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128
Chapter 7 Summary and Recommendation
for Future Development
129
7.1 Summary
Radiation hardening is a phenomenon that can cause metals to become brittle over time. In order
to improve the brittleness of tungsten (W) and molybdenum (Mo), a novel method of toughening
metals based on SiC-based fibers reinforcement is proposed. This method is intended for use in
structural materials that are utilized in thermonuclear fusion reactors. The objective of this thesis
is to develop new composites that are reinforced by SiC-based fiber in order to exhibit ductile
behavior even after irradiation. The toughening mechanism of these composites is similar to that
of fiber-reinforced ceramic matrix composites, which results from the debonding and friction that
occurs at the fiber/matrix interface. The composites were fabricated using hot press technology. In
order to optimize the fiber reinforced composite, this study investigated the effect of fiber types,
matrix, and sintering temperature on mechanical and thermal properties, from both in-plane and
through-plane directions, as well as microstructure. Two sintering methods were used to sinter the
2D woven SiC fiber, with and without a powder sheet. The study also explored the potential
diffusion barrier between SiC and brittle metals. In the following, the main results of this study
will be discussed.
Effect of sintering temperature on properties and microstructure
Unidirectional (UD) Hi-Nicalon type S SiC fiber was used to reinforce W composites
manufactured by hot press successfully. The stress-strain curves obtained at room temperature,
lower than the ductile-brittle transition temperature (DBTT) of W, showed obvious pseudo-ductile
behavior for specimen sintered until 1700 °C. So SiC fiber as reinforcement to strengthen W is
feasible. But the ultimate tensile strength (UTS) is low and the highest value is 82.9 MPa in
composite sintered at 1700 °C. Considering thermal conductivity, higher sintering temperature led
to higher thermal conductivity. In addition, it is much higher in in-plane direction than throughplane direction. The composites sintered at 1700 °C showed the largest strength with pseud
ductility. Therefore, the suitable sintering temperature to fabricate SiC/W composite is 1700 °C.
Effect of matrix on properties and microstructure
Metal foils were used as matrix in this work. W foils with 0.05 mm and 0.08 mm thickness
and Mo foil with 0.08 mm were selected. The recrystallization temperature of foils was examined
by annealing at high temperature. For W foils, the temperature is between 1200 °C and 1400 °C
130
independent of their thickness. In addition, the temperature of Mo foil is between 1000 °C and
1200 °C. All foils have recrystallized during sintering. Therefore, there is no contribution for
ductility of composites from foil. Continuous unidirectional SiC fiber reinforced W composites
with 0.05 mm or 0.08 mm foil as matrix were sintered at 1700 °C, and reinforced Mo composite
with 0.08 mm foil was sintered at 1500 °C successfully. The tensile test implies that the W based
composites obtained with 0.08 foil showed better apparent pseudo-ductile behavior due to the short
pull-out fiber and foil with higher ultimate tensile strength (UTS) of 197 MPa. In addition, the
pseudo ductility can also be found in Mo-based composite with the UTS of 109 MPa, worse than
it of W composite. In addition, the thermal conductivity of Mo composite is also lower than W
composite. Therefore, the best composite is unidirectional SiC/W composite with 0.08 mm foil.
Besides, it is demonstrated that composite with suitable interface or diffusion barrier show better
property.
Effect of fiber types on properties and microstructure
Three kinds of fiber, woven SiC fiber, unidirectional SiC fiber, and 2D SiC and W fiber
reinforced W composites with foils were prepared. For SiC woven fiber by two methods, the
results show that the method without powder sheet is more appropriate than that with powder sheet
due to composite by powder sheet with foils delaminated during machining. In addition, all
composites displayed pseudo-ductile behavior at room temperature. In addition, the strength of
both composites by both 2D fibers were higher than the 50 % strength of UD composite, because
only half fibers work in composites reinforced by 2D fibers. Therefore, 2D fibers are better than
UD fiber. For thermal property, the composite reinforced by 2D W/SiC fiber shows higher
conductivity at high temperature especially in through-plane direction compare with composites
without W wire by introducing W wire with high conductivity. Therefore, it can be speculated that
SiC/W woven fiber might be more suitable combined the thermal property and mechanical
property.
The potential diffusion barriers between metals and SiC
Seven materials (oxides (ZrO2, TiO2, and Er2O3), carbides (ZrC and TiC), and nitrides
(ZrN and TiN)) produced by dipping method or sputtering method as diffusion barrier in W and
SiC system were investigated by joined with W foils to select a proper interface for SiC fiber
reinforced W composite. And for the dipping method after 5 times dipping, the Er2O3 coated CVD131
SiC joined with W showed the thinnest reaction zone. In addition, 29.5 µm thick Er2O3 can work
until 1950 °C. Er2O3 was also used in SiC/Mo joint, and it works at least 1800 °C. Unidirectional
SiC fiber reinforced W with 0.08 mm foil with Er2O3 composite was sintered at 1700 °C. The SiC
fiber can be protected well, but the pseudo ductility reduced due to strong interface between fiber
and matrix. After adding C, the pseudo ductility can also be found. However, the strength and
thermal conductivity is lower than composite without diffusion barrier. Therefore, the most
appropriate interface in composite needs to be studied further.
7.2 Recommendation for future development
In this work, a novel toughening method for brittle metals, retaining toughness even under
room temperature successfully, which is the embrittlement conditions, was proposed and
developed based on the reinforcement of SiC fiber. The best composite in this work is (UD)
SiCf/W with 0.08 mm foil without diffusion barrier fabricated by method without powder sheet at
1700 °C by hot press. Because 1700 °C is appropriate temperature to balance between high density
to obtain high strength and weak interface to acquire pseudo ductility. Fiber reinforcement can
ameliorate the brittle metals, and show pseudo-ductility at room temperature, lower than their
DBTT. Therefore, there is no need to care about the radiation embrittlement. In addition, it can be
assumed that pseudo-ductile behavior will be obtained in the composite even if metal as matrix is
brittle caused by neutron irradiation. This work provides a guideline to improve the toughness of
brittle metals as a structural material in fusion reactor. However, strength was sacrificed in some
extent caused by the reaction between W and SiC. For working as diffusion barrier, Er2O3 is
suitable. However, it is not suitable to contact to SiC directly, an inner thin (~200 nm) pyrolytic
carbon (PyC) before coating Er2O3 is required. In addition, dipping method to coat Er2O3 on fiber
is not appropriate. Chemical vapor deposition (CVD) might be suitable. Moreover, another idea is
to change the fabrication method from hot press to CVD by reducing the sintering temperature or
to spark plasma sintering by reducing the holding time. In addition, it is necessary to carry out the
neutron irradiation experiment after optimizing the composite.
132
Publication list
Y. Du, B. Wang, Y. Zhong, T. Hinoki, Assessment of the Potential Diffusion Barriers
between Tungsten and Silicon Carbide for Nuclear Fusion Application, Coatings 2022, Vol. 12,
Page 639. 12 (2022) 639. https://doi.org/10.3390/COATINGS12050639.
Y. Du, T. Hinoki, Effect of Sintering Temperature on Properties of SiC Fiber Reinforced
Tungsten
Matrix
Composites,
Mater.
Trans.
63
(2022)
1550–1556.
https://doi.org/10.2320/MATERTRANS.MT-M2022043.
Y. Du, T. Hinoki, Effect of tungsten matrix on the mechanical property of SiC fiber
reinforced tungsten composites with foils fabricated at 1700 °C, Nucl. Mater. Energy. 31 (2022)
101142. https://doi.org/10.1016/j.nme.2022.101142.
Conference list
1.Yina Du, Bo Huang, Kanjiro Kawasaki, Fujio Shinoda and Tatsuya Hinoki，Effect of tungsten
matrix on mechanical properties of SiC fiber reinforced W composite; 2020.9 AESJ, Oral.
2.Yina Du，Tatsuya Hinoki，Thermal stability of tungsten and SiC with or without diffusion
barrier (ZrN and TiN) at 1700 °C, 2021.3 AESJ, Oral.
3.Yina Du，Tatsuya Hinoki，Effect of Er2O3 interphase on SiC fiber reinforced W composites；
2021.9 AESJ. Oral.
4.Yina Du，Tatsuya Hinoki, Effect of the Thickness of Tungsten Foil on Mechanical Property of
SiC Fiber Reinforced W Composites；The International Conference on Fusion Reactor Materials
(ICFRM-20), 2021.10 AESJ. Poster.
133
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