Stresses developed within a thermal barrier coating (TBC) due to the mismatch in thermal expansion of different coating components can cause coating failure. Nanostructured materials have an increased volume fraction of grain boundaries and this microstructural attribute may allow coatings to relieve the strain in the coating structure thereby improving the effectiveness and the lifetime of the TBC. Multi – layered TBCs were prepared using two techniques; atmospheric pressure plasma spray (APS) using a commercial system, and reduced pressure plasma spray using the Triple Torch Plasma Reactor (TTPR). The coatings were deposited on mullite and on NiCrAlY-coated steel substrates, and consisted of an inter – layer of nano-phase partially stabilized zirconia (n – PSZ) and a layer of conventional partially yttria stabilized zirconia coating (c – YSZ) as the top thermal barrier coat. The coatings were heat treated at various temperatures and the microstructural changes analyzed using scanning electron microscopy (SEM) images. Mechanical properties of the coating were studied using four point bend testing to better understand the effect of the n-PSZ inter-layer on the strain relief mechanisms that may be operative within the TBC.

The significance of metal atomization in the wire arc spray process is directly related to the final coating quality produced. Since the early observations of the melting behavior of the wire tips by Steffens, relatively little has been done to further the understanding of the mechanisms involved. The primary atomization of the molten wire tips show existence of sheets and extrusions on both electrodes which are strongly dependent on the system operating parameters. High-speed imaging has been used in this study, for classification of sheet, membrane and extrusion formations as a function of atomizing gas pressure, voltage and current settings. The breakup of metal structures formed on the electrodes is further classified in a manner consistent with established classifications for the break-up of other liquids, e.g. water or fuel. Quantitative descriptions of metal sheet lengths and breakup times are presented. The improved understanding of the metal breakup mechanisms in the wire arc spray process may provide a basis for (a) modification of existing computational codes for prediction of particle sizes and trajectories, and (b) for modifications to torch designs for providing more uniform particle fluxes.

Wire arc spray is a process for producing coatings and forming structures through the deposition of metal droplets. Wire arc spray has the advantage of low material coast and low power requirements when compared to other thermal spray technologies. This article elaborates on the assumptions made for using single consumable wire geometry, discusses experiments performed to test these assumptions, presents droplet generation results, and addresses required future work. Experiments revealed a critical relationship between wire polarity, wire position, and droplet beam dispersion. The article identifies a critical relationship between wire polarity, wire position, and spray pattern divergence in a single wire arc spray device.