Main features of magnetron sputtered aluminium–transition metal alloy coatings

Aluminium-based alloy coatings are potential candidates for galvanic corrosion protection of mechanical steel parts. This paper presents the microstructure and morphological characteristics as well as mechanical and corrosion properties for Al–TM systems (TM=Ti, Cr, or Fe) sputtered from single composite targets. The low temperature, as-deposited aluminium rich coatings are metastable supersaturated single or two-phase fcc, or amorphous solid solutions. It is shown that transition metal solubility in aluminium depends on TM to Al atomic size ratios. A good compromise between microhardness (7000 MPa) and low open-circuit potential can be achieved in Al–Cr and Al–Ti systems, whereas Al–Fe alloys cannot be used as sacrificial coatings above 10 at.% Fe for which the microhardness is only approximately 3500 MPa. The structure–properties relationship of amorphous or microcrystalline aluminium-rich, as-deposited films are discussed in detail.