Deposition and Cyclic Oxidation Behavior of a Protective ( Mo , W ) ( Si , Ge ) 2 Coating on Nb‐Base Alloys

Abstract

A multicomponent diffusion coating has been developed to protect Nb‐base alloys from high‐temperature environmental attack. A solid solution of molybdenum and tungsten disilicide, , constituted the primary coating layer which supported a slow‐growing protective silica scale in service. Germanium additions were made during the coating process to improve the cyclic oxidation resistance by increasing the thermal expansion coefficient of the vitreous silica film formed and to avoid pesting by decreasing the viscosity of the protective film. The development of the halide‐activated pack cementation coating process to produce this coating on Nb‐base alloys is described. The results of cyclic oxidation for coupons coated under different conditions in air at 1370°C are presented. Many coupons have successfully passed 200 1 h cyclic oxidation tests at 1370°C with weight‐gain values in the range of 1.2 to 1.6 mg/cm2.