Measuring Substrate Temperature Variation During Application of Plasma-Sprayed Zirconia Coatings

Substrate temperature variation was measured during plasma spraying of ZrO₂ 7% Y₂O₃ powder using fast-response thermocouples embedded in the stainless steel surface. Coatings were deposited with both stationary and moving torches. The substrate was either kept at room temperature at the start of coating deposition or pre-heated to 270-300 °C. Peak temperature during spraying reached 450 °C for a surface initially at room temperature, and 680 °C for a surface preheated to 300 °C before coating deposition. Preheating the substrate reduced coating porosity by approximately 40%. The porosity at the center of the deposit was significantly lower than that at its periphery since particle temperature and velocity were lower at the edges of the plasma plume than along its axis. When a coating was applied with a moving torch the substrate temperature did not increase above 450 °C, at which temperature heat losses to the ambient equalled the heat supplied by the plasma plume and particles. Coating porosity decreased with distance from the substrate. As sequential layers of coating are applied surface temperature increases and roughness decreases. Both of these factors suppress break-up of particles landing on the substrate and thereby reduce coating porosity.