Effect of La2O3 particles on microstructure and cracking-resistance of NiO scale on electrodeposited nickel films

The effect of La2O3 particles on the microstructure and cracking-resistance of thermally grown NiO scale, formed at 900 and 1000°C, on electrodeposited nickel films has been investigated through in situ acoustic emission (AE) testing in conjunction with SEM and TEM/EDX. NiO scale, formed on electrodeposited films of pure nickel, has a porous duplex layered structure, which exhibits a more brittle behavior than the scale on films with the addition of La2O3, particles. They played a profound role in the procrastination of the occurrence of scale cracking and reduction of the cracking events. Upon the comparison of the microstructure of NiO scale on the nickel film with the NiO scale on the LaO-containing film, several points can be concluded. For example, the stresses in NiO scale on the film with La2O3 were partly relaxed by a nondestructive route, such as scale plastic deformation due to La2O3-induced formation of a single-layered and fine-grained NiO scale. Also, it is believed that the particles reduced the size of the defects at the scale–metal interface and probably enhanced the cohesion between NiO grains and the adhesion between the scale–metal interface by retardation of the inward diffusion of nickel vacancies.