Design and construction of a coating reactor for R.F. thermal plasma system

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2017
Yıldırım, Hatice Gözde
Thermal plasma is a common method of surface coatings applied in a number of applications; aerospace, automotive and medical. Among them, medical application is probably the most critical as the coating should satisfy a number of requirements. Although D.C. plasma is the most common method used in coatings in general, R.F. plasma has its advantages, namely an extremely clean deposition condition as no electrode is involved in the process. In this study, a coating reactor was designed and constructed which can be used with an R.F. plasma system. The system, as is, consists of an induction plasma torch with the required generator and control units, a nanopowder reactor, a powder collector and a scroll vacuum pump. The coating unit is designed such that it will replace the nanopowders reactor and will fit between the torch and the powder collector. The coater has a double wall cylindrical chamber of 460 mm in diameter and 860 mm in length. The chamber is water-cooled via water circulation between the double wall. It comprises a manipulator which allows the movement of substrate in three dimensions namely -200 < x < 200, -100 < y < 100 and -75 < z < 75 mm. The coating chamber as designed was tested with HA coating of Ti-6Al-4V. Coating was applied with 18-20 kW power loading. The particle feeding rate was in 1 to 18 g/min and the stand-off distance was varied between 80- 130 mm. Coatings obtained were compared with a conventional HA coated medical via implant in terms of both morphology and the phase make-up. It was found that HA coatings obtained with R.F. thermal plasma had Ca/P ratio of 1.80 or higher. This value is higher than Ca/P=1.67 which is the target value in plasma spraying. High value of Ca/P is a sign of thermal decomposition caused probably by a long residence time in the plasma. It is therefore concluded that power loading should be kept low so as to produce HA coating with an acceptable Ca/P ratio or alternatively the coating may be achieved by suspension plasma spraying.
Citation Formats
H. G. Yıldırım, “Design and construction of a coating reactor for R.F. thermal plasma system,” M.S. - Master of Science, Middle East Technical University, 2017.