On the Role of Bubbles in Metallic Splat Nanopores and Adhesion

Abstract

Highly nanoporous surfaces were observed on the underside of Ni splats. Experiments varying process parameters and substrate treatments were performed to determine the mechanism of pore formation. A theory of impact-induced bubble nucleation and freezing into pores is presented, and calculations are compared with experimental results. Pore formation and morphology is strongly dependent on substrate (a) thermal properties as they affect time for bubble growth before solidification into pores and (b) roughness as submicron scratches enhance nucleation by providing heterogeneous sites and several micron grooves reduce the driving force for nucleation. Splat pull-off experiments are shown that suggest bubble nucleation and pore formation strongly affect adhesion, and represent a strong contribution to the effectiveness of surface roughening. Finally, this observation shows the potential for the manufacturing of high-surface area materials using thermal spray.