Thermally induced bond failure of electroless nickel/IVD aluminum coatings deposited on steel

Experiments were conducted to investigate methods to improve the bond of an electroless nickel/ion vapor deposited (IVD) aluminum coating applied to a steel substrate for use at an elevated temperature of 482°C. Samples were prepared by coating steel panels with columnar IVD aluminum followed by the application of electroless nickel from a sodium hypophosphite plating solution. Surface treatment of the underlying aluminum was varied by physically compacting the columnar structure of the aluminum by glass-bead peening and/or grit blasting. The cooling atmosphere after application of the aluminum was also investigated by using nitrogen to reduce the potential of aluminum oxidation prior to application of the electroless nickel coating. Results show that at elevated temperatures atomic diffusion takes place creating voids that eventually lead to failure of the electroless nickel coating. Bond failure can be retarded, but not eliminated, by using a nitrogen atmosphere to prevent oxidation of the IVD aluminum and by retention of the aluminum columnar structure before overcoating with electroless nickel.