Preparation and tribological properties of plasma sprayed nano and micro-structure alumina-reinforced CuAl composite coatings

Preparation and tribological properties of plasma sprayed nano and micro-structure alumina-reinforced CuAl composite coatings

Tribology International 101 (2016) 255–263 Contents lists available at ScienceDirect Tribology International journal homepage: www.elsevier.com/loca...

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Tribology International 101 (2016) 255–263

Contents lists available at ScienceDirect

Tribology International journal homepage: www.elsevier.com/locate/triboint

Preparation and tribological properties of plasma sprayed nano and micro-structure alumina-reinforced CuAl composite coatings Xiaoqin Zhao, Yulong An n, Guoliang Hou, Huidi Zhou, Jianmin Chen State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People's Republic of China

art ic l e i nf o

a b s t r a c t

Article history: Received 24 December 2015 Received in revised form 26 April 2016 Accepted 26 April 2016 Available online 27 April 2016

Nano-structured alumina (n-Al2O3) powders prepared by spray drying granulation and micro-structured alumina (m-Al2O3) powders by fused-crushed method were used to fabricate Al2O3 reinforced CuAl coatings. The microstructure and phase composition of the coatings were analyzed, and their tribological properties were investigated in comparison with those of the CuAl coating. In both CuAl/n-Al2O3 and CuAl/m-Al2O3 coatings, the reinforcing Al2O3 exists as well-flattened splats, and shows good interfacial compatibility with the bronze matrix. The alumina existed as amorphous and γ-Al2O3 phase in the CuAl/ m-Al2O3 composite coating, while α-Al2O3 phase was contained in the CuAl/n-Al2O3 coating. Although the CuAl/n-Al2O3 composite coating gives a highest friction coefficient at all loads, it possesses superior wear-resistance as compared with the CuAl/m-Al2O3 coating and pure CuAl coating. & 2016 Elsevier Ltd. All rights reserved.

Keywords: CuAl composite coating Alumina-reinforced Plasma spray Tribological properties

1. Introduction Plasma sprayed aluminum-bronze (CuAl) coatings are widely used in frictional components, such as piston guide, hydraulic press sleeve, shifter fork, and brake drum, due to their good friction-reducing property and high strength [1–4]. Common CuAl alloy contains up to 15% of Al that can form intermetallic compounds with copper matrix to enhance the alloy strength and meanwhile retain the low shear strength of copper to benefit the friction-reducing performance [5,6]. However, the limited hardness inherently in bronze alloys makes them liable to wear [7]. It has been reported that the reinforcement phase (mainly ceramics) in the bronze matrix can be effective on improving the wearresistance of CuAl and other copper based composites [8–11]. Particularly, Al2O3 is often used as reinforcing phase to prepare CuAl-Al2O3 composite coating that performs obviously enhanced hardness and wear-resistance attribute to the high hardness and good load-bearing capacity of reinforcing Al2O3 [10,11]. Recent studies have shown that nano-structured ceramics possess higher hardness and strength than the conventional counterparts because of the further hindrance to dislocation in the former ones [12]. However, powders composed of nano-particles cannot be directly deposited into coating by plasma spraying because of too small momentum. As one of the possible routes, spraying drying can be used to agglomerate the fine nano-structured ceramic particles into micro-sized powders with enhanced density and flowability, and consequently the as-received powders can be effectively deposited as n

Corresponding author. E-mail address: [email protected] (Y. An).

http://dx.doi.org/10.1016/j.triboint.2016.04.034 0301-679X/& 2016 Elsevier Ltd. All rights reserved.

nano-structured coatings by plasma spraying [13,14]. During spraying drying process, slurry that contains well-dispersed nano-particles is transformed into massive fine droplets by an atomizing nozzle. These droplets are quickly evaporated by hot gas to obtain dried powders, which are then collected by the cyclone separator. Besides, the particles with too small or large dimensions are removed from the spray drying chamber. Therefore, the as-received powders not only retain the nano-structure, but also possess proper size-range for plasma spraying [13–15]. In the present work, nano-structured alumina (n-Al2O3) powders were prepared by spray drying granulation and used as reinforcements of plasma sprayed CuAl coating. As comparison, micro-structured alumina (m-Al2O3) powders by fused-crushed method were also used to fabricate m-Al2O3 reinforced CuAl coatings. Additionally, the microstructure and tribological properties of the two kinds of Al2O3 reinforced CuAl coatings were investigated in comparison with those of pure CuAl coating.

2. Experimental procedure 2.1 Preparation of spray dried n-Al2O3 powders Commercial nano-alumina feedstocks (n-Al2O3, HTAL-01, Nanjing Haitai nano materials Co., Ltd., China), with a size distribution from 80 nm to 500 nm, were used to prepare agglomerate powders by spray drying granulation. The n-Al2O3 feedstocks and organic binders (polyvinyl alcohol, PVA) were mixed with deionized water, and then underwent ball milling up to 72 h. The as-received slurry was transformed into micro-sized powders with

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Table 1 Main parameters of the spray drying process. Parameters

Values

Mass ratio of PVA to n-Al2O3 Solid content in slurry/% Peristaltic pump rate/r min Compressed air pressure/bar Temperature of inlet air/°C Temperature of outlet air/°C

1.5:100 35 30 2.4 230 120

sprayed coatings. All the sliding tests were conducted at 25 °C with a relative humidity maintained at 40%, an amplitude of 2.5 mm, a frequency of 5 Hz. The total sliding distance of the test was 100 m and the number of cycles was 10,000. After the friction and wear tests, wear volume-losses of the coating specimens were measured on a 3D non-contact surface mapping profiler (ADE Corporation, USA). The wear rates are calculated as W ¼V/DL, where V is the wear volume-losses in mm3, D is the sliding distance in m and L is the normal load in N. 2.4 Characterization of coatings

Table 2 Parameters for plasma spraying coatings. Coating

CuAl

CuAl/n-Al2O3

CuAl/m-Al2O3

Voltage/V Current/A Distance/mm Ar flow rate/L min  1 Carrier gas flow rate/L min  1

60 500 100 60 8

65 500 80 50 6

65 500 80 50 6

a spray dryer (YC-015A, Shanghai Pilotech Instrument and Equipment Co., Ltd., China), and the process parameters are listed in Table 1. 2.2 Fabrication of pure CuAl coating and Al2O3 reinforced CuAl coatings The spray dried n-Al2O3 powders were mechanical blended at mass fraction of 20% with commercial CuAl alloy powders (KF-130, 45–105 μm; Beijing Research Institute of Mining & Metallurgy, China). The mixed powders were used to fabricate n-Al2O3 reinforced CuAl coating (CuAl/n-Al2O3) by plasma spraying. As comparisons, pure CuAl coating and conventional fused-crushed micro-structure alumina (m-Al2O3) (SY121, Sunspraying Ltd., China) reinforced CuAl coating (CuAl/m-Al2O3) were also prepared. The additive amount of m-Al2O3 in the relevant composite coating is also 20% at mass fraction. Coating deposition processes were performed with an atmosphere plasma spraying system (APS-2000, Institute of Aeronautical Manufacturing Technology, China). Argon and hydrogen were used as primary and secondary plasma gas, respectively. A six-axis robot (IRB2400, Asea Brown Boveri Ltd., Switzerland) was used to fix the plasma gun so that consistent spraying distance and passes can be obtained. Austenitic stainless steel disks (1Cr18Ni9Ti; Φ24  7.5 mm) were used as substrates. Prior to spraying, the substrates were grit blasted using the silica sands to obtain a surface roughness of approximately 2.2 70.3 μm as determined using a surface profiler, and the roughened substrates were cleaned with acetone in an ultrasonic bath. Optimized spraying parameters for preparing pure CuAl and Al2O3 reinforced composite CuAl coatings are listed in Table 2. 2.3 Friction and wear tests Dry sliding friction and wear tests were carried out in a ball-ondisk configuration with a reciprocating tribometer (CSM, Switzerland). The polished coatings with the thickness range approximately from 207 μm to 239 μm were used as the lower specimens, and sintered alumina balls with a diameter of 9.525 mm and a hardness of HRA 89–91 were used as the upper specimens and the frictional counterpart. Normal loads of 2 N, 5 N, 8 N were selected to evaluate the tribological behaviors of the

Particle size distribution of the spray-dried powders was determined by using a Malvern 3000 laser diffraction analyzer (Malvern, UK). The morphologies of starting powders, as sprayed coatings and wear tracks were analyzed with a JSM-5600LV scanning electron microscope (SEM, JEOL Corporation, Japan) coupled with an energy dispersive X-ray analyzer (EDXA). Furthermore, the microstructure morphologies of nano-powders were analyzed with a TECNAI G2 TF 20 trans-mission electron microscope (TEM). Phase compositions of the composite coatings were characterized with a D/max 2400 X-ray diffractometer (XRD; Cu-Ka radiation, potential 40 kV, current 150 mA). The coatings' microhardness was measured on polished surfaces with a Vickers hardness tester under a load of 200g with a dwell time of 10 s. Hardness values were obtained from the average of 10 individual measurements performed on the each coating. An OLYCIA m3 image analysis software was used to determine the porosity of the coatings, and porosity values were obtained from the average of 5 individual measurements.

3. Results and discussion 3.1 Powders for plasma spraying SEM morphologies of the CuAl powders and m-Al2O3 powders used for plasma spraying are shown in Fig. 1. It can be seen that the CuAl powders exhibit spherical or near-spherical shape and have a size of 50–100 μm, which are beneficial for powder flowability and plasma spraying. While the conventional m-Al2O3 powders with a size of 20–50 μm have irregular morphology resulted from the fusing and crushing process as well as the inherent brittleness of ceramic material. The low degree sphericity of m-Al2O3 powders would certainly be negative for their flowability, which have some adverse effects on the coating’ performances [16]. Fig. 2 shows the morphologies and size distribution of the nAl2O3 feedstock and n-Al2O3 powders by spray drying agglomerate. The TEM image clearly shows that the n-Al2O3 feedstock is approximately from 80 nm to 500 nm (Fig. 2(b)). Since the agglomerate of nano-particles, the size of the powders seems to be even bigger than 1 μm in Fig. 2(a). The plotted curve clearly shows that the size distribution range of the n-Al2O3 feedstock is approximately from 0.8 to 90 μm, with d50 of 20.1 μm (Fig. 2(d)). Furthermore, the spray dried n-Al2O3 powder particles well retain the fine nano-structures as shown in Fig. 2(e). Therefore, it can be concluded that the agglomeration of the nano-particles is basically free of grain growth during the ball-milling and spray drying process, due to the relatively low temperature. 3.2 Microstructure of composite coating Fig. 3 gives the SEM morphologies of the sprayed pure CuAl and Al2O3 reinforced CuAl coatings. As can be seen in Fig. 3(a), pure CuAl coating exhibits dense lamellar structure formed by well

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Fig. 1. SEM images of (a) CuAl alloy powders; (b) fused-crushed m-Al2O3 powders.

molten and flattened particles. In both CuAl/m-Al2O3 and CuAl/nAl2O3 composite coatings, the reinforcing Al2O3 exist as wellflattened splats, of which the long axes are basically parallel to the substrate plane, and shows good interfacial compatibility with the bronze matrix, indicating that the reinforcements are well molten (seen in Fig. 3(b) and (c)). However, obvious difference is also observed between CuAl/m-Al2O3 coating and CuAl/n-Al2O3 coating. In CuAl/m-Al2O3 coating, the m-Al2O3 reinforcements show local agglomeration, while the distribution of n-Al2O3 reinforcements in CuAl/n-Al2O3 coating is more uniform. As seen above, nAl2O3 powders present much higher degree sphericity as compared with m-Al2O3 powders, which is conducive to obtain uniformly-mixing powders of n-Al2O3/CuAl and flowability of powders increased. Therefore, uniform distribution of n-Al2O3 reinforcements in CuAl/n-Al2O3 coating is obtained [17]. In addition, the porosity values are 4.7 70.6% for CuAl/m-Al2O3 coating and 4.3 70.3% for CuAl/n-Al2O3 coating. These prove that the CuAl/n-Al2O3 coatings possess denser lamellar structure than CuAl/m-Al2O3 coatings, owing to the uniform distribution of nAl2O3 reinforcements in CuAl/n-Al2O3 coating. Fig. 4 shows the magnified cross-sectional SEM images of the CuAl/m-Al2O3 and CuAl/n-Al2O3 composite coatings. It is clear that the interfaces between Al2O3 splats and CuAl matrix are free of cracks and pores (see Fig. 4(a) and (b)), which further indicates the good compatibility between the reinforcements and bronze matrix. On the other hand, the distribution patterns of the two kinds of reinforcing Al2O3 present significant differences. The mAl2O3 reinforcements exist as flattened splats in the resultant composite coating seen in Fig. 4(a). For the spray dried n-Al2O3 powders, the organic binders are burned in plasma torch during the spraying process, and the agglomerate n-Al2O3 powders undergo dissociation, resulting in the formation of both fine granules and splats in the bronze matrix. Few of the n-Al2O3 granules were insufficient melted and remained in the n-Al2O3 coating (the “bright” areas in Fig. 4(b)). The distribution of Cu, Al and O elements correspond to the zones of Fig. 4(b) is further presented in Fig. 5. It is proved that the black fine granules and splats are well consistent with Al and O elements. Fig. 6 presents the XRD patterns of sprayed CuAl, CuAl/m-Al2O3 and CuAl/n-Al2O3 coatings. The pure CuAl coating consist of copper-based solid solution (α phase), Cu3Al (β0 phase), and Cu9Al4 (γ00 phase), and these phases are also retained in both CuAl/mAl2O3 and CuAl/n-Al2O3 composite coatings [5,6]. However, the XRD patterns of CuAl/m-Al2O3 coating show broad and weak diffraction peaks, indicating that the reinforcing m-Al2O3 exists as amorphous and γ phase with low crystallinity in sprayed coating

[18]. This should be attributed to the ultra-high cooling rate of plasma spraying technique, which tends to retain the hightemperature structure of alumina in the sprayed coating as meta-stable amorphous [16,19–21]. A comparison between the XRD pattern of CuAl/m-Al2O3 and CuAl/n-Al2O3 coating indicates that α-Al2O3 phase was appeared in the CuAl/n-Al2O3 coating, it could be inferred that α-Al2O3 phase in the CuAl/n-Al2O3 coating was originated from the unmelted or partially melted nano-sized raw material powders [22]. According to the results in Fig. 4(b), it can be seen that a small quantity of unmelted particles were existed in the CuAl/n-Al2O3 coating. Part of the spray-dried nanosized alumina feedstock with low weight cannot achieve the center of the spray flame. In this case, part of the feedstock cannot melt sufficiently, which was contained within the CuAl/n-Al2O3 coating as α-Al2O3 phase. The average microhardness of the three kinds of coatings was measured on a Vickers hardness tester under a load of 200g with a dwell time of 10 s. The pure CuAl coating has a lowest microhardness of about 162722 HV0.2. The CuAl/m-Al2O3 and CuAl/nAl2O3 composite coatings have the microhardness values of 209 731 HV0.2 and 223 729 HV0.2, respectively. Therefore, it is concluded that the diffusely distributed fine n-Al2O3 particles in the CuAl/n-Al2O3 composite coating are more beneficial for the homogeneity of the harden effect on bronze matrix. 3.3 Friction and wear behaviors of sprayed coatings The average coefficient of friction (COF) values of plasma sprayed CuAl, CuAl/m-Al2O3 and CuAl/n-Al2O3 coatings under varied normal loads are shown in Fig. 7. The pure CuAl coating gives lowest COF values under all loads, and the COF decreases form 0.34 to 0.25 with the increase of load from 2 N to 8 N. Both CuAl/n-Al2O3 and CuAl/m-Al2O3 composite coatings possess higher COF as compared with pure CuAl coating, which would be attributed to the enhancement of surface hardness and shearstrength of the composite coatings by addition of reinforcing alumina into coatings. The hardest CuAl/n-Al2O3 coating presents highest COF, and the COF slightly increases and then decreases with increasing normal load. Differently, the CuAl/m-Al2O3 composite coating possesses reduced COF from 0.43 to 0.37 as normal load increased. Fig. 8 shows the wear rates of plasma sprayed CuAl, CuAl/mAl2O3 and CuAl/n-Al2O3 coatings under varied normal loads. It is clear that both CuAl/m-Al2O3 and CuAl/n-Al2O3 composite coatings have smaller wear rates than pure CuAl coating under all testing loads due to the higher hardness of composite coatings by

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Fig. 2. SEM and TEM morphologies of the n-Al2O3 feedstock (a), (b); SEM morphologies of the spray dried agglomerate n-Al2O3 powders (c), (e). and their size distribution (d).

addition of reinforcing alumina into coatings. Furthermore, it is seen that n-Al2O3 reinforcement is more effective than m-Al2O3 on enhancing the wear-resistance of the sprayed bronze-based coatings. Compared to pure CuAl coating, the wear rate of CuAl/mAl2O3 composite coating is reduced by about 30%, while that value of CuAl/n-Al2O3 composite coating is reduced by an order of magnitude at 2 N and 5 N, and also reduced approximately 95% at 8 N. Fig. 9 gives the optical micrographs of wear scars of counterpart alumina balls sliding against CuAl, CuAl/m-Al2O3 and CuAl/n-Al2O3 coatings under the normal load of 8 N. The apparent contact areas

of the alumina balls are all in oval shape instead of round shape, and alumina balls are all free of abrasive grooves and spalling except varied degrees of adhesion transfers from the coatings, which indicate that no remarkable volume losses occur in alumina balls during sliding wear test so no spherical cap is removed. Therefore, it can be concluded that all the sprayed coatings just cause relatively mild damage to their frictional counterpart. For pure CuAl coating, the soft CuAl tends to adhere on the surface of counterpart alumina ball (see Fig. 9(a)). The additions of both mAl2O3 and n-Al2O3 reinforcements can greatly inhibit the adhesion of CuAl matrix to the counterpart balls at varying degrees.

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Fig. 3. Cross-sectional SEM images of the sprayed coatings: (a) pure CuAl coating; (b) CuAl/m-Al2O3 composite coating; (c) CuAl/n-Al2O3 composite coating.

Fig. 4. The magnified cross-sectional SEM images of (a) CuAl/m-Al2O3 and (b) CuAl/n-Al2O3 composite coatings.

Fig. 5. Distribution of Cu, Al and O elements on the cross-section of CuAl/n-Al2O3 composite coating (presented in Fig. 4b).

Fig. 6. XRD patterns of the plasma sprayed CuAl, CuAl/m-Al2O3 and CuAl/n-Al2O3 coatings.

Fig. 7. The average COF values of the plasma sprayed CuAl, CuAl/m-Al2O3 and CuAl/ n-Al2O3 coatings under varied normal loads.

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Especially, CuAl/n-Al2O3 composite coating not only possesses the highest wear resistance, but also further alleviates the adhesion transfers to frictional counterpart as compared with CuAl/m-Al2O3 coating (see Fig. 9(b) and (c)). 3.4 Effects of Al2O3 reinforcements Fig. 10 shows the three-dimensional wear track profiles of CuAl/n-Al2O3 and CuAl/m-Al2O3 composite coatings. It is seen that the wear tracks of CuAl/m-Al2O3 composite coating are much

Fig. 8. Wear rates of the plasma sprayed CuAl, CuAl/m-Al2O3 and CuAl/n-Al2O3 coatings under varied normal loads.

larger in dimension as compared with those of CuAl/n-Al2O3 coating under all test loads. Moreover, obvious abrasive grooves can be observed in the wear tracks of CuAl/m-Al2O3 coating, while only shallow grooves existed in CuAl/n-Al2O3 coating. In order to further illustrate the effect of n-Al2O3 and m-Al2O3 reinforcements on wear mechanism of sprayed bronze-based coatings, SEM wear morphologies and corresponding elementsdistributions on frictional surfaces of CuAl/n-Al2O3 and CuAl/mAl2O3 composite coatings are presented in Fig. 11. Local fracture and delamination in the frictional surface of CuAl/n-Al2O3 coating are much slighter than those of the CuAl/m-Al2O3 coating (shown in Fig. 10(a) and (b)). Elements-distribution suggests that n-Al2O3 reinforcements tend to diffusely distributed on the frictional surface of CuAl/n-Al2O3 coating, while m-Al2O3 reinforcements are located as islands of particles with typical dimension of about 10 μm on the frictional surface of CuAl/m-Al2O3 coating. Fig. 12 shows the SEM images and EDS analysis of wear debris for CuAl, CuAl/m-Al2O3 and CuAl/n-Al2O3 coatings under the normal load of 8N. The delamination-flake-like wear debris more or less flattened in terms of the shape could be found in the three different kinds of coatings, which was caused by plastic deformation during the sliding contact, proving the severe occurrence of the bulk splats delamination under dry sliding. The wear debris generated from CuAl/m-Al2O3 and CuAl/n-Al2O3 coating were composed of flattened and finely granular from reinforcement of alumina in the coatings. And interestingly, the particle size of CuAl/n-Al2O3 coating wear debris were smaller than that of CuAl/ m-Al2O3 coating due to finer original particle size of CuAl/n-Al2O3 coating (Fig. 12(b) and (c)). As a result, the degree of three body abrasion is very slight and nearly no abrasive grooves can be observed in the wear track of CuAl/n-Al2O3 coating as seen in Fig. 9 (c). The EDS analysis results were given in Fig. 12, corresponding to the images of wear debris for the coatings. It can be seen that the

Fig. 9. Optical micrographs of wear scars of alumina balls under the normal load of 8N sliding against: (a) CuAl coating; (b) CuAl/m-Al2O3 coating; (c) CuAl/n-Al2O3 coating.

Fig. 10. Three-dimensional profiles of frictional surface of (a), (b), (c) CuAl/n-Al2O3 and (d), (e), (f) CuAl/m-Al2O3 composite coatings under the normal loads of (a) and (d) 2 N, (b) and (e) 5 N, (c) and (f) 8 N.

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Fig. 11. SEM wear morphologies and corresponding elements-distributions on frictional surfaces of (a) CuAl/n-Al2O3 and (b) CuAl/m-Al2O3 composite coatings.

alumina content was higher for the reinforced coating than pure CuAl coating. Particularly, the Cu/Al (or O) atomic ratio of the wear debris generated from CuAl/n-Al2O3 coating was lower than that of the wear debris generated from CuAl/m-Al2O3 coating, which indicated that the spalling of the CuAl matrix phase is more serious for CuAl/m-Al2O3 coating. Under dry sliding condition, the actual contact area is located at a small amount of surface asperities, generating quite high Hertzian stress and plowing force [23]. With inherent low hardness and shear-strength, the surface of pure CuAl coating is liable to

local yielding under the normal load and repeated tangential friction force, and suffers plastic deformation and flow. Besides, the interfacial bonding between the deposited splats in thermally sprayed coating is relatively weak, and the friction-induced cracks are apt to propagate along the splats interfaces [24,25]. Therefore, severe bulk splats delamination should occur during dry sliding proved by the large lamellar debris in Fig. 12(a), and should be the main reason for relatively higher wear rate of pure CuAl coating. In both CuAl/m-Al2O3 and CuAl/n-Al2O3 composite coatings, the reinforcing Al2O3 exist as well-flattened splats distributed in

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Element

Wt%

At%

O

12.96

30.81

Al

21.07

29.71

Cu

65.97

39.49

Element

Wt%

At%

O

26.79

46.86

Al

35.02

36.32

Cu

38.19

16.82

Element

Wt%

At%

O

33.65

53.36

Al

37.25

35.02

Cu

29.10

11.62

Fig. 12. SEM images and EDS analytical of wear debris for (a) CuAl coatings (b) CuAl/m-Al2O3 coatings and (c) CuAl/n-Al2O3 coatings.

bronze matrix, partly restrains cracks propagation inside bronze matrix, and consequently alleviate the bulk splats delamination of CuAl matrix. So, the wear debris of both CuAl/m-Al2O3 and CuAl/nAl2O3 composite coatings have obviously reduced dimension, and the wear rate are much lower than that of pure CuAl coating. However, as can be seen above, the n-Al2O3 and m-Al2O3 reinforcements exhibit significant differences in distribution patterns, crystal phases and hardening effect, which can also greatly influence the wear behaviors of composite coatings. In comparison, the uniform and fine distribution of n-Al2O3 reinforcements in bronze matrix are more effective on enhancing the coating's resistance against adhesion and abrasive wear by increasing the hardness and shear-strength of the bronze matrix, resulting lowest wear rate under dry sliding condition seen in Fig. 8.

4. Conclusion Nano-structured alumina powders for plasma spraying were prepared by spray drying granulation. The as-received powders were used to fabricate n-Al2O3 reinforced CuAl coating (CuAl/nAl2O3). Microstructure, phase composition, and hardness of the composite coating were investigated. Additionally, friction and wear behaviors of the CuAl/n-Al2O3 composite coating were studied in comparison with those of pure CuAl coating, and conventional micro-sized alumina reinforced CuAl coating (CuAl/mAl2O3). Main conclusions can be drawn as follow:

1.The spray dried n-Al2O3 powders consisted of agglomerate micro-sized particles which retained the fine nano-structure very well. Besides, the spray dried powders possessed much higher degree of sphericity than those of the conventional fused and crushed micro-sized alumina powders. 2. In CuAl/n-Al2O3 composite coating, the n-Al2O3 reinforcements were transformed into well flattened spats and diffusely distributed quite fine particles, possessing good interfacial compatibility with the bronze matrix and better harden effect than conventional micro-sized alumina reinforcement. 3. The two kinds of reinforcing alumina exhibited significant differences in crystal morphology. The alumina existed as amorphous and γ-Al2O3 phase in the CuAl/m-Al2O3 composite coating, while α-Al2O3 phase was contained in the CuAl/n-Al2O3 coating which was mainly resulted from the unmelted or partially melted nano-sized raw material powders due to their lower weight. 4. Although the CuAl/n-Al2O3 composite coating gives a highest friction coefficient at all loads, it possesses superior wearresistance as compared with the CuAl/m-Al2O3 coating and pure CuAl coating, which may be due to the more uniform distribution of reinforcing n-Al2O3 on frictional surface.

Acknowledgments This project is financially supported by the Youth Innovation Promotion Association of the Chinese Academy of Sciences and the

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National Natural Science Foundation of China (Grant no. 51302272) and the Youth Innovation Promotion Association CAS (Grant no. 2014378). The authors also appreciate constructive comments of reviewers.

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