Abstract
This work reports research concerning the production of powder, suitable for reactive HVOF spraying, produced by mechanically alloying Ni(Cr), Ti and C elemental powder constituents. Powder mixing was achieved using a high-energy Uni-Ball II Mill and optimisation of the milling parameters are reported. The composition of the powder, at 50wt.%NiCr-40wt.%Ti-10wt.%C, was such that the application of heat has the potential to cause a SHS (Self propagating High Temperature Synthesis) reaction to take place. The utilisation of SHS reactions to produce TiC particles within metallic matrices is well known in bulk systems. However, this work describes carrying out this reaction in individual powder particles on exposure to the high temperature within the HVOF gun. The powder having undergone the SHS reaction during the spray process was deposited onto mild steel substrates to form a dense, coherent coating. The coatings thus formed were shown to contain nanoscale TiC in a Ni(Cr) matrix, indicating a SHS reaction had taken place. This TiC is much finer than that produced in conventional SHS reactions, which is typically ~5ìm. The percentage of TiC formed, and retained in the coating, was lower than expected from the constituent proportions and explanations for this observation are proposed. The microstructure of the coating is described and compared with a Ni(Cr)-TiC cermet coating sprayed using conventional SHS powder generated from reacted compacts which were crushed, sieved and classified to give sprayable feedstock powder.
