Effect of EBPVD coated nanoceria thickness on the isothermal oxidation of AISI 304 stainless steel

Stainless steels such as AISI 304 are widely used as structural material but are prone to degradation at high operating temperature due to thermal coefficient mismatch and thermal stress between the oxide scale and alloy. Coating of rare earth oxides such as cerium oxide on AISI 304 prevents the high temperature degradation by the formation of protective oxide layer on the surface. In the present work we report the effect of cerium oxide with thickness varied from 10 to 2200 nm on the high temperature oxidation protection. Cerium oxide nanoparticles synthesized by precipitation method and sintered at 1473 K was used as a target in electron beam physical vapour deposition. Structural and surface properties of the coatings were analysed prior and post-oxidation with respect to thickness using X-ray diffraction, Raman spectroscopy and scanning electron microscopy. Independent of coating thickness, the resultant films had a crystallite size of less than 8 nm which increased to about 40 nm upon oxidation at 1243 K. The increase in cerium oxide coating thickness resulted in a decrease in oxidation rate constant by 3-4 orders. Though bare AISI 304 exhibited severe oxidation, the presence of cerium oxide with 10 nm thickness reduced the spallation on exposure to high temperature. Oxygen vacancy concentration calculated from Raman spectroscopic analysis showed one order reduction in defect concentration upon oxidation but the changes were found to be minimal with the increase in coating thickness. Surface studies by scanning electron microscopy showed a flake like morphology for the bare AISI 304 on oxidation. But the size of the flakes reduced in the presence of 10 nm thick coating which indicates the beneficial effect of cerium oxide even at lower thickness towards the protection against high temperature oxidation. (C) 2015 Elsevier B.V. All rights reserved.