Analysis of Thermally Oxidized Surfaces of Additive Manufacturing Metal Powders Using Triboelectric Charging

In powder-based additive manufacturing (AM), thermal exposure during processing can modify powder surface chemistry and promote oxide growth, even under inert atmospheres from residual oxygen contamination. This study investigates the effect of thermal oxidation on the surface chemistry and triboelectric charging behavior of three AM metal alloy powders: AlSi10Mg, 316L stainless steel (SS 316L), and Ti6Al4V. The work examines the evolution of the oxide layer during baking at 100 and 300 °C using triboelectric charging corroborated by X-ray photoelectron spectroscopy (XPS), diffuse-reflectance spectroscopy, and work-function measurements. The results show that heating modifies the surface oxide state of all powders, with changes dependent on the alloy composition and baking temperature. For AlSi10Mg, heating modified the Al2O3-rich surface oxide, with changes consistent with increased oxide ordering and γ-Al2O3-like characteristics, with the work function increasing from 4.34 ± 0.01 eV in the as-received (AR) condition to 4.92 ± 0.03 eV after baking at 300 °C. The corresponding triboelectric n-exponent increased from 0.91 to 1.38. For SS 316L, transformation of Cr(OH)3 to Cr2O3 reduced triboelectric charge accumulation, while the oxygen concentration increased from 49.92 to 54.87 at.% and the work function decreased from 5.74 ± 0.02 to 5.28 ± 0.04 eV after baking at 300 °C. This reflected a drop in the n-exponent from 0.82 for AR to 0.73 at 300 °C. For Ti6Al4V, charging variations were associated with titanium oxide evolution and surface modifications consistent with rutile-related titanium oxide characteristics, with the work function increasing from 5.33 ± 0.01 to 5.44 ± 0.03 eV after baking at 300 °C. The corresponding triboelectric n-exponent increased from 0.49 to 0.52. Overall, triboelectric charging is a sensitive approach for detecting thermally driven surface oxide modifications in additive manufacturing powders.