On the stability of amorphous silica coatings for high-temperature service

Amorphous silica coatings significantly improve the resistance of Incoloy 800H to attack in aggressive atmospheres at temperatures up to 1000 °C. Eventually, however, failure occurs due to crystallisation of the silica and solid-solid interactions between the coating and substrate. Crystalline silica nucleated beneath a layer of alloy, which had been depleted in chromium on forming a pre-coating oxide layer. Due to the difference in chemical stability of amorphous and crystalline silica, silicon and oxygen diffused through the depleted alloy layer resulting in migration of this alloy layer towards the coating surface. Diffusion calculations suggest migration rates that are reasonably consistent with those observed experimentally.