Inter‐Nanoparticle Bonds in Agglomerates Studied by Nanoindentation

Abstract

Nanoparticles tend to agglomerate during synthesis or delivery processes owing to their high surface area and surface energy. It is extremely important to investigate stabilizing techniques that result in reducing the strength of internanoparticle bonds in agglomerates. However, the present measurement techniques of agglomerate strength can only deal with relatively large particles, in the micrometer range, and do not have the resolution to measure directly the interparticle forces in the nanometer regime. Therefore, new tools and techniques for such investigations are needed. In this work, small agglomerates of silver nanoparticles were deposited and fixed on hard substrates using an adhesion promoter. The deformation behavior of the agglomerates under low external loads was studied using the nanoindentation technique. It was proposed that the strength of agglomerates can be correlated to the shape of the nanoindentation load-displacement curves. The interparticle forces within the agglomerate stem from the van der Waals, capillary, and electrostatic forces. [1] To study these forces locally, one should apply a tool with spatial resolution of less than a nanometer, and with appropriate force resolution, of the order of micronewtons. Modern nanoindenters provide such a tool, but there are some problems that are yet to be overcome. Kendall [2] measured the strength of zirconia spray-dried agglomerates using a modified nanoindenter. Agglomerates with a typical size between 10 and 70 lm were crushed between rigid and parallel plates, and the tensile fracture strength was calculated from the load-displacement curves. Kendall and Weihs [3] continued Kendall’s work and observed elastic and plastic deformations of agglomerates during compression tests. In those pioneering experiments, the total macroscopic behavior of the agglomerates was observed but no information could be obtained regarding the interparticle bonds in the agglomerates. Adams et al. [4] performed nanoindentation tests on a layer of polymerbonded nanoparticulate films of silica deposited on a poly(methylmethacrylate) substrate. They used a Berkovich tip to penetrate a particulate coating and suggested that the residual load, which is the difference between the measured force and a third-order polynomial approximation of the load versus time dependence, could be related to the breaking of individual interparticle junctions. It should be emphasized that special care is required when measuring the mechanical response of particle aggregates or thin films deposited on substrates, since the latter can strongly contribute to the overall mechanical response of the film/substrate system. In a detailed study by Saha and Nix [5] the influence of the substrate hardness on the measured mechanical properties of sputtered Al and W films was investigated. They found that the contribution of the substrate’s hardness to the measured one is small when the substrate is much harder than the film. When dealing with ever-smaller agglomerates, fixation to the substrate becomes an increasing problem for nanoindentation tests. In this research, the fixation problem was addressed using adhesion promoters. This enabled us to study individual agglomerates placed on the nanoindenter stage. A Triboscope nanoindenter of Hysitron attached to an Autoprobe CP scanning force microscope equipped with a cube