Understanding suspension D.C. plasma spraying of nanostructured coatings

Summary form only given. Injecting nanosized particles in a D.C. plasma jet would require, to give them a high enough, momentum, a huge carrier gas flowrate, which would drastically perturb the plasma jet. To overcome this problem a suspension of these particles is mechanically injected with in the plasma jet. The high momentum of the latter tears the liquid jet in tiny droplets (1-3 /spl mu/m) which evaporate in a few microseconds. This scenario has been checked by emission spectroscopy of the plasma jet and simple calculations. The agglomerates of nanoparticles contained in the evaporated droplets are then melted and accelerated before flattening onto the substrate. They result in splats which diameter is between 100 and 2000 nm and which thickness varies between 60 and 300 nm. The way particles flatten and splats form is tested by collecting the latters at different distances downstream of the nozzle exit. The calculations of momentum and heat transfers to agglomerates confirm the experimental results related to the sand-off distance and the required plasma jet enthalpy and velocity. The process has been tested on stabilized zirconia and perovskite coatings sprayed on smooth substrates (Ra <0.05 /spl mu/m) preheated at different temperatures. Coatings can be very dense with less than 1% porosity or highly porous (>20%). To achieve LaMnO3 coatings which are not partially decomposed the particles have been doped with MnO2. With deposition rates of about 15 /spl mu/m/h.m2 and very intricated interface between zirconia and perovskite, the process seems very promising for SOFC's manufacturing.