CLASS 7 - THERMAL SPRAY MATERIALS
Examples include powder spray coatings, ceramic coatings, intermetallic coatings.
Thermal spraying techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. Thermal spraying can provide various coating thicknesses (20 microns to several mm) over a large area. Coating materials available for thermal spraying include metals, alloys, ceramics, plastics and composites. They are fed in powder or wire form, heated to a molten or semi-molten state and accelerated towards the substrates in the form of micron-size particles. Combustion or electrical arc discharge is usually used as the source of energy for thermal spraying. The coating quality is usually assessed by measuring its
porosity, oxide content, macro and microhardness, bond strength
and surface roughness.
Metallographic preparation of thermal spray coatings can be tricky; however, minimizing damage and understanding the chemical and mechanical properties of the coating and substrate will greatly improve successful specimen preparation. Note for WC-Co coatings, the cobalt is reactive with water so final polishing with 0.25 micron diamond replaces traditional alumina water based suspensions/slurries.