Much is known about the fundamental mechanisms of material failure, such as fracture and crack propagation, but much less about the reverse effect of material formation, i.e., how bulk materials form or consolidate via the coalescence of their constituent molecules, nanoparticles or surfaces as occurs during material processing or crack healing. Using the Surface Forces Apparatus (SFA) force and various optical, microscopy and x-ray imaging techniques we have studied how gold and platinum films "sinter" or "cold-weld" at the nano-scale to form continuous bulk films when two initially rough surfaces (composed of 5-10 nm asperities) are pressed together. We find that coalescence of these ductile materials occurs abruptly, like a first order phase transition, once a critical local pressure or interparticle separation is reached. Simple thermodynamic reasons are given for this apparently general effect which suggest that it may be a more general phenomenon for ductile materials interacting at the nano-scale, where high local curvatures, extremely small sizes and confinement effects produce large local forces, and where rapid molecular rearrangements (short equilibration times) can occur even at low diffusion rates. We also make some qualitative comparisons with the very different behavior observed with hard, brittle materials.
Keywords: ductile nanoparticles, cold-welding.