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Synthesis, structure, and properties of the high Nb–TiAl alloy after Ni coatings by plasma surface alloying technique
Journal Pre-proof Synthesis, structure, and properties of the high Nb-TiAl alloy after Ni coatings by plasma surface alloying technique Y.R. Wang, K. Zheng, R.Y. Wang, H.J. Hei, Y.S. Wang, J. Gao, Y.F. Liang, Y. Ma, B. Zhou, S.W. Yu, B. Tang, Y.L. Wang, J.P. Lin, Y.C. Wu PII:
S0042-207X(19)32283-3
DOI:
https://doi.org/10.1016/j.vacuum.2019.109029
Reference:
VAC 109029
To appear in:
Vacuum
Received Date: 9 September 2019 Revised Date:
15 October 2019
Accepted Date: 18 October 2019
Please cite this article as: Wang YR, Zheng K, Wang RY, Hei HJ, Wang YS, Gao J, Liang YF, Ma Y, Zhou B, Yu SW, Tang B, Wang YL, Lin JP, Wu YC, Synthesis, structure, and properties of the high Nb-TiAl alloy after Ni coatings by plasma surface alloying technique, Vacuum, https://doi.org/10.1016/ j.vacuum.2019.109029. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Synthesis, structure, and properties of the high Nb-TiAl alloy after Ni coatings by plasma surface alloying technique Y.R. Wanga, K. Zhenga, R.Y. Wanga, H.J. Heia, Y.S. Wanga*, J. Gaoa, Y.F. Liangb, Y. Maa, B. Zhoua, S.W. Yua*, B. Tanga, Y.L. Wangb, J.P. Linb, Y.C. Wua a
Institute of New Carbon Materials, Taiyuan University of Technology, Taiyuan, 030024, China
b
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, China
Abstract： ：Ni coatings were prepared on the high Nb-TiAl alloy by the plasma surface alloying technique. The results found that surface morphology and microstructure exhibited a strong dependence on the deposited temperature. The elements diffusion during the metalizing stage leaded to the high hardness value and adhesion force between the Ni coating and substrate. The corrosion resistance of the Ni-coated specimens depended on the fabrication temperature related to the deposition and diffusion layers. For the oxidation, the weight gain rate was involved in the chemical reaction in the initial oxidation stage. The inter-diffusion among elements such as the nickel, titanium, and aluminum resulted in the high adhesion force after the oxidation, improving the bonding force between the Ni coating and substrate. Keywords: Ni coating, TiAl, adhesion force
1. Introduction High Nb-TiAl alloys have received increasing attention due to their advantages such as low density, high yield strength and excellent creep resistance [1,2], providing them a promising future in aerospace, auto, and energy industry. However, several shortcomings including weak oxidation and corrosion resistance are still limiting their applications [3-5]. The surface modifications as the important approach have attracted numerous interests from research in laboratory to industry application. Today, many coatings have been explored e.g. ceramic coatings [6-9], multi-component alloyed coatings [10-13], and ceramic composite coatings [14-16]. However, these coatings always display a high residual stress and low adhesion strength caused by the mismatch thermal expansion coefficient, and fabrication process, leading to easily peel off during the service process. Therefore, the novel approach to attain the high-quality coating will be appreciated. Plasma surface alloying technique (PSAT) provides an effective way to improve surface properties through preparing a gradient coating and achieve metallurgical bonding between coating and substrate [17,18]. The technique utilizes a glow discharge phenomenon to sputter the source of the element under ion bombardment and speed to the surface of the cathode (workpiece). The atoms sputtered from the source are adsorbed to the surface of the workpiece at a high temperature using the diffusion process, forming a coating with the gradient composition distribution [17,18]. Up to now, it has been successfully employed to fabricate a variety of coatings including pure metal coatings [19,20], multi-component coatings [21,22],
composite coatings [23,24], etc. on the various substrates such as Ti-6Al-4V, steels, and γ-TiAl alloy, etc., to improve the surface properties of the alloy. For instance, Zhang et al. [21] prepared the Fe-Al-Nb coating by PSAT on the Q235 steel and got the superior surface mechanical property including the hardness and friction performance. Wei et al. [23] successfully prepared an anti-corrosion Ti-Ta alloy coating on pure titanium surface by PSAT, and found that the coating prevented the erosion in strong and high concentrated reducing acid. Cui et al. [22] fabricated the NiCoCrAlY coating on the γ-TiAl substrate by PSAT, and got the improved oxidation resistance due to the formation protective Al2O3 layer. Zhang et al. [19, 20] prepared the Ta coating on the γ-TiAl surface to improve the surface properties of γ-TiAl alloy. Hence, the PSAT offers a promising way to obtain the high-performance coating. Due to the good wear resistance, large plasticity, excellent corrosion and heat resistance [25,26], nickel and its alloys have been researched and applied extensively on corrosion and oxidation resistance, e.g. Ni coating [27, 28, 29], nanocrystalline nickel [30], Ni alloys [31-36] coatings. However, few results refer to fabricate the pure Ni coating on the γ-TiAl based intermetallic. In this paper, the nickel coatings were successfully prepared on the γ-typed Ti-45Al-8.5Nb substrate at different temperature by PSAT. The composition distribution, microstructure, micro-hardness, and adhesion property were investigated. Moreover, the corrosion and oxidation resistance were explored. 2. Experiments
The casting TiAl alloy with a nominal composition of Ti-45Al-8.5Nb-(W, B, Y) was employed as the experiment substrate. All specimens with a dimension of 10 mm×10 mm×5 mm were grounded to a mirror finish with silicon carbide papers. Nickel-plated with a purity of 99.99 % was utilized as the target material to prepare on the substrate by PSAT (LS-450, Self-assembly) at 800 ℃, 900 ℃, 1000 ℃ under the high purity argon atmosphere (purity 99.99 %), which were named as Ni/S.(800℃), Ni/S.(900℃) and Ni/S.(1000℃), respectively. The argon gas pressure is of 4.8 Pa, the source and substrate voltage is of 785 V and 535 V, respectively. The microstructure, surface morphology and elemental distribution of the Ni-coated specimens were characterized by X-ray diffraction (XRD, UltimalV, Topvendor Technology, Japan), scanning electron microscope (SEM, TESCAN MIRA3, TESCAN, Czechia) and its energy-dispersive X-ray spectroscopy (EDS). The adhesion force was tested by a scratch tester (HT-5001, Huijin Tier Plating Technology, China) with a load range from 5 N to 200 N. Loading speed is 80 N/min and scratch speed is 2 mm/min. The micro-hardness was tested with 0.05 kgf and 0.5 kgf load (HM-102, Mitutoyo, Japan). The corrosion experiments were conducted by electrochemical Workstation (CHI660E, Chenhua, China). The high-temperature isothermal oxidation experiments were carried out on box furnace at 900 ℃ for 90 h(SR2X, Kewei, China). Discontinuous weighing method was used to measure the quality of samples at least 5 times every 10 h by the electrical balance with an accuracy of 0.0001 g (ME104E, Mettler Toledo, Switzerland). 3. Results and Discussion
3.1 Morphology and composition Fig.1 shows the morphology and composition of Ni-coated specimens. The surface morphology depended strongly on the fabricated temperature. For the Ni/S. (800 ℃) specimen, the surface nickel particles displayed the spherical-like morphology within the nanometer scale [Fig. 1(a)]. But for the Ni/S. (900 ℃) specimen, the nickel particles aggregated to form the convex particles [Fig. 1(b)]. Especially, the island-like nickel particles with a diameter of ~ 0.5-5 µm formed under 1000 ℃ [Fig. 1(c)]. A probable reason is that Ni atoms released heat of crystallization during the depositing process brought on the negative temperature gradient on the surface of the substrate, leading to the growth as the island-like manner [37]. The surface chemical composition of Ni-coated specimens consisted of Ni, Ti, and Al elements, where the Ni was from the target but the Al and Ti were from the substrate, as the EDS analysis results displayed in Table 1. It indicated that these elements diffused toward each other during the fabricating process. Obviously, these Ni coatings were free from defects such as pores and micro-cracks caused by the metallurgical bonding between the coating to the substrate [Fig. 1(a1) and (b1)]. Moreover, the island growth character can be observed on the surface of Ni coated specimen prepared at 1000 ℃ [Fig. 1(c1)]. During the metalizing process, the element diffusion occurred variously in the interface of Ni coating and substrate due to the difference of element concentration and high-temperature environment [17]. These Ni coatings could be roughly divided into two zones including the deposited Ni-layer and interdiffusion region according to their line-scan
results, [Fig. 1(a2)-(c2)]. The Ti element diffused toward the deposited layer to form the TiNi phase [Fig. 1(a2)], which was identified by the results of XRD pattern in Fig. 2. Under the diffusion region, it is the substrate for the Ni/S. (800 ℃) specimen; but a Ni-poor area and Al-rich area existed for the Ni/S. (900 ℃) specimen [Fig. 1(b2)], which is similar to Yang’s study [38]. It indicates that the Ni-Al phases such as Al3Ni formed in combination with the XRD results. In the pre-sputtering stage, the ion, atom, and particle such as Ti and Al atoms were ejected from the substrate surface due to the bombardment by Ar+, inducing the generation of vacancy and defects in the substrate surface [17]. Those Ti and Al atoms of the internal substrate diffused toward the substrate surface due to the difference of elemental concentration [17,39]. So, several vacancies and defects were created in the vicinity of the surface caused by the diffusion and sputtering process. In the infiltration stage, nickel atoms preferentially occupy these vacancies and defects [37], then diffuse into the sub-surface. With the rising of Ni atoms, the number of vacancies and defects decreased, causing the lattice distortion and/or even solid solution [17,38]. So, the inward diffusion process of Ni element became harder and harder. In the late stage of sputtering, the inward diffusion of Ni atoms continued to be hindered and its adsorption energy was lowered, hence the concentration of Ni atoms on the surface rises as it begun to deposit on the substrate surface [38]. Besides, the rising temperature promoted the sputtering rate of the Ni element, resulting in the increased thickness of the Ni coating. 3.2 Microstructure
Fig. 2 shows the microstructure results from the XRD pattern of the Ni-coated specimens. According to the binary phase diagrams of Al-Ni [39], Ni-Ti [40] and Al-Ti [39], Ni coatings could include pure Ni, NiTi, Ni3Al, Al3Ni, TiAl, Ti3Al phases. For the Ni/S. (800 ℃) specimen, TiAl phase can be detected by X-rays due to the thin Ni coating. With the metalizing temperature increased, the Al3Ni phase appeared and the peak intensity of Ni3Al increased. The formation of Gibbs free energy of these intermetallics is displayed in table 2 [41]. The Al and Ni elements have the large Gibbs free energy such as -135.95 KJ/mol for Ni3Al at 900 °C, and -133.30 KJ/mol for Al3Ni at 900 °C. During the metallizing process, the nickel atoms reacted firstly with the Al atom in the substrate to generate Ni3Al and Al3Ni compounds. However, as the Al atoms in the substrate was consumed, the content of Ti atoms raised relatively, leading to the Ti atom diffused outward which reacted with the Ni atom to form a NiTi phase in the deposited layer and the surface of substrate. So, the Al and Ti elements were detected by EDS line analysis [Fig. 1]. 3.3 Hardness and adhesion property Fig. 3 shows the micro-Vickers hardness of Ni-coated specimen at the load of 0.05 kgf and 0.5 kgf with a dwelling time of 10 s. The micro-hardness of the substrate was ~ 350 HV at both loads. In comparison, the hardness of Ni coatings fabricated at 800 ℃, 900 ℃, and 1000 ℃ were of 551±4 HV at the load of 0.05 kgf. Since these Ni coatings were of 2.5-8.0 µm, the hardness is the coating’s value at a load of 0.05 kgf corresponding to 0.49 N. However, the hardness of Ni/S.(800 ℃) was of ~ 380 HV which was higher than the hardness of the substrate under the load of 0.5 kgf. In
contrast, the hardness of Ni-coated specimens increased to ~ 520 HV as the raised temperature. Because the indenter passed through the Ni coatings and hit the substrate at 0.5 kgf (4.9 N), these values were influenced by the Ni coatings and substrate. Due to the formation of various phases during the metallization process, the intermetallic phases such as Ni3Al compound acted as a dispersion strengthening in the Ni coating [42]. Moreover, the Ni atoms were partially dissolved in the TiAl substrate to function as a solid solution strengthening [42]. So, these Ni coatings had improved the surface hardness of the TiAl substrate. Adhesion force is one of the most important performance indicators for evaluating the quality of coatings. Fig. 4 shows the adhesion force of Ni-coated substrates investigated by the scratch test. The diamond indenter contacted the coating and slipped along the horizontal direction at a speed of 2 mm/min. At the same time, the vertical load gradually increased until the coating completely failed. Clearly, the first intense peak of the specimens prepared at different temperatures appears in 141 N to 151 N, which corresponds to the initial failure of the Ni coating. The load of the initial failure of present results was far larger than the value of other’s reports from techniques of cathode plasma electrolysis deposition[14], magnetron sputtering [43], preparation of micro-arc oxidation film [44], etc., indicating the strong metallurgical bonding between these Ni coatings and the TiAl substrate. Subsequently, various acoustic emission signals mean the generation of cracks and failure process of these coatings under the load of 172 N to186 N. A group of micro-cracks vertical to the sliding direction can be seen on the scratching surface and no obvious peeling off,
indicating the well bonded and good toughness between the Ni coating and the substrate. 3.4 Electrochemical performance Fig. 5 shows the potentiodynamic linear polarization curves of the Ni coating obtained in a 3.5 mol/L NaCl salt solution. The electrochemical parameters about Ni coating were present in table 4. The Ni coatings show the Ecorr values of -115.8 mV, -467.1 mV and -312.2 mV, respectively. The higher the potential indicated the better the corrosion resistance of Ni coating. However, the smaller Icorr value means the materials possessed well corrosion resistance. Here, the Icorr value was 24.27 /uAcm2 for the Ni/S. (800 ℃) specimen, which was two orders of magnitude higher than the value of 0.3911 /uAcm2 for the Ni/S. (900 ℃) specimen and 0.6732 /uAcm2 for the Ni/S. (1000 ℃) specimen. It indicated that the Ni/S. (900 ℃) and Ni/S.(1000 ℃) specimens have a better corrosion resistance than the Ni/S. (800 ℃) sample. The larger the passivation interval corresponds to the better the corrosion resistance of the material. Here, the passivation interval of Ni/S.(900 ℃) and Ni/S.(1000 ℃) was significantly higher than Ni/S.(800 ℃). The thickness of the deposited Ni layer increased (Fig. 1) with the rising fabrication temperature, improving its corrosion resistance. Moreover, since the Al element diffused easily toward the Ni coating at a high diffusion rate at high temperature, the content of Al element on the surface raised (Table 1), promoting its corrosion resistance. The catholic Tafel slope of Ni coating significantly exhibits higher than its anodic Tafel slopes (Table 4), which indicated that the electrochemical corrosion
behaviors of the coating were dominated by the anode reaction. It not facilitated to forming stable anode passivation layers on the surface, so the Ni coating exhibits active dissolution characteristics [45]. On the contrary, the anodic Tafel slope of Ni/S.(900 ℃) and Ni/S.(1000 ℃) was higher than their catholic Tafel slope, which illustrates the electrochemical corrosion behaviors of the coating were dominated by the cathode reaction. 3.5 Oxidation performance The oxidation kinetics curves of the coated high Nb-TiAl alloy are shown in Fig. 6(a). The oxidation weight rate of specimens increased quickly, in the beginning, ~10 h, especially for Ni/S.(800 ℃) due to the chemical reactions in the initial stage (table 5). During the period from 10 h to 90 h, the oxidation mass gain was 0.55 mg•cm-2 for the Ni/S. (800 ℃), 0.34 mg•cm-2 for Ni/S.(900 ℃) and 0.49 mg•cm-2 for Ni/S.(1000 ℃) specimens, respectively. Combined with the XRD patterns of Ni/S. samples [Fig. 6(b)], the peaks of oxides including Al2O3, TiO2 and NiTiO3 were the main phases after oxidation at 900 ℃ for 90 h instead of the peaks of intermetallic compounds. For the Ni/S. (1000 ℃) specimen, the oxide film included more NiO, TiO2, NiTiO3, and Al2O3. In comparison, the intensity of Al2O3 and NiTiO3 peaks enhanced but the intensity NiO peaks lowered. Oxidation morphology and composition distribution of Ni-coated specimens are presented in Fig. 7. The oxide layer of the Ni/S. (800 ℃) specimen displayed a surface character with the uniform nano-particles and nano-holes. This oxide film has a thickness of ~ 6.3 µm. The Ni and Ti elements are concentrated on the outer layer to
form nickel and titanium oxides. The Al-rich layer exists under the Ti-rich and Ni-rich zones, corresponding to the Al2O3 films. It plays a key role block oxygen from air diffusing into the substrate. But for the oxide layer of Ni/S.(900 ℃) specimen, the outermost oxide is mainly Al2O3 with a size of ~ 1 µm and nano-holes, the subsurface mixed with the TiO2, NiO, Al2O3 and NiTiO3. In the case of Ni/S.(1000 ℃) specimen, the dense oxide with the size of ~ 1 µm particles formed. The outermost layer consisted of nickel and titanium oxides, the Al2O3 and TiO2 films formed under the outermost layer to protect the substrate. Due to the interdiffusion of elements during the metallization process, the Al and Ti elements diffused into the Ni coatings to form new phases such as Ni-Ti and Ni-Al. Moreover, these elements continued to diffuse during the oxidation stage at 900 ℃. The chemical equations involved in the entire oxidation process and the formation of Gibbs free energy (∆G) of oxides are shown in table 5 [41]. The smaller the ∆G of oxide formation, the greater the affinity with the oxygen. The oxygen absorbed in the surface of Ni coating preferentially reacted with the Al element to form Al2O3 due to its smallest ∆G. But the critical Al concentration required at least 15 % to form a continuous dense alumina film [46]. Therefore, it hard to form dense Al2O3 because of the low Al content of the Ni/S. (800 ℃) specimen (Table 1), leading to the high oxidative weight gain rate [Fig. 6(a)]. However, the Al concentration of mapping result is of 12.1 % for the Ni/S. (900 ℃) specimen and 25.3 % the Ni/S. (1000 ℃) specimen (Table 1). Moreover, the content of oxygen and Al element had a similar trend for the Ni/S. (900 ℃) and Ni/S.(1000 ℃) specimens [Fig. 7(b) and (c)],
supporting to form the continuous alumina film. Because of the limited Al content and the fast growth rate of NiO on the surface, the NiO would cover the Al2O3 , leading to the decreasing Al content of the EDS result (table 4). Besides, the X-rays are hard to detect Al2O3 on the sub-surface due to the outermost thicken oxide film, resulting in the weaken Al2O3 peak. Besides, the spinel structure NiTiO3 formed because of the reaction of NiO with TiO2 at the high temperature, which slowed down the movement rate of ions in the oxide film. So, the oxidation resistance of the alloy also can be improved [42]. The adhesion force between Ni coatings and the substrate after oxidation had been investigated by a scratch tester [Fig. 8]. The first and last sharp acoustic signal peaks were thought of as the initial and final failure of the Ni coating, which related to the fabrication temperature. The initial and final failure load of the Ni/S. (800 ℃) specimen is of 90 N and 128 N, but for the Ni/S. (1000 ℃) specimen, the loads increased to 150 N and 155 N. These adhesion force values were higher than other’s reports for the resistance oxidation of TiAl alloys [14,39]. Numerous fine cracks vertical to the scratch direction can be seen on the scratching surface [Fig. 8(a1)-(c1)]. In the metalizing pre-sputtering stage of preparing for Ni coating, the argon ions bombard the surface of substrates and then a high concentration vacancy defect layer was introduced, where the defects distributed with a gradient way [30,31]. The defects contribute to element diffusion, leading to the strong adhesion force. 4. Conclusion Ni coatings were prepared on the TiAl alloy with the high Nb content at different
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[45] G. Venkatasubramanian , S. Mideen, A.- K. Jha, Effect of pH on the corrosion behavior of aluminum alloy welded plate in chloride solutions, J. Chem. Sci., 3(2013) 74-80. [46]Y. Sun, J. Hu, Metal corrosion and control, Harbin Institute of Technology Press, China, 2004.
Sample
Ni/S.(800 )
Ni/S.(900 )
Ni/S.(1000 )
Ni K
67.1
62.4
46.1
Table 1. The chemical composition of Ni-coated specimens in Fig. 1 analyzed by EDS maps (at. %).
Ti K
24.6
25.6
28.6
Al K
8.3
12.1
25.3
Phase
Ni/S.(800)
Ni/S.(900)
Ni/S.(1000)
NiTi
-57.21
-55.85
-54.25
Ni3Al
-139.14
-135.96
-132.93
Al3Ni
-134.68
-133.29
-
AlTi
66.36
-64.38
-62.17
Ti3Al
-
-
-88.39
Table 2. The Gibbs free energy of intermetallic compounds (KJ/mol).
Sample
Ecorr (mVvs CE)
Icorr
βc
βα
(uAcm2)
(mV·dec−1 )
(mV·dec−1 )
Ni/S.(800)
-115.8
24.27
73.48
65.96
Ni/S.(900)
-467.1
0.3911
62.29
81.53
Ni/S.(1000)
-312.2
0.6732
53.34
83.17
Table 3. Result of electrochemical parameters for the potentiodynamic polarization test of Ni-coatings in 3.5mol/L NaCl solution.
Sample
Ni/S.(800)
Ni/S.(900)
Ni/S.(1000)
OK
68.39
68.38
51.46
Ni K
9.78
12.17
46.76
Al K
13.7
5.63
0.59
Ti K
7.35
13.82
1.19
Table 4. The chemical composition of Ni-coated specimens in Fig. 7 analyzed by EDS maps (at. %).
Oxides
Chemical equation
△Gf (KJ/mol)
NiO
Ni+1/2O2=NiO
-132.158
TiO2
Ti+O2=TiO2
-727.229
Al2O3
Al+3/2O2=Al2O3
-1262.044
NiTiO
NiO+TiO2=NiTiO
-870.746
3
3
Table 5. The formation Gibbs free energy of oxides.
Fig. 1 The SEM results of Ni-coated specimens: (a) surface morphology, (a1) cross-sectional morphology, and (a2) its corresponding EDS line-scan profile Ni/S.(800
); (b) surface
morphology, (b1) cross-sectional morphology, and (b2) its corresponding EDS line-scan profile Ni/S.(900
); (c) surface morphology, (c1) cross-sectional morphology, and (c2) its
corresponding EDS line-scan profile Ni/S.(1000
).
Fig. 2 XRD patterns of Ni-coated specimens.
Fig. 3 The hardness of Ni-coated specimens tested at 0.05 and 0.5 /kgf.
Fig. 4 Results of scratch tests on the Ni-coated specimens: Acoustic signal-Load curves of (a) Ni/S.(800
), (b) Ni/S.(900
specimens (a1) Ni/S.(800
) and (c) Ni/S.(1000
), (b1) Ni/S.(900
) specimens; Surface scratch of the
) and (c1) Ni/S.(1000
).
Fig. 5 Potentiodynamic polarization curves of Ni-coated specimens.
Fig. 6 (a) Mass gain curves measured of Ni-coated specimens oxidized at 900 its corresponding XRD patterns.
for 90 h, and (b)
Fig.7 The SEM results of Ni-coated specimen after oxidation for 90 h. (a) surface morphology, (a1) cross-sectional morphology of Ni/S.(800
) specimen, and its corresponding elemental map;
(b) surface morphology, (b1) cross-sectional morphology of Ni/S.(900
) specimen, and its
corresponding elemental map; (c) surface morphology, (c1) cross-sectional morphology of Ni/S.(1000
) specimen, and its corresponding elemental map.
Fig. 8 Results of the scratch tests on the Ni-coated samples after oxidation for 90 h. Acoustic signal-Load curves of (a) Ni/S.(800
), (b) Ni/S.(900
Surface scratch of the specimens (a1) Ni/S.(800
) and (c) Ni/S.(1000
), (b1) Ni/S.(900
) specimens.
) and (c1) Ni/S.(1000
).
Highlights: Ni-coatings were prepared on Ti-45Al-8.5Nb substrate by plasma alloying technique. Surface morphology and microstructure exhibited a strong dependence on the deposited temperature. Ni-coatings presented the considerable corrosion resistance. Ni-coatings displayed the good adhesion force before and after oxidation caused by the metallurgical bonding.
Conflict of interest statement Dear editor,
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 service or insititution that could be construed as influencing the position presented in, or the review of, the manuscript entitled Thank you!
Your sincerely,
Yongsheng Wang,
College of Materials Science and Engineering,
Taiyuan University of Technology, Taiyuan 030024, P. R. China
Email: [email protected]
S0042-207X(19)32283-3
DOI:
https://doi.org/10.1016/j.vacuum.2019.109029
Reference:
VAC 109029
To appear in:
Vacuum
Received Date: 9 September 2019 Revised Date:
15 October 2019
Accepted Date: 18 October 2019
Please cite this article as: Wang YR, Zheng K, Wang RY, Hei HJ, Wang YS, Gao J, Liang YF, Ma Y, Zhou B, Yu SW, Tang B, Wang YL, Lin JP, Wu YC, Synthesis, structure, and properties of the high Nb-TiAl alloy after Ni coatings by plasma surface alloying technique, Vacuum, https://doi.org/10.1016/ j.vacuum.2019.109029. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Synthesis, structure, and properties of the high Nb-TiAl alloy after Ni coatings by plasma surface alloying technique Y.R. Wanga, K. Zhenga, R.Y. Wanga, H.J. Heia, Y.S. Wanga*, J. Gaoa, Y.F. Liangb, Y. Maa, B. Zhoua, S.W. Yua*, B. Tanga, Y.L. Wangb, J.P. Linb, Y.C. Wua a
Institute of New Carbon Materials, Taiyuan University of Technology, Taiyuan, 030024, China
b
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, China
Abstract： ：Ni coatings were prepared on the high Nb-TiAl alloy by the plasma surface alloying technique. The results found that surface morphology and microstructure exhibited a strong dependence on the deposited temperature. The elements diffusion during the metalizing stage leaded to the high hardness value and adhesion force between the Ni coating and substrate. The corrosion resistance of the Ni-coated specimens depended on the fabrication temperature related to the deposition and diffusion layers. For the oxidation, the weight gain rate was involved in the chemical reaction in the initial oxidation stage. The inter-diffusion among elements such as the nickel, titanium, and aluminum resulted in the high adhesion force after the oxidation, improving the bonding force between the Ni coating and substrate. Keywords: Ni coating, TiAl, adhesion force
1. Introduction High Nb-TiAl alloys have received increasing attention due to their advantages such as low density, high yield strength and excellent creep resistance [1,2], providing them a promising future in aerospace, auto, and energy industry. However, several shortcomings including weak oxidation and corrosion resistance are still limiting their applications [3-5]. The surface modifications as the important approach have attracted numerous interests from research in laboratory to industry application. Today, many coatings have been explored e.g. ceramic coatings [6-9], multi-component alloyed coatings [10-13], and ceramic composite coatings [14-16]. However, these coatings always display a high residual stress and low adhesion strength caused by the mismatch thermal expansion coefficient, and fabrication process, leading to easily peel off during the service process. Therefore, the novel approach to attain the high-quality coating will be appreciated. Plasma surface alloying technique (PSAT) provides an effective way to improve surface properties through preparing a gradient coating and achieve metallurgical bonding between coating and substrate [17,18]. The technique utilizes a glow discharge phenomenon to sputter the source of the element under ion bombardment and speed to the surface of the cathode (workpiece). The atoms sputtered from the source are adsorbed to the surface of the workpiece at a high temperature using the diffusion process, forming a coating with the gradient composition distribution [17,18]. Up to now, it has been successfully employed to fabricate a variety of coatings including pure metal coatings [19,20], multi-component coatings [21,22],
composite coatings [23,24], etc. on the various substrates such as Ti-6Al-4V, steels, and γ-TiAl alloy, etc., to improve the surface properties of the alloy. For instance, Zhang et al. [21] prepared the Fe-Al-Nb coating by PSAT on the Q235 steel and got the superior surface mechanical property including the hardness and friction performance. Wei et al. [23] successfully prepared an anti-corrosion Ti-Ta alloy coating on pure titanium surface by PSAT, and found that the coating prevented the erosion in strong and high concentrated reducing acid. Cui et al. [22] fabricated the NiCoCrAlY coating on the γ-TiAl substrate by PSAT, and got the improved oxidation resistance due to the formation protective Al2O3 layer. Zhang et al. [19, 20] prepared the Ta coating on the γ-TiAl surface to improve the surface properties of γ-TiAl alloy. Hence, the PSAT offers a promising way to obtain the high-performance coating. Due to the good wear resistance, large plasticity, excellent corrosion and heat resistance [25,26], nickel and its alloys have been researched and applied extensively on corrosion and oxidation resistance, e.g. Ni coating [27, 28, 29], nanocrystalline nickel [30], Ni alloys [31-36] coatings. However, few results refer to fabricate the pure Ni coating on the γ-TiAl based intermetallic. In this paper, the nickel coatings were successfully prepared on the γ-typed Ti-45Al-8.5Nb substrate at different temperature by PSAT. The composition distribution, microstructure, micro-hardness, and adhesion property were investigated. Moreover, the corrosion and oxidation resistance were explored. 2. Experiments
The casting TiAl alloy with a nominal composition of Ti-45Al-8.5Nb-(W, B, Y) was employed as the experiment substrate. All specimens with a dimension of 10 mm×10 mm×5 mm were grounded to a mirror finish with silicon carbide papers. Nickel-plated with a purity of 99.99 % was utilized as the target material to prepare on the substrate by PSAT (LS-450, Self-assembly) at 800 ℃, 900 ℃, 1000 ℃ under the high purity argon atmosphere (purity 99.99 %), which were named as Ni/S.(800℃), Ni/S.(900℃) and Ni/S.(1000℃), respectively. The argon gas pressure is of 4.8 Pa, the source and substrate voltage is of 785 V and 535 V, respectively. The microstructure, surface morphology and elemental distribution of the Ni-coated specimens were characterized by X-ray diffraction (XRD, UltimalV, Topvendor Technology, Japan), scanning electron microscope (SEM, TESCAN MIRA3, TESCAN, Czechia) and its energy-dispersive X-ray spectroscopy (EDS). The adhesion force was tested by a scratch tester (HT-5001, Huijin Tier Plating Technology, China) with a load range from 5 N to 200 N. Loading speed is 80 N/min and scratch speed is 2 mm/min. The micro-hardness was tested with 0.05 kgf and 0.5 kgf load (HM-102, Mitutoyo, Japan). The corrosion experiments were conducted by electrochemical Workstation (CHI660E, Chenhua, China). The high-temperature isothermal oxidation experiments were carried out on box furnace at 900 ℃ for 90 h(SR2X, Kewei, China). Discontinuous weighing method was used to measure the quality of samples at least 5 times every 10 h by the electrical balance with an accuracy of 0.0001 g (ME104E, Mettler Toledo, Switzerland). 3. Results and Discussion
3.1 Morphology and composition Fig.1 shows the morphology and composition of Ni-coated specimens. The surface morphology depended strongly on the fabricated temperature. For the Ni/S. (800 ℃) specimen, the surface nickel particles displayed the spherical-like morphology within the nanometer scale [Fig. 1(a)]. But for the Ni/S. (900 ℃) specimen, the nickel particles aggregated to form the convex particles [Fig. 1(b)]. Especially, the island-like nickel particles with a diameter of ~ 0.5-5 µm formed under 1000 ℃ [Fig. 1(c)]. A probable reason is that Ni atoms released heat of crystallization during the depositing process brought on the negative temperature gradient on the surface of the substrate, leading to the growth as the island-like manner [37]. The surface chemical composition of Ni-coated specimens consisted of Ni, Ti, and Al elements, where the Ni was from the target but the Al and Ti were from the substrate, as the EDS analysis results displayed in Table 1. It indicated that these elements diffused toward each other during the fabricating process. Obviously, these Ni coatings were free from defects such as pores and micro-cracks caused by the metallurgical bonding between the coating to the substrate [Fig. 1(a1) and (b1)]. Moreover, the island growth character can be observed on the surface of Ni coated specimen prepared at 1000 ℃ [Fig. 1(c1)]. During the metalizing process, the element diffusion occurred variously in the interface of Ni coating and substrate due to the difference of element concentration and high-temperature environment [17]. These Ni coatings could be roughly divided into two zones including the deposited Ni-layer and interdiffusion region according to their line-scan
results, [Fig. 1(a2)-(c2)]. The Ti element diffused toward the deposited layer to form the TiNi phase [Fig. 1(a2)], which was identified by the results of XRD pattern in Fig. 2. Under the diffusion region, it is the substrate for the Ni/S. (800 ℃) specimen; but a Ni-poor area and Al-rich area existed for the Ni/S. (900 ℃) specimen [Fig. 1(b2)], which is similar to Yang’s study [38]. It indicates that the Ni-Al phases such as Al3Ni formed in combination with the XRD results. In the pre-sputtering stage, the ion, atom, and particle such as Ti and Al atoms were ejected from the substrate surface due to the bombardment by Ar+, inducing the generation of vacancy and defects in the substrate surface [17]. Those Ti and Al atoms of the internal substrate diffused toward the substrate surface due to the difference of elemental concentration [17,39]. So, several vacancies and defects were created in the vicinity of the surface caused by the diffusion and sputtering process. In the infiltration stage, nickel atoms preferentially occupy these vacancies and defects [37], then diffuse into the sub-surface. With the rising of Ni atoms, the number of vacancies and defects decreased, causing the lattice distortion and/or even solid solution [17,38]. So, the inward diffusion process of Ni element became harder and harder. In the late stage of sputtering, the inward diffusion of Ni atoms continued to be hindered and its adsorption energy was lowered, hence the concentration of Ni atoms on the surface rises as it begun to deposit on the substrate surface [38]. Besides, the rising temperature promoted the sputtering rate of the Ni element, resulting in the increased thickness of the Ni coating. 3.2 Microstructure
Fig. 2 shows the microstructure results from the XRD pattern of the Ni-coated specimens. According to the binary phase diagrams of Al-Ni [39], Ni-Ti [40] and Al-Ti [39], Ni coatings could include pure Ni, NiTi, Ni3Al, Al3Ni, TiAl, Ti3Al phases. For the Ni/S. (800 ℃) specimen, TiAl phase can be detected by X-rays due to the thin Ni coating. With the metalizing temperature increased, the Al3Ni phase appeared and the peak intensity of Ni3Al increased. The formation of Gibbs free energy of these intermetallics is displayed in table 2 [41]. The Al and Ni elements have the large Gibbs free energy such as -135.95 KJ/mol for Ni3Al at 900 °C, and -133.30 KJ/mol for Al3Ni at 900 °C. During the metallizing process, the nickel atoms reacted firstly with the Al atom in the substrate to generate Ni3Al and Al3Ni compounds. However, as the Al atoms in the substrate was consumed, the content of Ti atoms raised relatively, leading to the Ti atom diffused outward which reacted with the Ni atom to form a NiTi phase in the deposited layer and the surface of substrate. So, the Al and Ti elements were detected by EDS line analysis [Fig. 1]. 3.3 Hardness and adhesion property Fig. 3 shows the micro-Vickers hardness of Ni-coated specimen at the load of 0.05 kgf and 0.5 kgf with a dwelling time of 10 s. The micro-hardness of the substrate was ~ 350 HV at both loads. In comparison, the hardness of Ni coatings fabricated at 800 ℃, 900 ℃, and 1000 ℃ were of 551±4 HV at the load of 0.05 kgf. Since these Ni coatings were of 2.5-8.0 µm, the hardness is the coating’s value at a load of 0.05 kgf corresponding to 0.49 N. However, the hardness of Ni/S.(800 ℃) was of ~ 380 HV which was higher than the hardness of the substrate under the load of 0.5 kgf. In
contrast, the hardness of Ni-coated specimens increased to ~ 520 HV as the raised temperature. Because the indenter passed through the Ni coatings and hit the substrate at 0.5 kgf (4.9 N), these values were influenced by the Ni coatings and substrate. Due to the formation of various phases during the metallization process, the intermetallic phases such as Ni3Al compound acted as a dispersion strengthening in the Ni coating [42]. Moreover, the Ni atoms were partially dissolved in the TiAl substrate to function as a solid solution strengthening [42]. So, these Ni coatings had improved the surface hardness of the TiAl substrate. Adhesion force is one of the most important performance indicators for evaluating the quality of coatings. Fig. 4 shows the adhesion force of Ni-coated substrates investigated by the scratch test. The diamond indenter contacted the coating and slipped along the horizontal direction at a speed of 2 mm/min. At the same time, the vertical load gradually increased until the coating completely failed. Clearly, the first intense peak of the specimens prepared at different temperatures appears in 141 N to 151 N, which corresponds to the initial failure of the Ni coating. The load of the initial failure of present results was far larger than the value of other’s reports from techniques of cathode plasma electrolysis deposition[14], magnetron sputtering [43], preparation of micro-arc oxidation film [44], etc., indicating the strong metallurgical bonding between these Ni coatings and the TiAl substrate. Subsequently, various acoustic emission signals mean the generation of cracks and failure process of these coatings under the load of 172 N to186 N. A group of micro-cracks vertical to the sliding direction can be seen on the scratching surface and no obvious peeling off,
indicating the well bonded and good toughness between the Ni coating and the substrate. 3.4 Electrochemical performance Fig. 5 shows the potentiodynamic linear polarization curves of the Ni coating obtained in a 3.5 mol/L NaCl salt solution. The electrochemical parameters about Ni coating were present in table 4. The Ni coatings show the Ecorr values of -115.8 mV, -467.1 mV and -312.2 mV, respectively. The higher the potential indicated the better the corrosion resistance of Ni coating. However, the smaller Icorr value means the materials possessed well corrosion resistance. Here, the Icorr value was 24.27 /uAcm2 for the Ni/S. (800 ℃) specimen, which was two orders of magnitude higher than the value of 0.3911 /uAcm2 for the Ni/S. (900 ℃) specimen and 0.6732 /uAcm2 for the Ni/S. (1000 ℃) specimen. It indicated that the Ni/S. (900 ℃) and Ni/S.(1000 ℃) specimens have a better corrosion resistance than the Ni/S. (800 ℃) sample. The larger the passivation interval corresponds to the better the corrosion resistance of the material. Here, the passivation interval of Ni/S.(900 ℃) and Ni/S.(1000 ℃) was significantly higher than Ni/S.(800 ℃). The thickness of the deposited Ni layer increased (Fig. 1) with the rising fabrication temperature, improving its corrosion resistance. Moreover, since the Al element diffused easily toward the Ni coating at a high diffusion rate at high temperature, the content of Al element on the surface raised (Table 1), promoting its corrosion resistance. The catholic Tafel slope of Ni coating significantly exhibits higher than its anodic Tafel slopes (Table 4), which indicated that the electrochemical corrosion
behaviors of the coating were dominated by the anode reaction. It not facilitated to forming stable anode passivation layers on the surface, so the Ni coating exhibits active dissolution characteristics [45]. On the contrary, the anodic Tafel slope of Ni/S.(900 ℃) and Ni/S.(1000 ℃) was higher than their catholic Tafel slope, which illustrates the electrochemical corrosion behaviors of the coating were dominated by the cathode reaction. 3.5 Oxidation performance The oxidation kinetics curves of the coated high Nb-TiAl alloy are shown in Fig. 6(a). The oxidation weight rate of specimens increased quickly, in the beginning, ~10 h, especially for Ni/S.(800 ℃) due to the chemical reactions in the initial stage (table 5). During the period from 10 h to 90 h, the oxidation mass gain was 0.55 mg•cm-2 for the Ni/S. (800 ℃), 0.34 mg•cm-2 for Ni/S.(900 ℃) and 0.49 mg•cm-2 for Ni/S.(1000 ℃) specimens, respectively. Combined with the XRD patterns of Ni/S. samples [Fig. 6(b)], the peaks of oxides including Al2O3, TiO2 and NiTiO3 were the main phases after oxidation at 900 ℃ for 90 h instead of the peaks of intermetallic compounds. For the Ni/S. (1000 ℃) specimen, the oxide film included more NiO, TiO2, NiTiO3, and Al2O3. In comparison, the intensity of Al2O3 and NiTiO3 peaks enhanced but the intensity NiO peaks lowered. Oxidation morphology and composition distribution of Ni-coated specimens are presented in Fig. 7. The oxide layer of the Ni/S. (800 ℃) specimen displayed a surface character with the uniform nano-particles and nano-holes. This oxide film has a thickness of ~ 6.3 µm. The Ni and Ti elements are concentrated on the outer layer to
form nickel and titanium oxides. The Al-rich layer exists under the Ti-rich and Ni-rich zones, corresponding to the Al2O3 films. It plays a key role block oxygen from air diffusing into the substrate. But for the oxide layer of Ni/S.(900 ℃) specimen, the outermost oxide is mainly Al2O3 with a size of ~ 1 µm and nano-holes, the subsurface mixed with the TiO2, NiO, Al2O3 and NiTiO3. In the case of Ni/S.(1000 ℃) specimen, the dense oxide with the size of ~ 1 µm particles formed. The outermost layer consisted of nickel and titanium oxides, the Al2O3 and TiO2 films formed under the outermost layer to protect the substrate. Due to the interdiffusion of elements during the metallization process, the Al and Ti elements diffused into the Ni coatings to form new phases such as Ni-Ti and Ni-Al. Moreover, these elements continued to diffuse during the oxidation stage at 900 ℃. The chemical equations involved in the entire oxidation process and the formation of Gibbs free energy (∆G) of oxides are shown in table 5 [41]. The smaller the ∆G of oxide formation, the greater the affinity with the oxygen. The oxygen absorbed in the surface of Ni coating preferentially reacted with the Al element to form Al2O3 due to its smallest ∆G. But the critical Al concentration required at least 15 % to form a continuous dense alumina film [46]. Therefore, it hard to form dense Al2O3 because of the low Al content of the Ni/S. (800 ℃) specimen (Table 1), leading to the high oxidative weight gain rate [Fig. 6(a)]. However, the Al concentration of mapping result is of 12.1 % for the Ni/S. (900 ℃) specimen and 25.3 % the Ni/S. (1000 ℃) specimen (Table 1). Moreover, the content of oxygen and Al element had a similar trend for the Ni/S. (900 ℃) and Ni/S.(1000 ℃) specimens [Fig. 7(b) and (c)],
supporting to form the continuous alumina film. Because of the limited Al content and the fast growth rate of NiO on the surface, the NiO would cover the Al2O3 , leading to the decreasing Al content of the EDS result (table 4). Besides, the X-rays are hard to detect Al2O3 on the sub-surface due to the outermost thicken oxide film, resulting in the weaken Al2O3 peak. Besides, the spinel structure NiTiO3 formed because of the reaction of NiO with TiO2 at the high temperature, which slowed down the movement rate of ions in the oxide film. So, the oxidation resistance of the alloy also can be improved [42]. The adhesion force between Ni coatings and the substrate after oxidation had been investigated by a scratch tester [Fig. 8]. The first and last sharp acoustic signal peaks were thought of as the initial and final failure of the Ni coating, which related to the fabrication temperature. The initial and final failure load of the Ni/S. (800 ℃) specimen is of 90 N and 128 N, but for the Ni/S. (1000 ℃) specimen, the loads increased to 150 N and 155 N. These adhesion force values were higher than other’s reports for the resistance oxidation of TiAl alloys [14,39]. Numerous fine cracks vertical to the scratch direction can be seen on the scratching surface [Fig. 8(a1)-(c1)]. In the metalizing pre-sputtering stage of preparing for Ni coating, the argon ions bombard the surface of substrates and then a high concentration vacancy defect layer was introduced, where the defects distributed with a gradient way [30,31]. The defects contribute to element diffusion, leading to the strong adhesion force. 4. Conclusion Ni coatings were prepared on the TiAl alloy with the high Nb content at different
temperatures by PSAT. These Ni coatings are free from defects such as pores and micro-cracks due to the metallurgical bonding of this technique. The surface morphology of Ni coatings depended on the sputtering temperature. Elements diffusion during the metalizing stage played a key role in the increased hardness and adhesion force. The corrosion resistance of Ni/S.(900 ℃) and Ni/S.(1000 ℃) specimens dominated by the cathode reaction is better than that of the Ni/S. (800 ℃) in the 3.5 mol/L NaCl solution. For the oxidation at 900 ℃, the weight gain rate was determined by the chemical reaction to the oxidation process. During the oxidation stage, the inter-diffusion of the main elements leads to the high adhesion force after oxidation. Acknowledgments The authors acknowledge all financial supports for this research including the State Key Lab of Advanced Metals and Materials from University of Science and Technology Beijing (Grant No. 2019-ZD02, startup fund), National Natural Science Foundation of China (Grant No. 51071135 and 51601124), Science and Technology Major Project of Shanxi (Grant No. 20181102013), and “1331 Project” Engineering Research Center of Shanxi (Grant No. PT201801). References [1] M. Yamaguchi, H. Inui, K. Ito, High-temperature structural intermetallics, Acta Mater., 48 (2000) 307-322. [2] Y.-W. Kim, Intermetallic alloys based on gamma titanium aluminize, JOM, 41(1989) 24-30.
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Sample
Ni/S.(800 )
Ni/S.(900 )
Ni/S.(1000 )
Ni K
67.1
62.4
46.1
Table 1. The chemical composition of Ni-coated specimens in Fig. 1 analyzed by EDS maps (at. %).
Ti K
24.6
25.6
28.6
Al K
8.3
12.1
25.3
Phase
Ni/S.(800)
Ni/S.(900)
Ni/S.(1000)
NiTi
-57.21
-55.85
-54.25
Ni3Al
-139.14
-135.96
-132.93
Al3Ni
-134.68
-133.29
-
AlTi
66.36
-64.38
-62.17
Ti3Al
-
-
-88.39
Table 2. The Gibbs free energy of intermetallic compounds (KJ/mol).
Sample
Ecorr (mVvs CE)
Icorr
βc
βα
(uAcm2)
(mV·dec−1 )
(mV·dec−1 )
Ni/S.(800)
-115.8
24.27
73.48
65.96
Ni/S.(900)
-467.1
0.3911
62.29
81.53
Ni/S.(1000)
-312.2
0.6732
53.34
83.17
Table 3. Result of electrochemical parameters for the potentiodynamic polarization test of Ni-coatings in 3.5mol/L NaCl solution.
Sample
Ni/S.(800)
Ni/S.(900)
Ni/S.(1000)
OK
68.39
68.38
51.46
Ni K
9.78
12.17
46.76
Al K
13.7
5.63
0.59
Ti K
7.35
13.82
1.19
Table 4. The chemical composition of Ni-coated specimens in Fig. 7 analyzed by EDS maps (at. %).
Oxides
Chemical equation
△Gf (KJ/mol)
NiO
Ni+1/2O2=NiO
-132.158
TiO2
Ti+O2=TiO2
-727.229
Al2O3
Al+3/2O2=Al2O3
-1262.044
NiTiO
NiO+TiO2=NiTiO
-870.746
3
3
Table 5. The formation Gibbs free energy of oxides.
Fig. 1 The SEM results of Ni-coated specimens: (a) surface morphology, (a1) cross-sectional morphology, and (a2) its corresponding EDS line-scan profile Ni/S.(800
); (b) surface
morphology, (b1) cross-sectional morphology, and (b2) its corresponding EDS line-scan profile Ni/S.(900
); (c) surface morphology, (c1) cross-sectional morphology, and (c2) its
corresponding EDS line-scan profile Ni/S.(1000
).
Fig. 2 XRD patterns of Ni-coated specimens.
Fig. 3 The hardness of Ni-coated specimens tested at 0.05 and 0.5 /kgf.
Fig. 4 Results of scratch tests on the Ni-coated specimens: Acoustic signal-Load curves of (a) Ni/S.(800
), (b) Ni/S.(900
specimens (a1) Ni/S.(800
) and (c) Ni/S.(1000
), (b1) Ni/S.(900
) specimens; Surface scratch of the
) and (c1) Ni/S.(1000
).
Fig. 5 Potentiodynamic polarization curves of Ni-coated specimens.
Fig. 6 (a) Mass gain curves measured of Ni-coated specimens oxidized at 900 its corresponding XRD patterns.
for 90 h, and (b)
Fig.7 The SEM results of Ni-coated specimen after oxidation for 90 h. (a) surface morphology, (a1) cross-sectional morphology of Ni/S.(800
) specimen, and its corresponding elemental map;
(b) surface morphology, (b1) cross-sectional morphology of Ni/S.(900
) specimen, and its
corresponding elemental map; (c) surface morphology, (c1) cross-sectional morphology of Ni/S.(1000
) specimen, and its corresponding elemental map.
Fig. 8 Results of the scratch tests on the Ni-coated samples after oxidation for 90 h. Acoustic signal-Load curves of (a) Ni/S.(800
), (b) Ni/S.(900
Surface scratch of the specimens (a1) Ni/S.(800
) and (c) Ni/S.(1000
), (b1) Ni/S.(900
) specimens.
) and (c1) Ni/S.(1000
).
Highlights: Ni-coatings were prepared on Ti-45Al-8.5Nb substrate by plasma alloying technique. Surface morphology and microstructure exhibited a strong dependence on the deposited temperature. Ni-coatings presented the considerable corrosion resistance. Ni-coatings displayed the good adhesion force before and after oxidation caused by the metallurgical bonding.
Conflict of interest statement Dear editor,
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 service or insititution that could be construed as influencing the position presented in, or the review of, the manuscript entitled Thank you!
Your sincerely,
Yongsheng Wang,
College of Materials Science and Engineering,
Taiyuan University of Technology, Taiyuan 030024, P. R. China
Email: [email protected]