Microstructure, hardness, and surface cracks evaluation of HVOF-sprayed stellite-1 coating applied on steam turbine blade

Thermal spray is often applied on steam turbine blade leading edge to increase abrasion resistance. Stellite 1 is one of the commonly used material as it is known to wear protection against abrasion, oxidation, and corrosion at elevated temperature. This paper evaluated microstructure, hardness, and surface cracks of HVOF-sprayed Stellite-1 coating applied to the steam turbine blade. Optical microscopy reveals that all cracked and no-cracked coating have similarity in the microstructure. Typical 1.89% porosity was found on the specimen. The unbonded interface between the coating and substrate was also located to about 38% in length. All samples have 4.1% different in coating hardness with an average value of 718 HV. Coating thickness has a relation with the amount of porosity produced. Analysis of variance showed that both of the thickness and the hardness of coating are influencing the coating in making the penetrant indication. The microstructure showed a less dense coating with apparent porosity and unbonded interface when compared with other HVOF experiment. Blasting with excessive pressure and or the improper angle have made an alumina deposit at and below interface which may interfere with coating adhesion. Chromium Carbide and Silicon Oxide are formed near the porosity of coating.