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Characteristics and high temperature oxidation behavior of Ni-Cr-Y2O3 nanocomposite coating prepared by cathode plasma electrolytic deposition
Journal of Alloys and Compounds 793 (2019) 170e178
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Characteristics and high temperature oxidation behavior of Ni-Cr-Y2O3 nanocomposite coating prepared by cathode plasma electrolytic deposition Cheng Quan b, Shunjie Deng a, *, Yidong Jiang a, Chi Jiang a, Maobing Shuai a a b
China Academy of Engineering Physics, Mianyang 621900, China Research Institute of Aerospace Special Materials and Processing Technology, Beijing 100074, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2019 Received in revised form 28 March 2019 Accepted 5 April 2019 Available online 16 April 2019
Ni-Cr-Y2O3 nanocomposite coating, with Y2O3 nano-particles finely dispersed, was prepared by cathode plasma electrolytic deposition from environmental-friendly nickel-trivalent chromium sulfate electrolyte directly. The coating is dense and uniform with typically molten morphology, possessing a nanocrystalline structure with an average grain size of about 30e50 nm. Owing to the synergy effect of the fine-grained structure of the coating and the reactive-element effect of the addition of Y2O3 nanoparticles, a thin, compact, uniform and continuous Cr2O3 oxide scale can be formed during 200 h oxidation at 750 C. Such Cr2O3 oxide scale provides effective protection to the inner alloy and substrate. © 2019 Elsevier B.V. All rights reserved.
Keywords: Cathode plasma electrolytic deposition Microstructure Nanocomposite High temperature oxidation
1. Introduction With the development of science and technology, materials are applied in more severe environment and required for better properties. The metallic materials used under high temperature should be able to form a stable, slow-growing and thin surface oxide, in order to provide good service performance. As a high temperature material, the oxidation of nickel-chromium binary alloy coating has been studied extensively in the past decades [1,2]. With sufficient concentration of chromium in the Ni-Cr alloy coating at high temperature, chromium can be oxidized selectively to form a Cr2O3 oxide scale with good oxidation resistance on the surface of the coating, which can protect the substrate from further oxidation. According to Wagner's theory, the critical concentration of chromium required to develop an external, intact and continuous Cr2O3 oxide scale of the nickel-chromium alloy coating during the high temperature oxidation process is more than approximately 20 wt% [3,4]. Because of the good oxidation resistance, nickel-chromium alloy coatings have been widely used in turbine blades, engine parts and high temperature resistant components to protect the substrate
* Corresponding author. E-mail address: [email protected] (S. Deng). https://doi.org/10.1016/j.jallcom.2019.04.063 0925-8388/© 2019 Elsevier B.V. All rights reserved.
materials from oxidation in high temperature environment [5,6]. Dong Wu et al. investigated microstructural stability and impacttoughness evolution of a Ni-Fe-based weld metal with different Cr contents during long-term thermal exposures up to 10000 h at 700 C [7]. Heng Zhang et al. found homogenizing the microstructure could improve the high temperature oxidation [8]. In addition, it is reported that small additions of reactive elements (Y, Ce, La, etc) as metallic or oxide dispersoid components have been shown to cause favorable characteristics of protective coatings, especially in improving oxidation resistance and scale adherence [9e11]. A variety of methods have been used to prepare nickelchromium alloy coatings, such as conventional electroplating [12], co-electrodeposition [13], laser cladding [14] and thermal spraying [15], etc. Among these methods, thermal spraying is the most widely used, as an effective way to prepare coatings without affecting any other properties [16e18]. However, the interconnected porosity in the coating and the oxidation during spraying process can't be easily avoided and have negative effects on coating oxidation resistance [19]. In recent years, cathode plasma electrolytic deposition (CPED), as an advanced coating preparation and surface modification technique, has been developed to enhance the oxidation and corrosion resistance of the metals [20]. It is a combination of conventional electrolysis and atmospheric plasma processing [21], which applies a high potential between
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two electrodes in an aqueous or non-aqueous media. As increasing the voltage above a certain critical value, the gas bubbles on the cathode surface will be ionized and the plasma discharge can be formed. With the influence of the plasma discharge, the thermal and physical/chemical effects can take place on the cathode surface. Different metal [22e26], ceramic and composite coatings [27e29] with specific properties, such as good adhesion to the substrate, high hardness, nanocrystalline structure, good corrosion and wear resistance, can be prepared on the cathode (sample) surface [30,31]. In this work, a novel Ni-Cr alloy coating with the dispersion of Y2O3 nano-particles was prepared on T91 steel by CPED in environmental-friendly trivalent chromium sulfate electrolyte, avoiding the toxicity of hexavalent chromium [32]. Characteristics and high temperature oxidation behavior of the composite coating were investigated, and the relationship between the coating microstructure and oxidation resistance was investigated and discussed. 2. Material and methods 2.1. Preparation of the coatings The experimental device for cathode plasma electrolytic depositing Ni-Cr-Y2O3 nanocomposite coatings was similar to that in Ref. [23]. However, some improvements in the core working part are shown in Fig. 1, to realize the co-deposition of the alloy coatings and the Y2O3 nano-particles preferably. Firstly, the electrolyte with Y2O3 nano-particles can be concentrated in the polytetrafluoroethylene (PTFE) case to avoid the splashing of the particles in plasma discharge process. Secondly, the silicone seal on the bottom of the case can restrict the flowing of the electrolyte, to prevent the coating from being deposited on the areas where there's electrolyte without the effect of plasma arcs. The difference in height between the electrolyte inlet and outlet was to ensure that the PTFE case can be full of electrolyte all the time in the depositing process. T91 steels (with the actual chemical composition by wt.%: C
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0.11%, Si 0.42%, S 0.11%, Cr 9.50%, Mo 1.05%, Ni 0.09%, Fe bal.) were used as the cathode samples, with dimensions of 15 mm 10 mm 4 mm prepared by usual machining and polished with SiC papers to 2000-grit. NiSO4 as the source of nickel ions, Cr2(SO4)3 as the source of trivalent chromium ions, NH4COOH as the complexing agent, NH4Br as the inhibitor, H3BO4 as the buffering agent, K2SO4, Na2SO4 and (NH4)2SO4 as the conducting salts, were used in the electrolyte. The concentrations of NiSO4 and Cr2(SO4)3 should be regulated to make sure that the deposition potentials of the nickel and chromium ions are similar [33]. According to the previous work [34], the actual concentrations of NiSO4 and Cr2(SO4)3 were decreased to lower values, so as to obtain dense and uniform coatings by CPED. Y2O3 nano-particles with the size of 40e60 nm were dispersed in the electrolyte uniformly. The electrolyte composition and the operating conditions were listed in Table 1. In the experimental process, the PTFE case was filled with the electrolyte sufficiently at the beginning. Then the depositing voltage was increased to the critical value rapidly, at which there were bright micro-arcs forming on the sample surface. The result of the previous work shows that the critical value in this electrolysis system is about 45 V. Based on the previous study [22], a relatively lower voltage will result in that the deposits can't be modified sufficiently by the weak and discontinuous micro-arcs. However, if excessively higher voltage is applied, the dense and severe microarcs will form and do harm to the sample. Therefore, the value of the depositing voltage was decided to conduct at approximately 10 V above the critical value in this work. At the selected voltage (60 V), the micro-arcs are uniform and tiny, so the deposits on the samples could be modified effectively and an intact composite coating was prepared on the entire surface of the sample. 2.2. Characterization The scanning electron microscope (SEM, JSM-6480A) equipped with an energy dispersive spectrometer (EDS) was used to investigate the surface and cross-sectional morphologies, and the
Fig. 1. The schematic diagram of the CPED device.
Table 1 Composition of electrolyte and operating conditions for preparing Ni-Cr-Y2O3 coatings. Electrolyte composition
Function
Quantity (g$L1)
Operating conditions
NiSO4 Cr2(SO4)3 H2SO4 NH4COOH NH4Br H3BO4 K2SO4 Na2SO4 (NH4)2SO4 Y2O3
Source of Ni ions Source of Cr ions pH modifier Complexing agent Inhibitor agent Buffering agent Conducting salt Conducting salt Conducting salt Nano-particle additive
4 15 40 35e40 7e9 35e40 35 50 55 0.8
Voltage: 60 V Current density: 200e300 A dm2 Frequency: 2000 Hz Duty ratio: 80% Time: 10 min pH ¼ 1e1.5 Temperature: 35±1 C
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composition and element distribution of the deposited coatings. The X-ray photoelectron spectroscopy (XPS, PHI-5000VPII, ULVACPHI) analysis was performed to determine the chemical composition with a base pressure lower than 5 108 Pa using a monochromatic Mg ka radiation at 1253.6 eV. The layer surface was cleaned (sputtered) by 2 keV Arþ bombardment for 2 min before the measurements. The binding energy was calibrated by using the value of contaminant carbon C1s ¼ 284.8 eV as reference. The X-ray diffraction (XRD, PW3710, Phillips) analysis was performed to determine the phase constituent with Cu Ka radiation (l ¼ 1.54052 Å) at a scanning rate of 6 /min in the range of 20e100 . The high-resolution transmission electron microscope (HRTEM, TECNAI F20) was used to reveal the grain size of the coating.
the high temperature cyclic oxidation test, all of the quartz crucibles were pre-heated to a constant weight. Then the crucibles with different samples were put into the silicon carbide tube furnace for high temperature cyclic oxidation. After a certain oxidation period of 10 h, the crucibles were moved out from the furnace and cooled down to room temperature in 30 min. The weight gain (crucible with sample) and spallation (crucible with oxidation peelings in the crucible) of samples were weighed by an electronic balance with an accuracy of 102 mg [35]. After that, the crucibles with samples were put back to the furnace again for the next cycle. After 20 times oxidation cycles, the dates were recorded and used to evaluate the high-temperature oxidation and spallation resistance of the samples. The surface and cross-sectional morphologies, the composition of the oxide scales, as well as the phases in the oxide scales were investigated by SEM, EDS, XPS and XRD, respectively.
2.3. High temperature cyclic oxidation test 3. Results The silicon carbide tube furnace was used to carry out the high temperature cyclic oxidation tests at 750 C in air for 200 h. Before
Fig. 2 shows the surface and cross-sectional morphologies of the
Fig. 2. SEM micrographs of the deposited coating prepared by CPED: (a) (c) (e) the surface, the cross-section morphologies and EDS analysis of Ni-Cr coating; (b) (d) (f) the surface, the cross-section morphologies and EDS analysis of Ni-Cr-Y2O3 coating.
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deposited coating prepared by CPED. As shown in Fig. 2a and b, it can be found that the surface morphologies of the deposited coatings are different from those prepared by conventional electrodeposition, thermal spraying or other method. The surfaces of the Ni-Cr coating and Ni-Cr-Y2O3 coating show molten morphologies and are filled with many bulged nodules, which correspond to the molten drops and pits from the melting and solidification cycles during the CPED process. Owing to the co-deposition of Y2O3 nanoparticles with Ni and Cr, the number of bulged nodules on the surface of Ni-Cr-Y2O3 coating is fewer than that of Ni-Cr coating. And the Surface roughness (Ra) of the Ni-Cr coating and Ni-Cr-Y2O3 coating is about 1.81 ± 0.3 mm and 1.47 ± 0.5 mm, respectively. In addition, the prepared coatings with rough surfaces are still uniform and dense. From the cross-sectional morphology in Fig. 2c and d, it can be seen that the thickness of the prepared coatings is about 20 mm. No obvious pores and cracks can be found, which is in accord with the result from the coating surface morphology. As shown in Fig. 2e and f, the Ni-Cr coating consists of nickel and chromium, and the content of chromium in the deposited coating is about 20 wt%. For the Ni-Cr-Y2O3 coating, it mainly composes of nickel, chromium and a small amount of yttrium and oxygen, and the content of chromium is also approximately 20 wt%. Fig. 3 shows the element mapping analysis of the Ni-Cr-Y2O3 coating. It can be seen that the distributions of all the elements are very uniform. No local enrichment of elements can be observed. Fig. 4 shows the XRD patterns of the bare substrate, the Ni-Cr coating and Ni-Cr-Y2O3 coating prepared by CPED. The result of the prepared coating from the XRD pattern is consistent with that from the EDS analysis in Fig. 2e and f. It reveals that the prepared Ni-Cr coating is composed of Ni-Cr (JCPDS No. 65-6291) phase, and the prepared Ni-Cr-Y2O3 coating is composed of Ni-Cr (JCPDS No. 65-6291) phase and Y2O3 (JCPDS No. 20-1412) phase. All of the diffraction peaks can match with the standard peaks, indicating that both of the prepared coatings are well-alloyed. In addition, the grain size of the Ni-Cr (JCPDS No. 65-6291) phase in Ni-Cr-Y2O3 coating is calculated by Scherrer equation:
Dhkl ¼
Kl B,cos q
(1)
Where Dhkl is the average grain size in a direction perpendicular to the crystal plane that is identified by the Miller indices hkl; K is a constant equal to 1 in this experiment; l is the wavelength of the X-
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Fig. 4. XRD patterns of (a) the bare substrate; (b) the Ni-Cr coating; (c) Ni-Cr-Y2O3 coating.
radiation (Cu Ka is 1.54052 Å); B is the full-width at the half maximum (FWHM) of the corresponding diffraction peaks; q is the Bragg diffraction angle. As shown in Table 2, the grain sizes of Ni-Cr (JCPDS No. 65-6291) phase in Ni-Cr-Y2O3 coating are calculated for 4 crystallographic family planes using Scherrer's equation, and the average grain size is about 46.5 nm. Fig. 5 shows the TEM examination of the Ni-Cr-Y2O3 coating. In the TEM analysis, several Y2O3 nano-particles are marked to confirm that they are finely dispersed in the Ni-Cr alloy. Furthermore, from the diffraction ring patterns of the selected area in Fig. 5b, most area of the prepared coating is still of Ni-Cr alloy phase. The reduction of R2 (R, the radius of electron diffraction rings), approximately following the ratio of 3:4:8:11 …, corresponds to the R2 ratio of face-centered cubic. According to TEM image, the average grain size of the coating is about 30e50 nm, which is consistent with the average grain size calculated by Scherrer equation. Therefore, it reveals that the prepared Ni-CrY2O3 coating is a nanocrystalline coating with face-centered cubic crystal structure. All the results above indicate that the product prepared by CPED is a Ni-Cr alloy with finely dispersed Y2O3 nanocomposite coating. These are mainly attributed to the effect of plasmas generating in the CPED process. With the increase of voltage, the hydrogen evolution reaction on cathode becomes very severe, thereby resulting in the restriction of current rise by a partial shielding action of the hydrogen bubbles over cathode surface. A continuous gaseous envelope with low electrical conductivity is formed gradually and nearly all the voltage drop across the electrolytic cell is concentrated in this region. The transient electric field intensity can
Table 2 Grain size of the Ni-Cr phase in Ni-Cr-Y2O3 coating obtained by Scherrer equation.
Fig. 3. Element mapping analysis of the deposited Ni-Cr-Y2O3 coating prepared by CPED.
Sample
Dhkl (nm) (111)
(200)
(220)
(311)
Ni-Cr-Y2O3
34.2
37.5
49.7
64.5
Average (nm)
46.5
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Fig. 5. TEM examination of the Ni-Cr-Y2O3 coating: (a) TEM image; (b) selected area diffraction ring patterns of point A.
Fig. 6. Oxidation kinetic curves of the samples at 750 C for 200 h: (a) mass gain versus time; (b) spallation mass versus time.
reach up to 106e108 V/m, leading to the breakdown of the gaseous envelope with discharge phenomenon [20]. The local temperature in this near-electrode region is reported to be 2000 C [36], and the transient temperature of plasmas is about 8000 K [37]. These high temperature plasmas will result in the localized melting of the deposits. The molten state deposits are surrounded by the relatively cool electrolyte, which will lead to the freezing of them. This rapid freezing will cause a quenching effect, leading to form an ultra-fine microstructure of the deposited coating. With the repetition of this transient process, the defects are made up, and the deposited coatings are dense and uniform, demonstrating a typically molten morphology with nanocrystalline structure eventually. For a conventional electrodeposited Ni-Cr alloy coating, there are always micro-cracks with the increase of Cr content [38]. However, the coatings prepared by CPED are crack-free, with relatively good microstructure and mechanical properties [22,23]. Fig. 6 shows the oxidation mass gain and the spallation mass per unit area as a function of time for the cyclic oxidation test of the bare and coated samples at 750 C for 200 h. It can be found that severe oxidation has taken place on the bare substrate, and the
oxidation mass gain is 4.311 mg/cm2. Simultaneously, due to the high internal thermal stresses between the oxide scale and the bare substrate, severe peeling has taken place on the surface of the bare substrate after 150 h cyclic oxidation. Finally, a high spallation mass of 1.992 mg/cm2 is achieved. As a contrast, the mass gain and spallation mass of the samples with Ni-Cr coating and Ni-Cr-Y2O3 coating are significantly decreased. The sample with Ni-Cr-Y2O3 coating shows the lowest mass gain (0.245 mg/cm2) and spallation mass (0.0033 mg/cm2), exhibiting the best high temperature oxidation resistance. Therefore, it can be concluded from the cyclic oxidation test that the oxidation resistance of substrate is increased by the prepared coating, and the Ni-Cr-Y2O3 coating oxidation resistance is improved by the dispersed Y2O3 nano-particles. Fig. 7 shows SEM micrographs and EDS analysis of the samples after cyclic oxidation at 750 C for 200 h. As shown in Fig. 7a, it can be seen that the bare substrate is oxidized seriously after cyclic oxidation. The bare substrate surface is rough and loose, and some micro-cracks can also be observed in the oxide scale. In Fig. 7b, it can be also found that the thickness of the oxide scale is about 50e60 mm, and there are some pores existing in the oxide scale.
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Fig. 7. SEM micrographs and EDS analysis of the samples after cyclic oxidation at 750 C for 200 h: (a) (b) the bare substrate; (c) (d) the Ni-Cr coating; (e) (f) the Ni-Cr-Y2O3 coating.
According to the EDS analysis, the oxide scale consists of ferrum, chromium and oxygen, so it should be mainly composed of iron oxides for the relatively high ferrum content. Since the oxide scales of Ni-Cr and Ni-Cr-Y2O3 coatings are thin and hard to be observed, the coated samples were soaked in methanol solution with 10% (V/ V) iodine for 24 h to dissolve the substrates and reserve only the oxide scales. As shown in Fig. 7c and e, the oxide scales of the coated samples are much more compact and smoother by comparing with that of the bare sample, and no obvious cracks have been found. From the cross-sectional morphologies (Fig. 7d and f), the thicknesses of the oxide scales on Ni-Cr coating and Ni-Cr-Y2O3 coating are about 3e4 mm and 2 mm, respectively, which is far much thinner than that of the bare substrate. According to the EDS analysis, the oxide scales on the two coatings are only composed of chromium and oxygen, indicating that the selective oxidation film of Cr2O3 has been formed. Therefore, it reveals that the oxidation resistance of the substrate has been improved by the prepared coating, and the Ni-Cr-Y2O3 coating possesses better oxidation
resistance. Fig. 8 shows the XPS spectra of the deposited coating surface and oxidized coating surface. As shown in Fig. 8a, Cr-metal (574.2 eV) is found on the deposited coating surface, and Cr oxides peak (577.0 eV) mainly composed of Cr2O3 is observed on the oxidized coating surface. Fig. 8b shows the narrow scan spectra of Ni2p3/2 on the surface. Ni-metal (852.8 eV) is detected on the deposited coating surface. However, no obvious peaks of nickel are observed on the oxidized coating surface, which is attributed to the presence of Cr2O3 oxide scale. As shown in Fig. 8c, the O1s peaks at 531.2 eV and 530.8 eV are found on the surface of the deposited coating and oxidized coating, respectively [39]. According to the XPS results, the chemical composition of the deposited coating surface and the oxidized coating surface is calculated and shown in Table 3. A small amount of oxygen is found on the deposited coating surface. Owing to the adsorption of oxygen on coating surface and a shallow information depth of XPS (5e10 nm) compared with EDS, the concentration of oxygen obtained by XPS is more than that
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Fig. 8. XPS spectra of the deposited coating surface and oxidized coating surface: (a) Cr2p3/2; (B) Ni2p3/2; (C) O1s.
obtained by EDS. For the oxidized coating, the singles of nickel are shielded by the oxide scale (2e4 mm) and can't be detected. Since there is a difference in information depth between XPS and EDS, the concentration of oxygen obtained by EDS and XPS is a little different. However, the ratio of Cr to Ni obtained by XPS is close to the EDS result. Fig. 9 shows the XRD patterns of the bare substrate and the coated sample after the cyclic oxidation test. It reveals that the diffraction peaks of the bare substrate correspond to the phases of Fe2O3(JCPDS No. 33-0664), FeCr2O4(JCPDS No. 24-0511), Cr2O3(JCPDS No. 38-1479) and Fe (JCPDS No. 65-4899). While, the oxide scales of the two coated samples are composed of only Cr2O3 (JCPDS No. 38-1479). The results from the XRD patterns match well with that from EDS and XPS analysis. Furthermore, the oxidation and spallation resistance of the Ni-Cr-Y2O3 nanocomposite coating prepared by CPED is relatively good by comparing with the similar coatings prepared by conventional electrodeposition, even thermal
Table 3 Chemical composition of the deposited coating surface and the oxidized coating surface obtained by XPS. Deposited coating
Ni Cr O
Oxidized coating
(at.%)
(wt.%)
(at.%)
(wt.%)
65.13 16.52 18.35
75.21 18.72 6.07
/ 37.48 62.52
/ 67.11 32.89
spraying [12,40]. 4. Discussion According to the results above, it can be seen that the oxidation resistance of the bare sample has been significantly improved by the coatings. Here, a brief analysis on increasing oxidation resistance of the bare sample by Ni-Cr coating and Ni-Cr-Y2O3 coating is given as follows. Ni and Cr atoms on the surface of Ni-Cr binary alloy can react with oxygen at high temperature. Then a thin oxide scale composed of NiOx and CrOy will be formed on the surface. Since the growth rates of the two oxides are different, the NiOx with a faster growth rate will usually tend to overgrow CrOy. The oxidation of Cr requires a low oxygen partial pressure [41] and Gibbs free energy of formation [42], so atoms of Cr at the substrate/oxide scale interface will react with the oxide of Ni, resulting in the following reaction:
x x Cr ðin substrateÞ þ NiOx ¼ CrOy þ Ni ðin substrateÞ y y
(2)
Because of the above reaction, Cr generally becomes depleted in the region of substrate near the substrate/oxide scale interface. Owing to the different diffusion rates of the reacting atoms, concentration gradients occur in the oxide scale and alloy substrate. It is reported that the initial oxide of a conventional Ni-Cr binary alloy coating may be consisted of NiO, Cr2O3 and NiCr2O4. In order to protect the substrate, the ideal oxide scale is supposed to be only
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Fig. 9. XRD patterns of (a) the bare substrate; (b) the Ni-Cr coating; (c) the Ni-Cr-Y2O3 coating after cyclic oxidation at 750 C for 200 h.
composed of Cr2O3, which possesses a relatively low oxygen diffusion rate. According to Wagner's theory [4], selective oxidation usually take place only above a critical concentration of the active alloy component. The critical concentration NCr of Cr for the selective formation of Cr2O3 scale can be expressed as:
V NCr f zCr MO
pkp
!1=2
DL þ DB 2w d
(3)
Where V is the alloy molar volume; zCr is the Cr atom valence; MO is the atomic weight of oxygen; DL is the diffusion coefficient of Cr in the alloy grain; DB is the diffusion coefficient of Cr in the alloy grain boundary; w is the average width of grain boundary; d is the grain size; kp is the parabolic rate constant for exclusive formation of Cr2O3. According to the previous studies, the selective oxidation of Cr takes place in alloys with more than approximately 20 wt% Cr. In this paper, a Ni-20 wt% Cr binary alloy coating was prepared by CPED on T91 substrate. Due to the insufficient concentration of Cr in the bare sample without coating, T91 substrate is oxidized seriously (Fig. 7a and b). The reaction products are mainly composed of Fe2O3, FeCr2O4 and a small amount of Cr2O3. Nevertheless, the sample coated with the Ni-Cr alloy shows a good
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property of oxidation resistance. Due to the physical/chemical effects of plasmas, the coating is dense and uniform with nanocrystalline structure. The grain boundaries per unit area increase in the nanocrystalline coating, leading to a quick diffusion of Cr atoms along the grain boundary outwards to the surface. According to Eq. (3), The critical concentration NCr of Cr for the selective formation can be reduced by decreasing the grain size d. Consequently, the selective oxidation of Cr is more likely to take place and an intact Cr2O3 oxide scale forms, which is compact, uniform, thin and protective. Such continuous Cr2O3 oxide scale provides good protection to the inner substrate. Owing to the addition of dispersed Y2O3 nano-particles in Ni-CrY2O3 coating, the growth mechanism of the Cr2O3 oxide scale is different from that of the Ni-Cr coating. Fig. 10 shows the schematic illustration of the selective oxidation of the novel Ni-Cr-Y2O3 nanocomposite coating prepared by CPED. Due to the nanocrystalline structure of the deposited coating and the addition of finely dispersed Y2O3 nano-particles, the initial oxides will be formed in very small grain size [43]. This finer-grained structure can enhance the selective oxidation of chromium, for that it favors the rapid short-circuit diffusion of the chromium in the alloy and a corresponding rapid development of the Cr2O3 oxide scale on the surface [44]. As a result, the microstructure of the coating contributes to the formation of a continuous Cr2O3 oxide scale, which is protective to the inner matrix. In addition, the finely dispersed Y2O3 nano-particles have changed the scale growth mechanism by segregating to the grain boundaries in the oxides, which is known as the reactive-element effect [45]. The oxygen diffusion along the grain boundaries is enhanced and the outward chromium diffusion is decreased [9]. Therefore, the growth mechanism is changed to involve a predominant inward oxygen diffusion, and the growth rate of the oxide scale is reduced. Because of the synergy effect of the fine-grained microstructure of the novel coating and the reactive-element effect of the addition of Y2O3 nano-particles, a thin, compact, uniform and continuous Cr2O3 oxide scale is formed finally. It remains stable for the extremely slow growth rate, and provides effective protection to the inner alloy and substrate. In the cyclic oxidation process, the spallation of the oxides is mainly dependent on the generation and relief of the stresses in oxide scales [46]. For the bare T91 steel substrate, the spallation of the oxide scale is very severe. This is mainly because of the thermal stresses in the oxide scale, which is usually generated from the growth of oxide and the expansion mismatch between the thick oxide scale and the substrate. However, for the Ni-Cr and Ni-CrY2O3 nanocomposite coating, it shows good spallation resistance. As is discussed above, the nanocrystalline structure of the Ni-Cr and Ni-Cr-Y2O3 coating can result in the formation of a thin finergrained oxide scale. The thin finer-grained oxides formed on the
Fig. 10. The schematic illustration of the selective oxidation of the Ni-Cr-Y2O3 nanocomposite coating prepared by CPED.
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surface could relieve the high temperature stresses, instead of cracking or spalling, thereby leading to exhibiting relatively good high temperature spallation resistance [47]. Moreover, the addition of Y2O3 particles will also contribute to improving the scale adherence to the coating. As a result, the Cr2O3 oxide scale is dense and crack-free, exhibiting good mechanical properties.
[18]
[19]
[20]
5. Conclusions [21]
In conclusion, a novel Ni-Cr-Y2O3 nanocomposite coating was prepared by cathode plasma electrolytic deposition from nickeltrivalent chromium sulfate electrolyte directly, with finely dispersed Y2O3 nano-particles. Owing to the effect of plasmas, the coating is dense and uniform with a typically molten morphology, possessing a nanocrystalline structure with an average grain size of about 30e50 nm. The cyclic oxidation test reveals that the deposited coating exhibits good high temperature oxidation and spallation resistance. These are mainly attributed to the synergy effect of the fine-grained structure of the coating and the reactive-element effect of the addition of Y2O3 nano-particles. Acknowledgements
[22]
[23]
[24]
[25]
[26]
[27]
This work was supported by National Natural Science Foundation of China (Grant No. 51801191). The authors would like to thank Chuanhui Liang, Chao Lv and Pan Yang (China Academy of Engineering Physics) for their kindly help.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Characteristics and high temperature oxidation behavior of Ni-Cr-Y2O3 nanocomposite coating prepared by cathode plasma electrolytic deposition Cheng Quan b, Shunjie Deng a, *, Yidong Jiang a, Chi Jiang a, Maobing Shuai a a b
China Academy of Engineering Physics, Mianyang 621900, China Research Institute of Aerospace Special Materials and Processing Technology, Beijing 100074, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2019 Received in revised form 28 March 2019 Accepted 5 April 2019 Available online 16 April 2019
Ni-Cr-Y2O3 nanocomposite coating, with Y2O3 nano-particles finely dispersed, was prepared by cathode plasma electrolytic deposition from environmental-friendly nickel-trivalent chromium sulfate electrolyte directly. The coating is dense and uniform with typically molten morphology, possessing a nanocrystalline structure with an average grain size of about 30e50 nm. Owing to the synergy effect of the fine-grained structure of the coating and the reactive-element effect of the addition of Y2O3 nanoparticles, a thin, compact, uniform and continuous Cr2O3 oxide scale can be formed during 200 h oxidation at 750 C. Such Cr2O3 oxide scale provides effective protection to the inner alloy and substrate. © 2019 Elsevier B.V. All rights reserved.
Keywords: Cathode plasma electrolytic deposition Microstructure Nanocomposite High temperature oxidation
1. Introduction With the development of science and technology, materials are applied in more severe environment and required for better properties. The metallic materials used under high temperature should be able to form a stable, slow-growing and thin surface oxide, in order to provide good service performance. As a high temperature material, the oxidation of nickel-chromium binary alloy coating has been studied extensively in the past decades [1,2]. With sufficient concentration of chromium in the Ni-Cr alloy coating at high temperature, chromium can be oxidized selectively to form a Cr2O3 oxide scale with good oxidation resistance on the surface of the coating, which can protect the substrate from further oxidation. According to Wagner's theory, the critical concentration of chromium required to develop an external, intact and continuous Cr2O3 oxide scale of the nickel-chromium alloy coating during the high temperature oxidation process is more than approximately 20 wt% [3,4]. Because of the good oxidation resistance, nickel-chromium alloy coatings have been widely used in turbine blades, engine parts and high temperature resistant components to protect the substrate
* Corresponding author. E-mail address: [email protected] (S. Deng). https://doi.org/10.1016/j.jallcom.2019.04.063 0925-8388/© 2019 Elsevier B.V. All rights reserved.
materials from oxidation in high temperature environment [5,6]. Dong Wu et al. investigated microstructural stability and impacttoughness evolution of a Ni-Fe-based weld metal with different Cr contents during long-term thermal exposures up to 10000 h at 700 C [7]. Heng Zhang et al. found homogenizing the microstructure could improve the high temperature oxidation [8]. In addition, it is reported that small additions of reactive elements (Y, Ce, La, etc) as metallic or oxide dispersoid components have been shown to cause favorable characteristics of protective coatings, especially in improving oxidation resistance and scale adherence [9e11]. A variety of methods have been used to prepare nickelchromium alloy coatings, such as conventional electroplating [12], co-electrodeposition [13], laser cladding [14] and thermal spraying [15], etc. Among these methods, thermal spraying is the most widely used, as an effective way to prepare coatings without affecting any other properties [16e18]. However, the interconnected porosity in the coating and the oxidation during spraying process can't be easily avoided and have negative effects on coating oxidation resistance [19]. In recent years, cathode plasma electrolytic deposition (CPED), as an advanced coating preparation and surface modification technique, has been developed to enhance the oxidation and corrosion resistance of the metals [20]. It is a combination of conventional electrolysis and atmospheric plasma processing [21], which applies a high potential between
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two electrodes in an aqueous or non-aqueous media. As increasing the voltage above a certain critical value, the gas bubbles on the cathode surface will be ionized and the plasma discharge can be formed. With the influence of the plasma discharge, the thermal and physical/chemical effects can take place on the cathode surface. Different metal [22e26], ceramic and composite coatings [27e29] with specific properties, such as good adhesion to the substrate, high hardness, nanocrystalline structure, good corrosion and wear resistance, can be prepared on the cathode (sample) surface [30,31]. In this work, a novel Ni-Cr alloy coating with the dispersion of Y2O3 nano-particles was prepared on T91 steel by CPED in environmental-friendly trivalent chromium sulfate electrolyte, avoiding the toxicity of hexavalent chromium [32]. Characteristics and high temperature oxidation behavior of the composite coating were investigated, and the relationship between the coating microstructure and oxidation resistance was investigated and discussed. 2. Material and methods 2.1. Preparation of the coatings The experimental device for cathode plasma electrolytic depositing Ni-Cr-Y2O3 nanocomposite coatings was similar to that in Ref. [23]. However, some improvements in the core working part are shown in Fig. 1, to realize the co-deposition of the alloy coatings and the Y2O3 nano-particles preferably. Firstly, the electrolyte with Y2O3 nano-particles can be concentrated in the polytetrafluoroethylene (PTFE) case to avoid the splashing of the particles in plasma discharge process. Secondly, the silicone seal on the bottom of the case can restrict the flowing of the electrolyte, to prevent the coating from being deposited on the areas where there's electrolyte without the effect of plasma arcs. The difference in height between the electrolyte inlet and outlet was to ensure that the PTFE case can be full of electrolyte all the time in the depositing process. T91 steels (with the actual chemical composition by wt.%: C
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0.11%, Si 0.42%, S 0.11%, Cr 9.50%, Mo 1.05%, Ni 0.09%, Fe bal.) were used as the cathode samples, with dimensions of 15 mm 10 mm 4 mm prepared by usual machining and polished with SiC papers to 2000-grit. NiSO4 as the source of nickel ions, Cr2(SO4)3 as the source of trivalent chromium ions, NH4COOH as the complexing agent, NH4Br as the inhibitor, H3BO4 as the buffering agent, K2SO4, Na2SO4 and (NH4)2SO4 as the conducting salts, were used in the electrolyte. The concentrations of NiSO4 and Cr2(SO4)3 should be regulated to make sure that the deposition potentials of the nickel and chromium ions are similar [33]. According to the previous work [34], the actual concentrations of NiSO4 and Cr2(SO4)3 were decreased to lower values, so as to obtain dense and uniform coatings by CPED. Y2O3 nano-particles with the size of 40e60 nm were dispersed in the electrolyte uniformly. The electrolyte composition and the operating conditions were listed in Table 1. In the experimental process, the PTFE case was filled with the electrolyte sufficiently at the beginning. Then the depositing voltage was increased to the critical value rapidly, at which there were bright micro-arcs forming on the sample surface. The result of the previous work shows that the critical value in this electrolysis system is about 45 V. Based on the previous study [22], a relatively lower voltage will result in that the deposits can't be modified sufficiently by the weak and discontinuous micro-arcs. However, if excessively higher voltage is applied, the dense and severe microarcs will form and do harm to the sample. Therefore, the value of the depositing voltage was decided to conduct at approximately 10 V above the critical value in this work. At the selected voltage (60 V), the micro-arcs are uniform and tiny, so the deposits on the samples could be modified effectively and an intact composite coating was prepared on the entire surface of the sample. 2.2. Characterization The scanning electron microscope (SEM, JSM-6480A) equipped with an energy dispersive spectrometer (EDS) was used to investigate the surface and cross-sectional morphologies, and the
Fig. 1. The schematic diagram of the CPED device.
Table 1 Composition of electrolyte and operating conditions for preparing Ni-Cr-Y2O3 coatings. Electrolyte composition
Function
Quantity (g$L1)
Operating conditions
NiSO4 Cr2(SO4)3 H2SO4 NH4COOH NH4Br H3BO4 K2SO4 Na2SO4 (NH4)2SO4 Y2O3
Source of Ni ions Source of Cr ions pH modifier Complexing agent Inhibitor agent Buffering agent Conducting salt Conducting salt Conducting salt Nano-particle additive
4 15 40 35e40 7e9 35e40 35 50 55 0.8
Voltage: 60 V Current density: 200e300 A dm2 Frequency: 2000 Hz Duty ratio: 80% Time: 10 min pH ¼ 1e1.5 Temperature: 35±1 C
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composition and element distribution of the deposited coatings. The X-ray photoelectron spectroscopy (XPS, PHI-5000VPII, ULVACPHI) analysis was performed to determine the chemical composition with a base pressure lower than 5 108 Pa using a monochromatic Mg ka radiation at 1253.6 eV. The layer surface was cleaned (sputtered) by 2 keV Arþ bombardment for 2 min before the measurements. The binding energy was calibrated by using the value of contaminant carbon C1s ¼ 284.8 eV as reference. The X-ray diffraction (XRD, PW3710, Phillips) analysis was performed to determine the phase constituent with Cu Ka radiation (l ¼ 1.54052 Å) at a scanning rate of 6 /min in the range of 20e100 . The high-resolution transmission electron microscope (HRTEM, TECNAI F20) was used to reveal the grain size of the coating.
the high temperature cyclic oxidation test, all of the quartz crucibles were pre-heated to a constant weight. Then the crucibles with different samples were put into the silicon carbide tube furnace for high temperature cyclic oxidation. After a certain oxidation period of 10 h, the crucibles were moved out from the furnace and cooled down to room temperature in 30 min. The weight gain (crucible with sample) and spallation (crucible with oxidation peelings in the crucible) of samples were weighed by an electronic balance with an accuracy of 102 mg [35]. After that, the crucibles with samples were put back to the furnace again for the next cycle. After 20 times oxidation cycles, the dates were recorded and used to evaluate the high-temperature oxidation and spallation resistance of the samples. The surface and cross-sectional morphologies, the composition of the oxide scales, as well as the phases in the oxide scales were investigated by SEM, EDS, XPS and XRD, respectively.
2.3. High temperature cyclic oxidation test 3. Results The silicon carbide tube furnace was used to carry out the high temperature cyclic oxidation tests at 750 C in air for 200 h. Before
Fig. 2 shows the surface and cross-sectional morphologies of the
Fig. 2. SEM micrographs of the deposited coating prepared by CPED: (a) (c) (e) the surface, the cross-section morphologies and EDS analysis of Ni-Cr coating; (b) (d) (f) the surface, the cross-section morphologies and EDS analysis of Ni-Cr-Y2O3 coating.
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deposited coating prepared by CPED. As shown in Fig. 2a and b, it can be found that the surface morphologies of the deposited coatings are different from those prepared by conventional electrodeposition, thermal spraying or other method. The surfaces of the Ni-Cr coating and Ni-Cr-Y2O3 coating show molten morphologies and are filled with many bulged nodules, which correspond to the molten drops and pits from the melting and solidification cycles during the CPED process. Owing to the co-deposition of Y2O3 nanoparticles with Ni and Cr, the number of bulged nodules on the surface of Ni-Cr-Y2O3 coating is fewer than that of Ni-Cr coating. And the Surface roughness (Ra) of the Ni-Cr coating and Ni-Cr-Y2O3 coating is about 1.81 ± 0.3 mm and 1.47 ± 0.5 mm, respectively. In addition, the prepared coatings with rough surfaces are still uniform and dense. From the cross-sectional morphology in Fig. 2c and d, it can be seen that the thickness of the prepared coatings is about 20 mm. No obvious pores and cracks can be found, which is in accord with the result from the coating surface morphology. As shown in Fig. 2e and f, the Ni-Cr coating consists of nickel and chromium, and the content of chromium in the deposited coating is about 20 wt%. For the Ni-Cr-Y2O3 coating, it mainly composes of nickel, chromium and a small amount of yttrium and oxygen, and the content of chromium is also approximately 20 wt%. Fig. 3 shows the element mapping analysis of the Ni-Cr-Y2O3 coating. It can be seen that the distributions of all the elements are very uniform. No local enrichment of elements can be observed. Fig. 4 shows the XRD patterns of the bare substrate, the Ni-Cr coating and Ni-Cr-Y2O3 coating prepared by CPED. The result of the prepared coating from the XRD pattern is consistent with that from the EDS analysis in Fig. 2e and f. It reveals that the prepared Ni-Cr coating is composed of Ni-Cr (JCPDS No. 65-6291) phase, and the prepared Ni-Cr-Y2O3 coating is composed of Ni-Cr (JCPDS No. 65-6291) phase and Y2O3 (JCPDS No. 20-1412) phase. All of the diffraction peaks can match with the standard peaks, indicating that both of the prepared coatings are well-alloyed. In addition, the grain size of the Ni-Cr (JCPDS No. 65-6291) phase in Ni-Cr-Y2O3 coating is calculated by Scherrer equation:
Dhkl ¼
Kl B,cos q
(1)
Where Dhkl is the average grain size in a direction perpendicular to the crystal plane that is identified by the Miller indices hkl; K is a constant equal to 1 in this experiment; l is the wavelength of the X-
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Fig. 4. XRD patterns of (a) the bare substrate; (b) the Ni-Cr coating; (c) Ni-Cr-Y2O3 coating.
radiation (Cu Ka is 1.54052 Å); B is the full-width at the half maximum (FWHM) of the corresponding diffraction peaks; q is the Bragg diffraction angle. As shown in Table 2, the grain sizes of Ni-Cr (JCPDS No. 65-6291) phase in Ni-Cr-Y2O3 coating are calculated for 4 crystallographic family planes using Scherrer's equation, and the average grain size is about 46.5 nm. Fig. 5 shows the TEM examination of the Ni-Cr-Y2O3 coating. In the TEM analysis, several Y2O3 nano-particles are marked to confirm that they are finely dispersed in the Ni-Cr alloy. Furthermore, from the diffraction ring patterns of the selected area in Fig. 5b, most area of the prepared coating is still of Ni-Cr alloy phase. The reduction of R2 (R, the radius of electron diffraction rings), approximately following the ratio of 3:4:8:11 …, corresponds to the R2 ratio of face-centered cubic. According to TEM image, the average grain size of the coating is about 30e50 nm, which is consistent with the average grain size calculated by Scherrer equation. Therefore, it reveals that the prepared Ni-CrY2O3 coating is a nanocrystalline coating with face-centered cubic crystal structure. All the results above indicate that the product prepared by CPED is a Ni-Cr alloy with finely dispersed Y2O3 nanocomposite coating. These are mainly attributed to the effect of plasmas generating in the CPED process. With the increase of voltage, the hydrogen evolution reaction on cathode becomes very severe, thereby resulting in the restriction of current rise by a partial shielding action of the hydrogen bubbles over cathode surface. A continuous gaseous envelope with low electrical conductivity is formed gradually and nearly all the voltage drop across the electrolytic cell is concentrated in this region. The transient electric field intensity can
Table 2 Grain size of the Ni-Cr phase in Ni-Cr-Y2O3 coating obtained by Scherrer equation.
Fig. 3. Element mapping analysis of the deposited Ni-Cr-Y2O3 coating prepared by CPED.
Sample
Dhkl (nm) (111)
(200)
(220)
(311)
Ni-Cr-Y2O3
34.2
37.5
49.7
64.5
Average (nm)
46.5
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Fig. 5. TEM examination of the Ni-Cr-Y2O3 coating: (a) TEM image; (b) selected area diffraction ring patterns of point A.
Fig. 6. Oxidation kinetic curves of the samples at 750 C for 200 h: (a) mass gain versus time; (b) spallation mass versus time.
reach up to 106e108 V/m, leading to the breakdown of the gaseous envelope with discharge phenomenon [20]. The local temperature in this near-electrode region is reported to be 2000 C [36], and the transient temperature of plasmas is about 8000 K [37]. These high temperature plasmas will result in the localized melting of the deposits. The molten state deposits are surrounded by the relatively cool electrolyte, which will lead to the freezing of them. This rapid freezing will cause a quenching effect, leading to form an ultra-fine microstructure of the deposited coating. With the repetition of this transient process, the defects are made up, and the deposited coatings are dense and uniform, demonstrating a typically molten morphology with nanocrystalline structure eventually. For a conventional electrodeposited Ni-Cr alloy coating, there are always micro-cracks with the increase of Cr content [38]. However, the coatings prepared by CPED are crack-free, with relatively good microstructure and mechanical properties [22,23]. Fig. 6 shows the oxidation mass gain and the spallation mass per unit area as a function of time for the cyclic oxidation test of the bare and coated samples at 750 C for 200 h. It can be found that severe oxidation has taken place on the bare substrate, and the
oxidation mass gain is 4.311 mg/cm2. Simultaneously, due to the high internal thermal stresses between the oxide scale and the bare substrate, severe peeling has taken place on the surface of the bare substrate after 150 h cyclic oxidation. Finally, a high spallation mass of 1.992 mg/cm2 is achieved. As a contrast, the mass gain and spallation mass of the samples with Ni-Cr coating and Ni-Cr-Y2O3 coating are significantly decreased. The sample with Ni-Cr-Y2O3 coating shows the lowest mass gain (0.245 mg/cm2) and spallation mass (0.0033 mg/cm2), exhibiting the best high temperature oxidation resistance. Therefore, it can be concluded from the cyclic oxidation test that the oxidation resistance of substrate is increased by the prepared coating, and the Ni-Cr-Y2O3 coating oxidation resistance is improved by the dispersed Y2O3 nano-particles. Fig. 7 shows SEM micrographs and EDS analysis of the samples after cyclic oxidation at 750 C for 200 h. As shown in Fig. 7a, it can be seen that the bare substrate is oxidized seriously after cyclic oxidation. The bare substrate surface is rough and loose, and some micro-cracks can also be observed in the oxide scale. In Fig. 7b, it can be also found that the thickness of the oxide scale is about 50e60 mm, and there are some pores existing in the oxide scale.
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Fig. 7. SEM micrographs and EDS analysis of the samples after cyclic oxidation at 750 C for 200 h: (a) (b) the bare substrate; (c) (d) the Ni-Cr coating; (e) (f) the Ni-Cr-Y2O3 coating.
According to the EDS analysis, the oxide scale consists of ferrum, chromium and oxygen, so it should be mainly composed of iron oxides for the relatively high ferrum content. Since the oxide scales of Ni-Cr and Ni-Cr-Y2O3 coatings are thin and hard to be observed, the coated samples were soaked in methanol solution with 10% (V/ V) iodine for 24 h to dissolve the substrates and reserve only the oxide scales. As shown in Fig. 7c and e, the oxide scales of the coated samples are much more compact and smoother by comparing with that of the bare sample, and no obvious cracks have been found. From the cross-sectional morphologies (Fig. 7d and f), the thicknesses of the oxide scales on Ni-Cr coating and Ni-Cr-Y2O3 coating are about 3e4 mm and 2 mm, respectively, which is far much thinner than that of the bare substrate. According to the EDS analysis, the oxide scales on the two coatings are only composed of chromium and oxygen, indicating that the selective oxidation film of Cr2O3 has been formed. Therefore, it reveals that the oxidation resistance of the substrate has been improved by the prepared coating, and the Ni-Cr-Y2O3 coating possesses better oxidation
resistance. Fig. 8 shows the XPS spectra of the deposited coating surface and oxidized coating surface. As shown in Fig. 8a, Cr-metal (574.2 eV) is found on the deposited coating surface, and Cr oxides peak (577.0 eV) mainly composed of Cr2O3 is observed on the oxidized coating surface. Fig. 8b shows the narrow scan spectra of Ni2p3/2 on the surface. Ni-metal (852.8 eV) is detected on the deposited coating surface. However, no obvious peaks of nickel are observed on the oxidized coating surface, which is attributed to the presence of Cr2O3 oxide scale. As shown in Fig. 8c, the O1s peaks at 531.2 eV and 530.8 eV are found on the surface of the deposited coating and oxidized coating, respectively [39]. According to the XPS results, the chemical composition of the deposited coating surface and the oxidized coating surface is calculated and shown in Table 3. A small amount of oxygen is found on the deposited coating surface. Owing to the adsorption of oxygen on coating surface and a shallow information depth of XPS (5e10 nm) compared with EDS, the concentration of oxygen obtained by XPS is more than that
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Fig. 8. XPS spectra of the deposited coating surface and oxidized coating surface: (a) Cr2p3/2; (B) Ni2p3/2; (C) O1s.
obtained by EDS. For the oxidized coating, the singles of nickel are shielded by the oxide scale (2e4 mm) and can't be detected. Since there is a difference in information depth between XPS and EDS, the concentration of oxygen obtained by EDS and XPS is a little different. However, the ratio of Cr to Ni obtained by XPS is close to the EDS result. Fig. 9 shows the XRD patterns of the bare substrate and the coated sample after the cyclic oxidation test. It reveals that the diffraction peaks of the bare substrate correspond to the phases of Fe2O3(JCPDS No. 33-0664), FeCr2O4(JCPDS No. 24-0511), Cr2O3(JCPDS No. 38-1479) and Fe (JCPDS No. 65-4899). While, the oxide scales of the two coated samples are composed of only Cr2O3 (JCPDS No. 38-1479). The results from the XRD patterns match well with that from EDS and XPS analysis. Furthermore, the oxidation and spallation resistance of the Ni-Cr-Y2O3 nanocomposite coating prepared by CPED is relatively good by comparing with the similar coatings prepared by conventional electrodeposition, even thermal
Table 3 Chemical composition of the deposited coating surface and the oxidized coating surface obtained by XPS. Deposited coating
Ni Cr O
Oxidized coating
(at.%)
(wt.%)
(at.%)
(wt.%)
65.13 16.52 18.35
75.21 18.72 6.07
/ 37.48 62.52
/ 67.11 32.89
spraying [12,40]. 4. Discussion According to the results above, it can be seen that the oxidation resistance of the bare sample has been significantly improved by the coatings. Here, a brief analysis on increasing oxidation resistance of the bare sample by Ni-Cr coating and Ni-Cr-Y2O3 coating is given as follows. Ni and Cr atoms on the surface of Ni-Cr binary alloy can react with oxygen at high temperature. Then a thin oxide scale composed of NiOx and CrOy will be formed on the surface. Since the growth rates of the two oxides are different, the NiOx with a faster growth rate will usually tend to overgrow CrOy. The oxidation of Cr requires a low oxygen partial pressure [41] and Gibbs free energy of formation [42], so atoms of Cr at the substrate/oxide scale interface will react with the oxide of Ni, resulting in the following reaction:
x x Cr ðin substrateÞ þ NiOx ¼ CrOy þ Ni ðin substrateÞ y y
(2)
Because of the above reaction, Cr generally becomes depleted in the region of substrate near the substrate/oxide scale interface. Owing to the different diffusion rates of the reacting atoms, concentration gradients occur in the oxide scale and alloy substrate. It is reported that the initial oxide of a conventional Ni-Cr binary alloy coating may be consisted of NiO, Cr2O3 and NiCr2O4. In order to protect the substrate, the ideal oxide scale is supposed to be only
C. Quan et al. / Journal of Alloys and Compounds 793 (2019) 170e178
Fig. 9. XRD patterns of (a) the bare substrate; (b) the Ni-Cr coating; (c) the Ni-Cr-Y2O3 coating after cyclic oxidation at 750 C for 200 h.
composed of Cr2O3, which possesses a relatively low oxygen diffusion rate. According to Wagner's theory [4], selective oxidation usually take place only above a critical concentration of the active alloy component. The critical concentration NCr of Cr for the selective formation of Cr2O3 scale can be expressed as:
V NCr f zCr MO
pkp
!1=2
DL þ DB 2w d
(3)
Where V is the alloy molar volume; zCr is the Cr atom valence; MO is the atomic weight of oxygen; DL is the diffusion coefficient of Cr in the alloy grain; DB is the diffusion coefficient of Cr in the alloy grain boundary; w is the average width of grain boundary; d is the grain size; kp is the parabolic rate constant for exclusive formation of Cr2O3. According to the previous studies, the selective oxidation of Cr takes place in alloys with more than approximately 20 wt% Cr. In this paper, a Ni-20 wt% Cr binary alloy coating was prepared by CPED on T91 substrate. Due to the insufficient concentration of Cr in the bare sample without coating, T91 substrate is oxidized seriously (Fig. 7a and b). The reaction products are mainly composed of Fe2O3, FeCr2O4 and a small amount of Cr2O3. Nevertheless, the sample coated with the Ni-Cr alloy shows a good
177
property of oxidation resistance. Due to the physical/chemical effects of plasmas, the coating is dense and uniform with nanocrystalline structure. The grain boundaries per unit area increase in the nanocrystalline coating, leading to a quick diffusion of Cr atoms along the grain boundary outwards to the surface. According to Eq. (3), The critical concentration NCr of Cr for the selective formation can be reduced by decreasing the grain size d. Consequently, the selective oxidation of Cr is more likely to take place and an intact Cr2O3 oxide scale forms, which is compact, uniform, thin and protective. Such continuous Cr2O3 oxide scale provides good protection to the inner substrate. Owing to the addition of dispersed Y2O3 nano-particles in Ni-CrY2O3 coating, the growth mechanism of the Cr2O3 oxide scale is different from that of the Ni-Cr coating. Fig. 10 shows the schematic illustration of the selective oxidation of the novel Ni-Cr-Y2O3 nanocomposite coating prepared by CPED. Due to the nanocrystalline structure of the deposited coating and the addition of finely dispersed Y2O3 nano-particles, the initial oxides will be formed in very small grain size [43]. This finer-grained structure can enhance the selective oxidation of chromium, for that it favors the rapid short-circuit diffusion of the chromium in the alloy and a corresponding rapid development of the Cr2O3 oxide scale on the surface [44]. As a result, the microstructure of the coating contributes to the formation of a continuous Cr2O3 oxide scale, which is protective to the inner matrix. In addition, the finely dispersed Y2O3 nano-particles have changed the scale growth mechanism by segregating to the grain boundaries in the oxides, which is known as the reactive-element effect [45]. The oxygen diffusion along the grain boundaries is enhanced and the outward chromium diffusion is decreased [9]. Therefore, the growth mechanism is changed to involve a predominant inward oxygen diffusion, and the growth rate of the oxide scale is reduced. Because of the synergy effect of the fine-grained microstructure of the novel coating and the reactive-element effect of the addition of Y2O3 nano-particles, a thin, compact, uniform and continuous Cr2O3 oxide scale is formed finally. It remains stable for the extremely slow growth rate, and provides effective protection to the inner alloy and substrate. In the cyclic oxidation process, the spallation of the oxides is mainly dependent on the generation and relief of the stresses in oxide scales [46]. For the bare T91 steel substrate, the spallation of the oxide scale is very severe. This is mainly because of the thermal stresses in the oxide scale, which is usually generated from the growth of oxide and the expansion mismatch between the thick oxide scale and the substrate. However, for the Ni-Cr and Ni-CrY2O3 nanocomposite coating, it shows good spallation resistance. As is discussed above, the nanocrystalline structure of the Ni-Cr and Ni-Cr-Y2O3 coating can result in the formation of a thin finergrained oxide scale. The thin finer-grained oxides formed on the
Fig. 10. The schematic illustration of the selective oxidation of the Ni-Cr-Y2O3 nanocomposite coating prepared by CPED.
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surface could relieve the high temperature stresses, instead of cracking or spalling, thereby leading to exhibiting relatively good high temperature spallation resistance [47]. Moreover, the addition of Y2O3 particles will also contribute to improving the scale adherence to the coating. As a result, the Cr2O3 oxide scale is dense and crack-free, exhibiting good mechanical properties.
[18]
[19]
[20]
5. Conclusions [21]
In conclusion, a novel Ni-Cr-Y2O3 nanocomposite coating was prepared by cathode plasma electrolytic deposition from nickeltrivalent chromium sulfate electrolyte directly, with finely dispersed Y2O3 nano-particles. Owing to the effect of plasmas, the coating is dense and uniform with a typically molten morphology, possessing a nanocrystalline structure with an average grain size of about 30e50 nm. The cyclic oxidation test reveals that the deposited coating exhibits good high temperature oxidation and spallation resistance. These are mainly attributed to the synergy effect of the fine-grained structure of the coating and the reactive-element effect of the addition of Y2O3 nano-particles. Acknowledgements
[22]
[23]
[24]
[25]
[26]
[27]
This work was supported by National Natural Science Foundation of China (Grant No. 51801191). The authors would like to thank Chuanhui Liang, Chao Lv and Pan Yang (China Academy of Engineering Physics) for their kindly help.
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