GROWTH-KINETICS AND PESTING RESISTANCE OF MOSI2 AND GERMANIUM-DOPED MOSI2 DIFFUSION COATINGS GROWN BY THE PACK CEMENTATION METHOD

The rapid, destructive low-temperature oxidation (pesting) of MoSi2 is an important Limitation to its practical use. The growth of molybdenum silicide diffusion coatings by a halide-activated pack cementation method results in an adherent superficial, salt byproduct, whose composition depends on the halide activator used to grow the coating. As a consequence of the residual salt deposit, coatings grown by a NaF-activated pack did not pest after 2500 hours of isothermal oxidation or after cyclic oxidation for 600 1 hour cycles in air at 500 degrees C. An additional minor improvement in the pesting resistance was observed for the germanium-doped MoSi2 coatings. The growth kinetics for a three-layer Ge-doped Mo-5(Si, Ge)(2)/Mo-5(Si, Ge)(3)/Mo-3(Si, Ge) coating are compared with rates calculated for the diffusion-controlled growth of an undoped three-layer silicide and for three-layer germanide coatings. The activation energy for the solid-state diffusional growth of MoSi2 is the same as that for Mo(Si, Ge)(2), but the growth rates for Mo(Si, Ge)(2) are faster. The activation energies for the solid state diffusional growth of the inner Mo-5(Si, Ge)(3) and Mo-3(Si, Ge) layers are similar to Mo5Ge3 and Mo3Ge, but the rates are slower. The chemical demixing of Mo(Si, Ge)(2) under a chemical potential gradient creates a maximum in the Ge concentration at the Mo(Si, Ge)(2)/Mo-5(Si, Ge)(3) interface, which produces unusual growth kinetics for the Mo-5(Si, Ge)(3) and Mo-3(Si, Ge) layers.