Thermal shock characteristics of plasma sprayed mullite coatings

Commercially available mullite (3Al(2)O(3). 2SiO(2)) powders containing oxi des of calcium and iron as impurities, have been made suitable for plasma s praying by using an organic binder. Stainless steel substrates covered with Ni-22Cr-10Al-1.0Y bond coat were spray coated with mullite, The 425 mu m t hick coatings were subjected to thermal shock cycling under burner rig cond itions between 1000 and 1200 degrees C and less than 200 degrees C with hol ding times of 1, 5, and 30 min. While the coatings withstood as high as 100 0 shock cycles without failure between 1000 and 200 degrees C, spallation o ccurred early at 120 cycles when shocked from 1200 degrees C, The coatings appeared to go through a process of self erosion at high temperatures resul ting in loss of material. Also observed were changes attributable to meltin g of the silicate grains, which smooth down the surface. Oxidation of the b ond coat did not appear to influence the failure, These observations were s upported by detailed scanning electron microscopy and quantitative chemical composition analysis, differential thermal analysis, and surface roughness measurements.