A New High Velocity Oxy-Fuel Process for Making Finely Structured and HighlyBonded Alloy Layers from Liquid Precursors

摘要

High velocity oxygen-fuel (HVOF) thermal spray processesare used in applications requiring the highest density andadhesion strength, which is not achievable in most otherthermal spray processes. Like other thermal spray processes,however, a normal HVOF process is not able to apply finepowders less than 10 μm via a conventional powder feeder.The advantages of using smaller and even nano-sized particlesin a HVOF process include uniform coating, less defectivemicrostructure, higher cohesion and adhesion, full density,lower internal stress and higher deposition efficiency. A newprocess has been developed to realize HVOF forming of finegrainedalloy layers by using liquid precursors containing finemetallic particles. Process investigations have shown thebenefits for making single and duplex layered coatings withfull density and high bond strength attributing to the very highkinetic energy of particles striking on the substrate surface andthe better melting of the small particles. One of the targetedapplications is for the water walls of a fossil-fired boiler thatoperate in a high temperature and corrosive environment. Thenew coating system is based on material selection, structuredesign, process innovation and diagnostics, microstructure,and property evaluation. It is promising to provide betterprotection of the boilers against various types of degradationslike corrosion, oxidation, erosion and interfacial failure.