Oxygen dependent morphology and mechanical properties of AlCr(Fe)-based coatings

Fe-alloyed intermetallic Al–Cr coatings as well as substoichiometric oxides (with respect to M2O3 stoichiometry) thereof were synthesised by reactive arc evaporation in order to investigate the impact of Fe and the oxygen flow rate during deposition on the structural evolution and mechanical properties. All coatings prepared in non-reactive conditions crystallise in a γ2-type Al8Cr5 structure with Fe substitutionally incorporated in the lattice featuring hardnesses in the range of 11 ± 1 GPa. By introducing oxygen to the process, the growth morphology changes from columnar to a nano-composite-like structure of small Cr-enriched crystallites embedded in an amorphous matrix. Simultaneously, the number of droplets and therewith associated defect structures increases and accordingly the hardness values of these films decrease to 8.5 ± 1 GPa. For oxygen flow rates close to the transition from substoichiometric to stoichiometric sesquioxides (~0.7–0.9 Pa), the crystallinity as well as the fraction of strong oxide bonds increase and highest hardnesses among all coatings investigated are obtained (>20 GPa). Furthermore, small additions of Fe to Al0.7Cr0.3 cathodes effectively improve the structural and mechanical properties of substoichiometric oxide films. •Formation of substoichiometric nanocomposite AlCrFe oxide coatings•Detailed TEM investigation of metallic AlCrFe and substoichiometric AlCrFe oxide coatings•Improvement of structure and mechanical property relationship by alloying Fe to AlCr coatings