Effects of sulfur impurity on the scale adhesion behavior of a desulfurized Ni-based superalloy aluminized by chemical vapor deposition

The surface of a single-crystal Ni-based superalloy, which contained a bulk sulfur content of similar to 0.4 ppmw, was aluminized in a hot-wall chemical vapor deposition (CVD) reactor, using AlCl3 and H-2 as gaseous precursors, at 1100 degrees C. The chemical composition and microstructure of the resulting aluminide coating were characterized with particular emphasis on sulfur incorporation as an impurity during aluminizing. Depth profiling by glow-discharge mass spectroscopy (GDMS) was used as a qualitative means of assessing the level of sulfur in the coating structure. Sulfur contamination, which was initially observed at the coating surface and the substrate-coating interface, could be reduced by some minor reactor modifications. With the reduced sulfur content, scale adhesion on the surface of the aluminide grains of the coating was significantly improved during cyclic oxidation, whereas scale spallation at the coating's grain boundaries became more apparent.