Role of oxygen vacancy tuning in EBPVD deposited LaxCe1−xO2−δ films in high temperature oxidation protection

Abstract

Cerium oxide based nanostructure coatings are shown to be promising for high temperature oxidation protection of AISI 304 stainless steel due to the presence of oxygen vacancy defects. The present work focuses on the correlation of oxygen vacancies generated by varying lanthanum concentration in ceria to high temperature oxidation protection. La_xCe_1−xO_2−δ (x = 0, 0.05, 0.1, 0.2 & 0.4) synthesized using a chemical co-precipitation method was sintered at 1473 K for 5 hours and used as a target in electron-beam physical vapour deposition (EBPVD). The thickness of the coatings over the AISI 304 substrate was maintained at 2000 nm and all the samples were isothermally oxidized at 1243 K for 24 hours. The La³⁺ doping and the presence of oxygen vacancies were confirmed by using X-ray diffraction and Raman spectroscopy, respectively, in the as coated condition. Though the target had larger mean crystallite size (77 to 52 nm) but varied marginally (6.7 to 7.3 nm) in the as coated condition emphasizing the role of physical process during deposition. The rate constant value for the oxidation process was found to be 3–4 orders lower than that of bare AISI 304 indicating the effect of coating against high temperature oxidation. 5% La doped ceria coating provided better oxidation protection than pure ceria while 40% doping resulted in one order lower rate. This can be attributed to the higher oxygen vacancy concentration present in the sample. The presence of La in the coatings also helped in the retention of oxygen vacancy concentration after oxidation. The reported study indicates the importance of oxygen vacancy tuning and need for uniform coatings towards designing coatings for high temperature oxidation protection.