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The effect of Cu addition on the crystallization behavior and tribological properties of reactive plasma sprayed TiCN–Cu coatings
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
The effect of Cu addition on the crystallization behavior and tribological properties of reactive plasma sprayed TiCN–Cu coatings Hongjian Zhaoa,b,c, Fangfang Guoa,b,c, Lingyan Zhua,b,c, Jining Hea,b,c,∗, Fuxing Yina,b,c,∗∗ a
School of Materials Science and Engineering, Hebei University of Technology, Tianjin, 300132, China Research Institute for Energy Equipment Materials, Hebei University of Technology, Tianjin, 300132, China c Tianjin Key Laboratory of Materials Laminating Fabrication and Interface Control Technology, Hebei University of Technology, Tianjin, 300132, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: TiCN-Cu coatings Reactive plasma spraying Crystallization Tribological properties
TiCN–Cu coatings were fabricated by reactive plasma spraying using Ti-graphite aggregates and Cu powders. The crystallization, mechanical and tribological properties were investigated. The results showed that TiCN–Cu coatings shared a face-center cubic structure and they consisted of TiC0·7N0.3, TiN0·3O, Cu2O, Cu, amorphous CNx and residual graphite phases. The addition of Cu mainly changed the crystallization behavior of the coating surface and binding surface with the substrate. Compared with TiCN coating, TiCN–Cu coatings had the lower hardness and hardness distribution uniformity. The incorporation of Cu improved the coating fracture toughness, while failed to improve the coating tribological properties. The main wear mechanism of TiCN–Cu coatings in the present sliding test were adhesive wear, coupling with partial tribo-oxidation wear.
1. Introduction In the past years, TiCN ceramic coatings have shown potential as the protective materials in many fields (such as cutting tools and dies) owing to the excellent mechanical properties and good wear resistance, which combine the excellent properties of TiC and TiN [1-3]. So far, many methods have been used to prepare TiCN coatings, such as physical vapor deposition (PVD) [4-6], chemical vapor deposition (CVD) [7,8] and reactive plasma spraying (RPS) [9-11]. For PVD and CVD, the coating thickness is very limited and these techniques are expensive and tedious [12]. For instance, Liu et al. [13] prepared TiCN coating by PVD on Sialon ceramic cutting inserts and investigated the coating properties. Although the coating hardness was 22.56 ± 0.99 GPa and it improved the cutting performance of the inserts remarkably, only a thin coating thickness of 4 μm was obtained after 45min deposition. Similarly, Rie et al. [14] prepared TiCN coating by CVD and only a growth rate of 1.0–2.0 μm/h was measured. Recently, RPS, which combines the atmosphere spray process and high-temperature synthesis, has received overwhelming interest in fabricating thick TiCN coatings for industrial applications because of the fast deposition rate and simple process [15,16]. Mi et al. [17] prepared TiCN coatings by RPS using Ti and graphite powders. They found that the coating with a thickness of 300 μm could be prepared within 5 min. The hardness and fracture toughness of the coating were 1674 ± 197HV0.1 and ∗
3.76 ± 0.31 MPa m1/2, respectively. However, for RPS ceramic coatings, some defects (such as pores, voids and micro-cracks) are unavoidable in them because of the limit bonding strength between the deposited molten particles and this would weaken the coating service performance [18]. As reported, many metallic additives (such as Co, Ni and Al) were regarded as an effective way to improve the mechanical and tribological properties of the ceramic coatings by enhancing bonding strength [19-22]. Recently, Zhang et al. [23,24] prepared TiCN–Cr and TiCN–Mo coatings using RPS. They found that the addition of Cr (or Mo) enhanced the coating mechanical properties and improved the wear resistance. Similarly, Cu, as one of the metallic additives, has been successfully added into nitride/carbide based ceramics to improve their mechanical and tribological properties owing to the good wetting behavior and low shear strength [25,26]. In this context, the addition of Cu into RPS TiCN coatings could also improve the integrated performance. However, up to now, few studies on the preparation and properties of RPS TiCN–Cu coatings have been reported. In this paper, the TiCN–Cu composite coatings were prepared by RPS using Cu powders and Ti-graphite powders. The crystallization behavior, mechanical and tribological properties of TiCN–Cu coatings were investigated.
Corresponding author. School of Materials Science and Engineering, Hebei University of Technology, Tianjin, 300132, China. Corresponding author. School of Materials Science and Engineering, Hebei University of Technology, Tianjin, 300132, China. E-mail addresses: [email protected] (J. He), [email protected] (F. Yin).
∗∗
https://doi.org/10.1016/j.ceramint.2019.12.066 Received 5 October 2019; Received in revised form 23 November 2019; Accepted 5 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Hongjian Zhao, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.066
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Fig. 1. SEM morphology of Ti (a), Cu (b), graphite (c) and Ti-graphite aggregates (d).
2. Experimental details
ray diffraction (D8 FOUCUS). The chemical bond in the coatings was analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5700 ESCA System) after removing the surface impurities by sputtering with Ar+ ion beam. Scanning electron microscopy (SEM, JSM-7100F, JEOL Ltd.) with the energy dispersive X-ray spectrometer (EDS) was used to characterize the coating surface and cross-section morphologies. To better observe the crystalline morphology of the coating, the coating surface and cross-section were corroded for 25min by mixed corrosion liquid (the ratios of nitric acid, hydrofluoric acid and alcohol were 1:1:8, respectively). The elements distribution on the coating surface was analyzed by electron probe microanalysis (EPMA, JXA-8530F, JEOL). The microhardness of the coatings were measured using Vicker microhardness tester (HX-1000) under a load of 100g. The sliding tests of the coatings were conducted using M − 200 tribometer (Xuanhua Material Test Machine Co., Ltd., Xuanhua, China). The friction counterpart was GCr15 steel ring (Diameter:40 mm). The normal load was 200 N and the sliding speed was 200 rpm. The sliding time was 30min. The wear loss was weighted in scales. The wear tracks were observed by SEM.
The raw materials were Ti powder with a size of 30–45 μm and a purity of 99.9% (Fig. 1a), Cu powder with a size of 30–45 μm and a purity of 99.9% (Fig. 1b) and graphite powder with a size of 5 μm (Fig. 1c). Firstly, Ti-graphite aggregates with a weight ratio 6:1 were prepared by mixing Ti and graphite (Fig. 1d). The specific mixing processes could be found in Ref. [23]. Secondly, the feedstock was prepared by mixing Ti-graphite aggregates with different weights of Cu powders (2 wt%, 4 wt%, 6.7 wt% and 10 wt%) using Al2O3 ball milling for 24h. The substrates were the medium carbon steels (10 mm × 10 mm × 10 mm) and they would be blasting with grit before spraying. All the coatings were sprayed using the GP-80 plasma spray equipment. As a bond coating to improve the adherence, Ni-10 wt %Al was sprayed. Then, TiCN and TiCN–Cu coatings were prepared. During the spray process, the powders were introduced into plasma jet by N2 and reacted to each other. The process parameters are listed in Table 1. For convenience, TiCN, TiCN-2wt.%Cu, TiCN-4wt.%Cu, TiCN6.7 wt%Cu and TiCN-10 wt%Cu coatings were noted as 0CT, 2CT, 4CT, 6.7CT and 10CT, respectively. The phase structure of the polished coatings were examined by X-
3. Results and discussions 3.1. Microstructure observation
Table 1 Process parameters of reactive plasma sprayed coatings. Parameters
Ni-10 wt%Al
CT
Ar gas fluent (protection), L/min N2 gas fluent (reaction), L/min N2 gas fluent (carrier), L/min Current, A Voltage, V Power, kW Gun-substrate distance, mm
80 40 0.3 500 60 30 100
80 60 0.5 500 70 35 100
The XRD pattern of CT coatings with different Cu addition are shown in Fig. 2. As shown, the incorporation of Cu did not change the face-center cubic structure. Without Cu addition (0CT), the coating mainly consisted of TiC0·7N0.3 phase. After adding metal Cu, TiN0·3O and Cu2O phases were also detected. With the increase of Cu, the peak intensities of TiC0·7N0.3 and Cu2O increased, coupling with the peak intensities of TiN0·3O decreasing. As the added Cu content was 10 wt%, the peak of TiN0·3O phase at about 70° disappeared. During spraying, the added Cu was easier to react with oxygen compared to Ti and this 2
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(Fig. 3d), the peak at 397.3eV belonged to N–Ti bond. The peaks at 399.3eV and 400eV belonged to sp2 N–C and sp3 N–C bonds, respectively. The peak at 396.2eV belonged to N–O bond. The presence of sp2 C–N and sp3 C–N bonds implies that the coating contained some amorphous CNx. From Cu2p spectra (Fig. 3e), two peaks at 932.7eV and 952eV belonged to pure metallic Cu. The other peaks belonged to Cu–O bond. The surface morphologies of CT coatings with different Cu addition after corrosion are shown in Fig. 4. Some pores and voids are observed on all coating surfaces. For 0CT and 2CT coating (Fig. 4a and b), many dendritic grains parallel to the coating surface were observed (A and B area). For 4CT coating (Fig. 4c), more equiaxed grains were observed (C area) as compared with which in 2CT coating. For 6.7CT and 10CT coatings (Fig. 4d and e), the columnar grains perpendicular to the coating surface were observed (D and E area). Generally, all the particles became molten drops under the action of the flame and then cooled on the substrate, finally solidified layer upon layer. With the increase of Cu, the heat transfer perpendicular to the coating surface increased and this increased the temperature gradient between the coating and the atmosphere. Thus, this would increase the rapid crystallization of the deposited drops to form the columnar grains along with the perpendicular to the coating surface. Moreover, the difference in the cooling velocity of the deposited drops also resulted in the pores and voids forming [23]. The cross-section morphologies of CT coatings with different Cu addition after corrosion are shown in Fig. 5. All the coatings showed a typical layer-layer structure. The crystal structure of each layer were columnar (most), dendritic and equiaxed (a few). With the increase of Cu, the grain sizes decreased. The binding surface morphologies between CT coatings and substrate after corrosion are shown in Fig. 6. For 0CT coating (Fig. 6a), the columnar grains perpendicular to the substrate were observed, which was because of the huge temperature gradient between the coating and the substrate. For 4CT coating (Fig. 6b), the growth direction of columnar grains existed a small angle of inclination towards to the substrate. Besides, some equiaxed grains were also observed. For 6.7CT and 10CT coatings (Fig. 6c and d), many dendritic grains parallel to the
Fig. 2. The XRD pattern of CT coatings with different Cu addition.
decreased the formation of TiN0.3O. Metal Cu phase and other compounds containing C and N were not detected. In order to further study the chemical valence state and the corresponding phase composition, XPS spectra of 10CT coating and Ti2p, C1s, N1s, Cu2p spectra were measured and the results are shown in Fig. 3. From the coating XPS spectra (Fig. 3a), it can be found Ti, C, N, Cu and O elements were detected in the coating. From Ti2p spectra (Fig. 3b), three peaks at 454.6eV, 458.2eV and 464.2eV could be assigned to Ti2p3/2 and Ti2p1/2, respectively. Ti2p3/2 peak at 454.6eV could be fitted into two peaks at 454.3eV and 454.9eV, which were assigned to Ti–N and Ti–C bond, respectively. The other peaks belonged to Ti–O bond. From C1s spectra (Fig. 3c), the peaks at 281.8eV, 284.5eV, 285.3eV and 288eV could be assigned to C–Ti, sp2 C–C, sp2 C–N and sp3 C–N bonds, respectively. The presence of C–C bond implies that the coating contained some unreacted graphite. From N1s spectra
Fig. 3. The XPS spectra of 10CT coating. (a) all spectra, (b) Ti2p, (c) C1s, (d) N1s, (e) Cu2p. 3
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Fig. 4. The surface morphologies of CT coatings with different Cu addition after corrosion.
hardness (1415HV0.1 ± 100) and the hardness of CT coatings with Cu addition showed a decrease trend. As the added Cu content was in the range of 2–10 wt%, 6.7CT coating had a higher hardness than those of other CT coatings. According to the analysis in Figs. 4 and 6.7CT and 10CT showed the columnar grains perpendicular to the coating surface. This could make these two coatings have stronger load-carrying capacity. However, 10CT coating contained more Cu2O and metal Cu soft phases, which had the lower hardness. Thus, the hardness of the coating decreased with adding more metal Cu. In general, in order to assess the inhomogeneous hardness distribution resulting from the defects (pores, voids and cracks) in RSP coatings, Weibull distribution was often adopted to analyze the microhardness of RPS coatings [9,16,23,24,27]. The formula was represented as below:
substrate surface were observed. For 0CT coating, the heat transfer was perpendicular to the substrate surface and this resulted in the formation of columnar grains. With the increase of Cu, the heat transfer parallel to the substrate surface increased. This resulted in the formation of the dendritic grains parallel to the substrate surface. The EPMA elemental mappings of 10CT coating are shown in Fig. 7. As shown in SEI image, white area, black area and gray area were observed. In the elemental mappings, the red color level represents high content and the black color level represents few content. As shown in Ti, C, Cu, N and O mapping, white area showed the red color level of Cu and O (partial blue), black color level of Ti, and blue color level of C and N. This indicated that white area contained metal Cu and Cuoxides, and no reaction among Ti, C, N and Cu occurred. Black area showed the red color level of C, black color level of Ti, and blue color level of Cu, N (partial green) and O. This indicated that black area contained residual graphite and CNx, which was consistent with the XPS analysis. Part of gray area showed red color level of Ti and green color level of C and N, implying that the reaction among Ti, C and N occurred. Part of gray area showed red color level of Ti and O, and green color level of N, implying that the reaction among Ti, N and O occurred.
F (H ) = 1 − exp[−(H / η) β ]
(1)
Where H is hardness, η is normalized tested parameter, β is Weibull shape parameter. The transformed formula from Eq. (1) is represented as below:
ln[−ln(1 − F (H ))] = β [ln(H ) − ln(η)]
(2)
According to Eq. (2), a diagram of the relationship between ln (1-F (H)) and ln(H) can be obtained and the results are shown in Fig. 9. As shown, a approximately linear relationship could be observed for all the plots. The correlation coefficient (R) values were closed to 1.0, which indicated that the hardness values were reliable. The β values of
3.2. Mechanical properties of the coatings The average microhardness of CT coatings with different Cu addition are shown in Fig. 8. As shown, 0CT coating had a maximum
Fig. 5. The cross-section morphologies of CT coatings with different Cu addition after corrosion. 4
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Fig. 6. The binding surface morphologies between CT coatings and substrate after corrosion.
3.3. Tribological properties of the coatings
(2–10)CT coatings were lower than that of 0CT coating, indicating the hardness values had a high variability after adding Cu into the coatings. In order to characterize the toughness of the brittle coatings, the crack extension force were calculated according to the following formula:
Gc = 6.115 ∗ 10−4 (
Pa2 ) c3
The friction coefficient and wear weight loss of CT coatings with different Cu addition are shown in Fig. 10. As shown, the friction coefficient and wear weight loss of TiCN coating were about 0.5 and 0.006g. The friction coefficient and wear weight loss of (2–10)CT coatings were higher than that of 0CT coating. In this work, most of Cu reacted with O to form Cu2O during RPS process, which was adverse to improve the coating tribological properties. In order to study the wear mechanism, the wear-track morphologies and the corresponding EDS spot analysis of CT with different Cu addition (2%, 6.7% and 10%) are shown in Fig. 11. On all worn surfaces, a large amount of wear debris and some cracks were observed. In addition, brittle fracture was also observed on worn surfaces of 10CT coating. According to the EDS spot results, for 2CT coating, the worn surface contained Ti (9.85 at.%), C (12.33 at.%), N (1.16 at.%), O (44.68 at.%) and Fe (31.98 at.%). No Cu was detected in this spot. For
(3)
Where P is the load, N; a is half of the diagonal length of the indentation, m; c is half of the diagonal length of the crack at the tip of the indentation, m. According to Eq. (3), the calculated crack extension force of 0CT and10CT coatings were approximately 33.4 J/m2 and 45.8 J/m2, respectively. This indicated that the addition of Cu increased the coating fracture toughness.
Fig. 7. The EPMA elemental mappings of 10CT coating. 5
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Fig. 10. The friction coefficient and wear weight loss of CT coatings with different Cu addition. Fig. 8. The average microhardness of CT coatings with different Cu addition.
TiCN coating. TiCN–Cu coatings consisted of TiC0·7N0.3, TiN0·3O, Cu2O, Cu, amorphous CNx and residual graphite phases. With the increase of Cu, the peak intensities of TiC0·7N0.3 and Cu2O increased, coupling with the peak intensities of TiN0·3O decreasing. In addition, the addition of Cu mainly also changed the crystallization behavior of the coating surface and binding interface with the substrate. (2) The hardness of TiCN coating was 1415HV0.1 ± 100. After adding Cu, the hardness of TiCN–Cu coatings decreased and the coating hardness distribution uniformity also decreased. According to the calculated crack extension force, the addition of Cu increased the coating fracture toughness. (3) The friction coefficient and wear weight loss of TiCN coating were about 0.5 and 0.006g. As the added Cu content was in the range of 2–10 wt%, 6.7CT coating had the lowest wear weight loss. The main wear mechanism of TiCN–Cu coatings in the present sliding test were adhesive wear, coupling with partial oxidative wear.
6.7CT and 10CT coating, Cu was also detected besides Ti, C, N, Fe and O. During the sliding test, the hardness of friction ring was lower than those of the coatings and this could result in iron transferring from ring to coatings. Simultaneously, the instantaneous high temperature caused by friction also resulted in oxidation of the transfer layer and coating materials. These indicated that the adhesive and tribo-oxidation wear occurred on the wear track. 4. Conclusions In this work, TiCN–Cu coatings were fabricated by reactive plasma spraying using Ti-graphite aggregates and Cu powders. The crystallization, mechanical and tribological properties were investigated. The following conclusions were drawn: (1) TiCN–Cu coatings shared a similar structure (face-center cubic) to
Fig. 9. Weibull distribution of microhardness of CT coatings with different Cu addition. 6
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Fig. 11. The wear-track morphologies and the corresponding EDS spot analysis of CT with different Cu addition. (a) 2CT, (b) 6.7CT, (c) 10CT
Declaration of competing interest [7]
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.
[8]
[9]
Acknowledgements
[10]
This research is financially supported by the National Natural Science Foundation of China (Grant No. 51872073).
[11]
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
The effect of Cu addition on the crystallization behavior and tribological properties of reactive plasma sprayed TiCN–Cu coatings Hongjian Zhaoa,b,c, Fangfang Guoa,b,c, Lingyan Zhua,b,c, Jining Hea,b,c,∗, Fuxing Yina,b,c,∗∗ a
School of Materials Science and Engineering, Hebei University of Technology, Tianjin, 300132, China Research Institute for Energy Equipment Materials, Hebei University of Technology, Tianjin, 300132, China c Tianjin Key Laboratory of Materials Laminating Fabrication and Interface Control Technology, Hebei University of Technology, Tianjin, 300132, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: TiCN-Cu coatings Reactive plasma spraying Crystallization Tribological properties
TiCN–Cu coatings were fabricated by reactive plasma spraying using Ti-graphite aggregates and Cu powders. The crystallization, mechanical and tribological properties were investigated. The results showed that TiCN–Cu coatings shared a face-center cubic structure and they consisted of TiC0·7N0.3, TiN0·3O, Cu2O, Cu, amorphous CNx and residual graphite phases. The addition of Cu mainly changed the crystallization behavior of the coating surface and binding surface with the substrate. Compared with TiCN coating, TiCN–Cu coatings had the lower hardness and hardness distribution uniformity. The incorporation of Cu improved the coating fracture toughness, while failed to improve the coating tribological properties. The main wear mechanism of TiCN–Cu coatings in the present sliding test were adhesive wear, coupling with partial tribo-oxidation wear.
1. Introduction In the past years, TiCN ceramic coatings have shown potential as the protective materials in many fields (such as cutting tools and dies) owing to the excellent mechanical properties and good wear resistance, which combine the excellent properties of TiC and TiN [1-3]. So far, many methods have been used to prepare TiCN coatings, such as physical vapor deposition (PVD) [4-6], chemical vapor deposition (CVD) [7,8] and reactive plasma spraying (RPS) [9-11]. For PVD and CVD, the coating thickness is very limited and these techniques are expensive and tedious [12]. For instance, Liu et al. [13] prepared TiCN coating by PVD on Sialon ceramic cutting inserts and investigated the coating properties. Although the coating hardness was 22.56 ± 0.99 GPa and it improved the cutting performance of the inserts remarkably, only a thin coating thickness of 4 μm was obtained after 45min deposition. Similarly, Rie et al. [14] prepared TiCN coating by CVD and only a growth rate of 1.0–2.0 μm/h was measured. Recently, RPS, which combines the atmosphere spray process and high-temperature synthesis, has received overwhelming interest in fabricating thick TiCN coatings for industrial applications because of the fast deposition rate and simple process [15,16]. Mi et al. [17] prepared TiCN coatings by RPS using Ti and graphite powders. They found that the coating with a thickness of 300 μm could be prepared within 5 min. The hardness and fracture toughness of the coating were 1674 ± 197HV0.1 and ∗
3.76 ± 0.31 MPa m1/2, respectively. However, for RPS ceramic coatings, some defects (such as pores, voids and micro-cracks) are unavoidable in them because of the limit bonding strength between the deposited molten particles and this would weaken the coating service performance [18]. As reported, many metallic additives (such as Co, Ni and Al) were regarded as an effective way to improve the mechanical and tribological properties of the ceramic coatings by enhancing bonding strength [19-22]. Recently, Zhang et al. [23,24] prepared TiCN–Cr and TiCN–Mo coatings using RPS. They found that the addition of Cr (or Mo) enhanced the coating mechanical properties and improved the wear resistance. Similarly, Cu, as one of the metallic additives, has been successfully added into nitride/carbide based ceramics to improve their mechanical and tribological properties owing to the good wetting behavior and low shear strength [25,26]. In this context, the addition of Cu into RPS TiCN coatings could also improve the integrated performance. However, up to now, few studies on the preparation and properties of RPS TiCN–Cu coatings have been reported. In this paper, the TiCN–Cu composite coatings were prepared by RPS using Cu powders and Ti-graphite powders. The crystallization behavior, mechanical and tribological properties of TiCN–Cu coatings were investigated.
Corresponding author. School of Materials Science and Engineering, Hebei University of Technology, Tianjin, 300132, China. Corresponding author. School of Materials Science and Engineering, Hebei University of Technology, Tianjin, 300132, China. E-mail addresses: [email protected] (J. He), [email protected] (F. Yin).
∗∗
https://doi.org/10.1016/j.ceramint.2019.12.066 Received 5 October 2019; Received in revised form 23 November 2019; Accepted 5 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Hongjian Zhao, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.066
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Fig. 1. SEM morphology of Ti (a), Cu (b), graphite (c) and Ti-graphite aggregates (d).
2. Experimental details
ray diffraction (D8 FOUCUS). The chemical bond in the coatings was analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5700 ESCA System) after removing the surface impurities by sputtering with Ar+ ion beam. Scanning electron microscopy (SEM, JSM-7100F, JEOL Ltd.) with the energy dispersive X-ray spectrometer (EDS) was used to characterize the coating surface and cross-section morphologies. To better observe the crystalline morphology of the coating, the coating surface and cross-section were corroded for 25min by mixed corrosion liquid (the ratios of nitric acid, hydrofluoric acid and alcohol were 1:1:8, respectively). The elements distribution on the coating surface was analyzed by electron probe microanalysis (EPMA, JXA-8530F, JEOL). The microhardness of the coatings were measured using Vicker microhardness tester (HX-1000) under a load of 100g. The sliding tests of the coatings were conducted using M − 200 tribometer (Xuanhua Material Test Machine Co., Ltd., Xuanhua, China). The friction counterpart was GCr15 steel ring (Diameter:40 mm). The normal load was 200 N and the sliding speed was 200 rpm. The sliding time was 30min. The wear loss was weighted in scales. The wear tracks were observed by SEM.
The raw materials were Ti powder with a size of 30–45 μm and a purity of 99.9% (Fig. 1a), Cu powder with a size of 30–45 μm and a purity of 99.9% (Fig. 1b) and graphite powder with a size of 5 μm (Fig. 1c). Firstly, Ti-graphite aggregates with a weight ratio 6:1 were prepared by mixing Ti and graphite (Fig. 1d). The specific mixing processes could be found in Ref. [23]. Secondly, the feedstock was prepared by mixing Ti-graphite aggregates with different weights of Cu powders (2 wt%, 4 wt%, 6.7 wt% and 10 wt%) using Al2O3 ball milling for 24h. The substrates were the medium carbon steels (10 mm × 10 mm × 10 mm) and they would be blasting with grit before spraying. All the coatings were sprayed using the GP-80 plasma spray equipment. As a bond coating to improve the adherence, Ni-10 wt %Al was sprayed. Then, TiCN and TiCN–Cu coatings were prepared. During the spray process, the powders were introduced into plasma jet by N2 and reacted to each other. The process parameters are listed in Table 1. For convenience, TiCN, TiCN-2wt.%Cu, TiCN-4wt.%Cu, TiCN6.7 wt%Cu and TiCN-10 wt%Cu coatings were noted as 0CT, 2CT, 4CT, 6.7CT and 10CT, respectively. The phase structure of the polished coatings were examined by X-
3. Results and discussions 3.1. Microstructure observation
Table 1 Process parameters of reactive plasma sprayed coatings. Parameters
Ni-10 wt%Al
CT
Ar gas fluent (protection), L/min N2 gas fluent (reaction), L/min N2 gas fluent (carrier), L/min Current, A Voltage, V Power, kW Gun-substrate distance, mm
80 40 0.3 500 60 30 100
80 60 0.5 500 70 35 100
The XRD pattern of CT coatings with different Cu addition are shown in Fig. 2. As shown, the incorporation of Cu did not change the face-center cubic structure. Without Cu addition (0CT), the coating mainly consisted of TiC0·7N0.3 phase. After adding metal Cu, TiN0·3O and Cu2O phases were also detected. With the increase of Cu, the peak intensities of TiC0·7N0.3 and Cu2O increased, coupling with the peak intensities of TiN0·3O decreasing. As the added Cu content was 10 wt%, the peak of TiN0·3O phase at about 70° disappeared. During spraying, the added Cu was easier to react with oxygen compared to Ti and this 2
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(Fig. 3d), the peak at 397.3eV belonged to N–Ti bond. The peaks at 399.3eV and 400eV belonged to sp2 N–C and sp3 N–C bonds, respectively. The peak at 396.2eV belonged to N–O bond. The presence of sp2 C–N and sp3 C–N bonds implies that the coating contained some amorphous CNx. From Cu2p spectra (Fig. 3e), two peaks at 932.7eV and 952eV belonged to pure metallic Cu. The other peaks belonged to Cu–O bond. The surface morphologies of CT coatings with different Cu addition after corrosion are shown in Fig. 4. Some pores and voids are observed on all coating surfaces. For 0CT and 2CT coating (Fig. 4a and b), many dendritic grains parallel to the coating surface were observed (A and B area). For 4CT coating (Fig. 4c), more equiaxed grains were observed (C area) as compared with which in 2CT coating. For 6.7CT and 10CT coatings (Fig. 4d and e), the columnar grains perpendicular to the coating surface were observed (D and E area). Generally, all the particles became molten drops under the action of the flame and then cooled on the substrate, finally solidified layer upon layer. With the increase of Cu, the heat transfer perpendicular to the coating surface increased and this increased the temperature gradient between the coating and the atmosphere. Thus, this would increase the rapid crystallization of the deposited drops to form the columnar grains along with the perpendicular to the coating surface. Moreover, the difference in the cooling velocity of the deposited drops also resulted in the pores and voids forming [23]. The cross-section morphologies of CT coatings with different Cu addition after corrosion are shown in Fig. 5. All the coatings showed a typical layer-layer structure. The crystal structure of each layer were columnar (most), dendritic and equiaxed (a few). With the increase of Cu, the grain sizes decreased. The binding surface morphologies between CT coatings and substrate after corrosion are shown in Fig. 6. For 0CT coating (Fig. 6a), the columnar grains perpendicular to the substrate were observed, which was because of the huge temperature gradient between the coating and the substrate. For 4CT coating (Fig. 6b), the growth direction of columnar grains existed a small angle of inclination towards to the substrate. Besides, some equiaxed grains were also observed. For 6.7CT and 10CT coatings (Fig. 6c and d), many dendritic grains parallel to the
Fig. 2. The XRD pattern of CT coatings with different Cu addition.
decreased the formation of TiN0.3O. Metal Cu phase and other compounds containing C and N were not detected. In order to further study the chemical valence state and the corresponding phase composition, XPS spectra of 10CT coating and Ti2p, C1s, N1s, Cu2p spectra were measured and the results are shown in Fig. 3. From the coating XPS spectra (Fig. 3a), it can be found Ti, C, N, Cu and O elements were detected in the coating. From Ti2p spectra (Fig. 3b), three peaks at 454.6eV, 458.2eV and 464.2eV could be assigned to Ti2p3/2 and Ti2p1/2, respectively. Ti2p3/2 peak at 454.6eV could be fitted into two peaks at 454.3eV and 454.9eV, which were assigned to Ti–N and Ti–C bond, respectively. The other peaks belonged to Ti–O bond. From C1s spectra (Fig. 3c), the peaks at 281.8eV, 284.5eV, 285.3eV and 288eV could be assigned to C–Ti, sp2 C–C, sp2 C–N and sp3 C–N bonds, respectively. The presence of C–C bond implies that the coating contained some unreacted graphite. From N1s spectra
Fig. 3. The XPS spectra of 10CT coating. (a) all spectra, (b) Ti2p, (c) C1s, (d) N1s, (e) Cu2p. 3
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Fig. 4. The surface morphologies of CT coatings with different Cu addition after corrosion.
hardness (1415HV0.1 ± 100) and the hardness of CT coatings with Cu addition showed a decrease trend. As the added Cu content was in the range of 2–10 wt%, 6.7CT coating had a higher hardness than those of other CT coatings. According to the analysis in Figs. 4 and 6.7CT and 10CT showed the columnar grains perpendicular to the coating surface. This could make these two coatings have stronger load-carrying capacity. However, 10CT coating contained more Cu2O and metal Cu soft phases, which had the lower hardness. Thus, the hardness of the coating decreased with adding more metal Cu. In general, in order to assess the inhomogeneous hardness distribution resulting from the defects (pores, voids and cracks) in RSP coatings, Weibull distribution was often adopted to analyze the microhardness of RPS coatings [9,16,23,24,27]. The formula was represented as below:
substrate surface were observed. For 0CT coating, the heat transfer was perpendicular to the substrate surface and this resulted in the formation of columnar grains. With the increase of Cu, the heat transfer parallel to the substrate surface increased. This resulted in the formation of the dendritic grains parallel to the substrate surface. The EPMA elemental mappings of 10CT coating are shown in Fig. 7. As shown in SEI image, white area, black area and gray area were observed. In the elemental mappings, the red color level represents high content and the black color level represents few content. As shown in Ti, C, Cu, N and O mapping, white area showed the red color level of Cu and O (partial blue), black color level of Ti, and blue color level of C and N. This indicated that white area contained metal Cu and Cuoxides, and no reaction among Ti, C, N and Cu occurred. Black area showed the red color level of C, black color level of Ti, and blue color level of Cu, N (partial green) and O. This indicated that black area contained residual graphite and CNx, which was consistent with the XPS analysis. Part of gray area showed red color level of Ti and green color level of C and N, implying that the reaction among Ti, C and N occurred. Part of gray area showed red color level of Ti and O, and green color level of N, implying that the reaction among Ti, N and O occurred.
F (H ) = 1 − exp[−(H / η) β ]
(1)
Where H is hardness, η is normalized tested parameter, β is Weibull shape parameter. The transformed formula from Eq. (1) is represented as below:
ln[−ln(1 − F (H ))] = β [ln(H ) − ln(η)]
(2)
According to Eq. (2), a diagram of the relationship between ln (1-F (H)) and ln(H) can be obtained and the results are shown in Fig. 9. As shown, a approximately linear relationship could be observed for all the plots. The correlation coefficient (R) values were closed to 1.0, which indicated that the hardness values were reliable. The β values of
3.2. Mechanical properties of the coatings The average microhardness of CT coatings with different Cu addition are shown in Fig. 8. As shown, 0CT coating had a maximum
Fig. 5. The cross-section morphologies of CT coatings with different Cu addition after corrosion. 4
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Fig. 6. The binding surface morphologies between CT coatings and substrate after corrosion.
3.3. Tribological properties of the coatings
(2–10)CT coatings were lower than that of 0CT coating, indicating the hardness values had a high variability after adding Cu into the coatings. In order to characterize the toughness of the brittle coatings, the crack extension force were calculated according to the following formula:
Gc = 6.115 ∗ 10−4 (
Pa2 ) c3
The friction coefficient and wear weight loss of CT coatings with different Cu addition are shown in Fig. 10. As shown, the friction coefficient and wear weight loss of TiCN coating were about 0.5 and 0.006g. The friction coefficient and wear weight loss of (2–10)CT coatings were higher than that of 0CT coating. In this work, most of Cu reacted with O to form Cu2O during RPS process, which was adverse to improve the coating tribological properties. In order to study the wear mechanism, the wear-track morphologies and the corresponding EDS spot analysis of CT with different Cu addition (2%, 6.7% and 10%) are shown in Fig. 11. On all worn surfaces, a large amount of wear debris and some cracks were observed. In addition, brittle fracture was also observed on worn surfaces of 10CT coating. According to the EDS spot results, for 2CT coating, the worn surface contained Ti (9.85 at.%), C (12.33 at.%), N (1.16 at.%), O (44.68 at.%) and Fe (31.98 at.%). No Cu was detected in this spot. For
(3)
Where P is the load, N; a is half of the diagonal length of the indentation, m; c is half of the diagonal length of the crack at the tip of the indentation, m. According to Eq. (3), the calculated crack extension force of 0CT and10CT coatings were approximately 33.4 J/m2 and 45.8 J/m2, respectively. This indicated that the addition of Cu increased the coating fracture toughness.
Fig. 7. The EPMA elemental mappings of 10CT coating. 5
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Fig. 10. The friction coefficient and wear weight loss of CT coatings with different Cu addition. Fig. 8. The average microhardness of CT coatings with different Cu addition.
TiCN coating. TiCN–Cu coatings consisted of TiC0·7N0.3, TiN0·3O, Cu2O, Cu, amorphous CNx and residual graphite phases. With the increase of Cu, the peak intensities of TiC0·7N0.3 and Cu2O increased, coupling with the peak intensities of TiN0·3O decreasing. In addition, the addition of Cu mainly also changed the crystallization behavior of the coating surface and binding interface with the substrate. (2) The hardness of TiCN coating was 1415HV0.1 ± 100. After adding Cu, the hardness of TiCN–Cu coatings decreased and the coating hardness distribution uniformity also decreased. According to the calculated crack extension force, the addition of Cu increased the coating fracture toughness. (3) The friction coefficient and wear weight loss of TiCN coating were about 0.5 and 0.006g. As the added Cu content was in the range of 2–10 wt%, 6.7CT coating had the lowest wear weight loss. The main wear mechanism of TiCN–Cu coatings in the present sliding test were adhesive wear, coupling with partial oxidative wear.
6.7CT and 10CT coating, Cu was also detected besides Ti, C, N, Fe and O. During the sliding test, the hardness of friction ring was lower than those of the coatings and this could result in iron transferring from ring to coatings. Simultaneously, the instantaneous high temperature caused by friction also resulted in oxidation of the transfer layer and coating materials. These indicated that the adhesive and tribo-oxidation wear occurred on the wear track. 4. Conclusions In this work, TiCN–Cu coatings were fabricated by reactive plasma spraying using Ti-graphite aggregates and Cu powders. The crystallization, mechanical and tribological properties were investigated. The following conclusions were drawn: (1) TiCN–Cu coatings shared a similar structure (face-center cubic) to
Fig. 9. Weibull distribution of microhardness of CT coatings with different Cu addition. 6
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Fig. 11. The wear-track morphologies and the corresponding EDS spot analysis of CT with different Cu addition. (a) 2CT, (b) 6.7CT, (c) 10CT
Declaration of competing interest [7]
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.
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[9]
Acknowledgements
[10]
This research is financially supported by the National Natural Science Foundation of China (Grant No. 51872073).
[11]
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