A Parametric Suspension Plasma SprayingKent Van Every - Purdue Graduate Student
In this study we are looking coatings used as a thermal barrier most specifically a 7% Yttrium Stabilized Zirconia applied by plasma spray.
Typically the coating is injected into the plasma application system as a dry powder with particle size around 10 micro-meters. Size of the feed powder effects the coating in that smaller powder favors smaller lamellae (lamellae are the "plate-like splats" that form as the powder hits the substrate at high velocity), and also smaller grains. Both of these conditions are desirable for more photon scattering and better insulating. Going smaller than 10 micro-meters is undesirable however from the standpoint of getting the equipment to function in applying the coating. This study looks at attempts to get to smaller [nano] particle sizes by suspending the powder in a high pressure carrier for injection into the plasma. The project has three main components. Production of a nanoparticle suspension, deposition of the suspended coating, and characterization of the coating produced.
Ethanol was chosen as the carrier for this investigation. Coating powder of a nominal 30 nano meter size as procured but is was found to be highly agglomerated (as received) so was ball milled to get most of it to below 100 nano-meter sized agglomerates. It was also filtered to 230 nano-meters before suspending in the carrier and introduction into the plasma spray gun at Ames laboratories.
Several variables were adjusted to get to a coating that is satisfactory. These include standoff distance from the plasma to the substrate, power of the plasma generator, percent of powder in the suspension, and injection velocity of the suspension into the plasma.
Standoff was found to be optimal at 50 mm. Greater values caused more spherical deposition (non-lamellar), while lesser values trapped more unmelted powder in the coating. Power was optimal at 38 Kv with less power also caausing an increase of unmelted powder.
Powder percent in suspension was found best at about 5%. Lower lead to smaller lamellae put very long deposition times.
Injection pressure was best at 40 solution liters per minute. Variation of this were measured in suspension velocities where 20 m/sec resulted in the best coating. Low velocities did not center the suspension well in the plasma plume and high velocities caused it to overshoot the plume. 15 m/sec did allow deposition but the particles were more spherical (less lamellae like) than when 20 m/sec was obtained.
The coating obtained from Solution Plasma Spray (SPS) does obtain the much smaller (by two orders of magnitude) favorable lamellae and grain size. It also contains about 35% porosity (mostly open) as opposed to conventional plasma spray with has only 9% porosity (mostly closed). The most striking feature of the SPS coating is the formation of a columnar orientation to the lamellae microstructure. It is thought this might be the result of the nanosized particles being light enough to be swept parallel to the deposited surface (with the plasma flow) after some deposition has occurred. Microstructure evidence suggests that these sideways travelling particles built up on the sides of earlier deposited particles and other surface imperfections causing the columnar appearance.
In response to a question from the floor, it is not known what advantages or disadvantages may result from the more porous nature of SPS in actual application as a thermal barrier coating.