The evolution of interfacial morphology is investigated in joints bonded by a newly developed ambient temperature ultrasonic bonding process based on Ni nano-cone arrays. This confirmed two types of void at the interface: a Type I void in the concave area among the cone-like nanocrystals, and a Type II void on the top of the cone-like nanocrystals. By increasing the bonding pressure or time, the Type I voids can be eliminated; however, a pressure threshold exists for the Type II voids, below which increasing the bonding time accelerates the formation of Type II voids. At pressures above this threshold, on the other hand, increasing the bonding time results in shrinkage of the Type II voids. Varying the pressure and time was also found to induce a change in the interfacial morphology from cone-like to ladder-like and finally a planar structure. In this way, the critical pressure for obtaining a solid bonding joint was found to be 7 MPa. (C) 2014 Elsevier B.V. All rights reserved.