Microstructure and mechanical properties of sputtered intermetallic Al–Au coatings

Abstract

Intermetallic Al-based coatings such as Al–Ni, Al–Cr, and Al–Pt are widely used as bond coats and/or as oxidation protection of turbine blades. Investigations during earlier work indicated promising thermal stability and oxidation resistance of intermetallic Al2Au-based coatings. Hence, here we study in detail the microstructure and mechanical properties of unbalanced magnetron sputtered Al–Au films with Al∕Au at. % ratios of 1.85, 2.00, and 4.32. The film with an Al∕Au ratio of 2.00 has a single-phase Al2Au dense columnar structure in the as-deposited state and a hardness of ∼8GPa. With increasing Al and Au contents the morphology changes into a more equiaxed structure with additional Al, AlAu, and Au phases, respectively. During growth, excess Al of the film with an Al∕Au ratio of 4.32 forms lamellar nanosegregations (∼3nm thick) in the Al2Au domains. The simultaneous nucleation and growth of Al2Au, AlAu, and Au phases in the coatings with an Al∕Au ratio of 1.85, with their different growth mechanisms, leads to the formation of pores having an average size of ∼150nm. These Al- and Au-rich films, compared to Al2Au, have reduced hardnesses of ∼4 and ∼2GPa, respectively, due to the additional soft metallic phases and pores present. Our results show that within the Al–Au system the single-phase intermetallic Al2Au films exhibit the best structural and mechanical properties for protecting oxidation sensitive materials.