Elsevier

Advanced Powder Technology

Volume 31, Issue 8, August 2020, Pages 3657-3666
Advanced Powder Technology

Original Research Paper
Fabrication of W-Cu functionally graded composites using high energy ball milling and spark plasma sintering for plasma facing components

https://doi.org/10.1016/j.apt.2020.07.015Get rights and content

Highlights

  • W-Cu FGCs up to six layers were fabricated by high energy ball milling and spark plasma sintering.

  • All the W-Cu FGCs exhibited a gradual change in hardness, coefficient of thermal expansion and modulus.

  • The thermal conductivity of W-Cu composites increased with increasing Cu content.

  • W-Cu FGCs showed superior performance over directly bonded W-Cu sample in thermal cycling tests.

Abstract

W-Cu functionally graded composites (FGCs) up to six layers have been developed using high energy ball milling and spark plasma sintering (SPS) at a lower temperature of 900 °C. The relative density of W-Cu composites increased from 85.4% (W80Cu20 layer) to 95.7% (W20Cu80 layer) with increasing Cu content. All the W-Cu FGCs exhibited a graded structure even after SPS and showed a gradual change in hardness, Young’s modulus, and coefficient of thermal expansion (CTE). Furthermore, W-Cu composites showed a CTE and modulus between those of W and Cu and could be used as an intermediate layer between W and Cu in plasma facing components. The thermal cycle testing at 800 °C has confirmed that the W-Cu FGCs developed in this study can withstand thermal shock and showed a superior performance over directly bonded W-Cu sample. The W-Cu FGCs developed in the present study are not only suitable for plasma facing components but can also be used where the thermal stresses are introduced due to the large mismatch in CTE or elastic modulus.

Introduction

Nowadays, fusion power reactor has attracted tremendous attention as a future source of energy due to its safe, green and unlimited energy source. The biggest challenge is the development of plasma facing components for diverter region, which should withstand a surface temperature of 1500 °C and heat flux of at least 10 MW/m2 or higher [1], [2]. Tungsten (W) was selected as a potential candidate for armour material because of its superior thermo-physical properties such as high irradiation resistance, high melting point and low coefficient of thermal expansion (CTE) [3], [4]. Copper (Cu) and its alloys (Cu-Cr-Zr) are promising candidates for heat sink material due to their high thermal conductivity [5]. However, direct bonding of these two materials lead to high thermal and residual stresses at the interface due to the large mismatch of the CTE (αW: 4.59 × 10−6, αCu: 16.5 × 10−6/ °C) and elastic modulus (EW: 410, ECu: 128 GPa) of W and Cu [5], [6]. Therefore, these stresses can lead to cracking and delamination of the joint between W and Cu. Pintsuk et al. [7] observed a microcrack formation at the W-Cu interface after performing a high heat flux test for 1000 cycles at 20 MW/m2.

In order to reduce interface failures due to mismatch of the CTE and elastic modulus, it is recommended to use the W-Cu functionally graded composites (FGCs) as an interlayer between W and Cu. This feature has been demonstrated in many numerical simulations [8], [9]. Greuner et al. [10] performed a high heat flux test on three-layered W-CuCrZr FGC samples, all of which withstood heat flux up to 20 MW/m2 without cracking or delamination at the interface. Several processing methods have been reported to develop W-Cu FGCs, including activated sintering [11], chemical vapor deposition [12], explosive joining [13], high gravity combustion synthesis, melt-infiltration [14], hot pressing [15], infiltration [5], [10], [16], mechanical alloying [17], microwave sintering followed by infiltration [18], plasma spraying [19] and resistance sintering [20].

As most of these methods have been carried out at high temperature and high-pressure, Cu melting occurs. This leads to diffusion of Cu and is difficult to maintain the graded structure of W-Cu FGC. Therefore, a solid-state processing technique such as powder metallurgy-based techniques are better suitable for developing W-Cu FGCs because they can be carried out at lower temperatures. However, it is difficult to produce high-density W-Cu FGCs with uniform microstructure using conventional sintering techniques because of the large difference in melting point (W: 3420 °C, Cu: 1084 °C) and the immiscible behaviour of W-Cu system due to positive heat of mixing + 35.5 kJ.mol−1 [21]. Researchers developed FGCs using sintering aids to improve the sinterability of W-Cu FGCs [11]. However, the use of sintering aids reduces the overall thermal conductivity of W-Cu FGCs.

Another way to improve the sinterability of W-Cu composites is to use nanocrystalline and well-mixed powders. Ryu et al. [22] found that ball milled W-Cu nanocrystalline powder has an enhanced sinterability compared to simple mixed powder with coarse grains. The authors group demonstrated sintering of pure W in nanocrystalline form at temperatures as low as 1500 °C [23], [24], while microcrystalline W is usually sintered at about 2800 °C. Spark plasma sintering (SPS) is a promising technique to develop novel materials such as FGCs with improved properties at lower temperature and pressure with shorter sintering times as compared to conventional sintering techniques. Therefore, the synthesis of nanocrystalline W-Cu powders using high energy ball milling and consolidation by SPS is a suitable method to produce W-Cu FGCs without the addition of sintering aids. However, there are few reports available in the open literature on the development of W-Cu FGCs using SPS. Yusefi et al. [25] developed a three-layered FGM (W-W50Cu50-Cu) using SPS and showed a 40% and 8.66% improvement in hardness and relative density, respectively, compared to the compacts synthesized by conventional sintering technique. Tang et al. [26] also fabricated three-layered FGM (W75Cu25-W50Cu50-W25Cu75) using SPS, but without a pure W and Cu layer.

However, majority of the researchers directly consolidated W with the layers of the W-Cu powder at a lower temperature (1000–1050 °C) compared to the sintering temperature of W, which obviously resulted in a lower density and hardness [25], [26], [27]. Although, Autissier et al. [2] used pre-sintered W, it took a longer sintering time of 40 min to obtain good interfacial bonding strength between the W and W80Cu20 layer, due to the small contact area between the layers. The weak localized Joule heating in the porous structure also a plausible reason. Joule heating is considered to be the predominant mechanism in SPS and creating a high localized Joule heating at the W/W-Cu/Cu interfaces can help to achieve good interfacial bonding strength with a shorter sintering time. The high localized Joule heating can be achieved by increasing the contact area at the interface, since contact points are the areas with high resistance [28], [29]. In the present study, we employed cold compacted layers of W-Cu composites instead of elemental powders along with a pre-sintered W layer in order to provide a high contact area at the W/W-Cu/Cu interfaces during the SPS. This has helped us to maintain the stoichiometry of W-Cu composition in all the layers. To the best of our knowledge, this approach is not reported for W-Cu FGCs so far.

The objective of the present work is to develop W-Cu FGCs (with a good interfacial bonding strength) for plasma facing component in a future fusion reactor. The ball milling (milling time) and SPS parameters (temperature, sintering time, and pressure) of W and W-Cu composites were optimized to minimize the contamination and maximize the composite density. The optimized parameters were used to develop W-Cu FGCs. Finally, the relative density, hardness, elastic modulus, thermal conductivity, CTE and thermal shock resistance of W-Cu FGCs were investigated.

Section snippets

Experimental details

W powder (purity 99.9%) with a particle size of 10 µm and an electrolytic Cu powder (purity 99.5%) with a particle size of 16 µm provided by Loba Chemie Pvt Ltd, were used as starting materials. To optimize the milling time, the ball milling of W was performed using a high energy planetary ball mill (Fritsch P-5, Germany). The milling of W was performed with varying ball milling time for 0, 5, 10, 15 and 20 h by keeping all other parameters constant. Ball to powder weight ratio (BPR) of 10:1

Optimization of ball milling and SPS parameters

XRD patterns of unmilled W and that milled for various period of times are shown in Fig. 2(a). The XRD pattern of W milled for 15 h sample shows small peaks of WC contamination and it increases with milling time. The previous work by the author’s group [30] also reported WC contamination in 20 h milled W sample, but not in 5 h milled sample. The peak broadening was observed with milling time due to lattice strain and crystallite size refinement during the milling. The effect of milling time on

Conclusions

In this study we developed a W-Cu FGCs containing 3, 4, 5 and 6 layers with improved thermal shock resistance by high energy ball milling and SPS at a lower temperature of 900 °C under a pressure of 50 MPa during 10 min. The ball milling parameters of 5 h, 300 rpm and 10:1 BPR were used for synthesizing nanocrystalline W and W-Cu powder. The SPS parameters of 1600 °C, 100 °C/min, 50 MPa and 10 min were used for fabricating pure W layer. The excessive localized Joule heating at the W/W-Cu/Cu

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was carried out with financial assistance from the Government of India, Department of Atomic Energy (DAE), Board of Research in Nuclear Sciences (BRNS) through the Sanction No# 39/22/2015-BRNS. The authors are thankful to Dr. Anirudha Karati (former Ph.D. student, IIT Madras) for his assistance in TEM imaging. The authors also thank Mr. Aroh Shrivastava (Scientific Officer-E, Institute for Plasma Research) for his assistance in thermal conductivity measurements and Mr. P. Priyesh

References (37)

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Present address: School of Engineering, The University of British Columbia-Okanagan, Kelowna, BC V1V 1V7, Canada.

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