Impact adhesion of iron at elevated temperatures

Small iron spheres were made to impinge normally on iron plates at several different temperatures in gases with various oxygen activities. The spheres welded permanently to the plate when the impact occurred under purified hydrogen and at low impact velocities but rebounded at impact velocities larger than a characteristic value which increased with an increase in either the temperature or the surface cleanliness of the sphere and plate. Permanent adhesion can be prevented, even at low impact velocities, by coating the iron with very thin films of wüstite or with thinner films of Al₂O₃. The linear increase in impact area with impact velocity can be understood from the theory of the hardness indenter, using the appropriate high strain rate flow stress. It is inferred that a bond always forms between colliding bodies but that permanent adhesion occurs only when the elastic energy stored during impact is less than the energy required to break the bond by crack propagation. The stored elastic energy increases with impingement velocity by more than does the energy required for the propagation of a crack to separate the bodies completely. Consequently, there exists a critical velocity beyond which permanent adhesion does not occur.