Suspension Plasma Spraying of Alumina Coatings: Process and Coating Structure

Summary form only given. Due to the large volume fraction of the internal interfaces, coatings structured at the nanoscale should exhibit better properties than conventional coatings structured at the microscale. However, when processing by thermal plasmas such feedstock, several questions arise: (i) how feeding the plasma jet with nanosized powders? (ii) how keeping their nanostructured structures when melting them? (iii) how controlling the growth of coating's grain? Suspension plasma spraying (SPS) appears as a technology permitting to circumvent those difficulties. It consists in mechanically injecting within the plasma flow a liquid suspension of particles of average diameter varying between 0.1 and 1 mum through an injector of diameter in the order of one hundred micrometers. Upon penetration within the DC plasma jet, two phenomena occur sequentially: droplet fragmentation and then evaporation (lasting, in average, two orders of magnitude longer than fragmentation). Particles are then processed by the plasma flow prior their impact, spreading and solidification upon the surface to be covered. Compared to plasma spraying of micron-sized particles, SPS exhibit several major differences: (i) a more pronounced sensitivity to arc root fluctuations requiring to adapt operating parameters in order to operate the spray gun in its take over mode unless to have inhomogeneous process of the suspension by the plasma: (ii) a shorter spray distance since small particles decelerate faster than bigger ones; (iii) a higher thermal flux transmitted from the plasma flow to the substrate, between 5 to 10 times higher than the heat flux transmitted in conventional plasma spraying. In consequence, coatings manufactured by SPS differ from conventional coatings by: (i) a more pronounced sensitivity to the particle size distribution: narrow-sized ones are absolutely required to produce dense layers; (ii) lamella characteristic dimensions smaller than the ones encountered in conventional plasma spraying (with lower flattening ratio due to lower particle Reynolds numbers upon impact) with almost no peripheral splashing (due to lower particle adjusted Sommerfeld numbers upon impact); (iii) lower residual stress levels within lamellae which limit the development of intralamellar cracks; (iv) a denser structure with almost no pore connectivity. This paper aims at presenting the SPS of alumina coatings in terms of: (i) suspension characteristics on the coating architecture; (ii) process optimization, emphasizing the predominant role of the arc root fluctuations on the coating architecture; (iii) resulting coating architectures, in particular in terms of their pore network architecture.