NANOMECHANICS OF POLYMERAND COMPOSITE PARTICLES
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This thesis concerns the nanomechanics of micron-sized polymer and composite particles used in electronic packaging technology. The aim of this PhD study is to develop a scientific methodology for characterizing the mechanical properties of single micron-sized particles produced by the well-known Ugelstad method. This includes pure polymer particles as well as metal coated polymer particles. Such particles have recently been exploited for new applications in microelectronics and microsystems. The mechanical properties of the particles are of crucial importance for these applications. However, due to the inherent complexity of the spherical geometry, mechanical characterization of single particles possesses great challenges. Because of large surface to volume ratio and in most cases lack of surfactants, the polymer particles tested in this work usually occur in a state of clusters. Accordingly, a particle dispersion procedure has been first developed to obtain individual particles suitable for testing. Thereafter a nanoindentation-based flat punch method for measuring the mechanical properties of single particles has been established. A diamond flat punch is specially designed for characterizing single particles, instead of the common sharp tip used for nanohardness measurements. Both polymer and metal-coated composite particles with various chemical compositions, crosslink densities, sizes and loading conditions have been studied using the nanoindentation-based flat punch methodology. All the polymers have been of an amorphous type. The contact load-displacement relationship of single micron-sized particles has been recorded and the stress-strain behavior has been determined. It has been shown that the slightly crosslinked polystyrene particles display a yielding behavior, and smaller surface cracks have been observed after deformation. However, the strongly crosslinked acrylic and polystyrene particles show a brittle fracture behavior. A striking particle size effect on the mechanical properties has been discovered for the first time. The results show that for the slightly crosslinked polystyrene particles the smaller the particle diameter is, the harder the particle behaves. The corresponding mechanisms of the particle size effect have been analyzed and are mainly contributed by a possible “core-shell” microstructure of this type of polymer particles. The effect of loading rate on the stressstrain behavior and the failure mechanism for both polymer particles and metal coated polymer particles has been investigated. The results indicate that the mechanical behavior of both particles is strongly dependent on the loading rate. The influence of the nanoscale metal coating has been revealed through comparing the mechanical properties of metal coated polymer particles with that of identical size, but uncoated ones. It has been found that within a range of relatively small deformations the metal coating plays a significant strengthening effect on the mechanical properties of the particles. The original findings in this PhD work have been presented in 6 international journal articles and 3 international conference papers. 5 published journal articles are attached in this thesis.
Består avHe, Y; Helland, T; Zhang, L; Kristiansen, H. Fracture of micrometre-sized Ni/Au coated polymer particles. Journal of Physics D - Applied Physics. (ISSN 0022-3727). 42(8), 2009. 10.1088/0022-3727/42/8/085405.
He, Y; Zhang, L; Kristiansen, H. Mechanical properties polymer particles for adhesives. INT J MATER RES. (ISSN 1862-5282). 98(5): 389-392, 2007.
He, Y; Zhang, L; Kristiansen, H. Nanomechanical Characterization of Single Micron-Sized Polymer Particles. Journal of Applied Polymer Science. (ISSN 0021-8995). 113(3): 1398-1405, 2009. 10.1002/app.29913.
He, Y; Zhang, L; Kristiansen, H. Compression properties of individual micron-sized acrylic particles. Materials letters (General ed.). (ISSN 0167-577X). 63(20): 1696-1698, 2009. 10.1016/j.matlet.2009.05.013.
He, Y; Zhang, L; Midttun, M; Fonnum, G; Modahl, I; Kristiansen, H; Redford, K. Size effect on mechanical properties of micron-sized PS-DVB polymer particles. Polymer. (ISSN 0032-3861). 49(18): 3993-3999, 2008. 10.1016/j.polymer.2008.07.015.