The study of refractory Ta10W and non-refractory Ni60A coatings deposited by wire electrical explosion spraying

Surface & Coatings Technology 374 (2019) 44–51

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The study of refractory Ta10W and non-refractory Ni60A coatings deposited by wire electrical explosion spraying

T

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Feng Han, Liang Zhu , Zong-han Liu, Lian Gong State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Wire electrical explosion spraying(WEES) Coating microstructure Deposition efficiency Coating thickness Adhesive strength

In conventional thermal spraying processes, it is difficult to employ one type of coating process to economically spray materials having greatly different melting points. In this study, wire electrical explosion spraying(WEES) was applied to prepare refractory Ta10W and non-refractory Ni60A coatings by using a self-designed WEES device. The coating microstructure, deposition efficiency and coating thickness, as well as the adhesive strength of both coatings were investigated. A uniform and dense Ta10W coating with the phase of Ta, Ta2N, and FeTaO4 can be obtained when the energy density was 151.6 J/mm3, while the uniform and dense Ni60A coating made up of FeNi and SiO2 phase was got at the energy density of 72.7 J/mm3. The maximum deposition efficiency of Ta10W and Ni60A were 53% and 47% respectively. The thickness increment of both coatings decreased with the explosion frequency. The critical load Lc in the scratch test can be regarded as a criterion of the adhesive strength, the average critical load of Ta10W was greater than 50 N, while the Ni60A was 38.3 N. Through developing a high-efficient spray device and choosing the appropriate process parameter, especially a suitable energy density, the WEES will be capable to produce the commercially available coatings with greatly different melting points.

1. Introduction Thermal spray describes a group of processes that employ the energy sources to melt the feedstock material, using process jets to accelerate the resultant particles toward a prepared surface [1]. In conventional thermal spray processes, in order to improve the adhesive strength between the coating and substrate, it is necessary to fully heat and/or propel the spray particles [2–4]. If the selected feedstock materials differ greatly in melting point, it is quite difficult to employ only one type of energy source to meet the requirements. Consequently, various spray processes(energy sources) must be properly chosen, the resulting high economic and labor cost is a considerable problem. Wire electrical explosion spraying(WEES), a new type of thermal spray technology, is used for fabricating conductive metallic or ceramic coatings on the surface of components [5,6]. In the WEES process, the high pulse current(about 30-50kA) generated by the discharge of a pulse capacitor bank is delivered into the conductive wire, the continuous energy deposition created by the Joule Heating will make the wire experience fusion, evaporation and further explosion. The resulting molten explosive products(spray particles) impact, deform, and solidify on the prepared substrate to build up a coating [7–10]. In order to enable the wire to completely melt and to explode at high speed, ⁎

meanwhile ensuring the energy conservation, the volume of the selected wires, in practice, are usually small, of which the diameter is often between 0.1 mm to 2 mm and the length always range from 20 mm to150 mm. Such a wire was often bound or welded on the electrodes in previous studies for the purpose that the thermal energy can be steadily deposited [11–13]. If the relatively thick coatings are to be applied, the operation of multiple electrical explosions(sprays) must be carried out, which means after the fixed wire is exploded, a new wire need to be fixed again between the electrodes preparing for the next spray. During the wire reloading process, the spraying system must be shut down to avoid electrical shock hazards, only if the wire replacement is done, the spraying system can be restarted and the spraying operation can be conducted, such the start/stop cycles must be carried out over and over again to obtain thicker coatings. It follows that the low feeding efficiency is one significant problem restricting its commercialization. The other reason limiting its development is the low deposition efficiency when the substrate is in the form of plate or flat surface. This is due to the explosive products will radially fly into the surrounding space after the explosion. Most of the products actually misses the plate substrate and is never deposited. The maximum deposition efficiency based on the previous report was not more than 25% [14].

Corresponding author. E-mail address: [email protected] (L. Zhu).

https://doi.org/10.1016/j.surfcoat.2019.05.065 Received 21 April 2019; Received in revised form 21 May 2019; Accepted 23 May 2019 Available online 24 May 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.

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For solving the problems mentioned above, our team developed a new mode of current injection, the essence of which lies in the wire does not direct contact the electrodes during spraying, there is a small air gap between the wire and the electrodes, the pulse current is pumped into the wire by means of gas discharge [15]. This mode of current injection is available for a relative move between wire and electrodes during spraying, thereby providing a potential for continuously feeding and exploding the wire. In addition, locating the wire into a self-designed constraint chamber to confine the explosive products, causing all the products can fly toward the substrate from only one direction. In this paper, a set of WEES device with the capability of continuous feeding wire and concentrate the explosive products was developed. The refractory Ta10W and non-refractory Ni60A coatings, which have a great potential in applications ranging from biomedical to aerospace, were prepared under different energy densities(the charging energy to the wire volume). The coating microstructure were investigated, and the explosive products were collected to understand the microstructure difference of both coatings. The deposition efficiency and coatings thickness were analyzed after continuous multiple explosions (spraying). At last, the adhesive strength of both coatings were characterized.

Table 1 Basic physical properties of both materials. Sample

Melting point (°C)

Hardness HV (kg/mm2)

Density (g/cm3)

Ta10W Ni60A

3080 990

227–243 687–700

16.78–16.90 7.73–7.53

by the constrained groove of the feeding rod traverse across the jet window and then impact on the substrate. The jet window area is 70 mm × 5 mm, which is also the coating area of a single electrical explosion. A complete layer that cover the entire substrate area can be accomplished in an x-y ladder mode by automatically moving the substrate. This device overcomes the weaknesses that the spray system must be shut down and the wire must be replaced manually after each single electrical explosion common in conventional electrical explosion spraying. The maximum feed velocity(spray rate) of this device related to the wire (diameter of 0.1–1 mm) can reach approximately 8–9 m/ min. 2.2. Experimental methods Commercially available Ni60A and Ta10W wire with 0.3 mm in diameter were used as the feedstock materials in this study. The basic physical properties and chemical composition of both materials are listed in Tables 1 and 2. 304 stainless steel plates were used as the substrate. Because the shock wave generated by the wire explosion can thoroughly clean and preheat the surface of the substrate before the explosive products impacted, only dry abrasive grit blasting treatment was conducted prior to the experiment, the carborundum grit with a size of 40 mesh(about 420 μm) was used, and the distance between nozzle orifice and substrate was approximately 50 mm, the angle of impingement of the blast was similar to that of the spraying(about 90° to the substrate). The ultrasonic cleaning was employed to remove residual entrapped grit after grit blasting. The optimum standoff distance (the distance between substrate and jet window) of 7 mm was chosen according to the reference [17]. The energy E stored in the capacitor bank is calculated as E = 1/2CU2, where the capacitance C is 8.88 μF, the initial charging voltage U ranged from 5 to 13 kV. Hence the energy density of the wires were accordingly calculated to be ranged from 22.4 J/mm3 to 151.6 J/mm3. A self-designed collection box, as schematically shown in Fig. 2, was mounted on the jet window of the spraying gun to collect the explosive products. In order to ensure the accuracy of the collection results, both wires were uninterruptedly exploded 20 times under each parameter. When the resulting explosive products subside in the collection box, they were sampled and then characterized by the Scanning Electron Microscope(Quanta 450 FEG), the size distribution was analyzed by the

2. Experimental procedure 2.1. Experimental device The schematic diagram of WEES device with independent intellectual property rights is illustrated in Fig. 1 [16]. This device is mainly composed of six parts: feeding rods(constraint chamber), spraying gun, copper electrodes, capacitor bank, high-voltage generator (H.V.), and driving unit. The feeding rods and the spraying gun are made of polyethylene. The length of each feeding rod is 1 m, which are pre-stacked in the cartridge box, the wire feedstock is preplaced in the constrained groove of the feeding rods before spraying. Two copper electrodes are wrapped in the spraying gun and connected to the capacitor bank respectively, the distance between the two electrodes is 70 mm(this means the length of the wire in a single electrical explosion is 70 mm). The capacitor bank was wired to the H.V. The working principle of this device is as follows: When the capacitor bank is charged to a desired voltage by the H.V., a high voltage electric field is first established between the two electrodes. The feeding rod is then continuously fed into the spraying gun from the cartridge box one after another by driving unit. When the wire is just located below the electrodes (the gap between the electrodes and the wire is about 2 mm), two breakdown channels are established simultaneously, causing the wire to explode. The resulting explosive products confined

Fig. 1. Schematic diagram of the self-designed WEES device. 45

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delamination, or spalling). Value of the Lc can be determined by the acoustic emission signal (AE), which is generated by the release of energy stored in a coating. The AE signal will show the obviously peak on the acoustic emission signal-load curve when the critical load Lc is reached. The Lc can be regarded as the criterion of the adhesive strength between the coating and substrate.

Table 2 Chemical compositions of both materials. Ta10W Ni60A

C 0.01 C 0.8

N 0.01 Cr 18

H 0.0015 Si 3.5

O 0.015 B 4

W 10 Fe 2.5

Ta Bal Ni Bal

3. Results and discussion 3.1. Characteristics of explosive products In WEES, the explosive products act as the basic elements for building up a coating. The formation of the explosive products should be considered as a stochastic event due to the nature of the explosion process. The characteristics of the explosive products usually reflect their thermal processing histories, which have a significant influence on the coating formation. Hence, in order to make a comprehensive understanding of the formation of both coatings, the characteristics of explosive products should be firstly investigated. The morphology and size distribution of the explosive products were characterized. From the morphology point, the explosive products of both materials were mainly spherical particles. When the energy density was relatively low(22.4 J/mm3–72.7 J/mm3), there were a large number of pores distribute on the Ta10W particles surface, while the surface of Ni60A particles were relatively smooth, the typical morphologies are shown in Fig. 3(a) and (b). As the energy density was raised from 72.7 J/mm3 to 151.6 J/mm3, the surface of Ta10W particles become smoother, whereas that of Ni60A particles appeared many pores, the typical features are illustrated in Fig. 3(c) and (d). The discrepancy in particle morphologies are mainly related to the evacuation mechanism of the material vapour dwelling in its host particle after the explosion. If the host particle surface had solidified into a solid shell before the material vapour was completely evacuated, such vapour will break through this solid shell during the evacuation process, causing the surface pores; if the material vapour had been totally evacuated before the host particle started to solidified, the resulting particles surface are of course smooth. It thus can be inferred that the duration of high temperature of the Ta10W particles became longer and that of the Ni60A turned shorter as the energy density was increased from 72.7 J/mm3 to 151.6 J/mm3. From the size point, with the increase of the energy density, the refinement of the Ta10W particles can be clearly indicated from Fig. 4(a). Especially when the energy density reached to 151.6 J/mm3, the proportion of particles less than 30 μm increased obviously, in which the proportion between 15 and 30 μm was about 40% and the proportion less than 15 μm was more than 10%. Fig. 4(b) illustrate the size distribution of Ni60A particles, the size of which was no longer refined when the energy density exceeded 72.7 J/mm3, the proportion of particles larger than 50 μm significantly increase when the energy density was lift up to 151.6 J/mm3. In the process of formation of the explosive products, as the continuous heating of the wire, the molten wire will gradually expand and then explode. If the expansion kinetic energy of the molten wire is large enough relative to its viscous force, the small and fast particles will be exploded; and if the case is opposite, the particles will develop into the large and slow ones. Therefore, it can be concluded from the above results that the expansion kinetic and temperature of the Ta10W wire is directly proportional to the energy density, the resulting particles became smaller and faster with the increase of the energy density, whereas the expansion kinetic and temperature of the Ni60A wire applies the opposite trend when the energy density was elevated from 72.7J/mm3 to 151.6 J/mm3, such that the viscous force of the molten wire was large enough relative to the expansion kinetic particularly at the energy density of 151.6 J/mm3, causing the particles have a larger size and lower speed. The difference in characteristics of explosive products can have the

Fig. 2. Schematic diagram of the explosive products collection box.

image-software of nano measurer. The SEM was also employed to investigate the microstructure of both coatings. The assessment of deposition efficiency and coating thickness in previous studies often focus on the single electrical explosion [11,18], which is hardly to reflect the true variation of the deposition efficiency and coating thickness under high-volume production spraying. In this study, the variations of the deposition efficiency and the thickness of both coatings after 10 times continuous electrical explosions were investigated. The deposition efficiency was determined in accordance with the ISO 17836:2017, which is defined as follows:

ηD =

Δms × 100% mf

(1)

where ηD, Δms, mf, denote, respectively, the deposition efficiency, the mass difference of substrate before and after spraying and the mass of wire fed through. Adopting image method to characterize the average thickness of the coatings. The adhesive strength test was conducted by using a CSM scratch tester. The process parameters of scratch tests as summarized in Table 3. The test consists in applying a continuously increasing vertical load by an indenter while the sample is moved at constant speed. The Critical Load(Lc) was defined as the minimum load at which the coating exhibited the first adhesive failure(such as coating chipping, Table 3 Process parameters of scratch tests. Parameter Testing mode Indenter radius (μm) Loading range (N) Loading rate (N/min) Scratching speed (mm/min) Scratching length (mm)

Value Acoustic emission signal 200 0–50 50 5 5

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Fig. 3. Typical morphology of the explosive products: (a)Ta10W 72.7 J/mm3, (b) Ni60A 72.7 J/mm3, (c) Ta10W 151.6 J/mm3, (d) Ni60A 151.6 J/mm3.

unflattened particles of Ta10W decreased, the substrate had been almost covered by the coating because of the increase in the amount of the flattened splats, while the Ni60A coating surface became denser and evener, the typical morphology of both coatings as shown in Fig. 5(c) and (d). When the energy density was risen to 151.6 J/mm3, the Ta10W coating got uniform and dense, which had a good integrity, whereas the Ni60A coating turned into rough and rugged, both unflattened particles and nanoscale particles spread on the coating surface, the typical features as shown in Fig. 5(e) and (f). This variation in microstructure of both coatings were caused mainly by the aforementioned difference in characteristics of both explosive products which is determined by the difference in the melting point of both materials. Ta10W has a higher melting point, in the case

most direct influence on the microstructure of coating. Based on the analysis of the morphology and the size distribution of the explosive products, the microstructure of both coatings were investigated in the next section. 3.2. Microstructure of coatings Fig. 5 shows the typical microstructure of Ni60A and Ta10W coatings under different energy densities. It is obvious from the Fig. 5(a) and (b) that the surface of both coatings were rather rough and nonuniform when the energy density was 22.4 J/mm3, there were a large number of unflattened particles scattered on both coating surfaces. As the energy density was increased to 72.7 J/mm3, the number of

Fig. 4. Volumetric-distribution histograms of particle size: (a) Ta10W, (b)Ni60A. 47

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Fig. 5. SEM image of microstructure of coatings surfaces: (a) Ta10W 22.4 J/mm3, (b) Ni60A 22.4 J/mm3, (c) Ta10W 72.7 J/mm3, (d) Ni60A 72.7 J/mm3, (e) Ta10W 151.6 J/mm3, (f) Ni60A 151.6 J/mm3.

of low energy density(22.4 J/mm3–72.7 J/mm3), the wire was only partially melted by the low rate Joule Heat before the explosion occurred, both temperature and velocity of the resulting explosive products were quite low that they cannot flatten well on the substrate. As the energy density increased gradually, the degree of melting of the wire was improved, more and more fast-speed molten explosive products flattened on the substrate, the resulting splats had a relatively high temperature and flowability when they bonded with the substrate and with each other, the coating formed by such splats became uniform and compactness. For the Ni60A, the lower melting point made it possible to be melt completely when the energy density got to 72.7 J/ mm3, at which the ideal temperature, velocity and trajectory of the explosive products for constituting a uniform and dense coating were probably reached(deduced from the analysis of the characteristics of

the explosive products). If the energy density continues to increase, the surface region of Ni60A wire will be rapidly vaporized during heating due to the skin effect, forming a structure of wire core inside and vapour corona outside, which was referred to as a “core–corona structure” [19–21]. The current rapidly flowed through the dense corona owing to the corona resistance was lower than that of the wire core, causing the inadequate heating of the wire core, there is no sufficient thermokinetic conditions to enable spreading of the explosive products on impact with the substrate. Additionally, a great deal of Ni60A vapour turned into nanoparticles during spraying [22], which will accompany the other explosive products deposit on the substrate, making the coating became rough and rugged. The phase analysis of both coating surfaces are given in Fig. 6. The XRD pattern of Ta10W coating (Fig. 6a) shows that the Ta, Ta2N, and 48

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Fig. 6. X-ray diffraction patterns of both coatings: (a) Ta10W, (b) Ni60A.

The deposition efficiency and coating thickness are the significant factors influencing the widely commercial acceptance of the WEES coatings. A high deposition efficiency is critical and desirable, because this ensures that the preparation of relatively thick coatings become more economical and efficient. In this section, the deposition efficiency and the coating thickness of both materials will be determined. Fig. 7 shows the deposition efficiency of both coatings under different energy densities. It is clearly visible from the figure that the deposition efficiency of both coatings show the similar trend of increase first and then decrease with the energy density. When the energy density was relatively low(less than 100.8 J/mm3), the variation of the deposition efficiency of both coatings were almost same, but when the energy density goes on increasing, the deposition efficiency of Ta10W

decreased slowly when it reached the maximum of about 53% at the energy density of 124.9 J/mm3, whereas the deposition efficiency of Ni60A had the rapidly decrease after it got to the peak value of 47% at the energy density of 100.8 J/mm3. The reasons of the decrease in deposition efficiency of both coatings can be explained by two parts: one is related to the vaporization of the wire when the energy density was high enough. Because such vaporized products are rapidly transformed into nanoparticles during the process of spraying, it is easier for these nanoparticles to disperse into the surrounding space than to deposit directly on the substrate owing to its low Stokes number [24,25]. In addition, the Ni60A was more easily to evaporate than the Ta10W under the same energy density, especially in the case of high energy density(larger than 100.8 J/mm3), resulting in the deposition efficiency of the Ni60A decrease faster than that of the Ta10W; The other reason is lie in the enhancement of explosive shock wave. Such strong shock wave struck the upper as-sprayed coating over and over again in the process of multiple continuous explosions, the coatings with poor cohesion were cleaned up to a certain extent. The average thickness of both coatings was determined under the energy density corresponding to their highest deposition efficiency, the results are shown in Fig. 8. As it is seen, the average thickness of both coatings is proportional to the explosion frequency. However, the increment of the thickness of both coatings decrease with the explosion frequency. The variation of coatings thickness prove once again that explosive shock wave has a significant influence on the formation of coatings. Because the propagation distance of the shock wave gradually decreases as the coating thickness increases, the shock strength for the as-sprayed coating increases. The destruction of both coatings becomes stronger, resulting in the increment of the coatings thickness decrease gradually. But in another way, the explosive shock wave can preheat the substrate or the upper as-sprayed coating, moreover the destroyed

Fig. 7. Deposition efficiency of both coatings.

Fig. 8. The average thickness of both coatings.

FeTaO4 presented on the coating surface. Both Ta and Ta2N have excellent corrosion resistance and biocompatibility, the formation of the FeTaO4 in coating surface means the substrate metal was found at the top surface of the coating, this indicate that there was an intense metallic interdiffusion between the substrate and coating, the metallurgical bonding can be created. This phenomenon was also found in the process of powder electrical explosion spraying, which called “BaseMetal Saturation Coating” [23]. Fig. 6(b) illustrate the XRD pattern of Ni60A coating surface, the FeNi and SiO2 phases were detected on the coating surface. The appearance of SiO2 phase reveal that deoxidizing elements silicon can well react with oxygen to form low-density SiO2 that float to the coating surface, which can prevent the coating from being oxidized during spraying. In order to reduce the oxide inclusions of the as-sprayed Ni60A coating after continuous multiple explosions by the WEES, the postcoating operation of fusion is recommended. 3.3. Deposition efficiency and coating thickness

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Fig. 9. Typical cross section of both coatings: (a) Ta10W 151.6 J/mm3, (b) Ni60A 72.7 J/mm3.

typical scratch mark morphology(several SEM pictures were merged together into a scratch mark morphology) and the corresponding acoustic emission signals of both coatings. The testing result of Ta10W coating, see from Fig. 10(a), revealed that there was no obvious coating fracture, delamination, and spalling in and around the scratch mark region, and the corresponding acoustic emission signals did not show obvious burst along the profile during the whole testing process. After 3 times repeated scratch tests in different positions of the Ta10W coating, the scratch mark morphology and acoustic emission signals shown the similar results. This indicates that the average critical load Lc of the Ta10W coating was greater than 50 N, which can be used to characterize the adhesive strength of the Ta10W coatings. Fig. 10(b) illustrates the typical testing result of Ni60A coating. It is apparent from the figure that a critical load Lc of about 37 N was determined from the acoustic emission signals, in which the first sharp signal peak corresponds to an adhesive failure event. The magnified morphology of the

coatings or some surface debris often have the relatively low adhesion/ cohesion, the coatings with highest adhesion/cohesion can always be left on the substrate. Thus, the appropriate selection of the energy density allows the advantages of the shock wave to be realized, while minimizing its disadvantages. Fig. 9 illustrate the typical cross section of both coatings. It can be seen that both coatings are quite dense, and there are no obviously crackings, inclusions or unmelted particles in both coatings. The interfaces between the coating and substrate are exhibit the typical anchoring effect. This means the coating and substrate have the outstanding adhesive strength.

3.4. Adhesive strength of coatings The Ta10W and Ni60A coatings with the same average thickness (about 20 μm) were selected for the scratch test. Fig. 10 shows the

Fig. 10. Typical scratch mark morphology and the corresponding acoustic emission signals of both coatings: (a) Ta10W, (b) Ni60A. 50

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the appropriate process parameter. Acknowledgements The authors would like to thank the financial support from the National Natural Science Foundation of China(Grant No.51765038). References [1] A. Vardelle, C. Moreau, J. Akedo, et al., The 2016 thermal spray roadmap, J. Therm. Spray Technol. 8 (2016) 1376–1440. [2] J.J. Tian, X.T. Luo, J. Wang, et al., Mechanical performance of plasma-sprayed bulklike NiCrMo coating with a novel shell-core-structured NiCr-Mo particle, Surf. Coat. Technol. 353 (2018) 179–189. [3] P.B. Mi, T. Wang, F.X. Ye, Influences of the compositions and mechanical properties of HVOF sprayed bimodal WC-co coating on its high temperature wear performance, Int. J. Refract. Met. Hard Mater. 69 (2017) 158–163. [4] S. Sugimura, J.S. Liao, Long-term corrosion protection of arc spray Zn-Al-Si coating system in dilute chloride solutions and sulfate solutions, Surf. Coat. Technol. 302 (2016) 398–409. [5] L. Zhu, H.T. Qiao, P.F. Zhang, Characteristics of molybdenum coating prepared by electro-thermal explosion spraying with constraining tube, Rare Metal Mater. Eng. 7 (2013) 1488–1491. [6] D.A. Romanov, E.A. Budovskikh, Y.D. Zhmakin, Surface modification by the EVU 60/10 electroexplosive system, Steel. In. Trans. (6) (2011) 464–468. [7] J. Vilys, Coating obtained by the metal directional explosion spraying technique, Proceedings of the ASME 2010 10th Biennial Conference on Engineering Systems Design and Analysis, July 12–14, 2010 (Istanbul, Turkey). [8] S.X. Hou, Z.D. Liu, D.Y. Liu, The study of NiAl–TiB2 coatings prepared by electrothermal explosion ultrahigh speed spraying technology, Surf. Coat. Technol. 205 (2011) 4562–4568. [9] Z.D. Liu, S.X. Hou, D.Y. Liu, L.P. Zhao, B. Li, J.J. Liu, An experimental study on synthesizing submicron MoSi2-based coatings using electrothermal explosion ultrahigh speed spraying method, J. Surf. Coat. Technol. 202 (2008) 2917–2921. [10] J.J. Liu, Z.D. Liu, An experimental study on synthesizing TiC–TiB2–Ni composite coating using electro-thermal explosion ultra-high speed spraying method, Mater. Lett. 64 (2011) 684–687. [11] Q. Li, Q.Z. Song, J.Z. Wang, Y.X. Duo, Effect of charging energy on droplet diameters and properties of high-carbon steel coatings sprayed by wire explosion spraying, J. Surf. Coat. Technol. 206 (2011) 202–207. [12] X.L. Jiang, Y.N. Wang, X. Lu, Research on the mechanism of extending artillery barrel life by electrical explosion spraying technology, Adv. Mater. Res. 429 (2012) 19–24. [13] J.Z. Yang, Z.Y. Liu, D.W. XU, T.J. LIU, Electrical conducting coating on glass substrate deposited by wire exploding spray coating method, Key Eng. Mater. 373-374 (2008) 367–370. [14] X.M. Fan, J.J. Liu, Molybdenum coating on inner surfaces of steel tube produced by eccentric mounted wire electrothermal explosion spraying, Rare Metal Mat. Eng. 2 (2012) 792–796. [15] L. Zhu, X.S. Bi, CN Patent, (2012) (2011100547927). [16] L. Zhu, F. Han, CN Patent (2017) (2015106480098). [17] L. Zhu, M.H. Shi, Y. Wang, Coating forming by electrical explosion spraying of WC powder in the constraining tube, Rare Metal Mat. Eng. 4 (2014) 968–972. [18] X.M. Fan, J.J. Liu, Process and properties of molybdenum coatings prepared by electro-thermal explosion spraying on aluminum alloy surface, Trans. Mater. Heat Treat. 9 (2012) 134–139. [19] M.J. Taylor, Formation of plasma around wire fragments created by electrically exploded copper wire, J. Phys. D. Appl. Phys. 35 (2002) 700–709. [20] K. Wang, Z.Q. Shi, Y.J. Shi, Z.G. Zhang, D. Zhang, Characteristics of electrical explosion of single wire in a vacuum and in the air, Acta Phys. Sin. 18 (2017) 185203. [21] V.M. Romanova, G.V. Ivanenkov, A.R. Mingaleev, et al., Electric explosion of fine wires: three groups of materials, Plasma Phys. Rep. (8) (2015) 617–636. [22] Y.S. Kwon, V.V. An, A.P. Ilyin, D.V. Tikhonov, Properties of powders produced by electrical explosions of copper–nickel alloy wires, Mater. Lett. 61 (2007) 3247–3250. [23] H. Tamura, M. Itaya, Base-metal saturation of refractory carbide coatings produced by enhanced ceramic jets in electrothermally exploded powder spray, J. Therm. Spray Technol. (3) (2000) 389–393. [24] C.T. Crowe, R.A. Gore, T.R. Troutt, Particle dispersion by coherent structures in free shear flows, Part. Sci. Technol. (3–4) (1985) 149–158. [25] K. Zhang, H.B. Xiong, X.M. Shao, Dynamic modeling of micro- and nano-sized particles impinging on the substrate during suspension plasma spraying, J. Zhejiang Univ-Sci A (Appl Phys & Eng) 9 (2016) 733–744.

Fig. 11. Magnified morphology of the adhesive failure location of the Ni60A coating.

adhesive failure location of the Ni60A coating as shown in Fig. 11, it was found that the delamination failure can be observed in the scratch mark, and the substrate becomes partly exposed. The average critical load of the Ni60A coating was 38.3 N under 3 times repeated tests. 4. Conclusions In this study, the refractory Ta10W and non-refractory Ni60A coatings were prepared using a self-designed device by the WEES technology, several following conclusions could be drawn: (1) A uniform and dense Ta10W coating with the phase of Ta, Ta2N, and FeTaO4 can be obtained when the energy density was 151.6 J/ mm3, while the uniform and dense Ni60A coating made up of FeNi and SiO2 phase was got at the energy density of 72.7 J/mm3. (2) Increasing the energy density from 22.4 J/mm3 to 151.6 J/mm3, the deposition efficiency of both coatings under 10 times continuous electrical explosions show the similar trend of increase first and then decrease. The maximum deposition efficiency of Ta10W is 53%, while that of the Ni60A is 47%. (3) The average thickness of both coatings is proportional to the explosion frequency. However, the increment of the thickness of both coatings decrease with the increase of the explosion frequency. (4) The adhesive strength of both coatings was evaluated by using the scratch test. The average critical load Lc of the Ta10W coating is larger than 50 N and that of the Ni60A coating is 38.3 N, which can be the criterion of the adhesive strength of both coatings. (5) By using the self-designed WEES device, the stable and efficient coating process can be realized. In the next stage, in order to make the feedstock transportation can be balanced against the energy density to obtain optimal coating performance, a closed-loop, output-based controls WEES device which have been successfully integrated into a control scheme is under development. As reviewed above, the WEES technology, although currently limited in use, have the potential to become more commercially significant in the future by developing a high-efficient spray device and choosing

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