TWI Industrial Member Report Summary 634/1998
F J Blunt
High velocity oxyfuel spraying (HVOF) is a well established process for the deposition of wear resistant coatings. With the HVOF process it is possible to produce coatings which are dense, adhere well to the substrate and have excellent cohesion. The amount of oxidation of the material which takes place during spraying is low, and the retention of particles such as carbides is excellent.
Tungsten carbide-cobalt is the material used for the majority of wear resistant coatings. However, this material is relatively costly, and there is increased interest in alternative hardfacing materials. One material, NiCr-Cr 3C 2, has received considerable attention because it can outperform tungsten carbide-cobalt at high temperatures, and is free from cobalt. It is commonly used for such applications as exhaust flaps on turbine engines and has shown promise in circulating fluidised bed combustor boilers in power generation.
Previous work at TWI had indicated that in the case of tungsten carbide-cobalt, there was a relationship between the power input to the HVOF gun, and the retention of hard particles of tungsten carbide in the sprayed coating. Several types of NiCr-Cr 3C 2 powder are available, and it was appreciated that the powder morphology and type could have a profound influence on the spraying behaviour. This work therefore consisted of a screening programme to assess the generic quality of the coatings produced from a range of powders, followed by a systematic investigation of the effects of stoichiometry and power input to the spray gun on the coating efficiency, morphology and hardness. Hydrogen fuel was chosen, since it was already known that this fuel gave coatings with the best adhesion to the substrate.
- To determine the generic characteristics of NiCr-Cr 3C 2 coatings produced from a range of different commercial powders.
- To optimise the spraying parameters for the most promising powder, using hydrogen fuel.