This study investigates relationships between in-flight particle characteristics and the microstructural properties of air plasma sprayed YSZ. Particle velocity and temperature were measured at different stand-off distances and contour plots were created. Coating porosity and hardness were measured and plotted over the same area. It is shown that in-flight particle characteristics are strongly affected by particle size distribution. Smaller particles in trajectories above the torch central axis were found to be 10 m/sec slower on average than larger particles in the center and lower section of the plume. The section of the plume with the highest temperatures and velocities is about 2 mm below the central axis and is shown to generate dense, hard layers with less than 7% porosity and hardness values on the order of 700 HV. Reynolds numbers determined from in-flight particle data also provide an indication of microstructural properties; where Reynolds numbers exceed 1000, coating porosity is less than 5%.