SiC and integrated assemblies for nuclear reactor applications

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Short communication

Fabrication of SiCf/SiC and integrated assemblies for nuclear reactor applications Amit Siddharth Sharma, Pipit Fitriani, Dang-Hyok Yoon

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School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Electrophoretic deposition Hot-pressing Toughness and toughening SiCf/SiC joining Integrated assembly

This study examined the processing feasibility of electrophoretic deposition (EPD), as an effective matrix infiltration method, in conjunction with hot-pressing to fabricate dense and tough SiCf/SiC materials. Flat and tubular specimens were fabricated by hot pressing at 1750 °C and 20 MPa after infiltrating a SiC-based matrix phase into Tyranno® SA3 SiC fabrics, which can be used as the core structural components for the next generation fission reactors and as blanket materials for future fusion reactors. For the tubular specimens, two types of preforms were compared: filament wound and jelly-rolled with different woven structures. The incorporation of SiC green tapes between the successive layers of SiC fabrics allowed better control over the composite density and pore distribution. The macro-architecture of the composites was optimized in terms of the slurry composition, sintering additives, and phase evolution. Fractography revealed considerable debonding at the SiCfiberPyCcoating interface and fiber pull-out with pronounced tail extension behavior when tested in flexure. Joining of the flat-tube and flat-flat SiCf/SiC was performed using a range of filler systems; the structures integrated using the Ti-based MAX fillers at 1750 °C and 3.5 MPa for 1–2 h soaking time are discussed.

1. Introduction The development of SiC fiber-reinforced SiC (SiCf/SiC) composites has attracted considerable attention on account of their excellent mechanical, chemical and thermal properties [1–7]. The low induced radioactivity of SiC under neutron irradiation highlights the potential of these composites as the structural components of the next generation fission and fusion reactors, which will require thermal stability at temperatures ≥ 1000 °C along with substantial mechanical properties [8]. Concerted efforts for the fabrication of these composites involve careful tuning of the key attributes, such as the structure, matrix chemistry, porosity, mechanical properties, and processing time [9,10]. A range of processing techniques have been used for the fabrication of these composites, including chemical vapor infiltration (CVI), polymer impregnation and pyrolysis (PIP), reactive sintering (RS)/melt infiltration (MI), and liquid phase sintering (LPS)/nano-infiltrated transient eutectoid (NITE) [9–12]. On the other hand, matrix densification, fiber degradation at high processing temperatures, and extended processing times are the key processing issues that limit the successful realization of any of these routes. In addition, the fabrication of hollow tubular SiCf/SiC requires a completely different approach with regard to the geometrical parameters, such as diameter, length, and wall thickness. As the mold used for flat SiCf/SiC fabrication cannot be

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employed for tubular SiCf/SiC, a separate cylindrical mold needs to be designed and fabricated. Commercially available SiC fabric woven from uncoated SiC fibers can be used as preform for flat and tubular specimens; however limited toughness, due to the absence of any weak coating, will be compromised [13]. The important desirable attributes for a tubular SiCf/SiC are lengths as large as possible with diameters preferably greater than 20 mm and a uniform wall thickness without cracks. As the next step in this research, the integration of simple geometries, such as flat and tubular, to achieve structures that can be employed in structural applications needs to be tested and extended for commercial purposes. This paper reports the salient features of a hybrid electrophoretic deposition (EPD) and hot pressing process to fabricate dense and tough SiCf/SiC. The adaptability of EPD for the infiltration of a SiC-based matrix into flat- and tubular-shaped SiCf/SiC preforms was studied. A mold for tubular SiCf/SiC with various design considerations was fabricated to obtain samples with the desired geometrical parameters. The flexural strength and fractography analysis were correlated to confirm the debonding tendency at the SiCfiber-PyCcoating interface and tail extension behavior indicating toughness. A joining procedure using green tape made from Ti-based MAX phase slurries was applied to obtain sound joints at various flat- and tubular-SiCf/SiCs to produce complex 3D geometries.

Corresponding author. E-mail address: [email protected] (D.-H. Yoon).

http://dx.doi.org/10.1016/j.ceramint.2017.09.126 Received 10 July 2017; Received in revised form 24 August 2017; Accepted 16 September 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Sharma, A.S., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.09.126

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Fig. 1. SE-SEM images of SiC fabric after EPD: (a) without ultrasonication, (b) with ultrasonication. (c) SiCf/SiC composite obtained after EPD and hotpressing. ‘+’ marks the location of the interfiber pores. M and F denote the matrix and fabric, respectively.

2. Materials and methods

3. Results and discussion

A SiC-based slurry comprising of β-SiC nanopowder (RNDKorea Corp., d50 = 52 nm) along with 5 wt% dispersant (Hypermer KD1, ICI), 10 wt% binder (PVB B-98, Solutia), and suitable sintering additives (10 wt% Al2O3-Y2O3 or Al2O3-Sc2O3 with respect to the β-SiC powder) dispersed in an organic solvent or water were prepared by ball milling for 24 h using SiC balls. This would ensure a homogeneous particulate dispersion with uniform infiltration through the inter-/intra-fiber pores in reinforcement fabric. The eutectic composition of the binary mixtures comprising the sintering additive was selected based on the standard phase diagrams [4,5]. The electrophoretic deposition of matrix particles into the SiC fabrics (0/90° 2D woven, Tyranno® SA3, Ube Industries, Japan) was performed using a dual electrode system under a DC or AC electric field after optimizing the zeta potentials of the particle constituents. Ultrasonic pulses (10 W, 20 kHz) with power density 10−2 W/mm2 were applied during DC- or AC-EPD to enhance the degree of matrix phase infiltration into the SiC fabric by preventing settling of the slurry and surface sealing in the SiC preform. Details of the EPD setup can be found elsewhere [14,15]. A 35 μm-thick SiC tape containing the sintering additives was also prepared using a table-top tape caster. Uniaxial lamination (10 MPa, 80 °C), debinding (350 °C, 2 h, air), and hot pressing (1750 °C, 20 MPa, 2 h, Ar) were performed for 20 layer stacks of alternating SiC fabric and SiC tape. Afterwards, the hot-pressed samples (dimension: 30 × 30 × 2 mm3) were then cut and polished for the three point bending test (bar dimension: 30 × 4 × 2 mm3) according to the ASTM C1161-13. Typically, 6–7 samples were subjected to a 3-point bending test under each condition to obtain statistically relevant and reproducible data, which were used to estimate the toughening behavior in the layered composites. Green tapes casted from MAX phase powders - Ti3AlC2 and Ti3SiC2 (Beijing Jinhezni Materials Co., Ltd., China) with a purity ≥ 98% and d50 ~ 32 and 21 µm, respectively were used for joining. The phase distribution, morphology, and fractured surfaces were examined by scanning electron microscopy (SEM; Hitachi S-4800). The density of the composites was measured using the Archimedes method whereas theoretical ones were estimated by rule of mixtures. The engineering drawings were generated using SOLIDWORKS® (Version 2010, Dassault Systemés SolidWorks Corporation).

3.1. Effective matrix infiltration The success and feasibility criteria of a SiCf/SiC fabrication process can be gauged by the extent of matrix infiltration in a commercially available SiC fabric, degradation of fibers in terms of the chemistry and structure, and the extent of fiber pullout. Because most studies used a 2D-woven SiC fabric with a 0/90° angular orientation between its warp and weft segments, each comprised of thousands of SiC fibers, several μm in-thickness, the infiltration of a SiC-matrix phase among the network of ‘interbundle’ and ‘interfiber’ pores of the SiC fabric is a critical step. As with existing techniques, the evolution of residual porosity due to inadequate and/or incomplete matrix infiltration is the main limitation for obtaining dense SiCf/SiC microstructures [11]. The applicability of EPD can be extended to fabrication of SiCf/SiC composites owing to reasonably high electrical conductivity of SiC because the EPD rests on administering the appropriate polarity to the SiC preform [16]. The infiltration of a chemically stabilized viscous slurry consisting of a SiC-based matrix into the fine voids of the SiC fabric can be a complex function of the slurry viscosity, applied voltage, and duration of the process. The viscosity of the slurry needs to be optimized to a level such that it can penetrate effectively in the pores but not settle to the bottom part of the preform after deposition due to gravity. If it is more viscous, the constituents of the slurry will block the pores in the preform once it is deposited without any further infiltration into the solid SiC fabric. This will result in a ‘surface sealing effect’ of the SiC fabric, which will severely limit further infiltration of the SiC-matrix. To counter this effect, the slurry needs to be agitated by an external source. For this purpose, ultrasonic pulses were applied during EPD. Fig. 1(a) and (b) presents cross-sectional SEM images of the SiC fabric after SiC-infiltration by EPD; (a) and (b) presents the sample without and with ultrasonication, respectively. The ultrasonication-assisted EPD process allowed more matrix infiltration, i.e., more in (b) than in (a) resulting in densities close to 90% and 95% of ρtheo without and with ultrasonication cases, respectively. The ‘unfilled’ interfiber pores, marked by ‘+’, will eventually result in closed porosity in the final composite. Fig. 1(c) presents a cross-sectional image of the SiCf/SiC after hot-pressing, showing fiber (F)- and matrix (M)-rich regions. SiC bundles running parallel and normal to the plane of paper were also evident (Fig. 1(c)).

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a larger flat (200 × 200 mm2) and tubular (inner diameter Φ = 20 mm, h = 50 mm, wall thickness = ~3 mm) specimens [15]. Larger EPD cells produced from Teflon with rectangular and cylindrical symmetry were designed separately for these specimens, whereas the electrodes were made from stainless steel. For the tubular specimens, two different preform structures, jelly-rolled and filament wound, were formed based on the number of rolled layers and the presence of a PyC coating. Formation of jelly-rolled structure, similar to the stacking formation for flat samples to produce layered composite, consist of winding 15 layers of SiC preform around a solid cylindrical graphite core of diameter 20 mm (Fig. 2(a)), whereas the filament wound preform used two spools of PyC-coated SiC fibers at an inter-ply angle of 55° to produce a single-layer preform. The slurry was infiltrated into the respective SiC preforms, dried, and alternate layers of SiC tape and infiltrated SiC fabric were then stacked and debonded [17]. Sintering of the tubular specimen was conducted in a specially designed graphiticsplit mold with pressure applied/transferred through graphite powder. Multiple ‘filling-cold pressing’ cycles was performed to ensure highdensity powder packing so that the top punch cannot move any further inside the mold; the whole assembly was then loaded inside the furnace (Fig. 2(a)) [17]. The geometry of this mold was optimized so that during hot-pressing, maximum transfer of the applied uniaxial pressure into the radial direction alongthe length of the tube can be facilitated. The inner surface of the bottom part of the mold and punch was angled with respect to the inner walls to impart the maximum pressure in an outward to inward manner using graphite powder, which was packed tightly around the SiC preform and acted as a ‘pressure-transfer medium’. Fig. 2(b) presents photomicrographs of the sintered tubularand flat-SiCf/SiC composites.

Fig. 2. (a) Graphitic split-mold for the fabrication of tubular specimens (left) with jellyrolled sample (right), and (b) sintered tubular- and flat-shaped SiCf/SiC.

A sufficiently larger volume of the SiC-matrix (ascertained by comparing weights before and after deposition) can be made to infiltrate into the fine voids of the SiC fabric using an ultrasonication-assisted EPD process. In addition, hot-pressing at 1750 °C and 20 MPa does not distort the initial circular cross-section of the SiC fibers.

3.3. Energy dissipation and toughening behavior The ‘toughening’ tendency of these layered ceramic composites can be predicted based on the tail extension behavior when tested in bending. The profuse debonding and fiber pull-out activity during fracture is a direct consequence of the energy dissipating tendency through the ‘weak’ interphases around the SiC fibers. The fractographs

3.2. Fabrication of different geometries The scalability and adaptability of the hybrid EPD and hot-pressing process was tested further by extending the developed methodology to

Fig. 3. SE-SEM fractographs: (a, b) planar and (c, d) tubular SiCf/SiC. (a, c) shows the general views, whereas the (b, d) locations show extensive coating debonding and fiber pull-out. The arrows in (c) mark the 55° relative orientation between fiber bundles.

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Fig. 4. Flexural stress-strain plot for SiCf/SiC. Various transitions have been roughly marked as I, II and III.

in Fig. 3(a)–(d) show the general topography of the fractured samples for flat (a–b) and tubular (c–d) SiCf/SiC with a 600–800 nm thick PyC coating. For the flat composites, the fractured surface (Fig. 3(a)) showed considerable ledge formation in each layer of the SiC fabric because 20 layers of SiC-infiltrated SiC fabric were stacked together. In addition, each SiC layer accommodated a larger fraction of the pulledout SiC fibers. Similarly, for the filament-wound tubular composite (Fig. 3(c)), both fiber bundles oriented at 55° in the single layer showed extensive fiber pull-out. The weak PyC layer serves as energy absorbing sites, which allows a crack to propagate, as shown by the extensive PyC rupturing marked by arrows in Fig. 3(b) and (d). The typical flexural stress versus strain plot (Fig. 4) of the SiCf/SiC bars carved out from the flat and tubular geometries exhibit strength values in the range of 350–400 MPa along with an extended tail behavior accompanied by an arrest of the sudden fall of the curve from the peak values.

Fig. 5. (a) SEM image of the SiCf/SiC joint using Ti3AlC2 filler tape and (b) photomicrograph of the flat samples after machined-out grooves with visibly no cracking.

multichannel structure was performed by carving out parallel rectangular grooves onto 3 flat SiCf/SiC samples to place the SiCf/SiC bars and fillergreen tape for joining (Fig. 5(b)). This machining of grooves, even though not cost effective, was performed to support the notion that fabricated composites possess enough toughness/strength to withstand the shock imparted by the machining tool. Also the grooving served the purpose of maintaining the precise spacing between successive bars. To join flat SiCf/SiC with tubular SiCf/SiC, a centrally-located circular hole with a diameter equal to the diameter of the tube (Φ = 20 mm) was drilled in the flat composite (Fig. 6). A through-thickness hole in the flat plate was not preferred to support the tube onto the flat and produce space for placing the filler material. Instead a step was formed with a thickness equal to that of the wall-thickness of the tube and height around half of the plate thickness. Fig. 6 presents the integrated assemblies obtained after performing pressureless joining with MAX phase tape as filler. Two flats were joined by a hollow SiCf/SiC tube, as shown in Fig. 6(a), with the supporting graphite rods at the four corners; the joints are marked. The multichannel structure shown in Fig. 6(b) has a dual layer of channels with rectangular cross-sections running from one end to the other. This structure will allow unidirectional fluid flow. By extending this joining methodology, a similar multilayered structure instead of a dual layer can also be fabricated.

3.4. Fabrication of integrated geometries For commercial applications, such as heat exchangers, flat- and tubular-SiCf/SiCs need to be integrated to fabricate components and assemblies. A sound joint is characterized by a pore- and crack-free interface with a homogenous distribution of phases in the joint region. High temperature stability and strength along with gas tightness are other important aspects that need to be considered. Consequently, a flat-tube and flat-flat SiCf/SiC were designed to obtain multichannel geometry for fluid transport applications. Different filler materials were selected based on their joining temperature, wetting affinity towards the base SiC substrates, and phase stability. Finally, joining of the SiCf/ SiC was carried out for selected filler materials, i.e., pre-ceramic polymers (polycarbosilane and polysilazane) with active fillers, β-SiC nanopowder with the appropriate sintering additives, MAX-based phases (Ti3AlC2 and Ti3SiC2) in the forms of powders and green tape (thickness ~60 µm). To mimic the conditions for pressureless joining, a nominal pressure of ~3.5 MPa was applied to the joining assembly to hold the components intact. A controlled temperature increase until the final limit of 1750 °C and a soak time of 1–2 h was optimized to produce sound joints [18]. Microstructural examination (Fig. 5(a)) and phase assemblage of the joint interface can be explained fromthe theoretical phase stability by calculating the Gibbs free energy of formation and correlated with the joint strength estimated using 3- and 4-point bending tests [18]. The joining mechanism can be attributed to the decomposition of Ti3AlC2 phase to yield Ti, Al and TiC phases followed by the solid state diffusion of these phases into the base composites. High temperature plasticity of Ti3AlC2 phase ensured pore-and crackfree joints at the base/filler/base interfaces with a joining strength close to 161 ± 12 MPa [18]. A machining schedule for a dual layer

4. Closing remarks and future directions The research related to the ‘fabrication of SiCf/SiC and integrated assemblies’ encompasses many important facets starting from composite fabrication by optimization of EPD and slurry parameters, tailorable high densities and toughening behavior, process extension to fabricate larger flat samples and adaptability to produce tubular SiCf/ SiC. Further efforts were directed to produce sound SiCf/SiC joints using different filler systems; Ti-based MAX (Ti3AlC2/Ti3SiC2) phase fillers were found to be quite promising. Therefore, an optimized pressureless joining schedule was developed and found to be applicable for 4

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Fig. 6. Complex assemblies produced by joining: (a) tube-flat and (b) flat-flat SiCf/SiC. Channels in the dual layer multi-channel structure are marked by arrows.

producing integrated assemblies. In particular, ultrasonication-assisted EPD can be used successfully for efficient SiC-based matrix infiltration in a SiC fabric. Around 15–20 layers of SiC tape and SiC-infiltrated SiC fabric for both the flat and tubular specimens were stacked. Subsequent hot-pressing at 1750 °C, 20 MPa for 2 h produced dense microstructures (~ 94–98% of ρtheo) without any degradation of SiC fibers. The EPD process can be adapted further to produce flat- and tubular- SiCf/SiC. A better response in flexure was also observed in the composites in terms of the debonding at coating-fiber interface, fiber pullout activity as well as the tail extension behavior. This can also be confirmed from intricate machining of the composites performed using conventional techniques. Fillers based on Ti-based MAX tape were used for pressureless joining at 1750 °C to produce complex integrated assemblies. Efforts are currently underway to diagnose the damage resistance of as-fabricated SiCf/SiC under ion- and proton-irradiation as well as SiCf/ SiC joints produced using MAX and preceramic polymer-based fillers. A comparative assessment of the microstructural defects and the variations in mechanical properties of pre- and post-irradiated composites will be a primary subject in ongoing studies.

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Acknowledgements

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The authors would like to express their gratitude for the grant received under Basic Science Research Program funded by the Ministry of Education (Grant no. NRF-2015R1D1A1A09056751).

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