Metal foam stabilization by oxide network particles

The fundamental stabilization mechanism for foaming metal powder compacts (PM foams) is investigated. It is shown that PM foams belong to the class of stable foams where the stability is controlled by the balance of interfacial forces which equilibrate after drainage has been completed. The origin of these interfacial forces is traced back to the oxide content of the underlying metal powder. The oxides are not present as compact particles but form crumpled, voluminous aggregates, the oxide network particles, which form a tenuous global network. The effective volume fraction of these network particles is about 30-35%. Analogous to a gel-sol transition, bubble nucleation leads to a fragmentation of the global oxide network structure. The isolated oxide network fragments have particle character and act as mechanical barriers within the foam structure where they induce a strong stabilizing disjoining pressure.