UMS Al-Mo coatings as an alternative to electro-plated cadmium

For most aerospace applications, replacing cadmium plating requires sacrificial coatings having excellent adhesion to steel and that can be deposited using non-embrittling processes. Aluminum-based coatings, deposited using the Unbalanced Magnetron Sputtering (UMS) deposition process, meet these requirements. This paper discusses the development of UMS Al-Mo coatings from the perspective of their potential use as cadmium alternatives in applications requiring good corrosion resistance along with high lubricity. Results of microstructural examination and tribological and electrochemical tests are presented for a series of UMS Al-Mo coating produced over the composition range from zero to 100% Mo. The coatings have uniform thickness, a fully dense, featureless or slightly columnar structure and good adhesion to 4340 steel. The passivating effect of increasing molybdenum content resulted in a gradual increase of the corrosion and pitting potentials, indicating increasing resistance to pitting corrosion and decreasing sacrificial corrosion properties. The coefficients of friction of Mo rich coatings (with Mo contents between 90 and 100%) were found to be comparable to that of cadmium.