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.