Deposition of silicon enriched nickel aluminide coatings on internally cooled airfoils

Aluminum diffusion coatings enriched with silicon were prepared by chemical vapor deposition on various nickel-based materials such as almost pure nickel and Rene 80. Aluminum and silicon chloride served as precursors. They were generated in-situ by feeding gaseous hydrogen chloride (HCl) into a heated reaction chamber (1100 °C) which contained aluminum and silicon donating material. An excess of hydrogen gas flow was found to improve the coating growth rate. Due to the high reactivity of aluminum, high ratios of silicon to aluminum donator had to be maintained (at least 10: 1). The coating thickness was determined from glow discharge spectroscopy and metallographic cross sections. Co-deposition and two-step deposition (first aluminizing and then siliconizing) were performed. Silicon contents of up to 50 at.% could be achieved. In contrast, a two-step process including first aluminizing and then siliconizing leads to an undesired replacement of aluminide by silicide. Al–Si diffusion coatings could be realized on internal cooling channels of turbine airfoils by single-step co-deposition.