Abstract
Thermally sprayed coatings are formed through the successive impact of molten droplets and/or semi-molten particles followed by flattening, rapid cooling, and solidification. Individual droplets flatten to form splats of several micrometers in thickness upon impact and result in the formation of a coating that has a lamellar structure with limited interface bonding. The inclusion of semi-molten particles in the coating modifies its microstructure. The bonding between particles dominates coating properties and performance. This review paper examines the bonding formation at the interface between thin lamellae in the coating. The effect of spray parameters on the bonding ratio is presented to reveal the main droplet parameters controlling bonding formation. It is shown that spray particle temperature dominates the bonding formation more than particle velocity. Significant increases in ceramic particle temperature are not possible due to the inherent characteristics of thermal spray processing; therefore, the bonding ratio is limited to a maximum of about 32%. On the other hand, it was found that through controlling the surface temperature of coating prior to molten droplet impact, the bonding at the lamellar interface can be significantly increased. Consequently, with the proper selection of deposition conditions and control of surface temperature, the bonding ratio of ceramic deposits can be altered from a maximum of 32% for a conventional deposit to the maximum of 100%. Such wide adjustability of lamellar bonding extends the applicability of plasma spray coatings to applications requiring different microstructures and properties. Moreover, this bonding control makes it possible to fabricate porous surfaces and structures through the deposit of surface-molten particles, to deposit high temperature abradable ceramic coatings, and to form super-hydrophobic surfaces. Furthermore, the ability to deposit coatings with complete interface bonding allows crystalline structure control of individual splats through epitaxial grain growth.
