Growth kinetics and pesting resistance of MoSi 2 and germanium‐doped MoSi 2 diffusion coatings grown by the pack cementation method

The rapid, destructive low‐temperature oxidation (pesting) of MoSi 2 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 by‐product, 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°C. An additional minor improvement in the pesting resistance was observed for the germanium‐doped MoSi 2 Coatings. The growth kinetics for a three‐layer Ge‐doped Mo(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 MoSi 2 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 Mo 5 Ge 3 and Mo 3 Ge, 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.