Class 7 includes thermal spray coatings such as metal, ceramic, and intermetallic layers applied through combustion or electric-arc methods. These coatings are used for wear resistance, corrosion protection, and thermal barriers across aerospace, automotive, and industrial applications. Metallographic evaluation targets porosity, bonding, and phase integrity across both coating and substrate.
Weak bonding or excessive force may cause coating to detach from the substrate during grinding or polishing.
High-velocity impact and sintering effects create delicate microstructures that must be preserved during prep.
Coatings like WC-Co react poorly with water—alternative lubricants or dry methods are needed.
Pores may become clogged with abrasive or smeared metal, hiding true structure unless cleaned between steps.
Use low-speed diamond saws with adequate cooling. Clamp coated surfaces carefully to avoid flaking at edges.
Vacuum impregnation is recommended to support pores and avoid coating separation during prep.
Use alumina abrasives and moderate pressure. Avoid water if cobalt or reactive metals are present.
Use 0.25 µm diamond slurry on low-nap cloths. Final polish with colloidal silica to minimize relief and clean pores.
Bond interface remains intact and visible
Pores appear open and clean, without smearing
No cobalt oxidation or color change from water exposure
Fine structures and phase boundaries are preserved