Development of composite silver/nickel nanopastes for low temperature joining of yttria-stabilized zirconia to stainless steels

Abstract

Joining of yttria-stabilized zirconia (YSZ) to stainless steels is considered for various applications such as solid oxide fuel cells, thermal barrier coatings, and medical implants. In the present work, a series of brazing pastes utilizing silver and nickel nanoparticles (~80nm) mixed with carboxymethyl cellulose as the organic phase were developed to join 5mol% YSZ to AISI 420 steel. Effects of processing parameters including time and temperature of brazing and the composition of the nanopastes on the microstructure of joints and their shear rupture strength were studied. Elaboration of the joining mechanism by scanning electron microscopy and electron probe micro-analyzer showed solid-state sintering of the nanoparticles. Limited diffusion of the elements in the 420 SS part caused interface formation with a spongy structure. Increasing of the paste solid loading increased the density of the spongy structure at the interface region; thereby higher mechanical strength was attained. The joining was more successful when a silver nanopaste was utilized owing to the higher sintering activity of silver nanoparticles compared to nickel at relatively low brazing temperatures. Meanwhile, at high solid loadings, localized nanoparticle aggregation and microstructural heterogeneity at the interface region were observed. Increasing of the brazing time and temperature enhanced the diffusion of elements and improved the joint strength. Optimization of the processing parameters by the L16 Taguchi method determined that a silver paste with 72% solid loading processed at 270°C for 120min would yield a sound joint with shear strength of 23±1MPa.