Effect of different substrate conditions upon interface with plasma sprayed zirconia—a TEM study

Transmission electron microscopy (TEM) was used to examine the effect of different substrate conditions on the interface between a stainless steel substrate and plasma-sprayed yttria-stabilized zirconia (YSZ). The paper focuses on the first-to-form isolated single splats, for which interface with the substrate plays a critical role in determining adhesion of the whole coating. Preheated mirror-polished stainless steel substrates served as a reference point for further comparison with rough, cold or bond-coated substrates. A modified wedge-polishing technique enabled the production of high-quality cross-sectional TEM samples with minimum ion-induced damage and large observable areas. The substrate chemistry, roughness and temperature all play an important role in the splat formation process; nevertheless, the substrate temperature effect is the most prominent of these. It was proposed that adsorbed air molecules on the surface of cold substrates cause turbulence in the spreading of molten ceramic droplets, which ultimately leads to poor splat adhesion. At a certain elevated substrate temperature, air molecules desorb from the substrate surface, thereby allowing substantially better contact and adhesion between splats and the substrate.