The diagrams show a comparison of thermal spray coatings deposited on macroroughened and smooth surfaces: (a) Sprayed metal over grooves; shrinkage constrained by grooves (b) Sprayed metal on smooth surface; effect of shear stress on bond due to shrinkage.
Macroroughening is usually accomplished by machining grooves or threads into the surface to be sprayed. Typically, rough-machined surfaces are also grit blasted prior to spraying. Surfaces roughened to this magnitude are often used for thick coatings to restrict shrinkage stresses, and to disrupt the lamellar pattern of particle deposition in order to break up the shear stresses parallel to the substrate surface, as shown in the diagrams. Caution should be used with this technique. Threading reduces cross section and can affect load capability and increase fatigue.
Volume 5A, Thermal Spray Technology -> Thermal Spray Processes and Coatings -> Introduction to Coating Design and Processing -> Processing Prior to Coating
