PLASMA SYNTHESIS AND DEPOSITION OF NEW MATERIALS BY SUSPENSION PLASMA SPRAYING (SPS)
Suspension Plasma Spraying (SPS) is a new process based on a suspension of fine (<10 μm) or even ultra-fine (<100 nm) powders axially fed into the RF plasma through an atomization probe.
The suspension
is brought into the plasma discharge by an atomizing probe using a pressurized
gas to shear the suspension and thus atomize it into a stream of fine droplets.
The plasma discharge vaporizes the carrier substance and agglomerates the small
solid particles into partially or totally melted drops, which are accelerated
to hit the substrate and form thereon the coating or near net shape body. Alternatively,
the molten drops can be solidified in-flight and collected into a vessel to
produce a powder.
The atomization probe is water-cooled and adequately designed to resist the extreme temperature of the plasma. The atomization probe is like the standard injection probe of our torch except for the inside, which has an additional tube to allow the insertion of the atomization liquid and the atomization gas. The suspension liquid is exposed to a high-velocity gas stream as it leaves the nozzle.
Suspension Plasma Spray (SPS) processes can be used for synthesis of both ceramic and metallic materials (dense and spherical) starting from very fine powders. There are a number of potential benefits to using a plasma system fed with a material in suspension (liquid phase). For instance, the very fine particles can be protected against decomposition or evaporation in the plasma or against oxidation in the case of pure metals, owing to the presence of liquid film surrounding the particles. Also, the suspension can be arranged so that the chemical reaction between the liquid and particles occurs in the plasma, making it possible to synthesize advanced ceramics and composites.
For
example the Suspension Plasma Spray (SPS) process has been successfully applied
to synthesis of hydroxyapatite (HA, Ca10(PO4)6(OH)2),
a human-bone compatible biomedical material, and to deposition of oxide ceramics
(e.g. perovskites). One of the major advantages of suspension plasma spraying
over conventional techniques for powder synthesis is its simplicity. SPS is
basically a one-step process incorporating atomization, drying, melting and
solidification into the 10 milliseconds or so in-flight process under plasma
conditions.