Development of the Particle Interface Bonding in Thermal Spray Coatings for Expanding High Performance Applications

Thermally sprayed coatings are formed through the successive impact of molten droplets or/and semi-melted particles following by flattening, rapid cooling and solidification of melt. Individual droplets flatten to form splats of several micrometers in thickness on impact and result in the formation of the coating of a lamellar structure with a limited interface bonding. The inclusion of semi-melted particles in the coating modifies microstructure. The bonding between particles dominates coating properties and performances. In this review paper, the bonding formation at the interface between thin lamellae in the coating is examined. 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 than particle velocity. The limitation to increase significantly ceramic particle temperature inherent to thermal spray process leads to the observation of the maximum bonding ratio 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 the 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 the lamellar bonding makes plasma spray coatings be applicable to different applications requiring different microstructures and properties. Moreover, the bonding control makes it possible to fabricate porous deposits through surface-melted particles which can be applied to the fabrication of porous materials, deposition of high temperature abradable ceramic coating, and formation of super-hydrophobic surface. Furthermore, the complete interface bonding leads to the control of individual crystalline structure control through epitaxial grain growth.