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Microstructure and high temperature oxidation behavior of the Al2O3 CPED coating on TiAl alloy
Journal Pre-proof Microstructure and high temperature oxidation behavior of the Al2O3 CPED coating on TiAl alloy Shaoqing Wang, Faqin Xie, Xiangqing Wu, Tao Lv, Yong Ma PII:
S0925-8388(20)30634-4
DOI:
https://doi.org/10.1016/j.jallcom.2020.154271
Reference:
JALCOM 154271
To appear in:
Journal of Alloys and Compounds
Received Date: 1 November 2019 Revised Date:
28 January 2020
Accepted Date: 9 February 2020
Please cite this article as: S. Wang, F. Xie, X. Wu, T. Lv, Y. Ma, Microstructure and high temperature oxidation behavior of the Al2O3 CPED coating on TiAl alloy, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154271. 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. © 2020 Published by Elsevier B.V.
Microstructure and High Temperature Oxidation Behavior of the Al2O3 CPED Coating on TiAl Alloy
Shaoqing Wanga, Faqin Xiea,*, Xiangqing Wua, Tao Lva, Yong Mab
a
School of Aeronautics,
Northwestern Polytechnical University, Xi’an 710072, P. R.China
b
School of Material Science and Engineering,
Shandong University of Science and Technology, Qingdao 266590, P. R. China
*Corresponding author: E-mail: [email protected]
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Abstract Al2O3 coating was fabricated on the surface of γ-TiAl alloy via cathodic plasma electrolytic deposition (CPED) technique. The microstructure and the composition of the coating were elaborately characterized. The results revealed that, the uniform coating combined well with the substrate, which was mainly composed of α-Al2O3, γ-Al2O3 and trace amount of amorphous Al(OH)3. Moreover, the high temperature oxidation behaviors of the substrate, the coating and their interface were investigated in detail. It was found that the existence of the Al2O3 coating could substantially decline the oxidation rate of TiAl alloy, because of the significant increase in high temperature oxidation resistance. Keywords: Microstructure; Oxidation behavior; Al2O3 coating; TiAl alloy; CPED
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1. Introduction In the early of 21st century, I. Zhitomirsky and S. K. Yen et al separately prepared zirconia and alumina films on Ni substrate and MAR-M247 super alloy by using cathodic electro-deposition and sintering post-process [1, 2]. It was verified that there were cracks in the surface of the films after annealing treatment, due to the maniacal dehydration of the amorphous hydroxides. In order to avoid the inhomogeneous structure resulting from the post-processing, T. Paulmier and W. Xue et al combined electrolytic deposition with plasma electrolytic technique as a whole, and proposed cathodic plasma electrolytic deposition (CPED) technique [3, 4]. During CPED process, treated samples acted as cathode, and hydroxides were deposited and dehydrated almost simultaneously on the cathode surface to fabricate oxide ceramic coatings [5-8]. The in-situ CPED coatings were of fast growth rate, and combined well with substrates. Therefore, the CPED technique has drawn wide attention in recent years. E. Bahadori et al prepared porous and rough α-Al2O3 CPED coating on IN738 alloy, and the coating exhibited high micro-hardness of 965 Hv0.2 [9]. J. Lin et al studied the oxidation resistance of the Al2O3 CPED coating on Ti45Al8.5Nb alloy, and the results indicated that the coating effectively improved the oxidation resistance of the substrate [10]. S. Deng et al prepared CPED Ni-Cr-Y2O3 nano-composite coatings on T91 steels, and the morphologies and the composition of the coatings were investigated [11]. Z. Jiang et al deposited Mn doped Al2O3 coating on TA15 alloy via CPED method, and they found that the emissivity of the coatings were significantly affected by the doping content of Mn element [12]. J. Huang et al synthesized Al3C4-Al2O3-ZrO2 and ZrO2-free ceramic coatings on 1060 aluminium alloys, and coated samples exhibited higher wear and corrosion resistance [13]. In order to improve the oxidation resistance of TiAl alloy at high temperature over 800 °C, our previous work synthesized Al2O3 CPED coating on TiAl alloy
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and the results showed that the coated samples could remarkably improve the high temperature oxidation resistance and wear resistance of the substrate [14, 15]. In general, current studies about CPED technique mainly focused on the fabrication, the characterization and the properties testing of the coatings [16-20]. However, there were few reports about the phase transition mechanism of amorphous hydroxides inside the coating, and especially oxidation behavior at high temperature over 800 °C. Herein, the purpose of this paper is to investigate the microstructure, the composition and the oxidation behavior of the Al2O3 CPED coatings on the surface of TiAl alloy. The oxidation resistances of TiAl alloy with Al2O3 CPED coating and pristine TiAl alloy were compared at different high temperature over 800 °C in static air and different oxidation time. And the oxidation behavior of the Al2O3 CPED coating was researched at high temperature in particular. 2. Experimental section 2.1. Materials and the fabrication of the Al2O3 CPED coating The γ-TiAl alloys (Ti49.7Al47.5V1.7Cr1.1 (at.%)) served as the substrates for the following coating experiments. These starting materials alloy ingots were prepared by vacuum non-consumable arc melting under an argon atmosphere, and were re-melted five times for ensuring compositional homogeneity. All substrate samples were machined into a size of 20 mm × 10 mm × 5 mm by electro-discharge machining. After being polished by Grit #160, #240, #400, #600 and #800 SiC abrasive papers respectively, the samples were cleaned in an ultrasonic bath of acetone and then dried. The CPED system was schematically shown in Fig. 1. A pulsed electrical power supply (450 V/100 Hz) controlled by computer was used for the deposition, and the duty ratio was 20%. The prepared TiAl samples acted as cathode and a piece of 304 stainless steel worked as anode.
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0.3 M Al(NO3)3 of ethanol (30, vol/%)-water solution acted as electrolyte, and the electrolytes were cooled to below 10 °C by circulating water all the time for lowering the temperature of the reaction system. All samples were treated for 30 min. During the CPED process, the reactions were listed as follows [13, 14]. In the electrolyte solution, Al(NO3)3 was dissociated under the effect of the electric field, Al(NO3)3 → Al3+ + 3NO3-
(1)
On the cathode surface, Al3+ combined with OH- to form deposition, Al3+ + 3OH- → Al(OH)3
(2)
Al(OH)3 was dehydrated under the energy of sparks, 2Al(OH)3 → Al2O3 + 3H2O
(3)
2.2. High temperature oxidation measurement Samples were perpendicularly placed in alumina crucibles and oxidized in a tube type resistance furnace in static air. TiAl alloy samples were proceeded at 850 °C, and coated samples were separately carried out at 850 °C, 950 °C and 1050 °C. After 10 h oxidation, the parallel samples were taken out and air-cooled down to room temperature. Mass gains (crucible with samples) were obtained by an electronic balance with an accuracy of 10-4 g, in consideration of reducing errors. And then these samples were put back to the furnace again for another oxidation cycle. Mass gains of the oxidized samples were calculated, and the microstructures of them were also characterized. 2.3. Characterization The morphologies, the structures and the chemical compositions of the samples were observed and analyzed by scanning electron microscopy (SEM, Verios G4, Philips-FEI Corpocyclen, Netherlands) with energy dispersive spectrometer (EDS). The oxidation state of Al
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element was characterized by X-ray photoelectron spectroscopy (XPS, Axis Supra, Kratos, England). The phase compositions of the samples were test by X-ray diffraction (XRD, XRD-7000, Shimadzu, Japan) with Cu Kα radiation and in the scanning range of 20°-80°. The molecular structure of the CPED coating was obtained by Raman spectrometer (Alpha300R, WITec, Germany). The high-resolution transmission electron microscopy (HRTEM) micrographs and the diffraction patterns of the samples were tested by transmission electron microscopy (TEM, FEI Talos F200X G2, Philips-FEI Corpocyclen, Netherlands), and the micro-nano fabrication of TEM samples was processed by Focused Ion Beam system (FIB, FEI Helios G4 CX, Philips-FEI Corpocyclen, Netherlands). 3. Results and discussion 3.1. Characterization of the Al2O3 CPED coating SEM images of Fig. 2 showed the surface and the cross-sectional morphologies of the Al2O3 CPED coating on TiAl alloy. There were micro-cracks and micro-pores in the surface of the coating, as exhibited in Fig. 2(a). And the micro-pores were deemed to be electrical discharge channels during the electrolytic deposition process [21-24]. From Fig. 2(b), it could find that there was no gap between the CPED coating and the substrate, indicating that the coating has high combining strength with the TiAl alloy substrate. Besides, after 30 min deposition, the CPED coating had a mean thickness of about 34.5 µm. Meanwhile, transverse micro-pores inside the cross-sectional structure could be clearly observed at high magnification. It was reported that those transverse micro-pores were conductive to improve the thermal insulation performance of the ceramic coatings [25-27]. The element compositions of the CPED coating were analyzed by employing EDS, and the linear and the sectional element distributions were displayed in Fig. 3. From Fig. 3(a), it was
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observed that Al and O elements distributed uniformly in the CPED coating. There were a handful of Ti element detected, and most of them were concentrated at the interface of the CPED coating and the substrate. As for the substrate, the relative content of Ti element increased remarkably, while O and Al elements decreased obviously. These results powerfully demonstrated that the CPED coating was mainly composed of Al and O elements. In Fig. 3(b), (c) and (d), the sectional element distributions of Al, O and Ti elements further agreed with the above viewpoint. For the sake of defining the oxidation state of Al element, XPS spectrums of the CPED coating were identified in Fig. 4. In Fig. 4(a), the survey spectrum clearly showed that the CPED coating consisted of Al and O elements. The core-shell spectrum of Al 2p was shown in Fig. 4(b). A peak at 71.85 eV was assigned to Al3+ [28], which demonstrated the formation of Al2O3 in the coating. To determine the phase compositions of the CPED coating, the XRD pattern was exhibited in Fig. 5. The peaks at 32.9°, 38.7°, 57.2°, 65.5° and 67.1° separately corresponded to (104), (110), (116), (124) and (030) of α-Al2O3 (JCPDS: 10-0173, Trigonal, a=0.4758 nm, c=1.2991 nm). The peaks at 45.2°and 79.2° were successively attributed to (311) and (440) of γ-Al2O3 (JCPDS: 79-1558, Cubic, a=0.564 nm, c=2.265 nm). In addition, a weak diffraction peak appeared from 20° to 35°, which was of particular attention. Researchers [28, 29] thought that, the existence of amorphous phase could be proved by weak diffraction peaks in XRD patterns, and the XRD pattern in Fig. 5 indicated that the CPED coating consisted of a small number amorphous phase. Fig. 6 illustrated the HRTEM micrograph and the electronic diffraction of Point A shown in Fig. 2(a). The spacings of the lattice fringes were 0.238 nm, 0.350 nm and 0.141 nm, which were
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assigned to α-Al2O3 (110, d=0.2379 nm) (012, d=0.3479 nm) and γ-Al2O3 (440, d=0.1405 nm), respectively. The scattered diffraction patterns in the electronic diffraction pattern manifested that the Al2O3 CPED coating exhibited excellent crystalline structures. Nevertheless, the halo in the electronic diffraction pattern also suggested that the prepared coating contained a small number of amorphous structures, which was consistent with the analysis of Fig. 5. In order to further confirm the presence of the amorphous phase, the Raman spectrum of the Al2O3 CPED coating was shown in Fig. 7. The peak at 711 cm-1 was attributed to the stretching vibration of Al-O bond of Al2O3, and the peak at 625 cm-1 was assigned to the in-plane bending vibration of Al-OH of α-Al(OH)3 [31], stating that the Al2O3 CPED coating was composed of crystalline Al2O3 and amorphous α-Al(OH)3. The results powerfully demonstrated that the elaborately fabricated coating indeed had amorphous phase. Given the above, the CPED coating on TiAl alloy was of uniform thickness, mainly composed by Al2O3 and a small amount of amorphous phase Al(OH)3, and the bonding at the interface was well. 3.2 Kinetic curves The cyclic oxidation kinetics curves of TiAl alloy measured at 850 °C, and TiAl alloy with coating separately tested at 850 °C, 950 °C and 1050 °C in static air were displayed in Fig. 8. The oxidation kinetic curve of TiAl alloy was approximately linear. However, at the same temperature, the mass gain of TiAl alloy with coating was much lower than pristine TiAl alloy. In addition, the slope of cyclic oxidation kinetics curve between 20 h to 50 h was 0.564, and yet the slope between 50 h to 100 h was 0.348, identifying that the increasing trend became lower with oxidation time increasing at 850 °C. With regard to higher temperature of 950 and 1050 °C, the mass gain rate increased quicker than that of at 850 °C, but stills slower than that of TiAl alloy.
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It was noted that the mass gains of coated samples at 850 °C and 950 °C for 20 h were negative. This situation was a result of the dehydration of amorphous phase Al(OH)3 inside the coating at high temperature. The dehydration of Al(OH)3 was as follows [2, 21], Al(OH)3
250 °C
AlOOH
450 °C
γ-Al2O3
1250 °C
α-Al2O3 (4)
It was noticed that there was no decline at 1050 °C, which could be due to the higher oxidation mass gain rate making up for the dehydration loss. Besides, there were a handful of visible flake-like exfoliations from the coating at 1050 °C for 60 h. 3.3 High temperature oxidation behavior (1) TiAl alloy The morphology and the microstructure of TiAl alloy at 850 ºC in air for 100 h were shown in Fig. 9. In Fig. 9(a), there were large peeling-off areas viewed on the surface, and the high-magnification inset image showed that these areas were composed of loose and porous pebbly-like particles. From Fig. 9(b), one could find that the cross-sectional microstructure of oxidation layer was comprised of two parts, the outer layer and the inner layer. There were distinct cracks at the interface between the two layers, as well as between the inner layer and TiAl alloy substrate. The appearance of these cracks indicated that the oxidation layer on the surface of TiAl alloy possessed loose and porous structure, which easily peeled off from the substrate. In that case, the oxidation layer almost did not have the ability to protect the substrate. Moreover, the EDS linear scanning spectra exhibited the distribution of Ti, Al and O elements in oxidation layer and TiAl alloy. It was seen that the order of Ti content was the substrate > the outer layer > the inner layer, and that of Al content was the substrate > the inner layer > the outer layer, respectively. Meanwhile, in the inner layer, the content of Ti and Al was basically the same. This fact revealed that Ti element dominated the outer layer, while the Al and Ti element
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dominated the inner layer. It was reported that at high temperature Al element was oxidized to Al2O3 and Ti element was oxidized to TiO2 [32, 33]. The reactions were listed as follows, 4Al + 3O2 → 2Al2O3
(5)
Ti + O2 → TiO2
(6)
Because Gibbs free energy of the formation of Al2O3 was close to that of TiO2 at high temperature, it was difficult to form continuous and compact protective Al2O3 oxidation layer on the surface of TiAl alloy [34], which was the main failure mechanism of TiAl alloy substrate at high temperature. Moreover, the activation energy of TiO2 was 59.5 kJ/mol, while Al2O3 was 502.4 kJ/mol [35], leading to that the reaction rate of TiO2 was much higher than that of Al2O3.Thus, the surface of TiAl alloy was easily oxidized to form loose and porous TiO2 outer layer. Eventually, the freshly formed outer layer mainly consisted of TiO2, and the newly formed inner layer was made up of TiO2 and Al2O3. Because of thermal stress, with time went by, the oxidation layer on the surface of TiAl alloy peeled off repeatedly. This behavior caused severe oxidation and increased mass gain rate, which was in consistence with the result shown in Fig. 8. (2) Coating at 850 ºC The morphological and the micro-structural evolution of Al2O3 coatings at 850 ºC in air for different oxidation time was exhibited in Fig. 10. When oxidation time was 10 h, in Fig. 10(a) and (b), only a small number of tiny micro-cracks were observed on the surface, and the micro-structure was compact. Meanwhile, the mean thickness of the coating was about 27.4 µm, which was thinner than that in Fig. 2(b). As for 20 h oxidation, there were more and broader cracks on the surface, as shown in Fig. 10(c). The occurrence of this case resulted from the dehydration of Al(OH)3. With increasing the oxidation time, the dehydration effect became more
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violent, giving rise to the formation of big cracks and causing a remarkable decline in Fig. 8. In Fig. 10(d), with respect to 30 h, the micro-cracks were covered by spicules partially. The EDS analysis showed that these spicules were composed of Al and O elements, revealing that these spicules were made up of Al2O3 mainly. With the increase of oxidation time for 50 h, Al2O3 spicules increased remarkably and connected with each other to form chunk-peelings, as shown in Fig. 10(e) and (f). Once oxidation time was 100 h, chunk-peelings almost covered the entire surface, and there were few cracks in the surface, with the fact displayed in Fig. 10(g) and (h). And it was found that the binding between these chunk-peelings and coating was poor. Nevertheless, the oxidation layer bound well with TiAl alloy substrate, declaring that the Al2O3 CPED coating expressed excellent high temperature oxidation performance. In addition, there was a thin bright Ti-rich layer of composition Ti0.59Al0.47V0.03Cr0.01 (at. %, the same below) appeared at the interface. The EDS linear scanning of coated sample at 850 ºC for 100 h was demonstrated in Fig. 11. It could be seen that, the outer layer was mainly composed of Al and O elements, while the inner layer was dominated by Al, O and Ti elements. Moreover, it deserved special notice that the distributions of Al and Ti elements were complementary. According to relative concentrations in Fig. 11(b) and Fick’s second law [36], ∂C(x, t) ∂t
= D
∂2 C ∂x2
(7)
The diffusion coefficients of Al atoms (DAl) and Ti atoms (DTi) were calculated. The results showed that, when oxidation time was 100 h at 850 ºC, DAl was 3.12×10-3 m2/s and DTi was 1.67×10-5 m2/s. The latter was lower by 2 magnitudes than that of the former. It was reported that during diffusion and oxidation process, the higher diffusion coefficient was, the faster velocity was [37]. 11
For the faster diffusion velocity, Al atoms diffused to outer surface of the coating and were oxidized to spicules Al2O3, and Ti atoms diffused to the interior of the coating. Meanwhile, O2 molecules permeated through micro-cracks and got inside the coating, where combined Ti atoms to form TiO2. Moreover, due to lower diffusion velocity, the diffusion of Ti atoms was hindered. Ti atoms enriched and formed a Ti-rich layer at the interface. Referring to the environment temperature, the chemical contents, and the Ti-Al binary phase diagrams [38, 39], it could be concluded that the phase composition of the Ti-rich layer was Ti3Al based solid solution of α2-Ti. The XRD patterns of the coatings for different oxidation time at 850 ºC were displayed in Fig. 12, and the quantitative analysis of Fig. 5 was listed in Tab. 1. In Fig. 12(a), when oxidation time was 20 h, compared with original Al2O3 coating, the absorption intensity of γ-Al2O3 enhanced, and yet the diffraction peaks of α-Al2O3 and amorphous Al(OH)3 became weaker. Based on Formula (4), Al(OH)3 in the coating was dehydrated to γ-Al2O3 at 850 ºC, causing the relative content of γ-Al2O3 increased, and α-Al2O3 declined. That’s, the relative content of γ-Al2O3 increased from 25.8% to 34.6%, while the content of α-Al2O3 decreased from 74.2% to 65.4%. With the increase of oxidation time to 50 h, as displayed in Fig. 12(b) and Tab. 1, the relative content of α-Al2O3 increased to 84.9%, while γ-Al2O3 decreased to 10.8%. It could be ascribed to the oxidation of Al atoms on the surface, that was, Al atoms diffused to the outer surface and were oxidized to spicules α-Al2O3. In the meantime, weak crystal absorption peaks of TiO2 appeared. These absorption peaks were attributed to rutile-TiO2 (JCPDS: 21-1276, tetragonal, a=0.4594 nm, c=0.2958 nm) and anatase-TiO2 (JCPDS: 89-4921, tetragonal, a=0.3795 nm, c=0.9518 nm). The relative content of rutile-TiO2 was 2.9%, while anatase-TiO2 was 1.4%. Along with increasing oxidation time to 100 h, in Fig. 12(c), the absorption peaks
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intensity of TiO2 evidently increased. The oxidation layer was comprised of α-Al2O3 82.0%, γ-Al2O3 8.5 %, rutile-TiO2 7.3 % and anatase-TiO2 2.2 %. The HRTEM micrographs and the electronic diffraction patterns of Point A, B and C in Fig. 10(a), (c) and (g) were illustrated in Fig. 13. In Fig. 13(a), when oxidation time was 10 h, compared to the sample exhibited in Fig. 6, the clarity of lattice fringes and the scattered diffraction patterns increased. In the meantime, the halo became weaker, testifying that the crystalline ratio of the coating improved, for the dehydration of Al(OH)3. The spacings of the lattice fringes were 0.238 nm, 0.141 nm and 0.235 nm, corresponding to (110) (d=0.2379 nm) of α-Al2O3, and (440) (d=0.1405 nm), (012) (d=0.3479 nm) of γ-Al2O3, respectively. From Fig .13(b), it was found that, lattice fringes became much clearer and diffraction rings appeared, which was accompanied by increasing of oxidation time to 20 h. Besides, the spacings were 0.238 nm, 0.137 nm and 0.141 nm, attributing to (110) (d=0.2379 nm), (030) (d=0.1374 nm) of α-Al2O3 and (440) (d=0.1405 nm) of γ-Al2O3, respectively. In Fig. 13(c), with oxidation time increasing to 100 h, lattice fringes and diffraction rings became complicated, ascribing to different crystallographic planes. The spacings were 0.238 nm, 0.323 nm and 0.247 nm, assigning to (110) (d=0.2379 nm) of α-Al2O3, and (110) (d=0.3237 nm), (101) (d=0.2482 nm) of rutile-TiO2. (3) Coating at 950 ºC The morphological and the micro-structural evolution of the coatings at 950 ºC in air for different oxidation time was illustrated in Fig. 14. In Fig. 14(a), when the oxidation time was 10 h, there were micro-cracks and dispersed spicules Al2O3 observed on the surface. Based on Arrhenius equation, it was reported that the higher oxidation temperature was , the bigger diffusion coefficient was, and the faster diffusion rate was [40].
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Q
D= D0 exp - RT
(8)
In the case of 950 ºC, Al atoms diffused to the outer surface and were oxidized to Al2O3 spicules for a shorter oxidation time of 10 h. In addition, the formation of broader micro-cracks also suggested that the dehydration process was much more violent. In the wake of increasing of oxidation time to 20 h, there were more Al2O3 spicules and cracks appearing on the surface, as displayed in Fig. 14(b). From Fig. 14(c), when oxidation time was 50 h, it could be seen that these spicules connected with each other and formed loose chunk-peelings on the surface. The Ti-rich layer at the interface was composed of Ti0.62Al0.33V0.04Cr0.01, as shown in Fig. 14(d), and the relative content of Ti element was higher than that of sample at 850 ºC for 100 h. With the increase of oxidation time for 100 h, as the fact displayed in Fig. 14(e) and (f), chunk-peelings covered the entire surface. Moreover, the content of Ti was of a growing tendency with increasing oxidation time, and the Ti-rich layer was comprised of Ti0.71Al0.23V0.04Cr0.02. The liner and the mapping elemental distributions of the coating at 950 ºC for 100 h were displayed in Fig. 15. In Fig. 15(a) and (b), according to the liner distribution of elements, the cross-sectional microstructure of the sample was divided into four layers, the outer layer, the inner layer, the Ti-rich layer and TiAl alloy substrate. The outer layer was mainly composed of Al and O elements, and the inner layer was made up of Al, Ti and O elements. Accordingly, Al and Ti elements exhibited complementarities as a whole. As shown in Fig. 15(c), (d) and (e), the sectional element distributions of Al, O and Ti elements further agreed with the above viewpoint. What’s more, there were a small amount of V elements detected in Ti-rich layer and the interface, proving that V atoms diffused to the Al2O3 CPED coating and enriched at the interface under high temperature. However, there was no Cr element detected in the coating and the Ti-rich layer,
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as the fact displayed in Fig. 15 (f) and (g). (3) Coating at 1050 ºC The morphologies and the microstructures of Al2O3 coatings at 1050 ºC for different oxidation time were demonstrated in Fig. 16. In Fig. 16(a), when oxidation time was 10 h, there were more spicules but no micro-crack on the surface observed. It was indicated that micro-cracks formed by dehydration were covered by oxidized spicules in a shorter time, leading to no decline in the oxidation kinetics curve at 1050 °C. In addition, radial-pinprick structures appeared on the surface of these spicules, and EDS analysis showed these radial-pinprick structures were mainly comprised of Al and O elements. According to Arrhenius equation, the faster diffusion rate at higher temperature favored the increasing of oxidation reaction rate [41, 42]. It could be deduced that, Al atoms were oxidized severely into radial-pinprick structures Al2O3 at higher temperature [43, 44]. There were more radial-pinpricks and spicules Al2O3 existed on the surface when oxidation time was 20 h, as displayed in Fig. 16(b), indicating that more Al atoms were oxidized with increasing oxidation time. In Fig. 16(c) and (d), as for 60 h oxidation, visible flake-like exfoliations occurred on the surface and formed a three-layer structure, including the loose and porous outer layer, the dense and compact medium layer, and the inner layer. And the Ti-rich layer was made up of Ti0.68Al0.26V0.04Cr0.02. The XRD patterns of these three layers were displayed in Fig. 17, and the quantitative analysis was listed in Table 2. The results showed that the outer layer was composed of α-Al2O3 75.2%, γ-Al2O3 14.1%, rutile-TiO2 8.7%, and anatase-TiO2 2.0%, testifying that a small amount of Ti atoms diffused and were oxidized on the surface. The medium layer consisted of α-Al2O3 45.2%, rutile-TiO2 39.7%, γ-Al2O3 9.6% and anatase-TiO2 8.2%, that’s, the relative content of
15
α-Al2O3 and γ-Al2O3 decreased, while rutile-TiO2 and anatase-TiO2 increased. The inner layer was made up of rutile-TiO2 80.4%, α-Al2O3 8.9%, γ-Al2O3 7.3% and anatase-TiO2 3.4%. 3.4. Discussion The oxidation behavior and evolutionary process of the Al2O3 CPED coating on TiAl alloy at high temperature was illuminated in Fig. 18. As shown in Fig. 18(a), it could be noted that there were micro pores on the surface, the Al2O3 coating combined well with TiAl alloy substrate, and the microstructure was uniform. Under high temperature environment, the amorphous Al(OH)3 was dehydrated and the micro-structure of Al2O3 CPED coating became more dense and compact, and the micro pores on the surface were covered by micro-cracks, which were caused by the dehydration of amorphous Al(OH)3, as displayed in Fig. 18(b). Those behavior also caused the decrease of mass gain rate and the coating thickness. As further increasing the oxidation time, Al and Ti atoms from TiAl alloy substrate diffused towards the coating. Owing to bigger diffusion coefficient, Al atoms diffused much faster than Ti atoms. These Al atoms were easily oxidized to Al2O3 and formed spicules and radial-pinprick structures on the surface, as viewed in Fig. 18(c). In Fig. 18(d) and (e), these Al2O3 spicules connected with each other and fabricated loose Al2O3 chunk-peelings on the surface with increasing oxidation time. With increasing oxidation time, Ti atoms also diffused outer from TiAl alloy substrate. However, for the lower diffusion coefficient than Al atoms, the diffusion rate of Ti atoms was also lower than Al atoms and Ti atoms diffused to the interior of the Al2O3 CPED coating. In addition, O2 molecules permeated through micro-cracks and got inside the coating, where combined Ti atoms to form TiO2, which was followed by the formation of a TiO2/Al2O3 mixture layer. In the meantime, a Ti-rich layer appeared at the interface, duo to lower diffusion
16
coefficient of Ti atoms, as shown in Fig. 18(d) and (e). When oxidation time was long enough, visible flake-like exfoliations occurred on the surface, these Al2O3 spicules and chunk-peelings peeled off, and the TiO2/Al2O3 mixture layer flaked and fell off. Moreover, it was noticed that the Ti content in Ti-rich layer was obviously higher than that in Ti alloy, and increased accompanied by increasing oxidation time. Based on the gradient change, Fick’s second law was provided to calculate the relationship between diffusion coefficient and atomic concentration. The intriguing phenomenon was not in contradiction with Fick’s second law, but could be explained by the theory of ‘uphill diffusion’ [45, 46]. The ‘uphill diffusion’ theory held that, the diffusion process occurred spontaneously in the decline of the standard Gibbs free energy of formation, which was the essence of diffusion as well. The standard Gibbs free energy of formation of α2-Ti in Ti-rich layer was -269.819 kJ/mol (1000 ºC), which was much lower than that of γ-TiAl (-62.760 kJ/mol, 1000 ºC) [47, 48]. This case resulted in that the formation of α2-Ti was extremely easier than that of γ-TiAl at high temperature. Moreover, the content of Ti atoms in Ti-rich layer increased with the increase of temperature and oxidation time. The formation of Ti-rich layer was in favor of the protection of the Al2O3 CPED coating at high temperature. In consideration of the existence of negative values in the cyclic oxidation kinetics curves, the protection of the coating can be explained by the changes of slope instead of fitting equations. Take 950 ºC for example, in Fig. 8, the slope of cyclic oxidation kinetics curve between 50 h to 100 h was lower than that of between 20 h to 50 h. Combined with the fact shown in Fig. 14, the appearance of Ti-rich layer at the interface supported slower increasing trend of oxidation. In other word, the protection of Al2O3 CPED coating was obviously improved along with the appearance of Ti-rich layer.
17
4. Conclusions In here, the Al2O3 CPED coating was fabricated on the surface of TiAl alloy substrate. The microstructures and the compositions of the coating were characterized, the high temperature oxidation performances of TiAl alloy and the coating were evaluated, and the oxidation behavior of the coating was elaborately investigated. The main conclusions were listed as follows: (1) There were micro pores on the surface of the Al2O3 CPED coating, the coating combined well with TiAl alloy substrate, and the microstructure was uniform. The coating was mainly composed of α-Al2O3, and a small number of γ-Al2O3 and amorphous Al(OH)3. The amorphous Al(OH)3 was dehydrated at high temperature environment. (2) The high temperature oxidation behavior and evolutionary process of the Al2O3 coating was that, Al atoms diffused outwards and were oxidized to form Al2O3 spicules and chunk-peelings on the surface, while Ti atoms diffused and were oxidized to form a TiO2/Al2O3 mixture layer inside the coating, and a Ti-rich layer was formed at the interface. (3) The Al2O3 CPED coating significantly improved the oxidation performances of TiAl alloy substrate at high temperature, and the appearance of Ti-rich layer at the interface was conductive to the improvement of the protection. Acknowledgements: This work was financially supported by The Science and Technology Program for Research and Development of Shaanxi Province (2018JZ5004). References [1] I. Zhitomirsky, A. Petric, Electrolytic deposition of ZrO2-Y2O3 films, Materials Letters 50 (2001) 189-193. [2] S. K. Yen, C. C. Chang, Cathodic reactions of electrolytic Al2O3 deposition on MAR-M247 superalloy, Materials Chemistry and Physics 77 (2002) 836-840.
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List of figure captions Fig. 1 Schematic view of the CPED system Fig. 2 The surface (a) and the cross-sectional (b) morphologies of the Al2O3 CPED coating Fig. 3 The linear and the sectional element distributions of the cross-sectional Al2O3 CPED coating Fig. 4 The survey (a) and the Al 2p core-level (b) XPS spectra of the Al2O3 CPED coating Fig. 5 The XRD pattern of the Al2O3 CPED coating Fig. 6 The HRTEM micrograph and the electronic diffraction of Point A shown in Fig. 2 (a) Fig. 7 The Raman spectrum of the Al2O3 CPED coating Fig. 8 The cyclic oxidation kinetics curves of TiAl alloy and coated samples Fig. 9 The surface morphology (a) and the cross-sectional microstructure (b) of TiAl alloy at 850 ºC in air for 100 h Fig. 10 Morphological and micro-structural evolution of Al2O3 CPED coating at 850 ºC in air for different oxidation times: (a) (b) 10 h, (c) 20 h, (d) 30 h, (e) (f) 50 h, (g) (h) 100 h. Fig. 11 The cross-sectional microstructure (a) and the EDS linear scanning (b) of Al2O3 CPED coating at 850 ºC for 100 h Fig. 12 The XRD patterns of samples at 850 ºC for different oxidation time: (a) 20 h, (b) 50 h and (c) 100 h Fig. 13 HRTEMs micrographs and the electron diffraction patterns of points in Fig. 10: (a) Point A in Fig. 10(a), (b) Point B in Fig. 10(c), (c) Point C in Fig. 10(g) Fig. 14 Microstructures of coatings for different oxidation time at 950 ºC: (a) 10 h, (b) 20 h, (c) (d) 50 h, (e) (f) 100 h Fig. 15 The The liner (a) (b) and the mapping (c) (d) (e) (f) (g) elemental distributions of the coating at 950 ºC for 100 h Fig. 16 Microstructures of coatings for different oxidation time at 1050 ºC: (a) 10 h, (b) 20 h, (c) (d) 60 h Fig. 17 The XRD patterns of the coatings for 100 h oxidation time at 1050 ºC Fig. 18 Schematic illustration of the oxidation behavior of Al2O3 CPED coating at high temperature
25
List of table captions Table 1 The quantitative analysis of XRD pattern of Fig. 5 and Fig. 12 Table 2 The quantitative analysis of XRD pattern of Fig. 17
26
Fig. 1 Schematic view of the CPED system
27
Fig. 2 The surface (a) and the cross-sectional (b) morphologies of the Al2O3 CPED coating
28
Fig. 3 The linear and the sectional element distributions of the cross-sectional Al2O3 CPED coating
29
Fig. 4 The survey (a) and the Al 2p core-level (b) XPS spectra of the Al2O3 CPED coating
30
Fig. 5 The XRD pattern of the Al2O3 CPED coating
31
Fig. 6 The HRTEM micrograph and the electronic diffraction of Point A shown in Fig. 2 (a)
32
Fig. 7 The Raman spectrum of the Al2O3 CPED coating
33
Fig. 8 The cyclic oxidation kinetics curves of TiAl alloy and coated samples
34
Fig. 9 The surface morphology (a) and the cross-sectional microstructure (b) of TiAl alloy at 850 ºC in air for 100 h
35
Fig. 10 Morphological and micro-structural evolution of Al2O3 CPED coating at 850 ºC in air for different oxidation times: (a) (b) 10 h, (c) 20 h, (d) 30 h, (e) (f) 50 h, (g) (h) 100 h.
36
37
Fig. 11 The cross-sectional microstructure (a) and the EDS linear scanning (b) of Al2O3 CPED coating at 850 ºC for 100 h
38
Fig. 12 The XRD patterns of samples at 850 ºC for different oxidation time: (a) 20 h, (b) 50 h and (c) 100 h
39
Fig. 13 HRTEMs micrographs and the electron diffraction patterns of points in Fig. 10: (a) Point A in Fig. 10(a), (b) Point B in Fig. 10(c), (c) Point C in Fig. 10(g)
40
41
Fig. 14 Microstructures of coatings for different oxidation time at 950 ºC: (a) 10 h, (b) 20 h, (c) (d) 50 h, (e) (f) 100 h
42
Fig. 15 The The liner (a) (b) and the mapping (c) (d) (e) (f) (g) elemental distributions of the coating at 950 ºC for 100 h
43
44
Fig. 16 Microstructures of coatings for different oxidation time at 1050 ºC: (a) 10 h, (b) 20 h, (c) (d) 60 h
45
Fig. 17 The XRD patterns of the coatings for 100 h oxidation time at 1050 ºC
46
Fig. 18 Schematic illustration of the oxidation behavior of Al2O3 CPED coating at high temperature
47
Table 1 The quantitative analysis of XRD pattern of Fig. 5 and Fig. 12 Oxidation Time
α-Al2O3 (%)
γ-Al2O3 (%)
rutile-TiO2 (%)
anatase-TiO2 (%)
0h
74.2
25.8
/
/
20 h
65.4
34.6
/
/
50 h
84.9
10.8
2.9
1.4
100 h
82.0
8.5
7.3
2.2
48
Table 2 The quantitative analysis of XRD pattern of Fig. 17 Layers
α-Al2O3 (%)
γ-Al2O3 (%)
rutile-TiO2 (%)
anatase-TiO2 (%)
Outer layer
75.2
14.1
8.7
2.0
Medium layer
42.5
9.6
39.7
8.2
Inner layer
8.9
7.3
80.4
3.4
49
Highlights
1. Al2O3 coating was prepared onto the surface of TiAl alloy substrate via CPED technique. 2. The microstructure and the composition of the coating were elaborately characterized in detail. 3. The improved high temperature oxidation behaviors of the substrate, the coating and their interface were illuminated.
Declaration of interest statement: We 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. Yours, All authors
S0925-8388(20)30634-4
DOI:
https://doi.org/10.1016/j.jallcom.2020.154271
Reference:
JALCOM 154271
To appear in:
Journal of Alloys and Compounds
Received Date: 1 November 2019 Revised Date:
28 January 2020
Accepted Date: 9 February 2020
Please cite this article as: S. Wang, F. Xie, X. Wu, T. Lv, Y. Ma, Microstructure and high temperature oxidation behavior of the Al2O3 CPED coating on TiAl alloy, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154271. 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. © 2020 Published by Elsevier B.V.
Microstructure and High Temperature Oxidation Behavior of the Al2O3 CPED Coating on TiAl Alloy
Shaoqing Wanga, Faqin Xiea,*, Xiangqing Wua, Tao Lva, Yong Mab
a
School of Aeronautics,
Northwestern Polytechnical University, Xi’an 710072, P. R.China
b
School of Material Science and Engineering,
Shandong University of Science and Technology, Qingdao 266590, P. R. China
*Corresponding author: E-mail: [email protected]
1
Abstract Al2O3 coating was fabricated on the surface of γ-TiAl alloy via cathodic plasma electrolytic deposition (CPED) technique. The microstructure and the composition of the coating were elaborately characterized. The results revealed that, the uniform coating combined well with the substrate, which was mainly composed of α-Al2O3, γ-Al2O3 and trace amount of amorphous Al(OH)3. Moreover, the high temperature oxidation behaviors of the substrate, the coating and their interface were investigated in detail. It was found that the existence of the Al2O3 coating could substantially decline the oxidation rate of TiAl alloy, because of the significant increase in high temperature oxidation resistance. Keywords: Microstructure; Oxidation behavior; Al2O3 coating; TiAl alloy; CPED
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1. Introduction In the early of 21st century, I. Zhitomirsky and S. K. Yen et al separately prepared zirconia and alumina films on Ni substrate and MAR-M247 super alloy by using cathodic electro-deposition and sintering post-process [1, 2]. It was verified that there were cracks in the surface of the films after annealing treatment, due to the maniacal dehydration of the amorphous hydroxides. In order to avoid the inhomogeneous structure resulting from the post-processing, T. Paulmier and W. Xue et al combined electrolytic deposition with plasma electrolytic technique as a whole, and proposed cathodic plasma electrolytic deposition (CPED) technique [3, 4]. During CPED process, treated samples acted as cathode, and hydroxides were deposited and dehydrated almost simultaneously on the cathode surface to fabricate oxide ceramic coatings [5-8]. The in-situ CPED coatings were of fast growth rate, and combined well with substrates. Therefore, the CPED technique has drawn wide attention in recent years. E. Bahadori et al prepared porous and rough α-Al2O3 CPED coating on IN738 alloy, and the coating exhibited high micro-hardness of 965 Hv0.2 [9]. J. Lin et al studied the oxidation resistance of the Al2O3 CPED coating on Ti45Al8.5Nb alloy, and the results indicated that the coating effectively improved the oxidation resistance of the substrate [10]. S. Deng et al prepared CPED Ni-Cr-Y2O3 nano-composite coatings on T91 steels, and the morphologies and the composition of the coatings were investigated [11]. Z. Jiang et al deposited Mn doped Al2O3 coating on TA15 alloy via CPED method, and they found that the emissivity of the coatings were significantly affected by the doping content of Mn element [12]. J. Huang et al synthesized Al3C4-Al2O3-ZrO2 and ZrO2-free ceramic coatings on 1060 aluminium alloys, and coated samples exhibited higher wear and corrosion resistance [13]. In order to improve the oxidation resistance of TiAl alloy at high temperature over 800 °C, our previous work synthesized Al2O3 CPED coating on TiAl alloy
3
and the results showed that the coated samples could remarkably improve the high temperature oxidation resistance and wear resistance of the substrate [14, 15]. In general, current studies about CPED technique mainly focused on the fabrication, the characterization and the properties testing of the coatings [16-20]. However, there were few reports about the phase transition mechanism of amorphous hydroxides inside the coating, and especially oxidation behavior at high temperature over 800 °C. Herein, the purpose of this paper is to investigate the microstructure, the composition and the oxidation behavior of the Al2O3 CPED coatings on the surface of TiAl alloy. The oxidation resistances of TiAl alloy with Al2O3 CPED coating and pristine TiAl alloy were compared at different high temperature over 800 °C in static air and different oxidation time. And the oxidation behavior of the Al2O3 CPED coating was researched at high temperature in particular. 2. Experimental section 2.1. Materials and the fabrication of the Al2O3 CPED coating The γ-TiAl alloys (Ti49.7Al47.5V1.7Cr1.1 (at.%)) served as the substrates for the following coating experiments. These starting materials alloy ingots were prepared by vacuum non-consumable arc melting under an argon atmosphere, and were re-melted five times for ensuring compositional homogeneity. All substrate samples were machined into a size of 20 mm × 10 mm × 5 mm by electro-discharge machining. After being polished by Grit #160, #240, #400, #600 and #800 SiC abrasive papers respectively, the samples were cleaned in an ultrasonic bath of acetone and then dried. The CPED system was schematically shown in Fig. 1. A pulsed electrical power supply (450 V/100 Hz) controlled by computer was used for the deposition, and the duty ratio was 20%. The prepared TiAl samples acted as cathode and a piece of 304 stainless steel worked as anode.
4
0.3 M Al(NO3)3 of ethanol (30, vol/%)-water solution acted as electrolyte, and the electrolytes were cooled to below 10 °C by circulating water all the time for lowering the temperature of the reaction system. All samples were treated for 30 min. During the CPED process, the reactions were listed as follows [13, 14]. In the electrolyte solution, Al(NO3)3 was dissociated under the effect of the electric field, Al(NO3)3 → Al3+ + 3NO3-
(1)
On the cathode surface, Al3+ combined with OH- to form deposition, Al3+ + 3OH- → Al(OH)3
(2)
Al(OH)3 was dehydrated under the energy of sparks, 2Al(OH)3 → Al2O3 + 3H2O
(3)
2.2. High temperature oxidation measurement Samples were perpendicularly placed in alumina crucibles and oxidized in a tube type resistance furnace in static air. TiAl alloy samples were proceeded at 850 °C, and coated samples were separately carried out at 850 °C, 950 °C and 1050 °C. After 10 h oxidation, the parallel samples were taken out and air-cooled down to room temperature. Mass gains (crucible with samples) were obtained by an electronic balance with an accuracy of 10-4 g, in consideration of reducing errors. And then these samples were put back to the furnace again for another oxidation cycle. Mass gains of the oxidized samples were calculated, and the microstructures of them were also characterized. 2.3. Characterization The morphologies, the structures and the chemical compositions of the samples were observed and analyzed by scanning electron microscopy (SEM, Verios G4, Philips-FEI Corpocyclen, Netherlands) with energy dispersive spectrometer (EDS). The oxidation state of Al
5
element was characterized by X-ray photoelectron spectroscopy (XPS, Axis Supra, Kratos, England). The phase compositions of the samples were test by X-ray diffraction (XRD, XRD-7000, Shimadzu, Japan) with Cu Kα radiation and in the scanning range of 20°-80°. The molecular structure of the CPED coating was obtained by Raman spectrometer (Alpha300R, WITec, Germany). The high-resolution transmission electron microscopy (HRTEM) micrographs and the diffraction patterns of the samples were tested by transmission electron microscopy (TEM, FEI Talos F200X G2, Philips-FEI Corpocyclen, Netherlands), and the micro-nano fabrication of TEM samples was processed by Focused Ion Beam system (FIB, FEI Helios G4 CX, Philips-FEI Corpocyclen, Netherlands). 3. Results and discussion 3.1. Characterization of the Al2O3 CPED coating SEM images of Fig. 2 showed the surface and the cross-sectional morphologies of the Al2O3 CPED coating on TiAl alloy. There were micro-cracks and micro-pores in the surface of the coating, as exhibited in Fig. 2(a). And the micro-pores were deemed to be electrical discharge channels during the electrolytic deposition process [21-24]. From Fig. 2(b), it could find that there was no gap between the CPED coating and the substrate, indicating that the coating has high combining strength with the TiAl alloy substrate. Besides, after 30 min deposition, the CPED coating had a mean thickness of about 34.5 µm. Meanwhile, transverse micro-pores inside the cross-sectional structure could be clearly observed at high magnification. It was reported that those transverse micro-pores were conductive to improve the thermal insulation performance of the ceramic coatings [25-27]. The element compositions of the CPED coating were analyzed by employing EDS, and the linear and the sectional element distributions were displayed in Fig. 3. From Fig. 3(a), it was
6
observed that Al and O elements distributed uniformly in the CPED coating. There were a handful of Ti element detected, and most of them were concentrated at the interface of the CPED coating and the substrate. As for the substrate, the relative content of Ti element increased remarkably, while O and Al elements decreased obviously. These results powerfully demonstrated that the CPED coating was mainly composed of Al and O elements. In Fig. 3(b), (c) and (d), the sectional element distributions of Al, O and Ti elements further agreed with the above viewpoint. For the sake of defining the oxidation state of Al element, XPS spectrums of the CPED coating were identified in Fig. 4. In Fig. 4(a), the survey spectrum clearly showed that the CPED coating consisted of Al and O elements. The core-shell spectrum of Al 2p was shown in Fig. 4(b). A peak at 71.85 eV was assigned to Al3+ [28], which demonstrated the formation of Al2O3 in the coating. To determine the phase compositions of the CPED coating, the XRD pattern was exhibited in Fig. 5. The peaks at 32.9°, 38.7°, 57.2°, 65.5° and 67.1° separately corresponded to (104), (110), (116), (124) and (030) of α-Al2O3 (JCPDS: 10-0173, Trigonal, a=0.4758 nm, c=1.2991 nm). The peaks at 45.2°and 79.2° were successively attributed to (311) and (440) of γ-Al2O3 (JCPDS: 79-1558, Cubic, a=0.564 nm, c=2.265 nm). In addition, a weak diffraction peak appeared from 20° to 35°, which was of particular attention. Researchers [28, 29] thought that, the existence of amorphous phase could be proved by weak diffraction peaks in XRD patterns, and the XRD pattern in Fig. 5 indicated that the CPED coating consisted of a small number amorphous phase. Fig. 6 illustrated the HRTEM micrograph and the electronic diffraction of Point A shown in Fig. 2(a). The spacings of the lattice fringes were 0.238 nm, 0.350 nm and 0.141 nm, which were
7
assigned to α-Al2O3 (110, d=0.2379 nm) (012, d=0.3479 nm) and γ-Al2O3 (440, d=0.1405 nm), respectively. The scattered diffraction patterns in the electronic diffraction pattern manifested that the Al2O3 CPED coating exhibited excellent crystalline structures. Nevertheless, the halo in the electronic diffraction pattern also suggested that the prepared coating contained a small number of amorphous structures, which was consistent with the analysis of Fig. 5. In order to further confirm the presence of the amorphous phase, the Raman spectrum of the Al2O3 CPED coating was shown in Fig. 7. The peak at 711 cm-1 was attributed to the stretching vibration of Al-O bond of Al2O3, and the peak at 625 cm-1 was assigned to the in-plane bending vibration of Al-OH of α-Al(OH)3 [31], stating that the Al2O3 CPED coating was composed of crystalline Al2O3 and amorphous α-Al(OH)3. The results powerfully demonstrated that the elaborately fabricated coating indeed had amorphous phase. Given the above, the CPED coating on TiAl alloy was of uniform thickness, mainly composed by Al2O3 and a small amount of amorphous phase Al(OH)3, and the bonding at the interface was well. 3.2 Kinetic curves The cyclic oxidation kinetics curves of TiAl alloy measured at 850 °C, and TiAl alloy with coating separately tested at 850 °C, 950 °C and 1050 °C in static air were displayed in Fig. 8. The oxidation kinetic curve of TiAl alloy was approximately linear. However, at the same temperature, the mass gain of TiAl alloy with coating was much lower than pristine TiAl alloy. In addition, the slope of cyclic oxidation kinetics curve between 20 h to 50 h was 0.564, and yet the slope between 50 h to 100 h was 0.348, identifying that the increasing trend became lower with oxidation time increasing at 850 °C. With regard to higher temperature of 950 and 1050 °C, the mass gain rate increased quicker than that of at 850 °C, but stills slower than that of TiAl alloy.
8
It was noted that the mass gains of coated samples at 850 °C and 950 °C for 20 h were negative. This situation was a result of the dehydration of amorphous phase Al(OH)3 inside the coating at high temperature. The dehydration of Al(OH)3 was as follows [2, 21], Al(OH)3
250 °C
AlOOH
450 °C
γ-Al2O3
1250 °C
α-Al2O3 (4)
It was noticed that there was no decline at 1050 °C, which could be due to the higher oxidation mass gain rate making up for the dehydration loss. Besides, there were a handful of visible flake-like exfoliations from the coating at 1050 °C for 60 h. 3.3 High temperature oxidation behavior (1) TiAl alloy The morphology and the microstructure of TiAl alloy at 850 ºC in air for 100 h were shown in Fig. 9. In Fig. 9(a), there were large peeling-off areas viewed on the surface, and the high-magnification inset image showed that these areas were composed of loose and porous pebbly-like particles. From Fig. 9(b), one could find that the cross-sectional microstructure of oxidation layer was comprised of two parts, the outer layer and the inner layer. There were distinct cracks at the interface between the two layers, as well as between the inner layer and TiAl alloy substrate. The appearance of these cracks indicated that the oxidation layer on the surface of TiAl alloy possessed loose and porous structure, which easily peeled off from the substrate. In that case, the oxidation layer almost did not have the ability to protect the substrate. Moreover, the EDS linear scanning spectra exhibited the distribution of Ti, Al and O elements in oxidation layer and TiAl alloy. It was seen that the order of Ti content was the substrate > the outer layer > the inner layer, and that of Al content was the substrate > the inner layer > the outer layer, respectively. Meanwhile, in the inner layer, the content of Ti and Al was basically the same. This fact revealed that Ti element dominated the outer layer, while the Al and Ti element
9
dominated the inner layer. It was reported that at high temperature Al element was oxidized to Al2O3 and Ti element was oxidized to TiO2 [32, 33]. The reactions were listed as follows, 4Al + 3O2 → 2Al2O3
(5)
Ti + O2 → TiO2
(6)
Because Gibbs free energy of the formation of Al2O3 was close to that of TiO2 at high temperature, it was difficult to form continuous and compact protective Al2O3 oxidation layer on the surface of TiAl alloy [34], which was the main failure mechanism of TiAl alloy substrate at high temperature. Moreover, the activation energy of TiO2 was 59.5 kJ/mol, while Al2O3 was 502.4 kJ/mol [35], leading to that the reaction rate of TiO2 was much higher than that of Al2O3.Thus, the surface of TiAl alloy was easily oxidized to form loose and porous TiO2 outer layer. Eventually, the freshly formed outer layer mainly consisted of TiO2, and the newly formed inner layer was made up of TiO2 and Al2O3. Because of thermal stress, with time went by, the oxidation layer on the surface of TiAl alloy peeled off repeatedly. This behavior caused severe oxidation and increased mass gain rate, which was in consistence with the result shown in Fig. 8. (2) Coating at 850 ºC The morphological and the micro-structural evolution of Al2O3 coatings at 850 ºC in air for different oxidation time was exhibited in Fig. 10. When oxidation time was 10 h, in Fig. 10(a) and (b), only a small number of tiny micro-cracks were observed on the surface, and the micro-structure was compact. Meanwhile, the mean thickness of the coating was about 27.4 µm, which was thinner than that in Fig. 2(b). As for 20 h oxidation, there were more and broader cracks on the surface, as shown in Fig. 10(c). The occurrence of this case resulted from the dehydration of Al(OH)3. With increasing the oxidation time, the dehydration effect became more
10
violent, giving rise to the formation of big cracks and causing a remarkable decline in Fig. 8. In Fig. 10(d), with respect to 30 h, the micro-cracks were covered by spicules partially. The EDS analysis showed that these spicules were composed of Al and O elements, revealing that these spicules were made up of Al2O3 mainly. With the increase of oxidation time for 50 h, Al2O3 spicules increased remarkably and connected with each other to form chunk-peelings, as shown in Fig. 10(e) and (f). Once oxidation time was 100 h, chunk-peelings almost covered the entire surface, and there were few cracks in the surface, with the fact displayed in Fig. 10(g) and (h). And it was found that the binding between these chunk-peelings and coating was poor. Nevertheless, the oxidation layer bound well with TiAl alloy substrate, declaring that the Al2O3 CPED coating expressed excellent high temperature oxidation performance. In addition, there was a thin bright Ti-rich layer of composition Ti0.59Al0.47V0.03Cr0.01 (at. %, the same below) appeared at the interface. The EDS linear scanning of coated sample at 850 ºC for 100 h was demonstrated in Fig. 11. It could be seen that, the outer layer was mainly composed of Al and O elements, while the inner layer was dominated by Al, O and Ti elements. Moreover, it deserved special notice that the distributions of Al and Ti elements were complementary. According to relative concentrations in Fig. 11(b) and Fick’s second law [36], ∂C(x, t) ∂t
= D
∂2 C ∂x2
(7)
The diffusion coefficients of Al atoms (DAl) and Ti atoms (DTi) were calculated. The results showed that, when oxidation time was 100 h at 850 ºC, DAl was 3.12×10-3 m2/s and DTi was 1.67×10-5 m2/s. The latter was lower by 2 magnitudes than that of the former. It was reported that during diffusion and oxidation process, the higher diffusion coefficient was, the faster velocity was [37]. 11
For the faster diffusion velocity, Al atoms diffused to outer surface of the coating and were oxidized to spicules Al2O3, and Ti atoms diffused to the interior of the coating. Meanwhile, O2 molecules permeated through micro-cracks and got inside the coating, where combined Ti atoms to form TiO2. Moreover, due to lower diffusion velocity, the diffusion of Ti atoms was hindered. Ti atoms enriched and formed a Ti-rich layer at the interface. Referring to the environment temperature, the chemical contents, and the Ti-Al binary phase diagrams [38, 39], it could be concluded that the phase composition of the Ti-rich layer was Ti3Al based solid solution of α2-Ti. The XRD patterns of the coatings for different oxidation time at 850 ºC were displayed in Fig. 12, and the quantitative analysis of Fig. 5 was listed in Tab. 1. In Fig. 12(a), when oxidation time was 20 h, compared with original Al2O3 coating, the absorption intensity of γ-Al2O3 enhanced, and yet the diffraction peaks of α-Al2O3 and amorphous Al(OH)3 became weaker. Based on Formula (4), Al(OH)3 in the coating was dehydrated to γ-Al2O3 at 850 ºC, causing the relative content of γ-Al2O3 increased, and α-Al2O3 declined. That’s, the relative content of γ-Al2O3 increased from 25.8% to 34.6%, while the content of α-Al2O3 decreased from 74.2% to 65.4%. With the increase of oxidation time to 50 h, as displayed in Fig. 12(b) and Tab. 1, the relative content of α-Al2O3 increased to 84.9%, while γ-Al2O3 decreased to 10.8%. It could be ascribed to the oxidation of Al atoms on the surface, that was, Al atoms diffused to the outer surface and were oxidized to spicules α-Al2O3. In the meantime, weak crystal absorption peaks of TiO2 appeared. These absorption peaks were attributed to rutile-TiO2 (JCPDS: 21-1276, tetragonal, a=0.4594 nm, c=0.2958 nm) and anatase-TiO2 (JCPDS: 89-4921, tetragonal, a=0.3795 nm, c=0.9518 nm). The relative content of rutile-TiO2 was 2.9%, while anatase-TiO2 was 1.4%. Along with increasing oxidation time to 100 h, in Fig. 12(c), the absorption peaks
12
intensity of TiO2 evidently increased. The oxidation layer was comprised of α-Al2O3 82.0%, γ-Al2O3 8.5 %, rutile-TiO2 7.3 % and anatase-TiO2 2.2 %. The HRTEM micrographs and the electronic diffraction patterns of Point A, B and C in Fig. 10(a), (c) and (g) were illustrated in Fig. 13. In Fig. 13(a), when oxidation time was 10 h, compared to the sample exhibited in Fig. 6, the clarity of lattice fringes and the scattered diffraction patterns increased. In the meantime, the halo became weaker, testifying that the crystalline ratio of the coating improved, for the dehydration of Al(OH)3. The spacings of the lattice fringes were 0.238 nm, 0.141 nm and 0.235 nm, corresponding to (110) (d=0.2379 nm) of α-Al2O3, and (440) (d=0.1405 nm), (012) (d=0.3479 nm) of γ-Al2O3, respectively. From Fig .13(b), it was found that, lattice fringes became much clearer and diffraction rings appeared, which was accompanied by increasing of oxidation time to 20 h. Besides, the spacings were 0.238 nm, 0.137 nm and 0.141 nm, attributing to (110) (d=0.2379 nm), (030) (d=0.1374 nm) of α-Al2O3 and (440) (d=0.1405 nm) of γ-Al2O3, respectively. In Fig. 13(c), with oxidation time increasing to 100 h, lattice fringes and diffraction rings became complicated, ascribing to different crystallographic planes. The spacings were 0.238 nm, 0.323 nm and 0.247 nm, assigning to (110) (d=0.2379 nm) of α-Al2O3, and (110) (d=0.3237 nm), (101) (d=0.2482 nm) of rutile-TiO2. (3) Coating at 950 ºC The morphological and the micro-structural evolution of the coatings at 950 ºC in air for different oxidation time was illustrated in Fig. 14. In Fig. 14(a), when the oxidation time was 10 h, there were micro-cracks and dispersed spicules Al2O3 observed on the surface. Based on Arrhenius equation, it was reported that the higher oxidation temperature was , the bigger diffusion coefficient was, and the faster diffusion rate was [40].
13
Q
D= D0 exp - RT
(8)
In the case of 950 ºC, Al atoms diffused to the outer surface and were oxidized to Al2O3 spicules for a shorter oxidation time of 10 h. In addition, the formation of broader micro-cracks also suggested that the dehydration process was much more violent. In the wake of increasing of oxidation time to 20 h, there were more Al2O3 spicules and cracks appearing on the surface, as displayed in Fig. 14(b). From Fig. 14(c), when oxidation time was 50 h, it could be seen that these spicules connected with each other and formed loose chunk-peelings on the surface. The Ti-rich layer at the interface was composed of Ti0.62Al0.33V0.04Cr0.01, as shown in Fig. 14(d), and the relative content of Ti element was higher than that of sample at 850 ºC for 100 h. With the increase of oxidation time for 100 h, as the fact displayed in Fig. 14(e) and (f), chunk-peelings covered the entire surface. Moreover, the content of Ti was of a growing tendency with increasing oxidation time, and the Ti-rich layer was comprised of Ti0.71Al0.23V0.04Cr0.02. The liner and the mapping elemental distributions of the coating at 950 ºC for 100 h were displayed in Fig. 15. In Fig. 15(a) and (b), according to the liner distribution of elements, the cross-sectional microstructure of the sample was divided into four layers, the outer layer, the inner layer, the Ti-rich layer and TiAl alloy substrate. The outer layer was mainly composed of Al and O elements, and the inner layer was made up of Al, Ti and O elements. Accordingly, Al and Ti elements exhibited complementarities as a whole. As shown in Fig. 15(c), (d) and (e), the sectional element distributions of Al, O and Ti elements further agreed with the above viewpoint. What’s more, there were a small amount of V elements detected in Ti-rich layer and the interface, proving that V atoms diffused to the Al2O3 CPED coating and enriched at the interface under high temperature. However, there was no Cr element detected in the coating and the Ti-rich layer,
14
as the fact displayed in Fig. 15 (f) and (g). (3) Coating at 1050 ºC The morphologies and the microstructures of Al2O3 coatings at 1050 ºC for different oxidation time were demonstrated in Fig. 16. In Fig. 16(a), when oxidation time was 10 h, there were more spicules but no micro-crack on the surface observed. It was indicated that micro-cracks formed by dehydration were covered by oxidized spicules in a shorter time, leading to no decline in the oxidation kinetics curve at 1050 °C. In addition, radial-pinprick structures appeared on the surface of these spicules, and EDS analysis showed these radial-pinprick structures were mainly comprised of Al and O elements. According to Arrhenius equation, the faster diffusion rate at higher temperature favored the increasing of oxidation reaction rate [41, 42]. It could be deduced that, Al atoms were oxidized severely into radial-pinprick structures Al2O3 at higher temperature [43, 44]. There were more radial-pinpricks and spicules Al2O3 existed on the surface when oxidation time was 20 h, as displayed in Fig. 16(b), indicating that more Al atoms were oxidized with increasing oxidation time. In Fig. 16(c) and (d), as for 60 h oxidation, visible flake-like exfoliations occurred on the surface and formed a three-layer structure, including the loose and porous outer layer, the dense and compact medium layer, and the inner layer. And the Ti-rich layer was made up of Ti0.68Al0.26V0.04Cr0.02. The XRD patterns of these three layers were displayed in Fig. 17, and the quantitative analysis was listed in Table 2. The results showed that the outer layer was composed of α-Al2O3 75.2%, γ-Al2O3 14.1%, rutile-TiO2 8.7%, and anatase-TiO2 2.0%, testifying that a small amount of Ti atoms diffused and were oxidized on the surface. The medium layer consisted of α-Al2O3 45.2%, rutile-TiO2 39.7%, γ-Al2O3 9.6% and anatase-TiO2 8.2%, that’s, the relative content of
15
α-Al2O3 and γ-Al2O3 decreased, while rutile-TiO2 and anatase-TiO2 increased. The inner layer was made up of rutile-TiO2 80.4%, α-Al2O3 8.9%, γ-Al2O3 7.3% and anatase-TiO2 3.4%. 3.4. Discussion The oxidation behavior and evolutionary process of the Al2O3 CPED coating on TiAl alloy at high temperature was illuminated in Fig. 18. As shown in Fig. 18(a), it could be noted that there were micro pores on the surface, the Al2O3 coating combined well with TiAl alloy substrate, and the microstructure was uniform. Under high temperature environment, the amorphous Al(OH)3 was dehydrated and the micro-structure of Al2O3 CPED coating became more dense and compact, and the micro pores on the surface were covered by micro-cracks, which were caused by the dehydration of amorphous Al(OH)3, as displayed in Fig. 18(b). Those behavior also caused the decrease of mass gain rate and the coating thickness. As further increasing the oxidation time, Al and Ti atoms from TiAl alloy substrate diffused towards the coating. Owing to bigger diffusion coefficient, Al atoms diffused much faster than Ti atoms. These Al atoms were easily oxidized to Al2O3 and formed spicules and radial-pinprick structures on the surface, as viewed in Fig. 18(c). In Fig. 18(d) and (e), these Al2O3 spicules connected with each other and fabricated loose Al2O3 chunk-peelings on the surface with increasing oxidation time. With increasing oxidation time, Ti atoms also diffused outer from TiAl alloy substrate. However, for the lower diffusion coefficient than Al atoms, the diffusion rate of Ti atoms was also lower than Al atoms and Ti atoms diffused to the interior of the Al2O3 CPED coating. In addition, O2 molecules permeated through micro-cracks and got inside the coating, where combined Ti atoms to form TiO2, which was followed by the formation of a TiO2/Al2O3 mixture layer. In the meantime, a Ti-rich layer appeared at the interface, duo to lower diffusion
16
coefficient of Ti atoms, as shown in Fig. 18(d) and (e). When oxidation time was long enough, visible flake-like exfoliations occurred on the surface, these Al2O3 spicules and chunk-peelings peeled off, and the TiO2/Al2O3 mixture layer flaked and fell off. Moreover, it was noticed that the Ti content in Ti-rich layer was obviously higher than that in Ti alloy, and increased accompanied by increasing oxidation time. Based on the gradient change, Fick’s second law was provided to calculate the relationship between diffusion coefficient and atomic concentration. The intriguing phenomenon was not in contradiction with Fick’s second law, but could be explained by the theory of ‘uphill diffusion’ [45, 46]. The ‘uphill diffusion’ theory held that, the diffusion process occurred spontaneously in the decline of the standard Gibbs free energy of formation, which was the essence of diffusion as well. The standard Gibbs free energy of formation of α2-Ti in Ti-rich layer was -269.819 kJ/mol (1000 ºC), which was much lower than that of γ-TiAl (-62.760 kJ/mol, 1000 ºC) [47, 48]. This case resulted in that the formation of α2-Ti was extremely easier than that of γ-TiAl at high temperature. Moreover, the content of Ti atoms in Ti-rich layer increased with the increase of temperature and oxidation time. The formation of Ti-rich layer was in favor of the protection of the Al2O3 CPED coating at high temperature. In consideration of the existence of negative values in the cyclic oxidation kinetics curves, the protection of the coating can be explained by the changes of slope instead of fitting equations. Take 950 ºC for example, in Fig. 8, the slope of cyclic oxidation kinetics curve between 50 h to 100 h was lower than that of between 20 h to 50 h. Combined with the fact shown in Fig. 14, the appearance of Ti-rich layer at the interface supported slower increasing trend of oxidation. In other word, the protection of Al2O3 CPED coating was obviously improved along with the appearance of Ti-rich layer.
17
4. Conclusions In here, the Al2O3 CPED coating was fabricated on the surface of TiAl alloy substrate. The microstructures and the compositions of the coating were characterized, the high temperature oxidation performances of TiAl alloy and the coating were evaluated, and the oxidation behavior of the coating was elaborately investigated. The main conclusions were listed as follows: (1) There were micro pores on the surface of the Al2O3 CPED coating, the coating combined well with TiAl alloy substrate, and the microstructure was uniform. The coating was mainly composed of α-Al2O3, and a small number of γ-Al2O3 and amorphous Al(OH)3. The amorphous Al(OH)3 was dehydrated at high temperature environment. (2) The high temperature oxidation behavior and evolutionary process of the Al2O3 coating was that, Al atoms diffused outwards and were oxidized to form Al2O3 spicules and chunk-peelings on the surface, while Ti atoms diffused and were oxidized to form a TiO2/Al2O3 mixture layer inside the coating, and a Ti-rich layer was formed at the interface. (3) The Al2O3 CPED coating significantly improved the oxidation performances of TiAl alloy substrate at high temperature, and the appearance of Ti-rich layer at the interface was conductive to the improvement of the protection. Acknowledgements: This work was financially supported by The Science and Technology Program for Research and Development of Shaanxi Province (2018JZ5004). References [1] I. Zhitomirsky, A. Petric, Electrolytic deposition of ZrO2-Y2O3 films, Materials Letters 50 (2001) 189-193. [2] S. K. Yen, C. C. Chang, Cathodic reactions of electrolytic Al2O3 deposition on MAR-M247 superalloy, Materials Chemistry and Physics 77 (2002) 836-840.
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List of figure captions Fig. 1 Schematic view of the CPED system Fig. 2 The surface (a) and the cross-sectional (b) morphologies of the Al2O3 CPED coating Fig. 3 The linear and the sectional element distributions of the cross-sectional Al2O3 CPED coating Fig. 4 The survey (a) and the Al 2p core-level (b) XPS spectra of the Al2O3 CPED coating Fig. 5 The XRD pattern of the Al2O3 CPED coating Fig. 6 The HRTEM micrograph and the electronic diffraction of Point A shown in Fig. 2 (a) Fig. 7 The Raman spectrum of the Al2O3 CPED coating Fig. 8 The cyclic oxidation kinetics curves of TiAl alloy and coated samples Fig. 9 The surface morphology (a) and the cross-sectional microstructure (b) of TiAl alloy at 850 ºC in air for 100 h Fig. 10 Morphological and micro-structural evolution of Al2O3 CPED coating at 850 ºC in air for different oxidation times: (a) (b) 10 h, (c) 20 h, (d) 30 h, (e) (f) 50 h, (g) (h) 100 h. Fig. 11 The cross-sectional microstructure (a) and the EDS linear scanning (b) of Al2O3 CPED coating at 850 ºC for 100 h Fig. 12 The XRD patterns of samples at 850 ºC for different oxidation time: (a) 20 h, (b) 50 h and (c) 100 h Fig. 13 HRTEMs micrographs and the electron diffraction patterns of points in Fig. 10: (a) Point A in Fig. 10(a), (b) Point B in Fig. 10(c), (c) Point C in Fig. 10(g) Fig. 14 Microstructures of coatings for different oxidation time at 950 ºC: (a) 10 h, (b) 20 h, (c) (d) 50 h, (e) (f) 100 h Fig. 15 The The liner (a) (b) and the mapping (c) (d) (e) (f) (g) elemental distributions of the coating at 950 ºC for 100 h Fig. 16 Microstructures of coatings for different oxidation time at 1050 ºC: (a) 10 h, (b) 20 h, (c) (d) 60 h Fig. 17 The XRD patterns of the coatings for 100 h oxidation time at 1050 ºC Fig. 18 Schematic illustration of the oxidation behavior of Al2O3 CPED coating at high temperature
25
List of table captions Table 1 The quantitative analysis of XRD pattern of Fig. 5 and Fig. 12 Table 2 The quantitative analysis of XRD pattern of Fig. 17
26
Fig. 1 Schematic view of the CPED system
27
Fig. 2 The surface (a) and the cross-sectional (b) morphologies of the Al2O3 CPED coating
28
Fig. 3 The linear and the sectional element distributions of the cross-sectional Al2O3 CPED coating
29
Fig. 4 The survey (a) and the Al 2p core-level (b) XPS spectra of the Al2O3 CPED coating
30
Fig. 5 The XRD pattern of the Al2O3 CPED coating
31
Fig. 6 The HRTEM micrograph and the electronic diffraction of Point A shown in Fig. 2 (a)
32
Fig. 7 The Raman spectrum of the Al2O3 CPED coating
33
Fig. 8 The cyclic oxidation kinetics curves of TiAl alloy and coated samples
34
Fig. 9 The surface morphology (a) and the cross-sectional microstructure (b) of TiAl alloy at 850 ºC in air for 100 h
35
Fig. 10 Morphological and micro-structural evolution of Al2O3 CPED coating at 850 ºC in air for different oxidation times: (a) (b) 10 h, (c) 20 h, (d) 30 h, (e) (f) 50 h, (g) (h) 100 h.
36
37
Fig. 11 The cross-sectional microstructure (a) and the EDS linear scanning (b) of Al2O3 CPED coating at 850 ºC for 100 h
38
Fig. 12 The XRD patterns of samples at 850 ºC for different oxidation time: (a) 20 h, (b) 50 h and (c) 100 h
39
Fig. 13 HRTEMs micrographs and the electron diffraction patterns of points in Fig. 10: (a) Point A in Fig. 10(a), (b) Point B in Fig. 10(c), (c) Point C in Fig. 10(g)
40
41
Fig. 14 Microstructures of coatings for different oxidation time at 950 ºC: (a) 10 h, (b) 20 h, (c) (d) 50 h, (e) (f) 100 h
42
Fig. 15 The The liner (a) (b) and the mapping (c) (d) (e) (f) (g) elemental distributions of the coating at 950 ºC for 100 h
43
44
Fig. 16 Microstructures of coatings for different oxidation time at 1050 ºC: (a) 10 h, (b) 20 h, (c) (d) 60 h
45
Fig. 17 The XRD patterns of the coatings for 100 h oxidation time at 1050 ºC
46
Fig. 18 Schematic illustration of the oxidation behavior of Al2O3 CPED coating at high temperature
47
Table 1 The quantitative analysis of XRD pattern of Fig. 5 and Fig. 12 Oxidation Time
α-Al2O3 (%)
γ-Al2O3 (%)
rutile-TiO2 (%)
anatase-TiO2 (%)
0h
74.2
25.8
/
/
20 h
65.4
34.6
/
/
50 h
84.9
10.8
2.9
1.4
100 h
82.0
8.5
7.3
2.2
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Table 2 The quantitative analysis of XRD pattern of Fig. 17 Layers
α-Al2O3 (%)
γ-Al2O3 (%)
rutile-TiO2 (%)
anatase-TiO2 (%)
Outer layer
75.2
14.1
8.7
2.0
Medium layer
42.5
9.6
39.7
8.2
Inner layer
8.9
7.3
80.4
3.4
49
Highlights
1. Al2O3 coating was prepared onto the surface of TiAl alloy substrate via CPED technique. 2. The microstructure and the composition of the coating were elaborately characterized in detail. 3. The improved high temperature oxidation behaviors of the substrate, the coating and their interface were illuminated.
Declaration of interest statement: We 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. Yours, All authors