This work examines the fabrication and properties of silicon nitride / graphene based nanostructures composites densified by the spark plasma sintering technique with the aim of proving the mechanical reinforcement effect expected by the excellent properties of the graphene fillers. Dense and homogeneous Si3N4 composites containing different type of carbon nanofillers were processed by dispersion of pristine graphene nanoplatelets, graphene oxide and unzipped multiwall carbon nanotubes into the ceramic slurry containing the ceramic matrix powders and sintering additives. Spark plasma sintering was selected as consolidation technique due to the lower temperatures and shorter times required for densification, compared to conventional sintering methods, preventing the damage of the carbon nanostructures. Scanning and transmission microscopy combined with construction of Raman intensity maps revealed differences in the exfoliation degree of the nanofillers, which also influenced ceramic grain growth, and evidenced the orientation effect in the nanofillers due to uniaxial pressure applied during sintering stage. The strong orientation clearly differentiates the electrical and thermal behavior of the materials in the directions parallel and perpendicular to the graphene plane having also a significant role in these properties the crystalline quality of carbon lattice. The low shear resistance between graphene layers in the nanostructures affects elastic modulus and hardness of the materials compared to monolithic Si3N4 producing a certain softening, however, the toughening effect of the three types of nanostructures was demonstrated for crack propagating perpendicular to the nanofiller aligment direction, being the bridging mechanism the main responsible of the toughness increase. Reduced graphene oxide nanosheets were the most effective reinforcements achieving 135% higher fracture toughness than Si3N4 under the same sintering and testing conditions. On the top of that, the nanofillers induce an electrical response of the composites once they form a connected network for small concentrations indeed, therefore, being quite dependent of the exfoliation and again on the nanofiller orientation. Very similar behavior is observed for the thermal conductivity of the composites, and with a heat flow rate for the orientations matching the graphene plane. These results have shown that graphene nanoplatelets and nanoribbons can be used as effective fillers for ceramic composites with tailored properties intended not only for applications that require good mechanical performance but wear resistance, good heat conduction and electrical conductivity.

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Aplicada. Fecha de lectura: 16-06-2014