Wear, erosion and corrosion resistance of HVOF-sprayed WC and Cr3C2 based coatings for electrolytic hard chrome replacement

International Journal of Refractory Metals & Hard Materials 81 (2019) 242–252

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Wear, erosion and corrosion resistance of HVOF-sprayed WC and Cr3C2 based coatings for electrolytic hard chrome replacement

T

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Qun Wanga, , Sisi Luoa, Shaoyi Wangb, Hui Wangc, Chidambaram Seshadri Ramachandrand a

College of Materials Science and Engineering, Hunan University, Changsha, Hunan 410082, PR China Ganzhou Zhangyuan Tungsten New Materials Co., Ltd, Ganzhou, Jiangxi 341300, PR China c Zhuzhou AECC PST Nanfang Gas Turbine Co., LTD., Zhuzhou, Hunan 412008, PR China d Department of Materials Science and Engineering, The State University of New York (SUNY) @ Stony Brook, New York 11794 2275, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: HVOF Cermet coatings EHC replacement Wear Erosion Corrosion

Thermally sprayed carbide-based coatings are nowadays extensively considered as an alternative to electrolytic hard chrome (EHC) coatings to reduce the environmental impact and the overall cost associated with EHC process. In this investigation, high-velocity oxy-fuel (HVOF) spray process was employed to prepare coatings using the traditional carbide powders namely the WC-10Co4Cr, the Cr3C2-25NiCr and a new type of mixed carbide powder WC-40Cr3C2-25NiCr. The Powder deposition rate, basic mechanical properties, abrasive wear, slurry erosion and corrosion resistance of the three coatings were then compared with the EHC coating. The results show that WC-10Co4Cr coating exhibited the highest hardness, abrasive wear and slurry erosion resistance followed by WC-40Cr3C2-25NiCr, EHC, and Cr3C2-25NiCr coating. The deposition efficiency of the powders as per hierarchy was found to be WC-40Cr3C2-25NiCr > WC-10Co4Cr > Cr3C2-25NiCr and all the HVOF sprayed coatings exhibited higher corrosion resistance than EHC coating. The highest powder deposition efficiency coupled with low density, acceptable tribo-corrosion performance, as well as low post processing cost makes the HVOF sprayed WC-40Cr3C2-25NiCr coating a potential candidate to replace the EHC coating.

1. Introduction Electroplating hard chromium (EHC) is a process of depositing a layer of hard chromium coating with high wear and corrosion resistance on the surface of alloy parts (such as carbon steel) by electroplating process, which makes electroplated hard chromium components exhibit higher toughness and lower cost compared to the sinteredHIP (Hot Iso-statically Pressed) ones. The EHC coatings have been used to enhance wear and corrosion resistance in a wide variety of markets and applications, including hydraulics, tooling, and aerospace components especially for those parts with thin wall and (or) complex shapes. [1]. However, acid mist and waste residue generated in the EHC plating process contain a lot of hexavalent chromium (Cr6+), which is detrimental to health and the environment [2,3]. Extensive environmental laws have been formulated and implemented to limit EHC plating process; therefore, it is necessary to find some technically and economically feasible surface technologies to substitute or replace it. There are many surface engineering technologies namely the plasma transferred hardfacing process, laser hardfacing process, thermal sprayed hardfacing process etc., which can be potentially considered as

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alternatives/replacement to the EHC process [4]. Among the surface engineering processes meant for hardfacing applications, the High-Velocity Oxy-Fuel (HVOF) sprayed cermets have proven to be one of the most reliable alternatives to EHC coatings [5–7]. The most commonly used cermet coatings deposited using HVOF-spraying are WC-Co, WCCoCr, WC-Ni, WC-Cr3C2-Ni, and Cr3C2-NiCr. Compared to WC-Co coating, the WC-CoCr deposit has higher corrosion resistance because of having the Cr element in it, which can generate passivation layer on the surface of the WC-CoCr coating in a corrosive environment [8–10]. Besides this, the WC-Co and WC-Ni coatings available in different stoichiometry are usually used to protect the machine parts [11]. These traditional WC-based powders are often expensive and display low deposition efficiency during the HVOF spraying process because of the high volume percentage of ceramic phase present in these powders resulting in high powder production cost [12]. In addition, it is very difficult to grind and polish the WC-based coatings due to their inherent high hardness [13], which leads to high post-processing cost. In an attempt to identify a more suitable cermet coating having high performance and low cost in order to replace the EHC coating in practical application, a new composite powder having a composition of WC-

Corresponding author. E-mail addresses: [email protected], [email protected] (Q. Wang).

https://doi.org/10.1016/j.ijrmhm.2019.03.010 Received 1 November 2018; Received in revised form 8 March 2019; Accepted 11 March 2019 Available online 12 March 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.

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Table 1 Nominal composition, apparent density, size distribution and deposition rate of the three powders. Powder

WC wt%

Cr3C2 wt%

Co wt%

Cr wt%

Ni wt%

Apparent density (g/cm3)

WC-10Co4Cr WC-40Cr3C2-25NiCr Cr3C2-25NiCr

86 35 /

/ 40 75

10 / /

4 5 5

/ 20 20

4.6 3.26 2.44

[14,15]. The surface roughness of the samples was measured by a diamond stylus surface roughness tester (Mitutoyo Corp., SJ 210, Tokyo, Japan). All the results such as porosity, hardness, fracture toughness, and surface roughness were taken from an average of 10 independently measured values.

40Cr3C2-25NiCr was prepared. This powder was deposited using the Praxair's JP8000 HVOF spray system and its microstructures, mechanical properties, erosion, abrasive and corrosion resistance were investigated. The HVOF sprayed WC-10Co4Cr, Cr3C2-25NiCr coating and the EHC coating was also compared under identical tribo-corrosion testing conditions.

2.3. Abrasion wear tests 2. Experiment A wet sand rubber wheel abrasion tester (MLS-225, Zhangjiakou Integrity Experimental Equipment Manufacturing Co., Ltd., Zhangjiakou, China) was used to test the abrasive wear resistance of all the coatings. The Shore hardness of the rubber wheel was 72, the load was 100 N, the rotational speed of the rubber wheel was 240 rpm. The tribological pair was submerged in the slurry composed of 40–70-mesh quartz sand (1.5 kg) and freshwater (1 kg), the total revolutions for each sample was 3000. The mass difference before and after the wear test was divided by the theoretical density of the coatings. Together with this density value, the load and the distance covered by the samples during the test were used to calculate the wear rate of each coating.

2.1. Materials and coating preparation In this investigation, a new type of composite carbide powder having a composition of WC-40Cr3C2-25NiCr and the two traditional carbide powders namely the WC-10Co4Cr and the Cr3C2-25NiCr powders (Chongyi Zhangyuan Tungsten Co., Ltd. Co., Ltd., China) was produced by agglomeration and sintering method. The size distributions of all the three powders were 5–45 μm. The properties of the powders are listed in Table 1 and their morphologies are shown in Fig. 1. All the three spray powders exhibited good spherical shape with many pores present on their surfaces as seen in Fig. 1 (a, c and e). The WC and Cr3C2 particles are distinctively identifiable in the WC-10Co4Cr (Fig. 1b) and Cr3C2-25NiCr (Fig. 1f) powders, respectively. The WC40Cr3C2-25NiCr powder, however, was composed of two types of ceramic particles (~1.5 μm of WC and 3–5 μm of Cr3C2). Binder metals such as Co, Ni, and Cr encapsulated the ceramic particles and wedged them together to form a near spherical shaped composite powder. A balanced and identical spray parameter was used to spray deposit the powders except for the powder feed rate, which was adjusted to account the differences in the apparent density of the three distinct powders. The optimization of spray parameters can be referred elsewhere [14], and the parametric details are presented in Table 2. The powder deposition efficiency was determined by dividing the mass of the coating by the mass of the powder used. The difference in mass of the substrate plate before and after spraying was equal to coating mass (the spray plume was restricted to within the substrate plate during the HVOF spray deposition process in order to calculate the deposition efficiency). The deposition efficiencies of the powders were found to follow the order: WC-40Cr3C2-25NiCr > WC-10Co4Cr > Cr3C2-25NiCr.

2.4. Erosion tests In the erosion wear test, the slurry was composed of 40–70 mesh SiO2 (150 g/min) and freshwater (1500 g/min), which was accelerated to 40 m/s by the compressed air and impact on the surface of the coating at 30° and 90° respectively with an erosion distance of 25 mm. The diameter of the nozzle used was 8 mm and the total erosion time was 45 min. The mass difference before and after erosion was divided by the theoretical density and erosion time of the coatings, which was used to calculate the erosion rate of each coating. The details of the erosion test apparatus can be found in our previous work [16]. 2.5. Corrosion tests Electrochemical tests for each coated sample were carried out in static NaCl solution (3.5 wt%) at room temperature. A saturated calomel electrode (SCE) and a platinum probe were used as the reference and counter electrodes, respectively. The open-circuit potential was measured after the coated sample was soaked in 3.5 wt% NaCl solution for 30 min to stabilize its potential and the tests were performed at a scanning rate of 0.5 mV s−1. Two representative samples of each coating type were used to ensure the reproducibility of test results. In addition to this, the standard method for copper-accelerated acetic acid-salt spray (Fog) Testing (ASTM B 368–1997) [17] was also used to investigate the corrosion resistance of the coatings, in which the total corrosion time adopted was 168 h.

2.2. Characterization A Rigaku D/max-2550 diffraction meter with Cu-Ka radiation was used to analyze the phase compositions of powders and coatings. The scanning electron microscopy (FEI-Quanta200) was employed to examine the microstructure and the worn surfaces of the coatings. Porosity measurement was performed on the cross section of the coating with an OLYMPUS GX51 microscope fitted with an image analyzer. Hardness measurements were carried out on the transverse section of the coating with a Vickers indenter at a load of 0.3 kg and a dwell time of 10s. The elastic modulus measurements were conducted on the polished coating surface by nanoindentation method (MCT + UNHT Nano Hardness Tester, Swiss CSM Instruments Co., Ltd.) with a load of 0.02 N, the loading and unloading velocities were maintained at 0.04 N/min, the dwell time was 10s. An average of 5 independently measured values is reported in this manuscript. The fracture toughness of coatings was tested by the indentation method with the load of 5Kg in accordance to the Evans and Wilshaw equation

3. Results 3.1. Phase composition The XRD results of WC-10Co4Cr, WC-40Cr3C2-25NiCr, and Cr3C225NiCr coatings are shown in Fig. 2. The WC-10Co4Cr coating was composed of WC as the main phase and the minor phase was W2C (Fig. 2a), which can be attributed to slight decarburization of WC during the HVOF spray process [14]. The phase compositions of WC40Cr3C2-25NiCr and Cr3C2-25NiCr coatings were almost the same as 243

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Fig. 1. Low and high magnification morphologies of (a, b) WC-10Co4Cr, (c, d) WC-40Cr3C2-25NiCr and (e, f) Cr3C2-25NiCr powders.

3.2. Microstructure of the HVOF sprayed cermet and EHC coatings

their starting powders, respectively (Fig. 2b and c). A broad peak centered around 43° appeared in the XRD pattern for all the three coatings, which means that there is the formation of an amorphous phase. Compared to the starting powder, the metal binder phases (Co, Cr or Ni) in the coatings have disappeared or decreased and they were converted to amorphous metal phases in all the HVOF coatings due to the rapid cooling rate of the sprayed molten and semi-molten particles when they impact on the substrate during the spraying process [14,18].

The cross-sectional morphologies of HVOF sprayed cermet and EHC coatings are shown in Fig. 3. Unlike the entire crack free HVOF sprayed coatings, there were a lot of cracks running perpendicular to the interface between coating and substrate because of the high stress generated during the hard chrome plating process. Some of the cracks were found to run throughout the entire thickness of coating as denoted by white arrows shown in Fig. 3i. Similar cracks were also found by Bolelli

Table 2 Spraying parameters and deposition for all the three powders. Coating

Kerosene (L/h)

Oxygen (m3/h)

Carry gas (m3/ h)

Step (mm)

Velocity (mm/s)

Spraying distance (mm)

Feed rate (g/ min)

Powder deposition rate (%)

WC-10Co4Cr WC-40Cr3C2-25NiCr Cr3C2-25NiCr

23.8 23.8 23.8

56.1 56.1 56.1

0.65 0.65 0.65

5 5 5

500 500 500

380 380 380

70 55 45

42.3 53.4 37.2

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average hardness of about ~ 850 HV0.3, which were lower than those of the WC-10Co4Cr (1207 HV0.3) and WC-40Cr3C2-25NiCr (1060 HV0.3) based coatings. The lower hardness of the WC-40Cr3C2-25NiCr coating made its post grinding and polishing easier compared to the WC10Co4Cr coating, which can drastically decrease the post-processing cost. All the HVOF sprayed coatings showed compact microstructure without cracks. Porosities of all the sprayed cermet coatings were < 1%, even though the porosities present in the coatings containing Cr3C2 were slightly higher than that of WC-10Co4Cr coating. The WC10Co4Cr coating had the highest fracture toughness and elastic modulus followed by the WC-40Cr3C2-25NiCr and the Cr3C2-25NiCr coating and the EHC coting showed the lowest elastic modulus. In addition, the EHC coating showed the lowest surface roughness followed by the WC10Co4Cr, WC-40Cr3C2-25NiCr and Cr3C2-25NiCr coating respectively. The abrasive wear rates of all the coatings are shown in Fig. 4. It is observed in Fig. 4 that the WC-40Cr3C2-25NiCr coating exhibited ~77% higher wear rate than that WC-10Co4Cr coating, but its wear rate was ~ 80% lower than that of EHC coating, which shows that it exhibits a moderate abrasive wear resistance. The erosion rate of the HVOF sprayed cermet and EHC coatings are shown in Fig. 5. All the HVOF sprayed cermet coatings, except Cr3C2-25NiCr showed higher erosion resistance than EHC coating. In addition, the erosion rates of the Cr3C225NiCr coating at 90° impact angle was much higher than that of 30° exhibiting significantly different erosion phenomena compared to other coatings, whose erosion rates at 90° impact angle were slightly higher than those at 30°. The potentiodynamic polarization curves of the HVOF sprayed cermet and EHC coatings in the 3.5 wt% NaCl solution are displayed in Fig. 6. The data for open circuit potential and corrosion current density of the cermet and EHC coatings calculated from Fig. 6 are presented in Table 4. From Table 4 it is inferred that all the HVOF sprayed coatings showed lower corrosion current density and displayed nobler open circuit potential than that of the EHC coating. 4. Discussion 4.1. Abrasive wear mechanism of the HVOF sprayed cermet and EHC coatings The typical abrasive wear surface morphologies of the HVOF sprayed cermet and EHC coatings are shown in Fig. 7. From Fig. 7, it can be found that the WC-based coatings showed smoother worn surface than Cr3C2-NiCr and EHC coatings. Many high hardness WC particles and metal binder (Co, Cr or Ni) were co-deposited by HVOF flame without distinct decarburization resulting in the compact coating with high hardness and fracture toughness, which can effectively hinder the penetration of the quartz abrasive into the matrix [11] and absorb a part of energy generated by the abrasive attack with some degree of plastic deformation [20]. Therefore, the WC-based coating showed the smoothest worn surface and the highest abrasive wear resistance among all the coatings in this work. The extrusion of the binder phase and its removal by plastic deformation were observed on the surface of the worn coatings. The cyclic fatigue, carbide grain fracture, the undermining of the particles and subsequent particle pullout were found to be the dominant wear-related failure processes for WC-based coatings [12,14]. While on the EHC coating, there were many grooves on its worn surface, which were caused due to its low hardness. The hardness of SiO2 (~1050 HV0.3) is higher than that of the EHC (~853 HV0.3), which can penetrate the surface of the EHC causing distinct grooves and removal of materials. In addition, the original cracks of EHC coating can lead to the detachment of the coating and finally result in the creation of many pits on its worn surface. Therefore, the uneven worn surface and high wear rate of EHC coating can be associated with its low hardness and inherent net shaped through thickness cracks. The worn surface of the Cr3C2-25NiCr coating also showed some grooves and materials detachments, which was identical to that of the EHC coating. It was found that hardness and wear rate was inversely

Fig. 2. XRD patterns of the (a) WC-10CO4Cr, (b) WC-40Cr3C2-25NiCr and (c) Cr3C2-25NiCr powders and coatings.

et al. and Lei et al. [5,19].

3.3. Properties and performance of the HVOF and EHC coatings 3.3.1. Hardness, porosity, fracture toughness, and surface roughness of the HVOF and EHC coatings Hardness, porosity, fracture toughness, surface roughness and elastic modulus of the HVOF and EHC coatings are listed in Table 3. It is observable in Table 3 that the Cr3C2-25NiCr and EHC have the similar 245

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Fig. 3. Cross-sectional SEM images and EDS results of HVOF sprayed and EHC coatings (a, b) WC-10CO4Cr coating (low and high magnification), (c, d) WC-40Cr3C225NiCr coating (low and high magnification), (e, f) EDS of the particle marked by 1 and 2 in Fig. 3(d), (g, h) Cr3C2-25NiCr coating (low and high magnification) and (i) EHC (low magnification).

Table 3 Properties of the cermet and EHC coatings. Coating

Hardness (HV0.3)

Porosity (%)

Fracture toughness (MPa·m1/2)

Surface roughness (μm)

Elastic modulus (GPa)

H3/E2 (×104)

Hp/Ht

WC-10Co4Cr WC-40Cr3C2-25NiCr Cr3C2-25NiCr EHC

1207 ± 97 1060 ± 134 843 ± 98 853 ± 63

0.13 ± 0.02 0.90 ± 0.03 0.87 ± 0.03 /

5.14 ± 0.95 3.68 ± 0.85 2.40 ± 0.74 /

4.12 4.22 6.55 0.96

311.2 263.4 242.1 211.9

1.82 1.72 1.02 1.38

0.83 0.94 1.19 1.17

246

± ± ± ±

0.31 0.29 0.25 0.06

± ± ± ±

12.7 11.3 11.9 1.75

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Table 4 Open circuit potential and corrosion current density of the HVOF cermet and EHC coatings.

-5

3

Volume loss (×10 mm （ / N•m))

6 5 4 3

Open circuit potential (V)

Corrosion current density (×10−6 A/cm2)

WC-10Co4Cr WC-40Cr3C2-25NiCr Cr3C2-25NiCr EHC

−0.38 −0.50 −0.21 −0.52

2.70 2.58 2.48 3.01

2

high hardness WC particles in WC-40Cr3C2-25NiCr coating which can effectively hinder the cutting and penetration of the SiO2 particles during the abrasive wear.

1 0 WC-10Co4Cr WC-40Cr3C2-25NiCr Cr3C2-25NiCr

4.2. Slurry erosion mechanism of the HVOF sprayed cermet and EHC coatings

EHC

Coating

The typical slurry erosion morphologies of the HVOF sprayed cermet and EHC coatings are shown in Fig. 8. According to the observations in Fig. 8, the surfaces of the coatings which have undergone impingement tests at 30°, exhibited significant plastic deformation and many grooves along the erosion direction as marked by arrows shown in Fig. 8 a, c, e, and h. The main erosion mechanisms observed were micro scratching and micro-plowing for all the tested coatings. The grooves on the erosion surface indicated that the materials have been removed or pushed aside and caused flaking of materials as marked by ellipses in Fig. 8 a, c, e, and h. These flakes will be removed by subsequent impacts of erodent particles. Both the grooves and the flaking of materials on the erosion surfaces were induced by the repeated impact of the erodent on the coating surface [16]. In comparison to the EHC coating, the WC-10Co-4Cr coatings showed smaller and shallower grooves as well as less and smaller flaked-off materials, which can be attributed to its higher hardness. The higher hardness makes it difficult for the sand particles to penetrate the surface of the coating. Ramachandran et al. also found a similar phenomenon for the APS sprayed rare earth oxide coatings [21]. The Cr3C2-25NiCr and EHC showed more coarse/rougher erosion surfaces, which can be attributed to their lower hardness. The erosion surface of WC-40Cr3C2-25NiCr coating exhibited a medium coarse erosion surface. Erosion mechanism of the HVOF sprayed cermet coatings at lower impact angle (30°) are explained as follows: the grooves may be formed primarily in the low hardness of metal binder region, resulting in dislocations of hard WC and Cr3C2 particles as seen in Fig. 8 a, c and e. The grooves in the binder region acted as failure initiating regions and the carbide grains were then dislodged by the erodent [22]. While the erosion mechanism of EHC coating occurring at a low angle (30°) is identical to the surface material getting directly scratched and plowed by high hardness erodent. This occurs because of the EHC's inherent low hardness coupled with its pre-existing cracks, which resulted in high erosion rate compared to HVOF sprayed WC-based coatings. It is observed in the erosion surfaces of the coatings at 90° impact angle that both the carbide-based coating and the EHC coating exhibited a lot of highly deformed platelets. The occurrence of grooves on the worn surfaces was limited, which means that a different erosion mechanism is involved at a high impact angle (90°) compared to the lower impact angles (30°). The removal of the materials happened at high impact angle due to micro chiseling and cutting rather than scratching and plowing. For the carbide-based coatings, the main erosion mechanism was that the low hardness metal binder was first cut and chiseled by the SiO2 particles, which is accompanied by the fragmentation of carbide particles. Finally, the carbide particles were pulled off from the surface by detaching from the support of the binder enabled by the intense impact of the high-velocity slurry [16,23]. Wayne and Sampath [24] suggest that the erosion rate of thermally sprayed tungsten carbide-based coatings is inversely proportional to the 1/2 power of hardness and 3/8 power of fracture

Fig. 4. Abrasive wear rate of the HVOF sprayed cermet and EHC coatings.

Fig. 5. Erosion rate of the HVOF sprayed cermet and EHC coatings. 1.5

WC-10Co4Cr WC-40Cr3C2-25NiCr Cr3C2-NiCr EHC

1.0

Potential/V

Coating

0.5

0.0

-0.5

-1.0 1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

Current/A Fig. 6. Polarization curves of the HVOF sprayed cermet and EHC coatings in 3.5% NaCl solution.

proportional [12,14] for the liquid fuel HVOF sprayed WC-12Co coatings. Therefore, the coating having the lowest hardness, which is the Cr3C2-25NiCr had a very rough worn surface and the highest wear rate. The WC-40Cr3C2-25NiCr coating showed a medium smooth worn surface with some shallower grooves without cracks or material detachments resulting in moderate wear rate, which was less than that of the Cr3C2-25NiCr and EHC coatings. This is because of the presence of the 247

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Fig. 7. Abrasive wear surface morphologies of the HVOF sprayed (a) WC-10Co4Cr, (b) WC-40Cr3C2-25NiCr, (c) Cr3C2-25NiCr and (d) EHC.

cracks, however, were short in their lengths and were usually either contained within one lamella or had traversed to neighboring lamellae since a high degree of continuity existed within the HVOF coatings. Conversely, the 30° impingement angle was seen to promote widespread pluck out (WC grain removal) and relatively little subsurface cracking (microstructures taken at 30o impact angle are not shown due to space constraints). The WC-40Cr3C2-25NiCr coating displayed a moderate erosion rate, which was in between that of the WC-10Co4Cr and EHC coating. It is well known that the amount of damage in solid particles impact erosion largely depends on the ratio of particle hardness to the specimen hardness. The hardness of the SiO2 erodent was about HV1000 [27]. It was noticed that with the increasing ratio of particle hardness to target hardness (Hp/Ht) ratio the erosion rate increases [28]. In a similar manner to abrasive wear, it has been shown that the erosion rate reduces when Hp/Ht < 1. The erosion mechanisms like microcracking and microchipping mechanisms are observed when the value of the hardness ratio is less than unity. For brittle materials where Hp/Ht > 1 material is removed by the indentationinduced fracture. In general, if the hardness of an abrasive exceeds that of coating, the following processes take place: Penetration of abrasive into the coating, micro cutting, and failure of splats resulting in the detachment of small chips [29]. Since the erosion of brittle lamellae is primarily via a mechanism involving the initiation and propagation of microcracks, one expects that the fracture toughness of the material will affect the erosion rate. Fracture toughness is usually assumed to be one of the indicators of the wear resistance of these types of materials in environments where the brittle fracture is the predominant mode of

toughness. Therefore, the inherent high hardness can make the HVOF sprayed WC-based coatings exhibit effective resistance in hindering the micro-cutting and chiseling compared to the EHC coating. In addition, the moderate fracture toughness, the HVOF sprayed WC-based coating can also absorb some energy caused by the impacting erodent, which helps it to avoid the brittle spallation. While as for as the Cr3C2-25NiCr coating is concerned, the erosion surface presented in Fig. 8f was very coarse with a lot of enormous flakes, which can be attributed to its low hardness and fracture toughness as seen in Table 3. The typical crosssectional morphologies of the HVOF sprayed cermet and EHC coatings The typical cross-sectional morphologies of the HVOF sprayed cermet and EHC coatings after a slurry jet impingement at 90° impact angle are shown in Fig. 9. From Fig. 9, it can be found that sub-surface cracks are more likely to generate in the all the coatings after slurry erosion at 90°, but the Cr3C2-25NiCr coatings showed the most serious sub-surface crack (Fig. 9c) due to its lowest fracture toughness result in the highest erosion rate. Lopez et at. have also found this phenomenon [25]. As for the EHC coating, the direction of cracks is different from these in the HVOF sprayed cermet coatings. These pre-existing cracks perpendicular to the interface between coating and substrate (Fig. 9d) coupled with low hardness makes the EHC coating to exhibit a coarse erosion surface, which was similar to that of Cr3C2-25NiCr coating. Regardless of impingement/impact angles, it is seen that an approximate inversed loglinear relationship was found to exist between the slurry erosion wear loss and the hardness of the cermet materials. The microstructure of the coatings eroded at a normal angle of impact displayed a lot of subsurfaces cracking similar to the observations of Gant and Gee [26]. The

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Fig. 8. Slurry erosion morphologies of the HVOF sprayed cermet and EHC coatings at 30° of (a)WC-10Co4Cr, (c) WC-40Cr3C2-25NiCr, (e) Cr3C2-25NiCr, (f) EHC and at 90° of (b)WC-10Co4Cr, (c) (d)WC-40Cr3C2-25NiCr, (f) Cr3C2-25NiCr and (i) EHC. 249

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Fig. 9. Cross-sectional morphologies of the HVOF sprayed (a) WC-10Co4Cr, (b) WC-40Cr3C2-25NiCr, (c) Cr3C2-25NiCr and (d) EHC after a slurry jet impingement at 90° impact angle.

Ht: hardness of coating). Due to the specific mechanical properties of the WC-based coating such as low Hp/Ht ratio value, high-H3/E2 ratio, such coatings are good candidates for protection against solid particle erosion compared to other considered coatings. It can be summarized that with increasing severity of erosion conditions, the erosion rate showed a stronger dependence on Hp/Ht. The transition in wear rates from mild to moderate to severe erosion conditions was associated with a change in erosion mechanism under these conditions with an increase in the porosity of the coatings. Furthermore, the crystal structure can influence the erosion rate. Stacking fault energy and dislocation cell formation are thought to be factors, which influence erosion as a function of the crystal structure. Shipway and Hutchings [34] identified the following as being important target material characteristics to consider in solid particle erosion: hardness, strain rate sensitivity, grain orientation, grain size effects, thermal parameters, and toughness. Of these, hardness has been the most extensively studied. It has been found when comparing different pure materials that harder materials generally erode less.

material removal [30]. This fact is due to the higher energy needed to initiate and propagate cracks in the target material when there is an increase in the fracture toughness. For thermally sprayed coatings as well as metallurgically bonded hardfacings, better correlations were found between the hardness of the wearing material and the modes of erosion, where plastic deformation was a major mechanism [31]. The erosion rate increased sharply as the coating hardness decreased. This increase is due to a combination of inter-lamellar porosity and poor inter-splat bonding in these coatings, as neither factor by itself can account for it. Erosion of thermally sprayed coatings by hard particles usually involves both plastic flow and brittle fracture at the same time. The effect of fracture toughness on the wear resistance of brittle materials is usually related to the hardness [32]. The wear of brittle materials based on mechanical properties such as hardness, elastic modulus, and fracture toughness, which have proven inadequate for engineering cermets, are also unsuitable for predicting the wear of the thermally sprayed coatings [33]. In this study of the interrelation of the erosion behavior of thermally sprayed coatings to their microstructural and micromechanical characteristics, the best correlations were found between the material hardness, the level of porosity and the erosion rate. From Table 3, it could be inferred that the WC-based coating possesses high hardness compared to other coatings. The Young's modulus values (Table 3) and the fracture toughness values (Table 3) of the rest of the coatings deposited in this investigation are lower than that of the WC-based coating [34]. The calculated Hp/Ht and H3/E2 ratios for the coatings are shown in Table 3 (Hp: hardness of the SiO2,

4.3. Corrosion mechanism All the HVOF sprayed carbide based composite coatings showed lower corrosion current density than that of the EHC coating as listed out in Table 3. This is due to the compact microstructure produced by the HVOF process and because of the presence of the Cr in the metal binder. Although the EHC coating was made of Cr, the presence of 250

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coating. The poor corrosion resistance of EHC in Copper-Accelerated Acetic Acid-Salt Spray (Fog) Testing was also attributed to its inherent cracks. Some cracks were found to be penetrating perpendicular to the interface between the coating and substrate and were found to extend up to the carbon steel substrate, which can act as the infiltration paths for the corrosion solution [5,19]. The compact and crack free HVOF sprayed carbide-based coatings can effectively hinder the corrosion fog penetration into the coating resulting in high corrosion resistance. The HVOF sprayed WC-40Cr3C2-25NiCr coating has been used to improve the wear and corrosion resistance of rotor of a positive displacement motor in the petroleum industry whose service life in saturated saline environment reached 400 h, which is about 4 times higher than that of conventional EHC coating. The process of HVOF spraying WC coating on the surface of the rotor is shown in Fig. 11. Crack-free HVOF sprayed WC-40Cr3C2-25NiCr coating with higher wear and corrosion resistance was responsible for the longer lifespan of the rotor in comparison to the EHC coated counterpart. 5. Conclusions Two traditional WC-10Co4Cr and Cr3C2-25NiCr coatings, as well as a new type of WC-40Cr3C2-25NiCr coating, were successfully deposited by HVOF spray process and their basic mechanical properties, abrasive wear, erosion and corrosion performance were investigated and compared to that of an EHC coating. Salient observations are summarized below: 1) All the HVOF sprayed carbide-based coatings showed compact microstructure without distinct cracks and the corresponding coating phase composition was almost similar to their original feedstock powders. 2) All the HVOF sprayed carbide-based coatings except Cr3C2-25NiCr showed higher abrasive wear, erosion, and corrosion resistance than the EHC coating. 3) High powder deposition efficiency, low density and acceptable performance of WC-40Cr3C2-25NiCr coating resulted in reduced cost with respect to coating preparation, grinding and polishing. Thus, this coating has enormous potential to replace the EHC coating.

Fig. 10. Surface morphologies of (a) WC-10Co4Cr, (b) WC-40Cr3C2-25NiCr, (c) Cr3C2-25NiCr and (d) EHC coatings after Copper-Accelerated Acetic Acid-Salt Spray (Fog) Testing.

Acknowledgments This work was financially supported by the Changsha Science and Technology Project, China (Grant No. kq1801004) and Natural Science Foundation of Hunan Province, China (Grant No. 2019JJ40045 and 2019JJ40023). References [1] R. Tapphorn, H. Gabel, Exploring low-temperature alternatives to electrolytic hard chrome coatings, Adv. Mater. Process. 176 (2018) 38–40. [2] H. Masoumi, S.M. Safavi, M. S, S.M. Nahvi, Effect of grinding on the residual stress and adhesion strength of HVOF thermally sprayed WC–10Co–4Cr coating, J. Manuf. Process. (9) (2014) 1139–1151. [3] M. Bielewski, Replacing cadmium and chromium, the research and technology organization and NATO, AG-AVT-140, 2011, pp. 1–22 Chp. 23. [4] V. Balasubramanian, R. Varahamoorthy, C.S. Ramachandran, S. Babu, Effect of speed, load, and hardness of rollers on dry sliding wear behavior of PTA hardfaced Stellite surface, Int. Heat Treat. Surf. Eng. 1 (2007) 156–163. [5] G. Bolelli, R. Giovanardi, L. Lusvarghi, T. Manfredini, The corrosion resistance of HVOF-sprayed coatings for hard chrome replacement, Corros. Sci. 48 (2006) 3375–3397. [6] J.A. Picas, M. Punset, M.T. Baile, E. Martín, A. Forn, Tribological evaluation of HVOF thermal-spray coatings as a hard chrome replacement, Surf. Interface Anal. 43 (2011) 1346–1353. [7] C. Lyphout, S. Björklund, Internal diameter HVAF spraying for wear and corrosion applications, J. Therm. Spray Tech. 24 (2015) 235–243. [8] E.S. Puchi-Cabrera, M.H. Staia, Y.Y. Santana, E.J. Mora-Zorrilla, J. Lesage, D. Chicot, J.G. La Barbera-Sosa, E. Ochoa-Perez, C.J. Villalobos-Gutierrez, Fatigue behavior of AA7075-T6 aluminum alloy coated with a WC-10Co-4Cr cermet by HVOF thermal spray, Surf. Coat. Tech. 220 (2013) 122–130. [9] Š. Houdková, F. Zahálka, M. Kašparová, L.M. Berger, Comparative study of

Fig. 11. Photograph of the WC-40Cr3C2-25NiCr coating deposited on the rotor using the HVOF spraying process.

micro-cracks has a detrimental effect on its corrosion resistance, which resulted in the lowest open circuit potential among all the tested coatings. The surface morphologies of all the coatings after 168 h copper-accelerated acetic acid-salt spray testing are shown in Fig. 10. From Fig. 10, it can be found that HVOF sprayed cermet coatings showed much less rust on their surface in comparison to the EHC 251

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[10]

[11]

[12] [13]

[14]

[15] [16] [17] [18]

[19]

[20]

[21]

corundum particles, Ceram. Int. 39 (2013) 649–672. [22] I. Hussainova, J. Ku¨barsepp, I. Shcheglov, Investigation of the impact of solid particles against hard metal and Cermet targets, Tribol. Int. 32 (1999) 337–344. [23] H. Sheng, Y.P. Wu, W.W. Gao, J.F. Zhang, Y.G. Zheng, Y. Zheng, Slurry erosioncorrosion resistance and microbial corrosion electrochemical characteristics of HVOF sprayed WC-10Co-4Cr coating for offshore hydraulic machinery, Int. J. Refract. Met. H. 74 (2018) 7–13. [24] S.F. Wayne, S. Sampath, Structure/property relationships in sintered and thermally sprayed WC-Co, J. Therm. Spray Tech. 1 (1992) 307–315. [25] E. Lopez Cantera, B.G. Mellor, Fracture toughness and crack morphologies in eroded WC–Co–Cr thermally sprayed coatings, Mater. Lett. 37 (1998) 201–210. [26] A.J. Gant, M.G. Gee, Wear modes in slurry jet erosion of tungsten carbide hardmetals: their relationship with microstructure and mechanical properties, Int. J. Refract. Met. H 74 (2018) 7–13 49 (2015) 192–202. [27] H. Arabnejad, S.A. Shirazi, B.S. McLaury, H.J. Subramani, L.D. Rhyne, The effect of erodent particle hardness on the erosion of stainless steel, Wear 332–333 (2015) 1098–1103. [28] I. Hussainova, Microstructure and erosive wear in ceramic-based composites, Wear 258 (2005) 357–365. [29] C. Dogan, J. Hawk, Role of composition and microstructure in the abrasive wear of high-alumina ceramics, Wear 229 (1999) 1050–1058. [30] A.G. Evans, E.A. Charles, Fracture toughness determinations by indentation, J. Am. Ceram. Soc. 62 (1979) 371–382. [31] C.S. Ramachandran, V. Balasubramanian, R. Varahamoorthy, Effect of experimental parameters on erosive abrasive wear behavior of plasma transferred arc Hardfaced surfaces-a comparative evaluation, J. Manuf. Sci. Prod. 10 (2009) 91–108. [32] R.G. Wellman, M.J. Deakin, J.R. Nicholls, The effect of TBC morphology on the erosion rate of EB PVD TBCs, Wear 258 (2005) 349–356. [33] L.C. Erickson, R. Westergard, U. Wiklund, N. Axen, H.M. Hawthorne, S. Hogmark, Cohesion in plasma sprayed coatings—a comparison between evaluation methods, Wear 214 (1998) 30–37. [34] P.H. Shipway, I.M. Hutchings, The role of particle properties in the erosion of brittle materials, Wear 193 (1996) 105–113.

thermally sprayed coatings under different types of wear conditions for hard chromium replacement, Tribol. Lett. 43 (2011) 139–154. C.S. Ramachandran, V. Balasubramanian, R. Varahamoorthy, An investigation on the aqueous corrosion behavior of plasma transferred arc hardfaced alloys, J. Manuf. Eng. 6 (2011) 153–160. M.A. Zavareh, A.A.D.M. Sarhan, B.B.A. Razak, The tribological and electrochemical behavior of HVOF-sprayed Cr3C2–NiCr ceramic coating on carbon steel, Ceram. Int. 41 (2015) 5387–5396. D.E. Crawmer, Thermal spray processes, in: J.R. Davis (Ed.), Handbook of Thermal Spray Technology, ASM International, Materials Park, OH, 2004, pp. 54–76. S.Y. Luo, Y.S. Liao, C.C. Chou, J.P. Chen, Analysis of the wear of a resin-bonded diamond wheel in the grinding of tungsten carbide, J. Mater. Process Tech. 69 (1997) 289–296. Q. Wang, Z. Chen, L. Li, G. Yang, The parameters optimization and abrasion wear mechanism of liquid fuel HVOF sprayed bimodal WC–12Co coating, Surf. Coat. Tech. 206 (2012) 2233–2241. A.G. Evans, T.R. Wilshaw, Quasi-static solid particle damage in brittle solids—I. Observations analysis and implications, Acta Metall. 24 (1976) 939. Q. Wang, Z.X. Tang, L.M. Cha, Cavitation and sand slurry erosion resistances of WC10Co-4Cr coatings, J. Mater. Eng. Perform. 24 (2015) 2435–2443. Standard method for copper-accelerated acetic acid-salt spray (Fog) testing ASTM B 368-1997 2018. S. Matthews, M. Hyland, B. James, Long-term carbide development in high-velocity oxygen fuel/high-velocity air-fuel Cr3C2-NiCr coatings heats treated at 900°C, J. Therm. Spray Tech. 13 (2004) 526–536. Q. Lei, Y.P. Wu, H. Sheng, Y.J. Qin, W. Shi, G.Y. Li, Corrosion behavior of HVOFsprayed Fe-based alloy coating in various solutions, J. Mater. Eng. Perform. 26 (2017) 3813–3820. Q. Wang, S.Y. Zhang, Y.L. Cheng, Wear and corrosion performance of WC-10Co4Cr coatings deposited by different HVOF and HVAF spraying processes, Surf. Coat. Tech. 218 (2013) 127–136. C.S. Ramachandran, V. Balasubramanian, P.V. Ananthapadmanabhan, Erosion of atmospheric plasma sprayed rare earth oxide coatings under air suspended

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