Nanocrystalline ceria imparts better high–temperature protection

Nanomaterials have attracted considerable interest with numerous technological developments in the last decade. Nanomaterials exhibit different physicochemical properties compared with their bulk counterparts because the diameters of the nanoparticles are less than the Bohr exciton radius. Cerium oxide based materials have been extensively studied for various technological applications. In the present study, the application of nanocrystalline cerium oxide for improvement of high–temperature–oxidation resistance of stainless steel has been studied. The role of coating of nanocrystalline cerium oxide towards improvement of high–temperature–oxidation resistance has been investigated and compared with that of the micrometre–sized cerium oxide particles. It was observed that nanocrystalline ceria improved high–temperature–oxidation resistance of AISI 304 stainless steel to a large extent compared with the micrometre–sized ceria coating. Nanocrystalline ceria coating decreased the isothermal parabolic rate constant of oxidation by more than two orders of magnitude compared with that of the bare alloy. The resistance to oxide scale spallation was also found to improve with the coating of cerium oxide nanoparticles. Secondary ion mass spectroscopy (SIMS) study of nanocrystalline ceria–coated and oxidized specimens revealed the presence of nanoceria at the outermost oxide surface, indicating a change in the oxide scale growth mechanism from outward cation diffusion to inward oxygen diffusion in the bare alloy at high temperature in dry air. The oxide scale morphology was studied using a scanning electron microscope (SEM) while a focused ion beam (FIB) technique was used to study the oxide–alloy interface. X–ray photoelectron spectroscopy (XPS) study of nanocrystalline ceria showed the presence of Ce3+ and Ce4+ oxidation states. It is proposed that the presence of the Ce3+ oxidation state in nanocrystalline ceria improves the oxidation resistance of stainless steel, and the related mechanisms are discussed.