Five types of tungsten carbide based powders with different chemical compositions (WC-12Co,
WC-17Co, WC-10Ni, WC-10Co-4Cr and WC- 20Cr-7Ni) were deposited onto ST37 mild steel
substrate using high velocity oxy fuel (HVOF) spray technique. The feedstock powders and sprayed
coatings were studied by using X-ray diffraction (XRD), and differential thermal analyzing (DTA).
The results were shown during HVOF thermal spraying, WC-M powders become partially melted
before being sprayed on the surface of the substrate with supersonic speed. In these types of
coatings, the crystallographic structures are normally non equilibrium, because the cooling rates of
the deposited splats are very high due to the cold substrate acting as a thermal sink. These partially
melted powders are then rapidly solidified to an amorphous phase. XRD analysis showed that the
amorphous phase was existed in all of the as sprayed coatings. The amorphous phase in WC-12Co,
WC-17Co and WC-10Ni coatings was transformed to crystalline phases by heat treatment at high
temperature. Heat treatment of these coatings at high temperature also resulted in partially
dissolution of WC particles and formation of new crystalline phases. In cobalt base coatings, the
new phases were eta carbide phases like Co6W6C and Co3W3C but in WC-10Ni coating a NiW
intermetallic phase was formed. Heat treatment of WC-10Co-4Cr and WC-20Cr-7Ni coatings did
not change the amorphous phases in these coatings. Differential thermal analysis of cobalt
containing coatings revealed an exothermic reaction at approximately 880°C. This exothermic
reaction may be related to the transformation of the amorphous phase to eta phases. On the contrary,
DTA analysis of feedstock powders of these coatings showed an endothermic reaction at
approximately 1000°C. DTA analyses of nickel containing cermets also showed similar results.
Differential thermal analysis of chromium containing cermets did not show any noticeable
exothermic or endothermic reactions.
High-temperature phase in zirconia film fabricated by aerosol gas deposition and its change upon subsequent heat treatment
