ADHESION PROPERTIES OF THE PLASMA SPRAY COATINGS

Abstract

Plasma sprayed thermal barrier coatings (TBCs) are widely used in gas turbines to reduce thermal effect and increase turbine efficiency. Depending on the application of plasma spray coating, different characteristics are important but there are some characteristics that are the same for all plasma spray applications: thickness, porosity, microstructure, presence of unmelted particles, cracks and oxides, microhardness and bond strength. The poor bonding between splats and the imperfections in the form of pores or thermal cracks cause the thermo-mechanical property values of plasma spray coatings. Bond strengths of TBCs were tested with the standard test method ASTM C 633. Porosity and unmelted particles ratio generally decreases adhesion tensile test values since a pore of critical size can induce a macroscopic failure with tensile stress applied. It was observed that the spray parameters: plasma gas flow rates, plasma current, powder feed rate and spray distance are important effective parameters on the microstructure as well as bond strength of the coatings.