Adhesion of plasma sprayed ceramic coatings to metals

A basic requirement for a satisfactory plasma sprayed coating is that it should not be easily detached from the substrate during use, a property which is usually rather loosely defined as adhesion. In practice failure may take place entirely within the coating (cohesive failure) or close to the substrate interface (adhesive failure). Plasma sprayed coatings are formed by the impact, deformation and solidification of individual liquid droplets so that their structure consists of a series of overlapping lamellae and the properties of the coating depend upon the interactions between individual lamellae and between lamellae and substrate. The adhesion of plasma sprayed ceramics to metals is relatively poor but can be considerably improved if a sprayed bond-coat is used between the ceramic and metal. Although the high bond strength between the intermediate layer and steel may be explained, it is not clear why ceramic coatings should adhere so well to the bond coating