Microstructure and mechanical properties of plasma spraying coatings from YSZ feedstocks comprising nano-and submicron-sized particles

Abstract

Atmospheric plasma spraying (APS) is an attractive technique to obtain nanostructured coatings due to its versatility, simplicity and relatively low cost. However, nanoparticles cannot be fed into the plasma using conventional feeding systems, due to their low mass and poor flowability, and must be adequately reconstituted into sprayable micrometric agglomerates.

In this work, nanostructured and submicron/nanostructured powders of yttria-stabilised zirconia (YSZ) were deposited using APS, with a view to obtaining high-performance thermal barrier coatings (TBC). All powders were reconstituted by spray-drying from different solid loading suspensions, followed by a thermal treatment of the spray-dried granules to reduce agglomerate porosity and enhance powder sinterability. The reconstituted granules were characterised by XRD, SEM, pore sizing and flowability evaluation.

The reconstituted feedstocks were successfully deposited onto metallic substrates by APS. A metallic bond coat was sprayed between the substrate and the ceramic layer. The coating microstructure, characterised by SEM, was formed by partially melted zones, which retained the initial powder microstructure, embedded in a fully melted matrix which acts as a binder. It was shown that feedstock characteristics which in turn are very dependent of starting suspension characteristics, in particular agglomerate density and primary particle size, impact on coating microstructure (porosity and amount of partially-melted areas). For this reason mechanical properties of coatings are also strongly affected by feeding powder characteristics.