Internal surface protection of gas turbine blade by Si‐aluminide coating

A laboratory-scale slurry silicoaluminizing process has been developed to apply coating into cooling passageways of a first-stage gas turbine blade. Analytical techniques for characterizing the coatings comprised optical microscopy, SEM, EDS, and XRD. The results of metallographic examinations demonstrated that acceptable coatings had been formed in almost all of the passageways. The amounts of Al and Si in the topcoat were found to be in the ranges of 24–26 wt% and 4.5–6 wt%, respectively. The heat treated coating consisted largely of β-NiAl phase as the coating matrix with uniform distribution of <2 µm-size precipitates. In addition, oxidation and hot corrosion performance of the coatings were investigated by cyclic exposure to 1100 °C and the furnace method using a mixture of sodium sulfate-40 wt% sodium vanadate-10 wt% sodium chloride at 780 °C, respectively. It was found that the presence of Si in the coating improves both oxidation and hot corrosion Type II properties.