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Adhesion properties of MgO–ZrO2 insulation coatings for 5 Tesla HTS coils
Materials Science and Engineering B97 (2003) 25 /33 www.elsevier.com/locate/mseb
Adhesion properties of MgO ZrO2 insulation coatings for 5 Tesla HTS coils /
E. Celik a,b,c, , E. Avci d, Y.S. Hascicek a a
National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Dr., Tallahassee, FL 32310, USA Faculty of Engineering, Department of Metallurgical and Materials Engineering, Dokuz Eylul University, Bornova, Izmir 35100, Turkey c Superconductivity Research Laboratory, ISTEC, JFCC, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan d Faculty of Engineering, Department of Metallurgical and Materials Engineering, Esentepe Campus, Sakarya University, Sakarya 54187, Turkey b
Received 28 May 2002; accepted 23 September 2002
Abstract Self insulating substrate tapes (SIST) is the most promising insulation technique for high temperature MgO /ZrO2 coatings on Ag and AgMg sheathed Bi2Sr2Ca1Cu2Ox (Bi-2212) superconducting tapes and wires in applications of HTS/LTS coils and magnets. We have already reported successful results as to the synthesis, characterizations and applications of the insulation coatings using the SIST. In order to provide no electrical short circuit in Jc measurements of HTS/LTS coils, the bonding of the coatings onto the substrate is a very important issue. In this present research, the adhesion properties of high temperature MgO /ZrO2 coatings were scrutinized for different processing parameters. Lap joints were fabricated by laying fresh sol /gel coated silver tape samples over each other and then by heat-treating at temperature range of 500 /800 8C for several times in air. These joint samples were pulled to failure by using a mini tensile tester. MgO /ZrO2 was coated on Ag tapes by sol /gel process using Mg and Zr based precursors. The obtained results obviously pointed out that the best Mg precursor is Mg(C5H7O2) ×/2H2O to prepare solution and there is a strong relationship between film growth and adhesion properties. Also, MgO content in ZrO2 increased its bonding strength. The optimum heat treatment conditions are 600 8C and 15 min for best bonding for these high temperature insulation coatings on HTS tape conductor. The failure mode of all samples was in the form of a mixed type interfacial/cohesive defects in MgO /ZrO2 coating. # 2002 Elsevier Science B.V. All rights reserved. Keywords: MgO /ZrO2; Sol /gel; Insulation; Adhesion; Self insulating substrate tapes; HTS/LTS conductor
1. Introduction High temperature MgO /ZrO2 coatings have been insulated on Ag and AgMg sheathed Bi-2212 tapes using a reel-to-reel, continuous sol /gel technique, which is specifically called as ‘self insulating substrate tapes’ at the National High Magnetic Field Laboratory (NHMFL) in order to provide turn-to-turn electrical insulation for HTS coils [1 /7]. It is necessary to use high temperature compatible insulating materials for turn-toturn insulation in Wind and React (W&R) magnet built from the high temperature superconductor. A thin, thermally stable and well-adhered coatings must (a)
Corresponding author. Tel.: /1-90-232-388-2880; fax: /1-90232-388-7864. Internet: http://www.deu.edu.tr E-mail address: [email protected] (E. Celik).
insulate at the coil’s operating temperatures; (b) retain continuity when shaping the coil; (c) be permeable to oxygen; (d) not induce damage to the HTS material or any sheathing material it might be encased within; (e) not significantly increase the overall size of the coil; and (f) be compatible with the thermal processing of the HTS material [8 /11]. In these coatings, the most important problem is the bonding of coatings to the substrate during the winding process [12]. The mechanical stability and adhesion of the coatings signify their fundamental quality. In sol /gel insulation coatings, factors affecting to the adhesion are generally residual stresses, coating structure (porosity, micro and macro cracks, coating thickness) and deposition parameters [9,10,12 /14]. The sol /gel coating parameters, which contain solution properties (precursors, solvent, chelating agent, viscosity and dilution), withdrawal rate, drying, heat treatment and sintering or annealing
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 3 9 1 - 4
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E. Celik et al. / Materials Science and Engineering B97 (2003) 25 /33
conditions (temperature, time, atmosphere, heating/ cooling rates), play important roles in the bonding of coatings onto tape substrates [1 /10,14 /17]. Coating failures can occur by many mechanisms such as surface damage (e.g. wear, oxidation), elastic or plastic deformation, fracture, etc. The degradation or failure of coatings is fundamentally related to the adhesion and cohesion strength. The failure mode(s) can be described as interfacial, cohesive, or mixed interfacial/cohesive defects, as indicated in Fig. 1 [12 /15]. Previously [8,9], the adhesion properties of the high temperature coatings were basically demonstrated. In this present work, the adhesion properties of MgO / ZrO2 coatings on silver tape substrates were emphasized for sol /gel process parameters regarding precursor type, MgO contents, heat treatment temperature and time.
2. Experimental procedure The MgO /ZrO2 coatings were coated on commercial Ag tapes with nominal dimensions of 65 /9.62 /0.05 mm3 through a sol /gel precursor dipping technique. Substrate surfaces were cleaned with acetone, ethanol and water. Zirconium tetrabutoxide and several Mg based precursors were used as precursors, acetyleacetone as a chelating agent, and isopropanal as a solvent to prepare clear homogenous sols. The gel films were formed by means of dip coating and dried at 300 8C for 1 min in air to decompose the precursors and organic groups and then heat treated at a temperature of 650 8C for 10 min in air. Further information on solution preparation and coating process can be found in Refs. [1,2,4 /8,18]. The sample surfaces were ultrasonically cleaned in acetone. After dipping the samples into the solutions, lap joints were prepared as shown in Fig. 2a. A few pounds of weight was put on the samples to improve contact between the coated Ag samples and then dried
Fig. 2. (a) The tensile test sample with a joint mode by sandwiching two sol /gel dipped Ag tapes; (b) Tensile testing (sS, shear stress at the coating /Ag interface; N , load and A , over lapped area).
at room temperature for 1 day, and then heat-treated. Oxide films were formed in interfacial area between two silver tapes by annealing each sample at temperatures between 500 /800 8C for durations of 5/30 min in air. The optimum precursor type, MgO content, heat treatment temperature and time were determined for MgO /ZrO2 coating on Ag and AgMg sheathed Bi-2212 tapes thereby using the adhesion properties. These joint samples were pulled to failure using a mini tensile tester. The joint samples were prepared such that the failure was in shear mode of the interface of insulated Ag substrate, and not at the joint legs, as seen in Fig. 2b. Experiments were tentatively repeated until optimum overlap area in the samples was obtained. In these experiments, the optimum overlap area was found as a 28.86 mm2. Measurements were done after this optimization. The tests, which showed a shear failure at the overlap area, were taken into consideration. After tensile tests, the microstructures of failure surfaces of the joint samples were examined with the help of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-Ray mapping. Additionally, surface resistance of the joint samples was measured before and after tensile testing in order to
Fig. 1. Interfacial and cohesive failure modes [10].
E. Celik et al. / Materials Science and Engineering B97 (2003) 25 /33
visually describe whether there is insulation at interfacial area.
3. Results and discussion There are many methods to measure the bonding strength of the coatings. As we have reported earlier [8,9], a lap-shear test of the joint samples is one of them thereby utilizing a mini tensile testing. Normally, the tensile testing is performed in evaluation of the mechanical strength of metals and alloys. In these tests, the joint samples, as indicated in Fig. 2a, were relatively pulled to failure in a constant velocity and a short time like a few minutes. The variations depending on a constant area of applied load to the samples give shear stresses. In this context, the mini tensile testing, which was specifically designed, was performed to determine the adhesion properties of the joint samples. The bonding strength of the coatings was found as a result of these experiments. Notably, the determination of the bonding strength of Ag tapes joint with MgO /ZrO2 is a combination of versatile mechanical test mechanisms. The fundamental mechanism is tensile testing, as described in Eq. (1); ss
F A
(1)
Where, ss is a tensile strength and F is an applied load to a unit area (A ). The mechanism supporting this rule is a phenomenon that the peeling of the coatings realizes. Even though the joining of the coatings on metal substrates with another metal is provided with epoxy resin in the most of Refs. [13,19/22], the joint samples produced by dipping into the solutions of two Ag tapes are dried at room temperature and then heat-treated at the temperature range of 500/800 8C for several times in air. The ceramic joint samples were pulled by the mini tensile testing. The shear stress found after this process gives the bonding strength of the coating to the substrate. The bonding strength is determined as the shear stress at which point the joint yields abruptly. It is known that the adhesion behavior of the MgO / ZrO2 insulation coatings on Ag and AgMg sheathed Bi2212 tapes influences their functions, lifetime and productivity [2 /10,23,24]. Virtually, due to the fact that the insulation coatings are necessary to use high temperature compatible materials for turn-to-turn insulation in any magnet built from the high temperature superconductors, we indeed want that the thin, thermally stable and well-adhered insulating materials surrounding the conductors are prevented from electrical short circuits within the winding of a coil. On the other hand, the main point here is the well-adhered insulating coatings including the HTS coil performance
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at magnet application. Based on this, it can be deduced that the bonding strength of the coatings is a very important parameter at determination of the adhesion properties. Owing to this point of view, effects of the adhesion on sol /gel process parameters were scrutinized. The process parameters are precursor materials (type, content in solution, dopant and chemical structure), solvent, chelating agent, viscosity, dilution, withdrawal rate, drying, heat treatment and annealing conditions (temperature, time, atmosphere and heating/cooling rate). Since standard solutions were prepared, namely the same type and amount of solvent, chelating agent were utilized, their effects were not discussed in this work. Because same viscous and diluted solutions were prepared as a result of these parameters, there is also no need to explain their effects. Under these circumstances, we exclusively evaluated effects on adhesion of precursor, type, MgO content, temperature and time of heat treatment. The precursor materials such as Mg(C5H7O2) ×/2H2O (magnesium 2,4-pentanedionate), Mg(CO3)4 ×/Mg(OH)2 ×/ 5H2O (magnesium hydrocarbonate), Mg(OCH3)2 (magnesium metoxide) and MgO (magnesium oxide) directly influence not only stability of the coating structure, but also their adhesion properties. Particularly, the sizes of these chemical powder materials effect mixture of atomic level in states of liquid or solid phases owing to their high solubility in isopropanal solvent. In this case, these properties drastically influence the adhesion of the coatings onto the substrates. As comparatively seen in Fig. 3, the relationships between shear stress and displacement were explained the effects of Mg based precursors depending on precursors type of Ag samples with MgO /ZrO2 joint. Table 1 presents the mechanical properties of MgO / ZrO2 joint samples prepared from these Mg precursors. Here the samples were heat treated at 650 8C for 10 min in air. The bonding strength and maximum load of Ag sample with MgO /ZrO2 prepared from Mg(C5H7O2)×/ 2H2O were 1.3 MPa and 37.56 N, respectively. The first deformation of the sample started at shear stress of 1.3 MPa and then dropped abruptly. After that, the sample presented a strength against deformation at shear stress of 1.24 MPa and elongation of 0.024% again and was consequently separated into two pieces. In samples prepared from MgO, the bonding strength and maximum load were found as 1.045 MPa and 30.159 N, respectively. After the deformation of the samples started at interfacial shear stress of 1.045 MPa and then dropped, the deformation at shear stress of 0.5 MPa and elongation of 0.06% took place again. The sample was consequently divided into two parts at elongation of 0.072%. Even though the samples prepared from Mg(CO3)4 ×/Mg(OH)2 ×/5H2O and Mg(OCH3)2 generally had good bonding strengths and maximum loads, the usage probability of these two
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Fig. 3. Shear stress vs. displacement for Ag substrates with MgO /ZrO2 joint produced from several Mg based precursors as marked.
precursors in sol /gel coatings decreased because they broke into a short time and geometric area remained under curve of shear stress vs. displacement is small. Especially, the bonding strength and maximum load of sample produced from Mg(CO3)4 ×/Mg(OH)2 ×/5H2O are 1.21 MPa and 34.9 N, respectively. In this sample, four steps were determined. The first one is the first deformation which possesses shear stress of 1.18 MPa and elongation of 0.003%. The second deformation was occurred at values of 1.15 MPa and 0.0032%. The deformations after the second step gradually increased and the sample started to peel off. Then the sample was completely separated into two parts at elongation of 0.01%. In the samples prepared from Mg(OCH3)2, deformation was gradually seen and completely broken off at elongation 0.003%. The bonding strength and maximum load of this sample were 0.96 MPa and 27.72 N, respectively. The optimum adhesion was achieved from the samples produced from Mg(C5H7O2)×/2H2O precursor. These behaviors reflected that there is a parallelism the coatings showing the best film growth properties [2,3]. Therefore, it was proved that there is a
strong correlation between film growth and adhesion properties.
Fig. 4. Shear stress vs. displacement for Ag substrates with MgO / ZrO2 joint produced from different mol.% MgO as marked.
Table 1 The mechanical properties of MgO /ZrO2 joint samples prepared from the Mg precursors Precursors
Max. load (N)
Adhesion strength (MPa)
Over lapped area (mm2)
Resistance before tensile testing (V)
Resistance after tensile testing (V)
Mg(C5H7O2)× 2H2O Mg(CO3)4 × Mg(OH)2 × 5H2O MgO Mg(OCH3)2
37.56 34.9 30.159 27.72
1.3 1.21 1.045 0.96
28.86 28.86 28.86 28.86
0.49 0.40 0.22 0.38
0.66 0.40 0.20 0.40
The samples were heat treated at 650 8C for 10 min in air.
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Table 2 The bonding strengths and maximum loads of the joint samples prepared from the composition of several mol.% doped MgO /ZrO2 using Mg(C5H7O2)× 2H2O precursor MgO content (mol.%)
Max. load (N)
Adhesion strength (MPa)
Over lapped area (mm2)
Resistance before tensile testing Resistance after tensile testing (V) (V)
0 12 17 20 24
22.17 25.43 31.42 33.15 37.56
0.768 0.880 1.08 1.150 1.30
28.86 28.86 28.86 28.86 28.86
0.14 0.49 0.50 0.53 0.50
Fig. 4 depicts shear stress vs. displacement for Ag substrates with MgO /ZrO2 joint produced from different mol.% MgO as marked. Furthermore, Table 2 explains bonding strengths and maximum loads of the joint samples prepared from the composition of several mol.% doped MgO /ZrO2 using Mg(C5H7O2)×/2H2O precursor. The joint samples were heat treated at 650 8C for 10 min in air. MgO content added to ZrO2 improved mechanical and adhesion properties of the coatings. As MgO content in ZrO2 increased, the bonding strength increased. The bonding strengths of 0, 12, 17, 20 and 24 mol.% MgO /ZrO2 were 0.768, 0.880, 1.080, 1.150 and 1.300 MPa, respectively. In addition to these values, their maximum loads were 22.17, 25.43, 31.42, 33.15 and 37.56 N, respectively. These samples were directly separated two parts without peeling after breaking off. Although 17 and 20% MgO / ZrO2 samples had almost close values, 24 mol.% MgO / ZrO2 samples had good adhesion properties. Fig. 5 shows shear stress vs. displacement for Ag substrates with MgO /ZrO2 joint, which were heat
0.12 0.63 0.53 0.53 0.52
treated for 10 min at varying temperatures as marked. Deformation started from one side of the joint and propagated by peeling to other side for the MgO /ZrO2 joint samples which were heat treated at temperatures between 500/800 8C. Whereas, deformation of the joint samples which were heat treated at 800 8C started from both of sides in some samples, and other ones directly broke off by giving damages in Ag substrates. More detail results to compare can be seen in Table 3. The results of the tensile tests performed on these samples are given in Table 3, which shows the highest adhesive strength of 1.05 MPa was obtained for 5 the joints by heat-treating at 600 8C for 10 min. Fig. 6 shows shear stress vs. displacement for MgO / ZrO2 joint samples heat treated at 600 8C for varying times. The mechanical properties of MgO /ZrO2 joint samples are given in Table 4. In spite of the decrease in the adhesion strength and maximum load values of samples, which were heat treated more than 20 min at 600 8C, there is an improvement in bonding properties for samples, which were heat treated between 5 /15 min.
Fig. 5. Shear stress vs. displacement for Ag substrates with MgO /ZrO2 joint heat treated for 10 min at varying temperatures as marked.
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Table 3 The mechanical properties of MgO /ZrO2 joint samples heat treated for 10 min at varying temperatures Time (min)
Temperature (8C)
Max. load (N)
Adhesion strength (MPa)
Over lapped area (mm2)
Resistance before tensile testing (V)
Resistance after tensile testing (V)
10 10 10 10 10 10 10
500 550 600 650 700 750 800
9.62 22.68 30.65 22.81 22.34 19.52 17.99
0.333 0.785 1.050 0.780 0.774 0.676 0.632
28.86 28.86 28.86 28.86 28.86 28.86 28.86
0.125 0.18 0.17 0.18 0.10 0.12 0.13
0.09 0.18 0.17 0.17 0.12 0.11 0.12
As seen from the table, 15 min is the optimum treatment time at a temperature of 600 8C for high temperature insulation coatings on Ag and AgMg sheathed Bi /2122 tape conductor. For joint sample with this heat treatment time, adhesion strength and maximum load were 1.12 MPa and 32.59 N, respectively. Fig. 7 shows SEM micrographs of interfacial areas of the MgO /ZrO2 joint samples with varying heat treatment temperatures. The pictures were taken from shear fracture surfaces. The bonding seems to be weak for joints with lower heat treatment temperatures. Adhesion strength and maximum load of the joint sample, which was heat-treated at 500 8C, were very low indicating weak bonding. We conclude that 600 8C was the optimum heat treatment temperature for best bonding for these high temperature insulation coatings on HTS tape conductor. The failure mode of all samples was in the form of a mixed type interfacial/cohesive defects in MgO /ZrO2 coating. That is to say, cohesive failure
took place as conformal cracking and interfacial failure was in the form of spalling. The present results are confirming our earlier reports on mixed interfacial/ cohesive failure mode [24]. EDS analysis and X-ray maps reveal that adhesion failure occurs at Ag/MgO /ZrO2 interface from the fracture surface. Ag, Zr, and Mg were detected in interfacial areas of the MgO /ZrO2 joint which was heat treated at 600 8C for 15 min as seen in Fig. 8. The surface resistances of the joint area prior to tensile testing and the interfacial area after tensile testing contributed to results of SEM, EDS and X-ray maps in order to analyze the deformation. There is no strong relationship between the resistance and the adhesion strength, but yet it seems that the best adhesion has better surface resistance. For instance, the joint samples, which was prepared from Mg(C5H7O2)×/2H2O, possessed a resistance of 0.66 V after tensile testing. As opposed to this, surface resistance of Ag tape is lower
Fig. 6. Shear stress vs. displacement for Ag substrates with MgO /ZrO2 joint heat treated at 600 8C for varying times as marked.
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Table 4 The mechanical properties of MgO /ZrO2 joint heat-treated at 600 8C for varying times Time (min)
Temperature (8C)
Max. load (N)
Adhesion strength (MPa)
Over lapped area (mm2)
Resistance before tensile testing (V)
Resistance after tensile testing (V)
5 10 15 20 25 30
600 600 600 600 600 600
9.02 16.67 32.59 30.44 25.48 12.41
0.311 0.577 1.120 1.060 0.880 0.430
28.86 28.86 28.86 28.86 28.86 28.86
0.25 0.23 0.22 0.17 0.12 0.11
0.15 0.15 0.15 0.11 0.11 0.13
than this value. This result points out that there is still and considerably insulation at interfacial area as seen from surface resistances in Tables.
4. Conclusions MgO /ZrO2 were coated on Ag tapes by sol /gel process using Mg and Zr based precursors. Optimized lap joints were fabricated and pulled to failure by a mini tensile test machine. The effects on adhesion of precursor, type, MgO content, temperature and time of heat treatment were optimized.
The joint samples, which were produced from Mg(C5H7O2) ×/2H2O, Mg(CO3)4 ×/Mg(OH)2 ×/5H2O, Mg(OCH3)2 and MgO directly influenced adhesion properties. The best Mg precursor to prepare sol /gel coating was Mg(C5H7O2)×/2H2O. In this sample, the bonding strength and the maximum load were 1.3 MPa and 37.56 N, respectively. Furthermore, MgO content in ZrO2 increased the bonding strength. The bonding strengths of 0, 12, 17, 20 and 24 mol.% MgO /ZrO2 were found as 0.768, 0.880, 1.080, 1.150 and 1.300 MPa, respectively. The optimum heat treatment conditions were determined as 600 8C and 15 min. In joint sample with optimum heat treatment, the bonding strength and
Fig. 7. SEM micrographs of interfacial areas of the MgO /ZrO2 joint samples with varying heat treatment temperatures as marked. Micrographs were taken from the shear fracture surfaces. The scale bars in the all of the micrographs denote 5 mm.
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Fig. 8. (a) EDS and (b) X-ray maps of interfacial areas of the joint MgO /ZrO2 at 600 8C for 15 min after tensile testing.
maximum load were determined as 1.12 MPa and 32.59 N, respectively. The SEM observation pointed out that the bonding seems to be weak for joints with lower heat treatment temperatures. The failure mode of all samples was in the form of a mixed type interfacial/cohesive defects in MgO /ZrO2 coating. EDS analysis and X-ray maps reveal that adhesion failure occurs at Ag/MgO /ZrO2 interface from the fracture surface.
Acknowledgements
[4]
[5]
[6]
[7]
This work is based upon research carried out at the National High Magnetic Field Laboratory (NHMFL), which is supported by the National Science Foundation, under Award No. DMR /9527035. The authors would like to thank B.R. Goddard for his assistance in SEM pictures, and K.R. Marken at Oxford Superconducting Technology, Inc., for providing Ag and AgMg/Bi-2212 tapes.
[10] [11]
References
[12]
[1] E. Celik, I.H. Mutlu, Y.S. Hascicek, High temperature compatible for superconductors and method of applying insulation to conductors (self-insulating substrate tapes (SIST), USA Patent, Patent number: 6344287; Issued Date: Feb. 5, 2002. [2] E. Celik, E. Avci, Y.S. Hascicek, MgO /ZrO2 insulation coatings on Ag/AgMg sheathed Bi-2212 superconducting tapes by sol /gel technique, Advances in Cryogenic Engineering (Materials) 46 (Part A and B) (2000) 291 /296. [3] H.W. Weijers, Q.Y. Hu, Y.S. Hascicek, A. Godeke, Y. Viouchkov, E. Celik, K. Marken, W. Dai, J. Parrell, Development of 3 T
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class Bi-2212 insert coils for high field NMR, IEEE Transactions on Applied Superconductivity 9 (2) (1999) 563 /568. E. Celik, H.I. Mutlu, E. Avci, Y.S. Hascicek, Y2O3 /ZrO2 insulation coatings on AgMg sheathed Bi-2212 superconducting tapes by sol /gel technique, IEEE Transactions on Applied Superconductivity 10 (1) (2000) 1329 /1332. E. Celik, I.H. Mutlu, Y.S. Hascicek, Electrical properties of high temperature insulation coatings by sol /gel method for magnet technology, IEEE Transactions on Applied Superconductivity 10 (1) (2000) 1341 /1344. E. Celik, I.H. Mutlu, H. Okuyucu, Y.S. Hascicek, Electrical characterization of ceramic insulation coatings for magnet technology, IEEE Transactions on Applied Superconductivity 11 (1) (2001) 2881 /2884. E. Celik, I.H. Mutlu, Y.S. Hascicek, Ceramic insulation for Nb3Sn wires and magnets, Physica C 370 (2002) 125 /131. E. Celik, E. Avci, Y.S. Hascicek, High temperature sol /gel insulation coatings for HTS magnets and their adhesion properties, Physica C 340 (2000) 193 /202. E. Celik, J. Schwartz, E. Avci, Y.S. Hascicek, Evaluation of adhesive strength of sol /gel ceramic insulation for HTS magnets, IEEE Transactions on Applied Superconductivity 9 (2) (1999) 1916 /1920. E. Celik, Ph.D. Thesis, Sakarya University, 2000. E. Celik, I.H. Mutlu, Y. Akin, H. Okuyucu, Y. S. Hascicek, High voltage breakdown capabilities of high temperature insulation coatings for HTS and LTS conductors, Advances in Cryogenic Engineering (Materials) 48 (2002) 277 /286. I.H. Mutlu, E. Celik, Y.S. Hascicek, High temperature insulation coatings and their electrical properties for HTS/LTS conductors, Physica C 370 (2002) 113 /124. C.J. Brinker, G.W. Scherer, Sol /gel science, the physics and chemistry of sol /gel processing (ISBN 0-12-134970-5), Academic Press, Inc, 1990, pp. 839 /881. C.C. Berndt, C.K. Lin, Measurement of adhesion for thermally sprayed materials Adhesion Measurement of Films and Coatings, first ed., vol. 1. Utrecht, The Netherlands. 41 /70, 1995. L. Yang, J. Cheng, Preparation and characterization of zirconium oxide coatings prepared from ZrCl2(OC2H5)2 by the sol /gel process, Journal of Non-Crystalline Solids 112 (1989) 422 /448.
E. Celik et al. / Materials Science and Engineering B97 (2003) 25 /33 [16] M.I. Maleto, L.I. Solovjeva, E.P. Turevskaya, K.A. Vorotilov, M.I. Yanovskaya, Alkoxy-derived Y2O3-stabilized ZrO2 thin films, Thin Solid Films 249 (1994) 1 /5. [17] G. Emig, R. Wirth, R. Zimmermann-Chopin, Sol /gel based precursors for manufacturing refractory oxide fibers, Journal of Materials Science 29 (1994) 4559 /4566. [18] E.Celik, E. Avci, W.Sigmund, Y.S. Hascicek, CeO2 /ZrO2 insulation coatings on Ag/AgMg sheathed Bi-2212 superconducting tapes by the sol /gel process for magnet technology, K.L. Mittal (Ed.) ISBN 90-6764-182-0, IEEE Transactions on Applied Superconductivity, 12 (2002) 1223 /1226. [19] E. Celik, E. Avci, Y.S. Hascicek, Synthesis and characterization of high temperature compatible ZrO2 insulation coatings on Ag/ AgMg Sheathed Bi-2212 Wires and Tapes, Surface and Coating Technology, 161 (2002) 355 /360.
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[20] F.Ustel, E. Celik, E.Avci, Investigation on adhesive strengths of plasma sprayed coating, Proceedings of the Eighth International Metallurgy and Materials Congress,1153 /1161, June 6 /9 1995, Istanbul, Turkey. [21] ASTM Standard 633 /679, Standard Test Method for Adhesion Strength of Flame-Sprayed Coatings, 1999. [22] D.A.J. Ramm, I.M. Hucthing, T. Clyne, Journal de Physique 3 (1993). [23] W. Zhang, W. Liu, Q. Xue, Characterization and tribological investigation of Al2O3 and modified Al2O3 sol /gel films, Materials Research Bulletin 36 (2000) 1903 /1914. [24] E. Celik, Y.S. Hascicek, Evaluation of adhesive strength of sol / gel ceramic insulation for HTS magnets, NHMFL 1998 Annual Report Vol. 1, 89 /90, January 1999, Tallahassee, USA.
Adhesion properties of MgO ZrO2 insulation coatings for 5 Tesla HTS coils /
E. Celik a,b,c, , E. Avci d, Y.S. Hascicek a a
National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Dr., Tallahassee, FL 32310, USA Faculty of Engineering, Department of Metallurgical and Materials Engineering, Dokuz Eylul University, Bornova, Izmir 35100, Turkey c Superconductivity Research Laboratory, ISTEC, JFCC, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan d Faculty of Engineering, Department of Metallurgical and Materials Engineering, Esentepe Campus, Sakarya University, Sakarya 54187, Turkey b
Received 28 May 2002; accepted 23 September 2002
Abstract Self insulating substrate tapes (SIST) is the most promising insulation technique for high temperature MgO /ZrO2 coatings on Ag and AgMg sheathed Bi2Sr2Ca1Cu2Ox (Bi-2212) superconducting tapes and wires in applications of HTS/LTS coils and magnets. We have already reported successful results as to the synthesis, characterizations and applications of the insulation coatings using the SIST. In order to provide no electrical short circuit in Jc measurements of HTS/LTS coils, the bonding of the coatings onto the substrate is a very important issue. In this present research, the adhesion properties of high temperature MgO /ZrO2 coatings were scrutinized for different processing parameters. Lap joints were fabricated by laying fresh sol /gel coated silver tape samples over each other and then by heat-treating at temperature range of 500 /800 8C for several times in air. These joint samples were pulled to failure by using a mini tensile tester. MgO /ZrO2 was coated on Ag tapes by sol /gel process using Mg and Zr based precursors. The obtained results obviously pointed out that the best Mg precursor is Mg(C5H7O2) ×/2H2O to prepare solution and there is a strong relationship between film growth and adhesion properties. Also, MgO content in ZrO2 increased its bonding strength. The optimum heat treatment conditions are 600 8C and 15 min for best bonding for these high temperature insulation coatings on HTS tape conductor. The failure mode of all samples was in the form of a mixed type interfacial/cohesive defects in MgO /ZrO2 coating. # 2002 Elsevier Science B.V. All rights reserved. Keywords: MgO /ZrO2; Sol /gel; Insulation; Adhesion; Self insulating substrate tapes; HTS/LTS conductor
1. Introduction High temperature MgO /ZrO2 coatings have been insulated on Ag and AgMg sheathed Bi-2212 tapes using a reel-to-reel, continuous sol /gel technique, which is specifically called as ‘self insulating substrate tapes’ at the National High Magnetic Field Laboratory (NHMFL) in order to provide turn-to-turn electrical insulation for HTS coils [1 /7]. It is necessary to use high temperature compatible insulating materials for turn-toturn insulation in Wind and React (W&R) magnet built from the high temperature superconductor. A thin, thermally stable and well-adhered coatings must (a)
Corresponding author. Tel.: /1-90-232-388-2880; fax: /1-90232-388-7864. Internet: http://www.deu.edu.tr E-mail address: [email protected] (E. Celik).
insulate at the coil’s operating temperatures; (b) retain continuity when shaping the coil; (c) be permeable to oxygen; (d) not induce damage to the HTS material or any sheathing material it might be encased within; (e) not significantly increase the overall size of the coil; and (f) be compatible with the thermal processing of the HTS material [8 /11]. In these coatings, the most important problem is the bonding of coatings to the substrate during the winding process [12]. The mechanical stability and adhesion of the coatings signify their fundamental quality. In sol /gel insulation coatings, factors affecting to the adhesion are generally residual stresses, coating structure (porosity, micro and macro cracks, coating thickness) and deposition parameters [9,10,12 /14]. The sol /gel coating parameters, which contain solution properties (precursors, solvent, chelating agent, viscosity and dilution), withdrawal rate, drying, heat treatment and sintering or annealing
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 3 9 1 - 4
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conditions (temperature, time, atmosphere, heating/ cooling rates), play important roles in the bonding of coatings onto tape substrates [1 /10,14 /17]. Coating failures can occur by many mechanisms such as surface damage (e.g. wear, oxidation), elastic or plastic deformation, fracture, etc. The degradation or failure of coatings is fundamentally related to the adhesion and cohesion strength. The failure mode(s) can be described as interfacial, cohesive, or mixed interfacial/cohesive defects, as indicated in Fig. 1 [12 /15]. Previously [8,9], the adhesion properties of the high temperature coatings were basically demonstrated. In this present work, the adhesion properties of MgO / ZrO2 coatings on silver tape substrates were emphasized for sol /gel process parameters regarding precursor type, MgO contents, heat treatment temperature and time.
2. Experimental procedure The MgO /ZrO2 coatings were coated on commercial Ag tapes with nominal dimensions of 65 /9.62 /0.05 mm3 through a sol /gel precursor dipping technique. Substrate surfaces were cleaned with acetone, ethanol and water. Zirconium tetrabutoxide and several Mg based precursors were used as precursors, acetyleacetone as a chelating agent, and isopropanal as a solvent to prepare clear homogenous sols. The gel films were formed by means of dip coating and dried at 300 8C for 1 min in air to decompose the precursors and organic groups and then heat treated at a temperature of 650 8C for 10 min in air. Further information on solution preparation and coating process can be found in Refs. [1,2,4 /8,18]. The sample surfaces were ultrasonically cleaned in acetone. After dipping the samples into the solutions, lap joints were prepared as shown in Fig. 2a. A few pounds of weight was put on the samples to improve contact between the coated Ag samples and then dried
Fig. 2. (a) The tensile test sample with a joint mode by sandwiching two sol /gel dipped Ag tapes; (b) Tensile testing (sS, shear stress at the coating /Ag interface; N , load and A , over lapped area).
at room temperature for 1 day, and then heat-treated. Oxide films were formed in interfacial area between two silver tapes by annealing each sample at temperatures between 500 /800 8C for durations of 5/30 min in air. The optimum precursor type, MgO content, heat treatment temperature and time were determined for MgO /ZrO2 coating on Ag and AgMg sheathed Bi-2212 tapes thereby using the adhesion properties. These joint samples were pulled to failure using a mini tensile tester. The joint samples were prepared such that the failure was in shear mode of the interface of insulated Ag substrate, and not at the joint legs, as seen in Fig. 2b. Experiments were tentatively repeated until optimum overlap area in the samples was obtained. In these experiments, the optimum overlap area was found as a 28.86 mm2. Measurements were done after this optimization. The tests, which showed a shear failure at the overlap area, were taken into consideration. After tensile tests, the microstructures of failure surfaces of the joint samples were examined with the help of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-Ray mapping. Additionally, surface resistance of the joint samples was measured before and after tensile testing in order to
Fig. 1. Interfacial and cohesive failure modes [10].
E. Celik et al. / Materials Science and Engineering B97 (2003) 25 /33
visually describe whether there is insulation at interfacial area.
3. Results and discussion There are many methods to measure the bonding strength of the coatings. As we have reported earlier [8,9], a lap-shear test of the joint samples is one of them thereby utilizing a mini tensile testing. Normally, the tensile testing is performed in evaluation of the mechanical strength of metals and alloys. In these tests, the joint samples, as indicated in Fig. 2a, were relatively pulled to failure in a constant velocity and a short time like a few minutes. The variations depending on a constant area of applied load to the samples give shear stresses. In this context, the mini tensile testing, which was specifically designed, was performed to determine the adhesion properties of the joint samples. The bonding strength of the coatings was found as a result of these experiments. Notably, the determination of the bonding strength of Ag tapes joint with MgO /ZrO2 is a combination of versatile mechanical test mechanisms. The fundamental mechanism is tensile testing, as described in Eq. (1); ss
F A
(1)
Where, ss is a tensile strength and F is an applied load to a unit area (A ). The mechanism supporting this rule is a phenomenon that the peeling of the coatings realizes. Even though the joining of the coatings on metal substrates with another metal is provided with epoxy resin in the most of Refs. [13,19/22], the joint samples produced by dipping into the solutions of two Ag tapes are dried at room temperature and then heat-treated at the temperature range of 500/800 8C for several times in air. The ceramic joint samples were pulled by the mini tensile testing. The shear stress found after this process gives the bonding strength of the coating to the substrate. The bonding strength is determined as the shear stress at which point the joint yields abruptly. It is known that the adhesion behavior of the MgO / ZrO2 insulation coatings on Ag and AgMg sheathed Bi2212 tapes influences their functions, lifetime and productivity [2 /10,23,24]. Virtually, due to the fact that the insulation coatings are necessary to use high temperature compatible materials for turn-to-turn insulation in any magnet built from the high temperature superconductors, we indeed want that the thin, thermally stable and well-adhered insulating materials surrounding the conductors are prevented from electrical short circuits within the winding of a coil. On the other hand, the main point here is the well-adhered insulating coatings including the HTS coil performance
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at magnet application. Based on this, it can be deduced that the bonding strength of the coatings is a very important parameter at determination of the adhesion properties. Owing to this point of view, effects of the adhesion on sol /gel process parameters were scrutinized. The process parameters are precursor materials (type, content in solution, dopant and chemical structure), solvent, chelating agent, viscosity, dilution, withdrawal rate, drying, heat treatment and annealing conditions (temperature, time, atmosphere and heating/cooling rate). Since standard solutions were prepared, namely the same type and amount of solvent, chelating agent were utilized, their effects were not discussed in this work. Because same viscous and diluted solutions were prepared as a result of these parameters, there is also no need to explain their effects. Under these circumstances, we exclusively evaluated effects on adhesion of precursor, type, MgO content, temperature and time of heat treatment. The precursor materials such as Mg(C5H7O2) ×/2H2O (magnesium 2,4-pentanedionate), Mg(CO3)4 ×/Mg(OH)2 ×/ 5H2O (magnesium hydrocarbonate), Mg(OCH3)2 (magnesium metoxide) and MgO (magnesium oxide) directly influence not only stability of the coating structure, but also their adhesion properties. Particularly, the sizes of these chemical powder materials effect mixture of atomic level in states of liquid or solid phases owing to their high solubility in isopropanal solvent. In this case, these properties drastically influence the adhesion of the coatings onto the substrates. As comparatively seen in Fig. 3, the relationships between shear stress and displacement were explained the effects of Mg based precursors depending on precursors type of Ag samples with MgO /ZrO2 joint. Table 1 presents the mechanical properties of MgO / ZrO2 joint samples prepared from these Mg precursors. Here the samples were heat treated at 650 8C for 10 min in air. The bonding strength and maximum load of Ag sample with MgO /ZrO2 prepared from Mg(C5H7O2)×/ 2H2O were 1.3 MPa and 37.56 N, respectively. The first deformation of the sample started at shear stress of 1.3 MPa and then dropped abruptly. After that, the sample presented a strength against deformation at shear stress of 1.24 MPa and elongation of 0.024% again and was consequently separated into two pieces. In samples prepared from MgO, the bonding strength and maximum load were found as 1.045 MPa and 30.159 N, respectively. After the deformation of the samples started at interfacial shear stress of 1.045 MPa and then dropped, the deformation at shear stress of 0.5 MPa and elongation of 0.06% took place again. The sample was consequently divided into two parts at elongation of 0.072%. Even though the samples prepared from Mg(CO3)4 ×/Mg(OH)2 ×/5H2O and Mg(OCH3)2 generally had good bonding strengths and maximum loads, the usage probability of these two
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Fig. 3. Shear stress vs. displacement for Ag substrates with MgO /ZrO2 joint produced from several Mg based precursors as marked.
precursors in sol /gel coatings decreased because they broke into a short time and geometric area remained under curve of shear stress vs. displacement is small. Especially, the bonding strength and maximum load of sample produced from Mg(CO3)4 ×/Mg(OH)2 ×/5H2O are 1.21 MPa and 34.9 N, respectively. In this sample, four steps were determined. The first one is the first deformation which possesses shear stress of 1.18 MPa and elongation of 0.003%. The second deformation was occurred at values of 1.15 MPa and 0.0032%. The deformations after the second step gradually increased and the sample started to peel off. Then the sample was completely separated into two parts at elongation of 0.01%. In the samples prepared from Mg(OCH3)2, deformation was gradually seen and completely broken off at elongation 0.003%. The bonding strength and maximum load of this sample were 0.96 MPa and 27.72 N, respectively. The optimum adhesion was achieved from the samples produced from Mg(C5H7O2)×/2H2O precursor. These behaviors reflected that there is a parallelism the coatings showing the best film growth properties [2,3]. Therefore, it was proved that there is a
strong correlation between film growth and adhesion properties.
Fig. 4. Shear stress vs. displacement for Ag substrates with MgO / ZrO2 joint produced from different mol.% MgO as marked.
Table 1 The mechanical properties of MgO /ZrO2 joint samples prepared from the Mg precursors Precursors
Max. load (N)
Adhesion strength (MPa)
Over lapped area (mm2)
Resistance before tensile testing (V)
Resistance after tensile testing (V)
Mg(C5H7O2)× 2H2O Mg(CO3)4 × Mg(OH)2 × 5H2O MgO Mg(OCH3)2
37.56 34.9 30.159 27.72
1.3 1.21 1.045 0.96
28.86 28.86 28.86 28.86
0.49 0.40 0.22 0.38
0.66 0.40 0.20 0.40
The samples were heat treated at 650 8C for 10 min in air.
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Table 2 The bonding strengths and maximum loads of the joint samples prepared from the composition of several mol.% doped MgO /ZrO2 using Mg(C5H7O2)× 2H2O precursor MgO content (mol.%)
Max. load (N)
Adhesion strength (MPa)
Over lapped area (mm2)
Resistance before tensile testing Resistance after tensile testing (V) (V)
0 12 17 20 24
22.17 25.43 31.42 33.15 37.56
0.768 0.880 1.08 1.150 1.30
28.86 28.86 28.86 28.86 28.86
0.14 0.49 0.50 0.53 0.50
Fig. 4 depicts shear stress vs. displacement for Ag substrates with MgO /ZrO2 joint produced from different mol.% MgO as marked. Furthermore, Table 2 explains bonding strengths and maximum loads of the joint samples prepared from the composition of several mol.% doped MgO /ZrO2 using Mg(C5H7O2)×/2H2O precursor. The joint samples were heat treated at 650 8C for 10 min in air. MgO content added to ZrO2 improved mechanical and adhesion properties of the coatings. As MgO content in ZrO2 increased, the bonding strength increased. The bonding strengths of 0, 12, 17, 20 and 24 mol.% MgO /ZrO2 were 0.768, 0.880, 1.080, 1.150 and 1.300 MPa, respectively. In addition to these values, their maximum loads were 22.17, 25.43, 31.42, 33.15 and 37.56 N, respectively. These samples were directly separated two parts without peeling after breaking off. Although 17 and 20% MgO / ZrO2 samples had almost close values, 24 mol.% MgO / ZrO2 samples had good adhesion properties. Fig. 5 shows shear stress vs. displacement for Ag substrates with MgO /ZrO2 joint, which were heat
0.12 0.63 0.53 0.53 0.52
treated for 10 min at varying temperatures as marked. Deformation started from one side of the joint and propagated by peeling to other side for the MgO /ZrO2 joint samples which were heat treated at temperatures between 500/800 8C. Whereas, deformation of the joint samples which were heat treated at 800 8C started from both of sides in some samples, and other ones directly broke off by giving damages in Ag substrates. More detail results to compare can be seen in Table 3. The results of the tensile tests performed on these samples are given in Table 3, which shows the highest adhesive strength of 1.05 MPa was obtained for 5 the joints by heat-treating at 600 8C for 10 min. Fig. 6 shows shear stress vs. displacement for MgO / ZrO2 joint samples heat treated at 600 8C for varying times. The mechanical properties of MgO /ZrO2 joint samples are given in Table 4. In spite of the decrease in the adhesion strength and maximum load values of samples, which were heat treated more than 20 min at 600 8C, there is an improvement in bonding properties for samples, which were heat treated between 5 /15 min.
Fig. 5. Shear stress vs. displacement for Ag substrates with MgO /ZrO2 joint heat treated for 10 min at varying temperatures as marked.
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Table 3 The mechanical properties of MgO /ZrO2 joint samples heat treated for 10 min at varying temperatures Time (min)
Temperature (8C)
Max. load (N)
Adhesion strength (MPa)
Over lapped area (mm2)
Resistance before tensile testing (V)
Resistance after tensile testing (V)
10 10 10 10 10 10 10
500 550 600 650 700 750 800
9.62 22.68 30.65 22.81 22.34 19.52 17.99
0.333 0.785 1.050 0.780 0.774 0.676 0.632
28.86 28.86 28.86 28.86 28.86 28.86 28.86
0.125 0.18 0.17 0.18 0.10 0.12 0.13
0.09 0.18 0.17 0.17 0.12 0.11 0.12
As seen from the table, 15 min is the optimum treatment time at a temperature of 600 8C for high temperature insulation coatings on Ag and AgMg sheathed Bi /2122 tape conductor. For joint sample with this heat treatment time, adhesion strength and maximum load were 1.12 MPa and 32.59 N, respectively. Fig. 7 shows SEM micrographs of interfacial areas of the MgO /ZrO2 joint samples with varying heat treatment temperatures. The pictures were taken from shear fracture surfaces. The bonding seems to be weak for joints with lower heat treatment temperatures. Adhesion strength and maximum load of the joint sample, which was heat-treated at 500 8C, were very low indicating weak bonding. We conclude that 600 8C was the optimum heat treatment temperature for best bonding for these high temperature insulation coatings on HTS tape conductor. The failure mode of all samples was in the form of a mixed type interfacial/cohesive defects in MgO /ZrO2 coating. That is to say, cohesive failure
took place as conformal cracking and interfacial failure was in the form of spalling. The present results are confirming our earlier reports on mixed interfacial/ cohesive failure mode [24]. EDS analysis and X-ray maps reveal that adhesion failure occurs at Ag/MgO /ZrO2 interface from the fracture surface. Ag, Zr, and Mg were detected in interfacial areas of the MgO /ZrO2 joint which was heat treated at 600 8C for 15 min as seen in Fig. 8. The surface resistances of the joint area prior to tensile testing and the interfacial area after tensile testing contributed to results of SEM, EDS and X-ray maps in order to analyze the deformation. There is no strong relationship between the resistance and the adhesion strength, but yet it seems that the best adhesion has better surface resistance. For instance, the joint samples, which was prepared from Mg(C5H7O2)×/2H2O, possessed a resistance of 0.66 V after tensile testing. As opposed to this, surface resistance of Ag tape is lower
Fig. 6. Shear stress vs. displacement for Ag substrates with MgO /ZrO2 joint heat treated at 600 8C for varying times as marked.
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Table 4 The mechanical properties of MgO /ZrO2 joint heat-treated at 600 8C for varying times Time (min)
Temperature (8C)
Max. load (N)
Adhesion strength (MPa)
Over lapped area (mm2)
Resistance before tensile testing (V)
Resistance after tensile testing (V)
5 10 15 20 25 30
600 600 600 600 600 600
9.02 16.67 32.59 30.44 25.48 12.41
0.311 0.577 1.120 1.060 0.880 0.430
28.86 28.86 28.86 28.86 28.86 28.86
0.25 0.23 0.22 0.17 0.12 0.11
0.15 0.15 0.15 0.11 0.11 0.13
than this value. This result points out that there is still and considerably insulation at interfacial area as seen from surface resistances in Tables.
4. Conclusions MgO /ZrO2 were coated on Ag tapes by sol /gel process using Mg and Zr based precursors. Optimized lap joints were fabricated and pulled to failure by a mini tensile test machine. The effects on adhesion of precursor, type, MgO content, temperature and time of heat treatment were optimized.
The joint samples, which were produced from Mg(C5H7O2) ×/2H2O, Mg(CO3)4 ×/Mg(OH)2 ×/5H2O, Mg(OCH3)2 and MgO directly influenced adhesion properties. The best Mg precursor to prepare sol /gel coating was Mg(C5H7O2)×/2H2O. In this sample, the bonding strength and the maximum load were 1.3 MPa and 37.56 N, respectively. Furthermore, MgO content in ZrO2 increased the bonding strength. The bonding strengths of 0, 12, 17, 20 and 24 mol.% MgO /ZrO2 were found as 0.768, 0.880, 1.080, 1.150 and 1.300 MPa, respectively. The optimum heat treatment conditions were determined as 600 8C and 15 min. In joint sample with optimum heat treatment, the bonding strength and
Fig. 7. SEM micrographs of interfacial areas of the MgO /ZrO2 joint samples with varying heat treatment temperatures as marked. Micrographs were taken from the shear fracture surfaces. The scale bars in the all of the micrographs denote 5 mm.
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Fig. 8. (a) EDS and (b) X-ray maps of interfacial areas of the joint MgO /ZrO2 at 600 8C for 15 min after tensile testing.
maximum load were determined as 1.12 MPa and 32.59 N, respectively. The SEM observation pointed out that the bonding seems to be weak for joints with lower heat treatment temperatures. The failure mode of all samples was in the form of a mixed type interfacial/cohesive defects in MgO /ZrO2 coating. EDS analysis and X-ray maps reveal that adhesion failure occurs at Ag/MgO /ZrO2 interface from the fracture surface.
Acknowledgements
[4]
[5]
[6]
[7]
This work is based upon research carried out at the National High Magnetic Field Laboratory (NHMFL), which is supported by the National Science Foundation, under Award No. DMR /9527035. The authors would like to thank B.R. Goddard for his assistance in SEM pictures, and K.R. Marken at Oxford Superconducting Technology, Inc., for providing Ag and AgMg/Bi-2212 tapes.
[10] [11]
References
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