Experimental Testing of a Curvilinear Gas Shroud Nozzle for Improved Plasma Spraying

The performance of a new gas shroud nozzle attachment for plasma spraying was tested experimentally using particle diagnostics, flow visualization, and coating characterization techniques. A nozzle attachment with curvilinear inner walls was tested and compared with a commercially available conical nozzle. Particle temperatures were measured with a high-speed ratio pyrometer and particle velocities were measured with an intensified camera and a two-laser illumination system. Flow visualization was performed by seeding the surrounding air with smoke. Particle temperatures at the spraying distance were 300 K higher with the curvilinear insert. The plasma jet was narrower but the particle velocity distribution at the spraying distance was unchanged. Higher temperatures and improved particle melting with the curvilinear insert resulted in a reduction in coating porosity (from 7.0 to 7.2 to 4.5–5.1%) and an increase in coating adhesion strength (from 27.2 to 42 MPa). Shrouding as injected through a circular slot around the nozzle exit was also seen to provide better protection than gas injected with the standard sixteen-port configuration.