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Preparation and characterization of aluminum-based coatings deposited by very low-pressure plasma spray
Journal Pre-proof Preparation and characterization of aluminum-based coatings deposited by very lowpressure plasma spray Xiujuan Fan, Geoffrey Darut, Marie Pierre Planche, Chen Song, Hanlin Liao, Ghislain Montavon PII:
S0257-8972(19)31025-4
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125034
Reference:
SCT 125034
To appear in:
Surface & Coatings Technology
Received Date: 8 April 2019 Revised Date:
27 September 2019
Accepted Date: 30 September 2019
Please cite this article as: X. Fan, G. Darut, M.P. Planche, C. Song, H. Liao, G. Montavon, Preparation and characterization of aluminum-based coatings deposited by very low-pressure plasma spray, Surface & Coatings Technology (2019), doi: https://doi.org/10.1016/j.surfcoat.2019.125034. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Preparation and characterization of aluminum-based coatings deposited by very low-pressure plasma spray Xiujuan FANa, Geoffrey DARUTa, Marie Pierre PLANCHEa, Chen Song b, Hanlin Liaoa, Ghislain MONTAVONa a Université Bourgogne Franche-Comté, ICB-PMDM-LERMPS UMR6303, 90010 Belfort, France b Guangdong Institute of New Materials, National Engineering Laboratory for Modern Materials Surface Engineering Technology, Guangzhou 510651, China Corresponding author: Chen SONG
E-mail: [email protected]
Abstract: Aluminum-based coatings were manufactured by the very low-pressure plasma spray, using an F4-VB plasma torch under a working pressure of 150 Pa. The morphologies and phase compositions of the resultant coatings were investigated along the radial direction of the plasma jet, which was found that the Al-based coatings exhibited three different deposition states. At the center of plasma jet, an Al-Fe coating was formed due to the diffusion of substrate element, which had a packed columnar microstructure that deposited by the vapor and clusters. As the radial distance increases, a pure Al coating was obtained, with a lamellar binary microstructure that mainly formed by spread liquid droplets and plenty of vapor. At the outer edge of plasma jet, another pure Al coating was achieved, possessed a typical layer microstructure consisted of semi-molten particles, molten particles, and a little vapor. Moreover, the microhardness of the deposits was measured, respectively. The hardness of Al-Fe coating reached 448.74±23.7 HV0.025, while that of pure Al coating was only 46.37±3.18 HV0.025. In order to better explain the difference in coating microstructures and properties along the radial direction, a spatial distribution model for the depositions based on the melting state of Al powders was established. Keywords: Very low-pressure plasma spray, al-based coating, microstructure, microhardness, spatial distribution model.
Introduction As an advanced thermal spray process, very low-pressure plasma spray (VLPPS) differs from the traditional thermal spray processes in that the coatings are manufactured in a controlled atmosphere chamber under pressures typically less than 500 Pa [1-3]. By adopting appropriate spraying parameters, the VLPPS can melt the injected particles or even vaporize them. As a result, the coating will possess a lamellar, columnar or hybrid microstructure, which is deposited from very fine molten droplets, vapor phase deposition, or a mixture of them [4-6]. Base on this feature, a wide range of coatings (titanium, copper, yttria-stabilized zirconia, etc.) with different morphologies had been successfully prepared by VLPPS [7-10]. Some works have reported that these coatings with diverse microstructures will induce different properties. For example, dense yttria-stabilized zirconia coating can be applied as the electrolyte for solid oxide fuel cell, due to its high ionic conductivity and good air tightness [10-12]. While columnar yttria-stabilized zirconia coating can be used as the thermal barrier layer for gas turbine, because of its low thermal conductivity and excellent thermal shock resistance [13-15]. Obviously, the difference in coating microstructure and performance is closely related to deposition state of the powders. Since the plasma jet at very low pressure has a significant expansion in the axial and radial direction. The injected powders with different particle sizes are impossible to undergo the same heating and accelerating process and have the same molten or vaporization state. On the one hand, the powders with small size that sent into the center of plasma jet can be heated and accelerated more thoroughly. On the other hand, the powders with large size that injected into the periphery of the plasma jet will be heated incompletely. Hence, it is really meaningful to investigate the powders deposition state at different radial positions of the plasma jet [16, 17]. The research results will provide theoretical guidance for VLPPS to produce coatings with specific microstructures and properties. In this study, aluminum powders were chosen to be sprayed by VLPPS process, due to its corrosion resistance [18, 19] and proper vaporization enthalpy (i.e., 38.23 KJ·cm-3) [20]. The resultant coating characterization such as the microstructures, composition, and microhardness along the radial direction of the plasma jet was investigated. Based on that, a spatial distribution model for the deposits was built for demonstrating the coating formation process. 2. Experimental process As shown in Fig.1, the atomized spherical Al powders prepared by ICB-PMDM-LERMPS lab was used as the feedstock, whose size distribution ranged from 8 to 22 µm, with an average diameter of 13 µm. Disc-shaped stainless steels were employed (φ 25 mm × δ 10 mm) as the substrates, placed every 30 mm in the X direction (Fig. 2). In order to obtain a clean surface and improve the bonding strength of the coating, the substrates were degreased in ethanol and grit-blasted by alumina sands. For spraying, a VLPPS system equipped with an F4-VB torch (Oerlikon– Metco, Switzerland) operating at the pressure of 150 Pa was employed to deposit
coatings. The spraying parameters in detail were presented in Table 1. Before spraying, the chamber is pumped down to a few Pascal to avoid oxygen and other impurities. The plasma torch reciprocated along the Y-axis, its localization was detected and adjusted using a laser. An optical emission spectrometer (OES) instrument (Jobin Yon spectrometer, TRIAX190, United Kingdom) was applied to detect the visible light emission of evaporated particles inside the plasma jet. A charge-coupled device detector was placed at a focal length of X=190 mm. The coating surfaces and cross-sections were observed with a field emission scanning electron microscope (FESEM JEOL 7800F, Japan). The coating compositions were determined by energy-dispersive spectrometry (EDS, SDDX-Flash 6130, Bruker, Germany) and with an X-ray diffractometer (XRD, D8 Advance, Bruker, Germany) equipped with CoKα radiation (λ=0.179 nm). The average Vickers hardness (Miniload-2, Leitz, Germany) of coatings using 10 random indentations was measured on the polished cross section, with a 25g load and a dwell time of 15s.
Fig. 1 (a) Morphology and (b) size distribution of the Al powders.
Fig. 2 VLPPS experimental configuration
Table 1: Experimental parameters Parameters Value Pressure (Pa) 150 Plasma torch F4-VB Plasma forming gases (L.min-1) Ar/H2 (45/10) Arc current intensity (A) 650 Voltage (V) 50 -1 Carrier gas flow rate (L.min ) 2.5 Powders mass rate (g.min-1) 1 Torch speed (mm/s) 400 Substrate temperature of sample (a) (°C) 600 Spraying distance (mm) 900 Spraying time (min) 30
3. Results and discussions 3.1 OES analysis The OES system detected the visible light emission at the center of the plasma jet with Al powders in the wavelength range from 300 to 900 nm (Fig. 3). According to the pattern, the typical excited electronic states of neutral atomic Al II and Al III were observed in the wavelength between 600 and 900 nm. These signals were associated with the peak positions of 738.80, 763.56, 763.63, 810.37, 811.64, and 826.56 nm. Based on the atomic and molecular spectral database [20], it revealed that part of the Al powders were vaporized by the plasma jet.
Fig. 3 OES analysis of the plasma jet with injected Al particles. 3.2 Coating microstructures The cross-sectional morphologies of the coatings along the radial direction of the plasma jet are presented in Fig. 4. The coating located at the center of the plasma jet shown a columnar microstructure deposited from the vapor, with some
nanometer-size dispersed particles inside (Fig. 4(a)). When the radial distance increased to 60 mm, the columnar microstructure of the coating was replaced by a mixture of continuous stacking splats formed by melted particles and vapor condensates. Thus, the coating exhibited a hybrid microstructure (Fig. 4 (b)). When the radial distance increased to 90 mm, a lamellar microstructure was predominant compared to that in 60mm coating. This phenomenon was caused by the decreasing amount of vapor phase in this area. Finally, a very thin coating was obtained at 120 mm away from the real impact of the jet in Fig. 4 (d). This coating was formed by the flying particles that did not enter or pass through the plasma jet. It was also possible that the small melted powders moved there following the divergent plasma jet.
Fig. 4 Cross-section microstructures of the coatings at radial distances of (a) 0 mm, (b) 60 mm, (c) 90 mm and (d) 120 mm. The surface morphologies of the coatings along the radial direction of the plasma jet are shown in Fig. 5. For the coating located at the center of the plasma jet, the surface presented different sizes of needle-like grains and ultra-fine particles formed by the vapor deposition. For the 60 mm coating, the needle-like grains in the coating (a) were replaced by sub-micrometer size particles. When the radial distance increased to 90 mm, the coating revealed agglomerated ball-like particles with a size from 2 to 5 µm. For the 120 mm coating, its microstructure was significantly different from the 0 mm and 60 mm coating. It was nearly beyond the scope of the jet impact. The surface was mainly composed of some splats formed by the solidification of molten droplets. Also, some unmolten particles could be found inside.
Fig. 5 Surface morphologies of the coatings at radial distances of (a) 0 mm, (b)60 mm, (c) 90 mm and (d) 120 mm. Furthermore, Fig. 6 shows the fracture morphologies of the coatings at different radial distances. The 0 mm coating presented many nano-sized particles that could be attributed to vapor phase deposition due to a high temperature at the center of the plasma jet. The 60 mm coating was composed of some ultra-fine particles embedded between the lamellar splats, exhibited a binary microstructure. The 90 mm coating shown typical layer structure formed by the molten and semi-molten particles. The changing of fracture morphologies was consistent with the variation of the sectional and surface morphology. For the coating at the radial distance of 120 mm, the fracture morphology was not obtained because of the ultra-thin thickness.
Fig. 6 Fracture microstructures of the coatings at radial distances of (a) 0 mm, (b) 60 mm, (c) 90 mm 3.3 Coating compositions The XRD patterns of the coatings at different radial distances are displayed in
Fig. 7. The main phase of the 0 mm coating, placed at the center of the plasma jet, was an intermetallic material identified as Al3Fe. As the radial distance increased to 60~120 mm, the coatings were mainly composed of Al phase. Moreover, the intensity of phase Al peaks gradually decreased as radial distance increased. Because the density of deposited particles gradually decreased. In a word, the coating located on the center of the plasma jet possessed a unique composition compared to the other coatings. The variation of phases was consistent with the changing features of the coating morphologies. In order to further study the changing rule, the EDS analyses of the coatings were applied in the following.
Fig. 7 XRD patterns of the coatings for different radial distances of (a) 0 mm, (b) 60 mm, (c) 90 mm, and (d) 120 mm Elemental chemical analysis of the 0 mm and 60 mm coating was conducted by using the EDS along the coating thickness (Fig. 8 and Fig. 9). On the one hand, the 0 mm coating with a columnar microstructure contained 60 wt. % Al and 30 wt. % Fe (Fig. 8(d)). The cross-sectional mapping in Fig. 8(b) showed that the Fe element is evenly distributed throughout the coating, not just at the interface between the substrate and the coating. During the experiment, the substrate temperature at the center of plasma jet was kept at about 650-700 ℃. This high temperature was close to the melting point of Al, which not only allow the Fe element in the substrate diffuse into the coating but also produce a sufficient in-situ reaction and form a uniform intermetallic compound Al3Fe. On the other hand, the 60 mm coating with a binary microstructure consisted almost entirely of Al (Fig. 9(d)), only a small amount of Fe diffused into the coating through the interface (Fig. 9(b)). Because the 60 mm coating was far from the jet center, where the temperature was lower than the melting point of Al. Hence the deposited Al coating was always in solid-state, which would significantly reduce the diffusing rate of Fe in the coating. That is why the 60~120 mm coating can only find the Al phase (Fig. 7).
Fig. 8 Coating at a radial distance of 0 mm: (a) SEM picture, (b) (c) element surface mapping, (d) element composition along the scanning direction.
Fig. 9 Coating at a radial distance of 60 mm: (a) SEM picture, (b) (c) element surface mapping, (d) element composition along the scanning direction. 3.5. Coating microhardness
Microhardness of the deposits at different radial distances were investigated. Due to the distinct microstructures and phase compositions between the 0 mm and 60 mm coating, their microhardness values varied greatly. It was found that the microhardness of the Al3Fe coating manufactured at the center of the plasma jet reached 448.74±23.7 HV0.025, which was more than two times higher than the microhardness of the Al3Fe coating by conventionally casting [21]. Besides, the microhardness of the Al coating located at further radial distance (60mm) was 46.37±3.18 HV0.025, which was higher than the theoretical hardness of Al bulk. 3.6 Deposition mechanism Since the Al powders were radially injected into the high enthalpy plasma jet. The heating process was mainly affected by the broad particle size distribution and the temperature difference in the expanding plasma jet under low pressure. As a result, the Al powders along the radial direction of the plasma jet would have various deposition states: solid particles, liquid droplets, and vapor phases. Based on this, the deposition mechanism for the as-sprayed coatings at different radial distances was discussed as following (Fig 10): At the center of the plasma jet, the high-temperature region could offer enough energy to melt or even evaporate the injected Al powders. Thus, the columnar microstructure with some nano-sized ball-like particles was formed by condensation of the vapor phase. As the radial distance increased to 60 mm, the heating capacity of the middle-temperature zone was weakened, so the injected Al powders were melted to the droplets mixed with a little vapor. The binary microstructure was formed by re-solidification of liquid droplets and condensation of vapor phases. As the radial distance increased to 90 mm, the low-temperature region made the injected Al powders partially melted to semi-molten particles. Thus, the layer microstructure was formed by solidification of semi-molten particles.
Fig. 10 Illustrations of the deposition features of the VLPPS-Al coatings 4. Conclusion
In this study, the VLPPS sprayed Al powders was used to investigate the radial difference of resultant coatings along the plasma jet, including the varying coating morphology, phase composition, and microhardness. At the center of the plasma jet, an Al3Fe coating was manufactured due to the highest temperature, which possessed a dense columnar microstructure composed of much vapor and many nanometer particles. The microhardness of the Al3Fe coating reached 448.74±23.7 HV0.025. As the radial distance increased to 60 mm and 90 mm, the Al3Fe coating replaced by two pure Al coatings. The 60 mm coating had a binary microstructure mainly formed by liquid droplets and vapor, while the 90 mm coating had a layer microstructure packed by molten droplets and semi-molten particles. The higher microhardness of the two coatings was only 46.37±3.18 HV0.025. To better summarize the evolutionary rule, the deposition mechanism along the radial distance of the plasma jet was elaborated. Acknowledgments The authors are grateful to the financial supports by the China Scholarship Council (CSC-No. 201504490061) and Science and Technology Planning Project of Guangdong Province (No. 2017A070701027 and No. 2014B070705007) References [1] G. Mauer, R. Vassen, Plasma Spray-PVD: Plasma Characteristics and Impact on Coating Properties, in: A. Gleizes, E. Ghedini, M. Gherardi, P. Sanibondi, G. Dilecce (Eds.) 12th High-Tech Plasma Processes Conference, 2012. [2] M.F. Smith, A.C. Hall, J.D. Fleetwood, P. Meyer, Coatings, 1 (2011) 117-132. [3] Z. Salhi, D. Klein, P. Gougeon, C. Coddet, Vacuum, 77 (2005) 145-150. [4] K. von Niessen, M. Gindrat, A. Refke, Journal of Thermal Spray Technology, 19 (2010) 502-509. [5] G. Mauer, M.O. Jarligo, S. Rezanka, A. Hospach, R. Vaßen, Surface and Coatings Technology, 268 (2015) 52-57. [6] K. von Niessen, M. Gindrat, Journal of Thermal Spray Technology, 20 (2011) 736-743. [7] B. Vautherin, M.P. Planche, G. Montavon, F. Lapostolle, A. Quet, L. Bianchi, Surface and Coatings Technology, 275 (2015) 341-348. [8] C. Song, M. Liu, Z.-Q. Deng, S.-P. Niu, C.-M. Deng, H.-L. Liao, Materials Letters, 217 (2018) 127-130. [9] N. Zhang, F. Sun, L. Zhu, C. Verdy, M.P. Planche, H. Liao, C. Dong, C. Coddet, Journal of Thermal Spray Technology, 20 (2011) 351-357. [10] G. Mauer, M. Jarligo, D. Marcano, S. Rezanka, D. Zhou, R. Vaßen, in: IOP Conference Series: Materials Science and Engineering, IOP Publishing, 2017, pp. 012001. [11] C. Zhang, H. Liao, W. Li, G. Zhang, C. Coddet, C. Li, C. Li, X.J.J.o.t.s.t. Ning, 15 (2006) 598-603.
[12] D. Marcano, G. Mauer, R. Vassen, A. Weber, Surface & Coatings Technology, 318 (2017) 170-177. [13] P.-j. He, S. Yin, C. Song, F. Lapostolle, H.-l. Liao, Journal of Thermal Spray Technology, 25 (2016) 558-566. [14] X. Zhang, K. Zhou, C. Deng, M. Liu, Z. Deng, C. Deng, J. Song, Journal of the European Ceramic Society, 36 (2016) 697-703. [15] A. Hospach, G. Mauer, R. Vaßen, D. Stöver, Journal of Thermal Spray Technology, 20 (2010) 116-120. [16] J. Mao, Z. Deng, M. Liu, C. Deng, J. Song, K. Yang, Surface and Coatings Technology, 328 (2017) 240-247. [17] C. Li, H. Guo, L. Gao, L. Wei, S. Gong, H. Xu, Journal of Thermal Spray Technology, 24 (2015) 534-541. [18] E.A. Esfahani, H. Salimijazi, M.A. Golozar, J. Mostaghimi, L.J.J.o.t.s.T. Pershin, 21 (2012) 1195-1202. [19] H.-S. Lee, J. Singh, M. Ismail, C.J.M. Bhattacharya, 6 (2016) 55. [20] NIST Physical Measurement Laboratory, http://physics.nist.gov/PhysRefData/ASD/lines_form.html, 2017 [21] E. Karaköse, M. Keskin, in: AIP Conference Proceedings, AIP, 2011, pp. 645-649.
Highlights: Different microstructures, compositions, and microhardnesses of Al-based coating along the radial direction of the low-pressure plasma jet were investigated. The Al-based coatings exhibited three different microstructures along the radial direction, including the columnar, binary, and layer microstructure. The microhardness of Al-Fe coating reached 448.74±23.7 HV0.025, while that of pure Al coating was only 46.37±3.18 HV0.025.
Conflicts of interest The authors declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Preparation and characterization of aluminum-based coatings deposited by very low-pressure plasma spray”
S0257-8972(19)31025-4
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125034
Reference:
SCT 125034
To appear in:
Surface & Coatings Technology
Received Date: 8 April 2019 Revised Date:
27 September 2019
Accepted Date: 30 September 2019
Please cite this article as: X. Fan, G. Darut, M.P. Planche, C. Song, H. Liao, G. Montavon, Preparation and characterization of aluminum-based coatings deposited by very low-pressure plasma spray, Surface & Coatings Technology (2019), doi: https://doi.org/10.1016/j.surfcoat.2019.125034. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Preparation and characterization of aluminum-based coatings deposited by very low-pressure plasma spray Xiujuan FANa, Geoffrey DARUTa, Marie Pierre PLANCHEa, Chen Song b, Hanlin Liaoa, Ghislain MONTAVONa a Université Bourgogne Franche-Comté, ICB-PMDM-LERMPS UMR6303, 90010 Belfort, France b Guangdong Institute of New Materials, National Engineering Laboratory for Modern Materials Surface Engineering Technology, Guangzhou 510651, China Corresponding author: Chen SONG
E-mail: [email protected]
Abstract: Aluminum-based coatings were manufactured by the very low-pressure plasma spray, using an F4-VB plasma torch under a working pressure of 150 Pa. The morphologies and phase compositions of the resultant coatings were investigated along the radial direction of the plasma jet, which was found that the Al-based coatings exhibited three different deposition states. At the center of plasma jet, an Al-Fe coating was formed due to the diffusion of substrate element, which had a packed columnar microstructure that deposited by the vapor and clusters. As the radial distance increases, a pure Al coating was obtained, with a lamellar binary microstructure that mainly formed by spread liquid droplets and plenty of vapor. At the outer edge of plasma jet, another pure Al coating was achieved, possessed a typical layer microstructure consisted of semi-molten particles, molten particles, and a little vapor. Moreover, the microhardness of the deposits was measured, respectively. The hardness of Al-Fe coating reached 448.74±23.7 HV0.025, while that of pure Al coating was only 46.37±3.18 HV0.025. In order to better explain the difference in coating microstructures and properties along the radial direction, a spatial distribution model for the depositions based on the melting state of Al powders was established. Keywords: Very low-pressure plasma spray, al-based coating, microstructure, microhardness, spatial distribution model.
Introduction As an advanced thermal spray process, very low-pressure plasma spray (VLPPS) differs from the traditional thermal spray processes in that the coatings are manufactured in a controlled atmosphere chamber under pressures typically less than 500 Pa [1-3]. By adopting appropriate spraying parameters, the VLPPS can melt the injected particles or even vaporize them. As a result, the coating will possess a lamellar, columnar or hybrid microstructure, which is deposited from very fine molten droplets, vapor phase deposition, or a mixture of them [4-6]. Base on this feature, a wide range of coatings (titanium, copper, yttria-stabilized zirconia, etc.) with different morphologies had been successfully prepared by VLPPS [7-10]. Some works have reported that these coatings with diverse microstructures will induce different properties. For example, dense yttria-stabilized zirconia coating can be applied as the electrolyte for solid oxide fuel cell, due to its high ionic conductivity and good air tightness [10-12]. While columnar yttria-stabilized zirconia coating can be used as the thermal barrier layer for gas turbine, because of its low thermal conductivity and excellent thermal shock resistance [13-15]. Obviously, the difference in coating microstructure and performance is closely related to deposition state of the powders. Since the plasma jet at very low pressure has a significant expansion in the axial and radial direction. The injected powders with different particle sizes are impossible to undergo the same heating and accelerating process and have the same molten or vaporization state. On the one hand, the powders with small size that sent into the center of plasma jet can be heated and accelerated more thoroughly. On the other hand, the powders with large size that injected into the periphery of the plasma jet will be heated incompletely. Hence, it is really meaningful to investigate the powders deposition state at different radial positions of the plasma jet [16, 17]. The research results will provide theoretical guidance for VLPPS to produce coatings with specific microstructures and properties. In this study, aluminum powders were chosen to be sprayed by VLPPS process, due to its corrosion resistance [18, 19] and proper vaporization enthalpy (i.e., 38.23 KJ·cm-3) [20]. The resultant coating characterization such as the microstructures, composition, and microhardness along the radial direction of the plasma jet was investigated. Based on that, a spatial distribution model for the deposits was built for demonstrating the coating formation process. 2. Experimental process As shown in Fig.1, the atomized spherical Al powders prepared by ICB-PMDM-LERMPS lab was used as the feedstock, whose size distribution ranged from 8 to 22 µm, with an average diameter of 13 µm. Disc-shaped stainless steels were employed (φ 25 mm × δ 10 mm) as the substrates, placed every 30 mm in the X direction (Fig. 2). In order to obtain a clean surface and improve the bonding strength of the coating, the substrates were degreased in ethanol and grit-blasted by alumina sands. For spraying, a VLPPS system equipped with an F4-VB torch (Oerlikon– Metco, Switzerland) operating at the pressure of 150 Pa was employed to deposit
coatings. The spraying parameters in detail were presented in Table 1. Before spraying, the chamber is pumped down to a few Pascal to avoid oxygen and other impurities. The plasma torch reciprocated along the Y-axis, its localization was detected and adjusted using a laser. An optical emission spectrometer (OES) instrument (Jobin Yon spectrometer, TRIAX190, United Kingdom) was applied to detect the visible light emission of evaporated particles inside the plasma jet. A charge-coupled device detector was placed at a focal length of X=190 mm. The coating surfaces and cross-sections were observed with a field emission scanning electron microscope (FESEM JEOL 7800F, Japan). The coating compositions were determined by energy-dispersive spectrometry (EDS, SDDX-Flash 6130, Bruker, Germany) and with an X-ray diffractometer (XRD, D8 Advance, Bruker, Germany) equipped with CoKα radiation (λ=0.179 nm). The average Vickers hardness (Miniload-2, Leitz, Germany) of coatings using 10 random indentations was measured on the polished cross section, with a 25g load and a dwell time of 15s.
Fig. 1 (a) Morphology and (b) size distribution of the Al powders.
Fig. 2 VLPPS experimental configuration
Table 1: Experimental parameters Parameters Value Pressure (Pa) 150 Plasma torch F4-VB Plasma forming gases (L.min-1) Ar/H2 (45/10) Arc current intensity (A) 650 Voltage (V) 50 -1 Carrier gas flow rate (L.min ) 2.5 Powders mass rate (g.min-1) 1 Torch speed (mm/s) 400 Substrate temperature of sample (a) (°C) 600 Spraying distance (mm) 900 Spraying time (min) 30
3. Results and discussions 3.1 OES analysis The OES system detected the visible light emission at the center of the plasma jet with Al powders in the wavelength range from 300 to 900 nm (Fig. 3). According to the pattern, the typical excited electronic states of neutral atomic Al II and Al III were observed in the wavelength between 600 and 900 nm. These signals were associated with the peak positions of 738.80, 763.56, 763.63, 810.37, 811.64, and 826.56 nm. Based on the atomic and molecular spectral database [20], it revealed that part of the Al powders were vaporized by the plasma jet.
Fig. 3 OES analysis of the plasma jet with injected Al particles. 3.2 Coating microstructures The cross-sectional morphologies of the coatings along the radial direction of the plasma jet are presented in Fig. 4. The coating located at the center of the plasma jet shown a columnar microstructure deposited from the vapor, with some
nanometer-size dispersed particles inside (Fig. 4(a)). When the radial distance increased to 60 mm, the columnar microstructure of the coating was replaced by a mixture of continuous stacking splats formed by melted particles and vapor condensates. Thus, the coating exhibited a hybrid microstructure (Fig. 4 (b)). When the radial distance increased to 90 mm, a lamellar microstructure was predominant compared to that in 60mm coating. This phenomenon was caused by the decreasing amount of vapor phase in this area. Finally, a very thin coating was obtained at 120 mm away from the real impact of the jet in Fig. 4 (d). This coating was formed by the flying particles that did not enter or pass through the plasma jet. It was also possible that the small melted powders moved there following the divergent plasma jet.
Fig. 4 Cross-section microstructures of the coatings at radial distances of (a) 0 mm, (b) 60 mm, (c) 90 mm and (d) 120 mm. The surface morphologies of the coatings along the radial direction of the plasma jet are shown in Fig. 5. For the coating located at the center of the plasma jet, the surface presented different sizes of needle-like grains and ultra-fine particles formed by the vapor deposition. For the 60 mm coating, the needle-like grains in the coating (a) were replaced by sub-micrometer size particles. When the radial distance increased to 90 mm, the coating revealed agglomerated ball-like particles with a size from 2 to 5 µm. For the 120 mm coating, its microstructure was significantly different from the 0 mm and 60 mm coating. It was nearly beyond the scope of the jet impact. The surface was mainly composed of some splats formed by the solidification of molten droplets. Also, some unmolten particles could be found inside.
Fig. 5 Surface morphologies of the coatings at radial distances of (a) 0 mm, (b)60 mm, (c) 90 mm and (d) 120 mm. Furthermore, Fig. 6 shows the fracture morphologies of the coatings at different radial distances. The 0 mm coating presented many nano-sized particles that could be attributed to vapor phase deposition due to a high temperature at the center of the plasma jet. The 60 mm coating was composed of some ultra-fine particles embedded between the lamellar splats, exhibited a binary microstructure. The 90 mm coating shown typical layer structure formed by the molten and semi-molten particles. The changing of fracture morphologies was consistent with the variation of the sectional and surface morphology. For the coating at the radial distance of 120 mm, the fracture morphology was not obtained because of the ultra-thin thickness.
Fig. 6 Fracture microstructures of the coatings at radial distances of (a) 0 mm, (b) 60 mm, (c) 90 mm 3.3 Coating compositions The XRD patterns of the coatings at different radial distances are displayed in
Fig. 7. The main phase of the 0 mm coating, placed at the center of the plasma jet, was an intermetallic material identified as Al3Fe. As the radial distance increased to 60~120 mm, the coatings were mainly composed of Al phase. Moreover, the intensity of phase Al peaks gradually decreased as radial distance increased. Because the density of deposited particles gradually decreased. In a word, the coating located on the center of the plasma jet possessed a unique composition compared to the other coatings. The variation of phases was consistent with the changing features of the coating morphologies. In order to further study the changing rule, the EDS analyses of the coatings were applied in the following.
Fig. 7 XRD patterns of the coatings for different radial distances of (a) 0 mm, (b) 60 mm, (c) 90 mm, and (d) 120 mm Elemental chemical analysis of the 0 mm and 60 mm coating was conducted by using the EDS along the coating thickness (Fig. 8 and Fig. 9). On the one hand, the 0 mm coating with a columnar microstructure contained 60 wt. % Al and 30 wt. % Fe (Fig. 8(d)). The cross-sectional mapping in Fig. 8(b) showed that the Fe element is evenly distributed throughout the coating, not just at the interface between the substrate and the coating. During the experiment, the substrate temperature at the center of plasma jet was kept at about 650-700 ℃. This high temperature was close to the melting point of Al, which not only allow the Fe element in the substrate diffuse into the coating but also produce a sufficient in-situ reaction and form a uniform intermetallic compound Al3Fe. On the other hand, the 60 mm coating with a binary microstructure consisted almost entirely of Al (Fig. 9(d)), only a small amount of Fe diffused into the coating through the interface (Fig. 9(b)). Because the 60 mm coating was far from the jet center, where the temperature was lower than the melting point of Al. Hence the deposited Al coating was always in solid-state, which would significantly reduce the diffusing rate of Fe in the coating. That is why the 60~120 mm coating can only find the Al phase (Fig. 7).
Fig. 8 Coating at a radial distance of 0 mm: (a) SEM picture, (b) (c) element surface mapping, (d) element composition along the scanning direction.
Fig. 9 Coating at a radial distance of 60 mm: (a) SEM picture, (b) (c) element surface mapping, (d) element composition along the scanning direction. 3.5. Coating microhardness
Microhardness of the deposits at different radial distances were investigated. Due to the distinct microstructures and phase compositions between the 0 mm and 60 mm coating, their microhardness values varied greatly. It was found that the microhardness of the Al3Fe coating manufactured at the center of the plasma jet reached 448.74±23.7 HV0.025, which was more than two times higher than the microhardness of the Al3Fe coating by conventionally casting [21]. Besides, the microhardness of the Al coating located at further radial distance (60mm) was 46.37±3.18 HV0.025, which was higher than the theoretical hardness of Al bulk. 3.6 Deposition mechanism Since the Al powders were radially injected into the high enthalpy plasma jet. The heating process was mainly affected by the broad particle size distribution and the temperature difference in the expanding plasma jet under low pressure. As a result, the Al powders along the radial direction of the plasma jet would have various deposition states: solid particles, liquid droplets, and vapor phases. Based on this, the deposition mechanism for the as-sprayed coatings at different radial distances was discussed as following (Fig 10): At the center of the plasma jet, the high-temperature region could offer enough energy to melt or even evaporate the injected Al powders. Thus, the columnar microstructure with some nano-sized ball-like particles was formed by condensation of the vapor phase. As the radial distance increased to 60 mm, the heating capacity of the middle-temperature zone was weakened, so the injected Al powders were melted to the droplets mixed with a little vapor. The binary microstructure was formed by re-solidification of liquid droplets and condensation of vapor phases. As the radial distance increased to 90 mm, the low-temperature region made the injected Al powders partially melted to semi-molten particles. Thus, the layer microstructure was formed by solidification of semi-molten particles.
Fig. 10 Illustrations of the deposition features of the VLPPS-Al coatings 4. Conclusion
In this study, the VLPPS sprayed Al powders was used to investigate the radial difference of resultant coatings along the plasma jet, including the varying coating morphology, phase composition, and microhardness. At the center of the plasma jet, an Al3Fe coating was manufactured due to the highest temperature, which possessed a dense columnar microstructure composed of much vapor and many nanometer particles. The microhardness of the Al3Fe coating reached 448.74±23.7 HV0.025. As the radial distance increased to 60 mm and 90 mm, the Al3Fe coating replaced by two pure Al coatings. The 60 mm coating had a binary microstructure mainly formed by liquid droplets and vapor, while the 90 mm coating had a layer microstructure packed by molten droplets and semi-molten particles. The higher microhardness of the two coatings was only 46.37±3.18 HV0.025. To better summarize the evolutionary rule, the deposition mechanism along the radial distance of the plasma jet was elaborated. Acknowledgments The authors are grateful to the financial supports by the China Scholarship Council (CSC-No. 201504490061) and Science and Technology Planning Project of Guangdong Province (No. 2017A070701027 and No. 2014B070705007) References [1] G. Mauer, R. Vassen, Plasma Spray-PVD: Plasma Characteristics and Impact on Coating Properties, in: A. Gleizes, E. Ghedini, M. Gherardi, P. Sanibondi, G. Dilecce (Eds.) 12th High-Tech Plasma Processes Conference, 2012. [2] M.F. Smith, A.C. Hall, J.D. Fleetwood, P. Meyer, Coatings, 1 (2011) 117-132. [3] Z. Salhi, D. Klein, P. Gougeon, C. Coddet, Vacuum, 77 (2005) 145-150. [4] K. von Niessen, M. Gindrat, A. Refke, Journal of Thermal Spray Technology, 19 (2010) 502-509. [5] G. Mauer, M.O. Jarligo, S. Rezanka, A. Hospach, R. Vaßen, Surface and Coatings Technology, 268 (2015) 52-57. [6] K. von Niessen, M. Gindrat, Journal of Thermal Spray Technology, 20 (2011) 736-743. [7] B. Vautherin, M.P. Planche, G. Montavon, F. Lapostolle, A. Quet, L. Bianchi, Surface and Coatings Technology, 275 (2015) 341-348. [8] C. Song, M. Liu, Z.-Q. Deng, S.-P. Niu, C.-M. Deng, H.-L. Liao, Materials Letters, 217 (2018) 127-130. [9] N. Zhang, F. Sun, L. Zhu, C. Verdy, M.P. Planche, H. Liao, C. Dong, C. Coddet, Journal of Thermal Spray Technology, 20 (2011) 351-357. [10] G. Mauer, M. Jarligo, D. Marcano, S. Rezanka, D. Zhou, R. Vaßen, in: IOP Conference Series: Materials Science and Engineering, IOP Publishing, 2017, pp. 012001. [11] C. Zhang, H. Liao, W. Li, G. Zhang, C. Coddet, C. Li, C. Li, X.J.J.o.t.s.t. Ning, 15 (2006) 598-603.
[12] D. Marcano, G. Mauer, R. Vassen, A. Weber, Surface & Coatings Technology, 318 (2017) 170-177. [13] P.-j. He, S. Yin, C. Song, F. Lapostolle, H.-l. Liao, Journal of Thermal Spray Technology, 25 (2016) 558-566. [14] X. Zhang, K. Zhou, C. Deng, M. Liu, Z. Deng, C. Deng, J. Song, Journal of the European Ceramic Society, 36 (2016) 697-703. [15] A. Hospach, G. Mauer, R. Vaßen, D. Stöver, Journal of Thermal Spray Technology, 20 (2010) 116-120. [16] J. Mao, Z. Deng, M. Liu, C. Deng, J. Song, K. Yang, Surface and Coatings Technology, 328 (2017) 240-247. [17] C. Li, H. Guo, L. Gao, L. Wei, S. Gong, H. Xu, Journal of Thermal Spray Technology, 24 (2015) 534-541. [18] E.A. Esfahani, H. Salimijazi, M.A. Golozar, J. Mostaghimi, L.J.J.o.t.s.T. Pershin, 21 (2012) 1195-1202. [19] H.-S. Lee, J. Singh, M. Ismail, C.J.M. Bhattacharya, 6 (2016) 55. [20] NIST Physical Measurement Laboratory, http://physics.nist.gov/PhysRefData/ASD/lines_form.html, 2017 [21] E. Karaköse, M. Keskin, in: AIP Conference Proceedings, AIP, 2011, pp. 645-649.
Highlights: Different microstructures, compositions, and microhardnesses of Al-based coating along the radial direction of the low-pressure plasma jet were investigated. The Al-based coatings exhibited three different microstructures along the radial direction, including the columnar, binary, and layer microstructure. The microhardness of Al-Fe coating reached 448.74±23.7 HV0.025, while that of pure Al coating was only 46.37±3.18 HV0.025.
Conflicts of interest The authors declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Preparation and characterization of aluminum-based coatings deposited by very low-pressure plasma spray”