Contents
1. Introduction..................................................................................................... 2
2. Interface properties and characterization of REBCO CC...................................................................... 2
2.1. Experimental characterization................................................................................... 2
2.2. Theoretical and numerical methods............................................................................... 6
3. Failures related to interface.......................................................................................... 8
3.1. Interface failure in individual tapes............................................................................... 8
3.2. Interface failure in windings.................................................................................... 9
4. Optimization strategies............................................................................................. 10
4.1. Micro material process level.................................................................................... 10
4.2. Macro structural level......................................................................................... 11
5. Challenges and future outlook........................................................................................ 13
6. Conclusion...................................................................................................... 13
Declaration of Competing Interest..................................................................................... 14
Acknowledgements................................................................................................ 14
References...................................................................................................... 14
1. Introduction
Superconducting materials exhibit unique characteristics such as zero electrical resistance, ideal diamagnetism, and the Josephson effect. Since their discovery, they have garnered significant attention and continuously attracted research interest across various scientific disciplines. Their development has revolutionized high-tech industrial applications in extreme environments, as evidenced by the International Thermonuclear Experimental Reactor (ITER) project, one of the largest and most important international research collaborations [1], [2].
Since the discovery of YBaCuO high-temperature superconductors [3], [4], the significant advantages of their high critical magnetic field, high current density, and superconductivity at liquid nitrogen (LN2) temperatures have attracted significant research interest in the slitting-edge science and advanced technology fields. Particularly in high-energy physics and high-field science, these materials have vast potential for applications [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16].
Significant progress has been made in the industrial-scale production and application of RE-Ba-Cu-O (REBCO, where RE = Y, Gd, Sm, and other rare earth elements) coated conductor (CC) tapes, also known as second-generation (2G) high-temperature superconducting (HTS) tapes. Compared to conventional low-temperature superconducting wires and first-generation Bi-based (BiSrCaCuO) HTS tapes, REBCO CC tapes exhibit higher critical current ($I_c$), critical magnetic field, and mechanical strength [17], [18]. Their fabrication techniques mainly involve ion beam-assisted deposition and metal–organic chemical vapor deposition. Currently, these materials are commercially produced by various companies, including Superpower [19] and AMSC [20] in the United States, SuNAM [21] in South Korea, Shanghai Superconductor Technology [22] in China, SuperOx [23] in Russia, Fujikura in Japan [24], and Theva [25] in Germany. As illustrated in Fig. 1, REBCO CC tapes exhibit layered composite structures with unique mechanical, thermal, and electromagnetic properties.
Fig. 1. Schematic structures of typical REBCO CC tapes. |
The high-strength Hastelloy substrate enhances the axial tensile strength of REBCO CC tapes, enabling them to withstand circumferential stresses of over 700 MPa under high-magnetic field and high-current conditions [26], [27]. However, the layered composite structure limits its mechanical properties in the thickness direction. Numerous tests have revealed that the transverse delamination strengths of 2G HTS composite tapes under transverse tensile loads are substantially lower than their axial tensile strengths. Moreover, the composite layers’ disparate electromagnetic, thermal, and mechanical properties, coupled with the thermal mismatch stress that occurs during cooling and electromagnetic forces under high magnetic fields, can lead to delamination damage in the tapes. This reduces the mechanical load-bearing capacity, irreversibly degrades the superconducting current-carrying performance, and even leads to the failure of tapes due to electromechanical degradation [28], [29].
This study provides a comprehensive overview of the interface failure behavior of REBCO CC tapes and their structures in extreme multifield environments. It also presents novel interface property characterization methods and optimization strategies to mitigate interface failure risks. The findings are intended to provide a reference for fabricating HTS materials and designing advanced superconducting devices. They also aid researchers in related fields in broadening their perspectives and staying updated on the latest research advances.
2. Interface properties and characterization of REBCO CC
The interfacial properties and characterization methods of REBCO CC tapes have received widespread attention. In experimental studies, various methods have been employed to evaluate the interfacial strength of CC tapes, including transverse tension, shear, bending, peel, cleavage, and cleavage tests. In addition, composite beam configurations such as the double-cantilever beam (DCB), end-notched flexure (ENF), and climbing drum peel (CDP) have been adopted to characterize the interfacial toughness and other properties of the tape interface. A series of theoretical and numerical studies were conducted to reveal the interfacial failure mechanism of REBCO CC tapes.
2.1. Experimental characterization
(A) Interface strength
Because excessive radial stress in superconducting coil structures is the main cause of delamination failure in superconducting tapes, transverse tension strength testing has been extensively conducted. The anvil tension test is the most widely employed among various testing methods. As shown in Fig. 2, an anvil tension test was utilized to measure the mechanical and electromechanical delamination strengths of the REBCO CC tapes [30]. The mechanical delamination strength is suitable for evaluating the interface strength of noncurrent-carrying superconducting tapes at any temperature. By applying a transverse tensile load to induce delamination between the layers, the peak force ($F_{\max }^{m} $) at the point of the delamination test was measured and divided by the upper anvil contact area (S) to define the mechanical transverse tension delamination strength ($\sigma_{d e l}^{m} $).
$ \sigma_{d e l}^{m}=\frac{F_{\max }^{\mathrm{m}}}{S} $
Fig. 2. Experimental setup for transverse tension delamination strength testing [30]: (a) overall view of test system; (b) schematic of mechanical delamination strength test setup; (c) schematic of electromechanical delamination strength test setup. |
Electromechanical delamination strength can be considered when testing the interface strength of current-carrying superconducting tapes at low temperatures (below the critical superconducting temperature). In addition to the mechanical delamination strength testing, $I_c$ tests were conducted on the samples, and the maximum force ($\left(F_{\max }^{e m}\right)$) at the point of significant degradation in the $I_c$ was measured and divided by the upper anvil contact area to define the electromechanical transverse tension delamination strength ($\left(\sigma_{d e l}^{e m}\right. $):
$ \sigma_{\text {del }}^{e m}=\frac{F_{\max }^{e m}}{S} $
Van der Laan [31] conducted the first transverse tension experiments on REBCO CC tapes employing the anvil tension method, and the results indicated that the slitting process during tape fabrication significantly influenced delamination strength. Majkic et al. [32] performed transverse tension tests adopting the pin-pull method, determined the delamination interface properties and statistically analyzed the test results. Delamination mainly occurred within the REBCO film; however, the origin of the delamination may have been at the REBCO/buffer layer interface. Shin’s research group [30], [33], [34], [35], [36], [37], [38], [39] conducted systematic experimental and numerical studies on delamination behavior in REBCO CC tapes. The mechanical and electromechanical delamination strengths were determined through delamination strength tests in different loading areas. Significant scattering was observed in the mechanical and electromechanical delamination strength test results, where the delamination strength significantly depended on the loading area, as illustrated in Fig. 3 [33]. Furthermore, a statistical analysis of the discrete test data was performed utilizing a two-parameter Weibull distribution function to determine the statistical significance of the scattered data [33], [34], [35]. In addition, the effects of different loading positions [36], geometric boundary configurations [37], anvil contact methods [38], and tape fabrication processes [39] on the test results are discussed.
Fig. 3. Measured mechanical and electromechanical transverse tension delamination strengths at different anvil areas in REBCO CC tapes [33]. |
Although the anvil tension test has been widely utilized to evaluate the transverse tensile strength of REBCO CC tapes, the experimental data obtained employing this method exhibit poor reliability. For example, when repeated anvil tension tests were performed on multiple REBCO CC tapes at random locations, the delamination strength values of 11 samples at 77 K ranged from <5 to >60 MPa [32]. To address the test data scattering, a failure probability evaluation method based on a three-parameter Weibull distribution function specific to the structural characteristics of REBCO was proposed by Zhang et al. [40], [41] Based on this method, a novel criterion for determining interface delamination strength was proposed, as illustrated in Fig. 4. In addition, a modified anvil tension test method was proposed by a research team at Shanghai Jiao Tong University, which involved utilizing high-strength epoxy resin films instead of solder as an adhesive and altering the contact surface shape between the test sample and the anvil [42]. This modified method was adopted to study the effect of temperature on the transverse tension interface strength of the REBCO CC tapes. Furthermore, a novel method for measuring the interlayer delamination strength of REBCO CC tapes, known as the bending-peel method, was developed via collaborative research between Tsinghua University and Lanzhou University. This method directly measures the interlayer strength of REBCO CC tapes at room temperature and LN2 temperature [43], [44]. These methods provide novel approaches for characterizing the interfacial properties of REBCO CC tapes.
Fig. 4. Measured mechanical and electromechanical transverse tension delamination strengths at different anvil areas in REBCO CC tapes and statistical modeling results [40]. |
According to numerical results [45], the scattering in the interface delamination strength determined utilizing the anvil tension method is related to inherent defects within the coated conductor and non-interface elastic-plastic deformation induced by the anvil tension process. In transverse anvil tension tests, a solder is typically utilized to connect the metal anvil and REBCO CC. Thermal mismatch stresses accumulate during soldering and solidification. High-quality soldering techniques are necessary because any nonuniformity or overflow of the solder can significantly affect the test results. These factors contribute to the scattering of the results of the interface strength testing in REBCO CC tapes. To address these issues, Gao et al. [46], [47] proposed a novel sample-fixture integration method to prepare and characterize the interface strength. This method simplifies the sample preparation and load application of the laboratory by integrating a superconducting composite tape sample and a fixture. As illustrated in Fig. 5, the superconducting composite tape sample was embedded in epoxy resin utilizing 3D printing technology to create a loading mold and secure the sample, and scanning electron microscopy (SEM) was employed to observe the delamination characteristics. This integrated approach enables experimental testing of the transverse tension and shear interface strengths in REBCO CC tapes. This resolves the issue of data dispersion in conventional experiments utilized to determine the delamination strength of the interface in HTS composite tapes and effectively reduces the testing variability (less than 10%). The microstructural characterization of the debonding interface indicated that the interface failure mode was significantly affected by the loading conditions. Regarding the transverse tensile delamination, the debonding location was mainly at the interface between the REBCO superconducting and buffer layers. Although interfacial cracking occurred mainly in the REBCO superconducting film, for shear delamination, part of the buffer layer also peeled off. This method overcomes the interference of additional bending moments in shear delamination experiments by utilizing a multibody connecting rod loading device [48]. It provides the first measurement of the pure shear delamination strength and failure characteristics of HTS composite tapes.
Fig. 5. Integrated sample-fixture method for testing transverse tension and shear interface strengths in REBCO CC tapes: (a) schematic of preparation process for transverse tension samples integrated with fixture [46]; (b) transverse tension test results [46]; (c) schematic of preparation process for shear samples integrated with fixture [47]; (d) shear test results [47]. |
Conventional anvil tension experiments have the disadvantage of wide data dispersion, whereas peel tests (illustrated in Fig. 6) have demonstrated better repeatability in the obtained results [49]. Therefore, peel testing was used to characterize the interface properties of coated superconducting tapes. Fig. 7 presents the peel-strength test results obtained for REBCO CC tapes fabricated with various chemical compositions and samples prepared in environments with varying acidity. The experimental results demonstrate that peel tests can be employed to effectively evaluate the influence of the chemical composition of the material and preparation environment on the interface strength of tapes [50], [51]. In addition, considering the effects of cleavage stress in superconducting magnet applications, Yanagisawa et al. [52] conducted a comprehensive study on cleavage delamination. The results revealed that delamination caused by cleavage stress occurred between the buffer layer and the Hastelloy alloy, and severe degradation of the delamination strength was observed at the edge notch owing to microcracks.
Fig. 6. Schematic of peel testing principles. |
Table 1 summarizes research on the interlayer delamination strength of HTS tapes under various simple loading conditions. Although a certain level of dispersion was observed in the test results under various loading conditions and the corresponding interface strengths obtained from various testing methods demonstrated certain differences, the overall interface strength of the coated conductor tapes was significantly lower than the uniaxial tensile strength (approximately 700 MPa) [53], [54], [55], [56], [57], [58], [59], [60]. Therefore, it is crucial to consider interface properties, assess the risk of interface failure, and optimize interface stresses in the manufacturing of superconducting cables, processing of superconducting magnets, and extreme operating environments to prevent interface failures.
Table 1. Overview of research on interlayer delamination strength in REBCO CC tapes under various simple loading conditions [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [46], [47], [48], [49], [50], [51], [52]. |
| Load conditions | Schematic | Experimental results |
|---|---|---|
| Transverse tension |  | Normal interface strength: 2.19-170 MPa [46], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42] |
| Shear |  | Shear interface strength: 10.3-11.98 MPa [47] |
| Bending-Peel |  | Delamination strength: 3.93-29.87 MPa [43], [44] |
| Peel |  | Peeling force: 0.5-6 N [49], [50], [51] |
| Cleavage |  | Cleavage stress: 0.38 MPa [52] |
(B)Interface toughness
In addition to interface strength, interface fracture toughness is another important parameter for assessing the interface properties of REBCO CC tapes. It provides significant guidance for evaluating the performance of superconducting composite materials, designing advanced superconducting devices under extreme multifield conditions, and predicting failure risks. Owing to the specific structure of REBCO CC tapes, negligible research has been conducted on the interfacial toughness compared to the interfacial strength. Currently, the methods developed primarily include the construction of composite beams for conducting DCB tension, ENF, and CDP tests.
Several classical interfacial fracture toughness testing methods are available for laminated composite materials, such as the DCB tension test for determining Mode I fracture toughness and the ENF test for determining Mode II fracture toughness. However, 2G REBCO CC tapes have a thin strip structure with a large aspect ratio (thickness ≈ 0.1 mm; aspect ratio > 40). Moreover, they exhibit an asymmetric structure along the thickness direction, making it challenging to directly apply conventional composite material interfacial fracture toughness testing methods. To address this issue, researchers have proposed the construction of composite beams suitable for conducting DCB tension and ENF tests to determine the interfacial fracture toughness of Modes I and II, respectively. As illustrated in Fig. 8, rectangular cross-sectional reinforcement beams (oxygen-free copper [61], Hastelloy alloy [62], or epoxy resin [63]) with thicknesses substantially higher than that of the superconducting composite tape were welded or bonded to the upper and lower surfaces of the tape. This ensured a consistent width and parallelism between the superconducting tape and reinforcement beams, and the neutral plane of the constructed composite beam was positioned within the superconducting tape. DCB tension and ENF tests were performed on the constructed composite beams. According to the beam theory, Modes I and II interfacial fracture toughness values are determined as follows [63]:
$ G_{I C}=\frac{3 P \delta}{2 W(a+\Delta)}, $
$ G_{I I C}=\frac{9 a^{2} P \delta}{2 W\left(2 L^{3}+3 a^{3}\right)} $
where P denotes the applied load,δ denotes the displacement at the loading point, W denotes the width of the composite beam sample, L denotes the span length of the composite beam sample, a denotes the crack extension length, h denotes the thickness of the composite beam sample, and Δ denotes the measurement error caused by the tensile fixture. Fig. 9 presents Mode I fracture toughness results obtained through the DCB tension tests utilizing composite beams [61]. The CDP test method has also been utilized for the quantitative characterization of Mode I interfacial fracture toughness and has achieved good results for tapes provided by various manufacturers [64].
Fig. 8. Characterization of interfacial toughness utilizing bending method: (a) schematic of a composite beam structure containing REBCO CC tapes; (b) schematic of DCB tension test for determining Mode I fracture toughness; (c) schematic of ENF test for determining Mode II fracture toughness. |
Fig. 9. Mode I fracture toughness determined through DCB tension tests utilizing composite beams [61]. |
2.2. Theoretical and numerical methods
To explain the effects of different loading anvils on the delamination strength test results and the origin of delamination in REBCO CC tapes by anvil tests, a numerical analysis of the mechanical stress distribution within the CC tape was performed with a 2D elastic-plastic finite element (FE) model [65]. The simulation results demonstrated that the REBCO layer experienced the largest stress with an anvil covering the entire conductor width, which could result in a low delamination strength. In contrast, the copper stabilizer layer was subjected to the largest stress among all the constituent layers, which could result in a higher delamination strength. Similarly, a full 3D plastic FE model successfully explained the delamination phenomenon occurring at the interface between REBCO and Hastelloy [66]. The FE model analyses of stress distributions on the constituent layers of a REBCO conductor under transverse tension [65], [66], however, either did not include all the major constituent thin-film layers or were not based on delamination modeling techniques. To address these issues, Gao et al. [45], [67] developed a 3D/2D mixed-dimensional elastic-plastic delamination FE model based on the specific structure of REBCO CC tapes to simulate interface failure under transverse tension. The model not only simulates the real size of the REBCO CC tape containing all major constituent film layers, but also simulates the delamination behavior of the REBCO CC tape for the first time. According to this model, the formation of interfacial delamination was characterized by the bilinear traction separation cohesive zone model (CZM) [68]. For single-mode (normal or shear) interface delamination, the constitutive relation of the interface is illustrated in Fig. 10, which is expressed as
$ \sigma_{i}=K^{\prime} u_{i}, \quad i=n, s $
where $σ_i$ is the interfacial traction stress and $u_i$ is the relative separation displacement between two associated points located on an interface’s top and bottom surfaces. The subscripts $ i=n, s $ denote the normal and shear directions, $ K_{i}^{\prime} $ is the updated penalty stiffness of the interface related to ui, expressed as
$ K_{i}^{\prime}=\left\{\begin{array}{cl} K_{i}, & u_{i}^{\max } \leqslant u_{i}^{0} \\ \left(1-D_{i}\right) K_{i}, & u_{i}^{0}<u_{i}^{\max }<u_{i}^{f}, \quad i=n, s, u_{n}^{\max } \geqslant 0, \\ 0, & u_{i}^{\max } \geqslant u_{i}^{f} \end{array}\right. $
where $ u_{i}^{\max } $ is the updated maximum separation displacement; Superscripts 0 and f denote the damage initiation and ultimate failure, respectively; and $D_i$ is the damage evolution function defined as
$ D_{i}=\left\{\begin{array}{lc} 0, & u_{i}^{\max } \leqslant u_{i}^{0} \\ \frac{u_{i}^{f}\left(u_{i}^{\max }-u_{i}^{0}\right)}{u_{i}^{\max }\left(u_{i}^{f}-u_{i}^{0}\right)}, & u_{i}^{0}<u_{i}^{\max }<u_{i}^{f}, \\ 1, & u_{i}^{\max } \geqslant u_{i}^{f} \end{array}\right. $
Fig. 10. Schematic drawing of bilinear traction-separation law at cohesive zone interface. |
Under mixed-mode loading conditions, the quadratic nominal stress damage criterion was utilized to predict the onset of damage at the interface in REBCO CC tapes [69]:
$ \left(\frac{\left\langle\sigma_{n}\right\rangle}{\sigma_{n}^{0}}\right)^{2}+\left(\frac{\sigma_{s}}{\sigma_{s}^{0}}\right)^{2}=1, $
where the symbol 〈 〉 is the Macaulay operator, $ \sigma_{n}^{0} $ and $ \sigma_{s}^{0} $ are the critical normal stress and critical shear stress, respectively. Once the initiation criterion is reached, interface delamination propagation of REBCO CC tapes under mixed-mode loading can be predicted by satisfying the “power law” criterion [70]
$ \left(\frac{G_{n}}{G_{n c}}\right)^{\alpha}+\left(\frac{G_{s}}{G_{s c}}\right)^{\alpha}=1, $
and “Benzeggagh-Kenane” criterion [71]
$ G_{m c}=G_{n c}+\left(G_{s c}-G_{n c}\right)\left(\frac{\beta^{2}}{1+\beta^{2}}\right)^{\eta} $
where Gn (Gnc) and Gs (Gsc) are the normal fracture toughness (critical normal fracture toughness) and the shear fracture toughness (critical shear fracture toughness), respectively, Gmc is the critical normal fracture toughness under mixed-mode loading conditions, β is the mode mixity ratio, and η is the Benzeggagh-Kenane parameter. The simulation results revealed that the computational efficiency of the 3D/2D model was much higher than that of the full 3D model, while maintaining sufficient accuracy [45]. Discussions on the effects of anvil size and initial cracking on delamination behavior showed that the interfacial delamination strength determined by anvil stretching increased with decreasing anvil size and decreased with increasing pre-crack area [45], suggesting that the dispersion of the interfacial delamination strength determined utilizing the anvil method is related to the intrinsic defects within the coated conductor and the non-interfacial elastic-plastic deformation induced by the anvil stretching process, which can be illustrated by Fig. 11 and expressed by Eq. (10). Based on this, a simplified 2D CZM-based delamination model was utilized to analyze the joint debonding behavior of REBCO CC tapes under various mechanical loads [72], [73] and the effect of the substrate thickness on the interfacial strength of REBCO CC tapes under peel tests [74], [75].
$ \sigma_{d e l}^{m} S=\sigma_{0}\left(S-S_{d e f}\right)+f \cos \theta $
$ \sigma_{d e l}^{m}=\sigma_{0} \frac{S-S_{d e f}}{S}+\frac{f}{S} \cos \theta $
Fig. 11. Schematic representation of interface strength test results affected by intrinsic defects and non-interfacial elastic–plastic deformation. Here, $ \sigma_{d e l}^{m}, \sigma_{0} $ represent the measured transverse tension delamination strength and true interface strength, Sdef is the area of intrinsic defects, f and θ are the non-interfacial force and the corresponding angle, respectively. |
Because the interfacial delamination of REBCO CC tapes is usually accompanied by the fracture of the superconducting layer, to reveal the failure mechanism of REBCO CC tapes under mechanical loading, a failure model combining interfacial delamination and intralaminar fracture of REBCO CC tapes was developed and numerically analyzed utilizing the FE method [76]. The interfacial delamination and fracture behavior were captured with the cohesive zone model and extended finite element method, respectively. The quadratic nominal stress failure criterion defined in Eq. (8) was utilized to predict the damage initiation of interfacial delamination under mixed-mode loading, and the maximum principal stress criterion defined in Eq. (11) was utilized to capture the damage initiation of the intralaminar fracture behavior in the superconducting layer.
$ \frac{\left(\sigma_{\max }\right)}{\sigma_{\max }^{0}}=1 $
where $ \sigma_{\max } $ and $ \sigma_{\max }^{0} $ represent the maximum principal stress and the maximum allowable principal stress, respectively. The results illustrate that in the REBCO CC tapes, the combined interfacial delamination and fracture damage interacted and exhibited progressive mechanical damage behavior, as presented in Fig. 12. Crack propagation in the REBCO superconducting layer leads to delamination failure at corresponding locations on the interface between the superconducting layer and its adjacent layers; delamination failure at the interface releases interlayer constraints and further enhances crack propagation.
Fig. 12. Simulation results of REBCO CC under uniaxial tension [76]: (a) distribution of maximum principal stress and crack propagation in superconducting layer; (b) interfacial delamination with stiffness degradation. |
In addition to numerical simulations, theoretical studies on the delamination behavior of REBCO CC tapes have also been conducted. Gou et al. [77], [78], [79], [80] conducted a systematic study on the delamination behavior in a multilayer REBCO CC based on the displacement-energy model (DEM). The results of theoretical calculations demonstrated that delamination could start at any interface of the REBCO CC tape or more than one interface simultaneously [77], and that the loss of bonding energy was greater for samples with more layers [78]. The DEM was also utilized to characterize the evolution of localized defects, interfacial crack growth rates, and fatigue life in REBCO CC tapes [79], [80]. In addition, a series of theoretical studies on the interfacial fracture behavior of REBCO CC tapes was conducted by Wang et al. [81], [82]. They investigated the fracture behavior of microscale DCB and ENF made of REBCO conductors based on the strain gradient and thermodynamic theories, respectively. Closed-form solutions for Types I and II fracture energy release rates of REBCO CC tapes were obtained. These studies are beneficial for designing experiments for testing the fracture toughness of microscale REBCO CC tapes.
3. Failures related to interface
The distinct electromagnetic, thermal, and mechanical properties of the composite layers in REBCO CC tapes contribute to interface delamination, induced by the thermal mismatch stress that occurs during cooling and electromagnetic forces under high magnetic fields. This delamination decreases the mechanical load-bearing capacity and irreversibly degrades the superconducting transport properties, potentially resulting in damage and electromechanical failure of the superconducting tapes.
3.1. Interface failure in individual tapes
Owing to the weak c-axis strength, interface failure can occur in REBCO CC tapes subjected to various external factors, including mechanical processing, thermal stress, and electromagnetic forces. Fig. 13 illustrates the interface failure of individual REBCO CC tapes subjected to CORC cable winding, and torsional deformation [83], [84]. Moreover, interfacial delamination was also found under uniaxial tensile loading [55]. The results revealed that the interfacial delamination of REBCO CC tapes is usually accompanied by the rupture of the superconducting layer. Song et al. [85] investigated quenching in REBCO superconducting tapes and found that conductor edge defects resulted in silver delamination (Fig. 14). This leads to dendritic flux avalanches and localized high heating, which further induces silver delamination. They also observed an interlayer delamination within the REBCO superconducting layer. In addition, even in the absence of transport currents, the Lorentz force between the induced screening currents and external magnetic fields can cause interlayer delamination, resulting in detachment of the superconducting layer from the buffer layer [86], [87]. Interface cracking and superconducting film fracture are drawbacks that limit the processing and utilization of REBCO CC tapes. Rational optimization and deformation control are required to reduce the risk of interface failure.
Fig. 14. Microscopic appearance of superconducting film in REBCO CCs after quenching [85]. |
3.2. Interface failure in windings
To investigate the impact of various impregnation methods on the structure and superconducting performance of HTS coils, Takematsu et al. [88] conducted $I_c$ measurements of REBCO double-pancake coils under LN2 conditions utilizing three coil winding techniques—epoxy impregnation, paraffin impregnation, and dry winding. The results revealed a significant reduction in the $I_c$ (∼8 A) for the epoxy-impregnated REBCO double-pancake coils, which was only 18% of the $I_c$ of the dry-wound coils. SEM and energy-dispersive X-ray spectroscopy (EDS) confirmed that mixed-mode interlayer delamination occurred between the superconducting and buffer layers and within the superconducting layer (Fig. 15). Numerical simulations revealed that the radial tensile stress accumulated during cooling from room temperature to 77 K, caused by the significant difference in the thermal expansion coefficients (CTE) between the impregnation material and superconducting tapes, can exceed the interface strength of the REBCO CC tapes. This results in delamination failure, in which the conductor detaches from the interface between the buffer and the REBCO superconducting layers or fractures within the REBCO layer, leading to crack formation and significant irreversible degradation of the $I_c$.
Fig. 15. Interface failure behavior of REBCO coils induced by thermal mismatch stress [88]: (a) I-V curves of REBCO double-pancake coils fabricated via epoxy impregnation, paraffin impregnation, and dry winding; (b) SEM and EDS images depicting location of delamination failure in epoxy-impregnated coils. |
In addition to the thermal mismatch stress, the Lorentz force under a strong background magnetic field can cause interface damage in REBCO CC tapes, which may adversely affect the stability of REBCO HTS magnets. Yanagisawa’s team [89] conducted experiments with REBCO and Bi-2223 coils connected in series, charged at 4.2 K, to generate a central magnetic field of 28.2 T under a background magnetic field of 17.2 T. However, a premature normal voltage appeared on the REBCO coil, leading to the cessation of charging at 25 T. Upon unwinding the REBCO coil, the interlayer delamination and peeling of the welded joint conductors were observed, as illustrated in Fig. 16. In addition, charging experiments for testing epoxy-impregnated REBCO superconducting coils under a background field of 11 T at 4.2 K revealed the occurrence of a normal voltage at a current of 408 A (65% of the upper limit) and the irreversible degradation of the $I_c$ [90]. Furthermore, optical microscopy revealed that the combined effects of the electromagnetic forces and thermal mismatch stress caused the superconducting coils to bend outward. The outward bending and strong circumferential stress ruptured the REBCO layer, irreversibly degrading the superconducting performance of the REBCO coils. Despite the absence of insulating materials with significantly different CTE (such as epoxy) between non-insulated superconducting coils, structural damage to the superconducting layer can occur owing to nonuniform electromagnetic forces induced by screening currents under strong electromagnetic fields and imbalanced forces during quenching [91], [92], [93], [94], [95], [96], [97], [98], [99]. Testing of steady-state high-field magnets revealed the occurrence of quenching, along with evident microcracks and interface fractures in the edge region of the REBCO CC tapes after a central magnetic field of 45.5 T was reached (illustrated in Fig. 17). This indicates that mechanical damage and evolution are the direct causes of the electromechanical degradation and failure of the superconducting electromagnetic structure. These findings demonstrate the potential for enhancing the magnetic field intensity by effectively controlling the mechanical damage [100].
Based on the meso-mechanics homogenization method, the 2D axisymmetric homogeneous block FE model has been widely utilized for stress analysis and delamination failure risk assessment of REBCO superconducting coils [101], [102], [103], [104], [105]. Because only the approximate results for the equivalent average stress distribution can be obtained with an equivalent homogenous block FE model, a 2D axisymmetric multilayer delamination FE model with main layers of REBCO CC tapes and insulation materials was developed based on the bilinear CZM [106], [107]. Based on the 2D axisymmetric multilayer delamination FE model, the stress distribution and delamination properties of an epoxy-impregnated REBCO winding induced during the cooling process were investigated, and multiple delamination failure modes in a winding with an excessive radius ratio were predicted, as presented in Fig. 18 [107]. Furthermore, the current-carrying degradation associated with delamination due to thermal stress has been investigated [108], as well as the delamination mechanism of epoxy-impregnated REBCO pancake windings during quenching [99]. In addition, a coupled thermal-mechanical CZM was developed in the framework of a 2D plane FE model, and the effects of the geometric configuration of the epoxy and interfacial cohesive strength distributions of the REBCO conductor were discussed [109].
Fig. 18. Multiple delamination failures modes of epoxy-impregnated REBCO winding with large radius ratio ($R_{\text {out }} / R_{\text {in }}$= 3) at different cooling temperatures [107]. |
4. Optimization strategies
Owing to the weak interfacial properties of REBCO CC tapes, the electromechanical degradation caused by mechanical damage, such as interfacial delamination and superconducting film fracture, hinders the development of advanced technologies and devices. Therefore, researchers have been continuously exploring optimization strategies focusing on two aspects—micro-material process level and macro-structural level—aiming to reduce the risk of interface failure.
4.1. Micro material process level
Enhancing the interface strength of REBCO conductors will fundamentally solve the problem of interface cracking and provide high flexibility in the design of high-field superconducting magnets. Consequently, a few studies targeting microscopic material process modifications have been conducted to improve the interfacial delamination strength of REBCO CC itself. Yoshizumi et al. [110] explored the interfacial delamination strength of REBCO CC as a function of the calcination temperature of GZO in the buffer layer. Generally, heat treatment at 500 °C is usually chosen because it results in higher flatness and, thus, higher grain alignment and Ic performance, but the delamination strength is low. However, the experimental results presented in Fig. 19(a) indicate that the strength was improved at higher calcination temperatures. Therefore, the optimal calcination temperature to obtain high Ic and interfacial delamination strength is 600 °C. To investigate the effect of laser cleaning and silver annealing on the interfacial strength of REBCO CC, Shin et al. [111] tested the interfacial delamination strength of REBCO CC samples subjected to different treatments at room temperature. As illustrated in Fig. 19(b), the interfacial strength of the samples treated with both laser cleaning (LC) and silver annealing (SA), as well as the samples treated with only additional SA, was significantly enhanced compared with the samples treated without frequent LC and SA. In addition, by changing the deposition temperature, Wang et al. [112] proposed a method to improve the interfacial strength of REBCO CC by increasing the strength between the silver and YBCO layers. As illustrated in Fig. 19(c), the average interfacial tensile strength increased from 21.6 to 48.8 MPa when the growth temperature of the silver layer was increased from 30 to 100 °C.
Fig. 19. Interfacial delamination strength of REBCO CC affected by micro material process: (a) calcination temperature; (b) additional Ag annealing process; (c) Ag depositing temperatures. |
4.2. Macro structural level
Currently, research on improving the interfacial delamination strength of REBCO CC interfaces based on microscopic material processes is relatively scarce and does not solve the problem of low interfacial strength. Therefore, several studies have been conducted on the extreme working conditions of low temperatures and strong electromagnetic fields. The optimization of the structural design of the REBCO CC itself or the superconducting coil structure has been conducted to reduce the stress/strain of the tapes under extreme working conditions, thus reducing the risk of interfacial failure.
(A)Optimization of coated conductor
To prevent the delamination failure of REBCO CC tapes in epoxy-impregnated coils, Yanagisawa et al. [113] developed a polyimide coating manufacturing technique. As illustrated in Fig. 20, the polyimide insulation layer formed via electrochemical deposition has two main effects on the REBCO CC tapes. Thermal shrinkage can be absorbed via the plastic deformation of polyimide, and delamination failure can be replaced by debonding between polyimide and epoxy resin. Research results obtained for epoxy-impregnated REBCO superconducting coils prepared employing this technique have demonstrated that the delamination stress on REBCO CC tapes is reduced by the peeling of the epoxy resin from the polyimide coating; therefore, interfacial cracking failure of the coil is avoided [114]. This confirms that the electrochemically deposited polyimide-coated REBCO CC tapes are reliable and easy to handle. Moreover, this technique can extend to long wires, making it suitable for dry-type and polymer-impregnated coils.
Fig. 20. Illustration of effects of electrochemically deposited polyimide insulation [113]: (a) reducing stress concentration through plastic deformation of electrochemically deposited polyimide; (b) interfacial debonding between electrochemically deposited polyimide and epoxy resin; (c) interfacial debonding between electrochemically deposited polyimide and REBCO CC. |
Similarly, Mizuno et al. [115] proposed an epoxy-impregnated coil technology in which a REBCO CC tape was wound together with a polytetrafluoroethylene (PTFE) tape. The PTFE tape effectively prevented interfacial cracking and mechanical degradation of the coil, and this technique was successfully applied to racetrack-shaped REBCO-impregnated coils. Yin et al. [116] proposed coating REBCO CC tape with a demolding agent to reduce delamination failure. This method is cost-effective, easy to implement, and suitable for epoxy-impregnated REBCO superconducting coils. Jeong et al. [117] proposed a design scheme for partially coating the REBCO superconducting layer based on an FE analysis of the multilayer structure of HTS tapes to improve their transverse tensile strength.
(B)Optimization of winding structure
The radial force induced by thermal mismatch stress in epoxy-impregnated REBCO HTS coils is related to the outer-to-inner radius ratio of the coil. The larger the outer-to-inner radius ratio, the higher the peak radial stress in the coil [118], [119], [101], [102], [103], [104], [105], [106], [107]. In other words, larger outer-to-inner radius ratios are more likely to cause delamination failure during cooling; therefore, it is recommended to utilize small outer-to-inner radius ratios to design impregnated coil structures. Moreover, reducing the thicknesses of the Hastelloy substrate, copper stabilizer, and epoxy resin layer can partially reduce the radial stress [107]. However, in high-field applications, utilizing coils with large dimensions and large outer-to-inner radius ratios is inevitable. To achieve delamination-immune epoxy-impregnated REBCO coils under arbitrary size conditions, Miyazaki et al. [101] proposed a coil structure optimization method that divided the coil radially into several sub-coils to reduce the radius ratios of each sub-coil. This method was successfully applied to pancake [102] and racetrack [118] coils. Gao et al. derived quantitative analytical equations for coil turns and optimal segmentation schemes based on a detailed theoretical analysis [107]. As illustrated in Fig. 21, the coil is divided into several sub-coils with the same radius ratio to ensure that the maximum radial stress in each subcoil is the same. The outer-to-inner radius ratio of the first sub-coil is denoted as η. The number of turns of the first subcoil, (j + 1)th subcoil, and the entire coil satisfy the following equations:
$N^{1}=\frac{R_{\text {out }}^{1}-R_{\text {in }}^{1}}{t_{\text {RUT }}}=\frac{(\eta-1) R_{\text {in }}^{1}}{t_{R U T}}$
$N^{j+1}=N^{j} \eta=N^{1}(\eta)^{j},$
$N^{s u m}=\frac{N^{1}\left[1-(\eta)^{n}\right]}{1-\eta}$
Fig. 21. Schematic of dividing impregnated coil into several sub-coils. |
where η is the outer-to-inner radius ratio of each sub-coil, $R_{\text {out }}^{1}$ and $R_{i n}^{1}$ are the outer and inner radii of the jth sub-coil, respectively, and $t_{R U T}$ is the thickness of a representative unit. Fig. 22 illustrates the radial stress distribution and turn calculation results for epoxy-impregnated REBCO superconducting coils divided into sub-coils with various outer-to-inner radius ratios during cooling from 293 to 77 K [107]. The results demonstrate that, for coils with arbitrary numbers of turns, delamination failure can be avoided by appropriate coil segmentation and the control of the outer-to-inner radius ratios of each sub-coil, ensuring that the radial stress in each sub-coil is within an acceptable range.
Fig. 22. Radial stress distribution and turn calculation results for epoxy-impregnated REBCO superconducting coils divided into sub-coils with various outer-to-inner radius ratios [107]. |
The primary cause of thermal mismatch stress in epoxy-impregnated coils is the mismatch between the CTE of the impregnating material and the superconducting tape. Epoxy-impregnated materials typically have a high thermal expansion coefficient (approximately 40×10-6 K-1 at 300 K). Therefore, by incorporating powders with a low CTE into the epoxy resin, such as metal powders [120], [121], [122], [123], [124] and ceramic particles [125], [126], [127], [128], or by incorporating powders with a negative CTE [107], [129], [130], the thermal expansion coefficient of the impregnating material can be reduced to a certain extent, thereby lowering the risk of coil cracking during cooling. The uniform mixing of powders with negative CTE into epoxy resin to form composite epoxy materials can reduce the effective thermal expansion coefficient of the composite with an increasing proportion of expansion material [129], [130], [131], [132], [133]. In particular, when the mass fraction of zirconium tungstate, a negative-thermal-expansion material, reached 60% in the epoxy resin, the composite epoxy material exhibited negative thermal expansion characteristics below 100 K (Fig. 23) [134]. Numerical calculations indicate that when the CTE of the impregnating material is reduced to 5 × 10−6 K−1, the delamination failure of the coil due to thermal mismatch stress no longer occurs [107]. Therefore, by incorporating certain powders with low thermal expansion, particularly those with negative thermal expansion, into the epoxy resin, the CTE of the impregnating material can be effectively reduced, thereby lowering the risk of interfacial cracking failure caused by thermal mismatch stress in the impregnated superconducting coils. In addition, applying overband constraints to the outer surface of impregnated coils can regulate and reduce the peak radial stress in the coils [105]. Moreover, the “ice-impregnation of coils,” a novel technology to produce impregnated superconducting coils, demonstrates unique advantages in reducing coil thermal mismatch stress in terms of cost-effectiveness and convenience, as ice exhibits thermal shrinkage characteristics similar to those of REBCO CC tapes at low temperatures (273-77 K) [135], [136], [137]. Notably, when reducing the thermal expansion coefficient, the impregnated material's mechanical strength and thermal conductivity should also be considered.
Fig. 23. CTE of pure epoxy and composite epoxy materials incorporated with 60% zirconium tungstate from 25 to 300 K [134]. |
5. Challenges and future outlook
Although much work has been performed on the characterization of the interfacial properties of REBCO CC tapes and the improvement of interfacial strength and optimization of superconducting coil structures, the issues of low interfacial strength and easy interfacial delamination under the multifield coupling of cryogenic and electromagnetic fields have not been fundamentally solved. Therefore, interfacial delamination failure in REBCO CC tapes remains a bottleneck, limiting the development of high-field superconducting magnets.
Interfacial strength and toughness are two basic parameters that characterize interfacial properties, and accurate characterization of these two material interfacial parameters of REBCO CC is essential for guiding its high-field application and interfacial failure risk evaluation. The anvil tension method has been widely adopted to characterize the interfacial strength. However, the results obtained utilizing this method demonstrates significant dispersion, which limits their usefulness for evaluating material performance. There are several experimental methods for measuring interfacial delamination strength, each with its advantages and disadvantages. However, no efforts have been made to correlate the results of the different methods, resulting in unclear relationships between them. Measuring samples with different methods and comparing them is essential to clarify the correlations among the different test methods. Methods for characterizing interfacial fracture toughness are still in the exploratory stage, and the reliability of these methods, DCB, ENF, and CDP, needs to be further verified. FE models based on the CZM are effectively applied to the interface failure analysis in both REBCO CC tapes and coils, but these models make assumptions about the failure mechanism and are dependent on the choice of material parameters, that is, interfacial strength and toughness; the simulation results strongly depend on the choice of interface failure criteria and the selection of interface parameters. Therefore, establishing industry standards for reliable, stable, and widely applicable methods for characterizing the interface properties of REBCO CC tapes is urgently required. In addition, reports on interface studies at the nanometer scale for REBCO tapes are rare. However, studies on the nature of the interfaces as well as the mechanical properties of the individual layers, especially REBCO superconducting films, are very important for understanding the interfacial failure mechanism. Micro- and nanoscale studies are urgently required to reveal interfacial failures' onset and extension and guide their applications.
Developing a simultaneous in situ observation method for interfacial damage under mechanical loading will help reveal the interfacial failure mechanism of REBCO CC tapes, which remains unknown. In addition, the current interface theory and testing are mainly based on direct reference to macroscopic-scale composite and laminated structure methods, usually applied to spatial scales above the millimeter or submillimeter order of magnitude. However, most superconducting tapes have a major functional layer on the micrometer scale. There is a lack of mesoscale-based evaluation theory, and existing theoretical and numerical methods do not consider scale effects.
The micromaterial-based process improved the interfacial properties of the REBCO CC to some extent; however, it did not solve this fundamental problem. Currently, the interfacial strength of REBCO conductors is still far lower than the axial tensile strength, and it remains a great challenge to significantly increase the interfacial strength. Researchers have focused on optimizing superconducting tape or coil structures. These strategies include coating the tape surface with polyimide, applying demolding agents, grouping coils for impregnation, and incorporating powders with low or negative CTE into the impregnated material. These methods effectively reduce the forces acting on the tapes within the superconducting coils, lowering the interface failure risk. However, these optimization strategies restrict the structural configuration of large-scale high-field magnets to a certain extent. Therefore, it remains a great challenge to overcome superconducting electromagnetic and mechanical failures due to interfacial cracking, in terms of microscopic material preparation processes and the optimization of superconducting composite structures under multiple fields.
6. Conclusion
The applications of REBCO HTS tapes, which are typical layered composite materials, are limited by issues related to their interface properties, which hinder the development of advanced HTS technologies and devices. The mechanical stress induced by magnet processing, thermal mismatch stress during cooling, electromagnetic stress under a high field, and thermal stress induced by quenching can lead to interface delamination and coating fractures in superconducting tapes, significantly affecting their mechanical and superconducting performances. Interfacial failure in REBCO CC tapes has become a bottleneck limiting the development of high-field superconducting magnets and has received widespread attention from researchers.
Several efforts have been made to characterize the interfacial properties of REBCO CC tapes, improve the interfacial strength, and optimize the superconducting coil structure to reduce the risk of interfacial failure. Despite the many challenging issues that confront REBCO CC tapes in interfacial failure, we firmly believe that if the interfacial failure problem of REBCO CC tapes is solved, the development of high-field magnets will be a novel breakthrough in the field of high magnetic fields, which will contribute significantly to major discoveries and developments in many fields such as physics, biology, chemistry, materials, medicine, and engineering.
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 study was supported by the National Natural Science Foundation of China (12272156 and 11932008) and Shanghai Superconductor Technology Co., Ltd - Lanzhou University Superconducting Materials and Mechanics Open Foundation for Industry-University-Research.
