Young’s modulus and hardness of pyrolytic carbon coatings used for containment of nuclear fuels Huixing Zhang, Eddie.Lopez-Honorato, Athar Javed, Jill Meadows, Ping Xiao Material Science Centre, School of Materials, the University of Manchester, M1 7HS, UK

Application in TRISO coating The main purpose of the outside three layers is stop all the fission products from releasing. It’s important to keep their integrity! The reason PyC to fail is due to the anisotropy deformation. 2

Contents PyC coatings were deposited on spherical particle using in the TRISO fuel Deposition temperature was varied from 1250 - 1450 °C Mechanical properties were measured using Nanoindentation Microstructure was shown in as deposited, and heat treated PyC coatings, using SEM, XRD and TEM.

PyC coatings preparation and properties PyC coatings density under different deposition conditions Orientation angle Domain size La (in graphene plane) and Lc (c-direction)

PyC coatings preparation and properties (Continued) PyC orientation angle and density PyC domain size and density relationship

Mechanical properties in as-deposited PyC Density effect Low density PyC, the microstructure difference does not affect theYoung’s modulus. (c) (a) (b) (a) (b) (c) 2 μm Domain size: 1.7 nm domain size: 1.1 nm domain size: 1.1 nm

Density effect (continued) (a) However, in high density region, the mechanical properties do not correlate directly with the density. Three kinds of microstructure were found in high density PyC. In low density PyC, both the Young’s modulus and hardness increase with the density linearly. (b) (a) (b) (c) 2 μm Laminar Granular Mixture of granular and laminar

Orientation angle, domain size and microstructure effect The Young’s modulus and hardness in high density PyC are not dominate by any single element, e.g. density, orientation angle and domain size. The only trend is: Young’s modulus and hardness increase with microstructure change from granular to laminar, and then to mixture structure. Since the particle bonding area and bonding strength dominates the mechanical properties of porous material [1,2]. We presume that in laminar structure the bonding area is bigger than in granular structure, while granular structure has stronger bonding strength. There is a maximum point where the maximum bonding area and strength could be reached, then the mechanical properties are maximum. [1] J.L.Kaae, Carbon, 1971, Vol. 9, pp. 291-299 [2] J.C.Bokros and R.J.Price, carbon, 1966, vol 3, pp, 503-509

Thermal treatment effect on mechanics and structure Young’s modulus and hardness High density Low density Hardness 1300 High density sample mechanical properties decrease dramatically; Young’s modulus 1300 Slightly changes could be seen in low density PyC; the mediate density specimen are experiencing the decrease.

Domain size change Laminar Graphitization process in dense PyC is quicker than in porous structure, because porous structure has stronger cross link than dense PyC. 1300 1300 The increase of Lc in laminar structure is more than granular and mixture structure PyC;

Microstructure change In high density laminar structure PyC 1250 C 50% Acetylene 200 μm Microstructure change is not uniform through the whole layer in laminar structure PyC. After 1800 C heat treatment As-deposited 2 μm

Microstructure change (continued) In high density granular structure PyC, 1350 C 33% Acy/Prop Before heat treatment After heat treatment 2 μm SAED with 200 nm aperture; OA: 43° SAED with 200 nm aperture; the crystal phase nucleation, did not see the diffraction angle change.

Summary With low density (<1.9 g/cm3), the Young’s modulus and hardness increase with the increase of the density linearly. In high density PyC, the laminar and granular microstructure are more likely to dominate the mechanical properties; they are not the function of single element, such as density, domain size and orientation angle. Thermal treatment promotes the crystalline process in high density PyC, which decreased the mechanical properties dramatically. Non-uniform microstructure change was found in laminar structure high density PyC after heat treatment. 13