Adhesion and coalescence of ductile metal surfaces and nanoparticles

Much more is known about material failure, such as fracture and crack propagation, than the reverse effect of material formation, i.e., how bulk materials form or consolidate during material processing or crack healing. Using the Surface Forces Apparatus, optical interferometry, optical and scanning probe microscopy, and x-ray diffraction, 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 nanometer-scale 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 and kinetic considerations suggest that it may be a more general phenomenon for ductile materials interacting at the nano-scale. We also make some qualitative comparisons with the very different behavior observed with hard, brittle materials.