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Tribological properties of laser cladding NiAl intermetallic compound coatings at elevated temperatures
Tribology International 104 (2016) 321–327
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Tribological properties of laser cladding NiAl intermetallic compound coatings at elevated temperatures
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Youjun Yua, Jiansong Zhoua, , Shufang Rena, Lingqian Wanga, Benbin Xina,b, Silong Caoa,b a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People's Republic of China b Graduate School of Chinese Academy of Sciences, Beijing 100039, People's Republic of China
A R T I C L E I N F O
A BS T RAC T
Keywords: NiAl intermetallics coating Laser cladding Tribological properties Wear mechanism
Tribological properties, phase compositions and wear mechanisms of laser cladding NiAl coating were investigated from room temperature to 1000 °C under air atmosphere environment. The results showed that a glaze layer composed of NiO, Ni2O3, Al2O3 and NiAl2O4 phases as well as Ni3Al phase were formed on worn surface under high temperature, which act as a solid lubricant and anti-wear material improving the tribological properties of NiAl coating at elevated temperatures. The wear mechanism is dominated by abrasive wear below 500 °C, and transforms to plastic deformation and adhesive wear at 500 °C up to 700 °C and oxidation wear at 900 °C and above, owing to formation of new oxide phase during tribochemical reaction.
1. Introduction Thanks to the desired physical and mechanical properties such as high melting point, high thermal conductivity, low density, and excellent oxidation resistance at an elevated temperature, NiAl intermetallic compound is regarded as an attractive high temperature structural material and corrosion resistant material in relative sliding components including pistons and valves, turbochargers, gas turbine engine rotor blades and stator vanes [1,2]. Particularly, NiAl intermetallic compound may find potential application in turbine engines whose blade tips suffer from sliding type wear upon contact with surrounding gas path seals [3,4]. In the meantime, as an attractive high temperature surface protective coating, NiAl intermetallic coating can form thin and adherent α-Al2O3 scales on the surface under elevated temperature (above 1100 °C), thereby acquiring prolonged life-time in association with prevention of hot corrosion [5]. To date, numerous studies have been conducted to examine the cyclic oxidation resistance of NiAl coating [6,7], but few are currently available about the friction and wear behavior of NiAl intermetallic compounds over a wide range of temperatures [3,4]. Several methods can be adopted to fabricate NiAl intermetallic compounds, including self-propagating high-temperature synthesis [8,9], combustion synthesis and hot pressing technique [10], spraying technique [11], and laser cladding [12]. Among these methods, laser cladding is of special significance, because it can efficiently provide intermetallics and intermetallic composite coatings possessing compact microstructure as well as excellent metallurgical bonding with the ⁎
substrate [12–15]. It is widely accepted that the wear resistance and wear mechanism of materials are highly dependent on their surface microstructure, because wear fatigue damage and failure of sliding friction couple usually start from the material surface [8,16–18]. As to NiAl intermetallic compound coating, it undergoes a ductile-to-brittle transition in the temperature range of 300~600 °C [19,20], and it can form stable alumina oxide layer at further elevated temperatures [5,21,22]. This means it is imperative to reveal the effect of temperature on the mechanical strength as well as friction and wear behavior of NiAl intermetallic compounds in terms of its application under dry sliding at elevated temperatures in air atmosphere. In the present research, therefore, we make use of laser cladding technique to fabricate NiAl intermetallic compound coating on 1Cr18Ni9Ti stainless steel substrate. Furthermore, we investigate the friction and wear behavior of the as-fabricated NiAl intermetallic compound coating from room temperature to elevated temperatures up to 1000 °C. This article reports the preparation of the laser cladding NiAl intermetallic compound coating and the evaluation of its friction and wear behavior at elevated temperatures, with the hope to provide some technical reference to its application in high temperature tribology. 2. Experimental details 2.1. Laser cladding of NiAl coating
Corresponding author. E-mail address: [email protected] (J. Zhou).
http://dx.doi.org/10.1016/j.triboint.2016.09.014 Received 18 May 2016; Received in revised form 6 September 2016; Accepted 8 September 2016 Available online 09 September 2016 0301-679X/ © 2016 Elsevier Ltd. All rights reserved.
The 1Cr18Ni9Ti stainless steel disks with a size of Ф
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Fig. 1. Typical curves of friction coefficient of NiAl coating with sliding time at RT.
Fig. 2. Variation of friction coefficient of NiAl coating with temperature.
45 mm×16 mm were used as the substrates. Commercially available powders of Al (30–150 µm, purity 99.5%) and Ni (~75 µm, purity 99.0%) with a Ni/Al atomic ratio of 1:1 were used as the starting materials. Prior to laser cladding, the as-received Ni/Al powders were mechanically milled using a planetary ball mill to achieve homogeneous mixing. The mixed powders were placed onto the steel substrate to form a loose layer of approximately 1.5 mm thick. The resultant loose layer of the mixed powders was then placed in a 10 kW transverse-flow continuous-wave CO 2 laser processing system equipped with a 4-axis computer numerical controlling unit. The laser cladding process was conducted in an argon shielding atmosphere at a laser beam size of 10 mm×2 mm, a power of 2.0–4.0 kW, and a scanning speed of 200–400 mm/min to afford target NiAl intermetallic coating.
track of NiAl coating. The morphology and element composition of the worn coating surfaces were analyzed with the 3D non-contact surface mapping profiler and a scanning electron microscope (SEM, JSM5600LV, JEOL, Japan) equipped with an energy dispersive spectrometer (EDS, Kevex, USA). The chemical state of the major elements on the worn surface of the coating was determined by X-ray photoelectron spectroscopy (XPS, PHI-5702, USA; Al-Kα excitation source; reference: contaminated carbon C 1s=284.80 eV). A Renishaw's inVia Micro-Raman spectrometer working with laser light at 633 nm was performed to analyze the chemical feature of the worn coating surfaces.
3. Results and discussions 3.1. Tribological properties
2.2. Friction and wear tests
The friction coefficient of NiAl coating under dry sliding at different temperatures is presented in Fig. 2. The friction coefficient at RT and 300 °C is 0.67 and 0.68, respectively and it slightly decreases to 0.63 at 500 °C. As the temperature rises from 500 °C to 700 °C, the friction coefficient tends to rise slightly therewith. Above 700 °C, the friction coefficient continuously declines with elevating temperature up to 1000 °C. This indicates that temperature has an important influence on the friction behavior of laser cladding NiAl intermetallics coating, and its friction coefficient is reduced to some extent at elevated temperatures. The variation in the wear rate of laser cladding NiAl intermetallics coating with temperature is shown in Fig. 3. It can be seen that the wear rate of NiAl coating increases from 9.5×10−5 mm3/N m at room temperature to 11×10−5 mm3/N m at 300 °C. Above 300 °C, the wear rate decreases significantly with further increase of temperature and reaches the lowest value of about 3×10−5 mm3/N m at 500 °C. When the temperature is further raised from 500 °C to 1000 °C, the wear rate gradually increases from 3×10−5 mm3/N m to 9×10−5 mm3/N m. This demonstrates that temperature also has a prominent influence on the wear resistance of laser cladding NiAl intermetallics coating. Moreover, as the temperature is below 500 °C, the friction coefficient and wear rate of laser cladding NiAl coating tend to change with increasing temperature in a similar manner. Above 500 °C, however, they tend to change with elevating temperature in opposite directions. This means that the friction-reducing ability and wear resistance of the laser cladding NiAl intermetallic compound coating do not necessarily vary with elevating temperature monotonously. In other words, it could be significant to reach a compromise between the friction-reducing ability and wear resistance of the laser cladding NiAl intermetallic coating in terms of its application in high temperature tribology.
A CSEM high temperature tribometer was used to evaluate the friction and wear behavior of the as-prepared NiAl coatings in a ballon-disk contact configuration under dry sliding at room temperature (RT) and elevated temperatures of 300 °C, 500 °C, 700 °C, 900 °C and 1000 °C. Si3N4 ceramic balls with a diameter of 6 mm were used as the counterpart balls. All the sliding tests ran at a normal load of 5 N, a sliding speed of 0.21 m/s, and a duration of 60 min. Prior to sliding tests, the specimens were mechanically polished by 1000 grit emery paper to reach an initial surface roughness of about 0.8 µm (Ra), followed by ultrasonically cleaning in an acetone bath. The friction coefficient was continuously recorded by the tribometer and the typical curves of friction coefficient with sliding time of NiAl coating at RT are presented in Fig. 1. The wear volume losses were measured with a MicroXAM three dimensional (3D) non-contact surface mapping profiler (ADE Corporation, Massachusetts, USA). The volume wear rate of the specimens is calculated as ω=/FS, where V is the wear volume loss in mm3, F is the normal load in Newton, and S is the total sliding distance in meter. Three repeat friction and wear tests were performed at each test temperature. The averaged friction coefficients and wear rates of the three repeat measurements are presented in this article (the standard deviations of the friction coefficient and wear rate are ± 2% and ± 5%). 2.3. Characterization The phase components of the worn surfaces of laser cladding NiAl coating under different sliding conditions were identified by X-ray diffraction (XRD, X'Pert-MRD, Philips). The area exposed to X-ray beam in this article is about 1.0 mm2 and it was confined to the sliding 322
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SEM analysis of its worn surfaces. Fig. 5 represent the SEM images of the worn surfaces of laser cladding NiAl intermetallic coating after sliding tests at different temperatures. It can be seen that the worn surface image evolution of NiAl coating is consistent with that observed in Fig. 4. Namely, a large amount of flakes and delamination pits is present on the worn surface at room temperature (see Fig. 5a), which suggests that the NiAl intermetallic coating is dominated by delamination and abrasive wear as it slides against the ceramic ball at room temperature. With increasing temperature up to 300 °C, some fine grooves and flaky wear debris appear on the wear track (see Fig. 5b), which indicates that the dominant wear mechanisms in this case are microploughing and abrasive wear. When the temperature rises to 300 °C and above, some plastic yielding parallel to the frictional direction emerges after sliding at 500 °C and 700 °C (see Figs. 5c and d), while a glaze layer emerges on the worn coating surface after sliding at 900 °C and 1000 °C (see Figs. 5e and f). This could be related to the brittle-to-ductile transition in association with softening of NiAl coating in the temperature range of 300–600 °C [20,23,24] and to the formation of tribo-oxidation layer at elevated temperatures up to 900 ° C and 1000 °C [8,25]. Corresponding wear mechanisms are plastic deformation and adhesive wear (500 °C and 700 °C) as well as tribooxidation wear (900 °C and above). Metal materials often possess high ductility, which is advantageous in forming the glaze layers at elevated temperatures. The smooth and complete glaze layers is formed under plastic deformation and tribo-oxidation reaction, thereby reducing the shear force in the contact area between the mating surfaces at elevated temperatures and improving the friction-reducing ability and wear resistance of laser cladding NiAl coating. However, at 900 °C and above, the oxidation of the sliding surfaces accelerates and leads to partial delamination of the oxidation layers. As a result, although the friction coefficient of the laser cladding NiAl coating decreases significantly with elevating temperature up to 700 °C and above, the wear rate increases gradually therewith (see Figs. 2 and 3). The 3D profiling images of the worn surfaces of Si3N4 ball after sliding against laser cladding NiAl coating at different temperatures are shown in Fig. 6. It can be seen that the worn surface of Si3N4 ball at room temperature shows signs of brittle delamination and contains wear debris, which indicates that the counterpart Si3N4 ball undergoes severe abrasive wear as it slides against the laser cladding NiAl coating thereat (see Fig. 6a). When the temperature increases to 500 °C, a discontinuous glaze layer is formed on the worn surface of Si3N4 ball to
Fig. 3. Variation of wear rate of NiAl coating with temperature.
3.2. Worn surface morphology characteristics Fig. 4 shows the 3D profiling images of the worn surfaces of NiAl coating after sliding at different temperatures. It can be seen that the worn surfaces of NiAl coating tested below 300 °C show evidence of severe scratches and scrapes in association with partially removed coating as wear debris (see Figs. 4a and b). This might be attributed to the shearing by the hard asperities of Si3N4 ball. Namely, the hard asperities of the ceramic counterpart could plow into NiAl coating and results in stripping of the coating. In the meantime, as the temperature is below 300 °C, the local flash temperature at the contact interface of NiAl coating and Si3N4 ball might be not high enough to soften the asperities. As a result, the laser cladding NiAl coating exhibits relatively high friction coefficient and wear rate below 300 °C (see Figs. 2 and 3) and it is dominated by abrasive wear thereat. When the temperature is above 300 °C, the worn surfaces of NiAl coating show sings of continuous slight scratches (see Fig. 4c), plastic deformation (see Figs. 4d and e), and frictional polishing (see Fig. 4f), depending on elevating temperature. This indicates that the wear mechanism transforms from abrasive wear to adhesion wear and tribo-oxidation wear with increasing temperature up to 1000 °C. In order to better reveal the wear mechanisms of the laser cladding NiAl intermetallic coating at different temperatures, we conducted
Fig. 4. 3D profiling images of worn surfaces of NiAl coating after sliding at different temperatures: (a)-(f) corresponds to RT, 300 °C, 500 °C, 700 °C, 900 °C and 1000 °C.
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Fig. 5. SEM images of worn surfaces of laser cladding NiAl coating at different temperatures: (a)-(f) corresponds to RT, 300 °C, 500 °C, 700 °C, 900 °C and 1000 °C.
minimize the abrasive wear, and the wear scar becomes smooth and flat (see Fig. 6b). With increasing temperature up to 700 °C, a smooth film in association with some fine and flake wear debris is present on the wear track of Si3N4 ball (see Fig. 6c), which indicates that the Si3N4 ball undergoes tribochemical reaction and plastic deformation thereat. As the temperature reaches 1000 °C, the worn surface of Si3N4 ball is completely covered by a continuous and compact glaze layer in association with some fine and shallow grooves (Fig. 5d), which
demonstrates that the dominant wear mechanism in this case is oxidation wear. Considering the above results, we can infer that the formation of the smooth films or glaze layers on the worn surfaces during high temperature tribo-oxidation process has a significant effect on the friction-reducing ability and wear resistance of laser cladding NiAl coating. This will be further discussed in the forthcoming section.
Fig. 6. 3D profiling images of worn surfaces of Si3N4 ball sliding against laser cladding NiAl coating at different temperatures: (a)-(d) corresponds to RT, 500 °C, 700 °C and 1000 °C.
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Fig. 7. XPS spectra of elements (a) Ni and (b) Al on the worn surface of laser cladding NiAl coating after sliding tests at RT and 800 °C, respectively.
3.3. Phase composition of the worn surface The XPS spectra of Ni and Al elements on the worn surface of laser cladding NiAl coating after sliding tests at RT and 800 °C are presented in Figs. 7a and b, respectively. At RT, the Ni2p3/2 peak at 851.8 eV is assigned to metal Ni, and metal Ni is partly oxidized (peak at 854.6 eV is assigned to NiO, and at 856.0 eV to Ni2O3 and NiAl2O4). At 800 °C, metal Ni is completely oxidized to be Ni2O3 and NiAl2O4. A similar phenomenon occurs to Al element. At RT, the Al2p3/2 peak at 71.5 eV is assigned to metal Al, and that at 73.2 eV is assigned to Al2O3. At 800 °C, metal Al is completely oxidized to be Al2O3 and NiAl2O4 with the binding energy at 73.2 eV and 74.4 eV, respectively. The XPS result indicates that the laser cladding NiAl coating undergoes tribochemical reactions when it slides against the ceramic ball at 800 °C. Accordingly, it can be inferred that the glaze layer on the worn surface of NiAl coating at 800 °C is mainly composed of NiO, Ni2O3, Al2O3 and NiAl2O4. Fig. 8 shows the typical SEM image of the wear track of NiAl coating at 1000 °C. The EDS analysis was performed on the areas marked with A and B. It can be seen that area A consists of micrometer crystal grains and has a laminated structure, and there exists a large amount of microcracks around the laminated structure of area A. Differing from area A, area B has a cauliflower structure of fine flaky crystals with nanometer grain size. The above results indicate that the glaze layer in area A of the wear track is preferentially peeled off under the cyclic friction stress during high temperature dry sliding. Besides, the glaze layer in area B of the wear track tends to grow after the outer glaze layer in area A is peeled off, which is also supported by the EDS results of O and Si content in areas A and B (see Table 1). Table 1 lists the EDS element composition of areas A and B (see Fig. 8) on the wear track of NiAl coating after dry sliding at 1000 °C. It can be seen that O, Al, Si and Ni elements are present in areas A and B, and there are some differences in the content of these elements. Considering that the Ni and Al elements in the laser cladding NiAl intermetallic coating may undergo tribochemical reactions with Si element in Si3N4 and O element in air during dry sliding at elevated temperature, we can infer that areas A and B mainly contain NiO, Al2O3, NiAl2O4 and SiO2 which are generated through the tribochemical reactions. Furthermore, the glaze layer on the worn surface of NiAl coating after sliding at elevated temperatures mainly consists of NiO, Al2O3 and NiAl2O4 generated through the oxidation of NiAl phase, as evidenced by relevant XPS data (see Fig. 7). The Raman spectra of areas A and B on the wear track of NiAl coating after dry sliding at 1000 °C are shown in Fig. 9. It can be seen that NiAl2O4, Al2O3, NiO and SiO2 phases are present in area A of the
Fig. 8. Typical SEM image of the wear track of NiAl coating at 1000 °C. The EDS analysis was performed on the areas marked with A and B. Table 1 Element composition of the typically selected areas A and B on the wear track of laser cladding NiAl coating after sliding at 1000 °C (determined by EDS analysis). Element
O Al Si Ni
Composition (wt%) A area
B area
53.60 31.75 2.62 12.18 100.15
45.16 36.54 1.32 16.98 100
wear track of NiAl coating (see Fig. 9a), while Al2O3, NiO and SiO2 phases are present in area B (see Fig. 9b). Furthermore, a comparison of the intensities of various Raman peaks implies that the oxide layer is preferentially formed in area A rather than area B. In other words, the glaze layer on the worn surface of NiAl coating is mainly composed of NiAl2O4, NiO and Al2O3 phases, and the outer glaze layer is prone to stripping under the actions of cyclic friction stress and oxidation (see Fig. 8). On the one hand, the oxide film contributes to decreasing the friction coefficient of laser cladding NiAl coating. On the other hand, excessive oxidation leads to the delamination of the glaze layer and results in increased wear rate under high temperatures. This means that, in terms of the application of laser cladding NiAl intermetallics coating in high temperature tribology, it could be significant to reach a compromise between its friction-reducing ability and wear resistance.
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Fig. 9. Raman spectra of areas A and B on the wear track of NiAl coating after dry sliding at 1000 °C.
The XRD pattern of the worn surface of NiAl coating after dry sliding at 1000 °C is shown in Fig. 10. NiAl and Ni3Al phases as well as NiO, Al2O3 and NiAl2O4 phases are identified in the worn surface, which is consistent with corresponding XPS, EDS and Raman results. Under sliding at elevated temperatures, the laser cladding NiAl intermetallic coating can undergo chemical reaction with atmospheric O forming Al2O3 phase and γ-Ni phase. In the meantime, the released Al can react with γ-Ni through diffusion of atoms to afford γ′-Ni3Al phase. This observation well conforms to what is reported elsewhere in that γ′-Ni3Al phase can be precipitated along NiAl grain boundaries owing to the solid-state diffusion of Ni atoms at 800 °C and above [5,26,27]. Moreover, Ni3Al phase exhibits higher high-temperature yield strength and ductility than NiAl intermetallic, which is favorable for abating the wear of the mating surface (i.e., Si3N4 ball surface). This could well explain why the laser cladding NiAl intermetallic coating has a lower wear rate at high temperature than at room temperature (see Fig. 3). The evolution of various phases during the dry sliding process of NiAl coating against Si3N4 ball at elevated temperatures is shown in Fig. 11. It can be seen that the phase composition of the oxide films varies with increasing temperature, which is related to the Gibbs free energies (ΔG°) of various oxide species which contribute to improving the friction-reducing ability and wear resistance of the laser cladding NiAl coating [28]. Namely, the oxide film of NiO and Ni2O3 phases is formed at 500 °C and below, thereby decreasing the friction coefficient and wear rate of NiAl coating in the moderate-low temperature regions (see Figs. 2 and 3). When temperature rises from 500 °C to 700 °C, additional protective oxide film of Al2O3 phase is formed together with that of NiO and Ni2O3, thereby significantly increasing the wear resistance of NiAl coating in the moderate-high temperature regions (see Figs. 2 and 3). As the temperature is raised to 900 °C and above, the oxide film of NiAl2O4 phase is formed together with that of NiO, Ni2O3 and Al2O3; and the oxide films tend to gradually grow into glaze layers on the worn surface of NiAl coating, thereby improving the wear resistance of the NiAl coating at high temperatures. In one word, it is the formation of the oxide films and/or glaze layers on the wear track of the laser cladding NiAl coating that accounts for its greatly improved friction-reducing ability and wear resistance over a wide temperature range.
Fig. 10. XRD pattern of the worn surface of NiAl coating after sliding at 1000 °C.
Fig. 11. Schematic drawing showing the evolution of various phases during the dry sliding of NiAl coating against Si3N4 ball at elevated temperatures.
NiAl coating were investigated from room temperature to 1000 °C under air atmosphere environment. It was found that a glaze layer composed of NiO, Ni2O3, Al2O3 and NiAl2O4 phases as well as Ni3Al phase is formed on the worn surface of the NiAl coating after sliding at elevated temperatures. The glaze layer acts as a wear-resistant solid lubricant to reduce the friction and wear of the laser cladding NiAl intermetallics coating coupled with Si3N4 ball at elevated temperatures. Moreover, the NiAl intermetallics coating is dominated by abrasive wear when it slides against the ceramic ball below 500 °C. As the
4. Conclusions Commercially available Al (30–150 µm, purity 99.5%) and Ni powders (~75 µm, purity 99.0%) with a Ni/Al atomic ratio of 1:1 were used as the starting materials to fabricate laser cladding NiAl intermetallics coating on 1Cr18Ni9Ti stainless steel substrate. Tribological properties, phase compositions and wear mechanisms of laser cladding 326
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temperature rises from 500 °C up to 700 °C, the NiAl intermetallics coating is dominated by plastic deformation and adhesive wear. Further elevating temperature to 900 °C and above transforms the major wear mechanism of the NiAl intermetallics coating to oxidation wear in association with the formation of new oxide phases through tribochemical reactions. Furthermore, it could be significant to reach a compromise between the friction-reducing ability and wear resistance of the laser cladding NiAl intermetallic coating in terms of its application in high temperature tribology. Acknowledgments The authors are grateful to the National Natural Science Foundation of China (NSFC, No. 51475444) for financial support. References [1] N.S. Stoloff, C.T. Liu, S.C. Deevi, Emerging applications of intermetllics, Intermetallics 8 (2000) 1313–1320. [2] A. Lasalmonie, Intermetallics: why is it so difficult to introduce them in gas turbine engines??, Intermetallics 14 (2006) 123–129. [3] C. Sierra, A.J. Vázquez, Dry sliding wear behaviour of nickel aluminides coatings produced by self-propagating high-temperature synthesis, Intermetallics 14 (2006) 848–852. [4] O. Ozdemir, S. Zeytin, C. Bindal, Tribological properties of NiAl produced by pressure-assisted combustion synthesis, Wear 265 (2008) 979–985. [5] S.J. Hou, S.L. Zhu, T. Zhang, F.H. Wang, A magnetron sputtered microcrystalline β-NiAl coating for SC superalloys. Part I. Characterization and comparison of isothermal oxidation behavior at 1100 °C with a NiCrAlY coating, Appl Surf Sci 324 (2015) 1–12. [6] S. Yang, F. Wang, W. Wu, Effect of microcrystallization on the cyclic oxidation behavior of β-NiAl intermetallics at 1000 °C in air, Intermetallics 9 (2001) 741–744. [7] H. Guo, D. Li, L. Zgeng, S. Gong, H. Xu, Effect of co-doping of two reactive elements on alumina scale growth of β-NiAl at 1200 °C, Corros Sci 78 (2014) 369–377. [8] S.Y. Zhu, Q.L. Bi, M.Y. Niu, J. Yang, W.M. Liu, Tribological behavior of NiAl matrix composites with addition of oxides at high temperatures, Wear 274–275 (2012) 423–434. [9] C. Curfs, X. Turrillas, G.B.M. Vaughan, A.E. Terry, Å. Kvick, M.A. Rodríguez, Al-Ni intermetallics obtained by SHS: a time-resolved X-ray diffraction study, Intermetallics 15 (2007) 1163–1171. [10] H.L. Zhao, F. Qiu, S.B. Jin, Q.C. Jiang, High work-hardening effect of the pure NiAl intermetallic compound fabricated by the combustion synthesis and hot pressing technique, Mater Lett 65 (2011) 2604–2606.
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Tribological properties of laser cladding NiAl intermetallic compound coatings at elevated temperatures
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Youjun Yua, Jiansong Zhoua, , Shufang Rena, Lingqian Wanga, Benbin Xina,b, Silong Caoa,b a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People's Republic of China b Graduate School of Chinese Academy of Sciences, Beijing 100039, People's Republic of China
A R T I C L E I N F O
A BS T RAC T
Keywords: NiAl intermetallics coating Laser cladding Tribological properties Wear mechanism
Tribological properties, phase compositions and wear mechanisms of laser cladding NiAl coating were investigated from room temperature to 1000 °C under air atmosphere environment. The results showed that a glaze layer composed of NiO, Ni2O3, Al2O3 and NiAl2O4 phases as well as Ni3Al phase were formed on worn surface under high temperature, which act as a solid lubricant and anti-wear material improving the tribological properties of NiAl coating at elevated temperatures. The wear mechanism is dominated by abrasive wear below 500 °C, and transforms to plastic deformation and adhesive wear at 500 °C up to 700 °C and oxidation wear at 900 °C and above, owing to formation of new oxide phase during tribochemical reaction.
1. Introduction Thanks to the desired physical and mechanical properties such as high melting point, high thermal conductivity, low density, and excellent oxidation resistance at an elevated temperature, NiAl intermetallic compound is regarded as an attractive high temperature structural material and corrosion resistant material in relative sliding components including pistons and valves, turbochargers, gas turbine engine rotor blades and stator vanes [1,2]. Particularly, NiAl intermetallic compound may find potential application in turbine engines whose blade tips suffer from sliding type wear upon contact with surrounding gas path seals [3,4]. In the meantime, as an attractive high temperature surface protective coating, NiAl intermetallic coating can form thin and adherent α-Al2O3 scales on the surface under elevated temperature (above 1100 °C), thereby acquiring prolonged life-time in association with prevention of hot corrosion [5]. To date, numerous studies have been conducted to examine the cyclic oxidation resistance of NiAl coating [6,7], but few are currently available about the friction and wear behavior of NiAl intermetallic compounds over a wide range of temperatures [3,4]. Several methods can be adopted to fabricate NiAl intermetallic compounds, including self-propagating high-temperature synthesis [8,9], combustion synthesis and hot pressing technique [10], spraying technique [11], and laser cladding [12]. Among these methods, laser cladding is of special significance, because it can efficiently provide intermetallics and intermetallic composite coatings possessing compact microstructure as well as excellent metallurgical bonding with the ⁎
substrate [12–15]. It is widely accepted that the wear resistance and wear mechanism of materials are highly dependent on their surface microstructure, because wear fatigue damage and failure of sliding friction couple usually start from the material surface [8,16–18]. As to NiAl intermetallic compound coating, it undergoes a ductile-to-brittle transition in the temperature range of 300~600 °C [19,20], and it can form stable alumina oxide layer at further elevated temperatures [5,21,22]. This means it is imperative to reveal the effect of temperature on the mechanical strength as well as friction and wear behavior of NiAl intermetallic compounds in terms of its application under dry sliding at elevated temperatures in air atmosphere. In the present research, therefore, we make use of laser cladding technique to fabricate NiAl intermetallic compound coating on 1Cr18Ni9Ti stainless steel substrate. Furthermore, we investigate the friction and wear behavior of the as-fabricated NiAl intermetallic compound coating from room temperature to elevated temperatures up to 1000 °C. This article reports the preparation of the laser cladding NiAl intermetallic compound coating and the evaluation of its friction and wear behavior at elevated temperatures, with the hope to provide some technical reference to its application in high temperature tribology. 2. Experimental details 2.1. Laser cladding of NiAl coating
Corresponding author. E-mail address: [email protected] (J. Zhou).
http://dx.doi.org/10.1016/j.triboint.2016.09.014 Received 18 May 2016; Received in revised form 6 September 2016; Accepted 8 September 2016 Available online 09 September 2016 0301-679X/ © 2016 Elsevier Ltd. All rights reserved.
The 1Cr18Ni9Ti stainless steel disks with a size of Ф
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Fig. 1. Typical curves of friction coefficient of NiAl coating with sliding time at RT.
Fig. 2. Variation of friction coefficient of NiAl coating with temperature.
45 mm×16 mm were used as the substrates. Commercially available powders of Al (30–150 µm, purity 99.5%) and Ni (~75 µm, purity 99.0%) with a Ni/Al atomic ratio of 1:1 were used as the starting materials. Prior to laser cladding, the as-received Ni/Al powders were mechanically milled using a planetary ball mill to achieve homogeneous mixing. The mixed powders were placed onto the steel substrate to form a loose layer of approximately 1.5 mm thick. The resultant loose layer of the mixed powders was then placed in a 10 kW transverse-flow continuous-wave CO 2 laser processing system equipped with a 4-axis computer numerical controlling unit. The laser cladding process was conducted in an argon shielding atmosphere at a laser beam size of 10 mm×2 mm, a power of 2.0–4.0 kW, and a scanning speed of 200–400 mm/min to afford target NiAl intermetallic coating.
track of NiAl coating. The morphology and element composition of the worn coating surfaces were analyzed with the 3D non-contact surface mapping profiler and a scanning electron microscope (SEM, JSM5600LV, JEOL, Japan) equipped with an energy dispersive spectrometer (EDS, Kevex, USA). The chemical state of the major elements on the worn surface of the coating was determined by X-ray photoelectron spectroscopy (XPS, PHI-5702, USA; Al-Kα excitation source; reference: contaminated carbon C 1s=284.80 eV). A Renishaw's inVia Micro-Raman spectrometer working with laser light at 633 nm was performed to analyze the chemical feature of the worn coating surfaces.
3. Results and discussions 3.1. Tribological properties
2.2. Friction and wear tests
The friction coefficient of NiAl coating under dry sliding at different temperatures is presented in Fig. 2. The friction coefficient at RT and 300 °C is 0.67 and 0.68, respectively and it slightly decreases to 0.63 at 500 °C. As the temperature rises from 500 °C to 700 °C, the friction coefficient tends to rise slightly therewith. Above 700 °C, the friction coefficient continuously declines with elevating temperature up to 1000 °C. This indicates that temperature has an important influence on the friction behavior of laser cladding NiAl intermetallics coating, and its friction coefficient is reduced to some extent at elevated temperatures. The variation in the wear rate of laser cladding NiAl intermetallics coating with temperature is shown in Fig. 3. It can be seen that the wear rate of NiAl coating increases from 9.5×10−5 mm3/N m at room temperature to 11×10−5 mm3/N m at 300 °C. Above 300 °C, the wear rate decreases significantly with further increase of temperature and reaches the lowest value of about 3×10−5 mm3/N m at 500 °C. When the temperature is further raised from 500 °C to 1000 °C, the wear rate gradually increases from 3×10−5 mm3/N m to 9×10−5 mm3/N m. This demonstrates that temperature also has a prominent influence on the wear resistance of laser cladding NiAl intermetallics coating. Moreover, as the temperature is below 500 °C, the friction coefficient and wear rate of laser cladding NiAl coating tend to change with increasing temperature in a similar manner. Above 500 °C, however, they tend to change with elevating temperature in opposite directions. This means that the friction-reducing ability and wear resistance of the laser cladding NiAl intermetallic compound coating do not necessarily vary with elevating temperature monotonously. In other words, it could be significant to reach a compromise between the friction-reducing ability and wear resistance of the laser cladding NiAl intermetallic coating in terms of its application in high temperature tribology.
A CSEM high temperature tribometer was used to evaluate the friction and wear behavior of the as-prepared NiAl coatings in a ballon-disk contact configuration under dry sliding at room temperature (RT) and elevated temperatures of 300 °C, 500 °C, 700 °C, 900 °C and 1000 °C. Si3N4 ceramic balls with a diameter of 6 mm were used as the counterpart balls. All the sliding tests ran at a normal load of 5 N, a sliding speed of 0.21 m/s, and a duration of 60 min. Prior to sliding tests, the specimens were mechanically polished by 1000 grit emery paper to reach an initial surface roughness of about 0.8 µm (Ra), followed by ultrasonically cleaning in an acetone bath. The friction coefficient was continuously recorded by the tribometer and the typical curves of friction coefficient with sliding time of NiAl coating at RT are presented in Fig. 1. The wear volume losses were measured with a MicroXAM three dimensional (3D) non-contact surface mapping profiler (ADE Corporation, Massachusetts, USA). The volume wear rate of the specimens is calculated as ω=/FS, where V is the wear volume loss in mm3, F is the normal load in Newton, and S is the total sliding distance in meter. Three repeat friction and wear tests were performed at each test temperature. The averaged friction coefficients and wear rates of the three repeat measurements are presented in this article (the standard deviations of the friction coefficient and wear rate are ± 2% and ± 5%). 2.3. Characterization The phase components of the worn surfaces of laser cladding NiAl coating under different sliding conditions were identified by X-ray diffraction (XRD, X'Pert-MRD, Philips). The area exposed to X-ray beam in this article is about 1.0 mm2 and it was confined to the sliding 322
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SEM analysis of its worn surfaces. Fig. 5 represent the SEM images of the worn surfaces of laser cladding NiAl intermetallic coating after sliding tests at different temperatures. It can be seen that the worn surface image evolution of NiAl coating is consistent with that observed in Fig. 4. Namely, a large amount of flakes and delamination pits is present on the worn surface at room temperature (see Fig. 5a), which suggests that the NiAl intermetallic coating is dominated by delamination and abrasive wear as it slides against the ceramic ball at room temperature. With increasing temperature up to 300 °C, some fine grooves and flaky wear debris appear on the wear track (see Fig. 5b), which indicates that the dominant wear mechanisms in this case are microploughing and abrasive wear. When the temperature rises to 300 °C and above, some plastic yielding parallel to the frictional direction emerges after sliding at 500 °C and 700 °C (see Figs. 5c and d), while a glaze layer emerges on the worn coating surface after sliding at 900 °C and 1000 °C (see Figs. 5e and f). This could be related to the brittle-to-ductile transition in association with softening of NiAl coating in the temperature range of 300–600 °C [20,23,24] and to the formation of tribo-oxidation layer at elevated temperatures up to 900 ° C and 1000 °C [8,25]. Corresponding wear mechanisms are plastic deformation and adhesive wear (500 °C and 700 °C) as well as tribooxidation wear (900 °C and above). Metal materials often possess high ductility, which is advantageous in forming the glaze layers at elevated temperatures. The smooth and complete glaze layers is formed under plastic deformation and tribo-oxidation reaction, thereby reducing the shear force in the contact area between the mating surfaces at elevated temperatures and improving the friction-reducing ability and wear resistance of laser cladding NiAl coating. However, at 900 °C and above, the oxidation of the sliding surfaces accelerates and leads to partial delamination of the oxidation layers. As a result, although the friction coefficient of the laser cladding NiAl coating decreases significantly with elevating temperature up to 700 °C and above, the wear rate increases gradually therewith (see Figs. 2 and 3). The 3D profiling images of the worn surfaces of Si3N4 ball after sliding against laser cladding NiAl coating at different temperatures are shown in Fig. 6. It can be seen that the worn surface of Si3N4 ball at room temperature shows signs of brittle delamination and contains wear debris, which indicates that the counterpart Si3N4 ball undergoes severe abrasive wear as it slides against the laser cladding NiAl coating thereat (see Fig. 6a). When the temperature increases to 500 °C, a discontinuous glaze layer is formed on the worn surface of Si3N4 ball to
Fig. 3. Variation of wear rate of NiAl coating with temperature.
3.2. Worn surface morphology characteristics Fig. 4 shows the 3D profiling images of the worn surfaces of NiAl coating after sliding at different temperatures. It can be seen that the worn surfaces of NiAl coating tested below 300 °C show evidence of severe scratches and scrapes in association with partially removed coating as wear debris (see Figs. 4a and b). This might be attributed to the shearing by the hard asperities of Si3N4 ball. Namely, the hard asperities of the ceramic counterpart could plow into NiAl coating and results in stripping of the coating. In the meantime, as the temperature is below 300 °C, the local flash temperature at the contact interface of NiAl coating and Si3N4 ball might be not high enough to soften the asperities. As a result, the laser cladding NiAl coating exhibits relatively high friction coefficient and wear rate below 300 °C (see Figs. 2 and 3) and it is dominated by abrasive wear thereat. When the temperature is above 300 °C, the worn surfaces of NiAl coating show sings of continuous slight scratches (see Fig. 4c), plastic deformation (see Figs. 4d and e), and frictional polishing (see Fig. 4f), depending on elevating temperature. This indicates that the wear mechanism transforms from abrasive wear to adhesion wear and tribo-oxidation wear with increasing temperature up to 1000 °C. In order to better reveal the wear mechanisms of the laser cladding NiAl intermetallic coating at different temperatures, we conducted
Fig. 4. 3D profiling images of worn surfaces of NiAl coating after sliding at different temperatures: (a)-(f) corresponds to RT, 300 °C, 500 °C, 700 °C, 900 °C and 1000 °C.
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Fig. 5. SEM images of worn surfaces of laser cladding NiAl coating at different temperatures: (a)-(f) corresponds to RT, 300 °C, 500 °C, 700 °C, 900 °C and 1000 °C.
minimize the abrasive wear, and the wear scar becomes smooth and flat (see Fig. 6b). With increasing temperature up to 700 °C, a smooth film in association with some fine and flake wear debris is present on the wear track of Si3N4 ball (see Fig. 6c), which indicates that the Si3N4 ball undergoes tribochemical reaction and plastic deformation thereat. As the temperature reaches 1000 °C, the worn surface of Si3N4 ball is completely covered by a continuous and compact glaze layer in association with some fine and shallow grooves (Fig. 5d), which
demonstrates that the dominant wear mechanism in this case is oxidation wear. Considering the above results, we can infer that the formation of the smooth films or glaze layers on the worn surfaces during high temperature tribo-oxidation process has a significant effect on the friction-reducing ability and wear resistance of laser cladding NiAl coating. This will be further discussed in the forthcoming section.
Fig. 6. 3D profiling images of worn surfaces of Si3N4 ball sliding against laser cladding NiAl coating at different temperatures: (a)-(d) corresponds to RT, 500 °C, 700 °C and 1000 °C.
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Fig. 7. XPS spectra of elements (a) Ni and (b) Al on the worn surface of laser cladding NiAl coating after sliding tests at RT and 800 °C, respectively.
3.3. Phase composition of the worn surface The XPS spectra of Ni and Al elements on the worn surface of laser cladding NiAl coating after sliding tests at RT and 800 °C are presented in Figs. 7a and b, respectively. At RT, the Ni2p3/2 peak at 851.8 eV is assigned to metal Ni, and metal Ni is partly oxidized (peak at 854.6 eV is assigned to NiO, and at 856.0 eV to Ni2O3 and NiAl2O4). At 800 °C, metal Ni is completely oxidized to be Ni2O3 and NiAl2O4. A similar phenomenon occurs to Al element. At RT, the Al2p3/2 peak at 71.5 eV is assigned to metal Al, and that at 73.2 eV is assigned to Al2O3. At 800 °C, metal Al is completely oxidized to be Al2O3 and NiAl2O4 with the binding energy at 73.2 eV and 74.4 eV, respectively. The XPS result indicates that the laser cladding NiAl coating undergoes tribochemical reactions when it slides against the ceramic ball at 800 °C. Accordingly, it can be inferred that the glaze layer on the worn surface of NiAl coating at 800 °C is mainly composed of NiO, Ni2O3, Al2O3 and NiAl2O4. Fig. 8 shows the typical SEM image of the wear track of NiAl coating at 1000 °C. The EDS analysis was performed on the areas marked with A and B. It can be seen that area A consists of micrometer crystal grains and has a laminated structure, and there exists a large amount of microcracks around the laminated structure of area A. Differing from area A, area B has a cauliflower structure of fine flaky crystals with nanometer grain size. The above results indicate that the glaze layer in area A of the wear track is preferentially peeled off under the cyclic friction stress during high temperature dry sliding. Besides, the glaze layer in area B of the wear track tends to grow after the outer glaze layer in area A is peeled off, which is also supported by the EDS results of O and Si content in areas A and B (see Table 1). Table 1 lists the EDS element composition of areas A and B (see Fig. 8) on the wear track of NiAl coating after dry sliding at 1000 °C. It can be seen that O, Al, Si and Ni elements are present in areas A and B, and there are some differences in the content of these elements. Considering that the Ni and Al elements in the laser cladding NiAl intermetallic coating may undergo tribochemical reactions with Si element in Si3N4 and O element in air during dry sliding at elevated temperature, we can infer that areas A and B mainly contain NiO, Al2O3, NiAl2O4 and SiO2 which are generated through the tribochemical reactions. Furthermore, the glaze layer on the worn surface of NiAl coating after sliding at elevated temperatures mainly consists of NiO, Al2O3 and NiAl2O4 generated through the oxidation of NiAl phase, as evidenced by relevant XPS data (see Fig. 7). The Raman spectra of areas A and B on the wear track of NiAl coating after dry sliding at 1000 °C are shown in Fig. 9. It can be seen that NiAl2O4, Al2O3, NiO and SiO2 phases are present in area A of the
Fig. 8. Typical SEM image of the wear track of NiAl coating at 1000 °C. The EDS analysis was performed on the areas marked with A and B. Table 1 Element composition of the typically selected areas A and B on the wear track of laser cladding NiAl coating after sliding at 1000 °C (determined by EDS analysis). Element
O Al Si Ni
Composition (wt%) A area
B area
53.60 31.75 2.62 12.18 100.15
45.16 36.54 1.32 16.98 100
wear track of NiAl coating (see Fig. 9a), while Al2O3, NiO and SiO2 phases are present in area B (see Fig. 9b). Furthermore, a comparison of the intensities of various Raman peaks implies that the oxide layer is preferentially formed in area A rather than area B. In other words, the glaze layer on the worn surface of NiAl coating is mainly composed of NiAl2O4, NiO and Al2O3 phases, and the outer glaze layer is prone to stripping under the actions of cyclic friction stress and oxidation (see Fig. 8). On the one hand, the oxide film contributes to decreasing the friction coefficient of laser cladding NiAl coating. On the other hand, excessive oxidation leads to the delamination of the glaze layer and results in increased wear rate under high temperatures. This means that, in terms of the application of laser cladding NiAl intermetallics coating in high temperature tribology, it could be significant to reach a compromise between its friction-reducing ability and wear resistance.
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Fig. 9. Raman spectra of areas A and B on the wear track of NiAl coating after dry sliding at 1000 °C.
The XRD pattern of the worn surface of NiAl coating after dry sliding at 1000 °C is shown in Fig. 10. NiAl and Ni3Al phases as well as NiO, Al2O3 and NiAl2O4 phases are identified in the worn surface, which is consistent with corresponding XPS, EDS and Raman results. Under sliding at elevated temperatures, the laser cladding NiAl intermetallic coating can undergo chemical reaction with atmospheric O forming Al2O3 phase and γ-Ni phase. In the meantime, the released Al can react with γ-Ni through diffusion of atoms to afford γ′-Ni3Al phase. This observation well conforms to what is reported elsewhere in that γ′-Ni3Al phase can be precipitated along NiAl grain boundaries owing to the solid-state diffusion of Ni atoms at 800 °C and above [5,26,27]. Moreover, Ni3Al phase exhibits higher high-temperature yield strength and ductility than NiAl intermetallic, which is favorable for abating the wear of the mating surface (i.e., Si3N4 ball surface). This could well explain why the laser cladding NiAl intermetallic coating has a lower wear rate at high temperature than at room temperature (see Fig. 3). The evolution of various phases during the dry sliding process of NiAl coating against Si3N4 ball at elevated temperatures is shown in Fig. 11. It can be seen that the phase composition of the oxide films varies with increasing temperature, which is related to the Gibbs free energies (ΔG°) of various oxide species which contribute to improving the friction-reducing ability and wear resistance of the laser cladding NiAl coating [28]. Namely, the oxide film of NiO and Ni2O3 phases is formed at 500 °C and below, thereby decreasing the friction coefficient and wear rate of NiAl coating in the moderate-low temperature regions (see Figs. 2 and 3). When temperature rises from 500 °C to 700 °C, additional protective oxide film of Al2O3 phase is formed together with that of NiO and Ni2O3, thereby significantly increasing the wear resistance of NiAl coating in the moderate-high temperature regions (see Figs. 2 and 3). As the temperature is raised to 900 °C and above, the oxide film of NiAl2O4 phase is formed together with that of NiO, Ni2O3 and Al2O3; and the oxide films tend to gradually grow into glaze layers on the worn surface of NiAl coating, thereby improving the wear resistance of the NiAl coating at high temperatures. In one word, it is the formation of the oxide films and/or glaze layers on the wear track of the laser cladding NiAl coating that accounts for its greatly improved friction-reducing ability and wear resistance over a wide temperature range.
Fig. 10. XRD pattern of the worn surface of NiAl coating after sliding at 1000 °C.
Fig. 11. Schematic drawing showing the evolution of various phases during the dry sliding of NiAl coating against Si3N4 ball at elevated temperatures.
NiAl coating were investigated from room temperature to 1000 °C under air atmosphere environment. It was found that a glaze layer composed of NiO, Ni2O3, Al2O3 and NiAl2O4 phases as well as Ni3Al phase is formed on the worn surface of the NiAl coating after sliding at elevated temperatures. The glaze layer acts as a wear-resistant solid lubricant to reduce the friction and wear of the laser cladding NiAl intermetallics coating coupled with Si3N4 ball at elevated temperatures. Moreover, the NiAl intermetallics coating is dominated by abrasive wear when it slides against the ceramic ball below 500 °C. As the
4. Conclusions Commercially available Al (30–150 µm, purity 99.5%) and Ni powders (~75 µm, purity 99.0%) with a Ni/Al atomic ratio of 1:1 were used as the starting materials to fabricate laser cladding NiAl intermetallics coating on 1Cr18Ni9Ti stainless steel substrate. Tribological properties, phase compositions and wear mechanisms of laser cladding 326
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