The angle of impingement of the spray stream is a source of porosity, because of shadowing. The diagram illustrates the type of shadowing generally associated with coatings sprayed at angles below 45° from the optimal normal angle of incidence.
Coating porosity decreases (i.e., density increases) as the angle of spray approaches 90°, that is, normal to the surface being coated. It follows that more advanced coating systems are sprayed with tighter tolerances on fixture alignment.
Plasma coatings may be sprayed at ±15°, whereas low-end coatings may be sprayed at ±30°. The diagram shows how surface protrusions build up and then shadow interstices or voids adjacent to protrusions and behind them. These protrusions produce even more shadowing, particularly as the angle of incidence is lower relative to the surface, that is, <90°.
Volume 5A, Thermal Spray Technology ⇒ Thermal Spray Processes and Coatings ⇒ Coating Structures, Properties, and Materials ⇒ Coating Structures
