Mechanical characteristics of FeAl2O4 and AlFe2O4 spinel phases in coatings - A study combining experimental evaluation and first-principles calculations

FeAl2O4 and AlFe2O4 have similar spinel structures but different atomic arrangements, leading to different properties. In this study, mechanical properties and structures of the two spinel phases in composite coatings prepared by reactive plasma spraying of Fe2O3-Al composite powder were investigated by means of multi-mode atomic force microscopy, X-ray diffraction, and scanning electron microscopy. It was demonstrated that AlFe2O4 was stronger than FeAl2O4 with a higher electron work function (EWF). First-principles calculations were conducted to understand the difference in mechanical properties between the two phases. It was shown that FeAl2O4 and AlFe2O4 have “normal” and “inverse” spinel structures with different configurations of valence electrons: Fe2+tet(Al3+)2O4 and Fe3+tet(Fe2+Al3+)octO4. Compared to Fe2+tet(Al3+)2O4, Fe3+tet(Fe2+Al3+)octO4 has its O and Fe atoms in the octahedral sites forming covalent bonds, which increase the stability and strength of the crystal, corresponding to higher EWF and larger bulk, shear and Young's moduli.