High-temperature oxidation resistance of sputtered (Al,Cr,Nb,Ta,Ti,Si)N coatings

High-entropy metal-sublattice (Al,Cr,Nb,Ta,Ti)N coatings, with up to 15.0 at% additional Si content were developed and investigated for their oxidation resistance by exposing them for 3, 30, and 100 h to ambient air at 900, 950, and 1000 ∘ C. The Si-free coating is rapidly oxidized, but all of the Si-alloyed coatings survive even the harshest oxidation test. The oxides crystallize mostly in the rutile structure with some Ta 2 O 5 -type phase fractions at higher Si contents. Detailed TEM investigations reveal a varied microstructure across the oxide scales with a succession of Cr-, Al, and Ti-rich top oxide layers, which agrees with a reported thermodynamical calculation of oxide stabilities. • We oxidized (Al,Cr,Nb,Ta,Ti,Six)N coatings for up to 100 h. • The Si-free coating fails at 950 ∘ C. • The Si-alloyed coatings all survive 100 h at 1000 ∘ C. • The most protective coating has amorphous grain boundary regions in the oxide scale. • The succession of oxide layers at the surface agree with a thermodynamical model.