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MoO3 composite coatings over wide temperature range
Journal Pre-proof Effect of MoO3 on the microstructure and tribological properties of laser-clad Ni60/nanoCu/h-BN/MoO3 composite coatings over wide temperature range
Yue Zhao, Kai Feng, Chengwu Yao, Zhuguo Li PII:
S0257-8972(20)30146-8
DOI:
https://doi.org/10.1016/j.surfcoat.2020.125477
Reference:
SCT 125477
To appear in:
Surface & Coatings Technology
Received date:
8 December 2019
Revised date:
29 January 2020
Accepted date:
16 February 2020
Please cite this article as: Y. Zhao, K. Feng, C. Yao, et al., Effect of MoO3 on the microstructure and tribological properties of laser-clad Ni60/nanoCu/h-BN/MoO3 composite coatings over wide temperature range, Surface & Coatings Technology (2020), https://doi.org/10.1016/j.surfcoat.2020.125477
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© 2020 Published by Elsevier.
Journal Pre-proof
Title Page Effect of MoO3 on the microstructure and tribological properties of laser-clad Ni60/nanoCu/h-BN/MoO3 composite coatings over wide temperature range Yue Zhao a,b, Kai Feng a,b, Chengwu Yao a,b, Zhuguo Li a,b*
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a Shanghai Key laboratory of materials Laser Processing and Modification, School of materials Science and engineering, Shanghai Jiao Tong University, Shanghai 200240, China b Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai, 200240, China
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Yue Zhao, first author, https://orcid.org/0000-0002-6728-5149, Email: [email protected] Kai Feng, https://orcid.org/0000-0001-8198-1071, Email: [email protected]
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Chengwu Yao, https://orcid.org/0000-0002-3407-8717, Email: [email protected]
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Zhuguo Li, corresponding author, https://orcid.org/0000-0002-1311-0044 Email: [email protected]
*Corresponding author: Prof. Zhuguo Li E-mail address: [email protected] Postal address: E-302A, School of Materials Science and Engineering Shanghai Jiao Tong University No.800 Dongchuan Road Shanghai, China, 200240
Journal Pre-proof
Effect of MoO3 on the microstructure and tribological properties of laser-clad Ni60/nanoCu/h-BN/MoO3 composite coating over a wide temperature range Yue Zhao a,b, Kai Feng a,b, Chengwu Yao a,b, Zhuguo Li a,b* a
Shanghai Key laboratory of materials Laser Processing and Modification, School of materials Science and engineering, Shanghai Jiao
b
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Tong University, Shanghai 200240, China
Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai, 200240, China
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Abstract
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The samples of nickel-base self-lubricating composite (Ni60/nano-Cu/h-BN)
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(NSLC) with different additive amounts of MoO3 were manufactured by laser
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cladding on substrates of Q235 steel. The microstructure and hardness of the as-fabricated NSLCs were investigated. The wear and tribological behaviors of the
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NSLCs with and without MoO3 from 25°C to 800°C were discussed. The results revealed that the addition of MoO3 resulted in the hardness improvement of the
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NSLCs due to the formation of Mo2C and Cr (Mo)-rich reinforcements during laser cladding process. The NSLC with 4 wt. % MoO3 addition showed the best wear performance from 25°C to 800°C. At 600°C and 800°C, MoO3 solid lubricant was regenerated by oxide reaction during the wear process. Meanwhile, CuMoO4 was generated owing to a tribo-chemical reaction at high-temperature. These solid lubricants formed a lubricating transfer film, which improved the tribological properties of the coating. Keywords: laser cladding; self-lubricating composite; wide temperature range; tribological properties
1. Introduction 1
Journal Pre-proof High temperature components (engine bearings, transmissions, and turbine sealing, etc.) need to operate in the frequently changing temperatures. The high temperature environment significantly affects the mechanical properties
[2]
of the components, even cause the fatigue cracks
[3]
[1]
and friction
. It is crucial to
prepare a composite coating on components to ensure the wear properties from 25°C to high temperature. Laser cladding is a highly efficient method for composite coating fabrication on the components forming metallurgical bonding to the substrate [4].
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Nickel-based superalloy (Ni60) is extensively applied to enhance the wear [5]
. It shows excellent
performance such as oxidation resistance, corrosion resistance
[6]
and wear resistance
. However, the friction coefficient of Ni60 is high
[8]
. Therefore, more lubricating
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[7]
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resistance of the composite over a wide range of temperature
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phases are needed to improve the lubricating ability of the composite coating. Solid lubricants are sensitive to temperature, and a single solid lubricant can only
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depress the friction coefficient effectively over a certain narrow range of wear
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temperature [9]. In order to ensure the excellent lubricating property over a wide range of temperature, some researchers have proposed the concept of "adaptive" high-temperature composite lubrication
[10-12]
. Excellent adaptive-lubrication property
of Molybdenum has been found in the temperature range of 20-700 °C in the forms like Mo-Cs, Mo-Ag, Mo-Pb, and Mo-Cu molybdates
[13]
. In this adaptive-lubrication
system, soft metals in terms of Cs, Pb, Cu, or Ag were utilized as the lubricant at low temperatures. While at high temperatures, molybdenum and soft metals are oxidized into lamellar MoO3 and metal oxides. They suffer tribo-chemical reaction forming the molybdate, that reduces the friction coefficient effectively. Heng Tao et al.
2
[14]
found
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that the addition of Mo in CrAlSiN nanocomposite coating improved its high-temperature anti-wear and lubricating properties. During the wear process, molybdenum was oxidized to MoO3 at 600 °C, which enhanced the tribological property effectively. Lu Yuchu et al.
[15]
found that because of the formation of
molybdenum oxide, the friction coefficients of Cr-Mo-Si-N nanocomposites at 750 °C
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and 800 °C were effectively reduced. Li Fei et al. [16] studied the friction mechanisms of SiC-Mo-CaF2 composite from 25 °C-1000 °C. They discovered that the chemical
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production of CaF2 and CaMoO4 improved the lubricating property at 25°C and
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800°C-1000°C, respectively. The lubricating ability of the Cu-Mo system at a wide
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range of temperature has seldom been investigated. Molybdenum is mainly added in
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the form of metallic Mo and MoS2 in the composites. [17]
, the composite of nanoCu/h-BN solid
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According to our previous study
lubricant enhanced the friction property of the nickel-based composite coating from
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25 °C-500 °C. However, the wear loss at 600 °C was high. MoO3 can provide molybdenum in high temperature tribological reactions. Therefore, MoO3 was added to the nickel-based composite to improve the friction and wear properties of the composite at higher temperature (for example 600°C and 800 °C). In this study, Ni60/nano-Cu/h-BN composites with different additive amounts of MoO3 were fabricated by laser cladding. H-BN is prone to spattering during laser processing, which significantly decreases the wear properties of the coating
[17]
.
Therefore, nano-Cu was cladded on h-BN to improve the wettability between h-BN and Ni-matrix, which in turn, enhanced the proportion of h-BN in the coating. Cu-Mo 3
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molybdate was expected to provide excellent lubricating property at high temperature. The friction behaviors of the composites from 25 °C to 800 °C were investigated. The effect of copper and molybdenum elements at different wear temperatures were investigated.
2.
Experimental procedures
2.1 Materials self-lubricating
composites
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Ni60/nanoCu/h-BN/MoO3
(NSLC)
were
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manufactured on a substrate of Q235 steel by laser cladding. NSLC without MoO3 is
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denoted as S1; NSLCs with 2 %, 4 %, and 6 wt. % MoO3 are denoted as S2, S3, and S4, respectively. The nominal composition of Ni60 in weight percent is
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67.3Ni-16Cr-3.3B-4.5Si-0.9C-8Fe. High energy ball milling was used to coat the
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nano-Cu on h-BN. Then mixed the powder with varying amounts of MoO3 and Ni60 powder. The compositions and proportions of S1, S2, S3, and S4 are compared in
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Table 1.
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Table 1. The morphologies of the prepared powder were presented in Fig.1.
Composition (wt. %) of NSLCs without and with 2%, 4%, and 6% MoO3 No.
MoO3
nanoCu/h-BN (4:1)
Ni60
—
6.25
93.75
2
6.25
91.75
S3
4
6.25
89.75
S4
6
6.25
87.75
S1 S2
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Fig.1. Morphology of (a) Ni60, (b) h-BN, (c) nano-Cu coated h-BN, (d) MoO3; and (e)
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overall and (f) local enlarged detailed morphology of nano-Cu/h-BN mixed MoO3
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2.2 Laser cladding process
The raw powder was pre-coated on the substrates and dried for 2-3h at
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90-100 °C in a vacuum oven before laser cladding. The laser cladding process was
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conducted by a diode-laser unit (Laserline LDF-8000, Germany). The dimension of the focused laser beam is 19 mm×6 mm. The process parameters were used with
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scanning speed of 2-3 mm/s and laser power of 3-4 kW, respectively
[18]
. The
continuous argon gas flow was used to shield the molten pool. 2.3 Microstructure analysis
The phases in the as-deposited samples were determined by an X-ray diffraction (XRD) spectrometer (6100, Shimadzu). The microstructures of the as-deposited samples were characterized by scanning electron microscope (SEM, JEM7800, JEOL) equipped with EDS energy dispersive spectroscopy. 2.4 Tribological test The micro-hardness of the as-deposited coatings was tested by Zwick/Roell ZHμ Vickers micro-hardness detector with a load of 0.5 kg and dwelling time of 14 s. 5
Journal Pre-proof High-temperature dry sliding friction experiments were conducted on a ball-on-disk equipment (UMT-2, Bruker CETR). The friction pairs were Al2O3 balls (diameter of 5 mm) and the as-fabricated coating specimens. The roughness of the samples was polished to 0.05μm before the wear tests. The wear test was conducted with a load of 30N, a wear radius of 50mm and a rotation speed of 50rpm. Each sample (S1-S4) was tested at 25°C, 200°C, 400°C, 600°C, and 800°C for 30 min. The results of the friction coefficient were repeated for 3 times and average friction coefficient was
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calculated. A laser confocal microscope (OLS4000, Olympus) was applied to analyze
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the wear loss. The measurements of wear rate were calculated using the formula
𝛥𝑣 2𝜋𝑅𝑆 = 𝐹𝐿 𝐹𝐿
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𝜂=
-p
below.
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Where η is wear rate; Δv is loss of the volume; L is sliding distance; F is load; R is the radius of the worn scar and S is the cross-section area of the scuffed scar. Every
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experiment was repeated for 3 times and average wear rate was calculated.
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The worn surfaces of the NBLCs were analyzed by SEM. The oxide solid lubricants were detected by X-ray photoelectron spectroscopy XPS (AXIS UltraDLD) and Dispersive Raman Microscope (RAMAN, Senterra R200-L, Bruker Optics).
3. Results and discussion 3.1 Microstructures
As exhibited in Fig.2 (a-d), the as-deposited coatings show an excellent metallurgical bonding to substrates without defects such as pores and cracks. With the increase of MoO3 addition from 0 % to 4 %, the thicknesses of the as-deposited coatings increase from 1.2 to 2.6 mm. When the addition amount of MoO3 increases to 6 %, the thickness of the composite coating is further increased. The mechanism of the increasing of the thickness with the addition of MoO3 has been discussed in 6
Journal Pre-proof another investigation
[20]
. The dilution rates of S1, S2, S3 and S4 are 1.41%, 0.58%,
0.34% and 0.41% respectively, which are calculated by image-pro software. As demonstrated in Fig.2 (e), with the addition of MoO3, S2, S3, and S4 are mainly composed of γ-(Ni, M), FeNi3, h-BN, M23C6, Ni2Si, CrB and Mo2C. The same phases are also detected in S1 with the exception of Mo2C. MoO3 is not detected in the XRD results of S2, S3, and S4. It can be speculated that the reaction between carbon and MoO3 occurs only under the high heat during laser irradiation [21].
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The microstructure and phase distribution of S2, S3 and S4 are similar. The
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microstructure and EDS result of S3 are exemplified to investigate the phases after
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adding MoO3 into NSLC (Fig.3). The bulk phase in Fig. 3 (a) is rich in chromium and [22]
indicates that the solid
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molybdenum. The binary phase diagram of Cr-Mo
solubility of chromium and molybdenum is high. Molybdenum is easily dissolved
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into the bulk Cr-rich reinforcement by solid solution effect. It can be deduced that the
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added MoO3 is reacted with carbon to form the molybdenum and carbon dioxide under the high heat of laser irradiation. The carbon dioxide is overflowed from the
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molten pool during the fabricating process
[2]
. Then some part of molybdenum react
with carbon forming the Mo2C reinforcements, and another part of molybdenum dissolve into Cr-rich reinforcements to form solid solution. Molybdenum, carbon dioxide and Mo2C are formed by the reactions below [23]: 2 2
=2
(1)
=
(2)
The amount of MoO3 that participated in the reaction is determined by the content of carbon in the molten pool. The content of carbon in raw powder is limited. When 6% MoO3 was added, the excessive MoO3 which did not participate in the reduction reaction and then evaporated under the high heat of laser irradiation [23]. It is 7
Journal Pre-proof why the thickness of the coating increased from 2.3 mm to 2.6 mm but do not further increase with higher additive amount of MoO3. According to our previous research [17], it was proven that the black dots which enrich of nitrogen element are h-BN solid lubricant. The distribution of nickel and copper indicate that copper dissolve into the γ- (Ni, M) matrix, forming solid solution. Fig. 4 shows the microstructure and distribution of the Cr-rich reinforcements in S1, S2, S3, and S4 after 12 hours etching. The shape of the Cr-rich reinforcements in
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S1 are long strip. While the Cr-rich reinforcements in S2, S3 and S4 are in the shape
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of bulk. The volume fractions of Cr-rich reinforcements in S1, S2, S3, and S4 are
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calculated as 13.8%±0.5, 18.5%±0.6, 21.9%±0.5, and 20.3%±0.4, respectively,
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by Image-Pro software. It is indicated that the reactions of MoO3 during laser
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cladding process change the shape of the Cr-rich reinforcements and enhance the
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content of reinforcements in the coating.
Fig. 2. (a-d) Cross section morphologies of S1 to S4; (e) XRD patterns of S1 to S4
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Fig. 3. Microstructure and EDS mapping results of NSLC with 4 wt. % MoO3 (S3)
Fig. 4. Microstructures after 12 h etching with different content of MoO3 3.2 Micro-hardness
As Fig. 5 exhibits, the average hardness of S1 is approximately 503 HV. The average hardness of S2, S3, and S4 are 602 HV, 660 HV, and 650 HV, respectively, which are all higher than that of S1. With the increase of MoO3 addition, the hardness of S1 to S4 is firstly increased and then decreased slightly. S3 with 4% addition of MoO3 shows the highest hardness (660HV). As early mentioned, the addition of MoO3 increases the content of Cr-rich reinforcements in the NSLCs (S2, S3 and S4) and causes the dispersion strengthening 9
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[19]
. In the meantime, the dissolution of molybdenum element into
Cr-rich reinforcement changes the shape of reinforcements and results in the solid solution strengthening. Thus, the addition of MoO3 heightens the hardness of the NSLCs. S3 with 4% MoO3 has the highest content of reinforcement (21.9%) and therefore it has the highest dispersion strengthening of the coating. When the addition amount of MoO3 increases to 6%, the evaporation of excess MoO3 reduces the content of the reinforcements and weakens the dispersion strengthening. Therefore, the
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hardness decreases in S4.
Fig. 5. The average hardness with different addition of MoO3 3.3 Tribological properties at different temperatures The wear experiments were conducted from 25°C to 800°C to evaluate the effect of MoO3 on the tribological properties of the coating. It can be noticed from Fig.6 (a) that with the increase of MoO3, the friction coefficients are firstly reduced and then increased, and 4wt. % MoO3 (S3) has the lowest friction coefficient. The difference in friction coefficient between S2, S3 and S4 is small. With the raising of temperature, the friction coefficient of the as-deposited NSLCs reduces gradually. It reaches the lowest coefficient at 600 °C, then increases slightly when the temperature further 10
Journal Pre-proof raises up to 800°C. As demonstrated in Fig.6 (b), with the addition of MoO3, the wear rates of S2(2%), S3(4%) and S4(6%) are lower than that of S1(0%). The wear rates firstly decrease and then increase with the amount increase of MoO3. S3(4%) has the lowest wear rate among the as-deposited coatings. This specific phenomenon is more obvious at high
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wear temperatures 600°C and 800°C.
Fig. 6. Friction coefficients (a) and wear rates (b) of S1, S2, S3 and S4 at 25°C,
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200°C, 400°C, 600°C, and 800°C According to our previous research
[22]
. The addition of MoO3 can increase the
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amount of h-BN. H-BN is a refractory phase with poor wettability and high melting point (3000 °C), it is easy to spatter during the laser cladding process. MoO3 is a low melting point oxide (790 °C). It firstly melted and wrapped on the refractory h-BN powder. It reduced the spattering of h-BN powder during the laser cladding process. Therefore, the amount of h-BN in S2, S3 and S4 is higher than that in S1. Thus, the protection effect of MoO3 (in S2, S3 and S4) on h-BN reduces the friction coefficient at 25°C-400°C. At 600°C and 800°C, the high temperature oxide solid lubricants MoO3, CuMoO4, CuO, NiO (observed by XPS and Raman spectrum) formed a dense lubricating transfer film in the wear scar, which decreases the friction coefficients of the NSLCs. Solid lubricants MoO3 and CuMoO4 are not detected in S1. Therefore, the 11
Journal Pre-proof friction coefficient of S1 is greater than S2, S3 and S4. At lower temperature, no obvious softening phenomenon occurs at these temperatures
[17]
. Therefore, the difference between the wear rate of the as-deposited
NSLCs is not large. The high temperature softening phenomenon becomes serious at 600°C and 800°C. It reduces the strength of the coatings
[24]
and causes considerable
adhesion scuffed. Thus, the wear rates enhance significantly. The addition of MoO3 caused the formation of Cr(Mo)-rich reinforcement, which enhances the hardness and [25]
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S4 are much lower than that of S1.
. Therefore, the wear rates of S2, S3 and
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deformation resistance of S2, S3 and S4
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With the increase of MoO3 addition from 0wt.% to 4wt.%, more Mo elements
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are dissolved in Cr-rich reinforcements or formed the Mo2C reinforcement, which increases the hardness. However, due to the limited content of Cr and C elements in
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Ni60, when the addition amount of MoO3 reached 6wt.%, the excess MoO3 cannot
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further formed the reinforcement to increase the hardness and wear properties. Therefore, S3 with the addition amount of 4wt. % MoO3 has the best friction and
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wear performance.
3.4 Effect of MoO3 on the friction behavior of the NSLCs
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Fig. 7. Morphology of the worn surfaces of (a1)-(d1) S1 and (a2)-(d2) S3 at 25°C, 400°C, 600°C
Table 2.
and 800°C
EDS results of positions A-H in Fig.7 Position
N
C
O
Si
Cr
Ni
Fe
Mo
Cu
A
3.12
16.89
0.08
3.47
7.18
58.12
9.15
-
1.99
B
16.19
17.17
0.17
2.59
6.42
45.85
8.37
0.78
2.47
C
1.83
14.98
15.71
2.16
5.63
39.76
13.94
5.99
-
D
15.25
7.29
10.84
3.5
6.37
35.17
10.02
9.8
1.76
E
3.11
10.52
25.68
3.12
3.26
40.08
11.24
-
2.99
F
2.56
6.23
16.37
6.12
4.53
34.25
15.46
11.32
3.16
13
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2.12
5.5
39.37
4.29
8.46
25.79
12.35
-
2.12
H
1.06
7.47
30.43
2.76
6.9
26.87
11.34
9.86
3.31
S3 with the addition of 4% MoO3 has the best tribological properties among all the as-deposited coatings, and the worn surfaces of S2, S3 and S4 are similar. Therefore, S1 and S3 are presented to discuss the effect of MoO3 addition on the wear behavior of the NSLCs. Fig. 7 exhibits the morphologies of worn surfaces S1 and S3
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at 25°C, 400°C, 600°C and 800°C.
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The worn surfaces of S1 and S3 at 25°C and 200°C are similar, which means that
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the wear behavior is the same. At 25°C (Fig.7 (a1) and (a2)), grindings and large number of grooves are observed on S1 and S3. It implies that the friction mechanism
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of S1 and S3 is dominated by the abrasive wear at 25°C and 200°C. The grooves on
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the worn surface of S1 are more than that on S3. The addition of MoO3 increases the content of h-BN solid lubricant in the coating
[20]
, therefore mitigates the abrasive
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wear of the coating. The EDS results of A and B in Fig. 7 (a1) and (a2) reveals that
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the oxygen content of the grinding is very low. The worn surface do not undergo oxidation reaction at 25 °C.
At 400 °C, lamellar grindings are viewed on the worn surface of S1 and S3. Shallow grooves are observed only on the worn surface of S1. Apparently, with the addition of MoO3, lamellar grindings on the worn surface of S3 are more uniformed distributed than S1. The lamellar grindings are formed by the adhesion of solid lubricants under the force of ceramic ball during the wear process. It is revealed that the main friction mechanisms of S1 at 400°C are micro-ploughing wear and adhesive wear. S3 is dominated by the adhesive wear. The EDS results of positions C and D in Fig. 7 (b1) and (b2) show that the oxygen content in the wear debris rises significantly. This implies that the worn surfaces are oxidized at 400°C, γ-(Ni, M) is oxide to NiO 14
Journal Pre-proof (observed by Raman) during the wear process. The addition of MoO3 increases the content of h-BN solid lubricant in the coating. The more content of h-BN composes with NiO, more uniformly distributed of the lamellar grinding forms, which results in a decrease of abrasive wear on the worn surface. Therefore, at lower temperature range the addition of MoO3 increases the tribological properties of NSLCs. The high temperature softening phenomenon at 600°C decreased the wear resistance of the coating [17]. Lubricating transfer film is covered on the worn surfaces
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of S1 and S3 at 600°C (Fig. 7(c1) and (c2)). The oxygen content on the lubricating
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transfer film (point E and F in Fig.7) is increased, which means that the oxidation
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process at 600°C is severer than 400°C. The oxide solid lubricants are adhesive worn
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off from the matrix, which in turn forms the lubricating transfer film and reduces the friction coefficient. The formation of MoO3 (observed by XPS) solid lubricant also
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results in a more uniform distribution of lubricating transfer film in S3 and the
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grooves are disappeared. Therefore, the friction property is improved. Shallow grooves are observed on the worn surface of S1. This illustrates that the friction
wear.
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mechanisms of S1 are adhesive wear and micro-ploughing wear, while S3 is adhesive
The softening phenomenon of S1 and S3 is serious at 800 °C [24]. The worn surfaces of S1 and S3 (Fig.7 (d1) and (d2)) are both covered with thick lubricating transfer film and the delamination phenomenon is observed clearly. The friction mechanisms of S1 and S3 are serious adhesive wear and surface delamination. The EDS results of positions G and H in Fig.7 show that the oxygen content of the lubricating transfer film of S1 and S3 is highest. This indicated that the worn surface suffers from extreme oxidation reaction at 800 °C. The detached grindings are oxidized to form the oxide solid lubricants. The oxide solid lubricants re-attaches to 15
Journal Pre-proof the worn surface, forming the thick lubricating transfer film. With the addition of MoO3, a new molybdate (CuMoO4 observed by Raman and XPS) solid lubricant is formed by the tribo-chemical reaction. Molybdate (CuMoO4) has excellent lubricating property at 800 °C. It composites with other oxide solid lubricants, which then forms a denser lubricating transfer film and as a result, decreases the friction coefficient. XPS tests are performed on the wear debris of S1 and S3 after the wear tests at 400°C, 600°C and 800 °C to study the products of the tribo-chemical reaction. Fig.8
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(a) exhibits the binding energies of molybdenum element in S3 at different wear
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temperatures. The peak of Mo3d5 at approximately 228 eV can be determined to be [25]
. The peaks of
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those of the molybdenum solid solution in the reinforcement
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MoNtvOx and O3 at approximately 232.6 eV and 233.1 eV at 600 °C and 800 °C are determined to be those of molybdenum element in CuMoO4 and MoO3 [26-27].
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At 400 °C, only Mo3d5 peak is measured on the worn surface of S3. It indicates
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that Cr(Mo)-rich reinforcements do not oxide to form the molybdenum oxide at 400°C. At 600 °C, part of Cr(Mo)-rich reinforcements are oxidized to form MoO3.
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Meanwhile, CuMoO4 solid lubricant is formed by the tribo-chemical reaction between CuO and MoO3
[22]
. At 800 °C, the peaks of Mo3d5 disappear, and only the peaks of
MoNtvOx and O3 are detected. It is indicated that Cr(Mo)-rich reinforcements are completely oxidized to MoO3 and CuMoO4 solid lubricants at 800 °C. The binding energies of copper in S1 and S3 at 400°C, 600°C, and 800°C are exhibited in Fig. 8 (b) and (c), respectively. The peak of Cu2p3 at approximately 932.6 eV and Cu2p1 at approximately 952.2 eV belongs to the solid solution of copper in the γ-(Ni, M) matrix. The peaks of Cu2p at approximately 933.9 eV, 942.2 eV, 953 eV, and 962 eV are attributed to the copper in CuO [28]. At 400 °C, the peaks of copper in γ-(Ni, Cu) (Cu2p3 and Cu2p1) are observed in both S1 and S3. Both the 16
Journal Pre-proof peak of γ-(Ni, Cu) (Cu2p3 and Cu2p1) and that of CuO (Cu2p) are observed at 600°C. Only the peak of CuO (Cu2p) is observed in S1 at 800 °C. Cu2p3, Cu2p1 and Cu2p are detached in S3. It indicates that γ-(Ni, Cu) in S1 and S3 is not oxidized at 400 °C. Part of the copper in γ-(Ni, Cu) is oxidized to CuO at 600°C. All the γ-(Ni, Cu) on the surface of S1 is oxidized at 800 °C. However, only part of the γ-(Ni, Cu) is oxidized in S3. This means that the worn surface of S1 is more susceptible to oxidation than S3,
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the addition of MoO3 alleviates the oxidation reaction during the wear process.
Fig. 8. XPS results of (a) molybdenum in S3 (b) copper in S1 and (c) copper in S3 Fig. 9 exhibits the Raman spectra of the debris of S1 and S3 from 25 °C, to 800 °C. The peaks of h-BN are observed at all tested wear temperatures. The peaks of NiO and Cr2O3 are detected from 200 °C to 800 °C. This indicates that the oxide reaction of nickel and chromium commences at 200 °C. At 600°C and 800°C, the peaks of CuO appear in S1 and S3. In the meantime, MoO3, and CuMoO4 are detected 17
Journal Pre-proof in S3. It is illustrated that at 600 °C and 800 °C, the wear debris are composed of h-BN, NiO, Cr2O3, CuO in S1 and h-BN, NiO, Cr2O3, CuO, MoO3, and CuMoO4 in S3. These solid lubricants are mixed together under the friction force, resulting in the formation of the lubricating transfer film. The lubricating transfer film protects the worn surface and improves the tribological properties. CuO, MoO3 and CuMoO4 are
2
=2
(3)
2
=2
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produced through following reactions:
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(4)
=
(5)
-p
4
Via calculation, the Gibbs free energies ΔG of reaction (3) at 600 °C and 800 °C
re
are −201.7 kJ mol-1 and −166.786 kJ mol-1. The Gibbs free energies of reaction (4) at
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600 °C and 800 °C are −1182.592 kJ mol-1 and −1083.948 kJ mol-1. The Gibbs free
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energies of reaction (5) at 600 °C and 800 °C are −17.438 kJ mol-1 and −19.833 kJ mol-1. Each of these reaction energies ΔG<0, means that these reactions can occur at
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600 °C and 800°C spontaneously.
Fig. 9. Raman spectra of the wear debris of (a) S1 and (b) S3 from 25 °C to 800 °C
4. Conclusions In this research, NSLCs with varying additive amounts of MoO3 were 18
Journal Pre-proof fabricated by laser cladding. The effect of the MoO3 on the microstructure and tribological behavior of NSLC from 25°C to 800°C was analyzed. The following conclusions were drawn: The addition of MoO3 to the NSLC was beneficial to improve the tribological properties of the composite. The added MoO3 was mostly transformed to Cr-Mo solid solution boride and Mo2C reinforcements after laser treatment, which changed the shape and increased the reinforcement content in the NSLC. The rising content and
of
solid solution of Mo element in Cr-rich reinforcements increased the hardness of the
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as-deposited coating from 503HV to 660HV. The NSLC with the addition of 4 wt. %
-p
MoO3 displayed the best tribological properties among all the as-deposited composite
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coatings from 25°C to 800°C. After adding 4wt. % MoO3, the friction coefficient of the NSLC decreased from 0.379-0.463 to 0.351-0.307, and the wear rate reduced from
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0.23-5.4*10-5mm/Nm to 0.063-2.8*10-5mm/Nm.
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At lower temperature range, the effective solid lubricant of the as-deposited coating was h-BN. The main wear mechanism of the as-deposited NSLCs was
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abrasive wear. The addition of MoO3 mitigate the abrasive wear failure of the worn surface. At high temperature (600°C and 800°C), the high temperature soft phenomenon caused the coating adhesive wear. The MoO3 solid lubricant was regenerated by the oxide reaction, meanwhile, CuO and MoO3 reacted to form CuMoO4 solid lubricant during the high-temperature wear test. h-BN, NiO, CuO, MoO3 and CuMoO4 solid lubricants formed the lubricating transfer film at high temperature. MoO3 and CuMoO4 solid lubricants reduced the friction coefficient and made the lubricating transfer film denser and not easy to be destroyed during the wear process, which improved the wear property of the composite.
Acknowledgements 19
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The authors acknowledge funding from National Key Research and Development Program of China [Grant No.2018YFB0407300] and National Natural Science Foundation of China [Grant No. 51805483].
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Journal Pre-proof Author contributions Yue Zhao: Conceptualization, Investigation, Data Curation, Writing - Original Draft. Kai Feng: Validation, Writing - Review & Editing, Supervision. Chengwu Yao: Validation, Writing - Review & Editing, Supervision. Zhuguo Li: Validation, Writing - Review & Editing, Supervision.
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Declaration of interests The authors declare that they have no known competing financial
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interests or personal relationships that could have appeared to influence
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the work reported in this paper.
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Research Highlights
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1. MoO3 was novelly added to increase the tribological property of the nickel-base
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self-lubricating composite coating.
2. MoO3 was transformed into Mo2C and Cr(Mo)-rich boride reinforcements
coating.
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increased the wear resistance of the nickel-base self-lubricating composite
3. Solid lubricants MoO3, CuO and CuMoO4 were formed at 600°C and 800°C by tribo-oxide reaction, which reduced the friction coefficient of the nickel-base self-lubricating composite coating.
23
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Yue Zhao, Kai Feng, Chengwu Yao, Zhuguo Li PII:
S0257-8972(20)30146-8
DOI:
https://doi.org/10.1016/j.surfcoat.2020.125477
Reference:
SCT 125477
To appear in:
Surface & Coatings Technology
Received date:
8 December 2019
Revised date:
29 January 2020
Accepted date:
16 February 2020
Please cite this article as: Y. Zhao, K. Feng, C. Yao, et al., Effect of MoO3 on the microstructure and tribological properties of laser-clad Ni60/nanoCu/h-BN/MoO3 composite coatings over wide temperature range, Surface & Coatings Technology (2020), https://doi.org/10.1016/j.surfcoat.2020.125477
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© 2020 Published by Elsevier.
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Title Page Effect of MoO3 on the microstructure and tribological properties of laser-clad Ni60/nanoCu/h-BN/MoO3 composite coatings over wide temperature range Yue Zhao a,b, Kai Feng a,b, Chengwu Yao a,b, Zhuguo Li a,b*
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a Shanghai Key laboratory of materials Laser Processing and Modification, School of materials Science and engineering, Shanghai Jiao Tong University, Shanghai 200240, China b Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai, 200240, China
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Yue Zhao, first author, https://orcid.org/0000-0002-6728-5149, Email: [email protected] Kai Feng, https://orcid.org/0000-0001-8198-1071, Email: [email protected]
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Chengwu Yao, https://orcid.org/0000-0002-3407-8717, Email: [email protected]
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Zhuguo Li, corresponding author, https://orcid.org/0000-0002-1311-0044 Email: [email protected]
*Corresponding author: Prof. Zhuguo Li E-mail address: [email protected] Postal address: E-302A, School of Materials Science and Engineering Shanghai Jiao Tong University No.800 Dongchuan Road Shanghai, China, 200240
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Effect of MoO3 on the microstructure and tribological properties of laser-clad Ni60/nanoCu/h-BN/MoO3 composite coating over a wide temperature range Yue Zhao a,b, Kai Feng a,b, Chengwu Yao a,b, Zhuguo Li a,b* a
Shanghai Key laboratory of materials Laser Processing and Modification, School of materials Science and engineering, Shanghai Jiao
b
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Tong University, Shanghai 200240, China
Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai, 200240, China
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Abstract
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The samples of nickel-base self-lubricating composite (Ni60/nano-Cu/h-BN)
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(NSLC) with different additive amounts of MoO3 were manufactured by laser
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cladding on substrates of Q235 steel. The microstructure and hardness of the as-fabricated NSLCs were investigated. The wear and tribological behaviors of the
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NSLCs with and without MoO3 from 25°C to 800°C were discussed. The results revealed that the addition of MoO3 resulted in the hardness improvement of the
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NSLCs due to the formation of Mo2C and Cr (Mo)-rich reinforcements during laser cladding process. The NSLC with 4 wt. % MoO3 addition showed the best wear performance from 25°C to 800°C. At 600°C and 800°C, MoO3 solid lubricant was regenerated by oxide reaction during the wear process. Meanwhile, CuMoO4 was generated owing to a tribo-chemical reaction at high-temperature. These solid lubricants formed a lubricating transfer film, which improved the tribological properties of the coating. Keywords: laser cladding; self-lubricating composite; wide temperature range; tribological properties
1. Introduction 1
Journal Pre-proof High temperature components (engine bearings, transmissions, and turbine sealing, etc.) need to operate in the frequently changing temperatures. The high temperature environment significantly affects the mechanical properties
[2]
of the components, even cause the fatigue cracks
[3]
[1]
and friction
. It is crucial to
prepare a composite coating on components to ensure the wear properties from 25°C to high temperature. Laser cladding is a highly efficient method for composite coating fabrication on the components forming metallurgical bonding to the substrate [4].
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Nickel-based superalloy (Ni60) is extensively applied to enhance the wear [5]
. It shows excellent
performance such as oxidation resistance, corrosion resistance
[6]
and wear resistance
. However, the friction coefficient of Ni60 is high
[8]
. Therefore, more lubricating
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[7]
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resistance of the composite over a wide range of temperature
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phases are needed to improve the lubricating ability of the composite coating. Solid lubricants are sensitive to temperature, and a single solid lubricant can only
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depress the friction coefficient effectively over a certain narrow range of wear
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temperature [9]. In order to ensure the excellent lubricating property over a wide range of temperature, some researchers have proposed the concept of "adaptive" high-temperature composite lubrication
[10-12]
. Excellent adaptive-lubrication property
of Molybdenum has been found in the temperature range of 20-700 °C in the forms like Mo-Cs, Mo-Ag, Mo-Pb, and Mo-Cu molybdates
[13]
. In this adaptive-lubrication
system, soft metals in terms of Cs, Pb, Cu, or Ag were utilized as the lubricant at low temperatures. While at high temperatures, molybdenum and soft metals are oxidized into lamellar MoO3 and metal oxides. They suffer tribo-chemical reaction forming the molybdate, that reduces the friction coefficient effectively. Heng Tao et al.
2
[14]
found
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that the addition of Mo in CrAlSiN nanocomposite coating improved its high-temperature anti-wear and lubricating properties. During the wear process, molybdenum was oxidized to MoO3 at 600 °C, which enhanced the tribological property effectively. Lu Yuchu et al.
[15]
found that because of the formation of
molybdenum oxide, the friction coefficients of Cr-Mo-Si-N nanocomposites at 750 °C
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and 800 °C were effectively reduced. Li Fei et al. [16] studied the friction mechanisms of SiC-Mo-CaF2 composite from 25 °C-1000 °C. They discovered that the chemical
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production of CaF2 and CaMoO4 improved the lubricating property at 25°C and
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800°C-1000°C, respectively. The lubricating ability of the Cu-Mo system at a wide
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range of temperature has seldom been investigated. Molybdenum is mainly added in
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the form of metallic Mo and MoS2 in the composites. [17]
, the composite of nanoCu/h-BN solid
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According to our previous study
lubricant enhanced the friction property of the nickel-based composite coating from
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25 °C-500 °C. However, the wear loss at 600 °C was high. MoO3 can provide molybdenum in high temperature tribological reactions. Therefore, MoO3 was added to the nickel-based composite to improve the friction and wear properties of the composite at higher temperature (for example 600°C and 800 °C). In this study, Ni60/nano-Cu/h-BN composites with different additive amounts of MoO3 were fabricated by laser cladding. H-BN is prone to spattering during laser processing, which significantly decreases the wear properties of the coating
[17]
.
Therefore, nano-Cu was cladded on h-BN to improve the wettability between h-BN and Ni-matrix, which in turn, enhanced the proportion of h-BN in the coating. Cu-Mo 3
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molybdate was expected to provide excellent lubricating property at high temperature. The friction behaviors of the composites from 25 °C to 800 °C were investigated. The effect of copper and molybdenum elements at different wear temperatures were investigated.
2.
Experimental procedures
2.1 Materials self-lubricating
composites
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Ni60/nanoCu/h-BN/MoO3
(NSLC)
were
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manufactured on a substrate of Q235 steel by laser cladding. NSLC without MoO3 is
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denoted as S1; NSLCs with 2 %, 4 %, and 6 wt. % MoO3 are denoted as S2, S3, and S4, respectively. The nominal composition of Ni60 in weight percent is
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67.3Ni-16Cr-3.3B-4.5Si-0.9C-8Fe. High energy ball milling was used to coat the
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nano-Cu on h-BN. Then mixed the powder with varying amounts of MoO3 and Ni60 powder. The compositions and proportions of S1, S2, S3, and S4 are compared in
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Table 1.
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Table 1. The morphologies of the prepared powder were presented in Fig.1.
Composition (wt. %) of NSLCs without and with 2%, 4%, and 6% MoO3 No.
MoO3
nanoCu/h-BN (4:1)
Ni60
—
6.25
93.75
2
6.25
91.75
S3
4
6.25
89.75
S4
6
6.25
87.75
S1 S2
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Fig.1. Morphology of (a) Ni60, (b) h-BN, (c) nano-Cu coated h-BN, (d) MoO3; and (e)
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overall and (f) local enlarged detailed morphology of nano-Cu/h-BN mixed MoO3
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2.2 Laser cladding process
The raw powder was pre-coated on the substrates and dried for 2-3h at
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90-100 °C in a vacuum oven before laser cladding. The laser cladding process was
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conducted by a diode-laser unit (Laserline LDF-8000, Germany). The dimension of the focused laser beam is 19 mm×6 mm. The process parameters were used with
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scanning speed of 2-3 mm/s and laser power of 3-4 kW, respectively
[18]
. The
continuous argon gas flow was used to shield the molten pool. 2.3 Microstructure analysis
The phases in the as-deposited samples were determined by an X-ray diffraction (XRD) spectrometer (6100, Shimadzu). The microstructures of the as-deposited samples were characterized by scanning electron microscope (SEM, JEM7800, JEOL) equipped with EDS energy dispersive spectroscopy. 2.4 Tribological test The micro-hardness of the as-deposited coatings was tested by Zwick/Roell ZHμ Vickers micro-hardness detector with a load of 0.5 kg and dwelling time of 14 s. 5
Journal Pre-proof High-temperature dry sliding friction experiments were conducted on a ball-on-disk equipment (UMT-2, Bruker CETR). The friction pairs were Al2O3 balls (diameter of 5 mm) and the as-fabricated coating specimens. The roughness of the samples was polished to 0.05μm before the wear tests. The wear test was conducted with a load of 30N, a wear radius of 50mm and a rotation speed of 50rpm. Each sample (S1-S4) was tested at 25°C, 200°C, 400°C, 600°C, and 800°C for 30 min. The results of the friction coefficient were repeated for 3 times and average friction coefficient was
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calculated. A laser confocal microscope (OLS4000, Olympus) was applied to analyze
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the wear loss. The measurements of wear rate were calculated using the formula
𝛥𝑣 2𝜋𝑅𝑆 = 𝐹𝐿 𝐹𝐿
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𝜂=
-p
below.
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Where η is wear rate; Δv is loss of the volume; L is sliding distance; F is load; R is the radius of the worn scar and S is the cross-section area of the scuffed scar. Every
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experiment was repeated for 3 times and average wear rate was calculated.
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The worn surfaces of the NBLCs were analyzed by SEM. The oxide solid lubricants were detected by X-ray photoelectron spectroscopy XPS (AXIS UltraDLD) and Dispersive Raman Microscope (RAMAN, Senterra R200-L, Bruker Optics).
3. Results and discussion 3.1 Microstructures
As exhibited in Fig.2 (a-d), the as-deposited coatings show an excellent metallurgical bonding to substrates without defects such as pores and cracks. With the increase of MoO3 addition from 0 % to 4 %, the thicknesses of the as-deposited coatings increase from 1.2 to 2.6 mm. When the addition amount of MoO3 increases to 6 %, the thickness of the composite coating is further increased. The mechanism of the increasing of the thickness with the addition of MoO3 has been discussed in 6
Journal Pre-proof another investigation
[20]
. The dilution rates of S1, S2, S3 and S4 are 1.41%, 0.58%,
0.34% and 0.41% respectively, which are calculated by image-pro software. As demonstrated in Fig.2 (e), with the addition of MoO3, S2, S3, and S4 are mainly composed of γ-(Ni, M), FeNi3, h-BN, M23C6, Ni2Si, CrB and Mo2C. The same phases are also detected in S1 with the exception of Mo2C. MoO3 is not detected in the XRD results of S2, S3, and S4. It can be speculated that the reaction between carbon and MoO3 occurs only under the high heat during laser irradiation [21].
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The microstructure and phase distribution of S2, S3 and S4 are similar. The
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microstructure and EDS result of S3 are exemplified to investigate the phases after
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adding MoO3 into NSLC (Fig.3). The bulk phase in Fig. 3 (a) is rich in chromium and [22]
indicates that the solid
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molybdenum. The binary phase diagram of Cr-Mo
solubility of chromium and molybdenum is high. Molybdenum is easily dissolved
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into the bulk Cr-rich reinforcement by solid solution effect. It can be deduced that the
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added MoO3 is reacted with carbon to form the molybdenum and carbon dioxide under the high heat of laser irradiation. The carbon dioxide is overflowed from the
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molten pool during the fabricating process
[2]
. Then some part of molybdenum react
with carbon forming the Mo2C reinforcements, and another part of molybdenum dissolve into Cr-rich reinforcements to form solid solution. Molybdenum, carbon dioxide and Mo2C are formed by the reactions below [23]: 2 2
=2
(1)
=
(2)
The amount of MoO3 that participated in the reaction is determined by the content of carbon in the molten pool. The content of carbon in raw powder is limited. When 6% MoO3 was added, the excessive MoO3 which did not participate in the reduction reaction and then evaporated under the high heat of laser irradiation [23]. It is 7
Journal Pre-proof why the thickness of the coating increased from 2.3 mm to 2.6 mm but do not further increase with higher additive amount of MoO3. According to our previous research [17], it was proven that the black dots which enrich of nitrogen element are h-BN solid lubricant. The distribution of nickel and copper indicate that copper dissolve into the γ- (Ni, M) matrix, forming solid solution. Fig. 4 shows the microstructure and distribution of the Cr-rich reinforcements in S1, S2, S3, and S4 after 12 hours etching. The shape of the Cr-rich reinforcements in
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S1 are long strip. While the Cr-rich reinforcements in S2, S3 and S4 are in the shape
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of bulk. The volume fractions of Cr-rich reinforcements in S1, S2, S3, and S4 are
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calculated as 13.8%±0.5, 18.5%±0.6, 21.9%±0.5, and 20.3%±0.4, respectively,
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by Image-Pro software. It is indicated that the reactions of MoO3 during laser
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cladding process change the shape of the Cr-rich reinforcements and enhance the
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content of reinforcements in the coating.
Fig. 2. (a-d) Cross section morphologies of S1 to S4; (e) XRD patterns of S1 to S4
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Fig. 3. Microstructure and EDS mapping results of NSLC with 4 wt. % MoO3 (S3)
Fig. 4. Microstructures after 12 h etching with different content of MoO3 3.2 Micro-hardness
As Fig. 5 exhibits, the average hardness of S1 is approximately 503 HV. The average hardness of S2, S3, and S4 are 602 HV, 660 HV, and 650 HV, respectively, which are all higher than that of S1. With the increase of MoO3 addition, the hardness of S1 to S4 is firstly increased and then decreased slightly. S3 with 4% addition of MoO3 shows the highest hardness (660HV). As early mentioned, the addition of MoO3 increases the content of Cr-rich reinforcements in the NSLCs (S2, S3 and S4) and causes the dispersion strengthening 9
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[19]
. In the meantime, the dissolution of molybdenum element into
Cr-rich reinforcement changes the shape of reinforcements and results in the solid solution strengthening. Thus, the addition of MoO3 heightens the hardness of the NSLCs. S3 with 4% MoO3 has the highest content of reinforcement (21.9%) and therefore it has the highest dispersion strengthening of the coating. When the addition amount of MoO3 increases to 6%, the evaporation of excess MoO3 reduces the content of the reinforcements and weakens the dispersion strengthening. Therefore, the
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hardness decreases in S4.
Fig. 5. The average hardness with different addition of MoO3 3.3 Tribological properties at different temperatures The wear experiments were conducted from 25°C to 800°C to evaluate the effect of MoO3 on the tribological properties of the coating. It can be noticed from Fig.6 (a) that with the increase of MoO3, the friction coefficients are firstly reduced and then increased, and 4wt. % MoO3 (S3) has the lowest friction coefficient. The difference in friction coefficient between S2, S3 and S4 is small. With the raising of temperature, the friction coefficient of the as-deposited NSLCs reduces gradually. It reaches the lowest coefficient at 600 °C, then increases slightly when the temperature further 10
Journal Pre-proof raises up to 800°C. As demonstrated in Fig.6 (b), with the addition of MoO3, the wear rates of S2(2%), S3(4%) and S4(6%) are lower than that of S1(0%). The wear rates firstly decrease and then increase with the amount increase of MoO3. S3(4%) has the lowest wear rate among the as-deposited coatings. This specific phenomenon is more obvious at high
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wear temperatures 600°C and 800°C.
Fig. 6. Friction coefficients (a) and wear rates (b) of S1, S2, S3 and S4 at 25°C,
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200°C, 400°C, 600°C, and 800°C According to our previous research
[22]
. The addition of MoO3 can increase the
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amount of h-BN. H-BN is a refractory phase with poor wettability and high melting point (3000 °C), it is easy to spatter during the laser cladding process. MoO3 is a low melting point oxide (790 °C). It firstly melted and wrapped on the refractory h-BN powder. It reduced the spattering of h-BN powder during the laser cladding process. Therefore, the amount of h-BN in S2, S3 and S4 is higher than that in S1. Thus, the protection effect of MoO3 (in S2, S3 and S4) on h-BN reduces the friction coefficient at 25°C-400°C. At 600°C and 800°C, the high temperature oxide solid lubricants MoO3, CuMoO4, CuO, NiO (observed by XPS and Raman spectrum) formed a dense lubricating transfer film in the wear scar, which decreases the friction coefficients of the NSLCs. Solid lubricants MoO3 and CuMoO4 are not detected in S1. Therefore, the 11
Journal Pre-proof friction coefficient of S1 is greater than S2, S3 and S4. At lower temperature, no obvious softening phenomenon occurs at these temperatures
[17]
. Therefore, the difference between the wear rate of the as-deposited
NSLCs is not large. The high temperature softening phenomenon becomes serious at 600°C and 800°C. It reduces the strength of the coatings
[24]
and causes considerable
adhesion scuffed. Thus, the wear rates enhance significantly. The addition of MoO3 caused the formation of Cr(Mo)-rich reinforcement, which enhances the hardness and [25]
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S4 are much lower than that of S1.
. Therefore, the wear rates of S2, S3 and
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deformation resistance of S2, S3 and S4
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With the increase of MoO3 addition from 0wt.% to 4wt.%, more Mo elements
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are dissolved in Cr-rich reinforcements or formed the Mo2C reinforcement, which increases the hardness. However, due to the limited content of Cr and C elements in
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Ni60, when the addition amount of MoO3 reached 6wt.%, the excess MoO3 cannot
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further formed the reinforcement to increase the hardness and wear properties. Therefore, S3 with the addition amount of 4wt. % MoO3 has the best friction and
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wear performance.
3.4 Effect of MoO3 on the friction behavior of the NSLCs
12
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Fig. 7. Morphology of the worn surfaces of (a1)-(d1) S1 and (a2)-(d2) S3 at 25°C, 400°C, 600°C
Table 2.
and 800°C
EDS results of positions A-H in Fig.7 Position
N
C
O
Si
Cr
Ni
Fe
Mo
Cu
A
3.12
16.89
0.08
3.47
7.18
58.12
9.15
-
1.99
B
16.19
17.17
0.17
2.59
6.42
45.85
8.37
0.78
2.47
C
1.83
14.98
15.71
2.16
5.63
39.76
13.94
5.99
-
D
15.25
7.29
10.84
3.5
6.37
35.17
10.02
9.8
1.76
E
3.11
10.52
25.68
3.12
3.26
40.08
11.24
-
2.99
F
2.56
6.23
16.37
6.12
4.53
34.25
15.46
11.32
3.16
13
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2.12
5.5
39.37
4.29
8.46
25.79
12.35
-
2.12
H
1.06
7.47
30.43
2.76
6.9
26.87
11.34
9.86
3.31
S3 with the addition of 4% MoO3 has the best tribological properties among all the as-deposited coatings, and the worn surfaces of S2, S3 and S4 are similar. Therefore, S1 and S3 are presented to discuss the effect of MoO3 addition on the wear behavior of the NSLCs. Fig. 7 exhibits the morphologies of worn surfaces S1 and S3
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at 25°C, 400°C, 600°C and 800°C.
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The worn surfaces of S1 and S3 at 25°C and 200°C are similar, which means that
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the wear behavior is the same. At 25°C (Fig.7 (a1) and (a2)), grindings and large number of grooves are observed on S1 and S3. It implies that the friction mechanism
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of S1 and S3 is dominated by the abrasive wear at 25°C and 200°C. The grooves on
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the worn surface of S1 are more than that on S3. The addition of MoO3 increases the content of h-BN solid lubricant in the coating
[20]
, therefore mitigates the abrasive
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wear of the coating. The EDS results of A and B in Fig. 7 (a1) and (a2) reveals that
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the oxygen content of the grinding is very low. The worn surface do not undergo oxidation reaction at 25 °C.
At 400 °C, lamellar grindings are viewed on the worn surface of S1 and S3. Shallow grooves are observed only on the worn surface of S1. Apparently, with the addition of MoO3, lamellar grindings on the worn surface of S3 are more uniformed distributed than S1. The lamellar grindings are formed by the adhesion of solid lubricants under the force of ceramic ball during the wear process. It is revealed that the main friction mechanisms of S1 at 400°C are micro-ploughing wear and adhesive wear. S3 is dominated by the adhesive wear. The EDS results of positions C and D in Fig. 7 (b1) and (b2) show that the oxygen content in the wear debris rises significantly. This implies that the worn surfaces are oxidized at 400°C, γ-(Ni, M) is oxide to NiO 14
Journal Pre-proof (observed by Raman) during the wear process. The addition of MoO3 increases the content of h-BN solid lubricant in the coating. The more content of h-BN composes with NiO, more uniformly distributed of the lamellar grinding forms, which results in a decrease of abrasive wear on the worn surface. Therefore, at lower temperature range the addition of MoO3 increases the tribological properties of NSLCs. The high temperature softening phenomenon at 600°C decreased the wear resistance of the coating [17]. Lubricating transfer film is covered on the worn surfaces
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of S1 and S3 at 600°C (Fig. 7(c1) and (c2)). The oxygen content on the lubricating
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transfer film (point E and F in Fig.7) is increased, which means that the oxidation
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process at 600°C is severer than 400°C. The oxide solid lubricants are adhesive worn
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off from the matrix, which in turn forms the lubricating transfer film and reduces the friction coefficient. The formation of MoO3 (observed by XPS) solid lubricant also
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results in a more uniform distribution of lubricating transfer film in S3 and the
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grooves are disappeared. Therefore, the friction property is improved. Shallow grooves are observed on the worn surface of S1. This illustrates that the friction
wear.
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mechanisms of S1 are adhesive wear and micro-ploughing wear, while S3 is adhesive
The softening phenomenon of S1 and S3 is serious at 800 °C [24]. The worn surfaces of S1 and S3 (Fig.7 (d1) and (d2)) are both covered with thick lubricating transfer film and the delamination phenomenon is observed clearly. The friction mechanisms of S1 and S3 are serious adhesive wear and surface delamination. The EDS results of positions G and H in Fig.7 show that the oxygen content of the lubricating transfer film of S1 and S3 is highest. This indicated that the worn surface suffers from extreme oxidation reaction at 800 °C. The detached grindings are oxidized to form the oxide solid lubricants. The oxide solid lubricants re-attaches to 15
Journal Pre-proof the worn surface, forming the thick lubricating transfer film. With the addition of MoO3, a new molybdate (CuMoO4 observed by Raman and XPS) solid lubricant is formed by the tribo-chemical reaction. Molybdate (CuMoO4) has excellent lubricating property at 800 °C. It composites with other oxide solid lubricants, which then forms a denser lubricating transfer film and as a result, decreases the friction coefficient. XPS tests are performed on the wear debris of S1 and S3 after the wear tests at 400°C, 600°C and 800 °C to study the products of the tribo-chemical reaction. Fig.8
of
(a) exhibits the binding energies of molybdenum element in S3 at different wear
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temperatures. The peak of Mo3d5 at approximately 228 eV can be determined to be [25]
. The peaks of
-p
those of the molybdenum solid solution in the reinforcement
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MoNtvOx and O3 at approximately 232.6 eV and 233.1 eV at 600 °C and 800 °C are determined to be those of molybdenum element in CuMoO4 and MoO3 [26-27].
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At 400 °C, only Mo3d5 peak is measured on the worn surface of S3. It indicates
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that Cr(Mo)-rich reinforcements do not oxide to form the molybdenum oxide at 400°C. At 600 °C, part of Cr(Mo)-rich reinforcements are oxidized to form MoO3.
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Meanwhile, CuMoO4 solid lubricant is formed by the tribo-chemical reaction between CuO and MoO3
[22]
. At 800 °C, the peaks of Mo3d5 disappear, and only the peaks of
MoNtvOx and O3 are detected. It is indicated that Cr(Mo)-rich reinforcements are completely oxidized to MoO3 and CuMoO4 solid lubricants at 800 °C. The binding energies of copper in S1 and S3 at 400°C, 600°C, and 800°C are exhibited in Fig. 8 (b) and (c), respectively. The peak of Cu2p3 at approximately 932.6 eV and Cu2p1 at approximately 952.2 eV belongs to the solid solution of copper in the γ-(Ni, M) matrix. The peaks of Cu2p at approximately 933.9 eV, 942.2 eV, 953 eV, and 962 eV are attributed to the copper in CuO [28]. At 400 °C, the peaks of copper in γ-(Ni, Cu) (Cu2p3 and Cu2p1) are observed in both S1 and S3. Both the 16
Journal Pre-proof peak of γ-(Ni, Cu) (Cu2p3 and Cu2p1) and that of CuO (Cu2p) are observed at 600°C. Only the peak of CuO (Cu2p) is observed in S1 at 800 °C. Cu2p3, Cu2p1 and Cu2p are detached in S3. It indicates that γ-(Ni, Cu) in S1 and S3 is not oxidized at 400 °C. Part of the copper in γ-(Ni, Cu) is oxidized to CuO at 600°C. All the γ-(Ni, Cu) on the surface of S1 is oxidized at 800 °C. However, only part of the γ-(Ni, Cu) is oxidized in S3. This means that the worn surface of S1 is more susceptible to oxidation than S3,
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the addition of MoO3 alleviates the oxidation reaction during the wear process.
Fig. 8. XPS results of (a) molybdenum in S3 (b) copper in S1 and (c) copper in S3 Fig. 9 exhibits the Raman spectra of the debris of S1 and S3 from 25 °C, to 800 °C. The peaks of h-BN are observed at all tested wear temperatures. The peaks of NiO and Cr2O3 are detected from 200 °C to 800 °C. This indicates that the oxide reaction of nickel and chromium commences at 200 °C. At 600°C and 800°C, the peaks of CuO appear in S1 and S3. In the meantime, MoO3, and CuMoO4 are detected 17
Journal Pre-proof in S3. It is illustrated that at 600 °C and 800 °C, the wear debris are composed of h-BN, NiO, Cr2O3, CuO in S1 and h-BN, NiO, Cr2O3, CuO, MoO3, and CuMoO4 in S3. These solid lubricants are mixed together under the friction force, resulting in the formation of the lubricating transfer film. The lubricating transfer film protects the worn surface and improves the tribological properties. CuO, MoO3 and CuMoO4 are
2
=2
(3)
2
=2
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produced through following reactions:
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(4)
=
(5)
-p
4
Via calculation, the Gibbs free energies ΔG of reaction (3) at 600 °C and 800 °C
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are −201.7 kJ mol-1 and −166.786 kJ mol-1. The Gibbs free energies of reaction (4) at
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600 °C and 800 °C are −1182.592 kJ mol-1 and −1083.948 kJ mol-1. The Gibbs free
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energies of reaction (5) at 600 °C and 800 °C are −17.438 kJ mol-1 and −19.833 kJ mol-1. Each of these reaction energies ΔG<0, means that these reactions can occur at
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600 °C and 800°C spontaneously.
Fig. 9. Raman spectra of the wear debris of (a) S1 and (b) S3 from 25 °C to 800 °C
4. Conclusions In this research, NSLCs with varying additive amounts of MoO3 were 18
Journal Pre-proof fabricated by laser cladding. The effect of the MoO3 on the microstructure and tribological behavior of NSLC from 25°C to 800°C was analyzed. The following conclusions were drawn: The addition of MoO3 to the NSLC was beneficial to improve the tribological properties of the composite. The added MoO3 was mostly transformed to Cr-Mo solid solution boride and Mo2C reinforcements after laser treatment, which changed the shape and increased the reinforcement content in the NSLC. The rising content and
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solid solution of Mo element in Cr-rich reinforcements increased the hardness of the
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as-deposited coating from 503HV to 660HV. The NSLC with the addition of 4 wt. %
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MoO3 displayed the best tribological properties among all the as-deposited composite
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coatings from 25°C to 800°C. After adding 4wt. % MoO3, the friction coefficient of the NSLC decreased from 0.379-0.463 to 0.351-0.307, and the wear rate reduced from
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0.23-5.4*10-5mm/Nm to 0.063-2.8*10-5mm/Nm.
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At lower temperature range, the effective solid lubricant of the as-deposited coating was h-BN. The main wear mechanism of the as-deposited NSLCs was
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abrasive wear. The addition of MoO3 mitigate the abrasive wear failure of the worn surface. At high temperature (600°C and 800°C), the high temperature soft phenomenon caused the coating adhesive wear. The MoO3 solid lubricant was regenerated by the oxide reaction, meanwhile, CuO and MoO3 reacted to form CuMoO4 solid lubricant during the high-temperature wear test. h-BN, NiO, CuO, MoO3 and CuMoO4 solid lubricants formed the lubricating transfer film at high temperature. MoO3 and CuMoO4 solid lubricants reduced the friction coefficient and made the lubricating transfer film denser and not easy to be destroyed during the wear process, which improved the wear property of the composite.
Acknowledgements 19
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The authors acknowledge funding from National Key Research and Development Program of China [Grant No.2018YFB0407300] and National Natural Science Foundation of China [Grant No. 51805483].
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Journal Pre-proof Author contributions Yue Zhao: Conceptualization, Investigation, Data Curation, Writing - Original Draft. Kai Feng: Validation, Writing - Review & Editing, Supervision. Chengwu Yao: Validation, Writing - Review & Editing, Supervision. Zhuguo Li: Validation, Writing - Review & Editing, Supervision.
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Declaration of interests The authors declare that they have no known competing financial
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interests or personal relationships that could have appeared to influence
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the work reported in this paper.
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Research Highlights
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1. MoO3 was novelly added to increase the tribological property of the nickel-base
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self-lubricating composite coating.
2. MoO3 was transformed into Mo2C and Cr(Mo)-rich boride reinforcements
coating.
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increased the wear resistance of the nickel-base self-lubricating composite
3. Solid lubricants MoO3, CuO and CuMoO4 were formed at 600°C and 800°C by tribo-oxide reaction, which reduced the friction coefficient of the nickel-base self-lubricating composite coating.
23
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9