Phase evolution in synthesis of nanocrystalline WC-η composite powder by solid-state in situ reactions

International Journal of Refractory Metals & Hard Materials 71 (2018) 21–27

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Phase evolution in synthesis of nanocrystalline WC-η composite powder by solid-state in situ reactions Haibin Wang, Chao Hou, Xuemei Liu, Xingwei Liu, Xiaoyan Song

MARK

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College of Materials Science and Engineering, Key Laboratory of Advanced Functional Materials, Education Ministry of China, Beijing University of Technology, Beijing 100124, China

A R T I C L E I N F O

A B S T R A C T

Keywords: In situ reactions Nanocrystalline WC-η composite powder Phase evolution Thermodynamic calculation

Replacement of Co with η phase (e.g. Co6W6C and Co3W3C) is proposed as an effective approach to resist against corrosion of WC-Co coatings in environment of molten zinc. In the present work, the nanocrystalline WC-η powder was firstly synthesized as the coating material by a novel method of in situ reactions using metal oxides and carbon black as raw materials. The phase evolution in the process of in situ synthesis was investigated by thermodynamic calculations and experiments. It was found that the initial WO3, Co3O4 and C transformed firstly to CoWO4, then to W and W2C, and eventually to η and WC. The formation of η phase decreases the energy barrier for WC formation and hence the synthesis temperature for the composite powder. The duplex phase constitution of WC and η was obtained with optimized conditions, and the composite powder had a mean particle size of about 190 nm with inner grain size of about 50 nm. The amount of Co in the composite is controllable by adjusting the carbon addition to the starting materials.

1. Introduction Wear and corrosion of the galvanizing components (e.g. the sink roll and stabilizing roll) have been a great challenge in the hot-dip galvanizing industry [1–6]. In the past two decades, the WC-Co coating was widely used as the surface protection material for these rolls. The corrosion resistance of WC-Co coating in molten zinc depends largely on the binder phase. As reported [7,8], the metallic Co could easily react with Al of the liquid zinc (including Zn, Al, Fe, Cr, etc.) and form the CoeAl intermetallic compound which would subsequently transit to AlFe-Zn-Co phase and eventually to stable Fe2Al5. The FeeAl compound may be attached to the coating and is difficult to be removed. Also, it's suggested that the failure of WC-Co coating is resulted from the melting corrosion of Co [9]. Replacement of Co with the η phase (e.g. Co6W6C and Co3W3C) is considered to be an effectively approach to resist against the fast corrosion of WC-Co coating [5]. However, brittleness of η phase will result in the degradation of mechanical properties of the WC-based coatings [10–12]. Particularly, there is a higher tendency for cracks to initiate in the conventional coarse-grained coatings as compared with the nanocrystalline coating. The nanostructured WC-based coating has higher hardness and much higher toughness [13–15]. Thus, the nanostructured WC-η coating may be a better candidate material resisting against the wear and corrosion in environment of molten zinc. In order

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to prepare the nanostructured WC-η coating, the synthesis of nanoscale WC-η raw powder is the first important step. For the W-C-Co system, the formation of η phase mainly depends on the carbon content and the temperature [16–18]. In case of lacking carbon and lower temperatures, W and W2C may appear in the product and otherwise Co will coexist with the η phase. The temperature not only affects the stability of these phases, but also the particle size of the synthesized powder. It is a great challenge to realize the co-existence of nanoscale WC and η phase. In this work, a unique technique utilizing in situ reactions of metal oxides and carbon was used to prepare the nanocrystalline WC-η composite powder. The factors that affect the phase constitution of the powder were investigated in terms of the carbon addition to the raw materials and the reaction conditions. The phase evolution behaviors in the synthesis process of the powder will be clarified for better understanding the formation mechanisms of WC-η composite. 2. Experimental The commercially available tungsten oxide (FSSS: 10–18 μm, > 99.95% purity, Ganzhou Grand Sea W & Mo Group Co. Ltd., China), cobalt oxide (D50: 1–5 μm, BET ≥ 1.0 m2/g, 72.6–73.6% purity, Jiangsu Cobalt Nickel Metal Co. Ltd., China) and carbon black (D50: ~0.3 μm, 99.8% purity, PetroChina Southwest Oil and Gasfield

Corresponding author. E-mail address: [email protected] (X. Song).

https://doi.org/10.1016/j.ijrmhm.2017.10.022 Received 21 July 2017; Received in revised form 27 October 2017; Accepted 28 October 2017 Available online 31 October 2017 0263-4368/ © 2017 Elsevier Ltd. All rights reserved.

International Journal of Refractory Metals & Hard Materials 71 (2018) 21–27

H. Wang et al.

(a)

-100

1-CoWO4 2-WO2 3-W 4-W2C 5-Co6W6C 6-Co3W3C

1 124 6 35

16.3%

-150

61

-200

1

G (kJ/mol)

43 5 6 1121 616 61 1 2 1 4 4 5116511 1 12 1 1312 4 11 6 6

16.1%

15.9%

15.7%

30

40

50 60 2 (deg.)

70

80

-250

(a)

(1)-Co3O4+3WO3+C=3CoWO4+CO (2)-Co3O4+3WO3+0.5C=3CoWO4+0.5CO2

(1) (2)

-300 -350

465oC

-400 90

-450 -500

1-WC 2-Co3W3C 3-W 4-W2C 5-Co6W6C 6-WO2 7-CoWO4

1 1 4 2 12 2 4 23 5 25 54

200

16.3%

4

32 4 13

1 4 2 11

1 27

15.9%

7

(kJ/mol)

6

15.7%

30

40

Intensity (a. u.)

(c)

50

80

2 12

1 23 2 1 1

1

30

55

5

5 5

5

Intensity (a. u.)

2

2

200

(4)-CoWO4+5C=WC+Co+4CO (5)-CoWO4+2.5C=W2C+Co+2CO2 (6)-CoWO4+4.5C=W2C+Co+4CO (7)-CoWO4+2C=W+Co+2CO2

790 oC 675 oC

727 oC

400

600 800 1000 Temperature (K)

1200

1400

Fig. 2. Changes of Gibbs free energy of formation (a) and decomposition (b) reactions of CoWO4 as a function of temperature.

2 2 3

60 2 (deg.)

70

80

-10

90

1-WC 2-Co6W6C 3-Co 4-Co2W4C

3

1 2 12

16.3%

1 2 2 11

1

2

16.1%

(1)-W+C=WC (2)-W+0.5C=0.5W2C (3)-0.5W2C+0.5C=WC

-20

(3) o

770 C

-30 15.9%

4 2

30

(3)-CoWO4+3C=WC+Co+2CO2

-200

4 4

4

1400

15.7%

1 2

1200

(8)-CoWO4+4C=W+Co+4CO

-100

15.9%

50

1 1

300 (7) 200 (5) (3) 100

16.1%

40

(d)

(8) (b) 500 (6) 400 (4)

600 800 1000 Temperature (K)

0

16.3%

3 34

5

90

1 2 2 2 3 34 3

32

70

1-WC 2-Co6W6C 3-Co3W3C 4-Co 5-Co2W4C

1 1

60 2 (deg.)

400

600

16.1%

G (kJ/mol)

Intensity (a. u.)

(b)

(2) (1)

15.7%

40

50 60 2 (deg.)

70

80

-40 200

90

Fig. 1. XRD patterns of the powder synthesized at target temperatures of (a) 850 °C, (b) 900 °C, (c) 950 °C and (d) 1000 °C and a constant holding time of 1 h.

400

600 800 1000 1200 1400 Temperature (K)

Fig. 3. Changes of Gibbs free energy of the carbonization reactions of W and W2C as a function of temperature.

Company) were used as raw materials. With respect to various stoichiometric ratios of the η phases, the Co6W6C compound with the lowest carbon content among η phase, is chosen as the target products. The ratio of the raw materials is determined according to the following reaction equation:

WO3 + Co3 O4 + C → WC + Co6 W6C + CO

(1)

The theoretical Co content in the synthesized powder is designed as 12 wt%. In order to optimize the composition of the synthesized powder, the amount of carbon in the raw materials is adjusted from 15.7 wt% to 16.5 wt%. The raw powders were mixed by ball milling for 22

International Journal of Refractory Metals & Hard Materials 71 (2018) 21–27

H. Wang et al.

0 (a)

(1)-W2C+3Co+W=Co3W3C (2)-W2C+6Co+4W=Co6W6C

-60

16.1% 15.9%

30

(4) (3)

200

-80

(b) -100

15.7%

400

627oC 600 800 1000 1200 1400 Temperature (K)

(b)

(5)-Co6W6C+C=6WC+6Co (6)-2Co3W3C+4C=6WC+6Co

50

60

70

80

90

1 2

1-WC 2-Co6W6C 3-Co2W4C 4-Co

1

1 2

2 2 4

2 11

16.3%

1 2 211

1

30

2

16.1%

4

3

o 617 C

-140

40

2 (deg.)

-120

3 3

15.9%

33 3

15.7%

40

50

60

70

80

90

2 (deg.) (5)

-180

(6)

200

(c)

400

600 800 1000 1200 1400 Temperature (K)

Fig. 4. Changes of Gibbs free energy of the formation (a) and decomposition (b) reactions of Co6W6C and Co3W3C as a function of temperature.

16.3% 16.1% 15.9% 15.7%

8

3

16.3%

2 12

2

121 1 2 21

16.1%

40

50

1 1 23

60

2 (deg.)

15.7%

2 70

80

90

1-WC 2-Co6W6C 3-Co2W4C 4-Co

1 22 3 3 432

2 11

2

12 11

16.3%

1

15.9%

3

15.7%

3

900 950 o Temperature ( C)

30

1000

40

2

16.1%

4

4 850

2

15.9%

(d)

6

2 2 2 3 343 4

30

Intensity (a. u.)

10

2

1-WC 2-Co6W6C 3-Co2W4C 4-Co

1

1 1

Intensity (a. u.)

-160

Carbon content (wt.%)

16.3%

45 24 3 145 3 33 1 4 1 4

5

-100 -120

1-WC 2-W 3-W2C 4-Co3W3C 5-Co6W6C 6-WO2 7-CoWO4

1 1 3 4 2 14 34 3 4 5 4 7 5 5 3

(2)

-80

G (kJ/mol)

Intensity (a. u.)

(1) (3)-3W+3Co+C=Co3W3C (4)-6W+6Co+C=Co6W6C

-40

6

Intensity (a. u.)

G (kJ/mol)

-20

(a)

50

60

70

80

90

2 (deg.) Fig. 6. XRD patterns of the powder synthesized at target temperatures of (a) 850 °C, (b) 900 °C, (c) 950 °C and (d) 1000 °C and a constant holding time of 3 h.

Fig. 5. Carbon contents of the powders synthesized under different carbon additions and reaction temperatures. The target temperature was kept for 1 h.

X-ray diffraction (XRD) method using Cu Kα radiation in a Rigaku Ultimate IV diffractometer. The carbon content of the powder was measured by the CS844 analyzer (LECO in US). The amount of Co phase of the synthesized powder was measured by the ACOMT-I cobaltmagnetism apparatus (Chang Sha Xianyou in China). Before tests, the powder was weighed and put into a plastic cylindrical container. Then

20 h with a ball-to-powder weight ratio of 3:1.Then the mixture was sent to a vacuum furnace for the in situ reactions. The target temperature of the reactions was designed as 850 °C, 900 °C, 950 °C and 1000 °C with the holding time of 1 h and 3 h, respectively. The phase constitution of the synthesized powder was detected by 23

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H. Wang et al.

Carbon content (wt. %)

5.0

16.3% 15.9%

At 900 °C, CoWO4 almost decomposes owing to the increased diffusion rate of carbon as compared to that at 850 °C. The synthesized powder mainly consists of WC, W2C, W and Co3W3C, as shown in Fig. 1(b). Fig. 3 shows ΔG of the carbonization reactions of W and W2C, as marked with (1)–(3). The probability that WC is formed through the direct carbonization of W is higher than through W2C as an intermediate product over the studied temperature range. It's seen from Fig. 1(b) that with the increase of the carbon addition, the W content of the synthesized powder gradually decreases and the content of WC is increased. However, W2C may be formed in the carbon-deficient regions of the powder according to Reaction (2). Moreover, W2C is less likely to transform into WC at above 770 °C as compared with its formation reaction. Therefore, a certain amount of W2C remains in the synthesized powder. At 950 °C, the synthesized powder mainly consists of WC and η phase including Co6W6C, Co2W4C and Co3W3C, as show in Fig. 1(c). Obviously, the decomposition products of CoWO4 have further changed into η phase. In fact, the transformation partly starts at 900 °C (see Fig. 1(b)). Fig. 4(a) shows ΔG of the formation process of η phase as a function of temperature. It's seen that Co6W6C and Co3W3C are probably formed from W, W2C, Co and C in thermodynamics, as indicated by Reactions (1)–(4). Moreover, Co3W3C has the highest priority to form at above 627 °C. This is consistent with the experimental result as shown in Fig. 1(b). For all the carbon additions, there is WC and Co6W6C in the powders synthesized at 950 °C. Under lower carbon additions (e. g. 15.7% and 15.9%), Co2W4C is also included. However, with the carbon addition increases to 16.1%, Co2W4C transforms into Co3W3C accompanied by the occurrence of Co. When the reaction temperature increases to 1000 °C, Co3W3C transforms back to Co2W4C under all carbon additions, as shown in Fig. 1(d). This indicates that Co2W4C has higher phase stability at elevated temperature. Fig. 4(b) shows ΔG of the transformation process of η phase into WC as a function of temperature. It's seen that Co6W6C and Co3W3C can also transform into WC and Co at low temperatures when the required carbon is available. The experimental result confirms that this process can be completely finished at above 950 °C for 1 h (see Fig. 1(c) and (d)). Actually, the temperature for the complete carbonization of W into WC is usually higher than 1400 °C [19,20]. The decrease in the synthesis temperature is attributed to the intermediate η phase that decreases the energy barrier for WC formation. Therefore, at a relatively low temperature, the WC-η composite powder without any impurity phase is obtained. Fig. 5 shows the measured carbon contents of the powders synthesized under different conditions. The carbon contents of the powders synthesized at 850 °C are much higher than those obtained at elevated temperatures. This is attributed to the fact that the main phase of the powders synthesized at 850 °C is CoWO4. In the formation process of CoWO4 phase, only a little carbon of the raw materials is consumed. Above 900 °C, most of the metal oxides have been reduced by carbon. Accordingly, the carbon contents of the synthesized powders almost remain constant regardless of the change of reaction temperature. With the carbon addition increases in the raw materials, the carbon content of the powder synthesized at the same temperature is increased. Correspondingly, the low‑carbon phase Co2W4C transforms into the high‑carbon phase Co3W3C. In addition, for the composite powder prepared with carbon addition of 15.7% and 15.9%, the final carbon contents in the powder are virtually the same at the reaction temperature of 850–900 °C, as shown in Fig. 5. This is due to that the additional carbon may promote the formation of CoWO4, and release in the form of CO/CO2. At 850 °C, the reaction products are mainly CoWO4 and W (see Fig. 1(a)). As the carbon addition increases to 16.1% and 16.3%, much higher amount of CoWO4 is formed in the powders, suggesting that the reactions regarding the formation of CoWO4 were activated due to enhanced carbon activity. At 900 °C, most CoWO4 decomposes into WC, W2C and W, in which a certain amount of W further transforms into WC. With

16.1% 15.7%

4.5

4.0

3.5

850

900 950 Temperature ( C)

1000

Fig. 7. Carbon contents of the powders synthesized with different carbon conditions and reaction temperatures. The target temperature was kept for 3 h.

Table 1 Amount of magnetic cobalt of the powders synthesized under different conditions. Reaction temperature

850 °C 900 °C 950 °C 1000 °C

Holding time

1h 3h 1h 3h 1h 3h 1h 3h

Carbon addition (wt%) 15.7

15.9

16.1

16.3

0 0 0 0.58% 0.53% 0.56% 0.54% 0.55%

0 0 0 1.72% 1.55% 1.69% 1.54% 1.64%

0 0 0.77% 3.61% 3.32% 3.64% 3.40% 3.59%

0 0 0.89% 5.07% 4.64% 5.09% 4.79% 4.99%

the amount of magnetic Co of the powder can be determined according to its saturation magnetization. The apparatus has a high sensitivity up to 1 mg Co. The morphology and microstructures of the powder were observed by scanning electron microscopy (SEM) and transmission electron microscope (TEM). The particle sizes of the powders were estimated by linear intercept method based on the SEM images. The thermodynamic calculations were performed by the FactSage software. 3. Results and discussion 3.1. Influences of carbon addition and reaction temperature Fig. 1 shows the phase constitution of the synthesized powders under various carbon additions and reaction temperatures. The target temperature was kept for 1 h. At 850 °C (see Fig. 1(a)), the main phases of the synthesized powders are CoWO4 and W. Besides, there are a small amount of W2C, Co3W3C, Co6W6C and WO2. Fig. 2 shows the changes of Gibbs free energy (ΔG) of formation and decomposition reactions of CoWO4 as a function of temperature. As shown in Fig. 2(a), ΔG of both Reactions (1) and (2) are below zero over the studied temperature range. This indicates that CoWO4 can be formed by reactions of the metal oxides and carbon at low temperatures in thermodynamics. The reaction with the gas product of CO is more likely to take place at above 465 °C. As indicated by Reactions (3) and (4) in Fig. 2(b), it is possible for CoWO4 to change into WC and Co at above 675 °C. Also, W2C and W are probably produced when the temperature is higher than 727 °C, as indicated by Reactions (5)–(8). However, it is W and W2C that firstly form in the powder rather than WC at 850 °C, as shown in Fig. 1(a). It is attributed to the insufficient diffusion of carbon at this temperature, leading to the slow rate of carbonization reactions of W. 24

International Journal of Refractory Metals & Hard Materials 71 (2018) 21–27

H. Wang et al.

(a)

Fig. 8. SEM morphology of the in situ synthesized WC-η composite powders under different reaction conditions, (a) 900 °C for 3 h, (b) 950 °C for 1 h, (c) 950 °C for 3 h, (d) 1000 °C for 1 h, (e) 1000 °C for 3 h. The carbon addition to the raw materials is 15.7%.

(b)

500nm (c)

500nm (d)

500nm

500nm

(f) Mean particle size (nm)

(e)

300

260 240 220 200 180

500nm

3h 1h

280

900

930 960 990 o Temperature ( C) different phases is different. Specifically, the carbon atoms may diffuse more quickly in CoWO4 than in WC at the same temperature. As a reason, the rates of reactions relating to CoWO4 are higher than those relating to WC. At above 900 °C for 3 h, the main phases of the powder remain constant as WC and Co6W6C, as shown in Fig. 6(b–d), indicating that these phases are in equilibrium state at high temperatures. However, the similar phase constitution is obtained at a critical temperature up to 1000 °C for 1 h. Insufficient carbon diffusion within the limited time is responsible for the incomplete reactions that probably take place at lower temperatures according to thermodynamic calculations (see Figs. 2–4). Therefore, in addition to increasing the temperature, extending the holding time is also favorable for obtaining the desirable phase constitution. Fig. 7 shows the carbon contents of the powders synthesized at different temperatures for 3 h. As compared to those synthesized with the holding time of 1 h, the carbon contents of the powders synthesized at 850 °C for 3 h significantly decrease. This is consistent with the phase transformation of the powders. Similarly, the carbon contents of these powders synthesized at elevated temperatures almost remain constant.

increasing the carbon addition, both of the transformation rates of CoWO4 and W are increased. It can be concluded that the carbon activity plays a critical role in triggering these reactions. 3.2. Influence of holding time According to the above results, the diffusion rate of carbon has a great effect on the carburization reactions. Besides the temperature, the holding time is also an important factor that influences the diffusion of carbon. In order to observe the phase evolution behavior of the powder with the holding time, the mixed raw materials are subjected to in situ reactions at different temperatures for 3 h. Fig. 6 shows the phase constitutions of the obtained powders. As compared with those synthesized at 850 °C for 1 h (see Fig. 1(a)), the powders synthesized with 3 h have significantly decreased amount of CoWO4, as shown in Fig. 6(a). The powder mainly consists of WC, W2C and Co3W3C, which is similar to that of the powder synthesized at 900 °C for 1 h. As the reaction time extends to 3 h, the transformation from W and W2C to WC can be completed at 900 °C, as shown in Fig. 6(b). The above results indicate that the formation of WC at 900 °C is dominated by carbon diffusion. The diffusion rate of carbon into 25

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H. Wang et al.

Fig. 9. TEM micrographs of the as-synthesized WC-η composite powder: (a) bright-field image of the powder particles; (b) HRTEM image of the nanocrystalline structure within the particle. The insets show the FFT patterns of the corresponding grains and the indexing.

A

(a)

(1011) (1100)

WCHCP B 50 nm (b)

(1010)

B WCHCP

1.948 Å

C

D A

(0001)

(0111)

C

(1010)

5 nm

WCHCP Fig. 10. Typical cross-sectional microstructure of a thermal spray feedstock particle and the EDS element maps of Co and W.

Co

W

5µm

addition at 900 °C for 1 h, and the magnetic saturation of the powder is zero (see Table 1). This indicates that the η phase doesn't exhibit any magnetism, and the measured magnetic saturation is contributed by Co in the powder. From Table 1, it is seen that Co is not detected for the powders synthesized at 850 °C no matter how the carbon addition and the holding time change. The powders with the same carbon addition have an approximately equivalent amount of Co except those synthesized at 900 °C for 1 h. Under the same reaction conditions, the amount of Co of the powders increases with increasing the carbon addition.

3.3. Amount of magnetic Co The existence of Co phase in the synthesized powder is detrimental to the subsequently fabricated WC-based coating. With respect to the limitation of XRD detection, it is proposed by the author that the saturation magnetization of the powder was measured for exactly detecting the amount of Co phase. Table 1 shows the measured amount of Co of the powders synthesized under different conditions. It contains Co3W3C and Co6W6C for the powder synthesized with 15.7% carbon 26

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However, there is a distinct difference in the amount of Co of the powders synthesized at 900 °C for 1 h and 3 h. The reason is that the reaction products for 1 h contain CoWO4 whereas the oxide transforms into the η phase and Co for 3 h. Accordingly, it can be concluded that both the carbon addition of the raw materials and the hold time at the target temperature are essential for obtaining the pure WC-η composite powder at lower reaction temperatures, whereas the carbon addition is the only significant factor at elevated temperatures (e. g. above 900 °C).

than that for tungsten carburization. (3) By means of measuring saturation magnetization, Co with very small amount can be distinguished in the synthesized WC-η composite powder. The amount of Co depends largely on the carbon addition to the starting materials. Due to synthesis by in situ reactions, the η phase distributes homogeneously among WC particles.

3.4. Morphology and microstructure of the WC-η composite powder

This work was supported by the National Natural Science Foundation of China (51601004) and the Key Program of National Natural Science Foundation of China (51631002).

Acknowledgments

Fig. 8 shows the SEM morphology and the mean particle size of the WC-η composite powders synthesized under different conditions. It's seen that the mean particle sizes of the powders are in the range of 190–300 nm. With increasing the temperature and holding time, the mean particle size of the powder increases following a linear tendency. Since WC and η phase are synthesized by in situ reactions, the η phase has a homogeneous distribution among WC particles, which results in a higher agglomeration tendency of particles. Fig. 9 shows the TEM microstructure of the powder synthesized at 900 °C for 3 h. From Fig. 9(a), it is seen that there are several nanograins of about 50 nm within the particle. Fig. 9(b) shows the high-resolution TEM image of a local region of the particle and the corresponding fast Fourier transformation (FFT) patterns and their indexing. According to the indexing, the outer grains are WC with a hexagonal structure, as marked with A, B and C. The interplanar spacing of a certain plane of the central grain (as marked with D) is measured as 1.948 Å. However, only a single orientation of the grain can be distinguished by TEM so that its crystal structure is not determined. According to the morphology and size of the central grain shown in Fig. 9(a), it is probably Co-rich phase. In general, With respect to the grain coarsening induced by high temperature and long holding time, the suitable preparation conditions for the nanocrystalline WC-η composite powder are 900 °C for 3 h and with the carbon addition of about 15.7%. Using the synthesized nanocrystalline WC-η composite powder as raw material, the thermal spray feedstock was prepared by spray granulation and a subsequent heat-treatment. A typical cross-sectional microstructure of the feedstock particle is shown in Fig. 10. The particle has a spherical shape and relatively high density. The EDS analysis result demonstrates that the Co-containing η phase has a homogeneous distribution among WC particles. This is favorable to improve the corrosion resistance of the fabricated coating against molten zinc.

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4. Conclusions In this study, the phase evolution in the process of in situ synthesis of WC-η composite powder was investigated, based on which the composition of the composite powder was optimized. The following conclusions are obtained: (1) The method of in situ reactions is suitable for synthesis of nanocrystalline WC-η composite powder. The phase constitution, carbon content and particle size of the composite powder are controllable. The mean grain size inside the powder particle is about 50 nm. (2) Both thermodynamic calculations and experimental investigations show that, the initial WO3, Co3O4 and C firstly transform to CoWO4, then to W and W2C, and eventually to η and WC. The formation of η phase decreases the energy barrier for WC formation. Therefore, the in situ reactions can be completed at a temperature much lower

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