Concurrent Growth of Kirkendall Pores and Vapor–Solid–Solid Protuberances on Ni Wires During Mo Vapor-Phase Deposition

During vapor-phase deposition at 1273 K (1000 °C), by pack cementation, of Mo onto 127-μm-diameter Ni wires, two phenomena are observed to occur concurrently, leading to strong surface roughening: (i) the inward radial growth of Kirkendall pores below the wire surface and (ii) the outward growth from the wire surface of protuberances with sizes as large as 15 μm. High-aspect-ratio Kirkendall pores as long as 21 μm are created because of imbalanced interdiffusion between Mo and Ni. These pores in turn, by reducing the flow of Mo into the wires, may enhance the outward growth of Mo-rich protuberances, further roughening the wire surface. These protuberances have faceted tips as well as terraces and steps, indicating that their growth is governed by the vapor–solid–solid mechanism.