（1）Characteristics of the films

The structure of film sprayed in the air is extremely complex and has the following characteristics:

[Fig.1] is the schematic diagram of a film structure.

（2）onding mechanism of the film

The substrate and spray particles are generally bonded by the combined actions of the following: mechanical bonding (anchor effect) where particles attach to the concave and convex portions of the substrate's rough surface, metallurgical bonding, and/or physical bonding caused by intermolecular attractions, such as Van der Waals force.
The bonding mechanism varies by the substrate materials and surface shapes. For example, when using plasma spraying of Mo, Ta, and W on a steel or aluminum substrate, there will be a localized area that is bonded by metallurgical reactions resulting from diffusion.In addition to the mechanical bonding, the spray particles are bonded by the diffusion of hydrogen ions or metallic ions that are reciprocally generated during collisions of the particles between the oxide films.
One of the methods of increasing the adhesion properties would be to spray in a thin layer of materials that are known to bind strongly between the substrate and the film, such as Mo, W, Ni-AL, Ni-Cr.This method is called "bonding coat". The films formed by this method will alleviate heat expansion between the substrate and the thermal spraying film, while improving corrosion resistance and adhesiveness.The bonding coat is usually applied before spraying oxide ceramics.