Silicon carbide for nuclear fusion applications

In developing designs for real fusion devices silicon carbide is being considered for multiple uses, including being a structural material. Its performance under the extreme fusion environment, with the added difficulties involved with neutron damage, must be known and mechanical properties quantified for designs to become a reality. This work has directly addressed this issue, by looking at ion irradiated materials and their mechanical properties up to operationally relevant fusion temperatures. By focussing on how these properties change to understand tolerances and behaviours in operationally relevant environments this project has produced some key results in understanding both SiC and radiation damage in ceramics.
Commercially available forms of silicon carbide, as well as Transient Eutectic-Phase (TEP) fabricated materials have been subjected to silicon ion irradiations to peaks of 0.24 and 2.4dpa, at 300◦C and 750◦C. An additional neon ion implantation was performed to 2.4dpa at 300◦C. Various materials were fabricated using the TEP method, focussing on the changes in properties between monolithic and composite samples with varying percentage weights of sintering agents. Changes in mechanical properties were then measured using nanoindentation and high temperature nanoindentation.
Consistent increases in hardness and decreases in elastic modulus were seen across all material types, under all irradiation conditions when tested at room temperature. In a direct sintered sample the largest increase in hardness and smallest decrease in modulus was seen under the high dose, high temperature condition, with the smallest increase in hardness and largest decrease in elastic modulus seen after the low temperature, low dose implantation. The neon implantation and silicon implantation under the same conditions showed almost identical changes in the direct sintered material. In the fabricated materials, the composite sample produced with 5%wt sintering agent was the most consistent across all irradiation conditions, while the 10%wt composite sample exhibited the largest increase in hardness of ∼ 30% in the high dose, low temperature condition, the 10%wt monolithic sample showed the largest reduction in modulus of ∼ 19% under the same condition. Distinct changes in optical properties were seen under all irradiation conditions, alongside suppression of surface fracture around indentations, and the mechanisms driving this combination of effects explored. Sub-surface indentation fracture was significant in virgin and irradiated material.
High temperature nanoindentation up to 675◦C on high dose, low temperature direct sintered material showed large reduction in elastic modulus and hardness with temperature, while the percentage increase of hardness in the irradiated material was maximum at the irradiation temperature. Elastic modulus which had shown a reduction with irradiation at room temperature showed increases in comparison to the unirradiated materials at temperature. Both hardness and elastic modulus plateaued above ∼ 300◦C in the unimplanted material.
The direct comparison between materials, ion irradiation dose, ion irradiation temperature, and nanoindentation temperature in this work has directly addressed the absence of this in the field, especially with regards to how higher temperature observations show more realistic properties for in-service components. This in combination with the correlation of the changes in fracture behaviour and optical properties with these different experimental variables gives a much more comprehensive understanding of the changes in mechanical properties and the mechanisms behind them due to irradiation damage.

Keywords: Hot Nanoindentation
Micromechanics
Self-ions
Material Fabrication
Ion Irradiation
Nanoindentation
Composite Fabrication
Radiation Damage
Advisor: Armstrong, D
Roberts, S