Relationships between arc plasma jet properties and plasma/liquid interaction mechanisms for the deposition of nanostructured ceramic coatings

Ceramic nanostructured coatings with intermediate thicknesses between 10 and 100 µm give improved thermal and mechanical properties when used as thermal barrier coatings or wear-resistant coatings. Such coatings comply with the technical requirements of aeronautical and automotive applications. This implies the development of deposition processes with high throughput and deposition rates to promote the formation of nanostructured coatings. The use of a liquid phase as a carrier medium for nanoparticles or solution precursors is of major interest for injection within a thermal plasma jet. The as-sprayed materials can form ceramic nanostructured coatings provided the liquid injection, encompassing the physicochemical properties of the liquid and its injection method, can withstand the plasma properties. The repeatability of the interaction phenomena between the liquid phase and the arc jet has a key role in the efficiency of deposition, and some research efforts are devoted to stabilizing the arc while the liquid jet is continuously injected within the plasma. Alternatively a pulsed arc plasma jet can be generated and is associated with a time-phased injection of droplets. This paper presents the different issues related to the arc plasma properties produced by direct plasma torches, including arc instabilities and their influence on plasma/liquid interaction mechanisms leading to the formation of nanomaterials. There is a focus on pulsed plasma spraying associated with synchronized injection of microsized droplets by means of an inkjet printing method.