Effects of hard water on tribological properties of DLC rubbed against stainless steel and brass

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Effects of hard water on tribological properties of DLC rubbed against stainless steel and brass M. Uchidate, H. Liu, K. Yamamoto, A. Iwabuchi

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S0043-1648(13)00522-X http://dx.doi.org/10.1016/j.wear.2013.10.002 WEA100844

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Received date: 9 April 2013 Revised date: 5 October 2013 Accepted date: 7 October 2013 Cite this article as: M. Uchidate, H. Liu, K. Yamamoto, A. Iwabuchi, Effects of hard water on tribological properties of DLC rubbed against stainless steel and brass, Wear, http://dx.doi.org/10.1016/j.wear.2013.10.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effects of hard water on tribological properties of DLC rubbed against stainless steel and brass M. Uchidate a,*, H. Liu a, K. Yamamoto b, A. Iwabuchi a a.Department of Mechanical Engineering, Iwate University, 3-5, Ueda 4-chome, Morioka, Iwate 020-8551, Japan b.Materials Research Laboratory, Kobe Steel Ltd., 5-5, Takatsukadai 1-chome, Nishi-ku, Kobe 651-2271, Japan *

Corresponding author. Tel.: +81 196 621 6417; fax: +81 196 621 6417 E-mail address: [email protected] (M. Uchidate)

Abstract This paper reports a study on the tribology of a diamond-like carbon (DLC) coating rubbed against stainless steel and brass in a hard water environment at 20, 50 and 80°C for water hydraulics applications. Hard water designed to simulate tap water in London containing 95 mg/L Ca2+ was used. DLC containing 30 at% hydrogen was deposited on AISI 630 stainless steel using an unbalanced magnetron sputtering system. The results showed that the wear on the DLC and the counter surfaces, in addition to the amount of friction, in hard water was smaller than that in soft water designed to simulate tap water in Tokyo containing a smaller amount of Ca2+ (23 mg/L). These superior tribological properties in hard water are attributed to a Ca-containing tribolayer formed on the steel and brass surfaces. Overall, the results suggest that attention must be paid to the quality of water used in water hydraulics applications. Keywords: DLC coatings, Water lubrication, Friction, Wear, Elevated temperature, Dissolved ions 1. Introduction Hydraulics using water instead of oil would be very beneficial for avoiding fire hazards as well as environmental and health risks related to leakage in service. It has been shown that diamond-like carbon (DLC) coatings exhibit superior tribological properties in water [1-5]. These earlier studies were carried out in pure water, although tap water would be the most convenient and cost effective water in many applications. The present authors studied the tribological behavior of DLC rubbing against steel and

brass in both pure water and soft water designed to simulate Tokyo tap water [6,7]. The results showed that dissolved ions in water have a significant influence on friction and wear. Costa et al. also showed the importance of dissolved ions on the tribological behavior of DLC in seawater and saline solutions [8]. Figure 1 shows the concentrations of dissolved ions in tap water sampled in Tokyo and some European cities (Leeds, London and Nottingham in the UK, and Wiesbaden in Germany). European hard water contains much larger amounts of minerals, primarily Na+, Mg2+, and Ca2+. Liu et al. [9] studied the effects of these dissolved ions on wear and friction of DLC rubbed against stainless steel in aqueous solutions, including MgSO4 and CaSO4, and found that Ca2+ in solution reduces wear of both materials. However, experimental data using hard water at elevated temperatures has not yet been reported. This paper reports a preliminary study of the tribology of DLC rubbed against brass and steel in hard water designed to simulate London’s tap water at temperatures

D issolved am ount,ppm

140 Leeds N ottingham Tokyo

120 100

London W iesbaden

80 60 40 20 0

C l-

N O 3- S O 42- N a+

K+

M g2+

C a2+

Ion

Fig. 1

Dissolved ion analysis of tap water sampled in Tokyo and selected European cities.

ranging from 20 to 80°C. 2. Experimental 2.1 Tribotester A ball-on-disk tribotester, shown schematically in Fig. 2, was used in this study. Three ball specimens were pressed against a disk specimen and rotated at a speed of 0.4 m/s. The turning radius of the ball specimens was 13 mm. Water, as described below, was filled in an autoclave and circulated at a rate of 20 mL/min. The water temperature was elevated using a constant temperature bath covering the autoclave. More details of the

Constant temp. bath Water

Autoclave Weight

Water for temp. control

Disk specimen

Ball specimens to PS.

Thermocouple

Water to the Potentiostat (PS.)

Motor

Fig. 2

Schematic diagram of the tribotester used in this study. Table 1

Experimental conditions used.

Sliding speed

0.4 m/s

Turning radius of the ball specimens

13 mm

Load

57 N

Number of revolutions

36000 cycles (Approx.)

Water pressure

0.1 MPa

Dissolved oxygen

7-8 ppm

Water temperature

（20, 50, 80）±2°C

Water

Hard water (Simulated London tap water) Soft water (Simulated Tokyo tap water)

tribotester have been reported in Ref. [6]. The experimental conditions used are summarized in Table 1. The average value of the coefficient of friction after running-in (800 m sliding distance) was calculated from friction data. Each run was repeated twice and an additional test was conducted if the test variations were larger than 40% in terms of wear and friction. 2.2 Specimens A 1-μm-thick DLC coating containing 30 at.% hydrogen was deposited using an

unbalanced magnetron sputtering system on disk specimens made of AISI 630 stainless steel (Fe-17Cr-4Ni-4Cu-1Nb) hardened to the H900 condition. This material is used in hydraulic components such as pumps and cylinders due to its high strength and good corrosion resistance. A mixed gas containing 10% CH4 and 90% Ar was introduced into the deposition chamber in order to add hydrogen to the DLC coating. A Cr interlayer and a Cr-C compositional gradient layer (50 nm and 200 nm thickness, respectively) were deposited between the substrate and the DLC to improve adhesion. The nanoindentation hardness of the DLC was 16 GPa. Details concerning the DLC have been described elsewhere [1, 6]. As counter materials, two kinds of balls were used in this research, a ball made from the hardened AISI 630 stainless steel described above and a P31CHL high tensile brass (Cu-26Zn-3Al-2Ni) ball. This high tensile brass is also used in hydraulic systems. The ball diameter was 9.5 mm for both materials. The Vickers hardness values for the steel and brass balls were HV485 (4.85 GPa) and HV196 (1.96 GPa), respectively. The surface roughness for all specimens was determined to be Ra 0.03 μm using a stylus profilometer. After the tests, the wear volumes for the DLC disks were measured from profiles obtained by the stylus profilometer. The wear volumes for the ball specimens were evaluated from the diameters of the wear scars. The specific wear rate was calculated by dividing the wear volume by the load and sliding distance. 2.3 Water In order to use water with the same concentration of dissolved ions consistently throughout the experiments, it was necessary to prepare hard water in our laboratory. Thus, hard water designed to simulate London’s tap water was prepared. The reason for choosing this form of water is that it has the largest concentration of Ca2+ among the various water samples explored, which could have a significant influence on the tribological behavior (Fig. 1) [9]. The results of an ion analysis of London tap water and the prepared hard water, formed using HCl, HNO3, H2SO4, KOH, Mg(OH)2, NaOH, and CaCO3, is shown in Table 2. It should be noted that a small amount of dry ice, solid CO2, was placed in the water during the mixing process in order to dissolve CaCO3, since the solubility of CaCO3 is quite low (1.4∼1.5mg/100g at 20°C). By adding the dry ice, Ca(HCO3)2, which is soluble, is formed as follows [10]. CaCO3 + CO2+H2O → Ca(HCO3)2 Ca(HCO3)2 → Ca2+ +

2HCO3−

(1) (2)

The concentrations of the dissolved ions in soft water designed to simulate Tokyo tap

Table 2 Composition of London tap water and two kinds of simulated tap water used in this study.

Cl−

Hard water

Hard water

Soft water

(London tap water)

(Simulated London

(Simulated Tokyo

tap water)

tap water )

45

45

31

3−

26

25

13

2−

54

54

37

Na+

38

41

24

+

5.7

5.6

2.5

4.5

4.1

5.3

97

95

23

E.C.(mS/m)

70

69

29

pH

8.3

8.2

7.6

Dissolved ion mg/L

NO

SO4

K

Mg Ca

2+

2+

water are also shown in Table 2. Experimental results using this soft water described in Refs 6 and 7 are also shown for The amount of dissolved oxygen in the water was controlled by aeration of N2 and O2 gas in a reserve tank. In this study, the amount of dissolved oxygen was maintained at 7−8 ppm, which is the saturated

condition

at

standard

atmospheric pressure. 3. Results 3.1 DLC vs. Stainless Steel Figure 3 shows the specific wear rates

S pecific w ear rate, m m 3/(N ⋅m )

comparison.

D LC in soft w . D LC in hard w .

S teelin soft w . S teelin hard w .

10-6

10-7

10-8

10-9

0

20

40

60

80

100

W ater tem perature,°C Fig. 3 Specific wear rates for DLC disks and stainless steel balls.

for the DLC disks and stainless steel balls in hard and soft water. The wear for DLC increased with increasing temperature in both types of water. The wear rate for DLC in hard water ranged from 6×10−9 to 2×10−8 mm3/(N⋅m) and was 3-20 times lower than that in soft water. SEM images and surface profiles of the DLC disks tested in hard water are shown in Fig. 4. The surface of the DLC wear scar became rough at 80°C. The wear depth in the DLC was smaller than 1 μm under all the conditions tested and no clear sign of delamination was observed. On the other hand, the wear for the steel counter

surfaces in hard water decreased at elevated temperatures [7×10−9 mm3/(N⋅m) at 20°C and 3×10−9 mm3/(N⋅m) at 80°C], whereas it stayed constant at around 1×10−8 mm3/(N⋅m) in soft water. SEM and EPMA images of the ball specimens after the tests in hard water at 20 and 80°C are shown in Figs. 5 and 6, respectively. Ca was found on the front side of the wear scar at 20 °C (Fig. 5). At 80°C (Fig. 6), a larger area around the wear scar was covered with Ca. Also, O, probably in the form of some kind of oxide, was detected in the wear scars, and a small amount of C was also observed. In the case of the DLC surfaces, no clear change in chemical composition was found by EPMA.

Sliding Direction

Sliding Direction of

of the Ball Specimens

the ball specimens

Contact Circle Contact Circle

(a) SEM (20 °C)

(b) SEM (80 °C)

0.3 μm 100 μm (d) Profile (80 °C)

(c) Profile (20 °C)

Fig. 4 SEM images and surface profiles of a DLC disk surface rubbed against

Sliding Direction

stainless steel balls.

100 μm

Contact Circle

Fig. 5

(a) SEM

(b) Fe

(c) C

(d) O

(e) Ca

SEM and EPMA images of the wear scar on the stainless steel ball tested at 20°C.

Sliding Direction 100 μm

Contact Circle

Fig. 6

(a) SEM

(b) Fe

(c) C

(d) O

(e) Ca

SEM and EPMA images of the wear scar on the stainless steel ball tested at 80°C.

The coefficient of friction as a function of sliding distance at various temperatures is shown in Fig. 7. In all cases, the coefficient of friction decreased during the initial running-in and then increased. The average coefficient of friction shown in Fig. 8 indicates that friction increased slightly in hard water at elevated temperatures (0.13 – 0.15). However, the effect of temperature was much weaker than in soft water wherein the coefficient of friction increased up to 0.3 [6].

0.3

50℃

in soft w ater

80℃

D LC vs.S tainless steel

D LC vs.S tainless steelin H ard w ater 50℃

0.2

80℃

0.1

20℃ 0.0

0

1000

2000

in hard w ater

0.3

C oefficient of friction

Coefficient of friction

20℃

3000

0.2

0.1

0.0 0

Sliding distance, m

20

40

60

80

100

W ater tem perature,℃

Fig. 7 Friction behavior in hard water; DLC vs. stainless steel.

Fig. 8 Average coefficient of friction in soft and hard water; DLC vs. stainless steel.

3.2 DLC vs. Brass Figure 9 shows the specific wear rate for the DLC coated disks and brass balls in hard and soft water. The wear rates for both materials in hard water were smaller than in soft water. More specifically, the wear rate for brass in hard water was about half that in soft water. The wear rate for the DLC and brass in hard water was almost constant at all the temperatures tested [about 6×10−8 mm3/(N⋅m) for DLC and 1×10−8 mm3/(N⋅m) for brass]. As shown in Fig. 10, the values of the coefficient of friction in hard water were in the range 0.07-0.08 at 20 and 50°C, and increased to 0.14 at 80°C, whereas those in soft water increased to 0.17 and 0.24 at 50 and 80°C, respectively. The dependence of the coefficient of friction on the sliding distance for DLC vs. brass was similar to that for

10

B rass in soft w . B rass in hard w .

in soft w ater

in hard w ater

0.3

-6

D LC vs.B rass C oefficient of friction

S pecific w ear rate,m m 3/(N ⋅m )

D LC in soft w . D LC in hard w .

10-7

10-8

0.2

0.1

0.0

10

0

-9

0

20

40

60

80

100

W ater tem perature,°C

Fig. 9 Specific wear rate for the DLC disks and brass balls.

20

40

60

80

100

W ater tem perature,℃

Fig. 10 Average coefficient of friction in soft and hard water as a function of t t t DLC b

DLC vs. steel; a decrease in the initial running-in followed by an increase. SEM and EPMA images of the brass ball after the tests in hard water at 20 and 80°C, shown in Figs. 11 and 12, reveal that the wear scar was covered with Ca and O. The larger area was covered at 80°C. Al, which caused clear abrasive wear of DLC in pure water as described in Ref. [7], was found on the back side of the wear scar. Again, no

Sliding Direction

change in chemical composition was found on the DLC surfaces by EPMA.

100 μm

Contact Circle (a) SEM

(c) C Fig. 11

(b) Cu

(d) O

(c) Al

(e) Ca

SEM and EPMA images of the wear scar on the brass ball tested at 20°C.

Sliding Direction

Contact Circle

100 μm (a) SEM

(b) Cu

(c) C

(d) O

Fig. 12

(c) Al

(e) Ca

SEM and EPMA images of the wear scar on the brass ball tested at 80°C.

4. Discussion The experimental results showed that hard water decreases wear and friction for DLC vs. stainless steel and DLC vs. brass, especially at elevated temperatures. As described in previous papers [1,3,5,6,7,11, 12], our results can be understood in terms of a tribolayer, which is a smeared layer that forms on the counter surfaces. Yamamoto reported that the tribolayer was a mixture of elements from the counter material and carbon from DLC, and had a thickness of 300-350 nm [1]. Our previous experiments using soft water have shown that not only the mating materials but also dissolved ions such as Ca and Mg are included in the tribolayer [6, 7]. In the present work, accumulation of Ca on or around the wear scars on the ball specimens was observed as shown in Figs. 5, 6 and 11. Formation of a Ca-rich tribolayer can be the result of the unique nature of the solubility of Ca at elevated temperatures. It is known that Ca2+ accumulates as CaCO3 at elevated temperatures [10]. Ca2+ + 2HCO3−→ CaCO3↓ + CO2 + H2O

(3)

This means that the solubility of Ca, unlike most elements, decreases with increasing temperatures. This is why a larger amount of Ca was detected on the wear scars at elevated temperatures. The Ca in the tribolayer may be in the form of CaCO3 or CaO, which are known to be lubricious [13-15]. This tribolayer may have been gel-like [7,16] and may have reduced wear and friction by preventing direct contact at asperities between DLC and the counter surfaces. The results obtained by Liu et al. [9] also suggest the importance thicker

tribolayer

at

elevated

temperatures protected the rubbing surfaces and reduced wear. However, the

thick

tribolayer

reduces

the

effective hardness and increases the contact real area [17]. This may result in an increase in shear resistance and friction [17]. We believe that this is the cause of the increase in friction at 80°C for both DLC vs. steel and DLC vs. brass.

This

also

explains

the

Specific w ear rate,m m 3/(N ⋅m )

of Ca in the solution (Fig. 13). The ×[10-8]

D LC

A ISI630

8 6 4 2 0

P ure Soft KC l KN O K SO N a SO M gSO C aSO 2 4 2 4 4 4 3 w ater w ater

Fig. 13 Specific wear rate for DLC disks and stainless steel counter surfaces in pure water, tap water (Tokyo), and aqueous solutions. Reworked data from Ref. 9.

dependence of the coefficient of friction on the sliding distance. The decrease during the initial running-in is attributed to the inhibition of direct contact between the DLC and counter material due to the growing tribolayer, and the subsequent increase is attributed to shear at the increased contact area due to the thick and soft tribolayer. According to a calculation based on analytical solution of a frictional heating problem [18], the temperature rise due to friction was estimated to be 50−120°C for DLC vs. stainless steel and 20−30°C for DLC vs. brass (the coefficient of friction was assumed to be 0.15, which corresponds to the values at 80°C, and the contact radius was assumed to be 250 μm based on SEM observations).The average surface temperature when the DLC disks are rubbed against stainless steel can reach 80 +120 = 200°C at 80°C and can be higher at the actual contact region, which may enhance graphitization at the top of the contact asperities [19, 20]. Therefore, the wear for DLC increased even in hard water. Furthermore, a thicker oxide layer on the steel surface due to a temperature rise can inhibit the formation of a C-rich transfer layer [11]. This can also increase the DLC wear and friction due to more frequent direct contact between the DLC and steel. In the case of DLC vs. brass, the temperature rise was much smaller and therefore the increase in wear was insignificant. 4. Conclusions This study examined the effects of the presence of hard water on the tribological behavior of DLC rubbed against stainless steel and brass at elevated temperatures (20−80°C). Hard water designed to simulate London tap water containing 95 mg/L Ca2+ was used. Simulated Tokyo tap water was used for comparison (23 mg/L Ca2+ concentration). It was found that Ca2+ in hard water has a favorable effect on the reduction of wear and friction due to the formation of a tribolayer containing Ca and O, especially at elevated temperatures. Thus, hard water can be better than soft water for water hydraulics from a tribological point of view. However, attention must be paid to the quality of the water during operation. Examination of other kinds of contaminants such as F+ (fluorine) and fine particles such as sand and biofilms (bacteria) will be carried out in future work. Also, tests using actual water hydraulic systems should be performed since the sliding motion (one directional or reciprocal), sliding speed, contact conditions including tilting and vibration, and water flow, which affect tribolayer formation, can vary depending on the actual applications.

Acknowledgement This work was performed as part of the Japanese national research project on Smart Materials for Tribo-Systems in Drive Units and was supported by NEDO. References [1] K. Yamamoto, K. Matsukado, Effect of hydrogenated DLC coating hardness on the tribological properties under water lubrication, Tribology International, 39 (2006) 1609-1614. [2] M Suzuki, T. Ohana, A. Tanaka, Tribological properties of DLC films with different hydrogen contents in water environment, Diamond and Related Materials, 13 (2004) 2216-2220. [3] T. Ohana, X. Wu, A. Tanaka, Formation of lubrication film of diamond-like carbon films in water and air environments against stainless steel and Cr-plated balls, Diamond and Related Materials, 16 (2007) 1336-1339. [4] H. Ronkainen, S. Varjus, K. Holmberg, Tribological performance of different DLC coatings in water-lubricated conditions, Wear, 249 (2001) 267-271. [5] F. Majdič, I. Velkavrh, M. Kalin, Improving the performance of a proportional 4/3 water-hydraulic valve by using a diamond-like-carbon coating, 297 (2013) 1016-1024. [6] M. Uchidate, H. Liu, A. Iwabuchi, K. Yamamoto, Effects of water environment on tribological properties of DLC rubbed against stainless steel, Wear, 263 (2007) 1335-1340. [7] M. Uchidate, H. Liu, A. Iwabuchi, K. Yamamoto, Effects of water environment on tribological properties of DLC rubbed against brass, Wear, 267 (2009) 1589-1594. [8] R.P.C Costa, F.R. Marciano, D.A. Lima-Oliveira, E.J. Corat V.J. Trava-Airoldi, Tribological effect of iron oxide residual on the DLC film surface under seawater and saline solutions, Surface Science, 605 (2011) 783-787. [9] H. Liu, A. Iwabuchi, M. Uchidate, T. Shimizu, The effect of dissolved ions on the

tribological properties of DLC coating against stainless steel under water lubrication, Proc. ASIATRIB 2006 Kanazawa, (2006) 167-168. [10] M. Joss, Leaching of concrete under thermal influence, Otto-Graf-Journal, 12, (2001)51-68. [11] J. Fontaine, T Le Mongne, J.L. Loubet, M. Belin, Achieving superlow friction with hydrogenated amorphous carbon: some key requirements, Thin Solid Films, 482 (2005) 99-108. [12] D.C. Sutton, G. Limbert, B. Burdett, R.J.K. Wood, Interpreting the effects of interfacial chemistry on the tribology of diamond-like carbon coatings against steel in distilled water, Wear, 302 (2013) 918-928. [13] M. Zhang, X. Wang, X. Fu, Y. Xia, Performance and anti-wear mechanism of CaCO3 nanoparticles as a green additive in poly-alpha-olefin, Tribol. Int., 42 (2009) 1029-1039. [14] A. Greenall, A. Neville, A. Morina, M. Sutton, Investigation of the interactions between a novel, organic anti-wear additive, ZDDP and overbased calcium sulphonate, Tribol. Int., 46 (2012) 52-61. [15] J. Wang, F. Yan, Q Xue, Tribological behavior of PTFE sliding against steel in sea waer, Wear, 267 (2009) 1634-1641. [16] M. Uchidate, A. Iwabuchi, T. Kanno, H. Liu, Nano-Indentation Measurement of the tribo-layer on stainless steel and brass surfaces rubbed against DLC in water, Tribology Online, 3, 2(2008)48-53. [17] J. Halling, Surface Coatings materials conservation and optimum tribological performance, Tribology International, 12, 3 (1979) 203-208. [18] J. F. Archard, The temperature of rubbing surfaces, Wear, 2 (1958/59) 438-455. [19] A. Vanhulsel, B. Blanpain, J.-P. Celis, J. Roos, E. Dekempeneer, J. Smeets, Study of the wear behaviour of diamond-like coatings at elevated temperatures, Surf. Coat. Technol., 98 (1998) 1047–1052.

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Highlights Wear and friction of DLC vs. stainless steel and DLC vs. brass in hard water were smaller than in soft water. Wear and friction in hard water at elevated temperatures were smaller than in soft water． Accumulation of Ca on or around the wear scars of the stainless steel and brass was observed. Ca2+ in hard water has a favorable effect on the reduction in wear and friction due to a tribo-layer formation. Attention must be paid to the quality of water used in water hydraulics applications.