Tuning HVOF-Spraying of Nickel-Aluminum Bronce Coatings for high Strength and Cavitation Resistance

Due to oxidation during the coating process, thermal spray coatings often still miss needed strengths to survive cyclic loads that occur under cavitation attack in marine environments. Nevertheless, spray techniques may offer solutions for ship or pump propeller repair, as well as for ship rudder protection.

By using a DJ 2700 spray system with ethylene as fuel for processing nickel-aluminum bronze coatings, HVOF conditions were varied with respect to the oxygen to the fuel ratio lambda, the spay distance and the alternative use of air or nitrogen as gun cooling gas. Respective experimental results were correlated with calculated impact conditions.

Maximum tensile strength and cavitation resistance comparable to bulk material are obtained for under-stoichiometric oxygen to fuel ratios of about 0.8, still guaranteeing sufficient heat during coating build-up. Despite that, an increase in oxygen content and a decrease in electrical conductivity already occurs at lambda of 0.6. The result that oxidation restricts coating properties less than expected can be attributed to the complex interplay during coating formation, needing the right balance between kinetic and thermal energy. In the present case, partially liquid like deformation at internal interfaces is favored to ensure required inter-splat bonding qualities for cavitation protection.