ENVIRONMENTALLY FRIENDLY ALTERNATIVES TO HARD CHROMIUM COATINGS

The objective of this research project is to develop High Velocity Air Fuel, HVAF, sprayed metallic coatings with enhanced functional performances. The coatings will be attractive both economically and environmentally as alternatives to Electrolytic Hard Chromium coatings.

High Velocity Air Fuel, HVAF, is the newest member of the high velocity combustion spray processes and it brings significant advantages over its predecessors (i.e. High Velocity Oxy-Fuel – HVOF). The HVAF process operates on the same principle as HVOF, but the equipment is simpler. Using air instead of oxygen reduces significantly the overall cost of the spraying process and allows the system to operate at lower temperatures (approximately 1800 0C). Newer gas supply and gun configuration result in increased spraying velocity i.e. propelling particles up to Mach 3. Lower thermal energy and higher kinetic energy of particles are bringing important changes in coating properties making the HVAF coatings have higher adhesion, lower porosity, and lower oxide content than the HVOF coatings. The residual stress in HVAF coatings is lower and preponderantly in compressive state. Lower temperatures in the HVAF process imply also a lower depletion ratio as well as a lower dissolution ratio of the particles during spraying. The roughness of the HVAF coatings is considerably lower making the post-spray machining step unnecessary.