Suspension DC plasma spraying of thick finely-structured ceramic coatings: Process manufacturing mechanisms

Due to the large volume fraction of the internal interfaces and reduced size of stacking defects, thick (from 20 to 100 µm) nano- or sub-micron structured coatings exhibit better properties than conventional micron structured ones (e.g. higher coefficients of thermal expansion, lower thermal diffusivity, higher hardness and toughness, better wear resistance, among other coating characteristics and functional properties). They could hence offer pertinent solutions to numerous emerging applications, in particular for energy production, energy saving, diffusion and environmental barriers, etc. sion plasma spraying (SPS) permits to manufacture such finely-structured layers and consists in mechanically injecting within the plasma flow a liquid suspension of sub-micrometric-sized or nano-sized particles through an injector of diameter of the order of one hundred micrometers. Upon penetration within the DC plasma jet, two phenomena occur sequentially: droplet fragmentation and then solvent evaporation. Particles are then processed by the plasma flow (heat and momentum transfers) prior to their impact, flattening and solidification upon the surface to be covered. ed to plasma spraying of micrometer-sized particles (APS), SPS exhibit several major differences : i) a more pronounced sensitivity to electric are root fluctuation requiring to operate the spray gun in a relatively stable mode (take over) unless to process inhomogeneously the suspension which would results in heterogeneous coating structure; ii) a shorter spray distance (since small particles decelerate faster than bigger ones) leading to higher thermal flux transmitted from the plasma flow to the substrate (5 to 10 times higher than conventional plasma spraying); iii) an emphasized thermophoresis effect; iv) a typical cohesive structure made of the stacking of granular and flattened particles with low density of stacking defects. aper aims at presenting recent developments carried-out on this process in terms of process optimization and coating manufacturing mechanisms.