Surface Interactions in Powder Injection Moulding
Powder Injection Moulding (PIM) is an attractive technique for producing complex components in large quantities my means of powder technology. The process includes mixing powder with a suitable binder, usually a thermoplastic polymer. Because of the large attractive forces between small particles the powder can be agglomerated in the binder. In order to avoid particle agglomeration in the powder-binder mixture, surface-active additives are used for enhancing the dispersion of the particles. To have a mixture free of agglomerates is important since agglomerates will affect the viscous behaviour of the mixture. Moreover, agglomerates that remain can cause defects in the final part. If the additive is to work properly as dispersant it must interact with the powder surface and ideally adsorb onto it. Thus, not only is the chemical character of the additive itself important, but also the chemical properties of the powder surface. Metal powder has a surface layer of one or more oxides. For some materials like atomised stainless steel powder, the fabrication process (water or gas atomisation) determines the type of oxides present on the powder surfaces. Thus, the process also impacts the way powder interacts with additives in PIM.
This doctoral study combines controlled adsorption experiments with surface chemical analysis by means of primarily X-ray Photoelectron Spectroscopy (XPS) as well as Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). The aim of this approach is to determine the interaction between the organic compounds and the powder surfaces. Furthermore, rheological characterisation of model feedstock materials provides information on how the additive acts during mixing and moulding. Powders used are ferrous PIM powder such as gas-atomised and water-atomised fine stainless steel powder and carbonyl iron powder. The model feedstock is composed of such powder and linear low-density polyethylene (LLDPE) as a binder with possible addition of surfactant. Adsorption of both basic and acidic molecules on ferrous powder in organic liquids was investigated. Specific attention was paid to the surfactant stearic acid, as this is a common additive in PIM.
By means of XPS analyses it was shown that the stearic acid, being an acidic molecule, is adsorbed on metal powders with basic oxides. By applying the Tougaard nano-structure analysis for thin film characterisation by XPS the thickness of the stearic acid layer was estimated at ~20 Å. From model experiments with flat oxidised samples of Fe, Cr and Mn, this interaction with their oxides was confirmed using angle-resolved XPS analysis. Also, it was indicated the XPS characterisation can provide information about the orientation of the adsorbed molecules on the surface. It was demonstrated for powder covered with Si oxide that this acidic oxide interacts with basic molecules (e.g. aniline) when the top surface is composed of hydroxide. Rheological testing showed that adding stearic acid to the model feedstock materials reduced the viscosity much more than if added to the binder alone. If enough stearic acid is added, the dispersion of particles is improved. This effect could also be achieved by lowering the mixing temperature (increased shear force).
powder injection moulding (PIM)