Effect of La2O3 Particles on the Oxidation of Electrodeposited Nickel Films

Abstract

Electrodeposited Ni-La₂O₃composite films with nanometer-sizeLa₂O₃ oxide inclusions werefabricated by the codeposition of nickel withLa₂O₃ particles. The comparativeoxidation behavior in air at 900 and 1000°C of nickel coated with theNi-La₂O₃ composite and films withand without nickel-plating was studied by TGA, AE,SEM/EDX, EPMA, and TEM/EDX. In general, theNi-La₂O₃ composite-coated nickelhad the slowest rate and the best resistance tothermal cycling. AE tests revealed that cracking eventsin NiO scales on Ni-La₂O₃composite-coated nickel was significantly reduced incomparison to that of the scale on nickel-coated nickel during thermalcycling at 900°C. SEM investigation showed that theLa₂O₃-free NiO scale was composedof outer coarse columnar grains and inner equiaxed ones.By contrast, the scale on the Ni-La₂O₃composite-coated nickel consisted of only fine equiaxedNiO grains. The scale on theLa₂O₃-free samples wascharacterized by cracks that originated at thescale-substrate interface and spanned the scale thickness. By contrast,no scale cracks formed at theLa₂O₃-doped NiO scale-substrateinterface, but small voids were created at the triplepoints of the grain boundaries of NiO. In the La₂O₃-doped NiOscale, segregation of La ions to the NiO grainboundaries near the scale-surface was observed by EDXmicroanalyses in the TEM. It is believed that the Laions segregated at the grain boundaries of NiO led to an increase in thecohesion between nickel oxides and in a reduction of thescaling rate and the formation of scale with fineequiaxed crystal structure by blocking the outward and lateral growth of scale. The latter was dueto the predominant outward diffusion of nickel along NiOgrain boundaries being inhibited effectively by thesegregated La ions. The mechanism of the effect of the added La₂O₃particles on the nickel electrodeposits is discussed indetail.