Latest Developments in Suspension and Liquid Precursor Thermal Spraying

The interest to manufacture onto large surfaces thick (i.e., 10 to 20 µm, average thickness) finely structured or nano-structured layers is increasingly growing since about 10 years. This explains the interest for suspension thermal spraying (STS) and solution precursor thermal spraying (SPTS), both allowing manufacturing finely structured layers of thicknesses varying between a few micrometers up to a few hundred of micrometers.
STS aims at processing a suspension of sub-micrometric-sized or even nano-metric-sized solid particles dispersed in a solvent. The liquid solvent permits to inject particles in the thermal flow (i.e., due to their size, a carrier gas cannot play this role).
SPTS aims at processing a solution of precursors under the same conditions. Upon evaporation of the liquid, the precursor concentration increases until precipitation, pyrolysis and melting of small droplets. Compared to conventional thermal spray routes, STS and SPTS are by far more complex because fragmentation and vaporization of the liquid control the coating build-up mechanisms. Numerous studies are still necessary to reach a better understanding of involved phenomena and to further develop the technology, among which injection systems, suspension and solution optimizations, spray kinematics, etc. This review presents some recent developments in this field and develops what is our present knowledge in this field together with the available tools implemented to characterize the plasma – liquid interaction and the coating formation.