On the phase formation of cathodic arc evaporated Al1−xCrx-based intermetallic coatings and substoichiometric oxides

Abstract

The phase evolution of Al1−xCrx-based intermetallic coatings and corresponding substoichiometric oxides grown by cathodic arc evaporation was investigated in order to obtain a better understanding of the relation between oxygen flow rate, Al and Cr content, and structural evolution of the coatings deposited.When using 20 sccm Ar, or 50 sccm O2, or 100 sccm O2 per active source (p.a.s.) the cathode reaction zone consists of various intermetallic Al-Cr-compounds, which are in good agreement with the binary Al-Cr phase diagram. This is generally also reflected in the phase composition of the metallic and substoichiometric oxide coatings. The Al-rich compositions, Al0.75Cr0.25 and Al0.70Cr0.30, show a strong tendency for the formation of γ1-Al8Cr5 phases. Mostly, the coating compositions of the metallic constituents of the synthesised intermetallic and substoichiometric oxide coatings deviate from the elemental compositions of the cathode, show enrichment in Cr. This deviation is more pronounced for Cr-rich cathodes using low O2 flow rates during deposition.The dense columnar structure of the intermetallic coatings (hardness values between 2.5 and 10.2 GPa) turns into a nano-composite-like morphology for depositions with 50 and 100 sccm O2 p.a.s., which in turn leads to a significant hardness increase up to ~24 GPa. Among all coatings investigated, the Cr-rich compositions have higher hardness and denser morphology than the Al-rich layers.