The suspension plasma spray (SPS) technique has been used to obtain dense Y 2 O 3 coatings and to overcome the drawbacks of the conventional air plasma spray (APS). SPS uses suspensions containing micrometer or sub-micrometer sized powders dispersed in liquid media. In this study, microstructure developments and mechanical properties have been investigated as functions of particle size of source material and plasma processing parameters such as plasma power and stand-off distance. The microstructure of the coating was found to be highly related to the particle size and the plasma processing parameters, and it was directly reflected in the hardness and the adhesion strength. When fine powder (BET 16.4 m 2 /g) was used as a raw material in the suspension, there was, with increasing stand-off distance, a change from a dense structure with a slightly bumpy surface to a porous structure with a cauliflower-like surface. On the other hand, when a coarse powder (BET 2.8 m 2 /g) was used, the coating density was lower, with microscopic splats on the surface. Using fine Y 2 O 3 powders, the coating layer with an optimum short stand-off distance showed a high hardness of approximately 90% of that of sintered Y 2 O 3 and an adhesion strength several times higher than that of the coating by conventional APS.

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The effects of water addition on Y 2 O 3 coatings or thick films prepared by plasma suspension spray (SPS) have been investigated. Water addition in suspension media was found to be effective to control the color of a Y 2 O 3 coating prepared by SPS. The color changed with water addition at the shortest stand-off distance of 50 mm even if samples had the same crystalline phase. Change was not correlated with fragmentation behavior of liquid suspension inside the plasma jet. Water content over 50 vol% was found to produce unmelted particles, indicating that water suppressed heat transfer to the particles. However, plasma jet temperature was not affected. Instead, the coating fabricated with water addition has higher oxygen and lower carbon content compared to these characteristics of the coating without water addition. This was attributed to the retarded complete evaporation of liquid media from the suspension droplet, resulting in inhibition of excessive heating and evaporation of the molten Y 2 O 3 droplet. In this regard, crystalline phase development with respect to stand-off distance and water addition was discussed.

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