APPLICATION OF REACTION SYNTHESIS PRINCIPLES TO THERMAL SPRAY COATINGS

Reaction synthesis principles have been extended to plasma spraying to obtain coatings consisting of mixed oxide phases and iron aluminides. Elemental powders of iron and aluminium were fed through a d.c. plasm a torch to deposit intermetallic coatings on carbon steel substrate. C arbon steel substrates were also pre-heated with a plasma flame to cre ate an iron oxide surface on the substrate such that an exothermic the rmite reaction takes place when molten splats of aluminium impinge the pre-heated substrate at sub- or supersonic velocities. A thermite rea ction between iron oxide and aluminium allowed the formation of alumin a, FeAl2O4, iron, and iron aluminide phases. The presence of FeAl2O4 a nd Al2O3 increased the surface hardnesses of the coating, and the hard nesses of the coatings are significantly higher than the hardnesses of steel substrate, and aluminium particles. X-ray analysis of the coati ngs, microstructural observations, and microhardness measurements sugg est that plasma spraying conditions can be tailored to obtain coatings with high hardness values with in situ synthesized reinforcements (sp inel and alumina) or iron aluminide phases. Aluminium-rich phases were observed in the as-deposited coatings when a mixture of aluminium and iron or aluminium and nickel were fed through the plasma gun in ratio s equivalent to Fe3Al, FeAl, Ni3Al, and NiAl. In some cases, annealing allowed the formation of iron-rich or nickel-rich aluminide phases. H igh solidification rates of molten splats allowed very limited diffusi onal reactions between the splats of aluminium and iron, or aluminium and nickel because the available diffusional time for exothermic inter facial reactions is limited to a fraction of a second at best. Oxidati on of part of the aluminium led to the formation of alumina in the as- deposited coatings, and therefore, a vacuum plasma spraying technique is desirable to obtain intermetallic phases. The results suggest that reactive spraying will allow deposition of coatings by utilizing the h eats of reaction between the constituents, and reactive spraying will broaden the engineering applications of reaction synthesis techniques.