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Spark plasma sintering of yttria stabilized zirconia ceramics doped with multiwall carbon nanotubes and lanthanum oxide

University of British Columbia

Yttria Stabilized Zirconia (YSZ) is a ceramic composite often used as a thermal barrier coating (TBC), with particular applications in the gas turbine and power generation systems. Recent literature suggests that the addition of Carbon Nanotubes (CNTs) to YSZ can further improve the composite’s room temperature mechanical properties, such as hardness and fracture toughness. Such CNT reinforced YSZ composites could also be considered for high thermal gradient and thermal shock applications, such as turbine engine assemblies or high pressure and high-temperature components of diverse systems. Manufacturing high integrity CNT reinforced YSZ composites is challenging. First, it is widely reported that the presence of CNT agglomerations in the YSZ ceramic matrix deteriorates the mechanical properties of the composite material. Additionally, conventional sintering methods often compromise the integrity of the CNT due to the composite material’s prolonged exposure to high sintering temperatures. Alternatively, additives which form a solid solution with YSZ can be introduced, such as Lanthanum Oxide (La₂O₃). The strengthening mechanism achieved with CNT or (La₂O₃) is unique and may contribute to different material response in different loading regimes. In the current study, YSZ composites reinforced with CNT (0.2, 0.5, and 1 wt.%) and (La₂O₃) (11.6 and 21.6 wt.%) were fabricated by the Spark Plasma Sintering (SPS) process. The influence of SPS process parameters (compaction pressure, sintering temperature, and time) on the densification behaviour and mechanical properties were investigated. The microstructure and selected mechanical properties of the as-sintered composites were characterized using X-ray Diffraction (XRD), optical microscopy, Scanning Electron Microscopy (SEM), Neutron Diffraction (ND ) and Vicker’s microhardness. The results show that an optimal combination of sintering temperature (1400°C), pressure (40 MPa), time (3 min), and the amount of CNT (0.5 wt.%) was required to fabricate CNT-YSZ composites with optimal density and hardness values. Additionally, the results indicated that (La₂O₃) addition to the YSZ matrix resulted in an increase in the hardness by 10-12 % if a homogeneous dispersion could be achieved. Neutron diffraction was seen as a valuable tool to measure the residual strain in the SPS composites. Additionally, neutron diffraction texture analysis indicated that SPS fabricated composites did not reveal significant texture, with only a few samples exhibiting elevated preferential orientation of grains when CNTs were present.

Text

2020-11-24

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/76573

Mechanical Engineering

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/