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A novel approach to fabricate hybrid materials with excellent tribological properties from spray-formed ceramic
Materials Letters 193 (2017) 199–202
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
A novel approach to fabricate hybrid materials with excellent tribological properties from spray-formed ceramic Wen Deng a,b, Shuangjian Li a,b, Xia Liu a,b, Xiaoqin Zhao a, Yulong An a,⇑, Huidi Zhou a,⇑, Jianmin Chen a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 18 December 2016 Received in revised form 16 January 2017 Accepted 28 January 2017 Available online 4 February 2017 Keywords: Thermal spray Ceramics In-situ synthesis Composite materials Wear and tribology
a b s t r a c t A novel method was introduced in this article to fabricate hybrid materials by virtue of thermal sprayed ceramic coatings as templets to improve their tribology performance. MoS2 was in-situ synthesized in the pores and micro-cracks of ceramic coatings using hydrothermal method coupled with vacuum impregnation. The resultant MoS2 appeared flowerlike microsphere structure and constructed with many ultrathin nanosheets, which were curly and interconnected to grow in the pores and micro-cracks of ZrO2 coatings. The results of tribological test revealed that the composite coatings have excellent tribological properties due to the formation of MoS2 lubricating film on frictional surfaces in comparison with pure ZrO2 coatings. Ó 2017 Published by Elsevier B.V.
1. Introduction Ceramic materials have been extensively used in industrial field due to their high hardness, excellent wear- and corrosion- resistant as well as good anti-oxidation ability [1,2]. Ceramic coatings fabricated by thermal spraying could endow metal substrates with various outstanding properties [3,4]. The high hardness of ceramic materials gives the coatings outstanding anti-wear properties but usually leads to the serious wear of metallic pairs [5], which would result in the diminution of the size or volume of metal components. Eventually, their excepted functionality would be deprived. Thus, it is crucial to reduce the wear between ceramic coatings and metallic pairs. The introduction of lubrication phases in the ceramic coatings could improve the lubricating property of the coatings [6,7]. However, for thermal spraying coatings, the traditional solid lubricants (graphite, MoS2, etc.) are apt to oxidize in the present of oxygen on account of the ultra-high temperature of plasma flame core [8]. Therefore, it is still a challenge to prepare thermal sprayed coatings including with traditional solid lubricants. Besides, pores and micro-cracks exist in the coatings inevitably due to the intrinsic characteristics of thermal sprayed technology. These defects decrease the mechanical properties, thereby affecting the friction behavior [9,10]. Thus, by means of ingenious design, introducing the solid lubricant into the micro-cracks and ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. An), [email protected] (H. Zhou). http://dx.doi.org/10.1016/j.matlet.2017.01.148 0167-577X/Ó 2017 Published by Elsevier B.V.
pores of ceramic coatings could improve the friction and wear behavior of friction pairs. Based on the above analysis, in the present research, the ZrO2 ceramic coatings were fabricated via atmospheric plasma spraying. The lubricant MoS2 was synthesized through a hydrothermal method in the pores and micro-cracks of as-sprayed ZrO2 coatings. The microstructure of synthesized MoS2 was studied. Afterwards, the tribological properties of the compound coatings were investigated compared with pure ZrO2 coating. 2. Experimental section The ZrO2 ceramic coatings were deposited on 316 stainless steel substrates (u25 7.8 mm) by an APS-2000A system (Institute of Aeronautical Manufacturing Technology, Beijing, China). The assprayed coatings were mechanically polished to a surface roughness Ra 0.21 lm, followed by ultrasonically cleaning in acetone. MoS2 was in-suit synthesized by the hydrothermal method. A mixture of 1.87 g Sodium molybdate and 2.77 g thiourea were dissolved in 80 mL deionized water and magnetic stirred for 30 min. The polished specimen were immersed in the homogeneous solution and put in an ultrasonic bath for 10 min and then placed in a vacuum chamber for 10 min under 60 mmHg vacuum level. After repeating the above steps for three times, the solution with the samples was transferred into a 150 mL Teflon-lined stainless steel autoclave and was heated at 220 °C for 48 h. After that, the autoclave cooled down to room temperature naturally.
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W. Deng et al. / Materials Letters 193 (2017) 199–202
The friction and wear tests were performed by a ball-on-disc tribometer (CSM Instrument, Switzerland) with a reciprocating mode. Metallic balls (1Cr18Ni9Ti) with a diameter of 6 mm were used as the counterpart. All experiments were conducted at room temperature of 20 ± 2 °C, relative humidity of 30 ± 5%, a sliding velocity of 5 cm/s, the load of 5 N, amplitude of 2.5 mm and a total sliding distance of 100 m. The wear morphologies of coupled balls were described using a Micro-XAM-3D non-contact surface profiler (USA). The wear rates (W) of the coatings are calculated as: W = V/ PL, where V is the wear volume loss (mm3), P is the load (N) and L is the sliding distance (m). The maximum depth of worn track (h) is calculated by the volume of spherical cap (V1) and wear volume (V2) of coupled ball:
V 1 ¼ ph ð3R hÞ=3
ð1Þ
V 2 ¼ pb =64R;
ð2Þ
2
4
where b is the wear scar diameter and R is the radius of the ball.
3. Results and discussion The XRD pattern of the powders synthesized via hydrothermal reaction is shown in Fig. 1(a). It can be observed that all the diffraction peaks are in good agreement with that of hexagonal MoS2 (JCPDS No. 73-1508, molybdenite). Besides, no characteristic peaks of other impurities are detected, demonstrating that the sample fabricated by hydrothermal reaction is highly pure. The morphology and size of MoS2 are elucidated by SEM (Fig. 1(b)) and TEM (Fig. 1(c)). From Fig. 1(b), the samples exhibit microsphere structure composed of many lamellae. High-magnification SEM image of region ‘‘A” and TEM indicate that the size of lamellae is very thin with a thickness of about 10 nm, which interweaved together and formed the microsphere structure. As shown in Fig. 2(a), the ZrO2 coatings with a thickness of about 345 mm are uniformly deposited on the substrate. The pores and micro-cracks display on the fracture surface of pure ZrO2 coating, and the fracture surface is relatively smooth (Fig. 2(b)). However, the fracture surface of the composite coating (Fig. 2(c)) is
Fig. 1. The XRD pattern (a), SEM (b) and TEM (c) images of the resultant MoS2.
Fig. 2. The SEM images of cross-section (a) and fracture surface (b) of ZrO2 coating and the fracture surface (c) of composite coating and EDS analyses (d).
W. Deng et al. / Materials Letters 193 (2017) 199–202
very rough and no obvious pores or cracks are detected. From the enlarge figure of ‘‘Ⅰ”, it is surprising and interesting to find that many lamellar-structured phases generate in the composite coat-
201
ings. The EDS analysis indicates that the lamellar contains many Mo and S, which is corresponding to MoS2 due to the atom percentage of Mo and S is about 1:2. The results imply that MoS2 is
Fig. 3. The friction coefficient curves (a), SEM images of wear tracks ((b) and (c)) and the 3D topographies of counterparts sliding against ZrO2 coating (d) and composite coating (e).
Fig. 4. The SEM images of wear surfaces and corresponding element mapping: (a) ZrO2 coating, (b) ZrO2/MoS2 composite coating; the wear morphology of the tribo-pair sliding against composite coating (c) and corresponding Raman spectrum (d).
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successfully synthesized in the pores and micro-cracks of the ZrO2 coatings. Limiting by the size of the pores and micro-cracks, it is difficult to assemble into microspheres. As a result, MoS2 presented lamellar structure in the micro-defects of as-sprayed ZrO2 coatings. The friction coefficient curves and the morphology of worn surfaces of the ZrO2 coatings, composite coatings and the 3D topographies of the coupled balls are given in Fig. 3. Before friction test, the MoS2 layer is carefully removed from the surface of composite coating with Ra 0.14 lm. From Fig. 3(a), the friction coefficient of composite coating is about 0.12 which is much lower than that of ZrO2 coating (l 0.53). It can be concluded that the resultant MoS2 in the composite coatings played an important role in the decrease of the friction coefficient, which could be further confirmed by wear morphologies of coatings and the 3D topographies of coupled balls (Fig. 3(b) and (d)). The width of wear scar of ZrO2 coating is about 2.5 times broader than that of composite coating. And the wear rate of ZrO2 coating (8.52 106 mm3/Nm) is approximately six times higher than that of composite coating (1.37 106 mm3/Nm). Meanwhile, the maximum depth of worn track of the tribo-ball coupled with composite coating is about 6.97 mm, much smaller than that of the tribo-ball coupled with ZrO2 coating (24.69 mm). The results display that the existence of MoS2 not only leads to the decline of the friction coefficient but also reduces the wear of both coatings and counterparts. The SEM micrographs of wear surfaces and corresponding elemental composition mappings are displayed in Fig. 4. From Fig. 4 (a), Fe is uniformly covered on the worn surface of ZrO2 coating, which indicates that the elements of counterpart are transferred onto the coating due to the higher hardness of ZrO2 coating, which caused severe adhesive wear. However, Mo and S are detected on the wear scar of composite coating (Fig. 4(b)). It can be inferred that MoS2 lubricating film is formed on the worn surface. Besides, Fe is not detected in the enriched regions of MoS2, indicate that MoS2 relieved adhesive wear of the frictional surfaces. Raman spectrum of tribo-ball sliding against composite coating is further confirmed that MoS2 lubricating film is indeed present (Fig. 4 (c) and (d)). The formation of MoS2 lubricating film hinders the direct contact of the couples, thereby greatly alleviating the adhesive wear. Meanwhile, the existence of lubricating film decreased the shearing stress and the plastic deformation of worn surface, and resulted in remarkable improvement of tribological properties of the composite coating.
4. Conclusion MoS2 with lamellar-like was synthetized via hydrothermal method in the micro-cracks and pores of thermal sprayed ZrO2 coatings. The ZrO2/MoS2 composite coating exhibited a low friction coefficient and few adhesions as well as plows on the worn surface of the specimen and metallic counterpart attributed to the formation of MoS2 lubricating film. Compared with pure ZrO2 coating, the composite coatings presented an outstanding tribological properties. Acknowledgements This research is financially supported by the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2014378), the West Light Foundation of the Chinese Academy of Sciences. The authors appreciate helpful comments of the reviewers. References [1] S.J. Li, X. Xi, G.L. Hou, Y.L. An, et al., Preparation of plasma sprayed mullite coating on stainless steel substrate and investigation of its environmental dependence of friction and wear behavior, Tribol. Int. 91 (2015) 32–39. [2] M.A. Zavareh, A.A.D.M. Sarhan, B.B.A. Razak, et al., Plasma thermal spray of ceramic oxide coating on carbon steel with enhanced wear and corrosion resistance for oil and gas applications, Ceram. Int. 40 (2014) 14267–14277. [3] L. He, Y. Tan, X. Wang, et al., Microstructure and wear properties of Al2O3CeO2/Ni-base alloy composite coatings on aluminum alloys by plasma spray, Appl. Surf. Sci. 314 (2014) 760–767. [4] M.J. Ghazali, S. Forghani, N. Hassanuddin, et al., Comparative wear study of plasma sprayed TiO2 and Al2O3-TiO2 on mild steels, Tribol. Int. 93 (2016) 681– 686. [5] G. Bolelli, V. Cannillo, L. Lusvarghi, et al., Wear behaviour of thermally sprayed ceramic oxide coatings, Wear 261 (2006) 1298–1315. [6] Z.J. Huang, D.S. Xiong, et al., MoS2 coated with Al2O3 for Ni-MoS2/Al2O3 composite coatings by pulse electrodeposition, Surf. Coat. Technol. 202 (2008) 3208–3214. [7] L.Q. Kong, Q.L. Bi, M.Y. Niu, et al., High-temperature tribological behavior of ZrO2-MoS2-CaF2 self-lubricating composites, J. Eur. Ceram. Soc. 33 (2013) 51– 59. [8] H. Herman, Plasma-Sprayed Coatings, Scientific American, USA, 1988, p. 259. [9] Z. Wang, A. Kulkarni, S. Deshpande, et al., Effects of pores and interfaces on effective properties of plasma sprayed zirconia coatings, Acta Mater. 51 (2003) 5319–5334. [10] C.J. Li, J. Zou, H.B. Huo, et al., Microstructure and properties of porous abradable alumina coatings flame-sprayed with semi-molten particles, J. Therm. Spray Technol. 25 (2016) 264–272.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
A novel approach to fabricate hybrid materials with excellent tribological properties from spray-formed ceramic Wen Deng a,b, Shuangjian Li a,b, Xia Liu a,b, Xiaoqin Zhao a, Yulong An a,⇑, Huidi Zhou a,⇑, Jianmin Chen a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 18 December 2016 Received in revised form 16 January 2017 Accepted 28 January 2017 Available online 4 February 2017 Keywords: Thermal spray Ceramics In-situ synthesis Composite materials Wear and tribology
a b s t r a c t A novel method was introduced in this article to fabricate hybrid materials by virtue of thermal sprayed ceramic coatings as templets to improve their tribology performance. MoS2 was in-situ synthesized in the pores and micro-cracks of ceramic coatings using hydrothermal method coupled with vacuum impregnation. The resultant MoS2 appeared flowerlike microsphere structure and constructed with many ultrathin nanosheets, which were curly and interconnected to grow in the pores and micro-cracks of ZrO2 coatings. The results of tribological test revealed that the composite coatings have excellent tribological properties due to the formation of MoS2 lubricating film on frictional surfaces in comparison with pure ZrO2 coatings. Ó 2017 Published by Elsevier B.V.
1. Introduction Ceramic materials have been extensively used in industrial field due to their high hardness, excellent wear- and corrosion- resistant as well as good anti-oxidation ability [1,2]. Ceramic coatings fabricated by thermal spraying could endow metal substrates with various outstanding properties [3,4]. The high hardness of ceramic materials gives the coatings outstanding anti-wear properties but usually leads to the serious wear of metallic pairs [5], which would result in the diminution of the size or volume of metal components. Eventually, their excepted functionality would be deprived. Thus, it is crucial to reduce the wear between ceramic coatings and metallic pairs. The introduction of lubrication phases in the ceramic coatings could improve the lubricating property of the coatings [6,7]. However, for thermal spraying coatings, the traditional solid lubricants (graphite, MoS2, etc.) are apt to oxidize in the present of oxygen on account of the ultra-high temperature of plasma flame core [8]. Therefore, it is still a challenge to prepare thermal sprayed coatings including with traditional solid lubricants. Besides, pores and micro-cracks exist in the coatings inevitably due to the intrinsic characteristics of thermal sprayed technology. These defects decrease the mechanical properties, thereby affecting the friction behavior [9,10]. Thus, by means of ingenious design, introducing the solid lubricant into the micro-cracks and ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. An), [email protected] (H. Zhou). http://dx.doi.org/10.1016/j.matlet.2017.01.148 0167-577X/Ó 2017 Published by Elsevier B.V.
pores of ceramic coatings could improve the friction and wear behavior of friction pairs. Based on the above analysis, in the present research, the ZrO2 ceramic coatings were fabricated via atmospheric plasma spraying. The lubricant MoS2 was synthesized through a hydrothermal method in the pores and micro-cracks of as-sprayed ZrO2 coatings. The microstructure of synthesized MoS2 was studied. Afterwards, the tribological properties of the compound coatings were investigated compared with pure ZrO2 coating. 2. Experimental section The ZrO2 ceramic coatings were deposited on 316 stainless steel substrates (u25 7.8 mm) by an APS-2000A system (Institute of Aeronautical Manufacturing Technology, Beijing, China). The assprayed coatings were mechanically polished to a surface roughness Ra 0.21 lm, followed by ultrasonically cleaning in acetone. MoS2 was in-suit synthesized by the hydrothermal method. A mixture of 1.87 g Sodium molybdate and 2.77 g thiourea were dissolved in 80 mL deionized water and magnetic stirred for 30 min. The polished specimen were immersed in the homogeneous solution and put in an ultrasonic bath for 10 min and then placed in a vacuum chamber for 10 min under 60 mmHg vacuum level. After repeating the above steps for three times, the solution with the samples was transferred into a 150 mL Teflon-lined stainless steel autoclave and was heated at 220 °C for 48 h. After that, the autoclave cooled down to room temperature naturally.
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W. Deng et al. / Materials Letters 193 (2017) 199–202
The friction and wear tests were performed by a ball-on-disc tribometer (CSM Instrument, Switzerland) with a reciprocating mode. Metallic balls (1Cr18Ni9Ti) with a diameter of 6 mm were used as the counterpart. All experiments were conducted at room temperature of 20 ± 2 °C, relative humidity of 30 ± 5%, a sliding velocity of 5 cm/s, the load of 5 N, amplitude of 2.5 mm and a total sliding distance of 100 m. The wear morphologies of coupled balls were described using a Micro-XAM-3D non-contact surface profiler (USA). The wear rates (W) of the coatings are calculated as: W = V/ PL, where V is the wear volume loss (mm3), P is the load (N) and L is the sliding distance (m). The maximum depth of worn track (h) is calculated by the volume of spherical cap (V1) and wear volume (V2) of coupled ball:
V 1 ¼ ph ð3R hÞ=3
ð1Þ
V 2 ¼ pb =64R;
ð2Þ
2
4
where b is the wear scar diameter and R is the radius of the ball.
3. Results and discussion The XRD pattern of the powders synthesized via hydrothermal reaction is shown in Fig. 1(a). It can be observed that all the diffraction peaks are in good agreement with that of hexagonal MoS2 (JCPDS No. 73-1508, molybdenite). Besides, no characteristic peaks of other impurities are detected, demonstrating that the sample fabricated by hydrothermal reaction is highly pure. The morphology and size of MoS2 are elucidated by SEM (Fig. 1(b)) and TEM (Fig. 1(c)). From Fig. 1(b), the samples exhibit microsphere structure composed of many lamellae. High-magnification SEM image of region ‘‘A” and TEM indicate that the size of lamellae is very thin with a thickness of about 10 nm, which interweaved together and formed the microsphere structure. As shown in Fig. 2(a), the ZrO2 coatings with a thickness of about 345 mm are uniformly deposited on the substrate. The pores and micro-cracks display on the fracture surface of pure ZrO2 coating, and the fracture surface is relatively smooth (Fig. 2(b)). However, the fracture surface of the composite coating (Fig. 2(c)) is
Fig. 1. The XRD pattern (a), SEM (b) and TEM (c) images of the resultant MoS2.
Fig. 2. The SEM images of cross-section (a) and fracture surface (b) of ZrO2 coating and the fracture surface (c) of composite coating and EDS analyses (d).
W. Deng et al. / Materials Letters 193 (2017) 199–202
very rough and no obvious pores or cracks are detected. From the enlarge figure of ‘‘Ⅰ”, it is surprising and interesting to find that many lamellar-structured phases generate in the composite coat-
201
ings. The EDS analysis indicates that the lamellar contains many Mo and S, which is corresponding to MoS2 due to the atom percentage of Mo and S is about 1:2. The results imply that MoS2 is
Fig. 3. The friction coefficient curves (a), SEM images of wear tracks ((b) and (c)) and the 3D topographies of counterparts sliding against ZrO2 coating (d) and composite coating (e).
Fig. 4. The SEM images of wear surfaces and corresponding element mapping: (a) ZrO2 coating, (b) ZrO2/MoS2 composite coating; the wear morphology of the tribo-pair sliding against composite coating (c) and corresponding Raman spectrum (d).
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W. Deng et al. / Materials Letters 193 (2017) 199–202
successfully synthesized in the pores and micro-cracks of the ZrO2 coatings. Limiting by the size of the pores and micro-cracks, it is difficult to assemble into microspheres. As a result, MoS2 presented lamellar structure in the micro-defects of as-sprayed ZrO2 coatings. The friction coefficient curves and the morphology of worn surfaces of the ZrO2 coatings, composite coatings and the 3D topographies of the coupled balls are given in Fig. 3. Before friction test, the MoS2 layer is carefully removed from the surface of composite coating with Ra 0.14 lm. From Fig. 3(a), the friction coefficient of composite coating is about 0.12 which is much lower than that of ZrO2 coating (l 0.53). It can be concluded that the resultant MoS2 in the composite coatings played an important role in the decrease of the friction coefficient, which could be further confirmed by wear morphologies of coatings and the 3D topographies of coupled balls (Fig. 3(b) and (d)). The width of wear scar of ZrO2 coating is about 2.5 times broader than that of composite coating. And the wear rate of ZrO2 coating (8.52 106 mm3/Nm) is approximately six times higher than that of composite coating (1.37 106 mm3/Nm). Meanwhile, the maximum depth of worn track of the tribo-ball coupled with composite coating is about 6.97 mm, much smaller than that of the tribo-ball coupled with ZrO2 coating (24.69 mm). The results display that the existence of MoS2 not only leads to the decline of the friction coefficient but also reduces the wear of both coatings and counterparts. The SEM micrographs of wear surfaces and corresponding elemental composition mappings are displayed in Fig. 4. From Fig. 4 (a), Fe is uniformly covered on the worn surface of ZrO2 coating, which indicates that the elements of counterpart are transferred onto the coating due to the higher hardness of ZrO2 coating, which caused severe adhesive wear. However, Mo and S are detected on the wear scar of composite coating (Fig. 4(b)). It can be inferred that MoS2 lubricating film is formed on the worn surface. Besides, Fe is not detected in the enriched regions of MoS2, indicate that MoS2 relieved adhesive wear of the frictional surfaces. Raman spectrum of tribo-ball sliding against composite coating is further confirmed that MoS2 lubricating film is indeed present (Fig. 4 (c) and (d)). The formation of MoS2 lubricating film hinders the direct contact of the couples, thereby greatly alleviating the adhesive wear. Meanwhile, the existence of lubricating film decreased the shearing stress and the plastic deformation of worn surface, and resulted in remarkable improvement of tribological properties of the composite coating.
4. Conclusion MoS2 with lamellar-like was synthetized via hydrothermal method in the micro-cracks and pores of thermal sprayed ZrO2 coatings. The ZrO2/MoS2 composite coating exhibited a low friction coefficient and few adhesions as well as plows on the worn surface of the specimen and metallic counterpart attributed to the formation of MoS2 lubricating film. Compared with pure ZrO2 coating, the composite coatings presented an outstanding tribological properties. Acknowledgements This research is financially supported by the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2014378), the West Light Foundation of the Chinese Academy of Sciences. The authors appreciate helpful comments of the reviewers. References [1] S.J. Li, X. Xi, G.L. Hou, Y.L. An, et al., Preparation of plasma sprayed mullite coating on stainless steel substrate and investigation of its environmental dependence of friction and wear behavior, Tribol. Int. 91 (2015) 32–39. [2] M.A. Zavareh, A.A.D.M. Sarhan, B.B.A. Razak, et al., Plasma thermal spray of ceramic oxide coating on carbon steel with enhanced wear and corrosion resistance for oil and gas applications, Ceram. Int. 40 (2014) 14267–14277. [3] L. He, Y. Tan, X. Wang, et al., Microstructure and wear properties of Al2O3CeO2/Ni-base alloy composite coatings on aluminum alloys by plasma spray, Appl. Surf. Sci. 314 (2014) 760–767. [4] M.J. Ghazali, S. Forghani, N. Hassanuddin, et al., Comparative wear study of plasma sprayed TiO2 and Al2O3-TiO2 on mild steels, Tribol. Int. 93 (2016) 681– 686. [5] G. Bolelli, V. Cannillo, L. Lusvarghi, et al., Wear behaviour of thermally sprayed ceramic oxide coatings, Wear 261 (2006) 1298–1315. [6] Z.J. Huang, D.S. Xiong, et al., MoS2 coated with Al2O3 for Ni-MoS2/Al2O3 composite coatings by pulse electrodeposition, Surf. Coat. Technol. 202 (2008) 3208–3214. [7] L.Q. Kong, Q.L. Bi, M.Y. Niu, et al., High-temperature tribological behavior of ZrO2-MoS2-CaF2 self-lubricating composites, J. Eur. Ceram. Soc. 33 (2013) 51– 59. [8] H. Herman, Plasma-Sprayed Coatings, Scientific American, USA, 1988, p. 259. [9] Z. Wang, A. Kulkarni, S. Deshpande, et al., Effects of pores and interfaces on effective properties of plasma sprayed zirconia coatings, Acta Mater. 51 (2003) 5319–5334. [10] C.J. Li, J. Zou, H.B. Huo, et al., Microstructure and properties of porous abradable alumina coatings flame-sprayed with semi-molten particles, J. Therm. Spray Technol. 25 (2016) 264–272.