On the evaluation of the adhesion of electroplated Ni coatings upon steel substrate with extended microbridge technique

The interfacial adhesion of electroplated Ni coatings on steel substrate was assessed using an extended microbridge technique (eMBT), and the fracture path of interfacial cracks was examined through cross-sectional high-resolution SEM observation in loading condition. The results indicated that the magnitude of interfacial toughness increased ten to hundred times as the cleaning time of substrate surface was prolonged in a limited duration. The cross-sectional SEM examination revealed that the weak interface fracture was related to brittle mechanism, whereas the strong interface was a reflection of ductile one. In case of the duration time of substrate pre-cleaning, which was commonly adopted in applications of electroplate engineering, the over-erosion of substrate surface appeared and the steel surface became rough. In this case, the mechanical stick force, in addition to the physical bonding force across the interface, was observed to play a crucial role in the coatings integrity and bonding reliability.