Deposition of Zirconium Carbide onto a substrate
Van der Walt, B.
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he incident at Japan's Fukushima Daiichi nuclear power plant was aggravated by the lack of resistance of the zirconium alloy to steam oxidation. Zirconium alloys have been known to oxidise and produce hydrogen in a steam environment which can explode as in the case of Fukushima. To avoid oxidation at high temperatures by steam, the use of a corrosion resistant coating has been suggested. ZrC is a coating material under consideration because it has a high melting point, good mechanical strength, good thermal conductivity, and good corrosion resistance. ZrC also has properties beneficial to the nuclear industry such as a low neutron absorption cross-section and fission product resistance. In order to deposit ZrC onto the Zr fuel rods coating techniques such as chemical vapor deposition, pulsed laser deposition, magnetron sputtering and plasma spraying have been identified as candidate techniques. The selected method should produce a dense uniform coating with excellent adhesion, be reproducible, and of reasonable cost. Therefore the plasma spraying method was chosen as it was thought to be the most flexible and easily adaptable for the purpose of depositing ZrC onto Zr-alloy fuel rods. Plasma spraying coatings are dependent on many parameters that can influence the coating integrity. The particle temperature and velocity were identified as two crucial parameters, therefore experimental parameters were chosen to optimise these properties. The parameters are plasma power input (11.0, 16.5, and 22.0 kW), spray distance (60, 80 and 100 mm) and particle injection velocity (10, 15 and 20 m/s). Additionally the spraying atmosphere was also considered because of the risk of oxygen contamination during atmospheric spraying. This research was done using a button type dc non transfer arc plasma torch to deposit ZrC coatings onto stainless 304 substrates. The feedstock powder was analysed to determine its size and composition to help determine the spray parameters. A simulation using the Jets&Poudres program was used to verify that the plasma sprayed ZrC particles temperature and velocity were high enough to deposit. A SEM was used to investigate the surface and cross-sections of the coatings, while EDS and XRD was used to confirm their composition. It was found that an increase in plasma power produces better coatings, while the ideal spray distance was 100 mm and injection velocity was 15 m/s. XRD confirmed that all atmospheric plasma sprayed coatings had oxygen contamination while the inert spray runs where oxide free. In contrast the atmospheric coating deposited and formed coatings while the inert samples had many un-melted particles with an inconsistent structure. All coatings displayed weak adhesion and some form of porosity in the coating structure.
- Engineering