Thermal spraying of metals or cermets is widely used for different purposes such as protection from corrosion or from wear phenomena. One of the main application methods is HVOF (high velocity oxy fuel). In this process, the temperature is lower as compared to other thermal spraying processes because of the supersonic velocity of the particles. The thermal energy combined with the kinetic energy of the powder particles enables the formation of a dense thermal spray coating on the surface of the substrate. The higher density of the HVOF deposited coating compared to conventional coatings as well as the lower thermal impact on the particles are two major advantages of the process which are additionally accompanied by lower oxidation phenomena (as compared to e.g. wire spraying).
Pure nickel coatings were deposited on steel substrates in order to determine the influence of different thermal spraying parameters. The variation in fuel gas is well known to influence the appearance of the coating. This could also be determined in the course of this study for the gases ethane, propane and natural gas. In further trials, also the shroud gas was varied in addition to the fuel gas, changing it from the usually used gas (compressed air) to pure nitrogen while using the standard HVOF equipment (DJ 2700 Hybrid). It could be determined for all fuel gases used that this specific change had a strong influence on the density of the coating as well as on the oxide content within the coating.
These different coatings were additionally evaluated in terms of their corrosion behavior. The performance of the nickel coating was strongly depending on the microstructure and therefore on the spraying parameters of the HVOF process. The spraying parameters to obtain the coating microstructure for optimum corrosion performance by using the existing equipment could be evaluated.