- Email: [email protected]
Bi2O3
Surface & Coatings Technology 349 (2018) 157–165
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Mechanical and tribological properties of plasma sprayed NiAl composite coatings with addition of nanostructured TiO2/Bi2O3 ⁎
T
⁎
Xinpeng Wanga,b, Xiaochun Fenga,b, Cheng Lua,b, Gewen Yia, , Junhong Jiaa,c, , Hongchun Lid a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730030, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China c State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, PR China d Shaanxi Engineering Research Center of Special Sealing Technology, Xi'an Aerospace Prolusion Institute, Xi'an 710100, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Plasma spraying TiO2/Bi2O3 agglomerate Bimodal microstructure Tribo-chemical reaction
Spray-dried nanostructured TiO2/Bi2O3 feedstock powders were successfully incorporated into plasma sprayed NiAl composite coatings. The tribological properties of the composite coatings from RT to 800 °C were investigated. The results show that the nanostructured TiO2/Bi2O3 phases performed the typical bimodal microstructure resulting in a significant enhancement of mechanical and tribological properties of the composite coatings. The lowered friction and wear at 800 °C is attributed to the synergistic lubricating effect of Bi4Ti3O12 and NiTiO3 formed on worn surfaces by tribo-chemical reactions together with TiO2 and NiO. A continuous lubricating layer including NiTiO3 and Al2TiO5 was formed on counterfaces, which further improved the tribological behavior of composite coating at high temperatures.
1. Introduction For elevated temperature tribological components include air-foil bearings, high speed machine tools, gas turbine seals, etc., the wear occurred on the running surface is difficult to avoid and eventually leads to their failure. Developing elevated temperature self-lubricating composite coatings is an effective means to prevent severe contact and provide excellent lubrication effect between the friction pair. [1–4]. However, Most liquid lubricants (oil and greases) and conventional solid lubricant (graphite, MoS2 and soft metal) have been not meeting the requirements for high-temperature lubricants due to volatilizing and oxidizing at high temperatures (> 400 °C) [5–9]. Numerous oxides have been researched and used as high temperature lubricants for their structural and chemical stability, in which mixed oxide systems, such as NiO–TiO2, AgO–MoO3, CoO–MoO3, etc., exhibit exceptional and promising advantage during high temperature test [10–15]. At present, there are two main theories for the choice of ternary lubrication oxides. The crystal chemical approach, putting forward by Erdemir, provided a criterion that the binary oxides could exhibit the better lubricity as the difference in ionic potential increases [16,17]. Dimitrov suggested a polarizability approach which expressed the anion and cation polarizability by the interaction parameter. It is suggested that the smaller interaction parameters in oxides correspond to the lower friction coefficient [18,19]. These two approaches still have
⁎
some limitations and not suitable to explain the friction behavior of all of oxides, however, these could serve as reference to the choice of oxides lubricants at high temperature, and provide certain guidance to the lubrication mechanism of high temperature for oxides [5,15]. In the TiO2-Bi2O3 binary system, the ionic potential difference is high and bismuth oxide possess lower interaction parameter than other oxides according to polarizability approach, implying the theoretically excellent lubricating property [17,19]. H. Kato et al. observed that the addition of nanometer-sized Bi2O3 on the rubbing surface could form wear-protective layer and exhibit the best wear property compared to other oxides [20]. At the same time, TiO2 coating is widely used in wear and corrosion protection due to high efficiency of powder deposition and low porosity of coating [21]. Titanium dioxide also possess great high temperature lubricity and its coating can provide better combination of ductility, hardness and wear resistance than that of hard ceramic coatings [22–24] However, there isn't report on tribological properties about TiO2-Bi2O3 binary system yet. Many oxides usually exhibit poor cohesive strength with metal substrate as coating material due to their inherent properties. The Nibased alloy with high bonding strength, low stress, and excellent mechanical and anti-oxidizing property could be ideal matrix material for high-temperature coatings [21,25]. In our previous works, a series of Ni-based composite coatings were fabricated using conventional powders, and the addition of reinforced phase and lubricating phase
Corresponding authors at: State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730030, PR China. E-mail addresses: [email protected] (G. Yi), [email protected] (J. Jia).
https://doi.org/10.1016/j.surfcoat.2018.05.055 Received 27 January 2018; Received in revised form 1 May 2018; Accepted 27 May 2018 Available online 29 May 2018 0257-8972/ © 2018 Published by Elsevier B.V.
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
improves the lubrication and mechanical properties of the coatings, but the increase of phase interface is a great disadvantageous for coatings and lead to more micro-defects, which would undoubtedly influence the microstructure and deteriorate adhesive strength [26–28]. The researches have indicated that the composite coating by using nanostructured powders could exhibit less defect, more compact microstructure and higher cohesive strength comparing with the conventional one [29,30]. Goberman reported that nanostructured coatings can enhance the durability and performance of conventional plasma sprayed coatings [31]. Zhao et al. found that the nano-structure alumina-reinforced CuAl composite coating exhibits superior wear-resistance than the micro-structure alumina-reinforced one [32]. In this study, the formulation of NiAl-TiO2-Bi2O3 was designed based on theoretical and experimental basis. Spray-dried nanostructured TiO2/Bi2O3 feedstock powders were adopted as reinforced phase and high temperature lubricant of NiAl coatings. The microstructures, mechanical and tribological properties of composite coatings containing different amounts of TiO2/Bi2O3 are investigated. Meanwhile, the lubricating mechanisms at elevated temperatures are analyzed and discussed.
Fig. 2. Particle size distributions of nanostructured TiO2/Bi2O3 feedstocks. Table 1 Composition and mechanical properties of NiAl-TiO2/Bi2O3 composite coatings.
2. Experimental details
Composite coating
2.1. Materials and coating preparation TiO2 and Bi2O3 were commercially purchased and used as received. Two kinds of oxides were first ball-milled for 10 h in Pulverisette 5 Planetary high-energy-mill (Fritsch, Germany) respectively, then they were used to constitute into micrometer sized agglomerates in the mass ratio of 3:2 (TiO2:Bi2O3) by spray drying method and sintering at 600 °C for 1 h. The feedstock powders of NiAl were obtained from Sulzer Metco. The SEM morphologies of NiAl and TiO2/Bi2O3 feedstocks were shown in Fig. 1. The NiAl and TiO2/Bi2O3 feedstocks appear spherelike with a size distribution range of 20–60 μm and 5–48 μm, respectively. The TiO2/Bi2O3 feedstock displays a porous microstructure with particle sizes in range of 60–130 nm (Fig. 1c).The size distributions of TiO2/Bi2O3 feedstocks (Fig. 2) were acquired using a Malvern 3000 laser diffraction analyzer (Malvern, UK). The average size (D50) of TiO2/Bi2O3 feedstock is 15 μm which is appropriate for the plasma spraying. The NiAl and TiO2/Bi2O3 feedstocks with different ratios were mechanically blended by a M10 three-dimensional mixing instrument (Grinder, CHINA) and served as prepared feedstocks. Inconel 718 alloy disks (Φ38 mm × 8 mm), used as substrates, were sands blasted and ultrasonically cleaned in acetone before spraying. The spraying coatings were deposited by a 9MC atmospheric plasma spraying system (SulZer Metco, US). NiAl powder was applied as bond coat prior to spraying composite coatings. The compositions of composite coatings and the spraying parameters were listed in Tables 1 and 2, respectively.
NATB1 NATB2 NATB3
Compositions (wt%) NiAl
TiO2/Bi2O3 (3/ 2)
80 70 60
20 30 40
Vickers hardness (HV)
Bonding strength (MPa)
174.6 ± 5.0 194.4 ± 7.6 234.5 ± 6.8
49.56 ± 3.42 43.40 ± 2.89 40.43 ± 3.66
Table 2 Plasma spraying parameters. Parameters
Bond coat
Coatings
Current (A) Voltage (V) Ar gas flow rate, L·min−1 H2 gas flow rate, L·min−1 Power feed rate, g·min−1 Spray distance, mm
500 55 40 5 42 100
550 60 40 5 35 85
2.2. Characterization The hardness and bonding strength of the composite coatings were conducted according to our previous research methods [30], and average values are given in Table 1. A UMT-3 pin-on-disk high temperature tribological tester (Bruker Corp, USA) was used to perform the friction tests at selected temperatures of RT, 200 °C, 400 °C, 600 °C and 800 °C. The commercial Al2O3 ceramic ball (10 mm in diameter, HV 16.5 GPa, SG 3.92 g·cm−3) was used as the counterpart. The tests were
Fig. 1. Morphologies of feedstock powders: (a) NiAl, (b) nanostructured TiO2/Bi2O3, (c) high magnification of (b). 158
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
Fig. 3. XRD pattern of agglomerated TiO2/Bi2O3 powers (a) as-received and (b) calcined.
Fig. 4. XRD patterns of as-sprayed NiAl-TiO2/Bi2O3 composite coatings with different TiO2/Bi2O3 content (inset is partial enlarged drawing).
conducted with 10 N load, 0.1 m·s−1 sliding speed and 60 min test time. The sectional profile of the wear scar was measured by a MicroXAM800 three-dimensional profilometer (KLA-tencor, USA). The wear rate of the specimen was calculated by common approach, i.e., W = V/DL [32]. The phase compositions and structural evolutions of the powders and corresponding coatings were characterized by X-ray diffraction (PANalytical B.V., The Netherlands) for Cu-Kα radiation. The crosssection and the worn surface morphologies of the coatings were determined by a JEM-5600LV scanning electron microscope (Tescan Mira 3, Czech). Simultaneously, the worn surface morphologies of the counterpart were observed by an optical microscope (Olympus, Japan). And the corresponding phase identifications on the worn surface were analyzed using the HR800 Raman spectrometer (Horiba Jobin Yvon, France) with laser radiation wavelength of 532 nm.
the technology characteristics of plasma spraying. The interfaces between NiAl and oxide phases were closely integration, which shows better interlamellar cohesion. According to the EDS analysis, the gray phase is NiAl, the deep gray phase is TiO2 in which the dispersed white phase is the Bi2O3. Furthermore, the composite coating registered the typical bimodal microstructure, consists of partially melted regions (PM) and fully melted regions (FM) (inset of Fig. 5b). During the plasma spraying, a part of nanoparticles cannot be melted completely due to a short residence time in high speed flames. Simultaneously, a large number of grain boundaries in agglomerated feedstocks retard the growth of grain in the deposition process. So the partially melted regions preserve unique nanostructures (Fig. 5d). The unique bimodal microstructure formed by nanostructure feedstock powders is beneficial to effectively improve mechanical and tribological properties of the coatings, including the adhesive strength and toughness of the coating and wear-resistance property, etc. [29,31,33,35].
3. Results and discussion 3.2. Mechanical properties and tribological properties 3.1. Characterization of feedstock powers and as-sprayed coating The Vickers hardness and bonding strength of NiAl-TiO2-Bi2O3 composite coatings are presented in Table 1. The hardness of composite coatings increases from 174.6 HV to 234.5 HV with the TiO2/Bi2O3 contents increasing. In the bonding test, the failure occurred in the interior of the composite coatings. The bonding strength of composite coatings decreased with the addition of TiO2/Bi2O3. Nevertheless, the bonding strength values still keep at higher level (> 40 MP) comparing to our previous works [26–28], which is mainly ascribed to the reduced phase interfaces of composite coating by using of nanostructured TiO2/ Bi2O3 agglomerated powder as feedstock (Fig. 5). Fig. 6 shows the average friction coefficients and wear rate of composite coatings with different TiO2/Bi2O3 contents sliding against Al2O3 ball from room temperature to 800 °C. The friction coefficients of all coatings exhibit the same trend, and that is increased first from RT to 200 °C and then decreased sharply with the further increasing of temperatures. It can be seen that the composite coatings exhibited higher COFs in the low temperatures, but coatings display excellent lubrication performance at elevated temperature. Especially for the composite coating with 30 wt% TiO2/Bi2O3, the friction coefficient is registered to 0.07 at 800 °C. Base on the theoretical basis of Erdemir, mixed oxides with higher difference in ionic potential are inclined to form a lowmelting-point or readily shearable or highly stable compound, which could exhibit much lower hardness and better lubrication effect at elevated temperatures [17]. It may be a good explanation for the continuous decreasing of friction coefficient at elevated temperatures.
Fig. 3 gives the XRD patterns of the TiO2/Bi2O3 feedstock as-received and after calcined at 600 °C for 1 h. The phases of agglomerated powders are mainly composed of anatase-TiO2 (JCPDS file no. 21-1272) and α-Bi2O3 (JCPDS file no.41-1449). After heat treatment, the intensities of diffraction peaks become stronger, demonstrating that the amorphization and internal stress in oxides particles caused by ballmilling reduced. A new phase of Bi4Ti3O12 (JCPDS file no. 47-0398) was detected after heat treatment, which may be attributed to the solid state reaction between nano-TiO2 and nano-Bi2O3 at high temperature. Fig. 4 displays the XRD patterns of as-sprayed NiAl-TiO2/Bi2O3 composite coatings with different nanostructured oxides contents. The composite coating mainly consists of NiAl compound (JCPDS file no. 20-0019), Ni-based solid solution (JCPDS file no. 04-0850), r-TiO2 (JCPDS file no. 21-1276), δ-Bi2O3 (JCPDS file no. 45-1344) and Bi3Ti4O12 (JCPDS file no. 47-0398). The phases of a-TiO2 and α-Bi2O3 in feedstock transferred to more stable phases of r-TiO2 and δ-Bi2O3 at high temperature during plasma spraying process. Nevertheless, the diffraction peaks of r-TiO2, δ-Bi2O3 and Bi4Ti3O12 become weaker and broad due to the amorphization caused by high cooling rate during coating deposition process (inset of Fig. 4). The SEM images of cross-section of composite coatings with different content of TiO2/Bi2O3 are presented in Fig. 5. It is observed that all composite coatings possess dense microstructure and uniform distribution of the phases with the exception of pores and cracks caused by 159
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
Fig. 5. SEM images of the cross-section of the composite coatings of (a) NATB1, (b) NATB2 and (c) NATB3, (a'), (b') and (c') are the high magnification images of (a), (b) and (c).
effectively prevent brittle fracture and peeling of coating during wear test. Additionally, the existence of partially melted regions would generate fine debris with nano or sub-micron scale under sliding process, which could be filled in spalling pits and other pits to prevent the worn surface falling out. Therefore, the nanostructured coatings with unique bimodal microstructure possess excellent wear-resistance property.
Meanwhile, it can be found that the wear rates of the composite coatings increase firstly and then maintain relatively high values from room temperature to 600 °C, when above 600 °C, the wear rates of composite coatings decrease sharply. However, the wear rates of all composite coatings hold low value (under 8 × 10−5 mm3·(N·m)−1) in the tested temperature range, in which the wear rate of NATB2 is below 5 × 10−5 mm3·(N·m)−1 in the whole test temperatures. The reason for this can be attributed to the fact that the bimodal microstructure formed by nanostructured TiO2/Bi2O3 can provide excellent wear resistance [29,31,33]. It has been proved that the bimodal microstructure possessed “performance complementary” property, in which the partially melted regions improved the toughness of the coating and could
3.3. Worn surface morphologies Fig. 7 shows the worn surface morphologies of NATB2 composite coating after wear tests at different temperatures. At room temperature,
Fig. 6. Friction coefficients and wear rates of composite coatings at different temperatures. 160
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
Fig. 7. SEM morphology of worn surfaces of NATB2 coating at different test temperatures: (a) 25 °C, (b) 200 °C, (c) 400 °C, (d) 600 °C and (e) 800 °C. 161
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
Fig. 8. SEM morphology of worn surfaces of composite coatings: (a)–(b) NATB1 coating, (c)–(d) NATB3 coating.
a relatively smooth tribo-layer with some cracks and peeling is present on the worn surface. The wear mechanism is characterized by microcracking and spalling (Fig. 7a). Owing to the addition of nanometersized oxides, wear debris particles will be compacted and formed tribolayer, which are beneficial to the decrease in friction and wear. However, bismish oxide has more advantage than other oxides in this aspect [20]. The crack occurred on worn surface may be attributed to the fatigue wear of the coatings or the residual stresses in the thermal spraying coatings [34]. At 200 °C, the worn surface become coarse and is covered with a lot of wear debris and several plastic smearing compared with that at RT (Fig. 7b), which is corresponding to the higher friction coefficient. When the temperature increases to 400 °C, the worn surface is characterized by ploughed grooves and some wear debris (Fig. 7c), which illustrates that the wear mechanisms is dominated by micro-ploughing and abrasive wear. At 600 °C, the worn surface becomes smooth and is covered by incomplete glaze layer with fine and slight grooves (Fig. 7d), suggesting that the wear mechanism is characterized by plastic deformation and micro-scuffing. It is reported that, in the case of binary/mixed oxides, the frictional behavior of complex system similar to that of its low melting point constituent [19]. Therefore, Bi2O3 with lower melting point (825 °C) could be
Fig. 9. XRD patterns of worn surfaces of NATB2 coating at different test temperatures. 162
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
extent. However, the trace of ploughing may be attributed to the less addition of TiO2/Bi2O3 whose could possess a great effect to form a readily lubricous and easily plastic interface. With the TiO2/Bi2O3 contents increasing, the worn surface of NATB3 is covered with more smooth and complete glaze layer (Fig. 8d), which are responsible for the lowered friction coefficient and wear rate at high temperature. 3.4. Phase composition of the worn surface Fig. 9 shows the phase compositions of the worn surface of NATB2 composite coatings at different temperatures. It can be clearly seen the diffraction peaks of composite coating after tests below 400 °C are similar to that of as-sprayed coating (Fig. 4). At 600 °C, the diffraction peaks of the NiO (JCPDS file no.47-1049) appeared. At 800 °C, the NiO peaks become more evident and the new phase NiTiO3 (JCPDS file no.33-0960) is detected. Meanwhile, the diffraction peaks of Bi4Ti3O12 were significantly enhancing, which are attributed to the further reaction between TiO2 and Bi2O3 at high temperature. The synergistic lubricating effect of numerous oxides including TiO2, NiO, NiTiO3 and Bi4Ti3O12 formed a continuous lubricating layer on worn surface at 800 °C is attributed to reduce the friction coefficient and wear rates of the composite coating at high temperatures. Raman spectra of the worn surfaces of NATB2 coating at different test temperatures are presented in Fig. 10. At RT, the Raman spectrum of outside wear track displayed broad peaks centering at 445 cm−1 and 610 cm−1, which is assigned to TiO2. The Bi2O3 peak can be also detected at outside of wear track. In contrast, the weaker and broaden peaks are observed in the worn surface, which is assigned to Magnelitype titanium oxides due to that the oxygen vacancies created under repeated sliding process [36]. However, the Raman spectra for inside and outside the wear tracks have no obvious change at 200 °C and 400 °C. At 600 °C, the peaks of TiO2 and Bi2O3 are detected within the wear track. As typical high temperature lubricant, the existences of TiO2 and Bi2O3 within the wear track obviously improve the lubrication behavior of composite coating at 600 °C (Fig. 6a). At 800 °C, besides the TiO2 peaks, it can be observed that a series of new peaks appeared at 717 cm−1, 400 cm−1, 356 cm−1 and 302 cm−1, which are corresponding to rhombohedral structure of NiTiO3 [37]. Meanwhile, the Bi4Ti3O12 peaks are obviously detected, which is consistent to XRD analysis (Fig. 9). However, there was unnoticeable change for the Raman spectrum of outside wear track, suggesting that the tribo-chemical reaction only occurred on the worn surface and formed the Bi4Ti3O12 and NiTiO3 at high temperature. Bi4Ti3O12 with typical layered structure is readily shearable and possess lower melting point (875 °C) [38]. Meanwhile, NiTiO3 has been proved to be a stable oxide at elevated temperatures and could exhibit lubricity on the friction interface [39]. Fig. 11 presented the Raman spectra of within the worn surfaces of NATB1, NATB2 and NATB3 coatings at 800 °C. For NATB1 coating, the Raman spectra of wear track displayed strong peaks of TiO2 and weak peaks of Bi4Ti3O12 and NiTiO3. However, the intensity of Bi4Ti3O12 and NiTiO3 peaks obviously enhanced at the Raman spectra of NATB2 coating. The lowed friction coefficient of NATB2 coating at 800 °C indicated that these two kinds of ternary oxides possessed better high temperature lubrication performance. With the increase of oxides content, the Raman spectra of the wear track for NATB3 coating displayed strong peaks of TiO2 again, and the peaks of Bi4Ti3O12 deservedly displayed stronger intensity than that of NATB1. The friction coefficient of the NATB3 coating is higher than that of NATB2 but lower than that of NATB1. Obviously, the large mount existence of Bi4Ti3O12 and NiTiO3 on the worn surface would exhibit excellent lubrication property. The formation of these ternary oxides on the worn surface and their excellent lubrication property are well consistent to the theoretical research of Erdemir and contributed to explain the variation of friction coefficient of the composite coatings at 800 °C. The worn surface morphologies of the counterpart ball sliding against NATB2 at different temperatures are presented in Fig. 12. At RT,
Fig. 10. Raman spectra of the worn surfaces of NATB2 coating at different test temperatures.
Fig. 11. Raman spectra of the worn surfaces of NATB1, NATB2 and NATB3 coatings at 800 °C.
contributed to explain the tribological property of composite coating. As high temperature lubricant, the present of Bi2O3 on the worn surface is benefit to the lowered friction. However, the lower hardness and softening of Bi2O3 at 600 °C may result in higher wear rate comparing to those in lower temperatures. With further increase in the test temperature, a continuous and complete glaze layer is observed in the wear track (Fig. 7e). The mild wear mechanism and the complete lubricious interface resulted in obvious improvement of tribological properties of the composite coating at 800 °C. The worn surfaces morphologies of NATB1 and NATB3 composite coatings at room temperature and 800 °C are shown in Fig. 8. At room temperature, comparing to NATB1, the worn surface of NATB3 becomes coarse and is covered with a large amount of fine wear debris (Fig. 8c), resulting in the higher friction coefficient and wear rate. However, the cracks occurred on the worn surface are obviously improved compared with the former, since the existence of bimodal microstructure could effectively improve the toughness and prevent crack propagation [35]. At 800 °C, the worn surface of NATB1 is covered by thin glaze layer with slight grooves (Fig. 8b). The formation of glaze layer improved the tribological behavior of composite coating to some 163
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
Fig. 12. Worn surface morphologies (a–e) and Raman spectra (f) of the counterpart ball against NATB2 coating at different temperatures.
become smoother with the increasing of test temperature, which is related to continuous reduction of friction coefficient at this temperature range. At 800 °C, it can be seen that the worn surface of ball is covered by a continuous lubricating film which can effectively reduce the direct contact between ball and disk (Fig. 12e). It can be concluded
the worn surface of ball presents milder wear with light scratches than other test temperature surfaces, which indicates that the composite coating possesses the best wear property at RT (Fig. 12a). From 200 to 600 °C, the worn surface of all balls exhibit severe wear leading to higher wear rates (Fig. 12b–d). However, the worn surface of ball 164
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
that the tribolayer on worn surface mainly consists of Al2TiO5 and NiTiO3 at 800 °C by Micro-Raman analysis (Fig. 12f). It easily understands that the NiTiO3 is transferred from coating disk, while the formation of Al2TiO5 might be attributed to the interface reaction between TiO2 and Al2O3 occurred on the rubbing surfaces. With the formation of frictional layer on worn surface of ball, interface shear take place easily and the tribological behavior of composite coating are dramatically improved at high temperature.
[9] A.P. Semenov, High-temperature solid lubricating substances, J. Frict. Wear 28 (2007) 476–484. [10] M.B. Peterson, S.Z. Li, S.F. Murray, Wear-resisting oxide films for 900°C, J. Mater. Sci. Technol. 13 (1997) 99–106. [11] M.B. Peterson, S.J. Calabrese, S.Z. Li, X. Jiang, Friction of alloys at high temperature, J. Mater. Sci. Technol. 10 (1994) 313–320. [12] S.F. Murray, S.J. Calabrese, Effect of solid lubricants on low speed sliding behavior of silicon nitride at temperatures to 800 °C, Lubr. Eng. 49 (1993) 955–964. [13] D.J. Taylor, P.F. Fleig, S.T. Schwab, R.A. Page, Sol-gel derived, nanostructured oxide lubricant coatings, Surf. Coat. Technol. 120–121 (1999) 465–469. [14] E. Liu, W. Wang, Y. Gao, J. Jia, Tribological properties of Ni-based self-lubricating composites with addition of silver and molybdenum disulfide, Tribol. Int. 57 (2013) 235–241. [15] W. Gulbinski, T. Suszko, Thin films of MoO3-Ag2O binary oxides-the high temperature lubricants, Wear 261 (2006) 867–873. [16] A. Erdemir, A crystal-chemical approach to lubrication by solid oxides, Tribol. Lett. 8 (2000) 97–102. [17] A. Erdemir, A crystal chemical approach to the formulation of self-lubricating nanocomposite coatings, Surf. Coat. Technol. 200 (2005) 1792–1796. [18] V. Dimitrov, T. Komatsu, Classification of simple oxides: a polarizability approach, J. Solid State Chem. 163 (2002) 100–112. [19] B. Prakash, J.P. Celis, The lubricity of oxides revised based on a polarisability approach, Tribol. Lett. 27 (2007) 105–112. [20] H. Kato, K. Komai, Tribofilm formation and mild wear by tribo-sintering of nanometer-sized oxide particles on rubbing steel surfaces, Wear 262 (2007) 36–41. [21] J. Wang, Y. Shan, H. Guo, W. Wang, G. Yi, J. Jia, The tribological properties of NiCrAl2O3-TiO2 composites at elevated temperatures, Tribol. Lett. 58 (2015) 1–10. [22] R.S. Lima, B.R. Marple, Superior performance of high-velocity oxyfuel-sprayed nanostructured TiO2 in comparison to air plasma-sprayed conventional Al2O313TiO2, J. Therm. Spray Technol. 14 (2005) 397–404. [23] M.N. Gardos, The effect of anion vacancies of the tribological properties of rutile (TiO2−x), Tribol. Trans. 33 (1989) 209–220. [24] M.N. Gardos, H.-s. Hong, W.O. Winer, The effect of anion vacancies on the tribological properties of rutile (TiO2−x), Part II: experimental evidence, Tribol. Trans. 33 (2012) 209–220. [25] E. Liu, Y. Gao, W. Wang, X. Zhang, X. Wang, G. Yi, Effect of Ag2Mo2O7 incorporation on the tribological characteristics of adaptive Ni-based composite at elevated temperatures, Tribol. Trans. 56 (2013) 469–479. [26] B. Li, Y. Gao, M. Han, H. Guo, J. Jia, W. Wang, Tribological properties of NiAl matrix composite coatings synthesized by plasma spraying method, J. Mater. Res. 32 (2017) 1–8. [27] B. Li, J. Jia, Y. Gao, M. Han, W. Wang, Microstructural and tribological characterization of NiAl matrix self-lubricating composite coatings by atmospheric plasma spraying, Tribol. Int. 109 (2017) 563–570. [28] B. Li, J. Jia, Y. Gao, H. Guo, M. Han, W. Wang, Influence of silver contents on the tribological properties of Ni-based self-lubricating coatings by atmospheric plasma spraying, Acta Metall. Sin. (2017) 801–808. [29] E.P. Song, J. Ahn, S. Lee, N.J. Kim, Microstructure and wear resistance of nanostructured Al2O3-8wt%TiO2 coatings plasma-sprayed with nanopowders, Surf. Coat. Technol. 201 (2006) 1309–1315. [30] B. Li, J. Jia, M. Han, Y. Gao, W. Wang, C. Li, Microstructure, mechanical and tribological properties of plasma-sprayed NiCrAlY-Mo-Ag coatings from conventional and nanostructured powders, Surf. Coat. Technol. 324 (2017) 552–559. [31] D. Goberman, Y.H. Sohn, L. Shaw, E. Jordan, M. Gell, Microstructure development of Al2O3-13wt%TiO2 plasma sprayed coatings derived from nanocrystalline powders, Acta Mater. 50 (2002) 1141–1152. [32] X. Zhao, Y. An, G. Hou, H. Zhou, J. Chen, Preparation and tribological properties of plasma sprayed nano and micro-structure alumina-reinforced CuAl composite coatings, Tribol. Int. 101 (2016) 255–263. [33] V.P. Singh, A. Sil, R. Jayaganthan, A study on sliding and erosive wear behaviour of atmospheric plasma sprayed conventional and nanostructured alumina coatings, Mater. Des. 32 (2) (2011) 584–591. [34] S. Ahmaniemi, M. Vippola, P. Vuoristo, T. Mäntylä, M. Buchmannb, R. Gadowb, Residual stresses in aluminium phosphate sealed plasma sprayed oxide coatings and their effect on abrasive wear, Wear 252 (7–8) (2002) 614–623. [35] X. Lin, Y. Zeng, S.W. Lee, C. Ding, Characterization of alumina-3wt% titania coating prepared by plasma spraying of nanostructured powders, J. Eur. Ceram. Soc. 24 (2004) 627–634. [36] A. Skopp, N. Kelling, M. Woydt, L.M. Berger, Thermally sprayed titanium suboxide coatings for piston ring/cylinder liners under mixed lubrication and dry-running conditions, Wear 262 (2007) 1061–1070. [37] T.T. Pham, S.G. Kang, E.W. Shin, Optical and structural properties of Mo-doped NiTiO3 materials synthesized via modified Pechini methods, Appl. Surf. Sci. 411 (2017) 18–26. [38] R.C. Oliveira, L.S. Cavalcante, J.C. Sczancoski, E.C. Aguiarb, J.W.M. Espinosa, J.A. Varela, P.S. Pizani, E. Longo, Synthesis and photoluminescence behavior of Bi4Ti3O12 powders obtained by the complex polymerization method, J. Alloys Compd. 478 (2009) 661–670. [39] D.J. Taylor, P.F. Fleig, R.A. Page, Characterization of nickel titanate synthesized by sol-gel processing, Thin Solid Films 408 (2002) 104–110.
4. Conclusions The NiAl composite coatings with different contains of nanostructured TiO2/Bi2O3 are fabricated by atmospheric plasma spraying. The microstructures, mechanical and tribological properties of composite coatings were investigated. The following conclusions can be drawn: 1. The hardness of composite coatings increases with the TiO2/Bi2O3 increasing. The adhesive strength of composite coatings decreased with the addition of TiO2/Bi2O3, while that still keep at higher level (> 40 MP). 2. The nanostructured TiO2/Bi2O3 phases perform typical bimodal microstructure consisted of partially melted regions (PM) and fully melted regions (FM), which results in a significant enhancement of mechanical and tribological properties of the composite coatings. 3. The composite coatings with nanostructured TiO2/Bi2O3 exhibit excellent tribologicial behavior. Especially for the composite coating with 30 wt% TiO2/Bi2O3, the friction coefficient is registered to 0.07 at 800 °C and the wear rate is below 5 × 10−5 mm3·(N·m)−1 in the whole test temperatures. 4. The lowered friction and wear at 800 °C is attributed to the synergistic lubricating effect of Bi4Ti3O12 and NiTiO3 formed on worn surfaces by tribo-chemical reactions together with TiO2 and NiO. A continuous lubricating layer including NiTiO3 and Al2TiO5 was formed on counterfaces, which further improved the tribological behavior of composite coating at high temperatures. Acknowledgements The authors acknowledge the financial supports by the National Natural Science Foundation of China (Grant No. 51471181, 51575505, 51675508). References [1] A. Pauschitz, M. Roy, F. Franek, Mechanisms of sliding wear of metals and alloys at elevated temperatures, Tribol. Int. 41 (2008) 584–602. [2] P. He, G. Ma, H. Wang, et al., Tribological behaviors of internal plasma sprayed TiO2-based ceramic coating on engine cylinder under lubricated conditions, Tribol. Int. 102 (2016) 407–418. [3] J. Chen, Y. An, J. Yang, et al., Tribological properties of adaptive NiCrAlY-Ag-Mo coatings prepared by atmospheric plasma spraying, Surf. Coat. Technol. 235 (2013) 521–528. [4] R. Tyagi, D.S. Xiong, J.-l. Li, J. Dai, High-temperature friction and wear of Ag/h-BNcontaining Ni-based composites against steel, Tribol. Lett. 40 (2010) 181–186. [5] S.M. Aouadi, H. Gao, A. Martini, T.W. Scharf, C. Muratore, Lubricious oxide coatings for extreme temperature applications: a review, Surf. Coat. Technol. 257 (2014) 266–277. [6] C. Muratore, A.A. Voevodin, Chameleon coatings: adaptive surfaces to reduce friction and wear in extreme environments, Annu. Rev. Mater. Res. 39 (2009) 297–324. [7] D.-s. Xiong, Lubrication behavior of Ni–Cr-based alloys containing MoS2 at high temperature, Wear 251 (2001) 1094–1099. [8] C. Muratore, A.A. Voevodin, J.J. Hu, J.G. Jones, J.S. Zabinski, Growth and characterization of nanocomposite yttria-stabilized zirconia with Ag and Mo, Surf. Coat. Technol. 200 (2005) 1549–1554.
165
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Mechanical and tribological properties of plasma sprayed NiAl composite coatings with addition of nanostructured TiO2/Bi2O3 ⁎
T
⁎
Xinpeng Wanga,b, Xiaochun Fenga,b, Cheng Lua,b, Gewen Yia, , Junhong Jiaa,c, , Hongchun Lid a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730030, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China c State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, PR China d Shaanxi Engineering Research Center of Special Sealing Technology, Xi'an Aerospace Prolusion Institute, Xi'an 710100, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Plasma spraying TiO2/Bi2O3 agglomerate Bimodal microstructure Tribo-chemical reaction
Spray-dried nanostructured TiO2/Bi2O3 feedstock powders were successfully incorporated into plasma sprayed NiAl composite coatings. The tribological properties of the composite coatings from RT to 800 °C were investigated. The results show that the nanostructured TiO2/Bi2O3 phases performed the typical bimodal microstructure resulting in a significant enhancement of mechanical and tribological properties of the composite coatings. The lowered friction and wear at 800 °C is attributed to the synergistic lubricating effect of Bi4Ti3O12 and NiTiO3 formed on worn surfaces by tribo-chemical reactions together with TiO2 and NiO. A continuous lubricating layer including NiTiO3 and Al2TiO5 was formed on counterfaces, which further improved the tribological behavior of composite coating at high temperatures.
1. Introduction For elevated temperature tribological components include air-foil bearings, high speed machine tools, gas turbine seals, etc., the wear occurred on the running surface is difficult to avoid and eventually leads to their failure. Developing elevated temperature self-lubricating composite coatings is an effective means to prevent severe contact and provide excellent lubrication effect between the friction pair. [1–4]. However, Most liquid lubricants (oil and greases) and conventional solid lubricant (graphite, MoS2 and soft metal) have been not meeting the requirements for high-temperature lubricants due to volatilizing and oxidizing at high temperatures (> 400 °C) [5–9]. Numerous oxides have been researched and used as high temperature lubricants for their structural and chemical stability, in which mixed oxide systems, such as NiO–TiO2, AgO–MoO3, CoO–MoO3, etc., exhibit exceptional and promising advantage during high temperature test [10–15]. At present, there are two main theories for the choice of ternary lubrication oxides. The crystal chemical approach, putting forward by Erdemir, provided a criterion that the binary oxides could exhibit the better lubricity as the difference in ionic potential increases [16,17]. Dimitrov suggested a polarizability approach which expressed the anion and cation polarizability by the interaction parameter. It is suggested that the smaller interaction parameters in oxides correspond to the lower friction coefficient [18,19]. These two approaches still have
⁎
some limitations and not suitable to explain the friction behavior of all of oxides, however, these could serve as reference to the choice of oxides lubricants at high temperature, and provide certain guidance to the lubrication mechanism of high temperature for oxides [5,15]. In the TiO2-Bi2O3 binary system, the ionic potential difference is high and bismuth oxide possess lower interaction parameter than other oxides according to polarizability approach, implying the theoretically excellent lubricating property [17,19]. H. Kato et al. observed that the addition of nanometer-sized Bi2O3 on the rubbing surface could form wear-protective layer and exhibit the best wear property compared to other oxides [20]. At the same time, TiO2 coating is widely used in wear and corrosion protection due to high efficiency of powder deposition and low porosity of coating [21]. Titanium dioxide also possess great high temperature lubricity and its coating can provide better combination of ductility, hardness and wear resistance than that of hard ceramic coatings [22–24] However, there isn't report on tribological properties about TiO2-Bi2O3 binary system yet. Many oxides usually exhibit poor cohesive strength with metal substrate as coating material due to their inherent properties. The Nibased alloy with high bonding strength, low stress, and excellent mechanical and anti-oxidizing property could be ideal matrix material for high-temperature coatings [21,25]. In our previous works, a series of Ni-based composite coatings were fabricated using conventional powders, and the addition of reinforced phase and lubricating phase
Corresponding authors at: State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730030, PR China. E-mail addresses: [email protected] (G. Yi), [email protected] (J. Jia).
https://doi.org/10.1016/j.surfcoat.2018.05.055 Received 27 January 2018; Received in revised form 1 May 2018; Accepted 27 May 2018 Available online 29 May 2018 0257-8972/ © 2018 Published by Elsevier B.V.
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
improves the lubrication and mechanical properties of the coatings, but the increase of phase interface is a great disadvantageous for coatings and lead to more micro-defects, which would undoubtedly influence the microstructure and deteriorate adhesive strength [26–28]. The researches have indicated that the composite coating by using nanostructured powders could exhibit less defect, more compact microstructure and higher cohesive strength comparing with the conventional one [29,30]. Goberman reported that nanostructured coatings can enhance the durability and performance of conventional plasma sprayed coatings [31]. Zhao et al. found that the nano-structure alumina-reinforced CuAl composite coating exhibits superior wear-resistance than the micro-structure alumina-reinforced one [32]. In this study, the formulation of NiAl-TiO2-Bi2O3 was designed based on theoretical and experimental basis. Spray-dried nanostructured TiO2/Bi2O3 feedstock powders were adopted as reinforced phase and high temperature lubricant of NiAl coatings. The microstructures, mechanical and tribological properties of composite coatings containing different amounts of TiO2/Bi2O3 are investigated. Meanwhile, the lubricating mechanisms at elevated temperatures are analyzed and discussed.
Fig. 2. Particle size distributions of nanostructured TiO2/Bi2O3 feedstocks. Table 1 Composition and mechanical properties of NiAl-TiO2/Bi2O3 composite coatings.
2. Experimental details
Composite coating
2.1. Materials and coating preparation TiO2 and Bi2O3 were commercially purchased and used as received. Two kinds of oxides were first ball-milled for 10 h in Pulverisette 5 Planetary high-energy-mill (Fritsch, Germany) respectively, then they were used to constitute into micrometer sized agglomerates in the mass ratio of 3:2 (TiO2:Bi2O3) by spray drying method and sintering at 600 °C for 1 h. The feedstock powders of NiAl were obtained from Sulzer Metco. The SEM morphologies of NiAl and TiO2/Bi2O3 feedstocks were shown in Fig. 1. The NiAl and TiO2/Bi2O3 feedstocks appear spherelike with a size distribution range of 20–60 μm and 5–48 μm, respectively. The TiO2/Bi2O3 feedstock displays a porous microstructure with particle sizes in range of 60–130 nm (Fig. 1c).The size distributions of TiO2/Bi2O3 feedstocks (Fig. 2) were acquired using a Malvern 3000 laser diffraction analyzer (Malvern, UK). The average size (D50) of TiO2/Bi2O3 feedstock is 15 μm which is appropriate for the plasma spraying. The NiAl and TiO2/Bi2O3 feedstocks with different ratios were mechanically blended by a M10 three-dimensional mixing instrument (Grinder, CHINA) and served as prepared feedstocks. Inconel 718 alloy disks (Φ38 mm × 8 mm), used as substrates, were sands blasted and ultrasonically cleaned in acetone before spraying. The spraying coatings were deposited by a 9MC atmospheric plasma spraying system (SulZer Metco, US). NiAl powder was applied as bond coat prior to spraying composite coatings. The compositions of composite coatings and the spraying parameters were listed in Tables 1 and 2, respectively.
NATB1 NATB2 NATB3
Compositions (wt%) NiAl
TiO2/Bi2O3 (3/ 2)
80 70 60
20 30 40
Vickers hardness (HV)
Bonding strength (MPa)
174.6 ± 5.0 194.4 ± 7.6 234.5 ± 6.8
49.56 ± 3.42 43.40 ± 2.89 40.43 ± 3.66
Table 2 Plasma spraying parameters. Parameters
Bond coat
Coatings
Current (A) Voltage (V) Ar gas flow rate, L·min−1 H2 gas flow rate, L·min−1 Power feed rate, g·min−1 Spray distance, mm
500 55 40 5 42 100
550 60 40 5 35 85
2.2. Characterization The hardness and bonding strength of the composite coatings were conducted according to our previous research methods [30], and average values are given in Table 1. A UMT-3 pin-on-disk high temperature tribological tester (Bruker Corp, USA) was used to perform the friction tests at selected temperatures of RT, 200 °C, 400 °C, 600 °C and 800 °C. The commercial Al2O3 ceramic ball (10 mm in diameter, HV 16.5 GPa, SG 3.92 g·cm−3) was used as the counterpart. The tests were
Fig. 1. Morphologies of feedstock powders: (a) NiAl, (b) nanostructured TiO2/Bi2O3, (c) high magnification of (b). 158
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
Fig. 3. XRD pattern of agglomerated TiO2/Bi2O3 powers (a) as-received and (b) calcined.
Fig. 4. XRD patterns of as-sprayed NiAl-TiO2/Bi2O3 composite coatings with different TiO2/Bi2O3 content (inset is partial enlarged drawing).
conducted with 10 N load, 0.1 m·s−1 sliding speed and 60 min test time. The sectional profile of the wear scar was measured by a MicroXAM800 three-dimensional profilometer (KLA-tencor, USA). The wear rate of the specimen was calculated by common approach, i.e., W = V/DL [32]. The phase compositions and structural evolutions of the powders and corresponding coatings were characterized by X-ray diffraction (PANalytical B.V., The Netherlands) for Cu-Kα radiation. The crosssection and the worn surface morphologies of the coatings were determined by a JEM-5600LV scanning electron microscope (Tescan Mira 3, Czech). Simultaneously, the worn surface morphologies of the counterpart were observed by an optical microscope (Olympus, Japan). And the corresponding phase identifications on the worn surface were analyzed using the HR800 Raman spectrometer (Horiba Jobin Yvon, France) with laser radiation wavelength of 532 nm.
the technology characteristics of plasma spraying. The interfaces between NiAl and oxide phases were closely integration, which shows better interlamellar cohesion. According to the EDS analysis, the gray phase is NiAl, the deep gray phase is TiO2 in which the dispersed white phase is the Bi2O3. Furthermore, the composite coating registered the typical bimodal microstructure, consists of partially melted regions (PM) and fully melted regions (FM) (inset of Fig. 5b). During the plasma spraying, a part of nanoparticles cannot be melted completely due to a short residence time in high speed flames. Simultaneously, a large number of grain boundaries in agglomerated feedstocks retard the growth of grain in the deposition process. So the partially melted regions preserve unique nanostructures (Fig. 5d). The unique bimodal microstructure formed by nanostructure feedstock powders is beneficial to effectively improve mechanical and tribological properties of the coatings, including the adhesive strength and toughness of the coating and wear-resistance property, etc. [29,31,33,35].
3. Results and discussion 3.2. Mechanical properties and tribological properties 3.1. Characterization of feedstock powers and as-sprayed coating The Vickers hardness and bonding strength of NiAl-TiO2-Bi2O3 composite coatings are presented in Table 1. The hardness of composite coatings increases from 174.6 HV to 234.5 HV with the TiO2/Bi2O3 contents increasing. In the bonding test, the failure occurred in the interior of the composite coatings. The bonding strength of composite coatings decreased with the addition of TiO2/Bi2O3. Nevertheless, the bonding strength values still keep at higher level (> 40 MP) comparing to our previous works [26–28], which is mainly ascribed to the reduced phase interfaces of composite coating by using of nanostructured TiO2/ Bi2O3 agglomerated powder as feedstock (Fig. 5). Fig. 6 shows the average friction coefficients and wear rate of composite coatings with different TiO2/Bi2O3 contents sliding against Al2O3 ball from room temperature to 800 °C. The friction coefficients of all coatings exhibit the same trend, and that is increased first from RT to 200 °C and then decreased sharply with the further increasing of temperatures. It can be seen that the composite coatings exhibited higher COFs in the low temperatures, but coatings display excellent lubrication performance at elevated temperature. Especially for the composite coating with 30 wt% TiO2/Bi2O3, the friction coefficient is registered to 0.07 at 800 °C. Base on the theoretical basis of Erdemir, mixed oxides with higher difference in ionic potential are inclined to form a lowmelting-point or readily shearable or highly stable compound, which could exhibit much lower hardness and better lubrication effect at elevated temperatures [17]. It may be a good explanation for the continuous decreasing of friction coefficient at elevated temperatures.
Fig. 3 gives the XRD patterns of the TiO2/Bi2O3 feedstock as-received and after calcined at 600 °C for 1 h. The phases of agglomerated powders are mainly composed of anatase-TiO2 (JCPDS file no. 21-1272) and α-Bi2O3 (JCPDS file no.41-1449). After heat treatment, the intensities of diffraction peaks become stronger, demonstrating that the amorphization and internal stress in oxides particles caused by ballmilling reduced. A new phase of Bi4Ti3O12 (JCPDS file no. 47-0398) was detected after heat treatment, which may be attributed to the solid state reaction between nano-TiO2 and nano-Bi2O3 at high temperature. Fig. 4 displays the XRD patterns of as-sprayed NiAl-TiO2/Bi2O3 composite coatings with different nanostructured oxides contents. The composite coating mainly consists of NiAl compound (JCPDS file no. 20-0019), Ni-based solid solution (JCPDS file no. 04-0850), r-TiO2 (JCPDS file no. 21-1276), δ-Bi2O3 (JCPDS file no. 45-1344) and Bi3Ti4O12 (JCPDS file no. 47-0398). The phases of a-TiO2 and α-Bi2O3 in feedstock transferred to more stable phases of r-TiO2 and δ-Bi2O3 at high temperature during plasma spraying process. Nevertheless, the diffraction peaks of r-TiO2, δ-Bi2O3 and Bi4Ti3O12 become weaker and broad due to the amorphization caused by high cooling rate during coating deposition process (inset of Fig. 4). The SEM images of cross-section of composite coatings with different content of TiO2/Bi2O3 are presented in Fig. 5. It is observed that all composite coatings possess dense microstructure and uniform distribution of the phases with the exception of pores and cracks caused by 159
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
Fig. 5. SEM images of the cross-section of the composite coatings of (a) NATB1, (b) NATB2 and (c) NATB3, (a'), (b') and (c') are the high magnification images of (a), (b) and (c).
effectively prevent brittle fracture and peeling of coating during wear test. Additionally, the existence of partially melted regions would generate fine debris with nano or sub-micron scale under sliding process, which could be filled in spalling pits and other pits to prevent the worn surface falling out. Therefore, the nanostructured coatings with unique bimodal microstructure possess excellent wear-resistance property.
Meanwhile, it can be found that the wear rates of the composite coatings increase firstly and then maintain relatively high values from room temperature to 600 °C, when above 600 °C, the wear rates of composite coatings decrease sharply. However, the wear rates of all composite coatings hold low value (under 8 × 10−5 mm3·(N·m)−1) in the tested temperature range, in which the wear rate of NATB2 is below 5 × 10−5 mm3·(N·m)−1 in the whole test temperatures. The reason for this can be attributed to the fact that the bimodal microstructure formed by nanostructured TiO2/Bi2O3 can provide excellent wear resistance [29,31,33]. It has been proved that the bimodal microstructure possessed “performance complementary” property, in which the partially melted regions improved the toughness of the coating and could
3.3. Worn surface morphologies Fig. 7 shows the worn surface morphologies of NATB2 composite coating after wear tests at different temperatures. At room temperature,
Fig. 6. Friction coefficients and wear rates of composite coatings at different temperatures. 160
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
Fig. 7. SEM morphology of worn surfaces of NATB2 coating at different test temperatures: (a) 25 °C, (b) 200 °C, (c) 400 °C, (d) 600 °C and (e) 800 °C. 161
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
Fig. 8. SEM morphology of worn surfaces of composite coatings: (a)–(b) NATB1 coating, (c)–(d) NATB3 coating.
a relatively smooth tribo-layer with some cracks and peeling is present on the worn surface. The wear mechanism is characterized by microcracking and spalling (Fig. 7a). Owing to the addition of nanometersized oxides, wear debris particles will be compacted and formed tribolayer, which are beneficial to the decrease in friction and wear. However, bismish oxide has more advantage than other oxides in this aspect [20]. The crack occurred on worn surface may be attributed to the fatigue wear of the coatings or the residual stresses in the thermal spraying coatings [34]. At 200 °C, the worn surface become coarse and is covered with a lot of wear debris and several plastic smearing compared with that at RT (Fig. 7b), which is corresponding to the higher friction coefficient. When the temperature increases to 400 °C, the worn surface is characterized by ploughed grooves and some wear debris (Fig. 7c), which illustrates that the wear mechanisms is dominated by micro-ploughing and abrasive wear. At 600 °C, the worn surface becomes smooth and is covered by incomplete glaze layer with fine and slight grooves (Fig. 7d), suggesting that the wear mechanism is characterized by plastic deformation and micro-scuffing. It is reported that, in the case of binary/mixed oxides, the frictional behavior of complex system similar to that of its low melting point constituent [19]. Therefore, Bi2O3 with lower melting point (825 °C) could be
Fig. 9. XRD patterns of worn surfaces of NATB2 coating at different test temperatures. 162
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
extent. However, the trace of ploughing may be attributed to the less addition of TiO2/Bi2O3 whose could possess a great effect to form a readily lubricous and easily plastic interface. With the TiO2/Bi2O3 contents increasing, the worn surface of NATB3 is covered with more smooth and complete glaze layer (Fig. 8d), which are responsible for the lowered friction coefficient and wear rate at high temperature. 3.4. Phase composition of the worn surface Fig. 9 shows the phase compositions of the worn surface of NATB2 composite coatings at different temperatures. It can be clearly seen the diffraction peaks of composite coating after tests below 400 °C are similar to that of as-sprayed coating (Fig. 4). At 600 °C, the diffraction peaks of the NiO (JCPDS file no.47-1049) appeared. At 800 °C, the NiO peaks become more evident and the new phase NiTiO3 (JCPDS file no.33-0960) is detected. Meanwhile, the diffraction peaks of Bi4Ti3O12 were significantly enhancing, which are attributed to the further reaction between TiO2 and Bi2O3 at high temperature. The synergistic lubricating effect of numerous oxides including TiO2, NiO, NiTiO3 and Bi4Ti3O12 formed a continuous lubricating layer on worn surface at 800 °C is attributed to reduce the friction coefficient and wear rates of the composite coating at high temperatures. Raman spectra of the worn surfaces of NATB2 coating at different test temperatures are presented in Fig. 10. At RT, the Raman spectrum of outside wear track displayed broad peaks centering at 445 cm−1 and 610 cm−1, which is assigned to TiO2. The Bi2O3 peak can be also detected at outside of wear track. In contrast, the weaker and broaden peaks are observed in the worn surface, which is assigned to Magnelitype titanium oxides due to that the oxygen vacancies created under repeated sliding process [36]. However, the Raman spectra for inside and outside the wear tracks have no obvious change at 200 °C and 400 °C. At 600 °C, the peaks of TiO2 and Bi2O3 are detected within the wear track. As typical high temperature lubricant, the existences of TiO2 and Bi2O3 within the wear track obviously improve the lubrication behavior of composite coating at 600 °C (Fig. 6a). At 800 °C, besides the TiO2 peaks, it can be observed that a series of new peaks appeared at 717 cm−1, 400 cm−1, 356 cm−1 and 302 cm−1, which are corresponding to rhombohedral structure of NiTiO3 [37]. Meanwhile, the Bi4Ti3O12 peaks are obviously detected, which is consistent to XRD analysis (Fig. 9). However, there was unnoticeable change for the Raman spectrum of outside wear track, suggesting that the tribo-chemical reaction only occurred on the worn surface and formed the Bi4Ti3O12 and NiTiO3 at high temperature. Bi4Ti3O12 with typical layered structure is readily shearable and possess lower melting point (875 °C) [38]. Meanwhile, NiTiO3 has been proved to be a stable oxide at elevated temperatures and could exhibit lubricity on the friction interface [39]. Fig. 11 presented the Raman spectra of within the worn surfaces of NATB1, NATB2 and NATB3 coatings at 800 °C. For NATB1 coating, the Raman spectra of wear track displayed strong peaks of TiO2 and weak peaks of Bi4Ti3O12 and NiTiO3. However, the intensity of Bi4Ti3O12 and NiTiO3 peaks obviously enhanced at the Raman spectra of NATB2 coating. The lowed friction coefficient of NATB2 coating at 800 °C indicated that these two kinds of ternary oxides possessed better high temperature lubrication performance. With the increase of oxides content, the Raman spectra of the wear track for NATB3 coating displayed strong peaks of TiO2 again, and the peaks of Bi4Ti3O12 deservedly displayed stronger intensity than that of NATB1. The friction coefficient of the NATB3 coating is higher than that of NATB2 but lower than that of NATB1. Obviously, the large mount existence of Bi4Ti3O12 and NiTiO3 on the worn surface would exhibit excellent lubrication property. The formation of these ternary oxides on the worn surface and their excellent lubrication property are well consistent to the theoretical research of Erdemir and contributed to explain the variation of friction coefficient of the composite coatings at 800 °C. The worn surface morphologies of the counterpart ball sliding against NATB2 at different temperatures are presented in Fig. 12. At RT,
Fig. 10. Raman spectra of the worn surfaces of NATB2 coating at different test temperatures.
Fig. 11. Raman spectra of the worn surfaces of NATB1, NATB2 and NATB3 coatings at 800 °C.
contributed to explain the tribological property of composite coating. As high temperature lubricant, the present of Bi2O3 on the worn surface is benefit to the lowered friction. However, the lower hardness and softening of Bi2O3 at 600 °C may result in higher wear rate comparing to those in lower temperatures. With further increase in the test temperature, a continuous and complete glaze layer is observed in the wear track (Fig. 7e). The mild wear mechanism and the complete lubricious interface resulted in obvious improvement of tribological properties of the composite coating at 800 °C. The worn surfaces morphologies of NATB1 and NATB3 composite coatings at room temperature and 800 °C are shown in Fig. 8. At room temperature, comparing to NATB1, the worn surface of NATB3 becomes coarse and is covered with a large amount of fine wear debris (Fig. 8c), resulting in the higher friction coefficient and wear rate. However, the cracks occurred on the worn surface are obviously improved compared with the former, since the existence of bimodal microstructure could effectively improve the toughness and prevent crack propagation [35]. At 800 °C, the worn surface of NATB1 is covered by thin glaze layer with slight grooves (Fig. 8b). The formation of glaze layer improved the tribological behavior of composite coating to some 163
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
Fig. 12. Worn surface morphologies (a–e) and Raman spectra (f) of the counterpart ball against NATB2 coating at different temperatures.
become smoother with the increasing of test temperature, which is related to continuous reduction of friction coefficient at this temperature range. At 800 °C, it can be seen that the worn surface of ball is covered by a continuous lubricating film which can effectively reduce the direct contact between ball and disk (Fig. 12e). It can be concluded
the worn surface of ball presents milder wear with light scratches than other test temperature surfaces, which indicates that the composite coating possesses the best wear property at RT (Fig. 12a). From 200 to 600 °C, the worn surface of all balls exhibit severe wear leading to higher wear rates (Fig. 12b–d). However, the worn surface of ball 164
Surface & Coatings Technology 349 (2018) 157–165
X. Wang et al.
that the tribolayer on worn surface mainly consists of Al2TiO5 and NiTiO3 at 800 °C by Micro-Raman analysis (Fig. 12f). It easily understands that the NiTiO3 is transferred from coating disk, while the formation of Al2TiO5 might be attributed to the interface reaction between TiO2 and Al2O3 occurred on the rubbing surfaces. With the formation of frictional layer on worn surface of ball, interface shear take place easily and the tribological behavior of composite coating are dramatically improved at high temperature.
[9] A.P. Semenov, High-temperature solid lubricating substances, J. Frict. Wear 28 (2007) 476–484. [10] M.B. Peterson, S.Z. Li, S.F. Murray, Wear-resisting oxide films for 900°C, J. Mater. Sci. Technol. 13 (1997) 99–106. [11] M.B. Peterson, S.J. Calabrese, S.Z. Li, X. Jiang, Friction of alloys at high temperature, J. Mater. Sci. Technol. 10 (1994) 313–320. [12] S.F. Murray, S.J. Calabrese, Effect of solid lubricants on low speed sliding behavior of silicon nitride at temperatures to 800 °C, Lubr. Eng. 49 (1993) 955–964. [13] D.J. Taylor, P.F. Fleig, S.T. Schwab, R.A. Page, Sol-gel derived, nanostructured oxide lubricant coatings, Surf. Coat. Technol. 120–121 (1999) 465–469. [14] E. Liu, W. Wang, Y. Gao, J. Jia, Tribological properties of Ni-based self-lubricating composites with addition of silver and molybdenum disulfide, Tribol. Int. 57 (2013) 235–241. [15] W. Gulbinski, T. Suszko, Thin films of MoO3-Ag2O binary oxides-the high temperature lubricants, Wear 261 (2006) 867–873. [16] A. Erdemir, A crystal-chemical approach to lubrication by solid oxides, Tribol. Lett. 8 (2000) 97–102. [17] A. Erdemir, A crystal chemical approach to the formulation of self-lubricating nanocomposite coatings, Surf. Coat. Technol. 200 (2005) 1792–1796. [18] V. Dimitrov, T. Komatsu, Classification of simple oxides: a polarizability approach, J. Solid State Chem. 163 (2002) 100–112. [19] B. Prakash, J.P. Celis, The lubricity of oxides revised based on a polarisability approach, Tribol. Lett. 27 (2007) 105–112. [20] H. Kato, K. Komai, Tribofilm formation and mild wear by tribo-sintering of nanometer-sized oxide particles on rubbing steel surfaces, Wear 262 (2007) 36–41. [21] J. Wang, Y. Shan, H. Guo, W. Wang, G. Yi, J. Jia, The tribological properties of NiCrAl2O3-TiO2 composites at elevated temperatures, Tribol. Lett. 58 (2015) 1–10. [22] R.S. Lima, B.R. Marple, Superior performance of high-velocity oxyfuel-sprayed nanostructured TiO2 in comparison to air plasma-sprayed conventional Al2O313TiO2, J. Therm. Spray Technol. 14 (2005) 397–404. [23] M.N. Gardos, The effect of anion vacancies of the tribological properties of rutile (TiO2−x), Tribol. Trans. 33 (1989) 209–220. [24] M.N. Gardos, H.-s. Hong, W.O. Winer, The effect of anion vacancies on the tribological properties of rutile (TiO2−x), Part II: experimental evidence, Tribol. Trans. 33 (2012) 209–220. [25] E. Liu, Y. Gao, W. Wang, X. Zhang, X. Wang, G. Yi, Effect of Ag2Mo2O7 incorporation on the tribological characteristics of adaptive Ni-based composite at elevated temperatures, Tribol. Trans. 56 (2013) 469–479. [26] B. Li, Y. Gao, M. Han, H. Guo, J. Jia, W. Wang, Tribological properties of NiAl matrix composite coatings synthesized by plasma spraying method, J. Mater. Res. 32 (2017) 1–8. [27] B. Li, J. Jia, Y. Gao, M. Han, W. Wang, Microstructural and tribological characterization of NiAl matrix self-lubricating composite coatings by atmospheric plasma spraying, Tribol. Int. 109 (2017) 563–570. [28] B. Li, J. Jia, Y. Gao, H. Guo, M. Han, W. Wang, Influence of silver contents on the tribological properties of Ni-based self-lubricating coatings by atmospheric plasma spraying, Acta Metall. Sin. (2017) 801–808. [29] E.P. Song, J. Ahn, S. Lee, N.J. Kim, Microstructure and wear resistance of nanostructured Al2O3-8wt%TiO2 coatings plasma-sprayed with nanopowders, Surf. Coat. Technol. 201 (2006) 1309–1315. [30] B. Li, J. Jia, M. Han, Y. Gao, W. Wang, C. Li, Microstructure, mechanical and tribological properties of plasma-sprayed NiCrAlY-Mo-Ag coatings from conventional and nanostructured powders, Surf. Coat. Technol. 324 (2017) 552–559. [31] D. Goberman, Y.H. Sohn, L. Shaw, E. Jordan, M. Gell, Microstructure development of Al2O3-13wt%TiO2 plasma sprayed coatings derived from nanocrystalline powders, Acta Mater. 50 (2002) 1141–1152. [32] X. Zhao, Y. An, G. Hou, H. Zhou, J. Chen, Preparation and tribological properties of plasma sprayed nano and micro-structure alumina-reinforced CuAl composite coatings, Tribol. Int. 101 (2016) 255–263. [33] V.P. Singh, A. Sil, R. Jayaganthan, A study on sliding and erosive wear behaviour of atmospheric plasma sprayed conventional and nanostructured alumina coatings, Mater. Des. 32 (2) (2011) 584–591. [34] S. Ahmaniemi, M. Vippola, P. Vuoristo, T. Mäntylä, M. Buchmannb, R. Gadowb, Residual stresses in aluminium phosphate sealed plasma sprayed oxide coatings and their effect on abrasive wear, Wear 252 (7–8) (2002) 614–623. [35] X. Lin, Y. Zeng, S.W. Lee, C. Ding, Characterization of alumina-3wt% titania coating prepared by plasma spraying of nanostructured powders, J. Eur. Ceram. Soc. 24 (2004) 627–634. [36] A. Skopp, N. Kelling, M. Woydt, L.M. Berger, Thermally sprayed titanium suboxide coatings for piston ring/cylinder liners under mixed lubrication and dry-running conditions, Wear 262 (2007) 1061–1070. [37] T.T. Pham, S.G. Kang, E.W. Shin, Optical and structural properties of Mo-doped NiTiO3 materials synthesized via modified Pechini methods, Appl. Surf. Sci. 411 (2017) 18–26. [38] R.C. Oliveira, L.S. Cavalcante, J.C. Sczancoski, E.C. Aguiarb, J.W.M. Espinosa, J.A. Varela, P.S. Pizani, E. Longo, Synthesis and photoluminescence behavior of Bi4Ti3O12 powders obtained by the complex polymerization method, J. Alloys Compd. 478 (2009) 661–670. [39] D.J. Taylor, P.F. Fleig, R.A. Page, Characterization of nickel titanate synthesized by sol-gel processing, Thin Solid Films 408 (2002) 104–110.
4. Conclusions The NiAl composite coatings with different contains of nanostructured TiO2/Bi2O3 are fabricated by atmospheric plasma spraying. The microstructures, mechanical and tribological properties of composite coatings were investigated. The following conclusions can be drawn: 1. The hardness of composite coatings increases with the TiO2/Bi2O3 increasing. The adhesive strength of composite coatings decreased with the addition of TiO2/Bi2O3, while that still keep at higher level (> 40 MP). 2. The nanostructured TiO2/Bi2O3 phases perform typical bimodal microstructure consisted of partially melted regions (PM) and fully melted regions (FM), which results in a significant enhancement of mechanical and tribological properties of the composite coatings. 3. The composite coatings with nanostructured TiO2/Bi2O3 exhibit excellent tribologicial behavior. Especially for the composite coating with 30 wt% TiO2/Bi2O3, the friction coefficient is registered to 0.07 at 800 °C and the wear rate is below 5 × 10−5 mm3·(N·m)−1 in the whole test temperatures. 4. The lowered friction and wear at 800 °C is attributed to the synergistic lubricating effect of Bi4Ti3O12 and NiTiO3 formed on worn surfaces by tribo-chemical reactions together with TiO2 and NiO. A continuous lubricating layer including NiTiO3 and Al2TiO5 was formed on counterfaces, which further improved the tribological behavior of composite coating at high temperatures. Acknowledgements The authors acknowledge the financial supports by the National Natural Science Foundation of China (Grant No. 51471181, 51575505, 51675508). References [1] A. Pauschitz, M. Roy, F. Franek, Mechanisms of sliding wear of metals and alloys at elevated temperatures, Tribol. Int. 41 (2008) 584–602. [2] P. He, G. Ma, H. Wang, et al., Tribological behaviors of internal plasma sprayed TiO2-based ceramic coating on engine cylinder under lubricated conditions, Tribol. Int. 102 (2016) 407–418. [3] J. Chen, Y. An, J. Yang, et al., Tribological properties of adaptive NiCrAlY-Ag-Mo coatings prepared by atmospheric plasma spraying, Surf. Coat. Technol. 235 (2013) 521–528. [4] R. Tyagi, D.S. Xiong, J.-l. Li, J. Dai, High-temperature friction and wear of Ag/h-BNcontaining Ni-based composites against steel, Tribol. Lett. 40 (2010) 181–186. [5] S.M. Aouadi, H. Gao, A. Martini, T.W. Scharf, C. Muratore, Lubricious oxide coatings for extreme temperature applications: a review, Surf. Coat. Technol. 257 (2014) 266–277. [6] C. Muratore, A.A. Voevodin, Chameleon coatings: adaptive surfaces to reduce friction and wear in extreme environments, Annu. Rev. Mater. Res. 39 (2009) 297–324. [7] D.-s. Xiong, Lubrication behavior of Ni–Cr-based alloys containing MoS2 at high temperature, Wear 251 (2001) 1094–1099. [8] C. Muratore, A.A. Voevodin, J.J. Hu, J.G. Jones, J.S. Zabinski, Growth and characterization of nanocomposite yttria-stabilized zirconia with Ag and Mo, Surf. Coat. Technol. 200 (2005) 1549–1554.
165