Enhanced formation of α-Al2O3 at low temperature on Cr/Al coating by controlling oxygen partial pressure

alpha-Al2O3 is one of the most promising hydrogen permeation barriers due to its high melting point, stable chemical properties, and good machinability. However, alpha-Al2O3 synthesis usually requires temperatures above 1200 degrees C, which weakens the properties of the substrate material. Therefore, low-temperature preparation of alpha-Al2O3 is crucial. Most studies have reported that using Cr2O3 as a template is a good method to promote alpha-Al2O3 formation. However, the formation of monolithic Cr2O3 is still a challenge. To address these problems, a Cr/Al coating was introduced that was successfully deposited on stainless steel. A new composite coating was formed by the subsequent oxidation conversion under ultra-low-partial-pressure conditions via decomposing the oxygen precursors (nickel/iron oxide) in a closed environment. The results from X-ray diffraction, Raman spectra and other methods showed that the content of alpha-Al2O3 was enhanced at 2.6 x 10(-24) Pa (theoretical calculation value) and promoted the formation of near-stoichiometric Cr2O3. In addition, the ultra-low partial pressure condition could promote the diffusion of defects on the interface between metastable Al2O3 and corundum crystal structure Cr2O3, by which the alpha-Al2O3 was successfully formed at 600 degrees C due to the template effect of Cr2O3. This work provides a new approach for the low-temperature preparation of alpha-Al2O3.