The angle of impingement of the spray stream is another source of porosity. The diagram shows porosity caused by shadowing. Figure 8 illustrates the type of shadowing generally associated with coatings sprayed at angles below 45 degrees from the optimal normal angle of incidence.
Coating porosity decreases (i.e., density increases) as the angle of spray approaches 90 degrees; 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 degrees, whereas low-end coatings may be sprayed at ±30 degrees.
Figure 8 shows how surface protrusions build up and then shadow interstices or voids adjacent to the 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 degrees.
This information is from ASM Handbooks Online, Vol. 5A Thermal Spray Technology, Coating Structures, Properties, and Materials. To find this information (subscription required), click on the link below and scroll down to Fig. 8.
