Doctor of Philosophy
Department of Materials Engineering
Nolan, David James, The influence of microstructure on the physico-chemical stability of WC-12% Co thermal spray coatings in galvanizing application, Doctor of Philosophy thesis, Department of Materials Engineering, University of Wollongong, 1997. http://ro.uow.edu.au/theses/1527
The primary objective of the present work was to investigate the influence of microstructure on the chemical and physical stability of WC-12%Co thermal spray coatings in galvanizing environments. A further aim was to study the effects of powder feed material characteristics on coating structural development.
Four different powder feed materials were chosen for deposition by a HVOF thermal spray process with the aim of depositing coating layers containing different cobalt-bearing phases, from metallic β-Co to cobalt-containing carbide phases. These various coatings were thoroughly characterized before being tested in various corrosive environments, designed to simulate galvanizing applications. Such environments included air at 300°C and 450°C, and contact with molten galvanizing alloy (Zn-0.2%A1) at 450°C. Techniques used in the characterization of as-sprayed and test materials included advanced metallographic preparation methods, optical and electron microscopy (SEM), x-ray diffraction and x-ray photoelectron spectroscopy.
The results show that resistance to corrosion by liquid galvanizing alloy is strongly influenced by the coating microstructure. Coatings containing β-Co matrix phase were much more susceptible to degradation than were coatings in which the cobalt was contained in complex carbide phases. Diffusion between the molten galvanizing alloy and these carbide phases still occurred, but at a much lower rate than for the β-Co containing structures. Reactions with the galvanizing alloy led to the development of β'-CoAl, preferentially, though other phases such as γ2-(Zn-6%Co) and a (Zn,Co)2xWxCy - type phase were possible.
Another important finding was the extensive oxidation of WC-Co composites at temperatures of less than 500°C in air (this was previously unreported and unexpected, and this oxidation of the coating surfaces led to improved performance in contact with molten galvanizing alloy. The formation of oxide phases, such as WO3, CoWO4 and ZnWO4 (or some combination of the two, such as (Zn,Co)WO4), appeared to insulate the coating from the corrosive galvanizing alloy.
In relation to structural development in the as-sprayed coatings, powder particle characteristics such as size distribution, density and phase content interactively influence the final properties of the coatings. For example, high levels of porosity and increased particle size result in increased porosity and reduced phase transformation in the coatings as a result of decreased particle heating efficiency. For the same reason, porosity and particle size were important in determining the extent of decarburization observed. Powders containing β-Co as the matrix phase (that is, Wl and W2) result in coatings with β-Co and Co6W6C as matrix phases. These are severely deformed (and β-Co may even be amorphous if quenched from the molten state) due to the impact forces and rapid cooling experienced during coating deposition. Powders containing Co3W3C and WC result in coatings with little change to phase content, other than those resulting from decarburization reactions (such as W2C and W). However, the Co3W3C phase is severely deformed during the coating process, such that it is not detectable by XRD in the as-sprayed condition.
Finally, implications of the results for commercial galvanizing applications are discussed and suggestions for further work are proposed.