Effects of spraying conditions on the microstructure and properties of NiCrBSi coatings prepared by internal rotating plasma spraying

Surface & Coatings Technology 374 (2019) 625–633

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Effects of spraying conditions on the microstructure and properties of NiCrBSi coatings prepared by internal rotating plasma spraying

T

Ling Tanga,b, Jia-jie Kanga, Peng-fei Heb, Shu-yu Dingb, Shu-ying Chenc, Ming Liub, ⁎ ⁎⁎ Yu-cheng Xiongb, Guo-zheng Mab, , Hai-dou Wanga,b, a

School of Engineering and Technology, China University of Geosciences, Beijing 100083, China National Key Lab for Remanufacturing, Army Academy of Armored Forces, Beijing 100072, China c National Key Laboratory of Human Factors Engineering, China Astronaut Research and Training Center, Beijing 100094, China b

ARTICLE INFO

ABSTRACT

Keywords: Internal rotating plasma spraying NiCrBSi alloy coatings Microstructure Bond strength Tribological properties

NiCrBSi alloy coatings were fabricated via internal rotating plasma spraying (IRPS) inside the cylinder liner and in open space using the same processing parameters. The effects of spraying condition on the microstructural, mechanical and tribological properties of the IRPS coatings were investigated. Simultaneously, the effects of the flow rate of primary gas (FRPG) on the coatings at a short spray distance were thoroughly evaluated. The microstructure, Vickers hardness, bond strength and tribological properties of the coatings were experimentally determined and characterized in detail. The results demonstrated that all coatings fabricated by IRPS had a dense structure and high microhardness. The bond strength and tribological properties were effectively promoted inside the cylinder, in contrast to open space condition. It is also found that coefficient of friction and wear volume of the coatings declined with the increasing of FRPG. Especially, the coatings prepared with FRPG at 100 L⸱min−1 exhibited the lowest friction and best wear resistance despite their higher porosity. The main wear mechanisms were abrasive and fatigue wear for the coatings deposited in open space, while the dominant wear mechanisms for the coatings prepared inside the cylinder were adhesive wear and tribolayer delamination.

1. Introduction Internal plasma spraying technology, known as a valid method used for coatings preparation on the walls of inner bores, has attracted increasing attention in recent years [1,2]. The high thermal energy density available within the plasma coupled with the ability to design guns for specific applications with short spray distance has significantly promoted its use in the preparation of inner diameter coatings, particularly in the automotive industry [3,4]. As far as the internal plasma spraying (powder) is concerned, there are two kinds of techniques, namely internal translating plasma spraying and internal rotating plasma spraying (IRPS) [5]. The spraying gun moves up and down along the axis of the workpiece and the workpiece to be sprayed rotates around its axis during the internal translating plasma spraying process, while the gun rotates around itself with making axial movement at a certain speed and the workpiece remains fixed during IRPS. Therefore, IRPS has superior application prospect due to its advantages in spraying the inner bore components which are asymmetric and not easy to rotate around the axis of the inner bore center at a high speed in the spraying process.

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Since the RotaPlasma 500® internal plasma spraying system was invented by Metco in 1994 [6], they have developed relatively mature SUMEBore® internal spraying solutions, including spray guns and suitable powder materials [1,7]. Many studies have been subsequently conducted on the internal plasma spraying coatings. Such as Barbezat [8] made a comparison of the thermal spray processes for the coating deposition in engine cylinder bores and found that the plasma coatings exhibited better wear resistance and more significant consumption reduction. Uozato et al. [9] fabricated ferrous-based coatings which showed a potential equivalent to current liner-equipped engine bores by Rota-Plasma spray. Thomas Wopelka et al. [10] successfully repaired the boat engine to reduce the lube oil consumption using the same technology. Besides, the bond coatings and thermal barrier coatings (TBC) were deposited by internal HVOF and APS, respectively. It proved that special cooling methods are necessary for mitigating the high amount of heat energy in these systems [11]. Furthermore, the effect of ambient atmosphere on the plasma flow was studied and compared at length [12]. Liu et al. studied the effects of dust extraction and cooling installation on plasma spraying in the inner bore. They

Corresponding author. Correspondence to: H. D. Wang, School of Engineering and Technology, China University of Geosciences, Beijing 100083, China. E-mail addresses: [email protected] (G.-z. Ma), [email protected] (H.-d. Wang).

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https://doi.org/10.1016/j.surfcoat.2019.06.056 Received 5 May 2019; Received in revised form 9 June 2019; Accepted 18 June 2019 Available online 19 June 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.

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found that the quality and properties of the coatings strongly depended on the spraying condition and parameters of the plasma spraying system [13]. It can be concluded that spraying condition plays an important role during internal plasma spraying because of heat and dust accumulation in the limited space of the inner bore [14]. Heat accumulation and excess temperature may lead to oxidation, dimensional errors and excessive residual stress, while dust inclusion has a direct impact on the coating quality [15,16]. In contrast, heat and dust can be easily controlled and eliminated to obtain better spraying condition in open space. Hence, the effects of them on IRPS coating qualities are necessary to be evaluated. Ni-based alloy coatings have been widely studied and applied for their excellent properties including low friction coefficient, high wear resistance, oxidation resistance and corrosion protection under variety of conditions [17–20]. As we know, the plasma spraying process is affected by many factors, including the spraying power, spraying distance, flow rate of plasma gas, powder feeding rate and substrate properties (such as surface roughness, temperature, wetting ability and chemical state) [21,22]. Many researchers have attempted to improve the Ni-based coating quality and properties via different techniques, substrate properties or powder material component [23–27]. In addition, the effects of different parameters on Ni-based coatings were investigated in detail [28]. With optimized processing parameters, IRPS appears to be a promising approach to strengthen and repair the surfaces of inner bore components. Nevertheless, maximum spraying power (current and voltage) is required to ensure that the powders are fully melted for the short spray distance inside the cylinder. Thus, the adjustable parameters were restricted and the effect of the main gas on the plasma sprayed coatings became particularly important [29]. The work presented here contributes to a better understanding of the effect of spraying condition, especially in terms of heat and dust accumulation, on the properties of the IRPS coatings. The specimens in open space were taken as a reference to investigate the effects of limited space inside the cylinder liner on the plasma sprayed coatings. In addition, the influences of different flow rate of primary gas (FRPG) and the flow rate of secondary gas which changed simultaneously at a short spraying distance on the properties of the coatings are systematically analyzed and discussed.

Table 1 Chemical composition of the NiCrBSi alloy powders. Powders

NiCrBSi

Chemical composition (wt%) Ni

Cr

Si

Fe

B

C

O

72.82

15.25

4.50

3.36

3.28

0.77

0.02

liner (Fig. 1a) and in open space (Fig. 1b), respectively. The spray gun rotated at a speed of 120 r/min and moved along with the axis at a speed of 8 mm/min. The distance of the spray gun relative to the cylinder wall D0 was optimized to 70 mm by adjusting the eccentric distance of the spray gun, and the distance in open space kept the same. The ASTM 1045 steel machined into a dimension of 60 mm × 10 mm × 5 mm and tensile test bar with a diameter of 25.4 mm were used as substrates. Prior to spraying, all the substrates were degreased with alcohol in an ultrasonic cleaner and grit blasted with brown fused alumina for surface roughening and activation to increase the bond strength of coatings and substrates. Then, NiCrBSi alloy powders (−45 + 15 μm) prepared by gas atomization were sprayed onto the substrates to a thickness of approximately ~300 μm. The composition of the powders is presented in Table 1. During the spraying process, Ar was employed as the primary gas and carrier gas, and H2 as the secondary gas owing to its high enthalpy. Meanwhile, the substrates were cooled by compressed air to reduce the risk of overburning, excessive residual stress and severe oxidation of coatings. Especially, a synchronous cooling device was designed to achieve efficient cooling for the coatings inside the cylinder. The temperature of the substrates was measured by an infrared thermometer (TECMAN 750, China) on their back every spraying time (each pass process), and the temperature of the coatings surface inside the cylinder was measured after each spraying time. As shown in Fig. 2, the temperature of the substrates inside the cylinder was much higher than that in open space, which were 182 °C and 51 °C, respectively. What is particularly striking is that the measured temperature on the coatings surface inside the cylinder exceeded 530 °C because of the heat accumulation in limited space. To keep the coatings deposited inside the cylinder away from overheated, all the spraying process paused every 6 times and the temperature of coatings declined a bit. In contrast, the temperature of coatings prepared in open space dropped rapidly and kept pace with their substrate. The spraying parameters inside the cylinder liner and in the open space were kept identical to investigate the effects of spraying conditions on coating quality. At a relatively short spraying distance, the plasma jet and coatings quality were significantly affected by the FRPG. Therefore, the effects of Ar flow rate on the coating properties were scientifically and systematically investigated by employing three different flow rate values. Correspondingly, the flow rate of H2 was adjusted to keep the spraying voltage and power at their maximum

2. Experiment procedure 2.1. Coatings preparation An internal rotating plasma spraying system invented by the National Key Laboratory for Remanufacturing of China was employed to fabricate coatings under different conditions. The spray gun is exquisitely designed and capable of spraying in the workpiece with a minimum bore diameter of 76 mm. As is depicted in Fig. 1, the flat substrates to be sprayed were installed on the inner wall of the cylinder

Fig. 1. Rotating plasma spraying inside the cylinder liner and in open space. 626

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accordance with the ASTM C633-01 standard. The cylinder samples with diameter of Φ25.4 mm were used as matched components. The specimens with coatings were bonded with the matched components using E-7 high temperature structural glue (E-7, Shanghai Huayi, China) whose theoretical strength is higher than 70 MPa. After solidification for 3 h at 100 °C in a drying chamber, the tensile tests were conducted on an E45 universal testing machine (MTS-SANS, China) at room temperature. The specimens were subjected to a tensile load at a constant rate of 1 mm/min until fracture. The obtained strength of each group was the mean value of five specimens sprayed in the same condition. The wear performance of the coatings was evaluated using a ballon-disk type tribometer UMT-3 (CETR, America) under ball-on-disk reciprocating sliding mode. The tests were carried out under normal atmosphere at room temperature as well as under dry condition. The wear tests for each coating were carried out for three times to avoid unnecessary error, and the representative results of each coating are shown here. All the samples were ground with 2000 mesh abrasive paper before the test. The steel balls (GCr15, diameter 5 mm, 63.5 HRC) were used as the friction counterpart. The wear tests were conducted for 20 min at 40 mm/s under 10 N load. The morphologies and section profiles of the worn surfaces were characterized by means of SEM and LEXT (Olympus OLS4000, Japan) 3D laser measuring microscope.

Fig. 2. Temperature evolution at different position during the spraying process. Table 2 Plasma spraying parameters used to prepare the coatings. Sample

Ar (L/min) H2 (L/min) Spraying distance (mm) Plasma voltage (V) Plasma current (A) Powder feeding rate (g/min) Rotational speed of the spray gun (r/min) Vertical velocity of the spray gun (mm/s)

Inside the cylinder

In open space

A

B

C

D

E

F

70 6.3 70 100 300 40 120

85 5.6 70 100 300 40 120

100 5.2 70 100 300 40 120

70 6.3 70 100 300 40 120

85 5.6 70 100 300 40 120

100 5.2 70 100 300 40 120

8

8

8

8

8

8

3. Results and discussion 3.1. Microstructure characterization Fig. 3 presents the morphology of the cross-section of the coatings prepared by IRPS inside the cylinder liner and in open space with different FRPG. The thickness of the coatings is in the range of 210–290 μm. As can be seen from Fig. 3(c) and (f), the coatings deposited with Ar flow at 100 L min−1 are distinctly thinner than others as a result of the low deposition efficiency caused by the decrease of the dwell time of the particles in the plasma jet, as well as high cooling rate of a large volume of primary gas. Besides, the melting state of the particles was affected by the flow rate of secondary gas which simultaneously changed with FRPG. With the decreasing of secondary gas, the melting state of particles and deposition efficiency decreased. All the coatings take on compact microstructure without obvious cracks, stratification or large pores. As is depicted in Fig. 4, the porosity of coatings deposited with FRPG at 85 L min−1 are both lower and exhibit lower deviation than the other two groups. Besides, the porosity values of the coatings prepared inside the cylinder is less than that in open space at each FRPG. The following reasons can account for the phenomena. The state and morphology of the deposited powder particles were distinctly influenced by dwell time in the plasma jet and the obtained thermal energy, kinetic energy. When the FRPG is 70 L min−1, the flying velocity of particles is relatively slower and insufficient to spread them out evenly on the substrate. As a result, there are lamellar structures and many pores in coatings A and D as can be observed from Fig. 3(a) and (d). Moreover, the interface cracks and pores of coating D (prepared in open space) are more obvious. However, the particles demonstrate good melting state since the higher flow rate of secondary gas (H2) at the same spraying power. When the FRPG is 85 L⸱min−1, the coating exhibits the densest and most homogeneous microstructure and fewest defects as depicted in Fig. 3(b) and (e) since the particles exhibit a good melting state and have enough kinetic energy to uniformly spread out on the substrate. Besides, the large amount of unmelted and semi-melted particles in the coatings prepared with the FRPG at 100 L min−1 in open space brought about the appearance of distinct interface cracks and numerous large pores as shown in Fig. 3(f). This fact results from the loss of thermal energy taken away by the excess primary gas and increment of gas mixing with melt droplets. And the flow rate of H2 which decreased simultaneously with increasing FRPG leads to worse melting state for lower enthalpy of the plasma jet.

allowable values. The rotational speed and axial velocity of the spray gun were also optimized. Table 2 summarizes the detailed process parameters. 2.2. Coatings characterization The sprayed specimens were mechanically divided and ground with 400–2000 mesh SiC abrasive paper under a small constant load and polished to a mirror surface with grinding paste whose particle size was 1.5 μm. Then the polished cross sections of the coatings were observed by a Nova NanoSEM50-type environmental scanning electron microscope (SEM, FEI, America), equipped with energy dispersive spectroscopy (EDS, OXFORD, England) to investigate the microtopography and porosity. To obtain the porosities of the coatings statistically, they were calculated with an image analysis software (ImageJ2x) via twenty randomly selected SEM photos collected from the coatings with 2400× magnification. The porosity was determined by calculating the percentage of the single-color area in the intensity image in accordance with the ASTM Standard E2109-01 [30]. The crystal structure of the powders and coatings sprayed inside the cylinder liner and in the open space were determined through XRD (Bruker, Germany, Co Kα = 1.79026 Å) operated at 40 kV and 40 mA at a scanning range of 10–90°. The scanning speed and scanning step were set at 6°/min and 0.02°, respectively. The XRD spectrum was analyzed via Jade 5.0 software. The hardness of each coating was evaluated by a Vickers hardness indenter (Micromet-6030, America) with a load of 3 N for 15 s. The recorded hardness was averaged from twenty indentations taken from each specimen. The adhesion strength of the coatings was measured in 627

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Fig. 3. Cross-sectional views of microstructure morphologies of coatings prepared by IRPS inside the cylinder liner and in open space with different FRPG: (a) coating A, (b) coating B, (c) coating C, (d) coating D, (e) coating E, (f) coating F.

Furthermore, the block made of many unmelted particles included some pores, which could not be filled by the post-deposited splats [31]. It is worth mentioning that the repeated heating of plasma jet inside the cylinder had largely eliminated these defects as shown in Fig. 3(c). This fact indicates that the effect of plasma heating in the short distance and limited space is similar to that of plasma remelting and heat treatment which improve the compactness remarkably [32,33]. In addition, there are exactly smoke dust contamination particles in the coatings inside the cylinder as exhibited in Fig. 3(b). Higher magnification SEM images also indicate that all the coatings deposited inside the cylinder have many micro contamination particles, which can reduce the coating quality. However, their influence on the coating quality is less than that of the high temperature induced by the plasma jet inside the cylinder. In addition to the microstructure of the coatings, the XRD spectrum of NiCrBSi powders and coatings are displayed in Fig. 5. Numerous of composite phases were observed due to the complexity of NiCrBSi powders. The powders mainly consisted of Ni (corresponding to γ-Ni, Ni2Si and Ni3B) and Cr (corresponding to Cr3Ni2Si and Cr3Ni5Si2) peaks. However, in contrast with the diffraction pattern of the powders,

there are a certain amount of amorphous phases in all of the coatings. Only some fundamental peaks of phases like Cr3Ni2Si could be determinately detected in these samples, which was consistent with the prior literature [34]. An important reason for the existence of amorphous phase is the high cooling rate and temperature gradient caused by the cold cooling gas during the spraying process. It is believed that the amorphous formation conditions are satisfied and crystallization becomes impossible as the cooling rate increases [35]. And it was confirmed that amorphous phase in plasma sprayed NiCrBSi coatings was metastable and it could lead to the generation of Ni3B, Ni4Si and Ni3Si phases at temperatures from 500 to 700 °C [36]. Consequently, the peak intensity of the XRD spectrum of the coating inside the cylinder is higher, and the width of the wave is larger for the heat accumulation in limited space. Thus, it can be inferred from the XRD patterns that the main phases of coatings deposited inside the cylinder feature smaller grain size according to the Debye-Scherrer formula [37]. Besides, the solid solution Ni3B phases, which typically generated during the slow cooling period and are beneficial to the hardening of the coatings were distinctly detected inside the cylinder [35]. 628

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Fig. 3. (continued)

3.2. Microhardness and bond strength As is displayed in Fig. 6, the hardness distribution on cross section of the coatings changes smoothly. The average microhardness of the coatings sprayed inside the cylinder is 5.9% lower than that in open space. This fact may not only be attributed to the lower content of amorphous phases and hard particles, but also richer dust contamination. Obviously, the highest hardness (687 HV0.3) belongs to coating E, whose microstructure were compact without obvious defect. However, it doesn't affect the general trend that the average hardness increases with increasing FRPG. This result indicates that the melted or partially melted particles have enough energy to strike the substrate and flatten more fully to form compact structure under the inertia effect due to the high velocity of the primary gas. Fig. 7 presents the ultimate bond strength values of the coatings with error bars. It shows a coincident trend with the change of coatings hardness in open space while an opposite trend appears inside the cylinder. And a maximum value of 57.4 MPa was measured at coatings E and the maximum average bond strength 50.76 MPa was in the possession of coatings A. Besides, the strength of coatings inside the

Fig. 4. Porosity of coatings deposited in different condition and FRPG.

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Fig. 7. Bond strength of the coatings in different condition and FRPG.

cylinder was obviously higher in contrast to coatings deposited with same FRPG in open space, except FRPG is 85 L min−1. The values of all three groups of coatings inside the cylinder demonstrate lower deviation compared with that in open space. The results indicate that plasma heating inside the cylinder has significant influence on the improvement of bond strength of IRPS coatings via eliminating the defects in the coatings. However, there is no further improvement to the coatings with homogeneous and compact structure like that prepared with the FRPG at 85 L⸱min−1. In addition, three kinds of macroscopic fracture morphologies of the coatings (prepared with different FRPG) are illustrated in Fig. 8. As is depicted the Fig. 8(a), the typical fracture morphology of coating D demonstrates complete detachment without residual coating which indicates that the melt particles did not properly wet and spread out on the substrate. The typical fracture morphologies of coating E and F displayed in Fig. 8(b) and (c) were observed with fracture taking place interior the coatings, reflecting the exact cohesion strength. Furthermore, the residual parts showed dispersed distribution on the substrates of sample F, indicating an inhomogeneous structure of the coating. In summary, it can be concluded from the results that the FRPG played an important role in the coating preparation.

Fig. 5. XRD spectrum of the powders and coatings. (a) Powder and coatings prepared inside the cylinder, (b) powder and coatings prepared in open space.

3.3. Tribological behaviors Fig. 9 demonstrates the friction coefficients evolution of the coatings during tribological tests. The coefficient of friction (COF) for all the coatings increased rapidly at the start of the test and gradually trended to steady state level. During the fluctuating stage (0–100 s), the COF of the coatings sprayed in open space (coatings A, B and C) increased much faster than those inside the cylinder (coatings D, E and F). And it's evident that the COF of coatings A and B (inside the cylinder) were lower than coatings D and E (in open space) during the steady-stage (after 100 s). They were in the range of 0.4–0.5 and 0.5–0.65, respectively. However, COF for both coatings prepared with FRPG at 100 L min−1 dropped sharply. The COF curve of coating F showed a steady declining trend and keep at 0.322 without obvious fluctuation in the end, while the curve of coating C fluctuated around 0.37. This is closely related to the wear mechanism and melting state of particles which are explained in this research. As is observed in Fig. 10, the wear volume was consistent with the average COF. They were mainly determined by different wear mechanism of the coatings inside the cylinder and in open space, which are affected by many factors, such as porosity, microhardness, phase composition etc. The wear volume and COF of coatings prepared inside the cylinder were both lower than those in open space. It implies that

Fig. 6. Comparison of hardness of the coatings prepared inside the cylinder liner and in open space.

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Fig. 8. Typical fracture morphology of the coatings. (a) Completely detached, (b) interior fracture, (c) interior fracture accompanied with dispersed residues.

mechanism and typical appearances of the tracks were illustrated in Fig. 12. The representative morphologies of the worn coatings prepared in open space were displayed in Fig. 12(a) and (b). Severe delamination and some pittings as well as unmelted particles were observed on the worn surface. It indicates that interlaminar debonding of the splats in the frictional layer could occur due to nucleation and propagation of subsurface cracks under the action of reciprocating extrusion and friction stress. The cracks expanded along the pores among the unmelted particles and gradually formed large cracks, as is depicted in Fig. 12(b). It is thought that cracks initiate and propagate preferentially alongside defective regions with low cohesion energy under cyclic shear stress, for instance, at microcracks and pores on the interiors of the coating, resulting in delamination flakes and pittings [38]. The cracks accelerated the generation and shedding of abrasive particles, and then more severe abrasive wear occurred. They were induced by high porosity and low cohesion in the coatings in general. The parallel scratches in the direction of sliding that were observed on the worn surface may be produced by the wear debris or steel ball. In a word, it shows the typical characteristics of abrasive wear and fatigue wear on the coatings prepared in open space. In contrast, the surface of the wear track of the coatings deposited inside the cylinder was relatively smooth and without obvious delamination flakes, as seen in Fig. 12(c). There are some pittings and cracks in the grinding marks that are similar to the coatings heat treated at 500 °C and in literature [39]. It can be inferred that the fine grain and dense microstructure obtained for high temperature could reduce wear effectively by alleviating the abrasive wear. And thin tribolayer was also observed owing to adhesive wear and plastic deformation. It could keep the coatings away from abrasive wear to a certain extent. Further magnification of the image, as shown in Fig. 12(d), indicates that cracks and fatigue delamination flakes should be responsible for the formation of pittings. It is believed that most of the spalling debris are generated by the delamination of semi-melted or unmelted particles, which are weakly bound with surrounding splats [31]. The wear resistance was dominated by the cohesion strength of the coatings and the main wear mechanisms were adhesive wear and tribolayer delamination. Regarding coatings C and F, which demonstrated excellent tribological performance, there were three main factors contributing to the results together. As the FRPG increased, the melted and semi-melted particles struck the substrate at high speed and fully spread out to fabricate the coatings without obvious lamellar structure. And high amorphous phases with fewer grain boundaries could reduce the risk of crack propagation. Hard unmelted spherical particles fixed in the coatings were also observed in all wear tracks on each specimen, as shown in Fig. 12(e). Good interface bond was achieved between the particles and surrounding splats. It is also believed that the spalling of the unmelted particles will not occur if the maximum shear stress is located at the top of the particles [31]. Thus, the particles exposed to the wear could enhance resistance to micro-cutting by decreasing the contact area between the coating and the friction counterpart, consequently leading to lower deformation, COF and wear of the coating. It

Fig. 9. Friction coefficient of the coatings during the wear tests.

Fig. 10. Wear volumes and average friction coefficient of the coatings.

the dense microstructure resulting from the recrystallization under high temperature inside the cylinder had effectively improved the tribological properties of the coatings. Both the COF and wear volume showed declining trends with the increasing of FRPG and it demonstrated sharp drop when the FRPG is 100 L min−1. To visually depict the degree of abrasion, the surfaces of the worn track of the coatings are displayed in Fig. 11. Corresponding to the wear volume, coatings D and E demonstrated more severe abrasion with deeper and wider tracks compared with coatings A and B. And coatings C and F exhibited excellent wear resistance under the same load, and the wear volume was approximately 10% compared to other two groups, indicating that the FRPG had noticeable effects on both the COF and wear volume. SEM analysis of the wear tracks was conducted to identify the wear 631

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Fig. 11. Surface morphologies of the wear tracks on the coatings. (a) Coating A, (b) coating B, (c) coating C, (d) coating D, (e) coating E, (f) coating F.

Fig. 12. Magnified images of SEM morphologies: (a), (b) coatings prepared in open space, (c), (d) coatings prepared inside the cylinder, (e) coatings deposited with FRPG at 100 L min−1 and (f) representative EDS spectrum of worn surface.

can also be found that the coating C, which was prepared inside the cylinder exhibited better wear resistance. The representative result of plane scan analysis of the worn track is depicted in Fig. 12(f). It is clear Ni, Cr, Si, O and Fe were the primary elements according to the EDS data. The mass and atomic fraction of all phase detected by EDS are calculated and listed in Table 3. The primary elemental distributions of the coatings are similar to that of the original powder, though the contents of boron and carbon are not accounted because of the resolution capability of EDS. The obvious increasement of O content indicates that slight oxidation occurred during the spraying process and wear test. The lowest O content was observed in the EDS spectrum of coatings B and E (FRPG = 85 L min−1) that implies better oxidation resistance because of their dense microstructure. The

increased Fe content came from the grinding ball, which adhered on the worn tracks. 4. Conclusions Ni-based alloy coatings were fabricated by IRPS and the effect of spraying condition, especially the heat and dust accumulation as well as FRPG on the microstructural, mechanical and wear performance are comprehensively investigated. The main conclusions can be drawn as follows: (1) All the coatings present dense microstructure with average porosity less than 1.34%. Amorphous phases were observed in all of the 632

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Table 3 Composition of worn surface measured by EDS. Element

Inside the cylinder A

Ni Cr Si O Fe

In open space B

C

D

E

F

wt%

at.%

wt%

at.%

wt%

at.%

wt%

at.%

wt%

at.%

wt%

at.%

80.62 5.22 3.78 10.39 –

60.83 4.45 5.96 28.76 –

67.88 14.45 3.99 6.21 7.47

55.10 13.25 6.77 18.50 6.38

70.23 15.43 4.41 6.24 3.70

56.79 14.09 7.46 18.52 3.14

68.58 14.89 4.31 8.67 3.56

52.78 12.94 6.93 24.48 2.88

73.29 9.74 5.56 6.34 5.07

58.69 8.83 9.33 18.69 4.28

67.81 15.36 4.04 8.89 3.90

52.03 13.31 6.48 25.03 3.15

prepared coatings and the content inside the cylinder are lower because of the high temperature caused by the plasma heating in limited space. (2) The heat accumulation and higher temperature in the cylinder are beneficial for eliminating the defects in the coatings. The coatings present denser microstructure with lower porosity compared with the ones in open space. The dust contamination has slight effect on the coating quality. Therefore, the tribological properties are improved for the coatings fabricated in the cylinder compared with that in open space. (3) The microstructural, mechanical and tribological properties of the coatings are significantly influenced by the FRPG since its effect on the kinetic energy and cooling rate of particles. And it's obvious that the coatings prepared with FRPG at 100 L min−1 demonstrate excellent tribological properties.

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