Effect of La2O3 particles on the oxidation of electrodeposited nickel films

Abstract

Electrodeposited Ni-La2O3 composite films with nanometer-size La2O3 oxide inclusions were fabricated by the codeposition of nickel with La2O3 particles. The comparative oxidation behavior in air at 900 and 1000 degrees C of nickel coated with the Ni-La2O3 composite and films with and without nickel-plating was studied by TGA, AE, SEM/EDX, EPMA, and TEM/EDX. In general, the Ni-La2O3 composite-coated nickel had the slowest rate and the best resistance to thermal cycling. AE tests revealed that cracking events in NiO scales on Ni-La2O3 composite-coated nickel was significantly reduced in comparison to that of the scale on nickel-coated nickel during thermal cycling at 900 degrees C. SEM investigation showed that the La2O3-free NiO scale was composed of outer coarse columnar grains and inner equiaxed ones. By contrast, the scale on the Ni-La2O3 composite-coated nickel consisted of only fine equiaxed NiO grains. The scale on the La2O3-free samples was characterized by cracks that originated at the scale-substrate interface and spanned the scale thickness. By contrast, no scale cracks formed at the La2O3-doped NiO scale-substrate interface, but small voids were created at the triple points of the grain boundaries of NiO. In the La2O3-doped NiO scale, segregation of La ions to the NiO grain boundaries near the scale-surface was observed by EDX microanalyses in the TEM. It is believed that the La ions segregated at the grain boundaries of NiO led to an increase in the cohesion between nickel oxides and in a reduction of the scaling rate and the formation of scale with fine equiaxed crystal structure by blocking the outward and lateral growth of scale. The latter was due to the predominant outward diffusion of nickel along NiO grain boundaries being inhibited effectively by the segregated La ions. The mechanism of the effect of the added La2O3 particles on the nickel electrodeposits is discussed in detail.