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Tribological behavior of NiCr-base blended and nanostructured composite APS coatings by rig test
Wear 265 (2008) 1565–1571
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Wear journal homepage: www.elsevier.com/locate/wear
Tribological behavior of NiCr-base blended and nanostructured composite APS coatings by rig test Jinhwan Cho a , Yuming Xiong a , Juneseob Kim a , Changhee Lee a,∗ , SoonYoung Hwang b a Kinetic Spray Coating Laboratory (NRL), Division of Materials Science & Engineering, College of Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, South Korea b Research Institute of Industrial Science & Technology (RIST), Pohang, South Korea
a r t i c l e
i n f o
Article history: Received 31 May 2007 Accepted 14 March 2008 Available online 9 May 2008 Keywords: NiCr–Cr2 O3 –Ag–BaF2 /CaF2 PS304 feedstock Nanostructured composite APS Tribological behavior Solid lubricants
a b s t r a c t In the present paper, a nanostructured composite powder containing nano-sized Cr2 O3 (<100 nm) is manufactured by spray drying and heat treatment. By wear and rig test at room temperature (RT), 200 ◦ C, and 350 ◦ C, respectively, the mechanical and tribological properties of atmospheric plasma spray (APS) blended (PS304) and composite coatings with same composition (60NiCr–20Cr2 O3 –10Ag–10BaF2 /CaF2 , wt.%) are compared. The results show that uniform microstructure of the composite coating could be obtained by eliminating the difference in physical and thermophysical properties of components in feedstock, which leads to the non-uniformity of blended coating due to their different in-flight trajectories of components in APS process. Therefore, the excellent wear resistance of composite coating benefits from its uniform and fine microstructure. Also, the wide and uniform distribution of plastic Ag lubricant could result in the formation of surface localized tribofilm as lubricant film to protect the coating from fracture and spallation. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, solid lubricant system, instead of conventional liquid-base lubricant system, has been widely used as lubrication in foil bearings, which are applied to refrigerator compressor, gas turbine engines, micro-turbines, pumps, and turbochargers due to their good lubrication and high thermochemical stability at the high temperatures [1–4]. In general, the coating on foil bearings plays an important role in wear and friction resistance. For wear-resistant aim, in 1994, NASA developed a blended PS304 feedstock, which consists of four components: 60 wt.% NiCr matrix, 20 wt.% Cr2 O3 for wear resistance, 10 wt.% Ag, and 10 wt.% eutectic BaF2 /CaF2 [5–6]. As a result, the APS PS304 coating could markedly increase the wear resistance of foil bearings. In this coating, silver (Ag) and fluorides (BaF2 /CaF2 ), work as low (RT ∼ 450 ◦ C) and high (400 ∼ 900 ◦ C) temperature solid lubricants, respectively [7–13]. Unfortunately, the non-uniform microstructure, which results from the differences in the physical and thermophysical properties of components in APS plume [14,15], would lower mechanical and tribological properties of PS304 coating in some harsh environments. In order to improve the microstructural uniformity of as-sprayed coatings, we proposed to replace the blended feedstock
∗ Corresponding author. Fax: +82 2 2299 0389. E-mail address: [email protected] (C. Lee). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.03.008
with a nanostructured composite one in a density dependent inflight particle trajectory viewpoint in our previous study [15]. In APS process of blended feedstock, each component reacts with hot gas dynamics in different way to form in-flight particle segregation within the mass flux. However, nanostructured composite particle can effectively diminish this segregation. The reaction only slightly occurs on the surface of larger composite particle which is added by nano-sized particle by spray drying method in advance. The results show that uniform coatings could be obtained by APS nanostructured composite feedstock. The coatings show excellent mechanical properties, such as high hardness and bond strength, and low porosity [15,16]. In this study, the tribological behavior of conventional blended PS304 and nanostructured composite APS coatings was evaluated by means of a rig tester which can simulate actual wear conditions. According to SEM and XRD analysis for the microstructural evolution of worn coatings the wear features of the two coatings were compared. 2. Experimental procedure 2.1. Feedstock The blended powder feedstock was manufactured by mechanical blending method. It consists of four components: Ni–20Cr (60 wt.%), Cr2 O3 (20 wt.%), Ag (10 wt.%), and eutectic BaF2 /CaF2
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Fig. 1. Characteristics of (a) blended feedstock with (b) constituent phases and (c) spray-dried composite feedstock with (d) constituent phases.
(10 wt.%). The morphologies of raw materials and blended powder are given in Fig. 1(a) and (b). It can be seen that NiCr particle is spherical, and others are irregular. Cr2 O3 is blocky and angular. Solid lubricants, Ag and BaF2 /CaF2 , particles are an aggregate of small round particles and a platelet-type shape, respectively. The size of the blended powder ranges −80 m + 30 m. The nanostructured composite feedstock was reconstituted by means of a spray drying method using slurry with the same composition as the above blended powder. Before spray drying, the size of the componential particles is less than 20 m. The slurry was prepared using NiCr–Cr2 O3 –Ag–BaF2 /CaF2 mixture with distilled water and dispersant solution by ball-milling with zirconia grinding media. The binder solution was added during the extra 3 h milling at 9000 rpm. The detailed slurry preparation and spray drying method are given elsewhere [15]. The morphologies of spray dried powder and raw materials are shown in Fig. 1(c) and (d). After spray drying, the composite particle is spherical with a size of −80 + 20 m. From Fig. 1(c), the constituents (nano-sized Cr2 O3 , Ag, and BaF2 /CaF2 ) are randomly distributed in NiCr-base. Additionally, in order to prevent the spray dried powder from breaking and cracking, heat treatment at 960 ◦ C for 2 h under an argon environment was used to release the thermal stress and for homogenization. During heat treatment, melted silver percolated inward along the
internal boundary of the spray dried composite particle. After heat treatment, the powder was then cooled at a rate of 5 min−1 to room temperature. 2.2. Coating preparation and characterization The two coatings (blended and nanostructured composite) were deposited by an atmospheric plasma spray system (Sulzer Metco 9 MB). An AISI 304 stainless steel (5 cm × 7 cm × 0.5 cm) after grit blasting and cleaning was used as substrate. The spraying distance between the substrate and nozzle was 100 mm. The mixture of argon gas (110 SCFH) and hydrogen gas (5 SCFH) was used as plasma process gas. The powder-feeding rate was fixed as 30 g/min by argon gas. The powder was injected along the vertical direction to plasma jet. The details of plasma spray process parameters are given in Table 1. Table 1 Coating process parameters Plasma gas Ar (SCFH)
H2 (SCFH)
110
5
Arc current (A)
Spraying distance (mm)
Powder feeding rate (gm−1 )
500
100
30
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Fig. 2. (a) Rig test tribotester, (b) friction current variation at start-up, lift, shut-down, and stationary state during one cycle test at RT and 350 ◦ C, and (c) schematic method of mounting coated journal to foil (foil bearing).
The microstructure and composition of coatings were characterized by scanning electron microscopy (SEM) equipped with energy dispersion spectroscopy (EDS) before and after test. Porosity was measured using an image analysis method by calculating the cross-sectional area fraction of pores in unit area of coating according to the images using Image Pro-Plus software. Vickers microhardness was obtained on the polished cross-sectional coatings using a Vickers indenter with a load of 2.942 N. Bond strength was measured using a Romulus Bond Strength Tester. An epoxy overlaid stud with a diameter of 2.7 mm was attached to the as-polished coating and cured at 150 ◦ C for 90 min. The wear test was conducted using pin-on-disk-type dry sliding wear tester. The counterpart was an Inconel 718 pin (5 mm in diameter × 100 mm in length). As-coated specimens and counterpart were polished with 600# grit silicon carbide abrasive paper before test. Then, pin-on-disk test had been performed for 1 h with a load of 47 N and a sliding distance of 430 m. The mean sliding speed was 0.12 m/s, and the test sliding temperatures were RT (room temperature) and 200 ◦ C. Thus, the wear resistance of coatings is evaluated by the friction coefficient and weight loss of specimens during test. The friction coefficient could be calculated by dividing the friction force by the applied normal load during test. The weight loss was obtained by the difference in mass of the specimen before and after test.
2.3. Rig test The tribological behavior of the coatings was also evaluated by a rig test which could be used to simulate actual application conditions. The coated rotating journal (SCM440) was finished to less than 0.1 m roughness through mechanical grinding before test. The counterparts (foil) are made of hardened HRC (35–40) Inconel 718 (55 mm × 35 mm × 0.15 mm). Fig. 2(c) shows the method of mounting foil to coated journal. The journal was pressed with a total load of 25.48 N against the foil counterpart (foil). One test cycle consists of four states: startup (3s), lift (1s), shutdown (3s), and stationary state (23s), as schematically shown in Fig. 2(b). The maximum rotating speed is up to 6000 rpm, and the test temperatures are RT and 350 ◦ C. The maximum test cycle is up to 10,000. The frictional torque for calculating the tangential load between journal and foil could be obtained from power loss and friction force, and then friction current was automatically computerized from a data acquisition system. During test, if the friction current is above 15 A, the rig tester should be stopped automatically. This critical value was chosen based on the observation that when the friction current was higher than 15 A, the friction coefficient was more than 0.5, and numerous wear debris formed on the surface resulting from the increase in friction force between journal and foil. Besides, the weight loss was also obtained by journal wear track depth, which would be measured after each 500-cycle test. Also, the tester should be com-
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Fig. 3. Cross-sectional SEM back-scatter electron images (BSE) and phase distribution of as-sprayed coatings: (a) and (b) blended (A, NiCr; B, Cr2 O3 ; C, Ag; D, BaF2 /CaF2 ), and (c) and (d) nano-structured composite (A, NiCr; B, Cr2 O3 , C, Ag; D, BaF2 /CaF2 ).
pulsorily interrupted when wear track depth of journal surpasses 30 m.
3. Results and discussions 3.1. Coating microstructure Fig. 3 shows cross-sectional SEM back-scatter electron images of blended and nanostructured composite as-sprayed coatings. The microstructure of blended coating shows typical characteristics of coarse lamellars and non-uniform structure (Fig. 3(a)). The black regions are pores and cracks, the dark gray regions are mainly composed of Cr2 O3 , the bright gray regions are NiCr matrix phase, the white regions are the Ag phase, and the eutectic BaF2 /CaF2 is always codeposited with Cr2 O3 (region marked as D). Cracks and porosity occurred at the boundaries between Cr2 O3 hard phase and metal phase. In addition, the non-homogeneous microstructure is induced by different trajectories during particle flight in the plasma plume due to the difference in density, size, viscosity and melting point of powder components. For example, Cr2 O3 ( = 5.21 g cm−3 ) and eutectic fluorides ( = 4.01 g cm−3 ) could be deposited in the same layer, but, the trajectories of Ag ( = 10.5 g cm−3 ) and NiCr ( = 8.9 g cm−3 ) particles overlap and are separated from those of Cr2 O3 and fluorides due to the difference in density. Also, the phase fraction of low melting phases (such
as NiCr = 1726 K, Ag = 1235 K, BaF2 /CaF2 < 1323 K) is relatively lower than that of Cr2 O3 (high melting point, 2708 K) in coatings by APS with the same gas enthalpy. This is consistent with the physics of momentum transfer between gas dynamics and particle properties. Nanostructured composite coating (Fig. 3(c) and (d)) has a uniform and fine microstructure, and low porosity in comparison with the blended one. The micro-cracks in the composite coating are always terminated within one or two splats. It reveals excellent fracture resistance of the composite coating due to the uniform distribution of components. Thus, it is expected that the composite coating with nanostructured and fine lamellar microstructures show the better mechanical and tribological properties than the coarse and non-uniform blended coating. Table 2 summarizes some relative properties of the two coatings. Nanostructured composite coating has higher microhardness and bond strength than blended one. According to wear test, the friction coefficient and weight loss of blended coating is larger than that of composite coating at the test temperatures (RT and 200 ◦ C). It indicates that wear resistance as well as the mechanical properties is markedly improved by uniformity of nanostructured composite coating. 3.2. Tribological behavior In order to simulate the actual applications of coatings, the tribological behavior was also evaluated by a rig test, which could assess the friction stability and wear state of rotating coated journal
Table 2 Comparison of properties of blended and nanostructured composite coatings Coating
Porosity (%)
Bond strength (kg/cm2 )
Microhardness (HV 300)
Friction coefficient/weight loss (g) RT 200 ◦ C
Blended Composite
6.1 2.3
105.5 242.69
128 289
0.36–0.55/0.45 0.35–0.51/0.42 0.24–0.32/0.38 0.30–0.35/0.40
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Fig. 4. Friction current of APS coatings during rig test.
against foil counterpart for up to 10,000-cycle. The friction current changes at the four test stages (as shown in Fig. 2(b)). It goes up to the maximum at start-up state due to the start from contact between journal and foil (as shown in Fig. 2(c)), and then decreases gradually in lift period when the journal rotates normally. It increases again in shutdown period till the journal increases the contact area with foil and stops. Finally, it comes back 5–6 A during the stationary state due to basically driving electrical current. Fig. 4 shows the friction current variation during rig test of the two coatings at RT and 350 ◦ C, respectively. The friction current would be measured at every 500-cycle up to 10,000-cycle by the computer system. At a few initial cycles, the friction currents of the two coatings are about 5–6 A. However, for blended coating at 350 ◦ C, the friction current increases linearly to about 9–11 A after 500-cycle, and then above 15 A after 880-cycle. Its final wear track depth is more than 30 m. Especially, the friction current of blended coating at RT sharply increases above 15 A after only 48cycle. Hence, the blended coating has failed because both friction current and wear track depth are larger than the critical values (15 A and 30 m), respectively. Nevertheless, as given in Table 3, both friction current and wear track depth of the nanostructured composite coating keep 5–8 A and less than 10 m, respectively during 10,000-cycle test, regardless of test temperature (RT and 350 ◦ C). In addition, one can find a slight decrease of friction current after 1000-cycle rig test. This is beneficial from the lubrication of solid lubricants. The surface morphologies of the two coatings after rig test are shown in Fig. 5. Worn and rough surface on blended coating after test at the two temperatures could be observed. Meanwhile, the worn surface of nanostructured composite coating remains smooth with very few shallow scratches. 3.3. Solid lubricants effect Fig. 6 shows the SEM surface images of coatings after rig test at 350 ◦ C, as well as silver mapping and phase fraction on the worn surfaces. Ag solid lubricant could increase the wear and fric-
Fig. 5. Surfaces of (a) blended coating after 880-cycle and (b) nanostructured composite coating after 10,000-cycle rig test at 350 ◦ C.
tion resistance of coatings at 350 ◦ C because of its ductility, which can plastically shear between the two sliding surfaces and then provides a thin lubricant film [7]. As shown in Fig. 6(a), irregular and poor distribution of Ag on worn surface of blended coating can be seen by Ag mapping, and the silver fraction is about 7% (Fig. 6(b)), which is lower than the original value in feedstock to induce the deep stick-slip tracks and high friction force. The low Ag content in the coating may result from the loss accompanying with the formation of debris during test, besides the evaporation of in-flight Ag particles in high temperature plasma stream during coating preparation process. Therefore, no lubricant film can be found on the worn surface due to the insufficient supply of Ag solid lubricant. Then, a rough surface with deep wear tracks and some spallations along the sliding direction can be observed (as shown in Fig. 6(a)). Under the friction force, the localized shear stress induced by friction heat would result in fracture and spallation of coating at the weak regions, such as micro-cracks, pores, and hard-Cr2 O3 /soft-matrix-splat boundary. However, nanostructured composite coating shows a good wear resistance and tribological behavior due to a wide Ag distribution on the worn surface according to the mapping (Fig. 6(c)). It is beneficial for the uniform distribution of sufficient Ag in the coating (Fig. 6(d)) to improve the friction and wear resistance by holding back fracture and spallation during test. Furthermore, a tribofilm can be locally formed on the composite coating as lubricant film between the two contact surfaces during test. Generally, it is difficult to maintain adequate bonding with silver due to the formation of an easily transferred film on the counterpart surface [7]. Thus, the transferred Ag tribofilm on the counterpart surface of Inconel 718 (foil) can be detected, as shown in Fig. 7. The Ag tribofilm contents (area fraction) on counterparts of both blended and composite coatings are 5.4% and 9%, respectively, which are different from the solid lubricant contents on the worn
Table 3 Rig test results of blended and nanostructured composite coatings Coating
Blended Blended Composite Composite
Foil bearing Journal
Foil
1 1 1 1
8 8 8 8
Test temperature
Accumulated rig cycles
Friction current (A)
Wear depth
Debris
RT 350 ◦ C RT 350 ◦ C
48 880 Up to 10,000 Up to 10,000
Final 16 A Final 16 A 6∼8 A 5∼7 A
5 m >30 m <15 m <10 m
Much Much Few Few
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Fig. 6. Worn surface with Ag mapping, and phase fraction (wt.%) on (a and b) blended and (c and d) nanostructured composite coatings.
Fig. 7. SEM morphologies of transferred tribofilm and Ag mapping on the counterpart (foil) surface of (a) blended and (b) nanostructured composite coatings, and (c) Ag tribofilm content (area fraction) on the two surfaces.
surface of the corresponding coatings. The tribofilm on the counterpart of blended coating is poor and discontinuous. Also, much debris induced by spallation and sticking is generated at uncovered solid lubricant regions. On the other hand, the counterpart surface
of the nanostructured composite coating shows a dispersed and continuous distribution of Ag tribofilm which benefits to wear and friction resistance, as shown in Fig. 7b. In addition, it is worth noting that wear resistance of the composite coating at 350 ◦ C is slightly
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increased in comparison with at RT due to the enhanced lubrication effect (Fig. 4). At 350 ◦ C, the fluorides (BaF2 /CaF2 ) begin to work as lubricants with the slight increase of friction temperature between the sliding surfaces. 4. Conclusions By spray drying method, the APS nanostructured composite coating shows a uniform and fine microstructure in comparison with the conventional non-uniform structural blended coating. Thus, the composite coating shows better mechanical and tribological properties than the conventional one, such as microhardness, toughness, strength, and wear/friction resistance. The existence of weak regions, such as pores, cracks, Cr2 O3 /matrix boundary, results in the low wear resistance of blended coating. No tribofilm forms on the coating due to the evaporation and loss of Ag lubricant with the formation of debris. However, the excellent wear resistance of nanostructured composite coating benefits from the uniform microstructure and wide distribution of solid lubricants on the worn surface during test. The formation of tribofilm resulting from the sufficient lubricants supply could protect the composite coating from fracture and spallation. Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (No. 2006-02289). References [1] H. Heshmat, P. Hryniewicz, J.F. Walton II, J.P. Willis, S. Jahanmir, C. Dellacorte, Low-friction wear-resistant coatings for high temperature foil bearings, Trib. Int. 38 (2005) 1059–1075.
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[2] I.M. Allam, Solid lubricants for applications at elevated temperatures, J. Mater. Sci. 26 (1991) 3977–3984. [3] C. DellaCorte, The effects of substrate material and thermal processing atmosphere on the strength of PS304: a high temperature solid lubricant coating, NASA TM-211483, 2002. [4] C.M. Han, H.S. Choi, C.H. Lee, H.J. Kim, S.Y. Hwang, Evaluation of solid lubricant coatings for low friction and wear resistance using the atmospheric plasma spray, Mater. Sci. Forum 449–452 (2004) 777–780. [5] C. DellaCorte, B.J. Edmonds, Preliminary evaluation of PS300: a new selflubricating high temperature composite coating for use to 800 ◦ C, NASA TM-107056, 1996. [6] C. DellaCorte, B.J. Edmonds, Self-lubricating composite containing chromium oxide, US Patent No. 5,866,518 (1999). [7] M. K. Stanford, C. Dellacorte, D. Eylon, Particle morphology effects on flow characteristics of PS304 Plasma Spray Coating feedstock powder blend, NASA TM-211206 (2002). [8] A. Erdemir, R.A. Erck, G.R. Fenske, H. Hong, Solid/liquid lubrication of ceramics at elevated temperatures, Wear 203–204 (1997) 588–595. [9] H.E. Sliney, The use of silver in self-lubricating coatings for extreme temperatures, ASLE Trans. 29 (1986) 370–376. [10] N. Marechal, E. Quesnel, P. Juliet, Y. Pauleau, Sputter deposition and properties of silver lubricant thin films for use to 500 ◦ C, in: Proceedings of 8th CIMTEC-World Ceramics Congress and Forum on New Materials, 1995, pp. 551– 558. [11] H.E. Sliney, T.N. Strom, G.P. Allen, Fluoride solid lubricants for extreme temperatures and corrosive environments, ASLE Trans. 8 (1965) 307– 322. [12] M.C. Jeng, Y.L. Soong, Wear behavior of solid lubricants Ag and BaF2 –CaF2 obtained by laser surface cladding, Wear behaviour of solid lubricants Ag and BaF2 –CaF2 obtained by laser surface cladding, Surf. Coat. Technol. 57 (1993) 145–150. [13] W. Wang, Application of a high temperature self-lubricating composite coating on steamturbine components, Surf. Coat. Technol. 177–178 (2004) 12–17. [14] G. Kim, H. Choi, C. Han, S. Uhm, C. Lee, Characterization of atmospheric plasma spray NiCr–Cr2 O3 –Ag–CaF2 /BaF2 coatings, Surf. Coat. Technol. 195 (2005) 107–115. [15] H. Yang, C. Lee, S.Y. Hwang, The effect of nano-sized Cr2 O3 addition on the characteristics of NiCr–Cr2 O3 –Ag–BaF2 /CaF2 coating, Surf. Coat. Technol. 201 (2006) 38–44. [16] A.A. Voevodin, J.S. Zabinski, Nanocomposite and nanostructured tribological materials for space applications, Comp. Sci. Technol. 65 (2005) 741– 748.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Tribological behavior of NiCr-base blended and nanostructured composite APS coatings by rig test Jinhwan Cho a , Yuming Xiong a , Juneseob Kim a , Changhee Lee a,∗ , SoonYoung Hwang b a Kinetic Spray Coating Laboratory (NRL), Division of Materials Science & Engineering, College of Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, South Korea b Research Institute of Industrial Science & Technology (RIST), Pohang, South Korea
a r t i c l e
i n f o
Article history: Received 31 May 2007 Accepted 14 March 2008 Available online 9 May 2008 Keywords: NiCr–Cr2 O3 –Ag–BaF2 /CaF2 PS304 feedstock Nanostructured composite APS Tribological behavior Solid lubricants
a b s t r a c t In the present paper, a nanostructured composite powder containing nano-sized Cr2 O3 (<100 nm) is manufactured by spray drying and heat treatment. By wear and rig test at room temperature (RT), 200 ◦ C, and 350 ◦ C, respectively, the mechanical and tribological properties of atmospheric plasma spray (APS) blended (PS304) and composite coatings with same composition (60NiCr–20Cr2 O3 –10Ag–10BaF2 /CaF2 , wt.%) are compared. The results show that uniform microstructure of the composite coating could be obtained by eliminating the difference in physical and thermophysical properties of components in feedstock, which leads to the non-uniformity of blended coating due to their different in-flight trajectories of components in APS process. Therefore, the excellent wear resistance of composite coating benefits from its uniform and fine microstructure. Also, the wide and uniform distribution of plastic Ag lubricant could result in the formation of surface localized tribofilm as lubricant film to protect the coating from fracture and spallation. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, solid lubricant system, instead of conventional liquid-base lubricant system, has been widely used as lubrication in foil bearings, which are applied to refrigerator compressor, gas turbine engines, micro-turbines, pumps, and turbochargers due to their good lubrication and high thermochemical stability at the high temperatures [1–4]. In general, the coating on foil bearings plays an important role in wear and friction resistance. For wear-resistant aim, in 1994, NASA developed a blended PS304 feedstock, which consists of four components: 60 wt.% NiCr matrix, 20 wt.% Cr2 O3 for wear resistance, 10 wt.% Ag, and 10 wt.% eutectic BaF2 /CaF2 [5–6]. As a result, the APS PS304 coating could markedly increase the wear resistance of foil bearings. In this coating, silver (Ag) and fluorides (BaF2 /CaF2 ), work as low (RT ∼ 450 ◦ C) and high (400 ∼ 900 ◦ C) temperature solid lubricants, respectively [7–13]. Unfortunately, the non-uniform microstructure, which results from the differences in the physical and thermophysical properties of components in APS plume [14,15], would lower mechanical and tribological properties of PS304 coating in some harsh environments. In order to improve the microstructural uniformity of as-sprayed coatings, we proposed to replace the blended feedstock
∗ Corresponding author. Fax: +82 2 2299 0389. E-mail address: [email protected] (C. Lee). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.03.008
with a nanostructured composite one in a density dependent inflight particle trajectory viewpoint in our previous study [15]. In APS process of blended feedstock, each component reacts with hot gas dynamics in different way to form in-flight particle segregation within the mass flux. However, nanostructured composite particle can effectively diminish this segregation. The reaction only slightly occurs on the surface of larger composite particle which is added by nano-sized particle by spray drying method in advance. The results show that uniform coatings could be obtained by APS nanostructured composite feedstock. The coatings show excellent mechanical properties, such as high hardness and bond strength, and low porosity [15,16]. In this study, the tribological behavior of conventional blended PS304 and nanostructured composite APS coatings was evaluated by means of a rig tester which can simulate actual wear conditions. According to SEM and XRD analysis for the microstructural evolution of worn coatings the wear features of the two coatings were compared. 2. Experimental procedure 2.1. Feedstock The blended powder feedstock was manufactured by mechanical blending method. It consists of four components: Ni–20Cr (60 wt.%), Cr2 O3 (20 wt.%), Ag (10 wt.%), and eutectic BaF2 /CaF2
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Fig. 1. Characteristics of (a) blended feedstock with (b) constituent phases and (c) spray-dried composite feedstock with (d) constituent phases.
(10 wt.%). The morphologies of raw materials and blended powder are given in Fig. 1(a) and (b). It can be seen that NiCr particle is spherical, and others are irregular. Cr2 O3 is blocky and angular. Solid lubricants, Ag and BaF2 /CaF2 , particles are an aggregate of small round particles and a platelet-type shape, respectively. The size of the blended powder ranges −80 m + 30 m. The nanostructured composite feedstock was reconstituted by means of a spray drying method using slurry with the same composition as the above blended powder. Before spray drying, the size of the componential particles is less than 20 m. The slurry was prepared using NiCr–Cr2 O3 –Ag–BaF2 /CaF2 mixture with distilled water and dispersant solution by ball-milling with zirconia grinding media. The binder solution was added during the extra 3 h milling at 9000 rpm. The detailed slurry preparation and spray drying method are given elsewhere [15]. The morphologies of spray dried powder and raw materials are shown in Fig. 1(c) and (d). After spray drying, the composite particle is spherical with a size of −80 + 20 m. From Fig. 1(c), the constituents (nano-sized Cr2 O3 , Ag, and BaF2 /CaF2 ) are randomly distributed in NiCr-base. Additionally, in order to prevent the spray dried powder from breaking and cracking, heat treatment at 960 ◦ C for 2 h under an argon environment was used to release the thermal stress and for homogenization. During heat treatment, melted silver percolated inward along the
internal boundary of the spray dried composite particle. After heat treatment, the powder was then cooled at a rate of 5 min−1 to room temperature. 2.2. Coating preparation and characterization The two coatings (blended and nanostructured composite) were deposited by an atmospheric plasma spray system (Sulzer Metco 9 MB). An AISI 304 stainless steel (5 cm × 7 cm × 0.5 cm) after grit blasting and cleaning was used as substrate. The spraying distance between the substrate and nozzle was 100 mm. The mixture of argon gas (110 SCFH) and hydrogen gas (5 SCFH) was used as plasma process gas. The powder-feeding rate was fixed as 30 g/min by argon gas. The powder was injected along the vertical direction to plasma jet. The details of plasma spray process parameters are given in Table 1. Table 1 Coating process parameters Plasma gas Ar (SCFH)
H2 (SCFH)
110
5
Arc current (A)
Spraying distance (mm)
Powder feeding rate (gm−1 )
500
100
30
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Fig. 2. (a) Rig test tribotester, (b) friction current variation at start-up, lift, shut-down, and stationary state during one cycle test at RT and 350 ◦ C, and (c) schematic method of mounting coated journal to foil (foil bearing).
The microstructure and composition of coatings were characterized by scanning electron microscopy (SEM) equipped with energy dispersion spectroscopy (EDS) before and after test. Porosity was measured using an image analysis method by calculating the cross-sectional area fraction of pores in unit area of coating according to the images using Image Pro-Plus software. Vickers microhardness was obtained on the polished cross-sectional coatings using a Vickers indenter with a load of 2.942 N. Bond strength was measured using a Romulus Bond Strength Tester. An epoxy overlaid stud with a diameter of 2.7 mm was attached to the as-polished coating and cured at 150 ◦ C for 90 min. The wear test was conducted using pin-on-disk-type dry sliding wear tester. The counterpart was an Inconel 718 pin (5 mm in diameter × 100 mm in length). As-coated specimens and counterpart were polished with 600# grit silicon carbide abrasive paper before test. Then, pin-on-disk test had been performed for 1 h with a load of 47 N and a sliding distance of 430 m. The mean sliding speed was 0.12 m/s, and the test sliding temperatures were RT (room temperature) and 200 ◦ C. Thus, the wear resistance of coatings is evaluated by the friction coefficient and weight loss of specimens during test. The friction coefficient could be calculated by dividing the friction force by the applied normal load during test. The weight loss was obtained by the difference in mass of the specimen before and after test.
2.3. Rig test The tribological behavior of the coatings was also evaluated by a rig test which could be used to simulate actual application conditions. The coated rotating journal (SCM440) was finished to less than 0.1 m roughness through mechanical grinding before test. The counterparts (foil) are made of hardened HRC (35–40) Inconel 718 (55 mm × 35 mm × 0.15 mm). Fig. 2(c) shows the method of mounting foil to coated journal. The journal was pressed with a total load of 25.48 N against the foil counterpart (foil). One test cycle consists of four states: startup (3s), lift (1s), shutdown (3s), and stationary state (23s), as schematically shown in Fig. 2(b). The maximum rotating speed is up to 6000 rpm, and the test temperatures are RT and 350 ◦ C. The maximum test cycle is up to 10,000. The frictional torque for calculating the tangential load between journal and foil could be obtained from power loss and friction force, and then friction current was automatically computerized from a data acquisition system. During test, if the friction current is above 15 A, the rig tester should be stopped automatically. This critical value was chosen based on the observation that when the friction current was higher than 15 A, the friction coefficient was more than 0.5, and numerous wear debris formed on the surface resulting from the increase in friction force between journal and foil. Besides, the weight loss was also obtained by journal wear track depth, which would be measured after each 500-cycle test. Also, the tester should be com-
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Fig. 3. Cross-sectional SEM back-scatter electron images (BSE) and phase distribution of as-sprayed coatings: (a) and (b) blended (A, NiCr; B, Cr2 O3 ; C, Ag; D, BaF2 /CaF2 ), and (c) and (d) nano-structured composite (A, NiCr; B, Cr2 O3 , C, Ag; D, BaF2 /CaF2 ).
pulsorily interrupted when wear track depth of journal surpasses 30 m.
3. Results and discussions 3.1. Coating microstructure Fig. 3 shows cross-sectional SEM back-scatter electron images of blended and nanostructured composite as-sprayed coatings. The microstructure of blended coating shows typical characteristics of coarse lamellars and non-uniform structure (Fig. 3(a)). The black regions are pores and cracks, the dark gray regions are mainly composed of Cr2 O3 , the bright gray regions are NiCr matrix phase, the white regions are the Ag phase, and the eutectic BaF2 /CaF2 is always codeposited with Cr2 O3 (region marked as D). Cracks and porosity occurred at the boundaries between Cr2 O3 hard phase and metal phase. In addition, the non-homogeneous microstructure is induced by different trajectories during particle flight in the plasma plume due to the difference in density, size, viscosity and melting point of powder components. For example, Cr2 O3 ( = 5.21 g cm−3 ) and eutectic fluorides ( = 4.01 g cm−3 ) could be deposited in the same layer, but, the trajectories of Ag ( = 10.5 g cm−3 ) and NiCr ( = 8.9 g cm−3 ) particles overlap and are separated from those of Cr2 O3 and fluorides due to the difference in density. Also, the phase fraction of low melting phases (such
as NiCr = 1726 K, Ag = 1235 K, BaF2 /CaF2 < 1323 K) is relatively lower than that of Cr2 O3 (high melting point, 2708 K) in coatings by APS with the same gas enthalpy. This is consistent with the physics of momentum transfer between gas dynamics and particle properties. Nanostructured composite coating (Fig. 3(c) and (d)) has a uniform and fine microstructure, and low porosity in comparison with the blended one. The micro-cracks in the composite coating are always terminated within one or two splats. It reveals excellent fracture resistance of the composite coating due to the uniform distribution of components. Thus, it is expected that the composite coating with nanostructured and fine lamellar microstructures show the better mechanical and tribological properties than the coarse and non-uniform blended coating. Table 2 summarizes some relative properties of the two coatings. Nanostructured composite coating has higher microhardness and bond strength than blended one. According to wear test, the friction coefficient and weight loss of blended coating is larger than that of composite coating at the test temperatures (RT and 200 ◦ C). It indicates that wear resistance as well as the mechanical properties is markedly improved by uniformity of nanostructured composite coating. 3.2. Tribological behavior In order to simulate the actual applications of coatings, the tribological behavior was also evaluated by a rig test, which could assess the friction stability and wear state of rotating coated journal
Table 2 Comparison of properties of blended and nanostructured composite coatings Coating
Porosity (%)
Bond strength (kg/cm2 )
Microhardness (HV 300)
Friction coefficient/weight loss (g) RT 200 ◦ C
Blended Composite
6.1 2.3
105.5 242.69
128 289
0.36–0.55/0.45 0.35–0.51/0.42 0.24–0.32/0.38 0.30–0.35/0.40
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Fig. 4. Friction current of APS coatings during rig test.
against foil counterpart for up to 10,000-cycle. The friction current changes at the four test stages (as shown in Fig. 2(b)). It goes up to the maximum at start-up state due to the start from contact between journal and foil (as shown in Fig. 2(c)), and then decreases gradually in lift period when the journal rotates normally. It increases again in shutdown period till the journal increases the contact area with foil and stops. Finally, it comes back 5–6 A during the stationary state due to basically driving electrical current. Fig. 4 shows the friction current variation during rig test of the two coatings at RT and 350 ◦ C, respectively. The friction current would be measured at every 500-cycle up to 10,000-cycle by the computer system. At a few initial cycles, the friction currents of the two coatings are about 5–6 A. However, for blended coating at 350 ◦ C, the friction current increases linearly to about 9–11 A after 500-cycle, and then above 15 A after 880-cycle. Its final wear track depth is more than 30 m. Especially, the friction current of blended coating at RT sharply increases above 15 A after only 48cycle. Hence, the blended coating has failed because both friction current and wear track depth are larger than the critical values (15 A and 30 m), respectively. Nevertheless, as given in Table 3, both friction current and wear track depth of the nanostructured composite coating keep 5–8 A and less than 10 m, respectively during 10,000-cycle test, regardless of test temperature (RT and 350 ◦ C). In addition, one can find a slight decrease of friction current after 1000-cycle rig test. This is beneficial from the lubrication of solid lubricants. The surface morphologies of the two coatings after rig test are shown in Fig. 5. Worn and rough surface on blended coating after test at the two temperatures could be observed. Meanwhile, the worn surface of nanostructured composite coating remains smooth with very few shallow scratches. 3.3. Solid lubricants effect Fig. 6 shows the SEM surface images of coatings after rig test at 350 ◦ C, as well as silver mapping and phase fraction on the worn surfaces. Ag solid lubricant could increase the wear and fric-
Fig. 5. Surfaces of (a) blended coating after 880-cycle and (b) nanostructured composite coating after 10,000-cycle rig test at 350 ◦ C.
tion resistance of coatings at 350 ◦ C because of its ductility, which can plastically shear between the two sliding surfaces and then provides a thin lubricant film [7]. As shown in Fig. 6(a), irregular and poor distribution of Ag on worn surface of blended coating can be seen by Ag mapping, and the silver fraction is about 7% (Fig. 6(b)), which is lower than the original value in feedstock to induce the deep stick-slip tracks and high friction force. The low Ag content in the coating may result from the loss accompanying with the formation of debris during test, besides the evaporation of in-flight Ag particles in high temperature plasma stream during coating preparation process. Therefore, no lubricant film can be found on the worn surface due to the insufficient supply of Ag solid lubricant. Then, a rough surface with deep wear tracks and some spallations along the sliding direction can be observed (as shown in Fig. 6(a)). Under the friction force, the localized shear stress induced by friction heat would result in fracture and spallation of coating at the weak regions, such as micro-cracks, pores, and hard-Cr2 O3 /soft-matrix-splat boundary. However, nanostructured composite coating shows a good wear resistance and tribological behavior due to a wide Ag distribution on the worn surface according to the mapping (Fig. 6(c)). It is beneficial for the uniform distribution of sufficient Ag in the coating (Fig. 6(d)) to improve the friction and wear resistance by holding back fracture and spallation during test. Furthermore, a tribofilm can be locally formed on the composite coating as lubricant film between the two contact surfaces during test. Generally, it is difficult to maintain adequate bonding with silver due to the formation of an easily transferred film on the counterpart surface [7]. Thus, the transferred Ag tribofilm on the counterpart surface of Inconel 718 (foil) can be detected, as shown in Fig. 7. The Ag tribofilm contents (area fraction) on counterparts of both blended and composite coatings are 5.4% and 9%, respectively, which are different from the solid lubricant contents on the worn
Table 3 Rig test results of blended and nanostructured composite coatings Coating
Blended Blended Composite Composite
Foil bearing Journal
Foil
1 1 1 1
8 8 8 8
Test temperature
Accumulated rig cycles
Friction current (A)
Wear depth
Debris
RT 350 ◦ C RT 350 ◦ C
48 880 Up to 10,000 Up to 10,000
Final 16 A Final 16 A 6∼8 A 5∼7 A
5 m >30 m <15 m <10 m
Much Much Few Few
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Fig. 6. Worn surface with Ag mapping, and phase fraction (wt.%) on (a and b) blended and (c and d) nanostructured composite coatings.
Fig. 7. SEM morphologies of transferred tribofilm and Ag mapping on the counterpart (foil) surface of (a) blended and (b) nanostructured composite coatings, and (c) Ag tribofilm content (area fraction) on the two surfaces.
surface of the corresponding coatings. The tribofilm on the counterpart of blended coating is poor and discontinuous. Also, much debris induced by spallation and sticking is generated at uncovered solid lubricant regions. On the other hand, the counterpart surface
of the nanostructured composite coating shows a dispersed and continuous distribution of Ag tribofilm which benefits to wear and friction resistance, as shown in Fig. 7b. In addition, it is worth noting that wear resistance of the composite coating at 350 ◦ C is slightly
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increased in comparison with at RT due to the enhanced lubrication effect (Fig. 4). At 350 ◦ C, the fluorides (BaF2 /CaF2 ) begin to work as lubricants with the slight increase of friction temperature between the sliding surfaces. 4. Conclusions By spray drying method, the APS nanostructured composite coating shows a uniform and fine microstructure in comparison with the conventional non-uniform structural blended coating. Thus, the composite coating shows better mechanical and tribological properties than the conventional one, such as microhardness, toughness, strength, and wear/friction resistance. The existence of weak regions, such as pores, cracks, Cr2 O3 /matrix boundary, results in the low wear resistance of blended coating. No tribofilm forms on the coating due to the evaporation and loss of Ag lubricant with the formation of debris. However, the excellent wear resistance of nanostructured composite coating benefits from the uniform microstructure and wide distribution of solid lubricants on the worn surface during test. The formation of tribofilm resulting from the sufficient lubricants supply could protect the composite coating from fracture and spallation. Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (No. 2006-02289). References [1] H. Heshmat, P. Hryniewicz, J.F. Walton II, J.P. Willis, S. Jahanmir, C. Dellacorte, Low-friction wear-resistant coatings for high temperature foil bearings, Trib. Int. 38 (2005) 1059–1075.
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