Study of in-flight particle stream and particle behavior for understanding the instability phenomenon in plasma spraying process

For understanding the effect of powder feeding fluctuation on coating structure, in-flight particles were investigated by means of image analyzing. Particle behavior was estimated through numerical calculation on the base of force equilibrium. High definition videos were captured focusing on the in-flight particle stream and transparent carrier tube. Yttria-stabilized Zirconia (YSZ) coating was prepared with vacuum plasma spraying for clarifying the effect of powder feeding instability on coating structure. Optic micro-morphology of coating was obtained for porosity ratio estimation. An imaginary ring of bent tube was proposed for understanding particle behavior when unavoidable bends exist. Results indicate that periodic fluctuations exist in in-flight particle trajectory and in powder feeding. A transition region was found for particles with different diameters in regions between 0° and 180° within which particles with large sizes are difficult to transport. As a consequence, accumulation of powder was thus found both on upslope and downslope. The effective cross-section of the carrier tube for powder carrying was found reduced. According to the calculation, reducing equivalent diameter and increasing carrier gas flowrate have the same effect on powder carrying. A threshold was proposed to understand the slug behavior as: growing, maturity and breaking. Due to the break of slug, 'powder flooding' happens and may cause mutation in in-flight particle stream. Obvious increase was found in injected particle quality to induce the three-layer structure of coating. An empirical equation was proposed to describe slug behavior as a synthesis of growth rate and loss rate.