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Hydrogen content variation for enhancing the lubricated tribological performance of DLC coatings with ester
Surface & Coatings Technology 205 (2011) S89–S93
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Hydrogen content variation for enhancing the lubricated tribological performance of DLC coatings with ester Kirsten Bobzin, Nazlim Bagcivan, Sebastian Theiss, Koray Yilmaz ⁎ Surface Engineering Institute, RWTH Aachen University, Kackertstrasse 15, D-52072 Aachen, Germany
a r t i c l e
i n f o
Article history: Received 10 September 2010 Accepted in revised form 28 February 2011 Available online 5 March 2011 Keywords: DLC Hydrogen content Pin-on-disk tribometer TMP ester
a b s t r a c t There are many types of DLC (diamond-like carbon) coatings, which mainly differ from each other according to their hydrogen content, sp2 and sp3-bonded carbon atoms and alloying elements (such as Ti, Cr, W or Zr). The lubricated tribological performance of these coatings depends on the lubricant. Controversially, the same lubricant delivers different tribological performances with differently produced DLC coatings. It has been observed that the presence of hydrogen has a remarkable effect on the tribological performance of DLC coatings in inert and vacuum environments. In this paper, the hydrogen content of two different types of DLC coatings, a-C:H:Me (metal containing hydrogenated amorphous carbon) and a-C:H, was varied, in order to obtain an optimized tribological behavior with a synthetic ester (TMP ester). The tribological performance of the coatings with TMP ester is examined in a pin-on-disk tribometer. It could be shown that increasing the hydrogen content in a DLC coating improves their tribological performance with TMP ester. Besides, a-C:H type of coatings is found to be more suitable for TMP ester regarding low friction coefficients. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Diamond-like carbon coatings (DLC) are probably one of the most important modern coatings, which are inevitable for many tribological systems, due to their excellent mechanical and tribological properties, such as high hardness, self lubricating capability and high resistance against corrosive degradations [1,2]. As a result, the excellent tribological behavior under dry and lubricated contacts makes them very attractive for several industrial applications [3]. It is known that hydrogen containing DLC coatings provide very low friction coefficients in inert and vacuum environments [4]. Contrary, the hydrogen free coatings exhibit very high friction coefficients in such environments [5]. But, if humidity is introduced, low friction coefficients are also achieved for hydrogen free DLC coatings [4,6]. This phenomenon is explained by elimination of dangling σ-bonds of carbon by hydrogen on the surface. This prevents the adhesion of the tribo pairs and thus allows low friction coefficients [7]. The same effect on hydrogen free coatings can also be achieved, if hydroxyl group containing additive, such as glycerol mono-oleate (GMO) is added to lubricant [3,8]. Barros Bouchet et al. proved the existence of –OH layer after tribological results of DLC coatings with GMO containing polyalphaolefin [9]. Therefore, the presence of hydrogen on the surface of the DLC coatings is essential for a good tribological performance.
⁎ Corresponding author. Tel.: +49 241 80 93692; fax: +49 241 80 92941. E-mail address: [email protected] (K. Yilmaz). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.02.065
Additional to ta-C, which delivers very low friction coefficients with GMO additive containing lubricants, hydrogenated DLC (a-C:H) coatings may also enable lower friction coefficients, if they are adapted to lubricants [10,11]. Hence, optimization of the tribological performance of these coatings for selected lubricants may offer an alternative to that of ta-C with GMO. One promising lubricant is TMP ester. TMP ester possesses a high amount of hydroxyl group (–OH) and this allows very low friction of DLC coatings [10,12]. Besides, these lubricants are mostly environmentally friendly and they can be completely degraded [12]. Therefore, the optimization of DLC and lubricant with each other offers decreasing friction losses in tribological systems. Furthermore, by means of this adaptation lower friction coefficients may be achieved for environmentally friendly systems. In this study, we tried to show the adaptation possibility of hydrogen containing DLC coatings with TMP ester in order to achieve a better tribological performance. For this, two different types of DLC coatings, aC:H and a-C:H:Me, are deposited. The mechanical properties of the same type of coatings are very similar to each other; mainly their hydrogen content is different. The DLC coatings with TMP ester are investigated under boundary lubrication in a pin-on-disk tribometer. 2. Experimental setup 2.1. Coating deposition The DLC coatings were deposited on case-hardened (HRC 62) 16MnCr5 (AISI 5115) polished round samples in an industrial CC800/ 9 coating facility from CemeCon AG Wuerselen, which is equipped
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K. Bobzin et al. / Surface & Coatings Technology 205 (2011) S89–S93
Table 1 Designation, deposition technology and the precursor gases of the DLC coatings.
a-C:H:Zr-1 a-C:H:Zr-2 a-C:H:Zr-3 a-C:H-1 a-C:H-2
Interlayer
Deposition technology
Top coat/ precursor gas
Deposition technology
ZrC ZrC ZrC CrN CrN
MSIP MSIP MSIP MSIP MSIP
a-C:H:Zr/C2H2 a-C:H:Zr/C2H2 + H2 a-C:H:Zr/CH4 + H2 a-C:H/C2H2 + H2 a-C:H/C2H2 + H2
MSIP MSIP MSIP PACVD PACVD
with two cathodes. The mean surface roughness (Ra) of the round samples, which were used for characterization of the coatings and the tribological tests in pin-on-disk tribometer, was better than 0.008 μm. Prior to deposition the samples were cleaned in a multi stage ultrasonic bath, which contains alkaline solvents for a better removal of corrosion inhibitor and/or polishing paste on round samples. The coating chamber was pumped to an initial pressure of 2 mPa. After reaching this initial pressure, the coating chamber was heated up for 3600 s in order to remove residual humidity and outgasing materials. The maximum observed temperature on samples was 160 °C. After the heating phase, the surface of the samples was cleaned by means of argon ion etching. The heating and etching phase were the same for a-C:H and a-C:H:Zr. a-C:H:Zr type of coatings was deposited by a reactive sputtering process of Zirconium with acetylene (C2H2) or methane (CH4). A DC type of the power supply was applied on the zirconium targets. In order to achieve a good adhesion, a ZrC interlayer deposited prior to deposition of the a-C:H:Zr top layer. During the deposition of a-C:H:Zr-1 and a-C:H:Zr-2 acetylene and a-C:H:Zr-3 methane was used as precursor gas. A unipolar pulse type bias voltage at a pulse frequency of 250 kHz was applied. Its magnitude was ramped from −50 V to −100 V during the deposition. The a-C:H type coatings were deposited using PACVD (Plasma Activated Chemical Vapor Deposition). For the deposition of the a-C:H top layer acetylene and hydrogen were used as precursor gas. The plasma is generated using a pulsed MF type power supply at −450 V and −550 V for a-C:H-1 and a-C:H-2 respectively. The pulse frequency and duty cycle were 350 kHz and 61% respectively. The C2H2 flow for a-C:H-1 was kept constant at 60 sccm, whereas H2 flow was 40 sccm. The gas flows for a-C:H-2 were set to 20 sccm for C2H2 and 60 sccm for H2. Prior to deposition of the a-C:H top layer, CrN and CrCN interlayers are deposited in order to attain a good adhesion on the substrate. The interlayers are deposited using the DC MSIP (Magnetron Sputter Ion Plating) technology and Cr and graphite targets. The dimension of the targets was 88 × 500 mm2. The purity of chromium and graphite targets was 99.95% and that of zirconium targets was 99.99%. The deposited coatings and their deposition technique as well as the material source during the deposition are summarized in Table 1.
2.3. Coating characterization methods The thickness of the coatings was determined by crater grinding method according to the standard DIN EN 1071–2 [13]. The mechanical properties, universal hardness and Young's modulus, of the coatings were determined by nanoindentation measurements (Nanoindenter XP, MTS Nano Instruments) according to the method of Oliver and Pharr [14]. During the measurements the indentation depth was kept below 1/ 10 of the coating thickness. The carbon and zirconium ratio of the a-C:H:Zr coatings is determined by GDOES (Glow Discharge Optical Emission Spectroscopy) measurements. The hydrogen content of a-C:H was determined by NRA (nuclear reaction analysis) at the Forschungszentrum Dresden-Rossendorf (FZD), Germany. The NRA uses a core reaction (see Eq. (1)) in order to determine the hydrogen content. Gamma radiation is emitted with a constant photon energy and the ray intensity is directly proportional to the quantity of gamma particles. These gamma particles are measured by means of a gamma detector and demonstrate the measurement signal. The measured quantity of the gamma particles is 15 proportional to the quantity of N ions (also called monitor pulses), dihedral angle of the gamma detector, efficiency of the gamma detector and the concentration of the hydrogen in the sample. In order to convert the measured gamma particle quantity into the hydrogen concentration, a comparison measurement on a reference material (polyimide) with a known hydrogen content is carried out. The actual hydrogen content is 15 obtained from the basic proportional calculations. The energy of the N ions was set at 6582 keV, which corresponds to a measurement depth of 100 nm. The measurement depth of 100 nm was chosen, in order to eliminate the influence of humidity. Ten measurements are carried out for each specimen. The measured hydrogen contents are medium values of these ten values. 15
12
ð1Þ
N + p→ C + alpha + gamma
2.4. Pin-on-disk tests The tribological performance of the plasma coatings was analyzed using a pin-on-disk tribometer. During the measurements the samples were put into a lubricant pot, with which a continuous lubrication during the measurements is ensured. An initial maximum Hertzian contact pressure of 1800 MPa was applied by means of static loads. As counterbody uncoated case-hardened 16MnCr5 (HRC 62) pins were used. During the tribological tests the temperature of the samples and oil was set to 100 °C. The linear speed of the pin on the test sample was set to 0.1 m/s. The chosen test parameters correspond to boundary lubrication regime. The total sliding distance for each experiment amounts to 1000 m. 3. Results and discussion
2.2. Characteristic of TMP ester
3.1. Coating characterization
For the tribological investigations a TMP ester was used. This lubricant contains 3% S–P additive and its density at 25 °C is 0.882 g/m3. It has at 40 °C and at 100 °C kinematic viscosities of 136.3 mm2/s and 19.5 mm2/s, respectively.
The results of the characterization of the samples, which were carried out in accordance with the aforementioned methods, are summarized in Table 2. In order to give a better overview, the coatings are designated according to their hydrogen content. Due to the
Table 2 Overview of the characteristics of the coatings.
Coating thickness (μm) Hardness (GPa) Young's modulus (GPa) Hydrogen content [at%] C/Zr ratio Ra (μm)
a-C:H:Zr-1
a-C:H:Zr-2
a-C:H:Zr-3
a-C:H-1
a-C:H-2
a-C:H:Zr-39.3
a-C:H:Zr-38.2
a-C:H:Zr-32.3
a-C:H-23.0
a-C:H-21.4
6.8 6.5 ± 0.8 71 ± 6 39.3 90/10 0.008
6.5 7.2 ± 1 76 ± 7 38.2 95/5 0.008
3.8 10.4 ± 1.6 155 ± 12 32.3 80/20 0.013
1.2 33.9 ± 3.5 287 ± 20 23 – 0.08
1 34.8 ± 2.1 305 ± 20 21.4 – 0.04
K. Bobzin et al. / Surface & Coatings Technology 205 (2011) S89–S93
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Fig. 1. Hydrogen content of a-C:H:Zr type (left) and a-C:H type (right) DLC coatings.
Fig. 2. Progression of friction coefficients on the a-C:H:Zr-39.3 (left) and a-C:H:Zr-38.2 (right) over the distance in pin-on-disk tribometer with TMP ester.
reactive sputtering of Zr, a-C:H:Zr types of coatings possess considerably higher coating thicknesses. Contrary, the two a-C:H coatings have thin top layers due to low deposition rates of the PACVD process and are about 1 μm. The hydrogen content of a-C:H:Zr and a-C:H types of coatings are shown in Fig. 1. With increasing monitor pulses, the measured hydrogen concentration decreases for a-C:H:Zr type of coatings. This points out that hydrogen is weakly bounded in the coatings. Actually, the hydrogen in a-C:H:Zr-32.3 possesses the weakest bonding, such that it decreases strongly. In contrast, the hydrogen concentration measurements for a-C:H type coatings stay constant with increasing monitor pulses, which indicates stable bonded hydrogen with carbon. The a-C:H:Zr type coatings possess very high hydrogen concentrations. The addition of H2 into the deposition process for a-C:H:Zr-38.2 decreased the hydrogen concentration slightly in the DLC coating. One possible explanation is that during deposition, acetylene (C2H2) is reduced by excited hydrogen and looses hydrogen. This also leads to high carbon concentration on the top layer, such that the C/Zr ratio of a-C:H:Zr-38.2 is higher than the one of a-C:H:Zr-39.3. In case of utilizing methane as precursor gas, the deposited a-C:H:Zr-32.3 possesses lower hydrogen concentration, than that of with acetylene. In addition, the C/Zr ratio of this coating is also lower than both with acetylene. If the two a-C:H coatings are considered, they possess lower hydrogen concentrations.
is considered, the two measurements show different behavior. During the first measurement, the friction coefficient shows a constant decrease and at the end approaches to 0.08. But the second measurement shows a different progression, such that it reaches 0.08 rapidly at the very beginning, immediately after it increases to 0.09 and stays till the end of the measurement approximately constant. Fig. 3 shows the friction coefficient progression of a-C:H:Zr-32.3 with TMP ester. Both of the measurements show similarities, such that at the beginning the friction coefficients increase to 0.14 and then start to decrease. At the end, the friction coefficient of the first measurement approaches to 0.10, whereas the second one to 0.09. The different progressions of friction coefficients between the measurements are believed to be the effect of weakly bounded hydrogen. Such that if hydrogen is weakly bounded, the friction reducing effect that is formed during measurement due to the presence of hydrogen, can be easily destroyed. This may lead to an increase in adhesion
3.2. Friction behavior of the DLC coatings with TMP-ester The progression of friction coefficients for a-C:H:Zr-39.3 and a-C:H:Zr-38.2 in pin-on-disk tribometer is shown in Fig. 2. The first measurement of a-C:H:Zr-39.3 reaches to a constant value of 0.07 after 400 m, whereas the second measurement reaches this value already at 200 m. After this running in behavior, both measurements show very good agreement with each other. If a-C:H:Zr-38.2
Fig. 3. Progression of friction coefficients on the a-C:H:Zr-32.3 over the distance in pinon-disk tribometer with TMP ester.
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Fig. 4. Progression of friction coefficients on the a-C:H:Zr-32.3 over the distance in pin-on-disk tribometer with TMP ester.
forces between the tribo pairs and in return the friction coefficient. The spikes, which occur during the progression of friction coefficient, can also be explained by means of this phenomenon. The progression of the friction coefficients for the measurements on a-C:H-21.4 and a-C:H-23.0 is shown in Fig. 4. The friction progression for both measurements of a-C:H-21.4 has a good agreement and the friction coefficients approaches to 0.05 at the end of the measurements. The progression of friction coefficient for a-C:H-23.0 has a slight deviation. The first measurement approaches after 200 m 0.030 and reaches 0.024 at the end, while the second one has a shorter running-in phase and decreases to 0.035 in 100 m. At the end the friction coefficient decreases to 0.030. If the average friction coefficients are considered (see Fig. 5), the effect of the hydrogen concentration on the friction behavior will be apparent. With increasing hydrogen content the average friction coefficients on DLC coatings decrease. In a former work, using XPS (X-Ray photoelectron spectroscopy) the presence of oxygen could be shown in wear track of a-C:H after the measurements in pin-on-disk tribometer. This indicates the existence of hydroxyl group [11]. Thus, increasing the hydrogen content may increase the concentration of this hydroxyl group on the coating surface. By means of that, a better separation of tribological contacts in boundary lubrication regime is possible, which in return improves the friction behavior. Nevertheless, it is also convenient to consider the effect of surface roughness on the friction coefficients for a-C:H:Zr type of coatings. Both a-C:H:Zr-39.3 and a-C:H:Zr-38.2 possess lower surface roughness than a-C:H:Zr-32.3 at the beginning. The friction coefficients for a-C:H:Zr-39.3 and a-C:H:Zr-38.2 after the running in period decrease to 0.07 and 0.08 respectively. Analogously, the friction coefficient for a-C:H:Zr-32.3 reduces to 0.09. Therefore, the difference between the friction coefficients for the a-C:H:Zr type of coatings after the running in is not any longer significant.
However, the friction coefficients of the a-C:H:Zr coatings with higher hydrogen content are still lower than those with lower hydrogen content after running in. But the high Zr content in a-C:H: Zr-32.3 can also be the reason for this slight difference. Therefore, further investigations are necessary to find out the effect of the Zr content on the friction coefficient under lubricated conditions for a-C:H:Zr type of coatings. Besides, the effect of hydrogen content on tribological behavior depends on the type of DLC coatings, such that, although the hydrogen content in a-C:H:Zr is higher than that of a-C: H, they deliver higher friction coefficient. One possible explanation is the differences in mechanical properties. Due to lower hardness of a-C:H:Zr, the contact area on a-C:H:Zr type coatings is higher than on a-C:H and this leads to increased frictional forces [15]. Another possible explanation is the effect of the Zr content, such that due to Zr the adhesion tendency of the coatings is increased, which in return increases the friction coefficients. The effect of adhesion can clearly be seen for the friction progression for a-C:H:Zr-32.3, which possesses the highest Zr content. 4. Conclusion The aim of this work was to find out the optimization potential of frictional behavior of hydrogenated DLC coatings with TMP ester. For the optimization, the hydrogen content of two different types of DLC coatings, a-C:H and a-C:H:Zr, was varied. It is found that by means of increasing the hydrogen content, the friction coefficient of the coatings with TMP ester could be decreased. Nevertheless, this improvement depends on the DLC type, and lower friction coefficients are achieved for a-C:H than a-C:H:Zr. A future work with increased – OH groups in TMP ester by of mixing GMO is intended. By means of that it is possible to emphasize the influence of hydrogen content increase in DLC coatings for the reduction of friction losses. Furthermore, it is also intended to investigate the effect of the Zr content whilst keeping the hydrogen content and mechanical properties constant in order to decouple the effect of Zr and hydrogen concentration on the friction coefficient. References [1] [2] [3] [4] [5] [6] [7]
Fig. 5. Average friction coefficients of the DLC coatings.
A. Erdemir, C. Donnet, J. Phys. D Appl. Phys. 39 (2006) 311. A. Erdemir, C. Donnet, in: G.W. Stachowiak (Ed.), Wiley, New York, 2006, p. 191. M. Kano, Tribol. Lett. 18 (2006) 245. A. Erdemir, O. Eryilmaz, G. Fenske, J. Vac. Sci. Technol., A 18 (2000) 1987. J. Andersson, R.A. Erck, A. Erdemir, Surf. Coat. Technol. 163–164 (2000) 535. C. Donnet, J. Fontaine, A. Grill, T. Le Mogne, Tribol. Lett. 9 (2000) 137. C. Matta, O.L. Eryilmaz, M.I. De Barros Bouchet, A. Erdemir, J.M. Martin, K. Nakayama, J. Phys. D Appl. Phys. 42 (2009)8 075307. [8] M.I. De Barros Bouchet, M. Kano, in: A. Erdemir, J.M. Martin (Eds.), Elsevier, Amsterdam, 2007, p. 471. [9] M.I. De Barros Bouchet, C. Matta, T. Le-Mogne, J.M. Martin, Q. Zhang, W. Goddard, M. Kano, Y. Mabuchi, J. Ye, J. Phys. Conf. Ser. 89 (2007)8 012003.
K. Bobzin et al. / Surface & Coatings Technology 205 (2011) S89–S93 [10] K. Bobzin, N. Bagcivan, S. Theiss, K. Yilmaz, Plasma Coatings CrAlN and a-C:H for High Efficient Power Train in Automobile, Surf. Coat. Techol. 205 (2010) 1502. [11] K. Bobzin, N. Bagcivan, N. Goebbels, K. Yilmaz, B.-R. Hoehn, K. Michaelis, M. Hochmann, Surf. Coat. Technol. 204 (2009) 1097. [12] W. Dresel, in: T. Mang, W. Dresel (Eds.), Synthetic Base Oils, Wiley-VCH, Weinheim, 2001, 69 and 146.
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[13] DIN EN 1071–2:2002 Advanced technical ceramics, Methods of test for ceramic coatings Part 2: Determination of coating thickness by crater grinding method. [14] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [15] K. Holmberg, A. Matthews, in: D. Dowson (Ed.), Tribology series, Coatings tribology, Vol. 28, Elsevier Science, Amsterdam, 1994, p. 93.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Hydrogen content variation for enhancing the lubricated tribological performance of DLC coatings with ester Kirsten Bobzin, Nazlim Bagcivan, Sebastian Theiss, Koray Yilmaz ⁎ Surface Engineering Institute, RWTH Aachen University, Kackertstrasse 15, D-52072 Aachen, Germany
a r t i c l e
i n f o
Article history: Received 10 September 2010 Accepted in revised form 28 February 2011 Available online 5 March 2011 Keywords: DLC Hydrogen content Pin-on-disk tribometer TMP ester
a b s t r a c t There are many types of DLC (diamond-like carbon) coatings, which mainly differ from each other according to their hydrogen content, sp2 and sp3-bonded carbon atoms and alloying elements (such as Ti, Cr, W or Zr). The lubricated tribological performance of these coatings depends on the lubricant. Controversially, the same lubricant delivers different tribological performances with differently produced DLC coatings. It has been observed that the presence of hydrogen has a remarkable effect on the tribological performance of DLC coatings in inert and vacuum environments. In this paper, the hydrogen content of two different types of DLC coatings, a-C:H:Me (metal containing hydrogenated amorphous carbon) and a-C:H, was varied, in order to obtain an optimized tribological behavior with a synthetic ester (TMP ester). The tribological performance of the coatings with TMP ester is examined in a pin-on-disk tribometer. It could be shown that increasing the hydrogen content in a DLC coating improves their tribological performance with TMP ester. Besides, a-C:H type of coatings is found to be more suitable for TMP ester regarding low friction coefficients. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Diamond-like carbon coatings (DLC) are probably one of the most important modern coatings, which are inevitable for many tribological systems, due to their excellent mechanical and tribological properties, such as high hardness, self lubricating capability and high resistance against corrosive degradations [1,2]. As a result, the excellent tribological behavior under dry and lubricated contacts makes them very attractive for several industrial applications [3]. It is known that hydrogen containing DLC coatings provide very low friction coefficients in inert and vacuum environments [4]. Contrary, the hydrogen free coatings exhibit very high friction coefficients in such environments [5]. But, if humidity is introduced, low friction coefficients are also achieved for hydrogen free DLC coatings [4,6]. This phenomenon is explained by elimination of dangling σ-bonds of carbon by hydrogen on the surface. This prevents the adhesion of the tribo pairs and thus allows low friction coefficients [7]. The same effect on hydrogen free coatings can also be achieved, if hydroxyl group containing additive, such as glycerol mono-oleate (GMO) is added to lubricant [3,8]. Barros Bouchet et al. proved the existence of –OH layer after tribological results of DLC coatings with GMO containing polyalphaolefin [9]. Therefore, the presence of hydrogen on the surface of the DLC coatings is essential for a good tribological performance.
⁎ Corresponding author. Tel.: +49 241 80 93692; fax: +49 241 80 92941. E-mail address: [email protected] (K. Yilmaz). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.02.065
Additional to ta-C, which delivers very low friction coefficients with GMO additive containing lubricants, hydrogenated DLC (a-C:H) coatings may also enable lower friction coefficients, if they are adapted to lubricants [10,11]. Hence, optimization of the tribological performance of these coatings for selected lubricants may offer an alternative to that of ta-C with GMO. One promising lubricant is TMP ester. TMP ester possesses a high amount of hydroxyl group (–OH) and this allows very low friction of DLC coatings [10,12]. Besides, these lubricants are mostly environmentally friendly and they can be completely degraded [12]. Therefore, the optimization of DLC and lubricant with each other offers decreasing friction losses in tribological systems. Furthermore, by means of this adaptation lower friction coefficients may be achieved for environmentally friendly systems. In this study, we tried to show the adaptation possibility of hydrogen containing DLC coatings with TMP ester in order to achieve a better tribological performance. For this, two different types of DLC coatings, aC:H and a-C:H:Me, are deposited. The mechanical properties of the same type of coatings are very similar to each other; mainly their hydrogen content is different. The DLC coatings with TMP ester are investigated under boundary lubrication in a pin-on-disk tribometer. 2. Experimental setup 2.1. Coating deposition The DLC coatings were deposited on case-hardened (HRC 62) 16MnCr5 (AISI 5115) polished round samples in an industrial CC800/ 9 coating facility from CemeCon AG Wuerselen, which is equipped
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K. Bobzin et al. / Surface & Coatings Technology 205 (2011) S89–S93
Table 1 Designation, deposition technology and the precursor gases of the DLC coatings.
a-C:H:Zr-1 a-C:H:Zr-2 a-C:H:Zr-3 a-C:H-1 a-C:H-2
Interlayer
Deposition technology
Top coat/ precursor gas
Deposition technology
ZrC ZrC ZrC CrN CrN
MSIP MSIP MSIP MSIP MSIP
a-C:H:Zr/C2H2 a-C:H:Zr/C2H2 + H2 a-C:H:Zr/CH4 + H2 a-C:H/C2H2 + H2 a-C:H/C2H2 + H2
MSIP MSIP MSIP PACVD PACVD
with two cathodes. The mean surface roughness (Ra) of the round samples, which were used for characterization of the coatings and the tribological tests in pin-on-disk tribometer, was better than 0.008 μm. Prior to deposition the samples were cleaned in a multi stage ultrasonic bath, which contains alkaline solvents for a better removal of corrosion inhibitor and/or polishing paste on round samples. The coating chamber was pumped to an initial pressure of 2 mPa. After reaching this initial pressure, the coating chamber was heated up for 3600 s in order to remove residual humidity and outgasing materials. The maximum observed temperature on samples was 160 °C. After the heating phase, the surface of the samples was cleaned by means of argon ion etching. The heating and etching phase were the same for a-C:H and a-C:H:Zr. a-C:H:Zr type of coatings was deposited by a reactive sputtering process of Zirconium with acetylene (C2H2) or methane (CH4). A DC type of the power supply was applied on the zirconium targets. In order to achieve a good adhesion, a ZrC interlayer deposited prior to deposition of the a-C:H:Zr top layer. During the deposition of a-C:H:Zr-1 and a-C:H:Zr-2 acetylene and a-C:H:Zr-3 methane was used as precursor gas. A unipolar pulse type bias voltage at a pulse frequency of 250 kHz was applied. Its magnitude was ramped from −50 V to −100 V during the deposition. The a-C:H type coatings were deposited using PACVD (Plasma Activated Chemical Vapor Deposition). For the deposition of the a-C:H top layer acetylene and hydrogen were used as precursor gas. The plasma is generated using a pulsed MF type power supply at −450 V and −550 V for a-C:H-1 and a-C:H-2 respectively. The pulse frequency and duty cycle were 350 kHz and 61% respectively. The C2H2 flow for a-C:H-1 was kept constant at 60 sccm, whereas H2 flow was 40 sccm. The gas flows for a-C:H-2 were set to 20 sccm for C2H2 and 60 sccm for H2. Prior to deposition of the a-C:H top layer, CrN and CrCN interlayers are deposited in order to attain a good adhesion on the substrate. The interlayers are deposited using the DC MSIP (Magnetron Sputter Ion Plating) technology and Cr and graphite targets. The dimension of the targets was 88 × 500 mm2. The purity of chromium and graphite targets was 99.95% and that of zirconium targets was 99.99%. The deposited coatings and their deposition technique as well as the material source during the deposition are summarized in Table 1.
2.3. Coating characterization methods The thickness of the coatings was determined by crater grinding method according to the standard DIN EN 1071–2 [13]. The mechanical properties, universal hardness and Young's modulus, of the coatings were determined by nanoindentation measurements (Nanoindenter XP, MTS Nano Instruments) according to the method of Oliver and Pharr [14]. During the measurements the indentation depth was kept below 1/ 10 of the coating thickness. The carbon and zirconium ratio of the a-C:H:Zr coatings is determined by GDOES (Glow Discharge Optical Emission Spectroscopy) measurements. The hydrogen content of a-C:H was determined by NRA (nuclear reaction analysis) at the Forschungszentrum Dresden-Rossendorf (FZD), Germany. The NRA uses a core reaction (see Eq. (1)) in order to determine the hydrogen content. Gamma radiation is emitted with a constant photon energy and the ray intensity is directly proportional to the quantity of gamma particles. These gamma particles are measured by means of a gamma detector and demonstrate the measurement signal. The measured quantity of the gamma particles is 15 proportional to the quantity of N ions (also called monitor pulses), dihedral angle of the gamma detector, efficiency of the gamma detector and the concentration of the hydrogen in the sample. In order to convert the measured gamma particle quantity into the hydrogen concentration, a comparison measurement on a reference material (polyimide) with a known hydrogen content is carried out. The actual hydrogen content is 15 obtained from the basic proportional calculations. The energy of the N ions was set at 6582 keV, which corresponds to a measurement depth of 100 nm. The measurement depth of 100 nm was chosen, in order to eliminate the influence of humidity. Ten measurements are carried out for each specimen. The measured hydrogen contents are medium values of these ten values. 15
12
ð1Þ
N + p→ C + alpha + gamma
2.4. Pin-on-disk tests The tribological performance of the plasma coatings was analyzed using a pin-on-disk tribometer. During the measurements the samples were put into a lubricant pot, with which a continuous lubrication during the measurements is ensured. An initial maximum Hertzian contact pressure of 1800 MPa was applied by means of static loads. As counterbody uncoated case-hardened 16MnCr5 (HRC 62) pins were used. During the tribological tests the temperature of the samples and oil was set to 100 °C. The linear speed of the pin on the test sample was set to 0.1 m/s. The chosen test parameters correspond to boundary lubrication regime. The total sliding distance for each experiment amounts to 1000 m. 3. Results and discussion
2.2. Characteristic of TMP ester
3.1. Coating characterization
For the tribological investigations a TMP ester was used. This lubricant contains 3% S–P additive and its density at 25 °C is 0.882 g/m3. It has at 40 °C and at 100 °C kinematic viscosities of 136.3 mm2/s and 19.5 mm2/s, respectively.
The results of the characterization of the samples, which were carried out in accordance with the aforementioned methods, are summarized in Table 2. In order to give a better overview, the coatings are designated according to their hydrogen content. Due to the
Table 2 Overview of the characteristics of the coatings.
Coating thickness (μm) Hardness (GPa) Young's modulus (GPa) Hydrogen content [at%] C/Zr ratio Ra (μm)
a-C:H:Zr-1
a-C:H:Zr-2
a-C:H:Zr-3
a-C:H-1
a-C:H-2
a-C:H:Zr-39.3
a-C:H:Zr-38.2
a-C:H:Zr-32.3
a-C:H-23.0
a-C:H-21.4
6.8 6.5 ± 0.8 71 ± 6 39.3 90/10 0.008
6.5 7.2 ± 1 76 ± 7 38.2 95/5 0.008
3.8 10.4 ± 1.6 155 ± 12 32.3 80/20 0.013
1.2 33.9 ± 3.5 287 ± 20 23 – 0.08
1 34.8 ± 2.1 305 ± 20 21.4 – 0.04
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Fig. 1. Hydrogen content of a-C:H:Zr type (left) and a-C:H type (right) DLC coatings.
Fig. 2. Progression of friction coefficients on the a-C:H:Zr-39.3 (left) and a-C:H:Zr-38.2 (right) over the distance in pin-on-disk tribometer with TMP ester.
reactive sputtering of Zr, a-C:H:Zr types of coatings possess considerably higher coating thicknesses. Contrary, the two a-C:H coatings have thin top layers due to low deposition rates of the PACVD process and are about 1 μm. The hydrogen content of a-C:H:Zr and a-C:H types of coatings are shown in Fig. 1. With increasing monitor pulses, the measured hydrogen concentration decreases for a-C:H:Zr type of coatings. This points out that hydrogen is weakly bounded in the coatings. Actually, the hydrogen in a-C:H:Zr-32.3 possesses the weakest bonding, such that it decreases strongly. In contrast, the hydrogen concentration measurements for a-C:H type coatings stay constant with increasing monitor pulses, which indicates stable bonded hydrogen with carbon. The a-C:H:Zr type coatings possess very high hydrogen concentrations. The addition of H2 into the deposition process for a-C:H:Zr-38.2 decreased the hydrogen concentration slightly in the DLC coating. One possible explanation is that during deposition, acetylene (C2H2) is reduced by excited hydrogen and looses hydrogen. This also leads to high carbon concentration on the top layer, such that the C/Zr ratio of a-C:H:Zr-38.2 is higher than the one of a-C:H:Zr-39.3. In case of utilizing methane as precursor gas, the deposited a-C:H:Zr-32.3 possesses lower hydrogen concentration, than that of with acetylene. In addition, the C/Zr ratio of this coating is also lower than both with acetylene. If the two a-C:H coatings are considered, they possess lower hydrogen concentrations.
is considered, the two measurements show different behavior. During the first measurement, the friction coefficient shows a constant decrease and at the end approaches to 0.08. But the second measurement shows a different progression, such that it reaches 0.08 rapidly at the very beginning, immediately after it increases to 0.09 and stays till the end of the measurement approximately constant. Fig. 3 shows the friction coefficient progression of a-C:H:Zr-32.3 with TMP ester. Both of the measurements show similarities, such that at the beginning the friction coefficients increase to 0.14 and then start to decrease. At the end, the friction coefficient of the first measurement approaches to 0.10, whereas the second one to 0.09. The different progressions of friction coefficients between the measurements are believed to be the effect of weakly bounded hydrogen. Such that if hydrogen is weakly bounded, the friction reducing effect that is formed during measurement due to the presence of hydrogen, can be easily destroyed. This may lead to an increase in adhesion
3.2. Friction behavior of the DLC coatings with TMP-ester The progression of friction coefficients for a-C:H:Zr-39.3 and a-C:H:Zr-38.2 in pin-on-disk tribometer is shown in Fig. 2. The first measurement of a-C:H:Zr-39.3 reaches to a constant value of 0.07 after 400 m, whereas the second measurement reaches this value already at 200 m. After this running in behavior, both measurements show very good agreement with each other. If a-C:H:Zr-38.2
Fig. 3. Progression of friction coefficients on the a-C:H:Zr-32.3 over the distance in pinon-disk tribometer with TMP ester.
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Fig. 4. Progression of friction coefficients on the a-C:H:Zr-32.3 over the distance in pin-on-disk tribometer with TMP ester.
forces between the tribo pairs and in return the friction coefficient. The spikes, which occur during the progression of friction coefficient, can also be explained by means of this phenomenon. The progression of the friction coefficients for the measurements on a-C:H-21.4 and a-C:H-23.0 is shown in Fig. 4. The friction progression for both measurements of a-C:H-21.4 has a good agreement and the friction coefficients approaches to 0.05 at the end of the measurements. The progression of friction coefficient for a-C:H-23.0 has a slight deviation. The first measurement approaches after 200 m 0.030 and reaches 0.024 at the end, while the second one has a shorter running-in phase and decreases to 0.035 in 100 m. At the end the friction coefficient decreases to 0.030. If the average friction coefficients are considered (see Fig. 5), the effect of the hydrogen concentration on the friction behavior will be apparent. With increasing hydrogen content the average friction coefficients on DLC coatings decrease. In a former work, using XPS (X-Ray photoelectron spectroscopy) the presence of oxygen could be shown in wear track of a-C:H after the measurements in pin-on-disk tribometer. This indicates the existence of hydroxyl group [11]. Thus, increasing the hydrogen content may increase the concentration of this hydroxyl group on the coating surface. By means of that, a better separation of tribological contacts in boundary lubrication regime is possible, which in return improves the friction behavior. Nevertheless, it is also convenient to consider the effect of surface roughness on the friction coefficients for a-C:H:Zr type of coatings. Both a-C:H:Zr-39.3 and a-C:H:Zr-38.2 possess lower surface roughness than a-C:H:Zr-32.3 at the beginning. The friction coefficients for a-C:H:Zr-39.3 and a-C:H:Zr-38.2 after the running in period decrease to 0.07 and 0.08 respectively. Analogously, the friction coefficient for a-C:H:Zr-32.3 reduces to 0.09. Therefore, the difference between the friction coefficients for the a-C:H:Zr type of coatings after the running in is not any longer significant.
However, the friction coefficients of the a-C:H:Zr coatings with higher hydrogen content are still lower than those with lower hydrogen content after running in. But the high Zr content in a-C:H: Zr-32.3 can also be the reason for this slight difference. Therefore, further investigations are necessary to find out the effect of the Zr content on the friction coefficient under lubricated conditions for a-C:H:Zr type of coatings. Besides, the effect of hydrogen content on tribological behavior depends on the type of DLC coatings, such that, although the hydrogen content in a-C:H:Zr is higher than that of a-C: H, they deliver higher friction coefficient. One possible explanation is the differences in mechanical properties. Due to lower hardness of a-C:H:Zr, the contact area on a-C:H:Zr type coatings is higher than on a-C:H and this leads to increased frictional forces [15]. Another possible explanation is the effect of the Zr content, such that due to Zr the adhesion tendency of the coatings is increased, which in return increases the friction coefficients. The effect of adhesion can clearly be seen for the friction progression for a-C:H:Zr-32.3, which possesses the highest Zr content. 4. Conclusion The aim of this work was to find out the optimization potential of frictional behavior of hydrogenated DLC coatings with TMP ester. For the optimization, the hydrogen content of two different types of DLC coatings, a-C:H and a-C:H:Zr, was varied. It is found that by means of increasing the hydrogen content, the friction coefficient of the coatings with TMP ester could be decreased. Nevertheless, this improvement depends on the DLC type, and lower friction coefficients are achieved for a-C:H than a-C:H:Zr. A future work with increased – OH groups in TMP ester by of mixing GMO is intended. By means of that it is possible to emphasize the influence of hydrogen content increase in DLC coatings for the reduction of friction losses. Furthermore, it is also intended to investigate the effect of the Zr content whilst keeping the hydrogen content and mechanical properties constant in order to decouple the effect of Zr and hydrogen concentration on the friction coefficient. References [1] [2] [3] [4] [5] [6] [7]
Fig. 5. Average friction coefficients of the DLC coatings.
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