Effect of NiCr content on the solid particle erosion behavior of NiCr-Cr3C2 coatings deposited by atmospheric plasma spraying

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Effect of NiCr content on the solid particle erosion behavior of NiCr-Cr3C2 coatings deposited by atmospheric plasma spraying ⁎

⁎

Zhen Li, Yanle Li , Jianfeng Li , Fangyi Li, Haiyang Lu, Jiyu Du, Xueju Ran, Xingyi Zhang Key Laboratory of High Efficiency and Clean Mechanical Manufacture (Ministry of Education), School of Mechanical Engineering, Shandong University, Jinan 250061, China National Demonstration Center for Experimental Mechanical Engineering Education, Shandong University, Jinan 250061, China

A R T I C LE I N FO

A B S T R A C T

Keywords: NiCr-Cr3C2 coating NiCr content Solid particle erosion Erosion behavior Atmospheric plasma spraying Impeller blade

Homogeneous coatings cannot meet the overall erosion-resistance requirements for an entire blade since materials present different erosion resistances at various impact angles. In this work, NiCr-Cr3C2 coatings with different NiCr contents were prepared on FV520B steels through atmospheric plasma spraying (APS) by using a double-channel powder feeder. The composition and microstructure of the as-sprayed coatings were examined by SEM, EDS, and XRD analyses, and microhardness was determined by microhardness tests. The erosion performance of NiCr-Cr3C2 coatings under different testing conditions (impact angle, solid particle size, solid particle velocity, and NiCr contents) were further evaluated with a gas–solid two-phase testing machine. Furthermore, the erosion wear mechanism of NiCr-Cr3C2 coatings with different NiCr contents at various impact angles was investigated. The results showed that the phase distribution inside the NiCr-Cr3C2 coatings became uneven with the increased NiCr content, leading to the fluctuation of microhardness. The erosion rate of 25 wt% NiCr-Cr3C2 coating initially increased and then decreased with increasing impact angle and solid particle size. The erosion rate was also improved rapidly with the increased erosion velocity for all impact angles. It was found that at low impact angles, the erosion resistance of NiCr-Cr3C2 coatings was increased with higher ceramic content because of the micro-cutting failure. By contrast, at medium and high impact angles, the erosion resistance of the coating could be improved by increasing metal content since coatings are eroded through fatigue spalling and rigid fracture.

1. Introduction Impeller blades are widely used in metallurgy, petrochemical, power generation, and other industries as the key components of centrifugal compressors, axial flow compressors, blowers, and other large equipment [1–4]. Impeller blades are subjected to severe working conditions, such as high temperature, high pressure, and high speed. Since transport media inevitably contain small solid particles, the impeller blade is prone to solid-particle erosion under the action of gas–solid two-phase flow, leading to blade damage failure and even the scrapping of the equipment [5]. Therefore, improving the erosion resistance of blades is significant for prolonging the service life of the equipment. The erosion resistance is related to erosion process parameters, such as the velocity, size, and impact angle of solid particles. Evstifeev et al. [6], Nan et al. [7], and Mothilal et al. [8] investigated the relationship

between erosion rate and velocity and concluded that erosion rate increases with the increase of erosion velocity. Nan et al. [7] and Lopez et al. [9] found that the erosion rate gradually increases with the increase of solid particle size. Moreover, the use of a high-precision filter could reduce the size of solid particles and the erosion wear of blades effectively. Liu et al. [10], Oka et al. [11], and Wang et al. [12] investigated the influence of impact angle on the erosion rate of metallic materials (e.g. stainless steel, Al, and gray cast iron), and found that erosion rate first increases and then decreases with the increase of impact angle. Finnel et al. [13] and Oka et al. [14] found that the erosion rate of ceramic materials (e.g. Al2O3, ZrO2, and MgO) increases with the increase of impact angle. Notably, the rule of erosion resistance of metallic materials and ceramic materials were different with the increase of impact angles [13]. As shown in Fig. 1, metallic material features with good erosion resistance at high impact angles, whereas ceramic material has good erosion resistance at low impact angles.

⁎ Corresponding authors at: Key Laboratory of High Efficiency and Clean Mechanical Manufacture (Ministry of Education), School of Mechanical Engineering, Shandong University, Jinan 250061, China. E-mail addresses: [email protected] (Y. Li), [email protected] (J. Li).

https://doi.org/10.1016/j.surfcoat.2019.125144 Received 14 August 2019; Received in revised form 4 November 2019; Accepted 5 November 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Zhen Li, et al., Surface & Coatings Technology, https://doi.org/10.1016/j.surfcoat.2019.125144

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behavior of 25wt% NiCr-Cr3C2 coating and found that abrasive grooving, delamination is the main wear mechanisms. Serhii et al. [29] found that the erosion resistance of NiCr-Cr3C2 coating prepared by APS is better than NiCrBSi coating and reference steel by the dry erosion testing. Murthy et al. [30] studied the influence of annealing treatment on the erosion performance of 50 wt% NiCr-Cr3C2 coating. They found that erosion rate decreases progressively with an increase in annealing temperature and affected by the elastic rebound characteristics, amorphous binder, and weak inter-splat boundaries of the coating. Kumar et al. [31] analyzed the erosion performance of 25 wt% NiCr-Cr3C2 and WC-10Co-4Cr coatings and found that abrasion and micro-cutting are the dominant material removal mechanisms at the 30° impact angle, whereas lip fracture caused by brittle extrusion is the main reason for the high erosion rate at the 90° impact angle. Current research on erosion resistant coatings focuses on homogeneous coating (metal or cermet coatings). Nevertheless, homogeneous coatings cannot meet the erosion resistance requirements of impeller blades at various impact angles (Fig. 1). It is of great significance to study the preparation of gradient coating in which the content of the metallic and ceramic materials can be matched to improve erosion resistance at low or high impact angles. Furthermore, atmospheric plasma spraying (high temperature and low velocity) provides enough energy to ensure good particle spreading behavior upon impact and a high deposit efficiency [32,33]. Moreover, the APS technique is convenient to change the proportion of metallic contents in the process of preparing the coating. Hench, cermet coatings (NiCrCr3C2) with different metallic (NiCr) contents were prepared through APS with a double-channel powder feeder and used as protective coatings against solid-particle erosion for centrifugal compressor blades in this work. The effects of different NiCr contents on the microstructure and physical properties of cermet coating were studied. The deposited coatings with different NiCr contents were tested under various erosion process parameters, and the erosion mechanism was analyzed in detail.

Fig. 1. Erosion rates of metallic and ceramic materials at different impact angles.

Therefore, selecting the appropriate material proportion in accordance with low or high impact angles is necessary for prolonging the service life of blades. Ceramic, metallic, and polymer coatings exhibit different solidparticle erosion resistances under the same working conditions [15,16]. Modifying coating materials is an effective way to improve erosion resistance. Mehmet et al. [17] investigated the resistance of GF/EP (glass fiber and epoxy resin)-Al2O3(15 wt% and 30 wt%) and found that adding of Al2O3 reduces the erosion rate at 30°, 60°, and 90° impact angles. Laguna et al. [18] investigated the erosion resistance of AISI 304, 306, and 420 stainless steel and found that the erosion resistance of 316 steel is the worst, whereas that of 420 steel is the best. Alman et al. [19] investigated the effect of MoSi2 on the erosion resistance of Si3N4. The erosion resistance of Si3N4-MoSi2 at high impact angles is better than that of Si3N4. Sima et al. [20] investigated the influence of WC ceramic particles on the erosion resistance of Ni materials. Ni-WC has better erosion resistance than Ni coating at low impact angles. Nevertheless, the erosion resistance of Ni-WC degraded at high impact angles. Therefore, the erosion resistance of ceramic (metallic) materials could be improved by adding metallic (ceramic) to form cermet materials. Thermal spraying technology has the advantages of rapid deposition, a wide range of material sources, and limited heating influence on the substrate. It is used to prepare cermet coatings with good erosion resistance and improve the service performance of components [21,22]. Recently, substantial research has been performed on the use of thermal spraying technology to prepare cermet coatings with different metal contents [23,24]. Hamilton et al. [25] prepared WC-10Co-4Cr coating and found that the erosion rate of this coating is less than that of the CA6NM substrate, which shows ductile behavior during erosion. However, the WC-10Co-4Cr coating is characterized by ductile and brittle failure modes during erosion. Babu et al. [26] investigated the effect of particle properties on the anti-erosion properties of WC-12Co coatings and found that the erosion rate of the coating increases with the increase of particle hardness. The coating eroded with SiO2 particles exhibited ploughing instead of subsurface cracking. However, extensive ploughing and subsurface cracking can be caused when SiC is used as the erosion particle. Cr3C2, as one of the carbide ceramic material, combine with NiCr alloy which has low density, excellent wear resistance and erosion resistance attracted more and more attention. Matikainen et al. [27] and Li et al. [28] studied the sliding wear

2. Methods 2.1. Coating and substrate materials Commercial NiCrAl, NiCr, and NiCr-Cr3C2 powders were used as feedstock powders in this study. The chemical composition of the feedstock powders is presented in Table 1. NiCrAl powders had an irregular shape with a particle size range of 45–75 μm and were used to prepare the bonding coating. Figs. 2a and 3a show the typical SEM photograph of the NiCr and NiCr-Cr3C2 powders. NiCr-Cr3C2 and NiCr powders were mixed with different ratios to prepare the functional coating and were predominantly spherical shapes and irregular shapes, respectively. The figures show that the sizes of powders NiCr-Cr3C2 and NiCr are around 15–45 μm and 45–75 μm, respectively. According to XRD results in Figs. 2b and 3b, (Ni, Cr) (PDF 26-0429), Cr3C2 (PDF 350804), and Cr7C3 (PDF 11-0550) are the dominant phases of the NiCrCr3C2 powders and (Ni, Cr) and Ni(PDF 04-0850) are the principal phases of the NiCr powder. Generally, the Cr7C3 phase is formed because of the decarburization during powder production. Martensitic stainless steel FV520B, which was widely used for air compressor impellers, was employed as the substrate in this study. Specimens with dimensions of 70 mm × 60 mm × 5 mm were prepared Table 1 Chemical composition of the feedstock powders.

2

Feedstock powder

Chemical composition, wt%

NiCrAl NiCr NiCr-Cr3C2

Ni 76, Cr 19, Al 5 Ni 80, Cr 20 NiCr 25, Cr3C2 75

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Fig. 2. The morphology and XRD pattern of NiCr-Cr3C2 powders.

and grit blasted with Al2O3 powders (grit 45) before the deposition of the NiCrAl bonding coating through APS process.

Table 2 Plasma spraying parameters.

2.2. Spraying processing The functional coating with different mixing ratios of NiCr-Cr3C2 and NiCr as well as the bonding coating of NiCrAl were produced by commercial APS guns (Xiuma Corp, XM-80JZ, CHN) with a double channel powder feeder (Xiuma Corp, XM-200SK, CHN). Table 2 presents the process parameters used in the plasma spraying process. To ensure the essential thickness of the functional coating, two layers have been sprayed. NiCr-Cr3C2 coatings with different NiCr contents were achieved by adjusting the rate of powder feeder, as listed in Table 3. NiCr-Cr3C2 coatings with four different NiCr contents were prepared and defined as C1 (25 wt% NiCr), C2 (55 wt% NiCr), C3 (70 wt% NiCr) and C4 (85 wt% NiCr), respectively. Fig. 4 schematically shows the spraying process.

Parameter

NiCrAl

NiCr

NiCr-Cr3C2

Arc voltage Arc current Primary gas (pressure); flowrate Auxiliary gas (pressure); flowrate Spray distance

60 V 550 A 600 A 600 A Argon (0.8 MPa); 30 L/min Hydrogen (0.72 MPa); 5 L/min 120 mm

Table 3 Process parameters of powder feed for NiCr-Cr3C2 coatings with different NiCr contents. Test no.

NiCrAl g/ min

NiCr g/ min

NiCr-Cr3C2 g/ min

NiCr wt%

Cr3C2 wt%

C1 C2 C3 C4

38 38 38 38

0 22.6 34.32 44.88

57 34.2 22.8 11.4

25 55 70 85

75 45 30 15

2.3. Microstructural and mechanical characterization 2.4. Erosion testing with solid particles The microstructure of the NiCr-Cr3C2 coatings was examined at cross sections by scanning electron microscopy (SEM, FEI QUANTAFEG 250, US) with energy dispersive spectroscopy (EDS, INCA Energy XMAX-50, UK). The phase composition of the NiCr-Cr3C2 coatings was examined by the X-ray diffraction technique (XRD, DMAX-2500PC, Japan), using the Cu Kα1. The results were then analyzed by using the MDI Jade software. The measured samples were compared with the PDF standard cards as the reference to retrieve all the phases in the samples. The microhardness of the coating was measured on polished cross-section using the Vickers indenter (Hengyi Corp MH-6, CN) with a load of 300 g for a dwell time of 15 s. An average of 6 measurements was recorded for each test.

Erosion tests were performed as the ASTM G76-13 standard by using a self-developed gas–solid two-phase testing machine. The testing equipment and the schematic of the erosion test are shown in Fig. 5. The high-velocity erosion flow required for the erosion tests was provided by a compressor. The working conditions for erosion tests in this study are listed in Table 4. Particle size, particle velocity, and impact angle were varied at different levels. Given that the working medium of the centrifugal air compressor was an industrial atmosphere, angular Al2O3 particles with diameters of 7, 10, and 14 μm were selected as the impact particles. The particle size distribution of Al2O3 erosion particles with different sizes is illustrated in Fig. 6. Velocities and impact angles were determined in accordance with the literature [10]. The erosion

Fig. 3. The morphology and XRD pattern of NiCr powders. 3

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Table 4 Erosion test condition. Solid particle material

Al2O3 (angular)

Solid particle average size (μm) Solid particle hardness (Hv) Particle velocity (m/s) Solid particle feed rate (g/min) Erosion time (min) Impact angle (°) Test temperature Nozzle diameter (mm)

7, 10, 14 2361 180, 200, 220, 240 4 25 12, 45, 60, 90 Room temperature 8.5

Fig. 4. Schematic view of the spraying process.

resistance of the NiCr-Cr3C2 coating was evaluated by using the mass erosion rate, which is defined as follows [19]:

ε=

mt m − mt2 = t1 mp mp

(1)

Fig. 6. Particle size distribution of Al2O3 erosion particles at different sizes.

where ε is the mass erosion rate of the coating (mg/g); mp is the number of solid particles involved in the testing(g); mt is the mass loss of the sample before and after testing (mg); and mt1 (mg) and mt2 (mg) are the masses of the samples before and after testing, respectively. mt1 and mt2 were measured by using an analytical electronic balance (Zhuojing Corp BSM-220.4) with an accuracy of 0.1 mg. During each test, the sample was exposed to particle flow for a fixed amount of particles (mp), and the mass loss (mt) of the sample was recorded. Each of the erosion

Fig. 5. Erosion test equipment. (a) Erosion test and erosion particle collection equipment; (b) Structure of the erosion chamber; (c) Schematic view of the erosion test. 4

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Fig. 7. Cross-sectional EBSD morphology of NiCr-Cr3C2/NiCrAl coating with different NiCr contents. (a) C1: 25 wt%; (b) C2: 55 wt%; (c) C3: 70 wt%; (d) C4: 85 wt%; (e) C1 at a high magnification; (f) C4 at a high magnification.

10.7%, 7%, 8.7%, and 9.5%, respectively. Compared with the C1 coating, the reduction in the porosity of the C2 coating is due to the reduction of layer-to-layer interface micro defects. However, the number of unmelted particles increased with the increase of NiCr content, which cannot fully combine with the surrounding layered structure. Therefore, the porosity of the NiCr-Cr3C2 coating is increased with the increase of NiCr content. The C1 coating primarily comprises dark gray block-shaped particles and light gray fillers (Fig. 7a,e). After adding NiCr powder, white stripes appeared inside the NiCr-Cr3C2 coating (Fig. 7b,c,d) and become increasingly pronounced as increasing NiCr content. Fig. 7f shows that NiCr-Cr3C2 coatings consist of gray block-shaped particles, light gray fillers, and white stripes. The EDS results for three marked zones of C4 coating (Fig. 7f) viewed under high magnification are shown in Fig. 8. As shown in Fig. 8a, dark gray block-shaped particles are rich in Cr and C elements, which may be produced by Cr3C2. Fig. 8b shows that light gray fillers are primarily comprised of Ni and Cr elements. The (Ni, Cr) alloy is produced by NiCr-Cr3C2 powders considering that light gray fillers exist

rates was the average of three measurements. 3. Results and discussion 3.1. Microstructure of the as-sprayed coating The EBSD morphology at the cross-section of the NiCr-Cr3C2/NiCrAl coating with different NiCr contents is illustrated in Fig. 7. Three distinct layers can be observed in these coatings from the bottom to top: the FV520B substrate, the NiCrAl bonding layer, and the NiCr-Cr3C2 functional layer. All coatings have a typical laminated structure that reflects the development of coating thickness through the deposition and resolidification of molten or semi-molten droplets [34]. Meanwhile, the small number of spherical structures observed inside the assprayed coating can be attributed to the insufficient thermal energy and kinetic energy for powder melting and spreading. Each layer of the coating has micro defects (i.e. cracks and pores) which lead to the decrease of coating compactness. The porosities of C1, C2, C3, and C4 are 5

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Fig. 8. EDS results for marked zones under high magnification for C4 coating: (a) zone marked as 1; (b) the zone marked as 2; (c) the zone marked as 3.

Fig. 9. Cross-sectional EDS line analysis of NiCr-Cr3C2/NiCrAl coating with different NiCr contents. (a) C1: 25 wt%; (b) C2: 55 wt%; (c) C3: 70 wt%; (d) C4: 85 wt%;

in all coatings and are surrounded by dark gray block-shaped particles (Fig. 7a-d). Compared with those shown in Fig. 8b and c, the white stripes contain a large amount of Ni element and can be found in C2, C3, and C4 coatings. Therefore, the white stripes are (Ni, Cr) phases and are formed by NiCr droplets. The EDS line analysis for the cross-section of NiCr-Cr3C2/NiCrAl coating with different NiCr contents is shown in Fig. 9. Sharp changes in Ni, Cr, C, and Fe elements can be observed between two interfaces (i.e. NiCr-Cr3C2 layer and NiCrAl layer, NiCrAl layer, and the substrate) in all cases. This indicates that elemental diffusion is limited and mechanical bonding is the main bonding form for the NiCr-Cr3C2/NiCrAl coating. Moreover, the existence of three substances (dark gray block-shaped particles, light gray fillers, and white stripe) and rapid change in elements (Ni, Cr, C) with the increase of NiCr content, indicate that the droplets produced by NiCr and NiCrCr3C2 powders are unevenly distributed (Fig. 9b–d). Therefore, the sprayed coatings could be featured with heterogeneous performance, such as microhardness performance.

Fig. 10. XRD patterns of NiCr-Cr3C2/NiCrAl coating with different NiCr contents.

6

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Fig. 11. Microhardness of NiCr-Cr3C2 coating with different NiCr contents.

Fig. 12. Erosion rate of C1 coating at different impact angles.

3.2. Phase composition

As mentioned above, the microhardness of FV520B (280 HV) is less than that of C1 coating (829 HV). Therefore, the impact angle of the maximum erosion rate of the FV520B is smaller than that of C1 coating. The erosion scar produced on the eroded surface of C1 coatings at impact angles of 12°, 45°, 60°, and 90° is shown in Fig. 13. As an increasing impact angle, the eroded area gradually transitions from a narrow oval to a circular shape. Moreover, eroded craters become increasingly pronounced. Fig. 14 shows the erosion rate of C1 coating at different solid particle velocities (particle size 14 μm) for various impact angles. The erosion rate of C1 coating increases with the increase of solid particle velocity at low, medium, and high impact angles (Fig. 14a). The erosion rate at 240 m/s is approximately twice as high as that at 180 m/s. Moreover, the relative error of the erosion rate at 240 m/s is larger than other parameters under the test impact angles, indicated that the coating erosion rate is more sensitive at 240 m/s. As shown in Fig. 14b, the erosion rate of the coating is approximately linear with the erosion velocity in the logarithmic coordinate system. Linear regressive formulas can be obtained between the erosion velocity (v) and the erosion rate (ε) at different impact angles in the logarithmic coordinate system:

The XRD diffractograms of the APS-deposited NiCr-Cr3C2 coatings with different NiCr contents are depicted in Fig. 10. The diffractograms of C1, C2, C3, and C4 coatings present similar trends. In all cases, NiCrCr3C2 coatings are mainly composed (Ni, Cr), Cr3C2, Cr7C3, Cr23C6(PDF 35-0783), Cr2O3(PDF 38-1479), and Ni phases. Moreover, the peak values of carbide phases (Cr3C2, Cr7C3, and Cr23C6) are gradually decreased, whereas those of the (Ni, Cr) and Ni phase are gradually enhanced with the increased NiCr proportion. Although no new phases are formed through adding NiCr powder, Cr23C6 and Cr7C3 phases are generated by the decomposition and decarbonization of the Cr3C2 phase at high temperatures [35,36]. Although NiCr and NiCr-Cr3C2 droplets cannot be readily oxidized during the spraying process due to the surrounding of inert gases, they could be oxidized to the Cr2O3 phase once exposed to the atmosphere. Compared with the Cr3C2 phase, the formation of carbides (Cr7C3, Cr23C6) and oxides (Cr2O3) reduce the microhardness and wear resistance of NiCr-Cr3C2 coatings. Therefore, it can be inferred that the increase of NiCr metal phase decreases the microhardness of the NiCr-Cr3C2 coating. 3.3. Microhardness Fig. 11 illustrates the variation in the microhardness of coatings with different NiCr contents. The microhardness of the coatings decreases with the increase of NiCr content. The average microhardness values of C1, C2, C3, and C4 are 829, 559, 379, and 323 HV, respectively. Compared with that of the FV520B substrate (280 HV), the microhardness of all coatings is greatly improved because of the presence of high hardness carbides (Cr3C2, Cr7C3, Cr23C6). Moreover, the maximum deviation of microhardness increases from 63 to 128.8 HV with the increase of NiCr content. This is consistent with the aforementioned morphology examination that the NiCr-Cr3C2 coating consists of dark gray block-shaped particles, light gray fillers, and white strips, leading to the heterogeneous distribution of hard phases (Fig. 7b-d).

lg ε = 2.52679 lg v − 6.14234

(2)

lg ε = 2.41758 lg v − 5.76915

(3)

lg ε = 2.23837 lg v − 5.32304

(4)

lg ε = 2.64659 lg v − 6.32947

(5)

where v is solid particle velocity. The slope of the line is the velocity index at different impact angles [38]. Consequently, the erosion rate can be expressed with solid particle velocity by

3.4. Erosion rate

ε = 7.20543 × 10−7v 2.52679

(6)

ε = 1.70157 × 10−6v 2.41758

(7)

ε = 4.75291 × 10−6v 2.23837

(8)

ε = 4.68306 × 10−7v 2.64659

(9)

Notably, velocity indices of the C1 coating are 2.52679, 2.41758, 2.23837, and 2.64659 at different impact angles. According to the literature [39], the velocity index of plastic materials is between 2 and 3.5 and that of brittle material is up to 6.5, indicating that C1 coating has plastic material properties. Furthermore, the velocity index of C1 coating at 90° impact angle is higher than that at other impact angles, meaning that the effect of erosion velocity on erosion rate is pronounced. In other words, the erosion rate increases more rapidly with the increase of velocity than with other conditions. Fig. 15 presents the erosion rate of C1 coatings at different solid

Fig. 12 shows average erosion rates of C1 coating at different impact angles. The erosion rate of C1 coating first increases and then decreases with the increase of impact angle. The variation trend of the erosion rate of C1 coating is consistent with that of plastic materials, such as FV520B [10] and Al [11]. Moreover, the maximum erosion rate of C1 coating is approximately 40°-60° impact angle. Compared with that of FV520B, the peak of the erosion rate of C1 coating changes from angle 24° to 60°. Published works have shown that the microhardness of materials affects the impact angle with the maximum erosion rate [37]. 7

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Fig. 13. The erosion damage on the surface of C1 coating at different impact angles (particle size 14 μm and velocity 240 m/s). (a) 12° impact angle; (b) 45° impact angle; (c) 60° impact angle; (d) 90° impact angle.

Fig. 14. Erosion rate of C1 coating at different particle velocities. (a) Normal coordinate coordinate (b) Logarithmic coordinate.

[14] and Laguna et al. [18] found that the erosion rate of the material increases with the decrease in microhardness at the low impact angle. As previously stated, the microhardness of NiCr-Cr3C2 coating follows a decreasing trend as increasing NiCr content (Fig. 11). This behavior indicates that the microhardness of the NiCr-Cr3C2 coating affects the erosion rate at the 12° impact angle. Therefore, the high microhardness of NiCr-Cr3C2 coating contributes to the improved erosion resistance of the coating at low impact angles. At an impact angle of 45°, the erosion rate of the 85 wt% NiCr-Cr3C2 coating is 1.23 times that of the 25 wt% NiCr-Cr3C2 coating. Moreover, the erosion rate of the coating with different NiCr contents shows an upward trend at the 45° impact angle. Microhardness may remain the main property that affects the erosion rate of coatings. The erosion rate fluctuates with the increased NiCr

particle sizes for various impact angles. The erosion rate of C1 coating first increases and then decreases with the increase of solid particle size. The maximum erosion rates with 10 μm particle size are 2.17, 3.12, 3.53, and 3.71 times those with 7 μm particle size at each impact angle. The experimental results show that the anti-erosion performance can be improved and the service life of compressor blades can be prolonged by using superior anti-erosion materials, such as cermet material. Moreover, the use of the high-precision filtration device is recommended to reduce the erosion of large solid particles in the transport medium. Fig. 16 presents the erosion rates of NiCr-Cr3C2 coating with different NiCr contents at various impact angles (particle size 14 μm and velocity 240 m/s). The erosion rate of the NiCr-Cr3C2 coating increases with the increase of NiCr content at the 12° impact angle. Oka et al. 8

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microhardness of the material was high value, the penetration of the material by erosion particles was hindered, thus decreasing the erosion rate. Moreover, the impact angle corresponding to the maximum erosion rate was higher for materials with higher microhardness. It was reported that the peak angle of the erosion rate of pure Al2O3 is 90° [43]. Since C1 coating had high carbide phase content, it was more difficult to erode than the NiCr phase. Therefore, the carbide phase accounted for the convexity of the erosion surface, which could effectively resist the erosion of alumina erosion particles and improved the erosion resistance of the coating. However, C4 coatings had low carbide phase content which could not effectively resist erosion, thus showing an increased erosion rate at low impact angles. Therefore, improving the microhardness of the coating could effectively reduce the erosion rate at low impact angles. At medium impact angles (45° and 60°), the horizontal force of solid particles was insufficiently large to finish a complete micro-cutting process. Therefore, plough marks were generated on the eroded surface [13]. As shown in Fig. 17c–f, cutting marks were shortened and deepened and exhibited a large number of lip patches on both sides at 45° and 60° impact angles. Material stacking also appeared at the back of some cutting marks. The horizontal force of erosion particles under this condition failed to provide sufficient energy to complete a cutting process. As the erosion progresses, subsequent particles continued to erode stacking materials. Finally, the stacking materials and lips were removed from the surface. With the increased NiCr content, the cutting marks on the erosion surface of the NiCr-Cr3C2 coatings were gradually decreased at the 45° impact angle, which indicated that the horizontal force of erosion particles could continue to complete the erosion of 25 wt% NiCr-Cr3C2 materials. Consequently, the erosion rate of C1 coatings continued to increase gradually with the increase of impact angle (Fig. 12). The cutting marks at each coating shrunk and deepened at a 60° impact angle. Moreover, the lips and stacking materials became increasingly pronounced. Additional fracture pits on the surfaces of erosion morphology at a 60° impact angle for C1 and C4 coatings were observed, which indicated that NiCr-Cr3C2 materials were removed by fatigue fracture (Fig. 17e,f). This was explained by that the interlamellar bonding of NiCr-Cr3C2 coating was characterized by weak bonding, which was weaker than the fracture strength inside the droplet. Beyond that, stress concentration could be generated because of the considerable amount of microcracks (e.g., cracks and pores). Consequently, fatigue cracks could rapidly expand along with the internal interface and subsurface of the coating under the continuous erosion of particles, resulting in the failure of NiCr-Cr3C2 coating at medium impact angles (Fig. 17e). At the high impact angle (90°), considerable craters with different depths are observed on the eroded surface as shown in Fig. 17g and h. Moreover, lips and material bulges could be found around the crater. The horizontal force was far less than the vertical force at high impact angles. Erosion particles were wedged into the coatings and act like nails. Meanwhile, lips were produced around the craters. Materials underwent plastic deformation and eventually ruptured under continuous erosion. Ultimately, NiCr-Cr3C2 was removed through fatigue spalling and fracture (Fig. 17g). Since NiCr-Cr3C2 coatings were prepared by APS with a layered structure, the mechanical bonding was the main bonding form. At high impact angles, the erosion rate of the coating was primarily determined by the capacity to resist deformation and cracking. With the increased metal content, the plasticity of the coating increased gradually, resulting in enhanced erosion resistance to deformation damage. When the coating contained considerable ceramic phases, cracks and other defects might be produced by the poor combination between bonding phases and hard phases. Therefore, high erosion rates could be resulted by the rigid fracture and lamellar shedding damage at the high impact angle. Since metallic materials had better plastic property than ceramic materials, lips formed by the NiCr material were not easy to break at the high impact angle. Consequently, improving the plasticity of the material (e.g., increasing NiCr content)

Fig. 15. Erosion rate of C1 coating at different solid particle sizes.

Fig. 16. Erosion rate of NiCr-Cr3C2 coating with different NiCr contents.

content at a 60° impact angle. Noticeably, at the 90° impact angle, the erosion rate of NiCr-Cr3C2 coating shows an opposite trend compared with that at 12° impact angle, which exhibits plastic material properties. Since NiCr alloy is featured with plastic properties, the plasticity of NiCr-Cr3C2 materials gradually increases with the increase of NiCr content [40]. It can be inferred that the ductile material has a lower erosion rate and better erosion resistance at high impact angles. 3.5. Erosion mechanism The SEM images of the surface morphology of C1 and C4 coatings eroded at various impact angles are presented in Fig. 17. The velocity of erosion particles could be divided into horizontal and vertical components, which generated horizontal force and vertical force. When erosion particles were irregularly ridged, the horizontal force caused the particles to act as a cutter and scratch the target surface, thus caused surface wear. Meanwhile, the vertical force would cause the particles to act like nails and wedge into the coating surface, thus caused plastic deformation. Therefore, at low impact angles, the horizontal force of the particles eroded the surface similar to a cutting process, which generated chips and cutting marks on the surface. This wear mechanism was called micro-cutting [41,42]. The cutting marks on C1 and C4 coatings presented the same direction and the shapes were shallow and long with labial pieces on both sides (Fig. 17a,b). The presence of several lip pieces behind the cutting marks indicated that a cutting process was completed by the erosion particles. The cutting lengths of coatings C1 and C4 were approximately 3.27 and 8.21 μm, respectively, which indicated that the NiCr-Cr3C2 coating with high NiCr content had weak erosion resistance at the same horizontal force. When the 9

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Fig. 17. SEM micrographs of the surface morphology of C1 and C4 coatings eroded at different impact angles. (a) C1: 12°; (b) C4: 12°; (c) C1: 45°; (b) C4:45°; (d)C4:45°; (e) C1: 60°; (f) C4: 60°; (g) C1: 90°; (h) C4: 90°.

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and reducing the internal cracks could effectively improve the erosion resistance of materials at high impact angles.

Foundation of China (51605258) and the Key Research and Development Program of Shandong Province (2019GGX104010).

4. Conclusions

References

(1) The microstructure and physical properties of NiCr-Cr3C2 coatings with different NiCr contents prepared by the APS process were examined. In particular, dark gray block-shaped particles, light gray fillers, and white stripes were observed based on EBSD morphology analysis, and rapid changes in Ni, Cr, and C elements were observed between two interfaces. These features indicated that the phase distribution inside the NiCr-Cr3C2 layer was uneven. Moreover, the porosity of NiCr-Cr3C2 coatings increased while the microhardness decreased with the increase of NiCr content. (2) The erosion rate of NiCr-Cr3C2 coatings under various testing conditions (impact angle, velocity, and particle size) and with different NiCr contents were tested by using a gas-solid two-phase tester. The erosion rate of the 25 wt% NiCr-Cr3C2 coating initially increased and then decreased as increasing the impact angle the maximum erosion rate peaks at 60° impact angle. The erosion rate at all impact angles increased rapidly when the erosion velocity was increased. Moreover, the erosion rates with the particle size of 10 μm were larger than that with other particle sizes (7 μm and 14 μm) for all the impact angles. Furthermore, by increasing the NiCr content, the erosion rate increased at low impact angles (12° and 45°) but decreased at the high impact angle (90°). (3) The wear mechanisms of NiCr-Cr3C2 coatings with different NiCr contents were analyzed at different impact angles. Long and narrow cutting marks were found on the coating surfaces eroded at the low impact angles (12°and 45°), and the cutting length increased with the increased NiCr content. The erosion mechanism of NiCr-Cr3C2 coatings at low impact angle was micro-cutting, indicating that increasing the microhardness of NiCr-Cr3C2 materials was effective to improve erosion resistance. Cutting marks decreased and plough marks increased gradually with the increased impact angle. Cracks were found in NiCr-Cr3C2 coatings with different NiCr content at medium and high impact angles (60°and 90°) according to SEM morphology. The failure forms were fatigue spalling and rigid fracture. Erosion resistance was improved by increasing NiCr contents and reducing internal cracking at high impact angles.

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Although erosion resistance of NiCr-Cr3C2 coating has been studied, surface characteristics (surface roughness, residual stress, etc.) of the coatings after solid particle erosion still need to be further investigated. Furthermore, Al2O3 was used as erosion particles in the present work. It is still of great significance to investigate other particle materials for evaluating the erosion resistance of NiCr-Cr3C2 coating. Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Zhen Li:Conceptualization, Writing - original draft, Formal analysis.Yanle Li:Writing - review & editing, Supervision.Jianfeng Li:Methodology, Project administration, Funding acquisition.Fangyi Li:Resources, Writing review & editing.Haiyang Lu:Conceptualization, Data curation.Jiyu Du:Data curation.Xueju Ran:Investigation.Xingyi Zhang:Validation. Acknowledgments This work is financially supported by the National Natural Science 11

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