Effect of the HVOF gun nozzle shape on the deposition efficiency of alumina coating

摘要

High Velocity Oxy-Fuel (HVOF) method using propylene as a fuel gas was employed to spray alumina coating. To improve the deposition efficiency, three HVOF gun nozzles of varying geometry were designed and tested. Numerical simulation of the spraying process was applied to discuss the effect of nozzle geometry to understand the influence on the velocity, temperature and degree of melting of the sprayed particles. Deposition efficiency, coating structure and hardness were examined experimentally. Numerical simulation suggested that the modified Laval type nozzle with a larger inlet diameter and longer conical converging section between the inlet and throat was capable to promote melting of the alumina particles since not only the combustion flame temperature was maintained high but also the flame stream velocity was reduced at the region from inlet to throat as compared to the straight type nozzle available in the market. The modified nozzle could be expected to promote fusion of the sprayed particles, however, the fused alumina particles actually agglomerated and re-solidified into coarse particles on the inside surface of the nozzle. These coarse alumina particles sputtered from the nozzle exit during spraying process could no longer contribute to deposit the coating. Consequently, maximum deposition efficiency stopped below a level of 45%. Alumina coatings of excellent hardness of 920-1290HV range, with a relatively dense microstructure could be obtained in this UVOF method irrespective of the gun nozzle geometry.