X-ray Photoelectron Spectroscopy (XPS) Investigation of the Surface Film on Magnesium Powders

Magnesium (Mg) and its alloys are attractive for use in automotive and aerospace applications because of their low density and good mechanical properties. However, difficulty in forming magnesium and the limited number of available commercial alloys limit their use. Powder metallurgy may be a suitable solution for forming near-net-shape parts. However, sintering pure magnesium presents difficulties due to surface film that forms on the magnesium powder particles. The present work investigates the composition of the surface film that forms on the surface of pure magnesium powders exposed to atmospheric conditions and on pure magnesium powders after compaction under uniaxial pressing at a pressure of 500 MPa and sintering under argon at 600 °C for 40 minutes. Initially, focused ion beam microscopy was utilized to determine the thickness of the surface layer of the magnesium powder and found it to be ∼10 nm. The X-ray photoelectron analysis of the green magnesium sample prior to sintering confirmed the presence of MgO, MgCO₃·3H₂O, and Mg(OH)₂ in the surface layer of the powder with a core of pure magnesium. The outer portion of the surface layer was found to contain MgCO₃·3H₂O and Mg(OH)₂, while the inner portion of the layer is primarily MgO. After sintering, the MgCO₃·3H₂O was found to be almost completely absent, and the amount of Mg(OH)₂ was also decreased significantly. This is postulated to occur by decomposition of the compounds to MgO and gases during the high temperature of sintering. An increase in the MgO content after sintering supports this theory.