"Formation of High Quality Ceramic Coating by Gas Tunnel Type Plasma Spraying"

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In this study, a gas tunnel type plasma spraying, which is a new type of thermal spraying, has been applied to the spraying of ceramics which have high melting point in order to obtain high quality coatings, and the characteristics of this new spraying method was clarified experimentally. The results are as follows :(1) The region of proper spraying conditions (P : input to plasma and L : spraying distance) where a good quality coating can be obtained was investigated by using alumina powder. In this case the region was rather larger than that of the case of conventional type of plasma spraying. As well as alumina coating, in zirconia coating and titania coating, the porosity is low and those are high density and very hard.(2) The relation between deposit characteristics of sprayed powder and formed coating quality was investigated. It showed that the good quality coating consisted of damaged deposit particles. And the critical spraying distance on the proper spraying conditions agreed with the specific spraying distance which was obtained from the characteristics of the deposit weight.(3) In a short distance spraying, a high hardness ceramic coating was formed : in the case of alumina coating, the Vickers hardness was Hv= 1500 at P= 30kW, L=30mm. These are high quality coatings of low porosity and high density, whose hardness was 20%-30% higher than that of the conventional type plasma spraying. The conventional one had many large pores and a few cracks.(4) According to the structural analysis of these high hardness ceramic coatings, in the case of alumina coating, the cell size was very fine in that high hardness layer, where alpha-alumina was rich. In the case of zirconia coating, the crystallization grew after spraying.