three interlayers were observed between silica and SUS. The innermost one was ～5 nm thick, Cr-rich, highly O-deficient, and formed a tenacious bond between SUS304 and silica; the two others were rather diffuse. The web-like development of ridges resulting from the oxidation of metallic elements (mostly Fe) diffused into the silica through nanoscale defects within the innermost interlayer was also observed. Many of the nanoscale defects seem to be related to SUS grain boundaries. Ni was almost absent in silica coating except in the innermost interlayer and ridges. The mechanisms responsible for these observed properties and phenomena are discussed.