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Diamond-like carbon coating for effective electrical insulation of Cu and Al wires
Diamond & Related Materials 103 (2020) 107731
Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Diamond-like carbon coating for effective electrical insulation of Cu and Al wires☆
T
⁎
Daichi Nakajimaa, , Hiroki Kuwabarab, Shigeru Annakab, Shinya Fujiic, Yoshikazu Tanakac, Kenji Hirakuria a
Department of Electrical and Electronic Engineering, Tokyo Denki University, Japan Nakadai Metals Co., Ltd., Japan c Nippon ITF, Inc., Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Diamond-like carbon (DLC) Electrical properties characterization Thermal stability Insulator Electronic device structure
The physical and mechanical properties, electrical insulation, chemical resistance, and heat resistance of diamond-like carbon (DLC) films fabricated using the radio frequency plasma-enhanced chemical vapor deposition method (RF-PECVD) on Cu (DLC/Cu) and Al (DLC/Al) substrates were evaluated in this study. The electrical insulation testing results showed that the dielectric breakdown voltage of both DLC/Cu and DLC/Al samples was approximately 145 kV/mm. For assessing the heat resistance, the samples were stored at a high temperature of 200 °C for a fixed time duration, and the changes in the insulation voltage resistance were evaluated. Based on the voltage resistance measurement results of the electrical insulation after heat resistance testing, the study confirmed that the DLC film is heat stable for 7 days of heating. In addition, testing confirmed that the DLC coating remained stable with no deterioration when immersed in both acidic and alkaline solutions, thus indicating high chemical resistance. These findings suggest that DLC is suitable for application as a thin and effective electrical insulation coating, and its properties support the realization of small coil lengths with square wires.
1. Introduction IN the modern world, there is an increasing demand for smaller and lighter electronic devices with high possibility for integration. Especially, to meet the demand for thinner and highly integrable smartphones, the mounted components need to be made smaller and lighter. Further evolution of ultra-compact technology is also essential for the components of wearable devices, a product group expected to grow into a major industry in the future [1]. The most fundamental components that constitute these electronic devices are resistance, capacitance, and inductance (coils). Resistance and capacitance are linked to the microelectronic states of the materials. The refinement of these components is progressing rapidly owing to foundational and practical researches being conducted frequently in this field [2]. On the other hand, coils have been used to prevent short circuits in round conducting wires made from materials with good conductivity, such as copper (Cu) and aluminum (Al), for approximately 100 years. Moreover, materials such as polyurethane and polyamidide with enamel coating over a wire
core are still being used today [3]. For this reason, despite ongoing attempts for refinements, such as miniaturization of conductive wires, thinner insulating films have not been achieved, which has prevented further progress in this field. In addition, attempts to reduce the size of these components by reducing the coil windings have decreased the inductance properties. To resolve this problem, a method has been proposed that involves winding more wire in a smaller space, in other words, increasing the space factor. Since the year 2000, the use of square wires instead of the standard round wires currently in use has been considered as a means for achieving increased space factor. Square wires are expected to show significant increase in the space factor when compared with round wires because round wires have contact points with one another when coiled but square wires have surface contact. However, when the standard enamel coating was applied to these square wires, various problems caused by surface tension, such as elliptical shape transformation of the insulating film, curvature of the film, and thickening of the insulation layer, hindered the application of the square wire technology. To resolve this problem, a thinner
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DLC: diamond-like carbon; RF-PECVD: radio frequency plasma-enhanced chemical vapor deposition; CVD: chemical vapor deposition; PVD: physical vapor deposition, PUW: polyurethane wire; PEW: polyethylene wire (PEW); SEM: scanning electron microscopy. ⁎ Corresponding author. E-mail address: [email protected] (D. Nakajima). https://doi.org/10.1016/j.diamond.2020.107731 Received 21 October 2019; Received in revised form 19 January 2020; Accepted 22 January 2020 Available online 23 January 2020 0925-9635/ © 2020 Elsevier B.V. All rights reserved.
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insulating film with superior electrical insulation properties and a square cross-sectional shape is desired. Diamond-Like Carbon (DLC) film is an amorphous carbon formed by graphite sp2 bonds and diamond sp3 bonds. In addition, the film contains between 0 and 40 atm% of hydrogen, and this property is dependent on the ratio of the sp2 and sp3 bonds as well as their hydrogen content [4]. Therefore, DLC coating can be prepared to suit the purpose and the properties of the surface materials with which it is used. A film with diamond-like hardness can be created by reducing the hydrogen content. With higher hydrogen content, it becomes a film with gas barrier properties and chemical resistance [5]. Because films with properties such as high hardness, low friction, and chemical resistance can be formed by introducing variations in the preparation method, the technology is widely used for practical applications such as surface coating of cutting tools, mainly for industrial use [6]. In addition, owing to the high electrical insulation properties of these films, there are reports of other uses of the film, for example, as an ultra-thin interlayer insulation for semiconductor IC units [7]. Two broad types of film formation methods are generally employed in the DLC manufacturing process. These methods are Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD), of which arc plating is an example. CVD uses a chemical reaction with gas raw materials for film formation, and PVD sublimates solid materials through the application of heat energy to form a film layer. Among the CVD methods, plasma CVD has the benefits of low film formation temperature, even film thickness, and the ability to form film layers with a 3-dimensional structure [8]. In this study, a CVD method capable of producing a coating on 3dimensional structural objects was used with the goal of developing a new DLC insulating film with functionalities such as electrical insulation, sliding properties, chemical resistance, and heat resistance. To achieve this goal, a direct current dielectric breakdown test was used to evaluate the electrical insulation properties of the DLC film through measurement of the maximum voltage the electrical insulation could withstand. In terms of the mechanical properties of the DLC film, a scratch test was used to determine the adhesion, and a ball-on-disc test was to measure the friction coefficient for evaluating the sliding properties. In addition, acid resistance and alkaline resistance tests were conducted to assess the chemical resistance properties considering the possible contact environment within electronic devices. A simulation in the form of a heat resistance test was also performed because applications of this technology would also require operation within the high-temperature environment inside electronic devices. By conducting the abovementioned evaluations of the DLC film, this study considered the usability of the technology for square wire coil insulation films.
Raman spectroscopy is generally used for structural evaluation of DLC; in this study, this method was employed with spectral shape peak ratio and peak intensity values of ID and IG, respectively. The adhesion of the DLC coating on metallic materials such as Cu and Al is considered poor, and DLC film formation is considered difficult [9]. For this reason, Raman spectroscopy (NRS-4100, JASCO) was used for evaluating the films on the flat DLC/Cu and DLC/Al test samples to confirm DLC film formation on the respective substrate surfaces. In addition, the breakdown voltages of insulating materials are highly dependent on the sample thickness of the materials [10]. Accordingly, the electrical insulation property of insulating materials is a value that considers the sample thickness of the material. Therefore, the minimum electrical field value required for insulation breakdown is often indicated. The main methods generally used for film thickness measurements are scanning electron microscopy (SEM) for cross-sectional measurements and light interference effect using a spectrophotometer. Cross-sectional observations using SEM are prone to errors owing to the effects of the viewing angles. Therefore, a UV–visible spectrophotometer (V-770, JEOL) was used for simple optical measurements of the film thickness on the DLC/Cu and DLC/Al samples. 2.2. Evaluation of film adhesion using scratch test In the case of insulating films on conducting wires, the adhesion of the film to the wire is crucial for the safety and reliability of electronic devices [11]. When adhesion is poor, the film can crack or peel when stressed during winding. This causes current leakage from the compromised areas of the insulating film, resulting in malfunction and failure of the electronic devices. Therefore, the adhesion property of the DLC film was evaluated using a scratch test machine (CSR5000, RHESCA) to ensure satisfactory adhesion to the wires (Cu, Al). In the testing conditions, scratch speed was 10 μm/s, rate of increase of applied load was 700mN/min, the humidity was 50%, and the temperature was 24 °C. The measurements were carried out according to the ISO 20502 standard [12]. Testing was conducted 3 times for DLC/Cu and DLC/Al substrates, and the results were averaged to determine the critical load. 2.3. Ball-on-disc testing for measurement of friction coefficient The winding properties of relatively flexible metal wires, such as Cu and Al, are considerably affected by the friction coefficient of the insulating film [13]. Owing to miniaturization and increased efficiency of electronic devices, a large amount of wire is coiled in a confined space, resulting in higher space factor; therefore, winding is considered to be an increasingly important property. Therefore, a ball-on-disc testing machine (Tribometer, CSEM) was used to measure the friction coefficient of the DLC/Cu and DLC/Al samples to determine the sliding property of the DLC film. In addition, the flat Cu and Al substrates without the DLC film, as well as a standard insulation-coated polyurethane wire (PUW), were used for the control group friction coefficient measurements. The object used was an SUJ2 ball with a diameter of 6 mm, and the load used was 3 N. In the testing conditions, the humidity was 50%, and the temperature was 24 °C. This measurement was conducted based on the ISO/DIS 18535 standard [14].
2. Experimental procedures 2.1. DLC formation and film structural analysis The radio frequency plasma-enhanced chemical vapor deposition method (RF-PECVD) was used to create a 13.56 MHz radio frequency electrical excitation source for DLC film formation. CH4 was used as the raw material gas. In this experiment, the following materials with excellent electrical conductivities, which are also used for manufacturing coils in real-world applications, were used as the raw materials: C1100 (10 mm × 10 mm × 0.5 mm) copper and A5052 (10 mm × 10 mm × 1.0 mm) aluminum. Film formation was performed on flat C1100 and A5052 substrates to evaluate the physical properties of the DLC film. The C1100 and A5052 base metal samples are denoted here as Cu and Al, whereas the flat C1100 and A5052 substrates treated with the DLC film are denoted as DLC/Cu and DLC/ Al, respectively. In each experiment, flat test pieces of DLC/Cu and DLC/Al substrates were used. The film thickness and structure were measured to analyze the fundamental properties of the DLC film formed on the test materials.
2.4. Measurement of dielectric breakdown voltage The dielectric breakdown voltage is the most important property to be considered when applying insulating DLC films to coil applications. To investigate this essential property, dielectric breakdown voltage was measured for the DLC/Cu and DLC/Al test pieces. In this experiment, voltages actually used in electronic devices were applied through direct current. The details of the experimental procedure are as follows: A direct current dielectric breakdown testing machine (SM7100, HIOKI) was used. 2
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1. Because surface contamination of the DLC/Cu and DLC/Al test pieces could cause creeping discharge [15], ethanol-soaked cotton swabs were used as necessary for cleaning. 2. The specimens were set atop a 100 × 200 × 0.5 mm Cu electrode, and a two-pin lead was used. One pin contacted the upper portion of the samples, and the other a Cu electrode. 3. Direct current was applied at a fixed voltage rate (2 V/s), and the voltage at which dielectric breakdown occurred was measured. 4. The dielectric breakdown field strength was then used to calculate the DLC film thickness. 2.5. Heat resistance testing of DLC films Owing to the increased miniaturization and enhanced performance of electrical components used in electronic devices, such as semiconductor elements and capacitors, the heat they generate has increased [16]. In particular, the heat output of semiconductor elements is extremely high in power devices with current limited to at least several hundred amperes [16]. In some cases, this heat released by electronic devices can affect the insulation performance of the insulating materials [17]. The declining insulation performance caused by heat deterioration is linked to device reliability issues such as malfunctions and failure [17]. Therefore, the heat resistance of the DLC film was evaluated using an electrical method. Both DLC/Cu and DLC/ Al substrates were stored under a temperature of 200 °C and humidity of 40 ± 2% in an enclosed space for a 7-day period (168 h). The substrate was then returned to room temperature (25 °C), and the insulation breakdown voltage was measured. The heat resistance was evaluated based on the rate of variability in the insulation breakdown voltage before and after this heating experiment. The electrical insulation voltage resistance was measured using the method described in Section 2.4.
Fig. 1. Raman spectra of each sample.
spectroscopy. The DLC was evaluated by verifying the presence or absence of the G band and D band by following the general process. The results of the Raman spectroscopy analysis are shown in Fig. 1. The horizontal axis in the graph in the figure indicates the Raman shift and vertical axis indicates the intensity. To compare with the general structure of the DLC film, Raman spectroscopy was also conducted on a Si substrate with a DLC film applied to the surface (DLC/Si). The results demonstrated that both the DLC/Cu and the DLC/Al samples had the same unique DLC properties. The G(graphite)-peak, caused by the internal oscillation of the graphite carbon atoms in a hexagonal lattice (E2g), was observed at approximately 1550 cm−1. The D(disorder)peak, caused by crystalline strain vibrations of the graphite structure (A1g), was observed at approximately 1330 cm−1. This confirmed that a DLC film had formed on both the Cu and Al substrate samples. In addition, a correlation can be obtained between the ID/IG ratio, which is calculated from these two peak intensities and the sp3 ratio. The higher the ID/IG ratio, the lower the sp3 ratio [19]. Further, the ID/IG of this DLC film is 0.49. Optical measurement of the film thickness was conducted for DLC/ Cu and DLC/Al test pieces using UV–visible spectroscopy. The film thickness was calculated from the obtained reflectance spectrum by fitting with the model formula. The estimated film thicknesses were 1.063 μm and 1.045 μm for DLC/Cu and DLC/Al, respectively.
2.6. Evaluation of chemical resistance of DLC films Insulating films in electronic devices are expected to protect the wiring and inductors from acidic and alkaline solutions formed by battery leakage. Therefore, the chemical resistance properties of insulating films are important [5]. Accordingly, the chemical resistance of the DLC film was investigated through an immersion test using acidic and alkaline solutions. This test used sulfuric acid (H2SO4, specific gravity 1.2) and sodium hydroxide (NaOH, 10%), which are commonly used as electrolyte solutions of batteries. For this test, the DLC/Cu and DLC/Al test pieces were immersed in the solutions at room temperature (25 °C) for 24 h, which represented one cycle. The samples were then washed for 10 min in pure water. A total of 7 cycles were implemented. In addition to the DLC/Cu and DLC/Al samples, PUW was also used as a contrast material in immersion testing. Because aluminum, the base material of DLC/Al, exhibits no chemical resistance properties and could dissolve if immersed in the solutions, these samples were tested by applying 50 μL of the solutions to the DLC surface. The chemical resistance of an insulating film meant for coils is generally determined by observing variations in the hardness of the insulating film after immersion [18]. Therefore, a nanoindenter (NanoGuru, NANOSTAR) was used to evaluate the variation in the film hardness, and a Raman spectroscopy device (NRS-4100, JASCO) was used to evaluate the variations in the fundamental structure of the film as a means of judging the chemical resistance of the DLC film. The results of the tests conducted for PUW, DLC/Cu, and DLC/Al were compared to verify the applicability of DLC films.
3.2. Evaluation of film adhesion using scratch test
3. Results and discussion
Film adhesion properties were measured for DLC/Cu and DLC/Al using a scratch test. The microphotographs of the samples after completing the scratch test are shown in Fig. 2. The film adhesion strengths of the samples were 554 mN for DLC/Cu and 422 mN for DLC/Al. The difference in film adhesion strength between DLC/Cu and DLC/Al is believed to be caused by the difference in hardness of the base materials, Cu and Al. The Vickers hardness test is a scale (HV) for measuring the hardness of a material. On this scale, Cu has a hardness of 46 HV, whereas Al has a hardness of 25 HV. Thus, Al has lower hardness than Cu. Accordingly, when applying a load and scratching the samples, the ratio of variations experienced by the base material is higher, which could cause lower adhesion strength. Further, the adhesion to the DLC film was strong, and its value on the Si substrate, which has been practically utilized, was 633.3 mN. Based on the above results, the study achieved a DLC film formation with sufficient adhesion on both Cu and Al substrate materials.
3.1. Spectroscopy analysis of DLC/CU and DLC/AL
3.3. Ball-on-disc testing for measurement of friction coefficient Friction coefficients for flat DLC/Cu, DLC/Al, Cu, and Al substrates
DLC/Cu and DLC/Al substrates were analyzed using Raman 3
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Fig. 4. Film hardness of each sample before and after immersion tests with H2SO4 and NaOH solutions.
Fig. 5. Raman spectra of DLC/Cu before and after immersion tests with H2SO4 and NaOH solutions.
Fig. 2. Microscopic images after the scratch tests of (a) DLC/Cu and (b) DLC/Al samples.
Fig. 3. Breakdown voltage after heating test.
were measured using a ball-on-disc test. Standard PUW used for coils served as the control material for comparison, and hence, its friction coefficient was also measured. The friction coefficient of DLC/Cu was approximately 1/3 that of Cu, namely, 0.239 and 0.643, respectively. The DLC coating also significantly reduced the friction coefficient of aluminum, as shown by the measurement results of 0.559 for Al and 0.229 for DLC/Al, which is once again approximately 1/3 of the value for the uncoated base material. Moreover, the friction coefficient for
Fig. 6. Raman spectra of DLC/Al before and after immersion tests with H2SO4 and NaOH solutions.
PUW was 0.848. These results confirmed that the friction coefficient of the DLC film is sufficiently low. This suggests that the DLC film could be used as a wire insulating coating for machinery applications that require sliding properties.
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Diamond & Related Materials 103 (2020) 107731
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stable and remained constant by both the highly acidic and alkaline solutions. This suggested that the application of this technology could improve the reliability of electronic devices.
3.4. Dielectric breakdown voltage measurement The dielectric breakdown voltages for DLC/Cu and DLC/Al substrates were measured using a direct current dielectric breakdown tester. Standard PUW coating used for coils again served as the control material for comparison, and hence, its dielectric breakdown voltage was measured. The dielectric breakdown voltages of the test materials were DLC/Cu: 143 kV/mm and DLC/Al: 146 kV/mm. Both test materials had values of approximately 145 kV/mm, confirming their high dielectric breakdown voltage properties. Based on the above, a DLC film formed under the same conditions considered in this study will possess identical dielectric breakdown voltage properties, irrespective of the base material or substrate material used. In addition, although the dielectric breakdown voltage of PUW was 247 kV/mm, the maximum value of the voltage actually used in miniaturized electronic devices ranges from 5 to 15 V. This suggests that the dielectric breakdown voltage of DLC films is sufficient to provide satisfactory electrical resistance in these devices.
4. Conclusions DLC coating was applied to flat Cu and Al substrates as an insulating film, using surface coating technology. The evaluation of properties such as sliding, chemical resistance, and heat resistance were conducted to verify the applicability of this coating to coils. The results demonstrated that the coating provided electrical insulation field strength of 145 kV/mm and friction coefficient of 0.23, confirming that its properties were sufficient for application to electronic devices. Strong chemical resistance to highly acidic and basic solutions was also verified. In addition, the coating was confirmed heat-stable in a 200 °C high-temperature environment. CRediT authorship contribution statement
3.5. Heat resistance testing of DLC film
Daichi Nakajima: Conceptualization, Writing - original draft, Data curation, Writing - review & editing. Hiroki Kuwabara: Data curation, Writing - review & editing. Shigeru Annaka: Data curation, Writing review & editing. Shinya Fujii: Data curation, Writing - review & editing. Yoshikazu Tanaka: Data curation, Writing - review & editing. Kenji Hirakuri: Data curation, Writing - review & editing.
Heat resistance testing was conducted on the DLC/Cu and DLC/Al samples. Fig. 3 shows the variations in the breakdown voltage with respect to the number of days heating is applied. In addition, the data for the heat-resistant properties of polyethylene wire (PEW), which are within the range used in machines that generally require heat resistance (200 °C), are also included in Fig. 3 [20]. The figure shows that this test confirmed that subjecting DLC/Cu and DLC/Al test samples to heating for 7 days (168 h) did not result in a decline in their electrical insulation properties. However, depending on the measurement time, the test confirmed that, in some cases, the dielectric breakdown voltage varied up or down by 10%. Environmental factors are believed to be the major cause of this variation. Dielectric breakdown voltage is known to be highly dependent on humidity. The inconsistency in the measured values between the different days is considered the result of humidity, which varied from 20% to 60%. The values of the dielectric breakdown voltage prior to heating and that after the 168-h heating period remained more or less constant. This suggested that environmental factors were the reason for the observed variations. In addition, because the standard deviation for all measurement data was no larger than 7, the test results were considered sufficiently reliable.
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. References [1] F.K. Horestani, M. Eshghi, M. Yazdchi, An ultra-low power amplifier for wearable and implantable electronic devices, Microelectron. Eng. 216 (Aug. 2019). [2] S.G. Farkoush, A. Wadood, T. Khurshaid, C.-H. Kim, S.-B. Rhee, Minimizing static VAR compensator capacitor size by using SMC and ASRFC controllers in smart grid with connected EV charger, Int. J. Elec. Power 107 (May 2019) 656–667. [3] J.K. Rainey, J.S. DeVries, B.D. Sykes, Estimation and measurement of flat or solenoidal coil inductance for radiofrequency NMR coil design, J. Magn. Reson. 187 (2007) 27–37, https://doi.org/10.1016/j.jmr.2007.03.016. [4] K. Sakurai, M. Hiratsuka, H. Nakamori, K. Namiki, K. Hirakuri, Evaluation of sliding properties and durability of DLC coating for medical devices, Diam. Relat. Mater. 96 (2019) 97–103, https://doi.org/10.1016/j.diamond.2019.03.021. [5] S. Alotaibi, K.N. Manjunatha, S. Paul, Stability of hydrogenated amorphous carbon thin films for application in electronic devices, Diam. Relat. Mater. 90 (2018) 172–180, https://doi.org/10.1016/j.diamond.2018.10.016. [6] A. Erdemir, J.M. Martin, Superior wear resistance of diamond and DLC coatings, Curr. Opin. Solid State Mater. Sci. 22 (2018) 243–254, https://doi.org/10.1016/j. cossms.2018.11.003. [7] K.M. Tan, M. Yang, T.Y. Liow, R. Tek Po Lee, Ultra high-stress liner comprising diamond-like carbon for performance enhancement of p-channel multiple-gate transistors, IEEE Trans. Electron Devices 56 (June 2009) 1277–1283. [8] R. Kato, M. Hasegawa, Fast synthesis of thin graphite film with high-performance thermal and electrical properties grown by plasma CVD using polycrystalline nickel foil at low temperature, Carbon 141 (2019) 768–773, https://doi.org/10.1016/j. carbon.2018.09.074. [9] H. Maruno, A. Nishimoto, Adhesion and durability of multi-interlayered diamondlike carbon films deposited on aluminum alloy, Surf. Coat. Technol. 354 (2018) 134–144, https://doi.org/10.1016/j.surfcoat.2018.08.094. [10] C. Neusel, G.A. Schneider, Size-dependence of the dielectric breakdown strength from nano- to millimeter scale, J. Mech. Phys. Solids 63 (2014) 201–213, https:// doi.org/10.1016/j.jmps.2013.09.009. [11] A. Kleinbichler, J. Zechner, M.J. Cordill, Buckle induced delamination techniques to measure the adhesion of metal dielectric interfaces, Microelectron. Eng. 167 (2017) 63–68, https://doi.org/10.1016/j.mee.2016.10.020. [12] Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics) Determination of Adhesion of Ceramic Coatings by Scratch Testing, ISO 20502, (2005). [13] L. Wang, L. Li, X. Kuang, Effect of substrate bias on microstructure and mechanical properties of WC-DLC coatings deposited by HiPIMS, Surf. Coat. Technol. 352 (2018) 33–41, https://doi.org/10.1016/j.surfcoat.2018.07.088. [14] Diamond-like Carbon Films – Determination of Friction and Wear Characteristics of Diamond-like Carbon Films by Ball-on-Disc Method, ISO18535, (2015). [15] M. Liang, K.L. Wong, Improving the long-term performance of composite insulators
3.6. Evaluation of chemical resistance of DLC film DLC/Cu and DLC/Al were subjected to immersion tests with H2SO4 and NaOH solutions for evaluating their chemical resistance. A nanoindenter was used to measure the surface hardness to evaluate the substrate durability after immersion of the test samples in H2SO4 and NaOH solutions. As the control material for comparison, the surface hardness of standard PUW used for coils was also measured. The variations in the hardness of DLC/Cu, DLC/Al, and PUW for each repetition of the experiment are shown in Fig. 4. Based on these data, although PUW exhibited no variation in the hardness before and after immersion in the NaOH alkaline solution, its hardness was confirmed to drop to 1/ 3 of its original value prior to immersion in the H2SO4 acidic solution. However, Fig. 4 shows that the hardness of the DLC/Cu and DLC/Al test samples did not decline after immersion in either H2SO4 or NaOH solutions, confirming that the surface conditions remained constant. The results for the Raman spectroscopy analyses of the films are shown in Figs. 5 and 6. The focus of this evaluation was calculating the ID/IG ratio by comparing the G-band with the D-band peak intensities. The Raman spectroscopy analysis results confirmed the Raman spectra specific to DLC in both the DLC/Cu and DLC/Al test samples after the H2SO4 and NaOH immersion tests. In addition, the ID/IG ratios for both DLC/Cu and DLC/Al before and after the immersion tests were confirmed to remain constant at approximately 0.49. The DLC film was 5
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[18] Winding Wires – Test Methods, IEC, 2016, p. 60851. [19] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Phys. Rev. B 61 (20) (2000) 95–107 https://journals.aps.org/ prb/pdf/10.1103/PhysRevB.61.14095. [20] T. Nagato, H. Kamibayashi, T. Zushi, H. Honjo, M. Fujii, T. Murakami, Development of high heat-resistant magnet wire, Mitsubishi Cable Industries Review 106 (2009) 21–24.
use nanocomposite: a review, Energy Procedia 110 (2017) 168–173, https://doi. org/10.1016/j.egypro.2017.03.123. [16] H. Matsumori, K. Urata, T. Shimizu, K. Takano, H. Ishii, Capacitor loss analysis method for power electronics converters, Microelectron. Reliab. 88–90 (2018) 443–446, https://doi.org/10.1016/j.microrel.2018.07.049. [17] Y. Xu, Y. Hou, H. Zhao, M. Zheng, M. Zhu, Enhanced high-temperature energy storage properties in fine-grained lead-free ceramic with high insulation resistivity, Mater. Lett. 249 (2019) 21–24, https://doi.org/10.1016/j.matlet.2019.04.057.
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Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Diamond-like carbon coating for effective electrical insulation of Cu and Al wires☆
T
⁎
Daichi Nakajimaa, , Hiroki Kuwabarab, Shigeru Annakab, Shinya Fujiic, Yoshikazu Tanakac, Kenji Hirakuria a
Department of Electrical and Electronic Engineering, Tokyo Denki University, Japan Nakadai Metals Co., Ltd., Japan c Nippon ITF, Inc., Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Diamond-like carbon (DLC) Electrical properties characterization Thermal stability Insulator Electronic device structure
The physical and mechanical properties, electrical insulation, chemical resistance, and heat resistance of diamond-like carbon (DLC) films fabricated using the radio frequency plasma-enhanced chemical vapor deposition method (RF-PECVD) on Cu (DLC/Cu) and Al (DLC/Al) substrates were evaluated in this study. The electrical insulation testing results showed that the dielectric breakdown voltage of both DLC/Cu and DLC/Al samples was approximately 145 kV/mm. For assessing the heat resistance, the samples were stored at a high temperature of 200 °C for a fixed time duration, and the changes in the insulation voltage resistance were evaluated. Based on the voltage resistance measurement results of the electrical insulation after heat resistance testing, the study confirmed that the DLC film is heat stable for 7 days of heating. In addition, testing confirmed that the DLC coating remained stable with no deterioration when immersed in both acidic and alkaline solutions, thus indicating high chemical resistance. These findings suggest that DLC is suitable for application as a thin and effective electrical insulation coating, and its properties support the realization of small coil lengths with square wires.
1. Introduction IN the modern world, there is an increasing demand for smaller and lighter electronic devices with high possibility for integration. Especially, to meet the demand for thinner and highly integrable smartphones, the mounted components need to be made smaller and lighter. Further evolution of ultra-compact technology is also essential for the components of wearable devices, a product group expected to grow into a major industry in the future [1]. The most fundamental components that constitute these electronic devices are resistance, capacitance, and inductance (coils). Resistance and capacitance are linked to the microelectronic states of the materials. The refinement of these components is progressing rapidly owing to foundational and practical researches being conducted frequently in this field [2]. On the other hand, coils have been used to prevent short circuits in round conducting wires made from materials with good conductivity, such as copper (Cu) and aluminum (Al), for approximately 100 years. Moreover, materials such as polyurethane and polyamidide with enamel coating over a wire
core are still being used today [3]. For this reason, despite ongoing attempts for refinements, such as miniaturization of conductive wires, thinner insulating films have not been achieved, which has prevented further progress in this field. In addition, attempts to reduce the size of these components by reducing the coil windings have decreased the inductance properties. To resolve this problem, a method has been proposed that involves winding more wire in a smaller space, in other words, increasing the space factor. Since the year 2000, the use of square wires instead of the standard round wires currently in use has been considered as a means for achieving increased space factor. Square wires are expected to show significant increase in the space factor when compared with round wires because round wires have contact points with one another when coiled but square wires have surface contact. However, when the standard enamel coating was applied to these square wires, various problems caused by surface tension, such as elliptical shape transformation of the insulating film, curvature of the film, and thickening of the insulation layer, hindered the application of the square wire technology. To resolve this problem, a thinner
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DLC: diamond-like carbon; RF-PECVD: radio frequency plasma-enhanced chemical vapor deposition; CVD: chemical vapor deposition; PVD: physical vapor deposition, PUW: polyurethane wire; PEW: polyethylene wire (PEW); SEM: scanning electron microscopy. ⁎ Corresponding author. E-mail address: [email protected] (D. Nakajima). https://doi.org/10.1016/j.diamond.2020.107731 Received 21 October 2019; Received in revised form 19 January 2020; Accepted 22 January 2020 Available online 23 January 2020 0925-9635/ © 2020 Elsevier B.V. All rights reserved.
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insulating film with superior electrical insulation properties and a square cross-sectional shape is desired. Diamond-Like Carbon (DLC) film is an amorphous carbon formed by graphite sp2 bonds and diamond sp3 bonds. In addition, the film contains between 0 and 40 atm% of hydrogen, and this property is dependent on the ratio of the sp2 and sp3 bonds as well as their hydrogen content [4]. Therefore, DLC coating can be prepared to suit the purpose and the properties of the surface materials with which it is used. A film with diamond-like hardness can be created by reducing the hydrogen content. With higher hydrogen content, it becomes a film with gas barrier properties and chemical resistance [5]. Because films with properties such as high hardness, low friction, and chemical resistance can be formed by introducing variations in the preparation method, the technology is widely used for practical applications such as surface coating of cutting tools, mainly for industrial use [6]. In addition, owing to the high electrical insulation properties of these films, there are reports of other uses of the film, for example, as an ultra-thin interlayer insulation for semiconductor IC units [7]. Two broad types of film formation methods are generally employed in the DLC manufacturing process. These methods are Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD), of which arc plating is an example. CVD uses a chemical reaction with gas raw materials for film formation, and PVD sublimates solid materials through the application of heat energy to form a film layer. Among the CVD methods, plasma CVD has the benefits of low film formation temperature, even film thickness, and the ability to form film layers with a 3-dimensional structure [8]. In this study, a CVD method capable of producing a coating on 3dimensional structural objects was used with the goal of developing a new DLC insulating film with functionalities such as electrical insulation, sliding properties, chemical resistance, and heat resistance. To achieve this goal, a direct current dielectric breakdown test was used to evaluate the electrical insulation properties of the DLC film through measurement of the maximum voltage the electrical insulation could withstand. In terms of the mechanical properties of the DLC film, a scratch test was used to determine the adhesion, and a ball-on-disc test was to measure the friction coefficient for evaluating the sliding properties. In addition, acid resistance and alkaline resistance tests were conducted to assess the chemical resistance properties considering the possible contact environment within electronic devices. A simulation in the form of a heat resistance test was also performed because applications of this technology would also require operation within the high-temperature environment inside electronic devices. By conducting the abovementioned evaluations of the DLC film, this study considered the usability of the technology for square wire coil insulation films.
Raman spectroscopy is generally used for structural evaluation of DLC; in this study, this method was employed with spectral shape peak ratio and peak intensity values of ID and IG, respectively. The adhesion of the DLC coating on metallic materials such as Cu and Al is considered poor, and DLC film formation is considered difficult [9]. For this reason, Raman spectroscopy (NRS-4100, JASCO) was used for evaluating the films on the flat DLC/Cu and DLC/Al test samples to confirm DLC film formation on the respective substrate surfaces. In addition, the breakdown voltages of insulating materials are highly dependent on the sample thickness of the materials [10]. Accordingly, the electrical insulation property of insulating materials is a value that considers the sample thickness of the material. Therefore, the minimum electrical field value required for insulation breakdown is often indicated. The main methods generally used for film thickness measurements are scanning electron microscopy (SEM) for cross-sectional measurements and light interference effect using a spectrophotometer. Cross-sectional observations using SEM are prone to errors owing to the effects of the viewing angles. Therefore, a UV–visible spectrophotometer (V-770, JEOL) was used for simple optical measurements of the film thickness on the DLC/Cu and DLC/Al samples. 2.2. Evaluation of film adhesion using scratch test In the case of insulating films on conducting wires, the adhesion of the film to the wire is crucial for the safety and reliability of electronic devices [11]. When adhesion is poor, the film can crack or peel when stressed during winding. This causes current leakage from the compromised areas of the insulating film, resulting in malfunction and failure of the electronic devices. Therefore, the adhesion property of the DLC film was evaluated using a scratch test machine (CSR5000, RHESCA) to ensure satisfactory adhesion to the wires (Cu, Al). In the testing conditions, scratch speed was 10 μm/s, rate of increase of applied load was 700mN/min, the humidity was 50%, and the temperature was 24 °C. The measurements were carried out according to the ISO 20502 standard [12]. Testing was conducted 3 times for DLC/Cu and DLC/Al substrates, and the results were averaged to determine the critical load. 2.3. Ball-on-disc testing for measurement of friction coefficient The winding properties of relatively flexible metal wires, such as Cu and Al, are considerably affected by the friction coefficient of the insulating film [13]. Owing to miniaturization and increased efficiency of electronic devices, a large amount of wire is coiled in a confined space, resulting in higher space factor; therefore, winding is considered to be an increasingly important property. Therefore, a ball-on-disc testing machine (Tribometer, CSEM) was used to measure the friction coefficient of the DLC/Cu and DLC/Al samples to determine the sliding property of the DLC film. In addition, the flat Cu and Al substrates without the DLC film, as well as a standard insulation-coated polyurethane wire (PUW), were used for the control group friction coefficient measurements. The object used was an SUJ2 ball with a diameter of 6 mm, and the load used was 3 N. In the testing conditions, the humidity was 50%, and the temperature was 24 °C. This measurement was conducted based on the ISO/DIS 18535 standard [14].
2. Experimental procedures 2.1. DLC formation and film structural analysis The radio frequency plasma-enhanced chemical vapor deposition method (RF-PECVD) was used to create a 13.56 MHz radio frequency electrical excitation source for DLC film formation. CH4 was used as the raw material gas. In this experiment, the following materials with excellent electrical conductivities, which are also used for manufacturing coils in real-world applications, were used as the raw materials: C1100 (10 mm × 10 mm × 0.5 mm) copper and A5052 (10 mm × 10 mm × 1.0 mm) aluminum. Film formation was performed on flat C1100 and A5052 substrates to evaluate the physical properties of the DLC film. The C1100 and A5052 base metal samples are denoted here as Cu and Al, whereas the flat C1100 and A5052 substrates treated with the DLC film are denoted as DLC/Cu and DLC/ Al, respectively. In each experiment, flat test pieces of DLC/Cu and DLC/Al substrates were used. The film thickness and structure were measured to analyze the fundamental properties of the DLC film formed on the test materials.
2.4. Measurement of dielectric breakdown voltage The dielectric breakdown voltage is the most important property to be considered when applying insulating DLC films to coil applications. To investigate this essential property, dielectric breakdown voltage was measured for the DLC/Cu and DLC/Al test pieces. In this experiment, voltages actually used in electronic devices were applied through direct current. The details of the experimental procedure are as follows: A direct current dielectric breakdown testing machine (SM7100, HIOKI) was used. 2
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1. Because surface contamination of the DLC/Cu and DLC/Al test pieces could cause creeping discharge [15], ethanol-soaked cotton swabs were used as necessary for cleaning. 2. The specimens were set atop a 100 × 200 × 0.5 mm Cu electrode, and a two-pin lead was used. One pin contacted the upper portion of the samples, and the other a Cu electrode. 3. Direct current was applied at a fixed voltage rate (2 V/s), and the voltage at which dielectric breakdown occurred was measured. 4. The dielectric breakdown field strength was then used to calculate the DLC film thickness. 2.5. Heat resistance testing of DLC films Owing to the increased miniaturization and enhanced performance of electrical components used in electronic devices, such as semiconductor elements and capacitors, the heat they generate has increased [16]. In particular, the heat output of semiconductor elements is extremely high in power devices with current limited to at least several hundred amperes [16]. In some cases, this heat released by electronic devices can affect the insulation performance of the insulating materials [17]. The declining insulation performance caused by heat deterioration is linked to device reliability issues such as malfunctions and failure [17]. Therefore, the heat resistance of the DLC film was evaluated using an electrical method. Both DLC/Cu and DLC/ Al substrates were stored under a temperature of 200 °C and humidity of 40 ± 2% in an enclosed space for a 7-day period (168 h). The substrate was then returned to room temperature (25 °C), and the insulation breakdown voltage was measured. The heat resistance was evaluated based on the rate of variability in the insulation breakdown voltage before and after this heating experiment. The electrical insulation voltage resistance was measured using the method described in Section 2.4.
Fig. 1. Raman spectra of each sample.
spectroscopy. The DLC was evaluated by verifying the presence or absence of the G band and D band by following the general process. The results of the Raman spectroscopy analysis are shown in Fig. 1. The horizontal axis in the graph in the figure indicates the Raman shift and vertical axis indicates the intensity. To compare with the general structure of the DLC film, Raman spectroscopy was also conducted on a Si substrate with a DLC film applied to the surface (DLC/Si). The results demonstrated that both the DLC/Cu and the DLC/Al samples had the same unique DLC properties. The G(graphite)-peak, caused by the internal oscillation of the graphite carbon atoms in a hexagonal lattice (E2g), was observed at approximately 1550 cm−1. The D(disorder)peak, caused by crystalline strain vibrations of the graphite structure (A1g), was observed at approximately 1330 cm−1. This confirmed that a DLC film had formed on both the Cu and Al substrate samples. In addition, a correlation can be obtained between the ID/IG ratio, which is calculated from these two peak intensities and the sp3 ratio. The higher the ID/IG ratio, the lower the sp3 ratio [19]. Further, the ID/IG of this DLC film is 0.49. Optical measurement of the film thickness was conducted for DLC/ Cu and DLC/Al test pieces using UV–visible spectroscopy. The film thickness was calculated from the obtained reflectance spectrum by fitting with the model formula. The estimated film thicknesses were 1.063 μm and 1.045 μm for DLC/Cu and DLC/Al, respectively.
2.6. Evaluation of chemical resistance of DLC films Insulating films in electronic devices are expected to protect the wiring and inductors from acidic and alkaline solutions formed by battery leakage. Therefore, the chemical resistance properties of insulating films are important [5]. Accordingly, the chemical resistance of the DLC film was investigated through an immersion test using acidic and alkaline solutions. This test used sulfuric acid (H2SO4, specific gravity 1.2) and sodium hydroxide (NaOH, 10%), which are commonly used as electrolyte solutions of batteries. For this test, the DLC/Cu and DLC/Al test pieces were immersed in the solutions at room temperature (25 °C) for 24 h, which represented one cycle. The samples were then washed for 10 min in pure water. A total of 7 cycles were implemented. In addition to the DLC/Cu and DLC/Al samples, PUW was also used as a contrast material in immersion testing. Because aluminum, the base material of DLC/Al, exhibits no chemical resistance properties and could dissolve if immersed in the solutions, these samples were tested by applying 50 μL of the solutions to the DLC surface. The chemical resistance of an insulating film meant for coils is generally determined by observing variations in the hardness of the insulating film after immersion [18]. Therefore, a nanoindenter (NanoGuru, NANOSTAR) was used to evaluate the variation in the film hardness, and a Raman spectroscopy device (NRS-4100, JASCO) was used to evaluate the variations in the fundamental structure of the film as a means of judging the chemical resistance of the DLC film. The results of the tests conducted for PUW, DLC/Cu, and DLC/Al were compared to verify the applicability of DLC films.
3.2. Evaluation of film adhesion using scratch test
3. Results and discussion
Film adhesion properties were measured for DLC/Cu and DLC/Al using a scratch test. The microphotographs of the samples after completing the scratch test are shown in Fig. 2. The film adhesion strengths of the samples were 554 mN for DLC/Cu and 422 mN for DLC/Al. The difference in film adhesion strength between DLC/Cu and DLC/Al is believed to be caused by the difference in hardness of the base materials, Cu and Al. The Vickers hardness test is a scale (HV) for measuring the hardness of a material. On this scale, Cu has a hardness of 46 HV, whereas Al has a hardness of 25 HV. Thus, Al has lower hardness than Cu. Accordingly, when applying a load and scratching the samples, the ratio of variations experienced by the base material is higher, which could cause lower adhesion strength. Further, the adhesion to the DLC film was strong, and its value on the Si substrate, which has been practically utilized, was 633.3 mN. Based on the above results, the study achieved a DLC film formation with sufficient adhesion on both Cu and Al substrate materials.
3.1. Spectroscopy analysis of DLC/CU and DLC/AL
3.3. Ball-on-disc testing for measurement of friction coefficient Friction coefficients for flat DLC/Cu, DLC/Al, Cu, and Al substrates
DLC/Cu and DLC/Al substrates were analyzed using Raman 3
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Fig. 4. Film hardness of each sample before and after immersion tests with H2SO4 and NaOH solutions.
Fig. 5. Raman spectra of DLC/Cu before and after immersion tests with H2SO4 and NaOH solutions.
Fig. 2. Microscopic images after the scratch tests of (a) DLC/Cu and (b) DLC/Al samples.
Fig. 3. Breakdown voltage after heating test.
were measured using a ball-on-disc test. Standard PUW used for coils served as the control material for comparison, and hence, its friction coefficient was also measured. The friction coefficient of DLC/Cu was approximately 1/3 that of Cu, namely, 0.239 and 0.643, respectively. The DLC coating also significantly reduced the friction coefficient of aluminum, as shown by the measurement results of 0.559 for Al and 0.229 for DLC/Al, which is once again approximately 1/3 of the value for the uncoated base material. Moreover, the friction coefficient for
Fig. 6. Raman spectra of DLC/Al before and after immersion tests with H2SO4 and NaOH solutions.
PUW was 0.848. These results confirmed that the friction coefficient of the DLC film is sufficiently low. This suggests that the DLC film could be used as a wire insulating coating for machinery applications that require sliding properties.
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stable and remained constant by both the highly acidic and alkaline solutions. This suggested that the application of this technology could improve the reliability of electronic devices.
3.4. Dielectric breakdown voltage measurement The dielectric breakdown voltages for DLC/Cu and DLC/Al substrates were measured using a direct current dielectric breakdown tester. Standard PUW coating used for coils again served as the control material for comparison, and hence, its dielectric breakdown voltage was measured. The dielectric breakdown voltages of the test materials were DLC/Cu: 143 kV/mm and DLC/Al: 146 kV/mm. Both test materials had values of approximately 145 kV/mm, confirming their high dielectric breakdown voltage properties. Based on the above, a DLC film formed under the same conditions considered in this study will possess identical dielectric breakdown voltage properties, irrespective of the base material or substrate material used. In addition, although the dielectric breakdown voltage of PUW was 247 kV/mm, the maximum value of the voltage actually used in miniaturized electronic devices ranges from 5 to 15 V. This suggests that the dielectric breakdown voltage of DLC films is sufficient to provide satisfactory electrical resistance in these devices.
4. Conclusions DLC coating was applied to flat Cu and Al substrates as an insulating film, using surface coating technology. The evaluation of properties such as sliding, chemical resistance, and heat resistance were conducted to verify the applicability of this coating to coils. The results demonstrated that the coating provided electrical insulation field strength of 145 kV/mm and friction coefficient of 0.23, confirming that its properties were sufficient for application to electronic devices. Strong chemical resistance to highly acidic and basic solutions was also verified. In addition, the coating was confirmed heat-stable in a 200 °C high-temperature environment. CRediT authorship contribution statement
3.5. Heat resistance testing of DLC film
Daichi Nakajima: Conceptualization, Writing - original draft, Data curation, Writing - review & editing. Hiroki Kuwabara: Data curation, Writing - review & editing. Shigeru Annaka: Data curation, Writing review & editing. Shinya Fujii: Data curation, Writing - review & editing. Yoshikazu Tanaka: Data curation, Writing - review & editing. Kenji Hirakuri: Data curation, Writing - review & editing.
Heat resistance testing was conducted on the DLC/Cu and DLC/Al samples. Fig. 3 shows the variations in the breakdown voltage with respect to the number of days heating is applied. In addition, the data for the heat-resistant properties of polyethylene wire (PEW), which are within the range used in machines that generally require heat resistance (200 °C), are also included in Fig. 3 [20]. The figure shows that this test confirmed that subjecting DLC/Cu and DLC/Al test samples to heating for 7 days (168 h) did not result in a decline in their electrical insulation properties. However, depending on the measurement time, the test confirmed that, in some cases, the dielectric breakdown voltage varied up or down by 10%. Environmental factors are believed to be the major cause of this variation. Dielectric breakdown voltage is known to be highly dependent on humidity. The inconsistency in the measured values between the different days is considered the result of humidity, which varied from 20% to 60%. The values of the dielectric breakdown voltage prior to heating and that after the 168-h heating period remained more or less constant. This suggested that environmental factors were the reason for the observed variations. In addition, because the standard deviation for all measurement data was no larger than 7, the test results were considered sufficiently reliable.
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. References [1] F.K. Horestani, M. Eshghi, M. Yazdchi, An ultra-low power amplifier for wearable and implantable electronic devices, Microelectron. Eng. 216 (Aug. 2019). [2] S.G. Farkoush, A. Wadood, T. Khurshaid, C.-H. Kim, S.-B. Rhee, Minimizing static VAR compensator capacitor size by using SMC and ASRFC controllers in smart grid with connected EV charger, Int. J. Elec. Power 107 (May 2019) 656–667. [3] J.K. Rainey, J.S. DeVries, B.D. Sykes, Estimation and measurement of flat or solenoidal coil inductance for radiofrequency NMR coil design, J. Magn. Reson. 187 (2007) 27–37, https://doi.org/10.1016/j.jmr.2007.03.016. [4] K. Sakurai, M. Hiratsuka, H. Nakamori, K. Namiki, K. Hirakuri, Evaluation of sliding properties and durability of DLC coating for medical devices, Diam. Relat. Mater. 96 (2019) 97–103, https://doi.org/10.1016/j.diamond.2019.03.021. [5] S. Alotaibi, K.N. Manjunatha, S. Paul, Stability of hydrogenated amorphous carbon thin films for application in electronic devices, Diam. Relat. Mater. 90 (2018) 172–180, https://doi.org/10.1016/j.diamond.2018.10.016. [6] A. Erdemir, J.M. Martin, Superior wear resistance of diamond and DLC coatings, Curr. Opin. Solid State Mater. Sci. 22 (2018) 243–254, https://doi.org/10.1016/j. cossms.2018.11.003. [7] K.M. Tan, M. Yang, T.Y. Liow, R. Tek Po Lee, Ultra high-stress liner comprising diamond-like carbon for performance enhancement of p-channel multiple-gate transistors, IEEE Trans. Electron Devices 56 (June 2009) 1277–1283. [8] R. Kato, M. Hasegawa, Fast synthesis of thin graphite film with high-performance thermal and electrical properties grown by plasma CVD using polycrystalline nickel foil at low temperature, Carbon 141 (2019) 768–773, https://doi.org/10.1016/j. carbon.2018.09.074. [9] H. Maruno, A. Nishimoto, Adhesion and durability of multi-interlayered diamondlike carbon films deposited on aluminum alloy, Surf. Coat. Technol. 354 (2018) 134–144, https://doi.org/10.1016/j.surfcoat.2018.08.094. [10] C. Neusel, G.A. Schneider, Size-dependence of the dielectric breakdown strength from nano- to millimeter scale, J. Mech. Phys. Solids 63 (2014) 201–213, https:// doi.org/10.1016/j.jmps.2013.09.009. [11] A. Kleinbichler, J. Zechner, M.J. Cordill, Buckle induced delamination techniques to measure the adhesion of metal dielectric interfaces, Microelectron. Eng. 167 (2017) 63–68, https://doi.org/10.1016/j.mee.2016.10.020. [12] Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics) Determination of Adhesion of Ceramic Coatings by Scratch Testing, ISO 20502, (2005). [13] L. Wang, L. Li, X. Kuang, Effect of substrate bias on microstructure and mechanical properties of WC-DLC coatings deposited by HiPIMS, Surf. Coat. Technol. 352 (2018) 33–41, https://doi.org/10.1016/j.surfcoat.2018.07.088. [14] Diamond-like Carbon Films – Determination of Friction and Wear Characteristics of Diamond-like Carbon Films by Ball-on-Disc Method, ISO18535, (2015). [15] M. Liang, K.L. Wong, Improving the long-term performance of composite insulators
3.6. Evaluation of chemical resistance of DLC film DLC/Cu and DLC/Al were subjected to immersion tests with H2SO4 and NaOH solutions for evaluating their chemical resistance. A nanoindenter was used to measure the surface hardness to evaluate the substrate durability after immersion of the test samples in H2SO4 and NaOH solutions. As the control material for comparison, the surface hardness of standard PUW used for coils was also measured. The variations in the hardness of DLC/Cu, DLC/Al, and PUW for each repetition of the experiment are shown in Fig. 4. Based on these data, although PUW exhibited no variation in the hardness before and after immersion in the NaOH alkaline solution, its hardness was confirmed to drop to 1/ 3 of its original value prior to immersion in the H2SO4 acidic solution. However, Fig. 4 shows that the hardness of the DLC/Cu and DLC/Al test samples did not decline after immersion in either H2SO4 or NaOH solutions, confirming that the surface conditions remained constant. The results for the Raman spectroscopy analyses of the films are shown in Figs. 5 and 6. The focus of this evaluation was calculating the ID/IG ratio by comparing the G-band with the D-band peak intensities. The Raman spectroscopy analysis results confirmed the Raman spectra specific to DLC in both the DLC/Cu and DLC/Al test samples after the H2SO4 and NaOH immersion tests. In addition, the ID/IG ratios for both DLC/Cu and DLC/Al before and after the immersion tests were confirmed to remain constant at approximately 0.49. The DLC film was 5
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[18] Winding Wires – Test Methods, IEC, 2016, p. 60851. [19] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Phys. Rev. B 61 (20) (2000) 95–107 https://journals.aps.org/ prb/pdf/10.1103/PhysRevB.61.14095. [20] T. Nagato, H. Kamibayashi, T. Zushi, H. Honjo, M. Fujii, T. Murakami, Development of high heat-resistant magnet wire, Mitsubishi Cable Industries Review 106 (2009) 21–24.
use nanocomposite: a review, Energy Procedia 110 (2017) 168–173, https://doi. org/10.1016/j.egypro.2017.03.123. [16] H. Matsumori, K. Urata, T. Shimizu, K. Takano, H. Ishii, Capacitor loss analysis method for power electronics converters, Microelectron. Reliab. 88–90 (2018) 443–446, https://doi.org/10.1016/j.microrel.2018.07.049. [17] Y. Xu, Y. Hou, H. Zhao, M. Zheng, M. Zhu, Enhanced high-temperature energy storage properties in fine-grained lead-free ceramic with high insulation resistivity, Mater. Lett. 249 (2019) 21–24, https://doi.org/10.1016/j.matlet.2019.04.057.
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