Columnar-structured thermal barrier coatings deposited via the water-based suspension plasma spray process

Suspension plasma spray (SPS) has been developed to be a rapid, facile and cost-effective process to deposit columnar-structured thermal barrier coatings (TBCs). Different than the most commonly used ethanol-based suspensions, water-based suspensions have not been used in the SPS process to deposit columnar-structured TBCs due to their high surface tension, although they are much cheaper and safer. In this work, a new water-based SPS process was prepared by adding surfactant to lower the surface tension. The optimum content of dispersant and surfactant added into the suspension was determined via the measurements of viscosity, particle size, surface tension, contact angles, and atomized droplet size. Coatings deposited using suspension with and without surfactant showed typical columnar-structured microstructures and vertically-cracked microstructures, respectively. The coatings deposited using suspension with surfactant also showed an evolution from columnar-structured microstructures to mixed microstructures of columns and cracks, and to homogeneous microstructures with the increase of standoff distance. The formation of different coating microstructures were correlated to the size of droplets after aerodynamic breakup and the Stokes number of in-flight particles. The new water-based suspension together with the water-based SPS process show high potential to be a cheap and effective alternative to the ethanol-based SPS process.

Corrosion characteristics of plasma spray, arc spray, high velocity oxygen fuel, and diamond jet coated 30MnB5 boron alloyed steel in 3.5 wt.% NaCl solution

30MnB5 boron alloyed steel surface is coated using different coating techniques, namely 60(Ni-15Cr-4.4Si-3.5Fe-3.2B 0.7C)-40(WC 12Co) metallic powder plasma spray, Fe-28Cr-5C-1Mn alloy wire arc spray, WC-10Co-4Cr (thick) powder high velocity oxy-fuel (HVOF), and WC-10Co-4Cr (fine) diamond jet HVOF. The microstructure of the crude steel sample consists of ferrite and pearlite matrices and iron carbide structures. The intermediate binders are well bonded to the substrate for all coated surfaces. The arc spray coated surface shows the formation of lamellae. The cross-section of HVOF and diamond jet HVOF coated surfaces indicates the formation of WC, W2C Cr, and W parent matrix carbide structures. The corrosion characteristic of the coated steel has been investigated in 3.5 wt.% NaCl solution using electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDAX) techniques. The results reveal that the steel corroded in the medium despite the coatings. However, the extent of corrosion varies. HVOF coated sample demonstrated the highest corrosion resistance while arc spray coated sample exhibited the least. EDAX mapping reveals that the elements in the coatings corroded in the order of their standard electrode potential (SEP). Higher corrosion resistance of HVOF coated sample is linked to the low SEP of tungsten.