Ni-base alloy composite coatings on aluminum alloys by plasma spray

Applied Surface Science 314 (2014) 760–767

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Microstructure and wear properties of Al2 O3 -CeO2 /Ni-base alloy composite coatings on aluminum alloys by plasma spray Long He, Yefa Tan ∗ , Xiaolong Wang, Ting Xu, Xiang Hong College of Field Engineering, PLA University of Science and Technology, Nanjing 210007, China

a r t i c l e

i n f o

Article history: Received 30 May 2014 Received in revised form 8 July 2014 Accepted 8 July 2014 Available online 15 July 2014 Keywords: Al2 O3 CeO2 Ni-base alloy Composite coating Wear

a b s t r a c t Al2 O3 and CeO2 particles reinforced Ni-base alloy composite coatings were prepared on aluminum alloy 7005 by plasma spray. The microstructure, microhardness, fracture toughness, critical bonding force and the wear behavior and mechanisms of the composite coatings were investigated. It is found that CeO2 particles can refine crystal grains, reduce porosity and unmelted Al2 O3 particles in the composite coatings. The microhardness, fracture toughness, critical bonding force and wear resistance of the composite coatings are enhanced due to synergistic strengthening effects of Al2 O3 and CeO2 particles. The friction coefficients and wear losses increase as loads increase. At the loads of 3–6 N, the composite coatings experience local plastic deformation and micro-cutting wear. At the loads in the range of 9–12 N, the calculated maximum contact stress and maximum tensile stress on friction surfaces increase leading to plastic deformation induced working hardening. The wear mechanisms change into micro-brittle fracture wear and slight oxidative wear. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Aluminum alloys are widely used in machine building, aerospace industry, national defense and offshore exploration due to their low density, high specific strength, and good thermal and electrical conductivity [1–4]. However, further applications of aluminum alloys in severe wear conditions are restricted owing to their low hardness and poor wear resistance. Since wear is primarily surface-related property, wear resistance can be improved by preparing strengthening coatings on aluminum alloys. Plasma spray is an important surface engineering technology with many advantages such as high spraying temperature, fast deposition rate, small heat effect on substrate and large variety of materials, so it is widely used in surface repairing and strengthening of mechanical parts [5–7]. As one of the most popular used coating materials in plasma spray, Ni-base alloy coatings own good wear properties both at room and high temperature [8,9], which exhibit well antiwear effects on surface strengthening of aluminum alloys, but the increasingly severe working conditions call for further enhancement in their wear resistance. Ceramic particles reinforced metal matrix composite coatings combine the high hardness and strength of ceramic particles with the good toughness of metal matrix to

∗ Corresponding author. Tel.: +86 25 80821055. E-mail addresses: [email protected], [email protected] (Y. Tan). http://dx.doi.org/10.1016/j.apsusc.2014.07.047 0169-4332/© 2014 Elsevier B.V. All rights reserved.

obtain better wear properties than the simple metal or ceramic coatings [10–12]. Martin [13] prepared WC particles reinforced Ni-base alloy composite coatings by plasma spray with 40% reduction in wear rate. The plasma spray TiC/Ni-base alloy composite coatings showed 30% less friction coefficient and 2/3 smaller wear loss than the Ni-base alloy coatings [14]. Obviously, ceramic particles, e.g. WC, TiC, are able to improve the wear resistance of Ni-base alloy coatings, but the production process of such particles is complex with high cost. Moreover, WC particles may be decomposed in spray process which will weaken the strengthening effects. In all kinds of ceramic particles, Al2 O3 owns high hardness and strength, good thermal stability and low cost, its thermal expansion coefficient is also close to Ni-base alloy coatings, which is suitable for industrial applications in particles reinforced composite coatings. In addition, researches have proved that rare earth, e.g. CeO2 , Y2 O3 and La2 O3 , are beneficial to meliorate the microstructure and performance of coatings due to special physical and chemical properties [15–17]. By now, reports on the preparation and wear performance of ceramic and rare earth particles synergistically reinforced composite coatings on aluminum alloys are rarely found. In this paper, the Al2 O3 -CeO2 /Ni-base alloy composite coatings were prepared on 7005 aluminum alloys by plasma spray. The microstructure, mechanical performance and wear properties of the composite coatings were investigated. The aim is to improve the wear resistance of Ni-base alloy coatings to extend the service life of aluminum alloy frictional parts.

L. He et al. / Applied Surface Science 314 (2014) 760–767

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Table 1 Compositions of composite materials. Composition (wt.%)

Coating no.

S1 S2 S3

Ni60

Al2 O3

CeO2

100 85 83

– 15 15

– – 2

2. Experimental The spraying materials, containing Ni60, Al2 O3 and CeO2 particles, were mechanically ball milled for 30 min according to the weight ratio listed in Table 1. The chemical composition of the Ni60 powder with a size distribution of 47–100 ␮m was 15.5 Cr, 3.5 B, 4.0 Si, 15.0 Fe, 3.0 W, 0.8 C and 58.2 Ni (wt.%). Al2 O3 and CeO2 particles had an average size of 30 ␮m and 10 ␮m, respectively. Aluminum alloy 7005 was selected as the substrates with the chemical composition of 0.35 Si, 0.4 Fe, 0.1 Cu, 0.2–0.7 Mn, 1.0–1.8 Mg, 0.06–0.2 Cr, 4.0–5.0 Zn, 0.08–0.2 Zr, 0.01–0.06 Ti and residue of Al (wt.%). Prior to spray, the substrates were sandblasted by quartz sand to the level of Sa 2.5 to remove rough oxidation layer and contaminations, and then were degreased by ultrasonic acetone bath to obtain clean surfaces. A DH-1080 plasma spray equipment was employed to deposit the Al2 O3 -CeO2 /Ni-base alloy composite coatings (S3). The spray parameters were electric current of 500 A, voltage of 50 V, argon flow rate of 70 L·min−1 , hydrogen flow rate of 10 L·min−1 and spray distance of 100 mm. The Ni-base alloy coatings (S1) and Al2 O3 /Ni-base alloy composite coatings (S2) were also fabricated using the same parameters as comparison. The sprayed specimens were ground by grinding wheel and diamond abrasive paste to the surface roughness of Ra = 0.5 ␮m, and then incised into test-pieces in the size of 10 × 10 × 6 mm. A DMM-330C optical microscope (OM) and a FEIQUANTA200 scanning electron microscopy (SEM) with energy-dispersive analysis of X-rays (EDAX) was used to observe coating morphologies and test elemental composition. Phase composition was identified by an X’TRA X-ray diffractometer (XRD) with CuK␣ radiation scanning from 25◦ to 85◦ at a step size of 0.02◦ s−1 . Microhardness was measured using a DHV-1000 microhardness tester at 4.9 N for a dwelling time of 15 s. Fracture toughness (KIC ) is calculated by the following equation [18]:

 E 1/2

KIC = ı

H

·

P c 3/2

(1)

where ı is the shape constant of 0.016 for standard Vickers indenter, E is the elastic modulus of the coatings (MPa), H is the microhardness of the coatings (MPa), P is the normal load (N), c is the half length of cracks (mm).

Fig. 1. XRD patterns of plasma spray coatings.

Critical bonding force was tested by a WS-2005 scratch tester for a length of 4 mm at 100 N with loading speed of 100 N·min−1 . Wear tests were carried out in an HT-500 ball-on-disc tribometer. The test-pieces slid against a GCr15 ball (˚ = 4 mm) under different loads of 3, 6, 9 and 12 N with a constant speed of 0.25 m·s−1 for a distance of 500 m at 25 ◦ C. Friction coefficients were tested by software attached to HT-500. Wear losses were measured by a TG328B analytical balance with precision of 0.1 mg. 3. Results and discussion 3.1. Microstructure The XRD patterns of the Ni-base alloy coating (S1), Al2 O3 /Nibase alloy composite coating (S2) and Al2 O3 -CeO2 /Ni-base alloy composite coating (S3) are shown in Fig. 1. It shows that the three coatings take -Ni as the matrix phase together with the reacted Cr-rich phases of Cr23 C6 and CrB. With the addition of Al2 O3 particles, Al2 O3 diffraction peaks appear in coatings S2 and S3, while the intensity of -Ni peak is weakened. A few CeNi5 and CeO2 phases also turn up in coating S3. Fig. 2 depicts the cross-section morphologies of the three coatings. It shows that coating S1 is uniform and dense except a few pores (Fig. 2a). For coatings S2 and S3, a typical lamellar structure of fully deformed dark Al2 O3 reinforcing phase alternately deposited with gray Ni-base alloy matrix phase is formed. Some pores and unmelted Al2 O3 inclusions exist in both coatings which are obviously reduced in coating S3 with CeO2 additions (Fig. 2b and c). Since the melting point of Al2 O3 (2100 ◦ C) is higher than that of

Fig. 2. Cross-section morphologies of coatings S1 (a), S2 (b) and S3 (c).

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L. He et al. / Applied Surface Science 314 (2014) 760–767

Fig. 3. OM images of microstructure of coatings S2 (a) and S3 (b).

Ni-base alloy (1268 ◦ C), the Ni-base alloy powder and small Al2 O3 particles can absorb sufficient heat for complete melting, extending and depositing to form lamellar structure in plasma spray process, but some big Al2 O3 particles are not fully melted and wrapped in coatings as inclusions. Similar coating microstructure was also reported in some papers [19,20]. As the temperature of plasma arc is higher than the decomposition temperature of CeO2 (2010 ◦ C) [21], some CeO2 particles are decomposed to generate atoms Ce. Rare earth elements can easily gather at phase boundary between Al2 O3 and Ni-base alloy, which accelerates the dissolution of Al2 O3 to reduce the unmelted particles and improve the microstructure of coating S3. The OM images of microstructure of coatings S2 and S3 are shown in Fig. 3. It can be seen that the grain size of coating S3 is smaller than that of coating S2 indicating obvious grain refinement. Research has proved that the relationship between critical nucleation energy (Gc ) and interface energy per unit area () is [22]: 3

Gc =

4 () 27 ( − ˛ )2

(2)

ˇ

where  is the nucleation shape factor, ˛ and ˇ are the chemical potential of ˛ and ˇ phase. Atoms Ce, with big diameter and high surface activity, can reduce surface tension and interface energy per unit area [23,24]. According to Eq. (2), the critical nucleation energy decreases to improve the nucleation of matrix. Besides, some atom Ce and formed tiny CeNi5 phase may act as nucleus to boost up the nucleation rate. Moreover, atoms Ce own low solubility in melting Ni, so they are probably pushed by freezing interface to phase boundary

Table 2 Mechanical properties of plasma spray coatings. Coating No.

S1

S2

S3

Microhardness (HV0.5 ) Fracture toughness (MPa·m1/2 ) Critical bonding force (N)

613.1 4.9 64.7

671.5 5.1 62.4

700.4 5.6 70.6

that restrains the fast growth of grains. Thus, the grain size of coating S3 is declined. According to the Hall-Petch formula [25]:  = 0 + kd−0.5

(3)

where  0 and k are constants, d is the average grain size. It can be inferred that the increment in yield strength of coating S3 is higher than coating S2 due to CeO2 particles induced refined microstructure, which are good for improving the wear properties. 3.2. Mechanical performance Table 2 lists the test results of microhardness, fracture toughness and critical bonding force of the coatings. It shows that the microhardness of coating S3 is 700.4 HV0.5 , which is increased by 4.3% and 14.3% compared with that of coatings S1 and S2. It is because Al2 O3 reinforcing phase with high hardness and compressive strength and CeO2 induced grain refinement effects improve the plastic deformation resistance of coating S3 under loading. For coating S3, its fracture toughness (5.6 MPa·m1/2 ) and critical bonding force (70.6 N) are also higher than coatings S1 and S2. This can be attributed to the following reasons. First, the Al2 O3 particles with high compression strength may act as impenetrable obstacles in the coatings. When the cracks encounter hard Al2 O3 particles, they deflect from the original direction to propagate either in the

Fig. 4. Friction coefficient (a) and wear loss (b) of three coatings and substrate at different loads.

L. He et al. / Applied Surface Science 314 (2014) 760–767

Fig. 5. Wear scar diameters of GCr15 steel balls sliding against three coatings.

matrix or along the interface between Al2 O3 particles and matrix, which extends the crack propagation length. The increasing length of crack propagation path demands higher crack tip energy to keep on propagating, that is to say, more crack tip energy will be consumed gradually. Thus, the cracks with low tip energy can hardly sustain the propagating process, and finally may be pinned at the Al2 O3 particles. Meanwhile, the increment in grain boundary length due to the reduced grain size benefits for increasing crack propagation resistance and improve the fracture toughness. Second, coating S3 is more dense with less pores and inclusions than coating S2. The stress concentration and crack initiation near the defects in the coatings can be alleviated to increase the fracture toughness. Third, atoms Ce enable to improve the surface fluidity of melting particles that means full deformation and deposition of spray particles [26]. The bonding area between coatings and substrates is enhanced indicating high critical bonding force. Thus, the addition of Al2 O3 and CeO2 particles can notably improve the mechanical properties of Ni-base alloy coatings. 3.3. Wear behavior Fig. 4 depicts the wear results of the three coatings and the substrates at different loads. It shows that the friction coefficients and wear losses of the coatings are much smaller than those of the substrates. The friction coefficients are in ascending trends as the loads rise from 3 N to 12 N. Coating S3 possesses the friction coefficients in the range of 0.18–0.32, which are reduced by 37.3–58.1%, 11.1–33.3% and 5.9–23.1% compared with the substrates, coatings

763

S1 and S2, respectively (Fig. 4a). The wear losses are also found to increase in ascending order with the loads. At the loads of 3–6 N, coating S3 is slightly worn with low wear losses of 0.7–0.9 mg, but goes into severe wear conditions with high wear loss to 5.2 mg when the loads are bigger than 9 N. Comparatively, the wear losses of coatings S1 and S2, changing in the range of 3.1–6.9 mg and 1.3–5.8 mg, are higher than those of coating S3. The aluminum alloy substrates suffer from severe wear with higher wear losses increasing from 7.6 mg to 16.4 mg (Fig. 4b). Fig. 5 exhibits the wear scar diameters of the GCr15 steel ball sliding against the three coatings. It can be seen that the wear scar diameters also increase as the loads increase. Coating S3 owns larger wear scar diameters varying among 993.9–1425.1 ␮m than coatings S1 (923.5–1142.4 ␮m) and S2 (966.2–1317.4 ␮m). Since the microhardness of aluminum alloy is much smaller than the GCr15 steel ball, the friction induced polishing surface is formed on the counterpart instead of round wear scar. Obviously, the Al2 O3 and CeO2 additions can enhance the wear properties of Ni-base alloy coatings. With the addition of Al2 O3 and CeO2 particles, the microhardness and plastic deformation resistance of coating S3 improve that limits serious dislocation and slippage caused by extrusion and friction of GCr15 steel ball. Besides, the uniformly distributed Al2 O3 hard particles act as anti-wear pivots to prevent severe abrasion of Ni-base alloy matrix. CeO2 particles reduce pores and inclusions in coating S3, so the friction-induced local stress concentration and crack initiation are restrained. As atoms Ce absorbed on the surface of Al2 O3 particles can enhance the wettability and mass transfer between Al2 O3 and Ni-base alloy, the bonding interface is intensified to prevent severe fell off of Al2 O3 particles, so the Al2 O3 -CeO2 /Ni-base alloy composite coatings have excellent wear behavior. 3.4. Wear mechanisms This part is to further investigate the wear mechanisms of Al2 O3 CeO2 /Ni-base alloy composite coatings under different loads. Generally, stress state of friction surfaces will influence the wear mechanisms of Al2 O3 -CeO2 /Ni-base alloy composite coatings. Simplifying the ball-on-disc wear test as the Hertz contact between a rigid sphere and an elastic half plane, the maximum contact stress (p0 ) and contact radius (a) in the contact area can be calculated by the equations below [27]:

 p0 =

6F  3 R2



1 − 12 E1

+

1 − 22

−2 1/3

E2

Fig. 6. OM image (a) and SEM morphology (b) of worn surface of composite coatings at 3 N.

(4)

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Fig. 7. Worn surface morphology (a) and EDAX spectrum (b) of composite coatings at 6 N.

Fig. 8. OM images of worn surface of composite coating (a) and GCr15 steel ball (b), and SEM morphology (c) and EDAX spectrum (d) of worn surface at 9 N.

 a=

3FR 4



1 − 12 E1

+

1 − 22 E2

 1/3 (5)

For bearing normal force alone, the stress distribution in contact area is as below [28]: 1 − 21 r = 3

where F is the normal force (N), R is the comprehensive curvature radius (m), E and  are elastic modulus (GPa) and Poisson’s ratio, 1 and 2 stands for coating S3 and GCr15 steel ball.



 − p0 1 −

a2 p0 r2 r2 a2



 1−

r2 1− 2 a

3/2 

1/2 ,

0
(6)

L. He et al. / Applied Surface Science 314 (2014) 760–767

1 − 21  = 3

 

− 21 p0

a2 p0 r2



r2 1− 2 a

 1−

r2 1− 2 a

3/2 

Table 3 Calculated maximum contact stress and maximum tensile stress of friction surface at different loads.

1/2 ,

0
(7)

Supposing the friction force as distributed shear force in contact area and satisfies the Coulomb law, the stress distribution along the sliding direction under friction force is: x =

p0 (4 − 1 ) x · , 16 a

0≤x≤a

(8)

According to the equations (6), (7) and (8), it is found that the maximum tensile stress appears at the margin of contact area (r = a) either under normal force or shear force. Based on the superposition principle, the maximum tensile stress can be expressed as below: m =

p0 p0 (4 − 1 ) (1 − 21 ) + 3 16

765

(9)

Table 3 lists the calculated maximum contact stress and maximum tensile stress at different loads. The worn surface morphologies of Al2 O3 -CeO2 /Ni-base alloy composite coatings at 3 N are shown in Fig. 6. It can be seen that some discontinuous furrows appear on the wear scar (Fig. 6a). The magnified worn surface shows some shallow stretches with local desquamation region (Fig. 6b). Since the maximum contact stress (829.0 MPa) and maximum tensile stress (230.6 MPa) are relatively

Loads (N)

3

6

9

12

Maximum contact stress (MPa) Maximum tensile stress (MPa)

829.0 230.6

1044.5 345.6

1195.7 445.0

1316.0 520.7

low, the composite coating suffers slight cutting and extrusion by the GCr15 steel ball that results in low wear loss. However, stress concentration is also prone to occur at the margin of pores and inclusions in wear process, which results in local slight desquamations. The wear mechanism is primary micro-cutting wear. The SEM morphology and EDAX spectrum of worn surface at the load of 6 N are shown in Fig. 7. There are wide micro-cutting traces accompanied with local loose deformation area (A) on the worn surface (Fig. 7a). EDAX result shows that area A contains the elements of Ni, Fe, Al, O, Si and Cr (Fig. 7b), it may be considered as the mechanical mixture layer (MML). In this friction condition, the calculated maximum contact stress (1044.5 MPa) and tensile stress (345.6 MPa) are higher than those at 3 N. The abrasion extent of worn surface by GCr15 steel ball is aggravated leading to obvious micro-cutting wear. Besides, some trapped wear debris is pressed and extruded resulting in MML. Nevertheless, it can be seen that no obvious desquamation appears at the loads of 3–6 N, so the wear losses of the composite coatings are relatively low. As the loads rise to 9 N, the wear scar becomes wide where red region and fractured desquamation appears as shown in Fig. 8a.

Fig. 9. SEM morphologies of worn surface (a), cross-section (b), wear debris (c) and EDAX spectrum (d) of wear debris at 12 N.

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Table 4 Microhardness of worn surfaces under different loads. Load (N)

3

6

9

12

Microhardness (HV0.5 )

705.4

718.8

760.8

810.6

From the magnified image, obvious delamination areas (B) with high content of Fe, Ni and O are observed (Fig. 8c and d). It can be inferred that the oxides mixture of Fex Oy and Nix Oy are formed during wear process, which suits for the red region on the worn surface. Meanwhile, the GCr15 steel ball generates big spherical segment further testify materials transfer (Fig. 8b). Owing to the high maximum contact stress of 1195.7 MPa, the composite coating is further extruded and ploughed by the CGr15 steel ball resulting in high friction temperature [29]. The mutual solubility between Ni and Fe is also increased to accelerate element diffusion, which results in materials transfer from GCr15 steel ball to coatings by repeating frictions. Moreover, the microhardness of worn surface (760.8 HV0.5 ) is higher than that of unworn surface (Table 4), indicating the working hardening of worn surface, so the brittleness of the worn surface rises. Under the maximum tensile stress of 445.0 MPa, microcracks and fractured fragments are formed in repeating friction process. The wear mechanisms are mainly microbrittle fracture wear and oxidation wear. The morphologies and EDAX spectrums of worn surface at 12 N are shown in Fig. 9. It shows that the composite coating is severely worn. Plenty of cracks and serious desquamation are generated on the worn surface (Fig. 9a). As shown in Fig. 9b, the fractured area on cross-section of the composite coating is smooth denoting a typical brittle fracture wear. A mixture of flake debris and particle debris is formed. EDAX spectrum shows that the wear debris contains Al, Ni, Fe, Si, Cr and a certain content of O (Fig. 9c and d), which may be the worn and oxidized Ni-base alloy matrix and Al2 O3 particles. Here, the calculated maximum contact stress and maximum tensile stress reach 1316.0 MPa and 520.7 MPa. The microhardness of the worn surface is increased by 15.7% compared with that of the unworn surface implying more severe working hardening. By repeating action of high tensile stress, the supporting effect of Ni-base alloy matrix on Al2 O3 particles is weakened resulting in stress concentration and dislocation accumulation at the interface between the Al2 O3 particles and Ni-base alloy matrix as well as the Ni-base alloy splats. As a result, cracks initiate and propagate along the bonding interface leading to brittle fracture of worn surface and formation of flake debris. Some flake debris wrapped on the contact area are pressed and cut into small particle debris. The composite coating is dominated by micro-brittle fracture wear and slight oxidative wear at 12 N. 4. Conclusions (1) CeO2 plays important roles in reducing pores and unmelted inclusions and refining grains of Al2 O3 -CeO2 /Ni-base alloy composite coatings. The microhardness, fracture toughness and critical bonding force of the composite coatings are 700.4 HV0.5 , 5.6 MPa·m1/2 and 70.6 N, which are higher than those of Ni-base alloy coatings and Al2 O3 /Ni-base alloy composite coatings. (2) As the loads rise from 3 N to 12 N, the friction coefficient and wear loss of Al2 O3 -CeO2 /Ni-base alloy composite coatings increases from 0.18 and 0.7 mg to 0.32 and 5.2 mg, which are lower than those of Ni-base alloy coatings and Al2 O3 /Ni-base alloy composite coatings. The excellent wear properties are attributed to the synergistic strengthening caused by Al2 O3 hard phase and CeO2 induced grain refinement effects. (3) At the loads of 3–6 N, the Al2 O3 -CeO2 /Ni-base alloy composite coatings are dominated by local plastic deformation and

micro-cutting wear. When the loads vary in the range of 9–12 N, the calculated maximum contact stress and maximum tensile stress are obviously increased. The composite coatings experience working hardening, and its primary wear mechanisms change into micro-brittle fracture wear and slight oxidative wear.

Acknowledgements The authors were grateful to Feng Jie for the help in specimen preparation. Thanked Mr. Weidi Dai and Mr. Yuda Ye for the use of the SEM and XRD.

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