Development of Plasma Parameters for the Manufacture of MCrAlY Bond Coats by Low-Pressure Plasma Spraying Using a Cascaded Torch

Abstract

MCrAlY bond coats (M-Ni, Co) for thermal barrier coating systems are often manufactured by low-pressure plasma spraying (LPPS) to achieve dense coatings with low oxygen uptake at high deposition efficiencies. Herein, the novel SinplexPro 03C plasma spray torch (Oerlikon Metco) is characterized regarding this application, and appropriate process parameters are developed. The mass-specific plasma enthalpy and the hydrogen–argon plasma gas ratio prove to be substantial factors. The best deposition efficiency and lowest porosity are achieved at a mass-specific plasma enthalpy between 17 and 19 MJ kg−1. The oxygen content increases degressively with the enthalpy. Already at small hydrogen fractions in the plasma gas, a fast route is established for the recombination of argon ions. Consequently, the thermal treatment of the feedstock is improved. Moreover, the porosity decreases at increasing hydrogen content of the plasma gas, as the gun is operated in constant current mode and thus the input power increases. However, the deposition efficiency decreases slightly. On the other hand, the oxygen content decreases due to the reducing effect of hydrogen. The deposition efficiency, porosity, and oxygen content are also reasonable if no hydrogen is admixed at all, as the fluctuations of the plasma are reduced.