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Effect of heat treatment on the microstructure, mechanical property and tribological property of plasma-sprayed high temperature lubricating composite coating from nanostructured powder
Journal Pre-proof Effect of heat treatment on the microstructure, mechanical property and tribological property of plasma-sprayed high temperature lubricating composite coating from nanostructured powder Bo Li, Yimin Gao, Cong Li, Zhiwei Liu, Hongjian Guo, Qiaoling Zheng, Yefei Li, Yunchuan Kang PII:
S0925-8388(19)33917-9
DOI:
https://doi.org/10.1016/j.jallcom.2019.152671
Reference:
JALCOM 152671
To appear in:
Journal of Alloys and Compounds
Received Date: 2 March 2019 Revised Date:
9 October 2019
Accepted Date: 13 October 2019
Please cite this article as: B. Li, Y. Gao, C. Li, Z. Liu, H. Guo, Q. Zheng, Y. Li, Y. Kang, Effect of heat treatment on the microstructure, mechanical property and tribological property of plasma-sprayed high temperature lubricating composite coating from nanostructured powder, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152671. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Effect of heat treatment on the microstructure， ，mechanical property and tribological property of plasma-sprayed high temperature lubricating composite coating from nanostructured powder Bo Lia,∗, Yimin Gaoa, Cong Lia,∗, Zhiwei Liua, Hongjian Guob, Qiaoling Zhenga, Yefei Lia, Yunchuan Kanga a
State Key Laboratory for Mechanical Behaviour of Materials, School of Materials Science and Engineering,
Xi’an Jiaotong University, Xi’an, People’s Republic of China b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou, People’s Republic of China
∗
Corresponding author at: State Key Laboratory for Mechanical Behaviour of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China E-mail:[email protected] ∗ Corresponding author at: State Key Laboratory for Mechanical Behaviour of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China E-mail:[email protected]
Abstract The nanostructured NiCrAlY-Mo-Ag composite coating was prepared by atmospheric plasma spraying (APS). The microstructure, mechanical property and tribological property of composite coatings before and after heat treatment were investigated. The coating had the least pores and cracks, the microstructure became more homogeneous and the precipitated phase increased obviously after 500 oC heat treatment. The size of precipitates became smaller ( ~10 nm) and the volume fraction increased. Meanwhile, heat treatment could effectively improve the mechanical property and tribological property of composite coatings. The composite coating had the highest adhesive strength and microhardness of 62.8 MPa and 478.8 HV as well as excellent tribological property over a wide temperature range after 500 oC heat treatment. The friction process promoted the tribo-chemical reaction and formed the Ag2MoO4, Ag2Mo2O7, NiO and NiMoO4 on the worn surface, which could act as the high temperature lubricant and effectively improved the tribological property of composite coating at high temperature. Keywords: Nanostructured powder; Dense microstructure; Precipitated phase; Heat treatment; Tribo-chemical reaction 1. Introduction With the development of aerospace industry, high temperature self-lubricating composite coatings with excellent tribological property are required widely [1-6]. Nevertheless, so far it is still difficult to seek effective lubricants over a wide range of temperature for composite coatings [6]. MCrAlY(M=Co or/and Ni) coatings show the
remarkable high-temperature oxidation resistance and corrosion, as well as it could withstand the high thermal and mechanical load [7-11]. Ag-Mo dual-lubricant presents excellent lubrication performance over a wide temperature range, which has been investigated by several researchers in recent years [2, 6, 12-14]. J. Chen et al. fabricated the plasma-sprayed NiCrAlY-Ag-Mo coating which had the excellent lubricating property over a wide temperature range [12]. Nevertheless, the plasma-sprayed coatings often present lamellar structure and contain many defects. The defects are very harmful to mechanical properties of coatings [15-18]. Heat treatment could obviously reduce the defects of coatings and effectively enhance the performance of coatings [19-25]. C. Zhang et al. investigated the effect of heat treatment on the phase composition, microstructure, tribological performance and microhardness of atmospheric plasma sprayed NiCrBSi coating. The results indicated that the microhardness of coating was obviously improved after heat treatment [23]. In recent years, some researchers fabricated the nanostructured composite coating by APS, which could eliminate the lamellar structure in the microstructured composite coating and shown superior adhesive strength, microhardness, corrosion resistant and tribological properties compared with their microstructured composite coating [26-33]. C. Bartuli et al. studied the high temperature mechanical properties of plasma-sprayed zirconia-yttria with conventional and nanostructured powders. The results indicated that nanostructured YSZ coating tended to preserve their porosity for higher temperature than the conventional coating. At the same time, the nanostructured YSZ coating shown the excellent high temperature mechanical properties [26]. Our
previous work investigated the microstructure, mechanical and tribological properties of plasma-sprayed NiCrAlY-Mo-Ag coating with conventional and nanostructured powders. The results indicated that the nanostructured coating had lower porosity, less defects, smaller grains and more dense microstructure and presented higher microhardness, adhesive strength as well as better tribological properties as compared with the microstructured coating [33]. In this work, the nanostructured NiCrAlY-Mo-Ag composite coating was prepared by APS. The effect of heat treatment on the microstructure, mechanical property and tribological property of composite coating were investigated. Meanwhile, the strengthen and wear mechanisms of composite coatings were also investigated in relation on its microstructure and chemical composition. 2. Materials and methods 2.1 Composite coating preparation The nanostructured NiCrAlY-Mo-Ag coating was prepared by APS (Metco 9M). The substrate material was Inconel 718 alloy and the dimension was Φ38 mm×8 mm. The substrate was cleaned in acetone by ultrasonic cleaner and then sand-cleaned before spraying. The nanostructured powder was fabricated by spray drying method. Firstly, the powders were mixed through high-energy ball milling for 20 h and contained 20 % Ag, 10 % Mo and 70 % NiCrAlY (mass fraction). The mixed powder was used to prepare the slurry and then fabricated the nanostructured powder. The spray drying powder had the same chemistry and constitution ratio as compared with the original mixed powder and had different size and shape. The size of spray drying
powder was 30-100 µm. The NiAl acted as the bond coating, which could efficiently enhance the adhesive strength of coating. The thickness of coating was about 300 µm and the bond coating was about 100 µm. Table Ⅰ presented the spraying parameters. Table 2.2 Heat treatment The composite coating was treated in a vacuum furnace and the dynamic vacuum was nearly 1×10-1 Pa. The temperatures of heat treatment were 400 oC, 500 oC and 600 oC respectively. The temperature raised to aim temperature at 10 oC/min and maintained 1 h then cooled by furnace cooling. 2.3 Characterization The microhardness of composite coating before and after heat treatment were measured by microhardness tester. The normal load was 300 g and the dwell time was 5 s on the surface of samples. Each specimen was measured at least ten times and then calculated the average value. Adhesive strength tests were measured according the standard of ASTM C633 for all composite coatings. The diameter of specimen was 25.4 mm (1 inch). The tensile speed was 0.5 mm/min. The phase composition and crystal structure of composite coatings were determined by XRD (40 kV operating voltage). The scan speed was 10 o/min and the angular range was 20º-90º. The morphologies and microstructure of composite coatings were observed by FE-SEM and HRTEM. The micro-Raman was used to study the chemical composition of worn surfaces. The detailed friction and wear tests are described elsewhere [19]. 3. Results and discussion
3.1 Microstructure and mechanical properties of composite coatings Fig. 1 Fig. 1 gives the microstructure of spray powders by spray drying. The powders show near-spherical or spherical shape and the dimension is 30-100 µm. The spray powders consist of many nanostructured powders and present the porous and roughly spherical (Fig.1b). At the same time, the powders display remarkable flowability, which could be effectively deposited on the sample surface during the process of APS. Fig. 2 Fig. 2 displays the XRD patterns of composite coatings. The composite coatings are mainly composed of Ni3Al (JCPDS file no.09-0097), NiAl (JCPDS file no.20-0019), Ag (JCPDS file no.04-0783) and Mo (JCPDS file no.42-1102). The single-phase Ag and Mo existed in the composite coatings demonstrate that the Ag and Mo don’t alloyed in matrix during plasma spraying process [19]. Compared with the microstructured powder composite coating, the nanostructured powder composite coating has weaker diffraction peaks of Mo [19]. It’s probably due to the Mo powder is very small in the nanostructured powders. The diffraction peaks of Ni3Al and Ag in the 500 oC heat treatment composite coating are weaker and wider than others, meaning that the crystallite sizes of Ni3Al and Ag are smaller after 500 oC heat treatment. The Ni3Al peak at 43.604o and Ag peak at 38.116o are chosen to figure the crystallite size. The crystallite sizes of Ni3Al and Ag are 8 nm and 13 nm respectively in the composite coating after 500 oC heat treatment. When the temperature of heat treatment increasing to 600 oC, the diffraction peaks of Ni3Al and Ag become stronger
and narrower and the crystallite sizes become bigger and respectively 12 nm and 18 nm, which is probably due to the higher heat treatment temperature. Meanwhile, it can be clearly seen that the diffraction peaks of Ni3Al shift to the right after 500 oC and 600 oC heat treatment. It is likely to the partial solid solution of Mo in the matrix. Fig. 3 Fig. 4 Fig. 3 shows the micrographs of polished cross-section of composite coatings. It can be obviously seen that the three components of composite coatings distribute uniformly and combine with each other tightly. The pores and cracks of composite coating effectively reduce through heat treatment. After 500 oC heat treatment, the composite coating has the least pores and cracks and the microstructure becomes more homogeneous, which could improve the performance of composite coating. The pores of composite coating has no obviously change with the increasing heat treatment temperature from 500 oC to 600 oC. The “healing” of the pore or the closure of the pore can be caused by localized transport of atoms or vacancies due to the thermal diffusion in the heat treatment, which has been widely reported in plasma-sprayed coatings [35,36]. Besides, the total pores will first decrease with the increasing time and then stabilized [37]. As the thermal diffusivity at 600 Ⅰ is higher than 500 Ⅰ, the diffusion rate of atoms and vacancies at 600 Ⅰ is higher, which can cause the sooner stabilize of the final porosity [36]. Therefore, the reason why the pores of composite coating has no obviously change with the increasing heat treatment temperature from 500 oC to 600 oC is that the porosity may be stabilized at
500 Ⅰ heat treatment for an hour. In order to further investigate the microstructure of composite coatings, the TEM analysis is conducted. Fig. 4 displays the TEM micrographs of composite coatings. After 500 oC heat treatment, the microstructure of composite coating becomes more homogeneous and more precipitated phase arises, which may be useful to improve the mechanical and tribological properties of composite coating. Meanwhile, the size of precipitates become smaller ( ~10 nm) and the volume fraction increases, as is clearly seen in Fig. 4b. These results are very in accordance with XRD results (Fig. 2). The 0.254 nm lattice distance is consistent with the (111) plane of cubic γ′-Ni3Al phase (JCPDS file no.09-0097) in the HRTEM image (Fig. 4c). This result is corresponding to the XRD results in Fig. 2. The diffraction spots remain with the cubic Ni3Al phase along its
1 10
orientation.
Nevertheless, it is distinctly seen that the lattice distortion occurs in the composite coating after 600 oC heat treatment (Fig. 4e and 4f), which may produce the residual stress in the composite coating and deteriorate the mechanical property of composite coating. Table Table Ⅰ indicates the microhardness of composite coatings. It can be evidently seen that the microhardness of composite coatings effectively increase after heat treatment. The composite coating shows the highest microhardness of 478.8 HV after 500 oC heat treatment, which may be attributed to the homogeneous and dense microstructure of composite coating (Fig. 3c). Nevertheless, the microhardness of composite coating decreases to 456.6 HV after 600 oC heat treatment, which is due to the lattice
distortion in the composite coating (4f). Fig. 5 The adhesive strength of composite coatings heat-treated at different temperatures are shown in Fig. 5. It can be distinctly seen that the change trend of adhesive strength versus heat treatment temperature is similar with that of microhardness. The composite coating presents the highest adhesive strength of 62.8 MPa after 500 oC heat treatment, which is contributed to the dense and homogeneous microstructure (Fig. 3c). Heat treatment could visibly enhance the adhesive strength of composite coatings, which is probably due to the decreased cracks and element diffusion between the composite coating and substrate after heat treatment. The adhesive strength of composite coating decreases to 52.9 MPa and the fracture surface occurs within the coating, which is presumably due to the lattice distortion in the coating after 600 oC heat treatment (Fig. 4f). 3.2 Tribological property of composite coatings Fig. 6 The friction coefficient and wear rate of composite coatings over a wide temperature range are presented in Fig. 6. It can be distinctly seen that the friction coefficient and wear rate of composite coatings before and after heat treatment consecutively decrease with the increase of temperature and the composite coating displays the lowest friction coefficient and wear rate at all test temperatures after 500 o
C heat treatment. It is probably attributed to the homogeneous and dense
microstructure of composite coating, which is useful to form the continuous glaze
film in the process of friction test. Meanwhile, the partial solid solution of Mo may be enhanced the tribological property of composite coatings after heat treatment. The friction coefficient and wear rate of composite coatings at all test temperatures below 0.6 and 6.5×10-5mm3/N.m respectively. The composite coatings present the excellent tribological property over a wide temperature range. Fig. 7 Fig. 7 displays the worn surface SEM images of as-sprayed and 500 oC heat treatment composite coatings at different test temperatures. At 25 oC, the worn surface of as-sprayed composite coating is covered with grooves and wear debris, compared with the 500 oC heat treatment coating, resulting in the higher friction coefficient and wear rate (Fig. 6). At 500 oC, the worn surfaces of all coatings become smoother than that at 25 oC. So the wear rate and friction coefficient of composite coatings decrease obviously (Fig. 6). The worn surface of composite coating after 500 oC heat treatment has less scratches than as-sprayed coating. So the composite coating shows lower friction coefficient and wear rate (Fig. 6). At 900 oC, the worn surfaces of all composite coatings are covered with continuous glaze film, which could stem the direct contact of composite coatings and Al2O3 counterpart ball and effectively decrease the wear rate and friction coefficient of composite coatings. So the composite coatings have the lowest wear rate and friction coefficient at 900 oC (Fig. 6). Fig. 8 The XRD is used to determine the phase composition of worn surfaces of
composite coating after 500 oC heat treatment at different friction test temperatures (Fig. 8). It can be found that the phase composition of worn surfaces have no distinctly change below 500 oC. The new phases of NiO and silver molybdates appear on the worn surfaces when friction tests at 700 oC and 900 oC (Fig. 8d and 8e). After 900 oC friction test, the NiO and silver molybdates diffraction peaks become more stronger. It has been reported that NiO and silver molybdates act as the lubricating role at high temperature and the synergistic lubricating effects of them could efficiently enhance the tribological property of composite coatings at high temperature [34]. So the composite coatings have lower wear rate and friction coefficient above 700 oC (Fig. 6). Fig. 9 For the purpose of further studying the tribo-chemical reaction mechanism on the worn surface of composite coatings at high temperature, micro-Raman is tested. Fig. 9 presents the Raman spectra of worn surfaces of 500 oC heat treatment composite coating after friction test at 700 oC and 900 oC. At 700 oC, the Ag2MoO4, Ag2Mo2O7 and NiO peaks have been detected within the wear track of 500 oC heat treatment coating. At 900 oC, besides Ag2MoO4, Ag2Mo2O7 and NiO peaks, the NiMoO4 is also observed within the wear track. Nevertheless, only weak Ag2Mo2O7 peak is detected outside the wear track of composite coating at 700 oC and 900 oC. It can be understood that the friction process facilitates the tribo-chemical reaction and forms the Ag2MoO4, Ag2Mo2O7, NiO and NiMoO4 on the worn surface, which could act as the high temperature lubricant and the synergistic lubricating effects of them can
efficiently improve the tribological property of composite coatings at high temperature [33,34]. So the composite coating has lower wear rate and friction coefficient at high temperature (Fig. 6).
4. Conclusions In this work, the composite coatings were fabricated with nanostructured powders by APS. The effect of heat treatment on the microstructure, mechanical property and tribological property of composite coatings were researched. The main results can be summarized as follows: (1) Heat treatment could efficiently reduce the defects of composite coatings. The composite coating has the least pores and cracks, the microstructure becomes more homogeneous and more precipitated phase arises after 500 oC heat treatment. The size of precipitates become smaller ( ~10 nm) and the volume fraction increases. (2) Heat treatment could efficiently improve the adhesive strength, microhardness and tribological property of composite coatings. The composite coating has the highest adhesive strength, microhardness and excellent tribological property over a wide temperature range after 500 oC heat treatment. (3) The friction process promotes the tribo-chemical reaction and forms the Ag2MoO4, Ag2Mo2O7, NiO and NiMoO4 on the worn surface, which could act as the high temperature lubricant and the synergistic lubricating effects of them could efficiently improve the tribological property of composite coatings at high temperature.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51805408, 51665026), the Natural Science Foundation of Shaanxi Province (Grant No. 2019JQ-283), the China Postdoctoral Science Foundation (Grant No. 2019M653597), the Shaanxi Province Postdoctoral Science Foundation, the Fundamental Research Funds for Central Universities (Grant No. xzy012019010) and the Guangxi Innovation Driven Development Project (GUIKEAA18242001).
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List of figure and table captions: Fig. 1 SEM images of spray drying powders. Fig. 2 XRD patterns of composite coatings: (a) as-sprayed, (b) 400 oC heat treatment, (c) 500 oC heat treatment and (d) 600 oC heat treatment. Fig. 3 Micrographs of polished cross-section of composite coatings: (a) as-sprayed, (b) 400 oC heat treatment, (c) 500 oC heat treatment and (d) 600 oC heat treatment. Fig. 4 TEM micrographs and corresponding SAED patterns of composite coatings: (a) as-sprayed, (b-d) heat-treated at 500 oC and (e, f) 600 oC. Fig. 5 Adhesive strength of composite coatings heat-treated at different temperatures. Fig. 6 Friction coefficient and wear rate of composite coatings over a wide temperature range. Fig. 7 Worn surface SEM images of composite coatings after friction test: (a-c) as-sprayed composite coating and (d-f) heat-treated at 500 oC composite coating. Fig. 8 XRD patterns of worn surfaces of 500 oC heat treatment composite coating after friction test at different temperatures: (a) 25 oC, (b) 300 oC, (c) 500 oC, (d) 700 oC and (e) 900 oC. Fig. 9 Raman spectra of worn surfaces of 500 oC heat treatment composite coating after friction test at (a) 700 oC and (b) 900 oC. Table Ⅰ Spray parameters Table Ⅰ Microhardness of composite coatings heat-treated at different temperatures.
Figures:
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Tables: Table Ⅰ Spray parameters Parameter
Value
Plasma gas flow Ar, L/min
40
Secondary gas flow H2, L/min
5
Spraying angle
90 º
Powder feed rate, g/min
42
Current, A
500
Voltage, V
55
Spray distance, mm
100
Table Ⅰ Microhardness of composite coatings heat-treated at different temperatures. Composite coating Vickers hardness (HV)
as-spray
400 oC
500 oC
600 oC
430.4±15.6 464.6±16.5 478.8±18.3 456.6±16.1
Highlights: 1. The microstructure becomes more dense and homogeneous after heat treatment. 2. After 500 oC heat treatment, the more precipitated phase arises. 3. Heat treatment improves the mechanical and tribological properties of coatings. 4. The friction process promotes the tribo-chemical reaction and forms the lubricants.
Declaration of Interest Statement None
S0925-8388(19)33917-9
DOI:
https://doi.org/10.1016/j.jallcom.2019.152671
Reference:
JALCOM 152671
To appear in:
Journal of Alloys and Compounds
Received Date: 2 March 2019 Revised Date:
9 October 2019
Accepted Date: 13 October 2019
Please cite this article as: B. Li, Y. Gao, C. Li, Z. Liu, H. Guo, Q. Zheng, Y. Li, Y. Kang, Effect of heat treatment on the microstructure, mechanical property and tribological property of plasma-sprayed high temperature lubricating composite coating from nanostructured powder, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152671. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Effect of heat treatment on the microstructure， ，mechanical property and tribological property of plasma-sprayed high temperature lubricating composite coating from nanostructured powder Bo Lia,∗, Yimin Gaoa, Cong Lia,∗, Zhiwei Liua, Hongjian Guob, Qiaoling Zhenga, Yefei Lia, Yunchuan Kanga a
State Key Laboratory for Mechanical Behaviour of Materials, School of Materials Science and Engineering,
Xi’an Jiaotong University, Xi’an, People’s Republic of China b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou, People’s Republic of China
∗
Corresponding author at: State Key Laboratory for Mechanical Behaviour of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China E-mail:[email protected] ∗ Corresponding author at: State Key Laboratory for Mechanical Behaviour of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China E-mail:[email protected]
Abstract The nanostructured NiCrAlY-Mo-Ag composite coating was prepared by atmospheric plasma spraying (APS). The microstructure, mechanical property and tribological property of composite coatings before and after heat treatment were investigated. The coating had the least pores and cracks, the microstructure became more homogeneous and the precipitated phase increased obviously after 500 oC heat treatment. The size of precipitates became smaller ( ~10 nm) and the volume fraction increased. Meanwhile, heat treatment could effectively improve the mechanical property and tribological property of composite coatings. The composite coating had the highest adhesive strength and microhardness of 62.8 MPa and 478.8 HV as well as excellent tribological property over a wide temperature range after 500 oC heat treatment. The friction process promoted the tribo-chemical reaction and formed the Ag2MoO4, Ag2Mo2O7, NiO and NiMoO4 on the worn surface, which could act as the high temperature lubricant and effectively improved the tribological property of composite coating at high temperature. Keywords: Nanostructured powder; Dense microstructure; Precipitated phase; Heat treatment; Tribo-chemical reaction 1. Introduction With the development of aerospace industry, high temperature self-lubricating composite coatings with excellent tribological property are required widely [1-6]. Nevertheless, so far it is still difficult to seek effective lubricants over a wide range of temperature for composite coatings [6]. MCrAlY(M=Co or/and Ni) coatings show the
remarkable high-temperature oxidation resistance and corrosion, as well as it could withstand the high thermal and mechanical load [7-11]. Ag-Mo dual-lubricant presents excellent lubrication performance over a wide temperature range, which has been investigated by several researchers in recent years [2, 6, 12-14]. J. Chen et al. fabricated the plasma-sprayed NiCrAlY-Ag-Mo coating which had the excellent lubricating property over a wide temperature range [12]. Nevertheless, the plasma-sprayed coatings often present lamellar structure and contain many defects. The defects are very harmful to mechanical properties of coatings [15-18]. Heat treatment could obviously reduce the defects of coatings and effectively enhance the performance of coatings [19-25]. C. Zhang et al. investigated the effect of heat treatment on the phase composition, microstructure, tribological performance and microhardness of atmospheric plasma sprayed NiCrBSi coating. The results indicated that the microhardness of coating was obviously improved after heat treatment [23]. In recent years, some researchers fabricated the nanostructured composite coating by APS, which could eliminate the lamellar structure in the microstructured composite coating and shown superior adhesive strength, microhardness, corrosion resistant and tribological properties compared with their microstructured composite coating [26-33]. C. Bartuli et al. studied the high temperature mechanical properties of plasma-sprayed zirconia-yttria with conventional and nanostructured powders. The results indicated that nanostructured YSZ coating tended to preserve their porosity for higher temperature than the conventional coating. At the same time, the nanostructured YSZ coating shown the excellent high temperature mechanical properties [26]. Our
previous work investigated the microstructure, mechanical and tribological properties of plasma-sprayed NiCrAlY-Mo-Ag coating with conventional and nanostructured powders. The results indicated that the nanostructured coating had lower porosity, less defects, smaller grains and more dense microstructure and presented higher microhardness, adhesive strength as well as better tribological properties as compared with the microstructured coating [33]. In this work, the nanostructured NiCrAlY-Mo-Ag composite coating was prepared by APS. The effect of heat treatment on the microstructure, mechanical property and tribological property of composite coating were investigated. Meanwhile, the strengthen and wear mechanisms of composite coatings were also investigated in relation on its microstructure and chemical composition. 2. Materials and methods 2.1 Composite coating preparation The nanostructured NiCrAlY-Mo-Ag coating was prepared by APS (Metco 9M). The substrate material was Inconel 718 alloy and the dimension was Φ38 mm×8 mm. The substrate was cleaned in acetone by ultrasonic cleaner and then sand-cleaned before spraying. The nanostructured powder was fabricated by spray drying method. Firstly, the powders were mixed through high-energy ball milling for 20 h and contained 20 % Ag, 10 % Mo and 70 % NiCrAlY (mass fraction). The mixed powder was used to prepare the slurry and then fabricated the nanostructured powder. The spray drying powder had the same chemistry and constitution ratio as compared with the original mixed powder and had different size and shape. The size of spray drying
powder was 30-100 µm. The NiAl acted as the bond coating, which could efficiently enhance the adhesive strength of coating. The thickness of coating was about 300 µm and the bond coating was about 100 µm. Table Ⅰ presented the spraying parameters. Table 2.2 Heat treatment The composite coating was treated in a vacuum furnace and the dynamic vacuum was nearly 1×10-1 Pa. The temperatures of heat treatment were 400 oC, 500 oC and 600 oC respectively. The temperature raised to aim temperature at 10 oC/min and maintained 1 h then cooled by furnace cooling. 2.3 Characterization The microhardness of composite coating before and after heat treatment were measured by microhardness tester. The normal load was 300 g and the dwell time was 5 s on the surface of samples. Each specimen was measured at least ten times and then calculated the average value. Adhesive strength tests were measured according the standard of ASTM C633 for all composite coatings. The diameter of specimen was 25.4 mm (1 inch). The tensile speed was 0.5 mm/min. The phase composition and crystal structure of composite coatings were determined by XRD (40 kV operating voltage). The scan speed was 10 o/min and the angular range was 20º-90º. The morphologies and microstructure of composite coatings were observed by FE-SEM and HRTEM. The micro-Raman was used to study the chemical composition of worn surfaces. The detailed friction and wear tests are described elsewhere [19]. 3. Results and discussion
3.1 Microstructure and mechanical properties of composite coatings Fig. 1 Fig. 1 gives the microstructure of spray powders by spray drying. The powders show near-spherical or spherical shape and the dimension is 30-100 µm. The spray powders consist of many nanostructured powders and present the porous and roughly spherical (Fig.1b). At the same time, the powders display remarkable flowability, which could be effectively deposited on the sample surface during the process of APS. Fig. 2 Fig. 2 displays the XRD patterns of composite coatings. The composite coatings are mainly composed of Ni3Al (JCPDS file no.09-0097), NiAl (JCPDS file no.20-0019), Ag (JCPDS file no.04-0783) and Mo (JCPDS file no.42-1102). The single-phase Ag and Mo existed in the composite coatings demonstrate that the Ag and Mo don’t alloyed in matrix during plasma spraying process [19]. Compared with the microstructured powder composite coating, the nanostructured powder composite coating has weaker diffraction peaks of Mo [19]. It’s probably due to the Mo powder is very small in the nanostructured powders. The diffraction peaks of Ni3Al and Ag in the 500 oC heat treatment composite coating are weaker and wider than others, meaning that the crystallite sizes of Ni3Al and Ag are smaller after 500 oC heat treatment. The Ni3Al peak at 43.604o and Ag peak at 38.116o are chosen to figure the crystallite size. The crystallite sizes of Ni3Al and Ag are 8 nm and 13 nm respectively in the composite coating after 500 oC heat treatment. When the temperature of heat treatment increasing to 600 oC, the diffraction peaks of Ni3Al and Ag become stronger
and narrower and the crystallite sizes become bigger and respectively 12 nm and 18 nm, which is probably due to the higher heat treatment temperature. Meanwhile, it can be clearly seen that the diffraction peaks of Ni3Al shift to the right after 500 oC and 600 oC heat treatment. It is likely to the partial solid solution of Mo in the matrix. Fig. 3 Fig. 4 Fig. 3 shows the micrographs of polished cross-section of composite coatings. It can be obviously seen that the three components of composite coatings distribute uniformly and combine with each other tightly. The pores and cracks of composite coating effectively reduce through heat treatment. After 500 oC heat treatment, the composite coating has the least pores and cracks and the microstructure becomes more homogeneous, which could improve the performance of composite coating. The pores of composite coating has no obviously change with the increasing heat treatment temperature from 500 oC to 600 oC. The “healing” of the pore or the closure of the pore can be caused by localized transport of atoms or vacancies due to the thermal diffusion in the heat treatment, which has been widely reported in plasma-sprayed coatings [35,36]. Besides, the total pores will first decrease with the increasing time and then stabilized [37]. As the thermal diffusivity at 600 Ⅰ is higher than 500 Ⅰ, the diffusion rate of atoms and vacancies at 600 Ⅰ is higher, which can cause the sooner stabilize of the final porosity [36]. Therefore, the reason why the pores of composite coating has no obviously change with the increasing heat treatment temperature from 500 oC to 600 oC is that the porosity may be stabilized at
500 Ⅰ heat treatment for an hour. In order to further investigate the microstructure of composite coatings, the TEM analysis is conducted. Fig. 4 displays the TEM micrographs of composite coatings. After 500 oC heat treatment, the microstructure of composite coating becomes more homogeneous and more precipitated phase arises, which may be useful to improve the mechanical and tribological properties of composite coating. Meanwhile, the size of precipitates become smaller ( ~10 nm) and the volume fraction increases, as is clearly seen in Fig. 4b. These results are very in accordance with XRD results (Fig. 2). The 0.254 nm lattice distance is consistent with the (111) plane of cubic γ′-Ni3Al phase (JCPDS file no.09-0097) in the HRTEM image (Fig. 4c). This result is corresponding to the XRD results in Fig. 2. The diffraction spots remain with the cubic Ni3Al phase along its
1 10
orientation.
Nevertheless, it is distinctly seen that the lattice distortion occurs in the composite coating after 600 oC heat treatment (Fig. 4e and 4f), which may produce the residual stress in the composite coating and deteriorate the mechanical property of composite coating. Table Table Ⅰ indicates the microhardness of composite coatings. It can be evidently seen that the microhardness of composite coatings effectively increase after heat treatment. The composite coating shows the highest microhardness of 478.8 HV after 500 oC heat treatment, which may be attributed to the homogeneous and dense microstructure of composite coating (Fig. 3c). Nevertheless, the microhardness of composite coating decreases to 456.6 HV after 600 oC heat treatment, which is due to the lattice
distortion in the composite coating (4f). Fig. 5 The adhesive strength of composite coatings heat-treated at different temperatures are shown in Fig. 5. It can be distinctly seen that the change trend of adhesive strength versus heat treatment temperature is similar with that of microhardness. The composite coating presents the highest adhesive strength of 62.8 MPa after 500 oC heat treatment, which is contributed to the dense and homogeneous microstructure (Fig. 3c). Heat treatment could visibly enhance the adhesive strength of composite coatings, which is probably due to the decreased cracks and element diffusion between the composite coating and substrate after heat treatment. The adhesive strength of composite coating decreases to 52.9 MPa and the fracture surface occurs within the coating, which is presumably due to the lattice distortion in the coating after 600 oC heat treatment (Fig. 4f). 3.2 Tribological property of composite coatings Fig. 6 The friction coefficient and wear rate of composite coatings over a wide temperature range are presented in Fig. 6. It can be distinctly seen that the friction coefficient and wear rate of composite coatings before and after heat treatment consecutively decrease with the increase of temperature and the composite coating displays the lowest friction coefficient and wear rate at all test temperatures after 500 o
C heat treatment. It is probably attributed to the homogeneous and dense
microstructure of composite coating, which is useful to form the continuous glaze
film in the process of friction test. Meanwhile, the partial solid solution of Mo may be enhanced the tribological property of composite coatings after heat treatment. The friction coefficient and wear rate of composite coatings at all test temperatures below 0.6 and 6.5×10-5mm3/N.m respectively. The composite coatings present the excellent tribological property over a wide temperature range. Fig. 7 Fig. 7 displays the worn surface SEM images of as-sprayed and 500 oC heat treatment composite coatings at different test temperatures. At 25 oC, the worn surface of as-sprayed composite coating is covered with grooves and wear debris, compared with the 500 oC heat treatment coating, resulting in the higher friction coefficient and wear rate (Fig. 6). At 500 oC, the worn surfaces of all coatings become smoother than that at 25 oC. So the wear rate and friction coefficient of composite coatings decrease obviously (Fig. 6). The worn surface of composite coating after 500 oC heat treatment has less scratches than as-sprayed coating. So the composite coating shows lower friction coefficient and wear rate (Fig. 6). At 900 oC, the worn surfaces of all composite coatings are covered with continuous glaze film, which could stem the direct contact of composite coatings and Al2O3 counterpart ball and effectively decrease the wear rate and friction coefficient of composite coatings. So the composite coatings have the lowest wear rate and friction coefficient at 900 oC (Fig. 6). Fig. 8 The XRD is used to determine the phase composition of worn surfaces of
composite coating after 500 oC heat treatment at different friction test temperatures (Fig. 8). It can be found that the phase composition of worn surfaces have no distinctly change below 500 oC. The new phases of NiO and silver molybdates appear on the worn surfaces when friction tests at 700 oC and 900 oC (Fig. 8d and 8e). After 900 oC friction test, the NiO and silver molybdates diffraction peaks become more stronger. It has been reported that NiO and silver molybdates act as the lubricating role at high temperature and the synergistic lubricating effects of them could efficiently enhance the tribological property of composite coatings at high temperature [34]. So the composite coatings have lower wear rate and friction coefficient above 700 oC (Fig. 6). Fig. 9 For the purpose of further studying the tribo-chemical reaction mechanism on the worn surface of composite coatings at high temperature, micro-Raman is tested. Fig. 9 presents the Raman spectra of worn surfaces of 500 oC heat treatment composite coating after friction test at 700 oC and 900 oC. At 700 oC, the Ag2MoO4, Ag2Mo2O7 and NiO peaks have been detected within the wear track of 500 oC heat treatment coating. At 900 oC, besides Ag2MoO4, Ag2Mo2O7 and NiO peaks, the NiMoO4 is also observed within the wear track. Nevertheless, only weak Ag2Mo2O7 peak is detected outside the wear track of composite coating at 700 oC and 900 oC. It can be understood that the friction process facilitates the tribo-chemical reaction and forms the Ag2MoO4, Ag2Mo2O7, NiO and NiMoO4 on the worn surface, which could act as the high temperature lubricant and the synergistic lubricating effects of them can
efficiently improve the tribological property of composite coatings at high temperature [33,34]. So the composite coating has lower wear rate and friction coefficient at high temperature (Fig. 6).
4. Conclusions In this work, the composite coatings were fabricated with nanostructured powders by APS. The effect of heat treatment on the microstructure, mechanical property and tribological property of composite coatings were researched. The main results can be summarized as follows: (1) Heat treatment could efficiently reduce the defects of composite coatings. The composite coating has the least pores and cracks, the microstructure becomes more homogeneous and more precipitated phase arises after 500 oC heat treatment. The size of precipitates become smaller ( ~10 nm) and the volume fraction increases. (2) Heat treatment could efficiently improve the adhesive strength, microhardness and tribological property of composite coatings. The composite coating has the highest adhesive strength, microhardness and excellent tribological property over a wide temperature range after 500 oC heat treatment. (3) The friction process promotes the tribo-chemical reaction and forms the Ag2MoO4, Ag2Mo2O7, NiO and NiMoO4 on the worn surface, which could act as the high temperature lubricant and the synergistic lubricating effects of them could efficiently improve the tribological property of composite coatings at high temperature.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51805408, 51665026), the Natural Science Foundation of Shaanxi Province (Grant No. 2019JQ-283), the China Postdoctoral Science Foundation (Grant No. 2019M653597), the Shaanxi Province Postdoctoral Science Foundation, the Fundamental Research Funds for Central Universities (Grant No. xzy012019010) and the Guangxi Innovation Driven Development Project (GUIKEAA18242001).
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List of figure and table captions: Fig. 1 SEM images of spray drying powders. Fig. 2 XRD patterns of composite coatings: (a) as-sprayed, (b) 400 oC heat treatment, (c) 500 oC heat treatment and (d) 600 oC heat treatment. Fig. 3 Micrographs of polished cross-section of composite coatings: (a) as-sprayed, (b) 400 oC heat treatment, (c) 500 oC heat treatment and (d) 600 oC heat treatment. Fig. 4 TEM micrographs and corresponding SAED patterns of composite coatings: (a) as-sprayed, (b-d) heat-treated at 500 oC and (e, f) 600 oC. Fig. 5 Adhesive strength of composite coatings heat-treated at different temperatures. Fig. 6 Friction coefficient and wear rate of composite coatings over a wide temperature range. Fig. 7 Worn surface SEM images of composite coatings after friction test: (a-c) as-sprayed composite coating and (d-f) heat-treated at 500 oC composite coating. Fig. 8 XRD patterns of worn surfaces of 500 oC heat treatment composite coating after friction test at different temperatures: (a) 25 oC, (b) 300 oC, (c) 500 oC, (d) 700 oC and (e) 900 oC. Fig. 9 Raman spectra of worn surfaces of 500 oC heat treatment composite coating after friction test at (a) 700 oC and (b) 900 oC. Table Ⅰ Spray parameters Table Ⅰ Microhardness of composite coatings heat-treated at different temperatures.
Figures:
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Tables: Table Ⅰ Spray parameters Parameter
Value
Plasma gas flow Ar, L/min
40
Secondary gas flow H2, L/min
5
Spraying angle
90 º
Powder feed rate, g/min
42
Current, A
500
Voltage, V
55
Spray distance, mm
100
Table Ⅰ Microhardness of composite coatings heat-treated at different temperatures. Composite coating Vickers hardness (HV)
as-spray
400 oC
500 oC
600 oC
430.4±15.6 464.6±16.5 478.8±18.3 456.6±16.1
Highlights: 1. The microstructure becomes more dense and homogeneous after heat treatment. 2. After 500 oC heat treatment, the more precipitated phase arises. 3. Heat treatment improves the mechanical and tribological properties of coatings. 4. The friction process promotes the tribo-chemical reaction and forms the lubricants.
Declaration of Interest Statement None