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Tribological properties of rare earth oxide added Cr3C2–NiCr coatings
Applied Surface Science 253 (2007) 4377–4385 www.elsevier.com/locate/apsusc
Tribological properties of rare earth oxide added Cr3C2–NiCr coatings Zhenyu Zhang *, Xinchun Lu, Jianbin Luo State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, PR China Received 21 July 2006; received in revised form 23 September 2006; accepted 24 September 2006 Available online 23 October 2006
Abstract A novel supersonic plasma spraying was used to prepare rare earth oxide added Cr3C2–NiCr coatings. X-ray diffractometer, contact surface profiler, hardness tester, micro-friction and -wear tester, environmental scanning electron microscope equipped with energy dispersive spectroscopy were employed to investigate the phase structure, surface morphology, microhardness, and friction properties of deposited coatings, respectively. The results show that surface roughness, microhardness, brittle fracture, friction extent and wear resistance of rare earth oxide added Cr3C2–NiCr coatings are effectively improved compared with that of unadded one. The friction and friction mechanism are also discussed. # 2006 Elsevier B.V. All rights reserved. Keywords: Supersonic plasma spraying; Cr3C2–NiCr coating; Rare earth oxide; Tribological property
1. Introduction Cr3C2–NiCr coating with high erosion resistance, high hardness and wear resistance [1], has been used to prevent oxidation effect up to 800 8C [2,3], so it is a promising coating applied for the strict service environment at high temperature. At present, the Cr3C2–NiCr coating is widely prepared using high-velocity oxy fuel (HVOF) method [4,5], while Cr3C2–NiCr coating prepared with supersonic plasma spraying (SPS) has not been reported as SPS is a novel developed technology. Compared with HVOF method, SPS has the higher melting temperature (up to 10,000 8C) and faster particle velocity (600 m/s), which can effectively prevent the disadvantages of HVOF coating with unmelted particles and higher porosity produced at the lower flame temperature (3000 8C) [6,7]. In our previous work, rare earth oxide was added into the Cobased coatings, leading to the decrease of friction coefficient at high temperature obviously [8], whereas the tribological properties at room temperature and small scale load were not presented. So in present study, the REO added Ni-based coatings are renewedly prepared using this novel SPS method
* Corresponding author. Tel.: +86 10 6278 8309; fax: +86 10 6278 1379. E-mail address: [email protected] (Z. Zhang). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.09.040
and the corresponding tribological properties at room temperature and small scale load are investigated. 2. Experimental 2.1. Novel supersonic plasma spraying system Novel supersonic plasma spraying system is composed of plasma torch, powder feeder, gas supply, water cooling circulator, control unit with PC interface and power supply unit. The key of the system is a novel supersonic plasma spraying gun (SPSG), as shown in Fig. 1, which presents the obvious difference between the SPSG and Plazjet gun [9]. 2.2. Coating and substrate materials The substrate is 2Cr13 stainless steel with chemical composition (wt.%) of 0.16–0.25 C, 12–14 Cr, 1.0 Si, 0.035 P, 0.03 S, 1 Mn, and Fe balance, which is cut into dimensions 30 mm 30 mm 5 mm. Prior to deposition, it is bombarded by Al2O3 grits for 10 min and then ultrasonically cleaned for 10 min. Cr3C2–25 wt.% NiCr powder (Beijing General Research Institute of Mining and Metallurgy, China) with sizes 5–45 mm and polygonal shape, as shown in Fig. 2, is used as coating material. Cr3C2–25 wt.% NiCr powder coated on 2Cr13 stainless steel with a thickness approximately
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Z. Zhang et al. / Applied Surface Science 253 (2007) 4377–4385 Table 1 Compound composition of coatings
Fig. 1. Comparison between SPSG and Plazjet gun.
Coatings
Compound composition
No. No. No. No. No. No. No. No.
Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr
1 2 3 4 5 6 7 8
coating added with added with added with added with added with added with added with
1% 2% 4% 1% 2% 4% 2%
La2O3 coating La2O3 coating La2O3 coating CeO2 coating CeO2 coating CeO2 coating CeO2 and 2% La2O3 coating
current 320 A, nitrogen powder feeding 0.6 m3/h, powder feed rate 50 g/min, and spraying distance 100 mm. For each coating preparation, rare earth oxide (REO), such as La2O3 or CeO2, is added into the Cr3C2–NiCr powder, then the mixed powder is put into a stirrer and stirred for 10 min for supersonic plasma spraying (SPS). Compound ingredients of eight coatings before deposition are shown in Table 1. It is noticed that the compound composition of following coatings named by No. 1 to No. 8 is listed in Table 1. 2.3. Measuring instruments
Fig. 2. SEM micrograph of Cr3C2–25 wt.% NiCr powder.
400 mm is carried out using the novel supersonic plasma spraying method. The spraying parameters used to deposit the coatings on the substrate were summarized as follows: argon flow rare 3.2 m3/h, hydrogen flow rare 0.3 m3/h, voltage 115 V,
Surface morphology, worn surface and electron back scattering pattern (EBSP) are obtained by environmental scanning electron microscope (ESEM, FEI Quanta 200 FEG, The Netherlands) with X-ray energy disperse spectroscopy (EDS). Average surface roughness (Ra) is measured using contact surface profiler (Taylor Hobson, UK). X-ray diffraction (XRD) analysis is carried out by a diffractometer (D/max2500, Rigaku, Japan) using Cu Ka radiation. The applied tube power is 15 kW, scanning angle resolution 0.018, and tube voltage 50 kV. Prior to the indentation tests, the coating surface is processed to less than 0.16 mm using a cubic BN grinding wheel, then ultrasonically cleaned in acetone solution for 10 min. Ten indents are performed using a 200 g force, which is standard force used in the thermal spraying industry [10,11]. Microhardness is measured by a hardness tester (DM-400, LECO, USA) cooperated with a optical microscope using
Fig. 3. SEM surface morphologies of (a) No. 1 and (b) No. 7 coatings.
Z. Zhang et al. / Applied Surface Science 253 (2007) 4377–4385 Table 2 Ra of coatings prepared using supersonic plasma spraying
3. Results and discussion
Coating
Ra (mm)
No. No. No. No. No. No. No. No.
9.55 0.5 6.76 0.3 6.19 0.2 6.17 0.4 7.26 0.3 6.53 0.3 6.94 0.5 6.51 0.4
1 2 3 4 5 6 7 8
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Vickers diamond tip for duration time of 15 s. Friction tests are conducted on a micro-friction and -wear tester (UMT, CETR, USA) under a reciprocating ball-on-block type, at room temperature under unlubricated condition in ambient air. The tungsten carbide ball with diameter of 4 mm, surface roughness Ra 25 nm and hardness of 1145 Hv is selected as counterpart. Prior to tribological tests, the surface of deposited coatings was machined to Ra 0.16 mm by cubic BN grinding wheel. For all the tribological tests of deposited coatings except for the special description, the duration time is 10 min, sliding speed 200 reci/min, sliding distance 5 mm and applied load 200 g.
3.1. Surface morphology and roughness Two typical surface SEM micrographs of sprayed coatings are shown in Fig. 3. It is observed that the two surfaces are dense, flat and free of crack, hole and unmelted particles as presented by Ji et al. [6] and Sidhu et al. [7]. So the coatings deposited by SPS has better surface quality, which results from the higher melting temperature and faster particle deposition velocity. Ra of eight different chemical composition coatings, measured from two parallel lines with length of 12.5 mm on the surface of coatings, is summarized in Table 2. It is seen that surface roughness of No. 1 coating without REO is obviously improved by the REO addition. Average value of Ra of seven REO coatings is 6.62 mm, which is 31% lower than that of unadded one. Additionally, Ra of unadded REO coating is at the level of 9 mm and REO added coating at the level of 6 mm, moreover among the added REO coatings, No. 4 coating has the lowest Ra of 6.17 mm. So the addition of REO can improve the
Fig. 5. EDS spectrum of No. 4 coating.
Fig. 4. XRD spectra of (a) Cr3C2–NiCr raw powder and (b) No. 4 coating.
Fig. 6. Variation of Vickers microhardness as a function of different coatings and substrate, under applied load of 200 g and duration of 15 s.
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Fig. 7. SEM worn surface morphologies, EBSP, EDS spectra of No. 1 coating for (a) wear track morphology, (b) EDS spectrum of (a), (c) magnified view of some location in (a), (d) EBSP of some location in (a), (e) EDS spectrum of 1# region marked in (d), and (f) EDS spectrum of 2# region marked in (d).
surface roughness effectively, due to the nature of the REO. As the rare earth is a surface active element and easily reacts with oxygen element forming stable rare earth oxide compound at melting state. During the process of crystallization, REO can
increase the amount of crystal nuclei and limit the growth of grain size, thus the microstructure of the coatings can be refined [12,13], leading to the improvement of surface morphology.
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Fig. 8. SEM worn surface morphology, EBSP, and EDS spectrum of No. 4 coating for (a) wear track morphology, (b) EBSP, and (c) EDS spectrum of 3# region marked in (b).
3.2. XRD analysis The XRD results of Cr3C2–NiCr raw powder and No. 4 coating are shown in Fig. 4(a) and (b), respectively. Both coatings have confirmed the presence of a large percentage of nickel and chromium metal phases, which is consistent with the reports of Sidhu et al. [7], Sahraoui et al. [14], and Vuoristo et al. [15]. While, according to the XRD result observed in Fig. 4(b), a new Cr7C3 phase is formed with a higher intensity, which results from the high melting temperature and sprayed particle velocity. For the presence of REO element, it can also be identified with EDS spectrum as shown in Fig. 5. There is no phase transformation occurring during the spraying process, the same conclusion can be drawn with the addition of CeO2. It is verified that REO does not transform to a new phase at the high melting temperature and sprayed particle velocity.
inherent property of sprayed coatings containing higher percentage of Cr3C2 and Cr7C3 hard phases and metallic nickel and chromium soft phases (Fig. 4). The average value of hardness of No. 1 coating without REO is 806 Hv, which is higher than that of conventional sprayed coatings, and is consistent with the HVOF reports of Wang and Shui [16], and Wang and Lee [17]. According to the average value of hardness, those of sprayed coatings are largely higher than that of substrate, and those of all the coatings added with REO are higher than that of unadded coating excluding No. 4 and No. 7 coatings. Among the coatings, No. 8 coating with 915.7 Hv has the highest hardness. It can be seen that the REO added coatings can increase the average microhardness due to the refinement effect of REO. As the REO can refine the microstructure, it makes the phase distribute uniformly, then the microhardness is increased, which is consistent with the improvement of surface roughness.
3.3. Microhardness measurements
3.4. Worn surface observation
The obtained data for microhardness tests is shown in Fig. 6. Compared with the change range of the hardness value of substrate, those of coatings is higher, which results from the
Worn surface SEM morphologies, EBSP and corresponding EDS spectra of No. 1, No. 4 and No. 7 coatings are shown in Figs. 7–9, respectively. It is observed that brittle fracture is the
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Fig. 9. SEM worn surface morphology, EBSP and EDS spectra of No. 7 coating for (a) worn surface morphology, (b) EBSP, (c) EDS spectrum of 3# region marked in (b), (d) EDS spectrum of 4# region marked in (b).
main friction mechanism for three SPS coatings. For No. 1 coating without REO, bigger micro-crack and delamination can be obviously seen compared with that of two REO added coatings. According to Figs. 7(a), 7(c), 8(a) and 9(a), the brittle fracture is the main friction mechanism. The corresponding EDS of Fig. 7(a) with a large percentage of nickel and cobalt element, agrees well with the XRD results (Fig. 4). From EBSP results of Figs. 7(d), 8(b) and 9(b) and its corresponding EDS analysis, It can be seen that SPS coating contains mainly two phases: gray black area marked with 1# region with large percentage of cobalt element, and gray white area marked with 2# region with large percentage of nickel element. La2O3 and CeO2 is also detected (Figs. 8(c) and 9(c)), respectively, which is consistent with the results of XRD. While in Fig. 9(d), it can be recognized that the white flake is tungsten carbide, which means that the wear also occurs in counterpart WC ball. As the refinement and purified effects of REO, the friction extent is reduced, as shown in Figs. 8(a) and 9(a) compared with that of Fig. 7(c). 3.5. Tribological properties For each coating and substrate, five curves of friction coefficient against sliding time, with interval distance of 3 mm,
are carried out. Figs. 10–12 show the variations of friction coefficient against sliding time of No. 1, No. 4 and No. 5 coatings, respectively. It is observed that three coatings have stable tribological properties with time going on. Also, friction curves of other coatings have the same trend, i.e. starting at a relatively low level of friction coefficient value, increasing to a peak and then achieving a steady state value, which is consistent with the reports of Carrasquero et al. [18] and Liu et al. [19]. Additionally, it can be seen that brittle fracture (Fig. 7(c)) takes place in No. 1 coating without REO when the sliding time is at the range of 200–300 s, as shown in Fig. 10(b)–(d). Furthermore, it is easy to find the brittle fracture starting, because during the friction process from the beginning to the first brittle fracture generating, the friction sound is mild and similar with that of machining cast iron, while when the brittle fracture occurs, a strident noise is heard and a large fluctuation can be seen in the friction curve as shown in Fig. 10(d). After the first brittle fracture, generally speaking, continuous brittle fracture will occur as shown in Fig. 10(b) and (c). However, there is no this kind of phenomenon generates in Figs. 11(b) and 12(b) corresponding to No. 4 and No. 5 two added REO coatings, respectively, moreover the other REO added coatings also does not have this kind of phenomenon. It is
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Fig. 10. Variation of friction coefficient as a function of sliding time for (a) No. 1 coating, (b) magnified view of (a), (c) magnified view of A marked in (b) and (d) magnified view of B marked in (c).
obvious that brittle fracture of REO added coatings is reduced during friction process, which results from refining effect of REO. Fig. 13 shows the variation of friction coefficient as a function of sprayed coatings and substrate. It is observed that only the friction coefficient of No. 4 REO added coating is
lower than that unadded coating, while approaching to a higher extent, the friction coefficient of other REO added coatings varies within the range of 0.69–0.64, which is almost at the same level with that of unadded one (0.66). Additionally, a trend that friction coefficient decreases as REO content increases takes place, is obvious seen in Fig. 13. While at
Fig. 11. Variation of friction coefficient as a function of sliding time for (a) No. 4 coating and (b) magnified view of (a).
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Fig. 12. Variation of friction coefficient as a function of sliding time for (a) No. 5 coating and (b) magnified view of (a).
Fig. 13. Variation of friction coefficient as a function of different coatings and substrate.
this applied load, the wear track is very shallow leading that the measurement of wear volume loss is very difficult. To compare the tribological property of sprayed coatings under high applied load, tribological property measurements under applied load of 30 N have been conducted at the same tribometer. The sliding distance is 6 mm, sliding speed 300 reci/min and duration 6 min. The variation of wear volume loss as a function of different coatings are shown in Fig. 14. It can be seen that wear resistance of REO added SPS coatings is largely enhanced, specially for No. 3, No. 4 and No. 8 coatings, whose wear volume loss is three orders magnitude smaller than that of unadded one. According to the experimental results, it can be drawn that the improvement of tribological property of REO to the SPS coatings under 30 N is largely resulted from the nature of REO that La2O3 has special hexagonal layered structure with better lubrication function [20], which can decrease the wear effectively, while at the load of 2 N the improvement is not obvious, as the lubrication function of REO is not obvious under very low load [12]. 4. Conclusions Through experimental verification, the contributions of REO to the Cr3C2–NiCr coatings can be concluded as follows: 1. REO can improve the surface average roughness effectively. 2. REO can enhance the average surface microhardness. 3. REO can reduce the fluctuation phenomenon of friction coefficient existing in the REO unadded coating, decrease the friction extent effectively and increase the wear resistance under high load largely, while at the vary low load, the improvement is not obvious. Acknowledgements
Fig. 14. Variation of wear volume loss as a function of different coatings.
The authors appreciate the financial support from the National Key Basic Research Program of China (Grant No. 2003CB716201) and the National Natural Science Foundation of China (Grant No. 50575121).
Z. Zhang et al. / Applied Surface Science 253 (2007) 4377–4385
References [1] J. Morimoto, M. Tomie, F. Kuriyama, N. Abe, Proc. Int. Therm. Spray Conf. (2001) 599–605. [2] D. Dalmas, S. Benmedhakene, H. Kebir, C. Richard, A. Laksimi, J.M. Roleandt, Surf. Coat. Technol. 173 (2003) 130–143. [3] L. Berger, M. Vuoristo, P. Mantyla, T.W. Gruner, Proc. Int. Therm. Spray Conf. 1 (1998) 75–82. [4] C.J. Li, K. Sonoya, G.C. Ji, Y.Y. Wang, Proc. Int. Therm. Spray Conf. 1 (1998) 287–292. [5] M. Uusitalo, M. Kaipiainen, P. Vuoristo, T. Mantyla, Mater. Sci. Forum 369–372 (2001) 475–482. [6] G.C. Ji, Y.Y. Wang, C.J. Li, K. Sonoya, J. Xi’an, Jiaotong Univ. 34 (2000) 59–63 (in Chinese). [7] H.S. Sidhu, B.S. Sidhu, S. Prakash, J. Mater. Process. Technol. 171 (2006) 77–82. [8] Z.Y. Zhang, X.C. Lu, B.L. Han, J.B. Luo, Mater. Sci. Eng. A., in press. [9] S. Zhu, B.S. Xu, Key Eng. Mater. 280–283 (2005) 1203–1206.
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[10] M. Factor, I. Roman, Surf. Coat. Technol. 132 (2000) 181–193. [11] L. Pawlowski, The Science and Engineering of Thermal Spray Coatings, John Wiley and Sons, 1995,, p. 182. [12] G.M. Zhao, K.L. Wang, C.G. Li, Tribology 24 (2004) 318–321 (in Chinese). [13] K.L. Wang, Q.B. Zhang, M.L. Sun, X.G. Wei, Y.M. Zhu, Corros. Sci. 43 (2001) 255–267. [14] T. Sahraoui, N.E. Fenineche, G. Montavon, C. Coddet, Mater. Des. 24 (2003) 309–313. [15] P. Vuoristo, K. Niemi, A. Makela, T. Mantyla, Proc. Natl. 7th Therm. Spray Conf., Boston, Massachusetts, vol. 20–24, (1994), pp. 121–126. [16] B.Q. Wang, Z.R. Shui, Wear 253 (2002) 550–557. [17] B. Wang, S.W. Lee, Wear 239 (2000) 83–90. [18] E. Carrasquero, A. Bellosi, M.H. Staia, Int. J. Refract. Met. Hard Mater., Sp. I. 23 (2005) 391–397. [19] W.M. Liu, J.L. Duda, E.E. Klaus, Wear 199 (1996) 217–221. [20] Q. Liang, Z.J. Zhang, Q.J. Xue, et al. Rare Earths 20 (1999) 32–34 (in Chinese).
Tribological properties of rare earth oxide added Cr3C2–NiCr coatings Zhenyu Zhang *, Xinchun Lu, Jianbin Luo State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, PR China Received 21 July 2006; received in revised form 23 September 2006; accepted 24 September 2006 Available online 23 October 2006
Abstract A novel supersonic plasma spraying was used to prepare rare earth oxide added Cr3C2–NiCr coatings. X-ray diffractometer, contact surface profiler, hardness tester, micro-friction and -wear tester, environmental scanning electron microscope equipped with energy dispersive spectroscopy were employed to investigate the phase structure, surface morphology, microhardness, and friction properties of deposited coatings, respectively. The results show that surface roughness, microhardness, brittle fracture, friction extent and wear resistance of rare earth oxide added Cr3C2–NiCr coatings are effectively improved compared with that of unadded one. The friction and friction mechanism are also discussed. # 2006 Elsevier B.V. All rights reserved. Keywords: Supersonic plasma spraying; Cr3C2–NiCr coating; Rare earth oxide; Tribological property
1. Introduction Cr3C2–NiCr coating with high erosion resistance, high hardness and wear resistance [1], has been used to prevent oxidation effect up to 800 8C [2,3], so it is a promising coating applied for the strict service environment at high temperature. At present, the Cr3C2–NiCr coating is widely prepared using high-velocity oxy fuel (HVOF) method [4,5], while Cr3C2–NiCr coating prepared with supersonic plasma spraying (SPS) has not been reported as SPS is a novel developed technology. Compared with HVOF method, SPS has the higher melting temperature (up to 10,000 8C) and faster particle velocity (600 m/s), which can effectively prevent the disadvantages of HVOF coating with unmelted particles and higher porosity produced at the lower flame temperature (3000 8C) [6,7]. In our previous work, rare earth oxide was added into the Cobased coatings, leading to the decrease of friction coefficient at high temperature obviously [8], whereas the tribological properties at room temperature and small scale load were not presented. So in present study, the REO added Ni-based coatings are renewedly prepared using this novel SPS method
* Corresponding author. Tel.: +86 10 6278 8309; fax: +86 10 6278 1379. E-mail address: [email protected] (Z. Zhang). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.09.040
and the corresponding tribological properties at room temperature and small scale load are investigated. 2. Experimental 2.1. Novel supersonic plasma spraying system Novel supersonic plasma spraying system is composed of plasma torch, powder feeder, gas supply, water cooling circulator, control unit with PC interface and power supply unit. The key of the system is a novel supersonic plasma spraying gun (SPSG), as shown in Fig. 1, which presents the obvious difference between the SPSG and Plazjet gun [9]. 2.2. Coating and substrate materials The substrate is 2Cr13 stainless steel with chemical composition (wt.%) of 0.16–0.25 C, 12–14 Cr, 1.0 Si, 0.035 P, 0.03 S, 1 Mn, and Fe balance, which is cut into dimensions 30 mm 30 mm 5 mm. Prior to deposition, it is bombarded by Al2O3 grits for 10 min and then ultrasonically cleaned for 10 min. Cr3C2–25 wt.% NiCr powder (Beijing General Research Institute of Mining and Metallurgy, China) with sizes 5–45 mm and polygonal shape, as shown in Fig. 2, is used as coating material. Cr3C2–25 wt.% NiCr powder coated on 2Cr13 stainless steel with a thickness approximately
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Z. Zhang et al. / Applied Surface Science 253 (2007) 4377–4385 Table 1 Compound composition of coatings
Fig. 1. Comparison between SPSG and Plazjet gun.
Coatings
Compound composition
No. No. No. No. No. No. No. No.
Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr
1 2 3 4 5 6 7 8
coating added with added with added with added with added with added with added with
1% 2% 4% 1% 2% 4% 2%
La2O3 coating La2O3 coating La2O3 coating CeO2 coating CeO2 coating CeO2 coating CeO2 and 2% La2O3 coating
current 320 A, nitrogen powder feeding 0.6 m3/h, powder feed rate 50 g/min, and spraying distance 100 mm. For each coating preparation, rare earth oxide (REO), such as La2O3 or CeO2, is added into the Cr3C2–NiCr powder, then the mixed powder is put into a stirrer and stirred for 10 min for supersonic plasma spraying (SPS). Compound ingredients of eight coatings before deposition are shown in Table 1. It is noticed that the compound composition of following coatings named by No. 1 to No. 8 is listed in Table 1. 2.3. Measuring instruments
Fig. 2. SEM micrograph of Cr3C2–25 wt.% NiCr powder.
400 mm is carried out using the novel supersonic plasma spraying method. The spraying parameters used to deposit the coatings on the substrate were summarized as follows: argon flow rare 3.2 m3/h, hydrogen flow rare 0.3 m3/h, voltage 115 V,
Surface morphology, worn surface and electron back scattering pattern (EBSP) are obtained by environmental scanning electron microscope (ESEM, FEI Quanta 200 FEG, The Netherlands) with X-ray energy disperse spectroscopy (EDS). Average surface roughness (Ra) is measured using contact surface profiler (Taylor Hobson, UK). X-ray diffraction (XRD) analysis is carried out by a diffractometer (D/max2500, Rigaku, Japan) using Cu Ka radiation. The applied tube power is 15 kW, scanning angle resolution 0.018, and tube voltage 50 kV. Prior to the indentation tests, the coating surface is processed to less than 0.16 mm using a cubic BN grinding wheel, then ultrasonically cleaned in acetone solution for 10 min. Ten indents are performed using a 200 g force, which is standard force used in the thermal spraying industry [10,11]. Microhardness is measured by a hardness tester (DM-400, LECO, USA) cooperated with a optical microscope using
Fig. 3. SEM surface morphologies of (a) No. 1 and (b) No. 7 coatings.
Z. Zhang et al. / Applied Surface Science 253 (2007) 4377–4385 Table 2 Ra of coatings prepared using supersonic plasma spraying
3. Results and discussion
Coating
Ra (mm)
No. No. No. No. No. No. No. No.
9.55 0.5 6.76 0.3 6.19 0.2 6.17 0.4 7.26 0.3 6.53 0.3 6.94 0.5 6.51 0.4
1 2 3 4 5 6 7 8
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Vickers diamond tip for duration time of 15 s. Friction tests are conducted on a micro-friction and -wear tester (UMT, CETR, USA) under a reciprocating ball-on-block type, at room temperature under unlubricated condition in ambient air. The tungsten carbide ball with diameter of 4 mm, surface roughness Ra 25 nm and hardness of 1145 Hv is selected as counterpart. Prior to tribological tests, the surface of deposited coatings was machined to Ra 0.16 mm by cubic BN grinding wheel. For all the tribological tests of deposited coatings except for the special description, the duration time is 10 min, sliding speed 200 reci/min, sliding distance 5 mm and applied load 200 g.
3.1. Surface morphology and roughness Two typical surface SEM micrographs of sprayed coatings are shown in Fig. 3. It is observed that the two surfaces are dense, flat and free of crack, hole and unmelted particles as presented by Ji et al. [6] and Sidhu et al. [7]. So the coatings deposited by SPS has better surface quality, which results from the higher melting temperature and faster particle deposition velocity. Ra of eight different chemical composition coatings, measured from two parallel lines with length of 12.5 mm on the surface of coatings, is summarized in Table 2. It is seen that surface roughness of No. 1 coating without REO is obviously improved by the REO addition. Average value of Ra of seven REO coatings is 6.62 mm, which is 31% lower than that of unadded one. Additionally, Ra of unadded REO coating is at the level of 9 mm and REO added coating at the level of 6 mm, moreover among the added REO coatings, No. 4 coating has the lowest Ra of 6.17 mm. So the addition of REO can improve the
Fig. 5. EDS spectrum of No. 4 coating.
Fig. 4. XRD spectra of (a) Cr3C2–NiCr raw powder and (b) No. 4 coating.
Fig. 6. Variation of Vickers microhardness as a function of different coatings and substrate, under applied load of 200 g and duration of 15 s.
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Fig. 7. SEM worn surface morphologies, EBSP, EDS spectra of No. 1 coating for (a) wear track morphology, (b) EDS spectrum of (a), (c) magnified view of some location in (a), (d) EBSP of some location in (a), (e) EDS spectrum of 1# region marked in (d), and (f) EDS spectrum of 2# region marked in (d).
surface roughness effectively, due to the nature of the REO. As the rare earth is a surface active element and easily reacts with oxygen element forming stable rare earth oxide compound at melting state. During the process of crystallization, REO can
increase the amount of crystal nuclei and limit the growth of grain size, thus the microstructure of the coatings can be refined [12,13], leading to the improvement of surface morphology.
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Fig. 8. SEM worn surface morphology, EBSP, and EDS spectrum of No. 4 coating for (a) wear track morphology, (b) EBSP, and (c) EDS spectrum of 3# region marked in (b).
3.2. XRD analysis The XRD results of Cr3C2–NiCr raw powder and No. 4 coating are shown in Fig. 4(a) and (b), respectively. Both coatings have confirmed the presence of a large percentage of nickel and chromium metal phases, which is consistent with the reports of Sidhu et al. [7], Sahraoui et al. [14], and Vuoristo et al. [15]. While, according to the XRD result observed in Fig. 4(b), a new Cr7C3 phase is formed with a higher intensity, which results from the high melting temperature and sprayed particle velocity. For the presence of REO element, it can also be identified with EDS spectrum as shown in Fig. 5. There is no phase transformation occurring during the spraying process, the same conclusion can be drawn with the addition of CeO2. It is verified that REO does not transform to a new phase at the high melting temperature and sprayed particle velocity.
inherent property of sprayed coatings containing higher percentage of Cr3C2 and Cr7C3 hard phases and metallic nickel and chromium soft phases (Fig. 4). The average value of hardness of No. 1 coating without REO is 806 Hv, which is higher than that of conventional sprayed coatings, and is consistent with the HVOF reports of Wang and Shui [16], and Wang and Lee [17]. According to the average value of hardness, those of sprayed coatings are largely higher than that of substrate, and those of all the coatings added with REO are higher than that of unadded coating excluding No. 4 and No. 7 coatings. Among the coatings, No. 8 coating with 915.7 Hv has the highest hardness. It can be seen that the REO added coatings can increase the average microhardness due to the refinement effect of REO. As the REO can refine the microstructure, it makes the phase distribute uniformly, then the microhardness is increased, which is consistent with the improvement of surface roughness.
3.3. Microhardness measurements
3.4. Worn surface observation
The obtained data for microhardness tests is shown in Fig. 6. Compared with the change range of the hardness value of substrate, those of coatings is higher, which results from the
Worn surface SEM morphologies, EBSP and corresponding EDS spectra of No. 1, No. 4 and No. 7 coatings are shown in Figs. 7–9, respectively. It is observed that brittle fracture is the
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Fig. 9. SEM worn surface morphology, EBSP and EDS spectra of No. 7 coating for (a) worn surface morphology, (b) EBSP, (c) EDS spectrum of 3# region marked in (b), (d) EDS spectrum of 4# region marked in (b).
main friction mechanism for three SPS coatings. For No. 1 coating without REO, bigger micro-crack and delamination can be obviously seen compared with that of two REO added coatings. According to Figs. 7(a), 7(c), 8(a) and 9(a), the brittle fracture is the main friction mechanism. The corresponding EDS of Fig. 7(a) with a large percentage of nickel and cobalt element, agrees well with the XRD results (Fig. 4). From EBSP results of Figs. 7(d), 8(b) and 9(b) and its corresponding EDS analysis, It can be seen that SPS coating contains mainly two phases: gray black area marked with 1# region with large percentage of cobalt element, and gray white area marked with 2# region with large percentage of nickel element. La2O3 and CeO2 is also detected (Figs. 8(c) and 9(c)), respectively, which is consistent with the results of XRD. While in Fig. 9(d), it can be recognized that the white flake is tungsten carbide, which means that the wear also occurs in counterpart WC ball. As the refinement and purified effects of REO, the friction extent is reduced, as shown in Figs. 8(a) and 9(a) compared with that of Fig. 7(c). 3.5. Tribological properties For each coating and substrate, five curves of friction coefficient against sliding time, with interval distance of 3 mm,
are carried out. Figs. 10–12 show the variations of friction coefficient against sliding time of No. 1, No. 4 and No. 5 coatings, respectively. It is observed that three coatings have stable tribological properties with time going on. Also, friction curves of other coatings have the same trend, i.e. starting at a relatively low level of friction coefficient value, increasing to a peak and then achieving a steady state value, which is consistent with the reports of Carrasquero et al. [18] and Liu et al. [19]. Additionally, it can be seen that brittle fracture (Fig. 7(c)) takes place in No. 1 coating without REO when the sliding time is at the range of 200–300 s, as shown in Fig. 10(b)–(d). Furthermore, it is easy to find the brittle fracture starting, because during the friction process from the beginning to the first brittle fracture generating, the friction sound is mild and similar with that of machining cast iron, while when the brittle fracture occurs, a strident noise is heard and a large fluctuation can be seen in the friction curve as shown in Fig. 10(d). After the first brittle fracture, generally speaking, continuous brittle fracture will occur as shown in Fig. 10(b) and (c). However, there is no this kind of phenomenon generates in Figs. 11(b) and 12(b) corresponding to No. 4 and No. 5 two added REO coatings, respectively, moreover the other REO added coatings also does not have this kind of phenomenon. It is
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Fig. 10. Variation of friction coefficient as a function of sliding time for (a) No. 1 coating, (b) magnified view of (a), (c) magnified view of A marked in (b) and (d) magnified view of B marked in (c).
obvious that brittle fracture of REO added coatings is reduced during friction process, which results from refining effect of REO. Fig. 13 shows the variation of friction coefficient as a function of sprayed coatings and substrate. It is observed that only the friction coefficient of No. 4 REO added coating is
lower than that unadded coating, while approaching to a higher extent, the friction coefficient of other REO added coatings varies within the range of 0.69–0.64, which is almost at the same level with that of unadded one (0.66). Additionally, a trend that friction coefficient decreases as REO content increases takes place, is obvious seen in Fig. 13. While at
Fig. 11. Variation of friction coefficient as a function of sliding time for (a) No. 4 coating and (b) magnified view of (a).
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Fig. 12. Variation of friction coefficient as a function of sliding time for (a) No. 5 coating and (b) magnified view of (a).
Fig. 13. Variation of friction coefficient as a function of different coatings and substrate.
this applied load, the wear track is very shallow leading that the measurement of wear volume loss is very difficult. To compare the tribological property of sprayed coatings under high applied load, tribological property measurements under applied load of 30 N have been conducted at the same tribometer. The sliding distance is 6 mm, sliding speed 300 reci/min and duration 6 min. The variation of wear volume loss as a function of different coatings are shown in Fig. 14. It can be seen that wear resistance of REO added SPS coatings is largely enhanced, specially for No. 3, No. 4 and No. 8 coatings, whose wear volume loss is three orders magnitude smaller than that of unadded one. According to the experimental results, it can be drawn that the improvement of tribological property of REO to the SPS coatings under 30 N is largely resulted from the nature of REO that La2O3 has special hexagonal layered structure with better lubrication function [20], which can decrease the wear effectively, while at the load of 2 N the improvement is not obvious, as the lubrication function of REO is not obvious under very low load [12]. 4. Conclusions Through experimental verification, the contributions of REO to the Cr3C2–NiCr coatings can be concluded as follows: 1. REO can improve the surface average roughness effectively. 2. REO can enhance the average surface microhardness. 3. REO can reduce the fluctuation phenomenon of friction coefficient existing in the REO unadded coating, decrease the friction extent effectively and increase the wear resistance under high load largely, while at the vary low load, the improvement is not obvious. Acknowledgements
Fig. 14. Variation of wear volume loss as a function of different coatings.
The authors appreciate the financial support from the National Key Basic Research Program of China (Grant No. 2003CB716201) and the National Natural Science Foundation of China (Grant No. 50575121).
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