Development of unbalanced magnetron sputtered Al–Mo coatings for cadmium replacement

Aluminum is emerging as a key material system to replace environmentally unacceptable cadmium, especially for application on high strength steels. This is a particularly challenging application, which requires deposition of highly corrosion resistant, sacrificial coatings having excellent adhesion properties and in many applications, high lubricity. Al–Mo coatings, deposited using the unbalanced magnetron sputtering (UMS) deposition process, can meet these requirements. Depending on the level of alloying, coatings with enhanced corrosion properties or coatings with a low coefficient of friction can be obtained. Results of microstructural examination and tribological and electrochemical tests are presented for a series of UMS Al–Mo coatings produced over the full binary range. The coatings have uniform thickness, a fully dense, featureless or columnar structure and a good adhesion to steel. The passivating effect of molybdenum results in a gradual increase of the corrosion potential (from −0.750 V for pure Al to −0.220 V for Al-90Mo) and pitting potentials (respectively, from −0.650 to +0.200 V). Coatings with the Mo content below 14 wt.% are sacrificial to AISI 4340 steel. The coefficient of friction of Al–Mo coatings was tested to be between 0.2 and 0.9, with the lowest value obtained for Al-95Mo composition. At this level, the coefficient of friction of UMS Al-95Mo is comparable to that reported for cadmium.