Fabrication and characterization of Al2O3 - Ni nanocomposites
Kannisto, Erkka Juhani
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Al2O3 - Ni nanokomposiittien valmistus ja karakterisointi
This master’s thesis is divided into two sections: A literature survey and an experimental part. The literature survey broadly reviews ceramic nanocomposites and gives the reader a basic understanding concerning their mechanical properties and the state-of-the-art research made in this area. The survey also reviews colloidal processing and manufacturing technology of ceramic nanomaterials. Based on the survey the reader will be able to analyse the results presented in the experimental part of this study, although it requires also basic understanding about materials science and ceramic materials. In the experimental part, Al2O3-Ni nanocomposite powder was synthesized using thermolysis and green compacts were slip casted from the powders. Sintering of the green compacts was done using pulsed electric current sintering (PECS) method, which helps to retain the nanostructure better than normal sintering. The nanocomposites were compared with pure alumina reference samples that were produced with the same methods. Compared to reference, nanocomposite hardness rose by 2 % and fracture toughness by 13 %. According to results and literature the hardening effect was found to relate to nickel nanoparticles under a critical size (<60 nm). Toughening was analysed to be a cause of large difference in thermal expansion between the matrix and second phase particles, which induce a residual stress state in the material after sintering. Additionally novel geometrical model was introduced which can be used to predict nanoparticle coarsening during sintering. New properties can arise from the size effect alone and therefore controlling the size during sintering becomes a necessity. Work was financed by TEKES and coordinated by Finnish Metals and Engineering Competence Cluster Ltd. as a part of the Demanding Applications (DEMAPP) research program. Work was done in close collaboration with Aalto University’s Materials and engineering department and Metso Paper Ltd.