Application of a simple surface nanocrystallization process to a Cu–30Ni alloy for enhanced resistances to wear and corrosive wear

Wear 271 (2011) 1224–1230

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Application of a simple surface nanocrystallization process to a Cu–30Ni alloy for enhanced resistances to wear and corrosive wear X.Y. Mao a,b , D.Y. Li b,∗ , F. Fang a , R.S. Tan a , J.Q. Jiang a a b

School of Materials Science and Engineering, Southeast University, Nanjing 211189, Jiangsu, China Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2V4

a r t i c l e

i n f o

Article history: Received 29 August 2010 Received in revised form 9 December 2010 Accepted 10 December 2010

Keywords: Wear Corrosion Corrosive wear Surface nanocrystallization Cu–30Ni

a b s t r a c t A simple surface nanocrystallization process combining repeated hammering and recovery treatment was applied to Cu–30Ni alloy for improved resistance to corrosive wear in a NaCl solution. This study was motivated by aquaculture applications of copper alloys, which require not only the resistance to pure corrosion but also that to corrosion involving wear such as scratching. The synergy of corrosion and wear may significantly accelerate surface failure of copper alloys in the marine environment. We nanocrystallized the surface of the copper alloy by repeatedly hammering the surface followed by a recovery treatment that turned dislocation cells into nano-sized grains, which considerably strengthened the surface. The high-density grain boundaries largely enhanced atomic diffusion and thus improved the passivation capability of the nanocrystallized surface. As a result, the nanocrystallized surface possessed higher resistances to corrosion, wear and corrosive wear. In this work, particular emphases were put on effects of the hammering duration on the thickness of nanocrystallized surface layer and variations in corrosion, wear and corrosive wear rates as a function of the distance from top surface to bulk. Variations in other relevant properties with the distance were also investigated, including the adhesive force, friction coefficient and hardness. The relationships between the properties and depth help to estimate the durability or service life of the nanocrystallized surface layer. Efforts were made to clarify the mechanisms responsible for observed phenomena and relationships. © 2011 Elsevier B.V. All rights reserved.

1. Introduction

nanocrystalline materials obeys the well-known Hall–Petch relationship [10,11]:

Materials in corrosive environments are often accompanied by mechanical attacks such as scratching or wear, which may considerably enhance the surface failure process due to the synergy of electrochemical and mechanical attacks. The mechanical action introduces defects, rendering the surface more anodic, and/or destroys protective surface layers such as a passive film. As a result, the corrosion is amplified. Extensive studies have demonstrated that nanocrystalline metallic materials with passivation capability possess higher resistance to corrosion, compared to their microcrystalline counterparts [1–3]. This is attributed to enhanced atomic diffusion along high-density grain boundaries, which accelerates the passivation process and improves the adherence of passive film to the substrate through possible oxide pegging [4–6]. On the other hand, the high-density grain boundaries in a nanocrystalline material result in elevated energy barriers to nucleation and propagation of dislocations. Consequently, the nanocrystalline material is harder and more wear resistant [7–9]. The hardness of

H = H0 + d−(1/2)

∗ Corresponding author. Tel.: +1 780 492 6750; fax: +1 780 492 2881. E-mail address: [email protected]. 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.12.063

(1)

where H and d are hardness and average grain size, respectively, and H0 and  are material constants. However, if the grain size is too small, e.g., below 10 nm, the hardness and grain size may not follow the Hall–Petch relation [12–14]. The material loss of a material caused by abrasion or sliding wear is often estimated using Archard’s equation [15]: W =K

P H

(2)

where W is the volume loss per unit sliding distance; P is the applied load; and K is a wear coefficient. Simply combining Eqs. (1) and (2), one may express the wear loss as a function of the grain size W =K

P H0 + d−1/2

(3)

Thus, the high-density grain boundaries in nanocrystalline passive materials suppress both wear and corrosion, which would also lead to increased resistance to synergistic attack of corrosion and wear.

X.Y. Mao et al. / Wear 271 (2011) 1224–1230

This article reports our recent study on the application of a simple surface nanocrystallization process to enhance Cu–30Ni alloy surface against corrosion and corrosive wear in the marine environment. The study was motivated by aquaculture applications of copper alloys, which require not only the resistance to pure corrosion but also that to corrosion involving wear such as scratching. The synergy of corrosion and wear may significantly accelerate surface failure of copper alloys in the marine environment. The surface nanocrystallization was achieved by repeated hammering and subsequent recovery heat treatment. The former resulted in severe plastic deformation associated with the formation of high-density dislocation cells and the recovery treatment turned the dislocation cells into nano-scale grains. Since prolonged hammering increased the thickness of deformed layer, a thicker nanocrystalline surface layer would improve the durability of the nanocrystalline surface for longer service life. The objective of this study is to investigate the effectiveness of the surface nanocrystallization process in improving the resistances of Cu–30Ni alloy to corrosion and corrosive wear in a dilute NaCl solution. Particular attention was paid to the resistances with respect to the depth from top surface and their correlation with the grain size.

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corrosion current and potential were recorded. A pin-on-disc tribometer (SCM Instrument) with a Si3 N4 ball of 6.0 mm diameter was used to evaluate the wear behavior of a sample respectively in air and in a 3.5 wt%NaCl solution. Wear tests were performed under a constant normal load of 5 N and at a constant sliding speed of 0.01 m/s over 3000 circles. During the wear tests, friction coefficients of the samples were also measured. A strain sensor was used to measure the lateral displacement of the pin and the lateral force was then calculated using a cantilever approach. The friction coefficient was determined by a data-logging system. After the wear tests, worn surfaces were examined under an optical microscope (Leica DMIL). The volume loss (V) was determined based on the following equation: V=

R 2

 −



 R2 arcsin(w1 /2r) − w1 (4r 2 − w1 2 ) 45

 R2 arcsin(w2 /2r) − w2 (4r 2 − w2 2 ) 45



 (4)

where R is the radius of worn track, r is the radius of ball, and w1 and w2 are the width of worn track on the sample and on the ball, respectively. Mode details about the calculation of volume loss are given in Appendix A.

2. Experimental details 3. Results and discussion 2.1. Sample preparation Samples were cut from a commercial Cu–30Ni alloy tube, consisting of (in wt%): Ni 30.57, Mn 0.87, Fe 0.98, C 0.04, Zn 0.10, P 0.005 and Cu balance. The final dimensions of samples were 30 mm in diameter and 3 mm in thickness. The samples were ground with silicon carbide papers up to 600-grit, rinsed with acetone, ethanol and deionized water successively, and dried in an air flow. The samples were then hammered for 1 h using a roto-hammer (Robert Bosch Tool Corporation, USA) at a frequency of 50 Hz. The head of the hammer made of high-speed steel has a semi-spherical shape of 5 mm in diameter. The impact energy is 1.6 ft–l bs (2.207 Nm). During the hammering process, the head scanned the surface at a speed around 5 mm/s. Fig. 1 schematically illustrates a hammering process, during which the hammer head is moved over the target surface to punch the surface at different locations. The roughness of the hammered surface was in the range of 10 ␮m. Hammered samples were then annealed at 350 ◦ C for 1 h in an argon atmosphere. All samples were polished using SiC papers of 1200-grit and then etched using a 4 wt.%HNO3 solution. The specimen for wear test was a disc with 20 mm in diameter and 3 mm in thickness. 2.2. Characterization The size of grains in the nanocrystallized surface layer was determined using an atomic force microscope (AFM, Digital Instruments, CA, USA). Variations in hardness from top surface to bulk were measured at cross-section using a microindenter (Fisher Technology Ltd, Winsor, CT) with a diamond tip under a maximum load of 50 mN. Each hardness value is an average of at least three measurements. Dynamic polarization measurements were carried out using a commercial electrochemical system (model Pc4-750, Gamry Instruments Inc., Warminster, PA) by sweeping the potential at a scan rate of 3 mv/s from −0.4 to 0.1 V versus open corrosion potential (OCP) in a 3.5 wt.%NaCl solution. Prior to the electrochemical tests, samples with electrical connecting wires were stabilized for 50 min in the solution. The exposed face of 5 mm × 5 mm was the working electrode. A saturated calomel electrode (SCE) was used as the reference electrode and a platinum plate of 20 mm × 20 mm was used as the counter electrode. During the polarization test, the

3.1. Grain size and hardness with respect to the depth from top surface In order to determine the thickness of a nanocrystallinzed layer, cross-sectional hardness profiles of treated samples were measured using the micro-indenter under a maximum load of 50 mN. Fig. 2 illustrates the cross-sectional hardness profile of a sample punched for 1 h and subsequent annealed at 350 ◦ C for 1 h. As shown, the hardness was about 430 Hv at the top surface, decreased to a stable value of 90 Hv after the depth reached 150 ␮m. The hardness of the top surface was about four times higher than that of the bulk. These results indicate that thickness of the deformed layer was about 150 ␮m. Approximate grain sizes at different depths are also shown in Fig. 2. One may see that the grain size increases gradually with increasing the depth from the top surface, where the average grain size is 45 nm, and the thickness of the nanocrystalline layer (with average grain size below 100 nm) is about 60 ␮m. Since there is no material loss during hammering, the thicknesses of deformed layer and thus nanocrystalline layer can be increased by increasing the hammering load and time. This process could be more controllable and feasible, compared to other surface nanocrystallization processes such as shot peening [16,17], sliding wear [18] and sandblasting [19,20]. The relation between the hardness and d−1/2 was examined and is plotted in Fig. 3, which represents a form of Hall–Petch equation: H = 51.0 + 2146.3.d−1/2

(5)

3.2. Wear behavior Wear performances of surface nanocrystallized samples during dry sliding wear and wear in a 3.5 wt%NaCl solution were evaluated, respectively. Fig. 4 illustrates the wear rates versus the depth from top surface to bulk in air and in the 3.5 wt%NaCl solution, respectively. The wear rate is the volume loss (mm3 ) per unit sliding distance (m). As shown, the dry wear rate increased with an increase in depth, and then became stable when the depth was larger than about 150 ␮m. The wear rate in the NaCl solution with the depth showed a similar trend but the wear loss was larger than that in air due to the synergistic attack of corrosion and wear.

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h

head

Deformed layer

Sample

Fig. 1. Schematic illustration of a repeated hammering process: the head of hammer is moved up and down at a frequency of 50 Hz to impact the sample surface and introduce severe plastic deformation.

Wear tracks were observed under an optical microscope. Fig. 5 shows the wear tracks on microcrystalline and nanocrystalline surface experienced wear tests in air. As shown, both the wear tracks were shiny with plowing grooves in the sliding direction. Microcracks were observed on the microcrystalline surface, as indicated

by arrows. The presence of the micro-cracks corresponded to a lower resistance of the microcrystalline surface to wear involving tearing or plowing, compared to the nanocrystallized surface. As shown, the wear rate was significantly lower at the top surface where the grain size was at the nanometer scale. Since the

Fig. 2. Hardness versus the depth from top surface to inside a sample, which was hammered for 1 h at a frequency of 50 Hz followed by annealing at 350 ◦ C for 1 h in an argon atmosphere.

500

5.0

450

4.5

400

4.0

350

3.5

wear rate, mm /m

300

3

Hardness, Hv

X.Y. Mao et al. / Wear 271 (2011) 1224–1230

250 200 150

2.5 2.0 1.5 1.0

50

0.5 0.02

0.04

0.06

0.08

0.10

0.12

-1/2

Grain size, d (nm

0.14

0.16

0.18

0.20

)

0.004

0.006

0.008

0.010

0.012 −1

Fig. 6. Wear rate with respect to hardness (Hv

0.014

).

relation between the wear rate and hardness for the Cu–Ni alloy is presented in Fig. 6. Based on a simple linear regression analysis of the wear data and the determined relation between hardness and grain size, the relationship between the wear rate (mm3 /m) in air (Wair ) and the grain size (nm) of the surface nanocrystallized Cu–30Ni alloy could be expressed as

5

4

Wair = 6.82 + 0.17d−1/2

3 in air in NaCl solution

2

1

0

0.0 0.002

-1/2

Fig. 3. Hardness versus grain size (d−1/2 ).

Wear rate,mm3 /m

3.0

100

0.00

1227

0

30

60

90

120

150

180

210

Depth from surface, μm Fig. 4. Wear rates versus the depth from top surface measured in air and in a 3.5 wt%NaCl solution, respectively.

wear resistance largely depends on the hardness, the effectiveness of surface nanocrystallization in reducing wear is clearly demonstrated. For dry sliding wear without corrosion involved, wear of a material is mainly dependent on its hardness. A plot representing the

(6)

It may need to mention that during dry sliding wear tests, frictional heat is generated, which may lead to oxidation and thus influence the wear mechanism. In the present case, the sliding speed (0.01 m/s) for the wear tests was not high and the tests were performed in open air, so that the generated frictional heat might not sufficient to cause oxidation. However, if local oxidation did occur during the wear tests, the high-density grain boundaries in the nanocrystallized surface layer, in addition to the increased strength of the substrate, were beneficial to the oxide scale with enhanced bonding between the oxide and the nanocrystallized substrate, attributed to the effect of oxide pegging at grain boundaries [6]. This helped to increase the load-bearing capability of the oxide scale and thus benefited the overall resistance of the nanocrystalline to wear, compared to a microcrystalline surface. The wear behavior of the surface nanocrystallized Cu–30Ni alloy sample in the 3.5%NaCl solution showed a similar trend and the wear rate of the surface layer was considerably lower than that of the bulk. The elevated hardness of the nanocrystallized surface layer is one of beneficial factors. Another beneficial factor is

Fig. 5. Wear tracks on (a) untreated surface and (b) a nanocrystallized surface, respectively, generated by dry wear test.

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Fig. 7. Wear tracks on (a) untreated surface and (b) a nanocrystallized surface, respectively, generated by wear test in a 3.5 wt%NaCl solution.

3.3. Corrosion behavior and its influence on wear Corrosion behavior of the surface nanocrystallized alloy was investigated by measuring the corrosion current and potential at various depths from the top surface. As shown in Fig. 8, the corrosion resistance decreased with an increase in the depth, indicating that the layer with smaller nano-grains possessed higher resistance to corrosion. The corrosion resistance of Cu–30Ni alloy benefits from the formation of a CuO/Cu(OH)2 /Cu2 O protective film of layer can form on surface of Cu–30Ni alloy in NaCl solution [23]. The high-density grain boundaries accelerated atomic diffusion and thus increased the passivation rate. This helps to suppress the synergy of corrosion and wear. When the passive film was damaged by wear, faster re-passivation or faster self-repair capability reduced the exposure of the target surface to the corrosive environment. The increases in both corrosion resistance and harness led to a considerable decrease in the overall wear rate as illustrated in Fig. 4, which is also demonstrated by the narrowing of wear track on the nanocrystallized surface in comparison with that on the microcrystalline one as shown in Fig. 7. The faster atomic diffusion in a nanocrystalline passive material also makes the passive film more

-150

40

Corrosion potential,mV(SCE)

-160

35

Corrosion potential Passivation current

-170

30

-180 25

-190 -200

20

-210

15

-220

Passivation current,μA

the positive effect of nanocrystallization on corrosion resistance. More discussion on corrosion is given in the next section. Wear tracks on microcrystalline and nanocrystallized surfaces experienced wear in the 3.5%NaCl solution were also observed under the optical microscope, and obtained images are presented in Fig. 7. More micro-cracks were observed (indicated by arrows) when the surfaces were worn in the corrosive NaCl solution, compared to the case of dry wear. It is noticed that at top surface the difference in wear rate between dry wear and wear in the NaCl solution is negligible, which however increases as the depth from the top surface increases and eventually reaches a stable value as shown in Fig. 4. This implies that the synergy of wear and corrosion was largely eliminated at the top surface where the grain size was in the range of 45 nm. As the grain size increases, the effect of NaCl solution on wear rate is enhanced. Thus, nanocrystallization of passive materials has the capability of significantly suppressing corrosion-wear synergy, which can be very pronounced in many cases [21,22]. We also estimated the service lives of surface nanocrystallized sample and untreated one subjected to wear in air and in NaCl solution respectively, represented as the time for removing a layer of 150 ␮m in thickness (details are given in Appendix B). Compared to the untreated sample, the service life of surface nanocrystallized sample under dry wear condition is 2 times as long as that of untreated one, while decrease to 1.2 times when tested in the NaCl solution.

10 -230 5

-240 -250

0

30

60

90

120

150

180

0 210

Depth from surface, μm Fig. 8. Corrosion potential and passivation current versus the depth from top surface to inside a surface nanocrystallized sample.

compacted and more protective with less defects such as voids or vacancies, thus resulting in enhanced surface stability, improved mechanical properties and its adherence to the substrate [24,25]. 3.4. Friction behavior Tangential surface force is largely responsible for material removal by wear. The frictional coefficients of the nanocrystallized surface during dry and wet sliding wear processes were measured with respect to the depth from top surface. Results of the friction measurements are presented in Fig. 9. As shown, the friction coefficients decreased as the depth from the top surface decreased, corresponding to a decrease in the grain size. It is known that the friction coefficient is inversely proportional to hardness due to the reduced contacting area. The observed changes in friction coefficient with the depth from top surface are thus explainable. The friction coefficient of Cu–30Ni alloy in the NaCl solution was higher than that in air, which should be attributed to the enhanced destructive process in the corrosive solution, though clarification of the mechanism needs further studies on details of the local failure events. Friction comes from two sources: mechanical and electrochemical interactions. The former is affected by hardness and the latter is determined by the adhesion behavior. In this work, the adhesive force with respect to the depth from the top surface was measured using an atomic force microscope with a standard Si3 N4 tip in the ambient environment. Results of the measurement are presented

X.Y. Mao et al. / Wear 271 (2011) 1224–1230

sponding to the decrease in grain size, following the Hall–Petch relationship. (2) The resistance to dry wear increased from inside a sample to its top surface with a decrease in the grain size, mainly due to the increase in hardness. The situation is similar when tested in a 3.5%NaCl solution. In this case, both the increased hardness and enhanced corrosion resistance by nanocrystallization effectively suppressed the synergistic attack of wear and corrosion. (3) Surface nanocrystallization also reduced the friction coefficient through the reduction of the contact area with an increase in hardness and the decrease in the surface adhesive force when a more protective passive film developed on the nanocrystallized surface.

0.40 0.35

friction coefficient, μ

0.30 0.25 0.20 0.15 in air in NaCl solution

0.10 0.05 0.00

0

30

60

90

120

150

180

1229

210

Acknowledgements

Depth from surface, μm Fig. 9. Friction coefficients versus the depth from top surface of a surface nanocrystallized sample measured in air and in 3.5 wt%NaCl solution, respectively. 9 8

This work is supported by the Natural Science and Engineering Research council of Canada, Major State Basic Research Development Program of China (Grant No. 2007CB616903), and China Scholar Council (CSC). The authors would like to thank Dr. Xinhu Tang and Bin Yu for their assistance in operating various instruments and valuable suggestions.

7

Appendix A.

Adhesive force, nN

6

Fig. A1 schematically illustrates the cross-section of a wear track on a sample. The volume loss (V) depends on the indention of the pin ball that is pressed into the sample. The ball also has a volume loss (Fig. A2), which should be subtracted when calculating the volume loss of a target sample. As a result, the following equation is given to calculate the volume loss of a sample.

5 4 3 2 1 0

30

60

90

120

150

180

210

Depth from surface, μm Fig. 10. Adhesive force versus the depth from top surface of a surface nanocrystallized sample.

in Fig. 10. As shown, from the bulk to surface the adhesive force decreased and reached the minimum at the top surface with the smallest grain size. Such changes are attributed to (1) the decrease in the contact area as the hardness increased as a result of the decrease in grain size, and (2) the formation of a more protective passive film that reduced the surface reactivity or blocked the interaction between the surface electrons and the surrounding medium. The decrease in surface reactivity by promoted passivation through nanocrystallization has been well demonstrated by other materials such as stainless steel [20,26].

Fig. A1. Schematic illustration of the cross-section of a wear track.

4. Conclusions The main conclusions of this work are listed as follows: (1) Repeated hammering and subsequent recovery treatment provide an effective process for surface nanocrystallization. This process was applied to Cu–30Ni alloy to nanocrystallize its surface. Under the current condition, the average size of grains at top surface is about 45 nm and the thickness of the nanocrystalline layer (with average grain size below 100 nm) is about 60 ␮m. The depth of the deformed layer is larger than 150 ␮m. Hardness increased when approaching the top surface, corre-

Fig. A2. Worn area of a pin ball.

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V =

X.Y. Mao et al. / Wear 271 (2011) 1224–1230

R 2



 −

 R2 arcsin(w1 /2r) − w1 (4r 2 − w1 2 ) 45

 R2 arcsin(w2 /2r) − w2 (4r 2 − w2 2 ) 45





where R is the radius of worn track on the sample, r is the radius of the pin ball, and w1 and w2 are widths of the worn track on the sample and on the ball, respectively. Appendix B. The service life of sample was calculated in the following way. The layer with thickness h can be divided into n layers of equal thickness which are donated as h0 , h1 , . . . , hn−1 , and thus the time (t) to remove layer of thickness h is the sum of the time to remove each layer i.e. t = t0 + t1 + · · · + tn−1 . For an untreated sample: t = t0 + t1 + · · · + tn−1 =

= =

2RW

vball v 2RW

vball v 2RW

vball v

(h0 + h1 + · · · + hn−1 )

lim

n−1 

n→∞

(A.1) hi

i=0

·h

where R is the radius of worn track, W is the width of worn track, vball is the velocity of the pin ball, and v is the stable wear rate. For a nanocrystallized sample, t = t0 + t1 + · · · + tn−1 =

=

=

=

2RW

h

0

vball 2RW

vball 2RW

vball 2RW

vball

v0 lim

0

h

v1

+ ··· +

hn−1



vn−1

i

i=0

vi

(A.2)

n−1   1

n→∞



h1

n−1  h

n→∞

lim

+

i=0

1

v

vi

×

h n



dx

where the wear rate v is a function of depth (x). References [1] X.Y. Wang, D.Y. Li, Mechanical, electrochemical and tribological properties of nano-crystalline surface of 304 stainless steel, Wear 255 (7–12) (2003) 836–845.

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