Influence of hydrogen on the microstructure of plasma-sprayed yttria-stabilized zirconia coatings

The influence of secondary hydrogen and current on the deposition efficiency (DE) and microstructure of yttria-stabilized zirconia (YSZ) coatings was evaluated. To better understand the influence of the spray process on coating consistency, a YSZ powder, -125 +44 mu m, was sprayed with nitrogen/hydrogen parameters and a 9 MB plasma gun from Sulzer Metco. DE and coating porosity, which were produced using two different spray gun conditions yielding the same input power, were compared. Amperage was allowed to vary between 500 and 560 A, and hydrogen was adjusted to maintain constant power, while nitrogen flow was kept at a fixed level. Several power conditions, ranging from 32 to 39 kW, were tested. Different injection geometries (i.e., radial with and without a backward component) were also compared. The latter was found to produce higher in-flight temperatures due to a longer residence time of the powder particles in the hotter portion of the plasma. Porosity was based on cross-sectional micrographs. In-flight particle temperature and velocity measurements were also carried out with a special sensor for each condition. Test results showed that DE and coating density could vary significantly when a different hydrogen flow rate was used to maintain constant input power. On the other hand, DE was found to correlate very well with the temperature of the in-flight particles. Therefore, to obtain more consistent and reproducible DE and microstructures, it is preferable to maintain the in-flight particle temperature around a constant value instead of keeping a constant input power by adjusting the secondary hydrogen flow rate.