- Email: [email protected]
On the influence of surfactant over friction properties of steel
Accepted Manuscript On the influence of surfactant over friction properties of steel R.K. Upadhyay, L.A. Kumaraswamidhas PII: DOI: Reference:
S0009-2614(14)00472-2 http://dx.doi.org/10.1016/j.cplett.2014.05.090 CPLETT 32235
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
7 April 2014 29 May 2014
Please cite this article as: R.K. Upadhyay, L.A. Kumaraswamidhas, On the influence of surfactant over friction properties of steel, Chemical Physics Letters (2014), doi: http://dx.doi.org/10.1016/j.cplett.2014.05.090
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 proof before it is published in its final 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.
On the influence of surfactant over friction properties of steel *1 R. K. Upadhyay, 1L.A. Kumaraswamidhas *1 Tribology Lab, Department of Mechanical & Mining Machinery Engineering, Indian School of Mines, Dhanbad Jharkhand-826004 (INDIA) *Corresponding Author: R. K. Upadhyay *Email: [email protected] Abstract Surfactant wetting properties are important for any tribological system. In order to explore these properties the influence of four surfactant i.e. sodium hexametaphosphate (SHMP); zinc dithiodiphosphate (ZDDP); sodium dodecyl sulphate (SDS); and cetrimonium bromide (C-TAB); on the friction behaviour of physical vapour deposition (PVD) coated surfaces were studied using pin on disk tribometer. Anionic, cationic and non-ionic surfactants are dispersed in oil/water medium. Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDX) and X-Ray Diffraction (XRD) analysis was performed to obtain the surface morphology, chemical composition and structure of the material. Keywords: Surfactant, Physical vapour deposition, Friction, XRD
1. Introduction There are many investigations on using surfactants with base oil and their applications on metal substrates. However, there are few studies focused on application of surfactants on PVD coated metallic surfaces and during their machining performance. Since the recognition of anionic, non-ionic, cationic and amphoteric surfactants as fluids, more efforts have been made to design new type of lubricants those are capable of nonvolatile, non-flammability and high thermo-oxidative stability [1-3]. Current research focuses on ionic liquids [4-6] and several authors reported its suitability where conventional lubricants fail [7-8]. Dispersion of nanoparticles [9] in to oil or water medium with surfactant led to produce homogeneous solution which directly affects the interface medium and changes the physical-chemical properties. Numerous authors have been reported a better tribological behaviour of surfactants like SDS, ZDDP, SHMP and C-TAB in aqueous medium [10-15]. These surfactants are act like additives on the metallic surfaces which significantly reduces friction and provides larger tool life. These surfactants wet the material surfaces throughout its nano gaps those are non-
1
visible to the naked eyes and reduces surface tension. The wetting performance of SDS and C-TAB were investigated by Bonaccurso et al., and found that spreading was dominated by the inertia whereas, the duration of the event does not affected by the addition of surfactants [16]. In this work, we added cationic (cetrimonium bromide, CTAB), anionic (sodium Hexametaphosphate, SHMP; sodium dodecyl sulfate, SDS) and non-ionic (zinc dithiodiphosphate, ZDDP) surfactants to PAO6 base oil and water to study the friction behaviour of PVD coated metal substrate under lubricated condition. Tests were repeated several times in order to check repeatability of the results and average value has been taken into consideration.
2. Experimental Two PVD coated vanadis 23 and orvar supreme (uddeholm Inc.) steel substrate were examined with the help of pin on disk tribometer. The following chemical composition was used for vanadis 23(C=1.28%, Cr=4.2%, Mo=5%, W=6.4%, V=3.1%) and orvar (C=0.39%, Cr=5.2%, Mo=1.4%, V=0.9%, Si=1%) steel substrate. Bearing steel (100Cr6) of dia 6mm with chemical composition of Cmax=1%, Si= 0.25%, Mn= 0.35% Cr= 0.15% and hardness of 55 HRC were used to test the samples. Further information about PVD test condition and deposition parameter can study in author’s recent work [17, 18]. Sliding friction test was conducted with pin on disk tribometer (CSM Instruments) under a fixed sliding speed, load of 9.4cm-s and 5N respectively against vanadis 23, orvar supreme steel disc of hardness 60HRC and 52 HRC. This load and sliding velocity kept constant throughout the measurement. A vertical pin consists of steel ball (100Cr6) of dia 6mm rotates against a steel disk and a holder clamps the pin tightly. To determine the phase in the coatings X-ray diffraction (XRD) analysis was performed using Phillips Diffractometer. The analysis was performed in the 2θ range of 20°–90° and incident angle was fixed at 0.2°. The scattering in the low angle regime solely arises from the chemical modulation of the structure the length scales are greater than the lattice spacing of the constituent layers. Four surfactant i.e. sodium dodecyl sulphate (SDS), zinc dithiodiphosphate (ZDDP), sodium Hexametaphosphate (SHMP), and Cetyl trimethylammonium bromide (C-TAB) as 1 wt% were analysed in this study with PAO6 as an base oil. The suspensions in oil/water were prepared by suspending 10ml of oil with 1 wt % surfactant using an Ultrasonication. 1 wt% surfactants was added into PAO6 oil/water and colloidal suspensions were prepared which is further ultrasonic treated for 15–min in order to break agglomerates. The
2
frequency and power of the ultrasound were constant 20 kHz and 100 W, respectively. Properties of these surfactants with base oil are listed in Table 1.
3. Results and Discussion Figure 1 a, b represents the average friction values of vanadis 23 and orvar steel substrates while slid against steel balls with and without surfactant (PAO6 alone) in oil medium. In both the cases PAO6 base oil alone gives higher results compared to surfactant based lubricant. Oil based coolant add lubricity to the contact zone which reduces friction and furthermore dissipated heat from the contacting zone. PAO6 oil alone gives higher coefficient of friction (COF) values. Addition of ZDDP which is used as engine oil additive reduces this value close to 0.14 which gives significant change in the COF value. For ZDDP tribo-films amorphous polyphosphate base layers are presumed to dominate the mechanical properties. Outer layer is surrounded by a sulphide which is better served to control the friction properties of the film. On other hand SDS outperform among all with the least COF value of 0.12. SDS possess soapiness behaviour at the metallic surface and due to this a slipping phenomenon developed rather than sticking at the surface. COF values for C-TAB and SHMP relatively higher than ZDDP and SDS for both the substrates. In case of orvar, C-TAB value reaches at its maximum of 0.21and 0.20 for SHMP. Whereas, for vanadis these values are of lower ones with 0.16 for SHMP and 0.17 for C-TAB. C-TAB, a cationic surfactant can be used as stabiliser in aqueous media. It prevents agglomeration to yield highly stable, well-dispersed metal nanoparticles and restricted the further growth of the suspended particle [19]. In case of water based lubricant (Figure 1 c, d), compared to oil medium COF values are measured relatively higher in case of C-TAB and attained 0.35, 0.40 for vanadis and orvar steel substrates respectively. SDS and ZDDP based lubricant gives varying COF values. In case of vanadis substrate COF values of 0.20 and 0.23 were measured for SDS and ZDDP. This value goes beyond in case of orvar and attained 0.22 and 0.27 for SDS and ZDDP based water lubricants. Whereas, COF values 0.26 and 0.28 were measured for SHMP in case of vanadis and orvar substrates. Although the measured friction values are of higher side compared to oil medium but still water based lubricants in the contact zone reduces friction and remove excessive generated heat which gives prolong operational life to the equipment where dry sliding action required. These lubricants are efficient cooler and environmentally friendly in nature which excessively increases its usefulness. Solubility of these surfactants depends on hydrophilic-lipophilic balance (HLB), lower HLB values represents lipophilic surfactants whereas, higher HLB ascribed to hydrophilic surfactants. Among these tested surfactants, SDS possesses higher HLB value of 40 which reflects hydrophobic in nature. In order to wet the surface
3
completely contact angle must be below 90°. SDS spreads well over the surface and making the contact angle below 90°. In a liquid medium each molecule are surrounded by another molecules and cohesion forces keeps the molecules close to each other. Each surface molecule directed towards inward due to strong attraction in perpendicular direction which is normal to the surface and causes the surface under state of lateral tension [20]. Surfactants favours temperature dependent solubility and it increases with temperature. After certain point rise in further temperature causes sudden change in solubility. For an industrial application of surfactants lower temperature variation recommended.
The SEM analysis for both steel substrates under oil and water lubricated contact is depicted in Figure 2 a-d. Figure 2a, c shows oil treated samples of vanadis and orvar steel respectively. The micro thermal cracks observed for Figure 2a and Figure 2b. Surfactants at coating interface as a coolant produces thermal shock due to its low thermal expansion [21]. Figure 2c and Figure 2d shows the adhesion of materials on the exposed surface of coated steel substrates during sliding test under water/surfactant lubricated contact. Figure 3a and Figure 4a shows an Energy Dispersive X-Ray Spectroscopy analysis to determine the chemical composition with their relative proportions of layer coating. The EDX analysis on the smooth surface revealed mainly W, N2 for vanadis 23 (Figure 3a) and W, O2, C, Fe for orvar (Figure 3b) steel substrate. High percentage of Fe content at orvar steel substrate indicates the material transfer between substrate and ball thus this material transfer leads to plastic deformation of coating. Lack of iron content at vanadis steel surface indicates materials resistance to plastic deformation/coating delamination. The cross section images of vanadis 23 (Figure 3b) and orvar supreme substrate (Figure 4b) depicted in order to obtained the cumulative wt.% of the coating content. The maximum deposited tungsten content of 98% and lower of 96% were obtained for vanadis23, orvar supreme steel substrate respectively. At certain instant lower friction values achieved with higher nitrogen wt.% with their lower compressive strength [18].
XRD pattern for tungsten nitride [18, 22] coatings are presented in Figure 5 and three main zones i.e. α-W, β-W and β-W/W2N revealed during the study. First, Low nitrogen content structure zone α-W with (110) preferential orientation at 2ϴ = 51̊. Secondly, structural zone β-W with intermediate nitrogen content represents portion of α-W and β-W2N zone. In this zone tungsten film shows a peak with lattice parameter which corresponds to (210) preferential orientation at 2ϴ = 47.5̊. Addition of nitrogen resulted in the formation of (200) preferential orientation at 2ϴ = 36.5̊. Further addition of nitrogen content leads formation of third
4
structural zone β-W2N with (111) preferential orientation at 2ϴ = 31̊ along with the presence of β-W2N (200) peak at 2ϴ = 45̊. XRD peak displacement and the peak broadening were used to measure the internal stresses and the particle size whereas, β-W2N (210) is considered for the calculations. At lower nitrogen content [18] (≤ 20%) internal stress is about 6GPa whereas, increase in nitrogen content (≥25%) further decreases the internal stresses and attains the value of 4GPa.
4. Discussion The coefficient of friction reduces and it depends on the type of surfactant dispersed in oil or water medium. In this study used surfactants have some unique characteristic which makes it excellent candidate of lubricants like high chemical and thermal stability and low melting point. Dry sliding friction behaviour of orvar, vanadis substrates without surfactant are shown in Figure 6. It is clear that with out surfactant under dry sliding condition friction values for vandis and orvar steel reached its maximum value of 0.24 and 0.255 respectively, and found to be higher compared to oil medium. During tribological contact between the rubbing surfaces lower friction values achieved due to thread like micelles form on the metallic surfaces. A thin monolayer surfactant film is responsible for the reduction of friction coefficient and it does not depend on the overall layer thickness. A proper concentration (preferably ≥1wt.%) of the surfactants would be advisable because low concentration have a negative effect on the friction and antiwear properties. One of the basic criteria to reduce friction is polarity of these surfactants with their fluid drag properties changing with time [23]. Surfactants have ability to repair mechanically degraded surfaces and it consists of amphiphiles with hydrophilic heads and hydrophobic tails which aggregates in water above a critical micelle concentration [24]. If chemically absorbed films on the substrate are in crystalline or semi-crystalline [25,26] form the material contact protected from severe wear-out. The Water is heavier than oil, a thin film of water always covers a hydrophilic steel surface tightly than oil and further it requires strong force of attraction to contact with steel. Although, friction response in case of SDS is rather complex due to change in layer structure and composition by the combined action of load and shear; nevertheless, SDS surfactant in the oil/water medium shows strong attractive interaction behaviour towards steel substrates with lower interfacial tension and hence gives lesser coefficient of friction values among the all tested samples.
5
5. Conclusion Friction properties of four surfactants were measured under oil and water lubricated contact. SDS gives less friction values among all tested surfactants whereas, C-TAB gives worst results in the both cases. An EDX analysis were performed to measure the chemical composition and we found iron content only for orvar supreme steel; for that only material transfer took place between ball and disk. Dry sliding friction values of both substrates are higher compared to oil lubricated ones due to the fact that during dry contact of both surfaces asperities rub against each other and this lead to detachment of material from one to another. The major part of the material transfer took place from the softer one due to the ploughing action. In case of water lubricants friction values are higher than dry condition due to solubility behaviour of these surfactants.
Acknowledgement Author gratefully acknowledges the support from the faculty members CTU-Prague and ISM-Dhanbad.
References 1.
Z. Wang, Y. Xia, Z. Liu, Z. Wen, Tribology Letters 46 (2012) 33–42.
2.
F. J. Carrion, S. Martınez, G. Nicolas, P. Iglesias, J. Sanes, M. D. Bermudez, International Journal of Molecular Sciences 10 (2009) 4102–4115.
3.
N. Gungor, A. Alemdar, O. Atici , I.O. Ece, Materials Letters 51 (2001) 250–254.
4.
I. Minami, Molecules 14 (2009) 2286–2305.
5.
A. E. Somers, P. C. Howlett, J. Sun, D. R. MacFarlane, M. Forsyth, Tribology Letters 40 (2010) 279– 284.
6.
A. Hernandez Battez, R.Gonzalez, J.L.Viesca, A. Fernandez-Gonzalez, M.Hadfield, Tribology International 58 (2013) 71–78.
7.
A. E. Jimenez,M. D. Bermudez, Tribology Letters 40 (2010) 237–246.
8.
D. Li, M. Cai, D. Feng, F. Zhou, W. Liu. Tribology International 44 (2011) 1111–1117.
9.
Z. Guoa, J. Xiong, M. Yang, S. Xiong, J. Chen, Y. Wu, H. Fan, L. Sun, J. Wang, H. Wang, Journal of Alloys and Compounds 493 (2010) 362–367.
10.
M. Halder, D. N. Nath, Chemical Physics Letters 387 (2004) 81–84.
11.
Z. Komeily-Nia, M. Montazer, M. Latifi, Colloids and Surfaces A: Physicochem. Eng. Aspects 439 (2013) 167– 175.
6
12.
J. Pang, G. Xu, Chemical Physics Letters 537 (2012) 118–125.
13.
F. Kılıç, H. Gül, S. Aslan, A. Alp, H. Akbulut, Colloids and Surfaces A: Physicochem. Eng. Aspects 419 (2013) 53– 60
14.
V. G. Rao, U. Brahmachari, S. Mandal, S. Ghosh, C. Banerjee, Chemical Physics Letters 552 (2012) 38–43.
15.
S. Park, G. R. Choi, J. H. Kim, J. C. Lee, Journal of the Korean Physical Society 61 (2012)1400-1403.
16.
X. Wang, L. Chen, E. Bonaccurso, J. Venzmer, Langmuir 29 (2013) 14855−14864.
17.
R. K. Upadhyay, L. A. Kumaraswamidhas, Materials at high Temperatures, 2014, DOI: 1878641314Y.0000000003 (in press).
18.
R.
K.
Upadhyay,
L.
A.
Kumaraswamidhas,
Surface
Engineering,
2014,
DOI:
1743294414Y.0000000260 (in press). 19.
M. Bicker, I. Sisman, Powder Technology 198 (2010) 279–284.
20.
H. Y. Erbil, Surface Chemistry of Solid and Liquid Interfaces, Blackwell publishing, 2006, India.
21.
J. Gu, S. Tung, G. Barber, Wear Process Manufacturing, ASTM STP1362 (1999) 31–47.
22.
S. Guruvenket, G. M. Rao, Material Science and Engineering B, 106 (2004) 172-176.
23.
G. aguilar, K. Gasljevic, E. F. Matthys, International Journal of Heat and Mass Transfer 44 (2001) 2835-2843.
24.
Y. Qi, E. Kesselman, D. J. Hart, Y. Talmon, Anthony Mateo, Jacques L. Zakin, Journal of Colloid and Interface Science 354 (2011) 691–699.
25.
D. Kumar, J. Daniel, S.K. Biswas, Journal of Colloid and Interface Science 345 (2010) 307–315.
26.
C.M. Mate, Tribology on the Small Scale: A Bottom up Approach to Friction, Lubrication and Wear, Oxford Univ. Press, New York, 2008.
7
Table 1 Structure and properties of surfactants and base oil Surfactant SDS
IUPAC/Other name Sodium lauryl sulfate
Cationic/Anionic Anionic
C-TAB
Hexadecyl-trimethylammonium bromide
Cationic
SHMP
Hexasodium metaphosphate
Anionic
ZDDP
ZnDTP
Nonionic
Structure
Density (15°C) = 0.826 g/cm3 PAO 6 Oil kinematic viscosity (40°C) = 30.5 cSt
8
a
b
c
d
Figure 1 COF values of (a): vanadis 23, (b): orvar steel substrates for oil based lubricants and COF values of (c): vanadis 23 (d): orvar steel substrates for water based lubricants.
9
ab
cd
b
d
Figure 2 SEM morphology of (a) vanadis 23, (b) orvar supreme under oil lubricated contact; and (c) vanadis 23, (d) orvar supreme under water lubricated contact
10
a
11
b
Figure 3 EDX spectrm of (a) vanadis 23 and (b) cross-section cumulative wt.% of content measured on substrate
12
a
13
b
Figure 4 EDX spectrm of (a) orvar supreme and (b) cross-section cumulative wt.% of content measured on substrate
14
Figure 5 XRD pattern for W/W2N coatings
15
Figure 6 Friction performance of vanadis 23 and orvar supreme steel substrate under dry sliding condition
16
Highlights Vanadis 23, Orvar Supreme samples were tested under wet and dry condition. SDS, C-TAB, SHMP and ZDDP are used as surfactant in oil/water medium. SDS outperforms whereas, C-Tab gives worst result among all surfactants.
18
Graphical Abstract
Dry sliding friction behaviour of vanadis 23, orvar supreme substrates without surfactant shows its maximum value of 0.24 and 0.255 respectively, and found to be higher compared to oil medium. These values are of higher side due to the fact that during dry contact of both surfaces asperities rub against each other and this lead to detachment of material from one to another. The major part of the material transfer took place from the softer one due to the ploughing action.
19
S0009-2614(14)00472-2 http://dx.doi.org/10.1016/j.cplett.2014.05.090 CPLETT 32235
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
7 April 2014 29 May 2014
Please cite this article as: R.K. Upadhyay, L.A. Kumaraswamidhas, On the influence of surfactant over friction properties of steel, Chemical Physics Letters (2014), doi: http://dx.doi.org/10.1016/j.cplett.2014.05.090
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 proof before it is published in its final 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.
On the influence of surfactant over friction properties of steel *1 R. K. Upadhyay, 1L.A. Kumaraswamidhas *1 Tribology Lab, Department of Mechanical & Mining Machinery Engineering, Indian School of Mines, Dhanbad Jharkhand-826004 (INDIA) *Corresponding Author: R. K. Upadhyay *Email: [email protected] Abstract Surfactant wetting properties are important for any tribological system. In order to explore these properties the influence of four surfactant i.e. sodium hexametaphosphate (SHMP); zinc dithiodiphosphate (ZDDP); sodium dodecyl sulphate (SDS); and cetrimonium bromide (C-TAB); on the friction behaviour of physical vapour deposition (PVD) coated surfaces were studied using pin on disk tribometer. Anionic, cationic and non-ionic surfactants are dispersed in oil/water medium. Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDX) and X-Ray Diffraction (XRD) analysis was performed to obtain the surface morphology, chemical composition and structure of the material. Keywords: Surfactant, Physical vapour deposition, Friction, XRD
1. Introduction There are many investigations on using surfactants with base oil and their applications on metal substrates. However, there are few studies focused on application of surfactants on PVD coated metallic surfaces and during their machining performance. Since the recognition of anionic, non-ionic, cationic and amphoteric surfactants as fluids, more efforts have been made to design new type of lubricants those are capable of nonvolatile, non-flammability and high thermo-oxidative stability [1-3]. Current research focuses on ionic liquids [4-6] and several authors reported its suitability where conventional lubricants fail [7-8]. Dispersion of nanoparticles [9] in to oil or water medium with surfactant led to produce homogeneous solution which directly affects the interface medium and changes the physical-chemical properties. Numerous authors have been reported a better tribological behaviour of surfactants like SDS, ZDDP, SHMP and C-TAB in aqueous medium [10-15]. These surfactants are act like additives on the metallic surfaces which significantly reduces friction and provides larger tool life. These surfactants wet the material surfaces throughout its nano gaps those are non-
1
visible to the naked eyes and reduces surface tension. The wetting performance of SDS and C-TAB were investigated by Bonaccurso et al., and found that spreading was dominated by the inertia whereas, the duration of the event does not affected by the addition of surfactants [16]. In this work, we added cationic (cetrimonium bromide, CTAB), anionic (sodium Hexametaphosphate, SHMP; sodium dodecyl sulfate, SDS) and non-ionic (zinc dithiodiphosphate, ZDDP) surfactants to PAO6 base oil and water to study the friction behaviour of PVD coated metal substrate under lubricated condition. Tests were repeated several times in order to check repeatability of the results and average value has been taken into consideration.
2. Experimental Two PVD coated vanadis 23 and orvar supreme (uddeholm Inc.) steel substrate were examined with the help of pin on disk tribometer. The following chemical composition was used for vanadis 23(C=1.28%, Cr=4.2%, Mo=5%, W=6.4%, V=3.1%) and orvar (C=0.39%, Cr=5.2%, Mo=1.4%, V=0.9%, Si=1%) steel substrate. Bearing steel (100Cr6) of dia 6mm with chemical composition of Cmax=1%, Si= 0.25%, Mn= 0.35% Cr= 0.15% and hardness of 55 HRC were used to test the samples. Further information about PVD test condition and deposition parameter can study in author’s recent work [17, 18]. Sliding friction test was conducted with pin on disk tribometer (CSM Instruments) under a fixed sliding speed, load of 9.4cm-s and 5N respectively against vanadis 23, orvar supreme steel disc of hardness 60HRC and 52 HRC. This load and sliding velocity kept constant throughout the measurement. A vertical pin consists of steel ball (100Cr6) of dia 6mm rotates against a steel disk and a holder clamps the pin tightly. To determine the phase in the coatings X-ray diffraction (XRD) analysis was performed using Phillips Diffractometer. The analysis was performed in the 2θ range of 20°–90° and incident angle was fixed at 0.2°. The scattering in the low angle regime solely arises from the chemical modulation of the structure the length scales are greater than the lattice spacing of the constituent layers. Four surfactant i.e. sodium dodecyl sulphate (SDS), zinc dithiodiphosphate (ZDDP), sodium Hexametaphosphate (SHMP), and Cetyl trimethylammonium bromide (C-TAB) as 1 wt% were analysed in this study with PAO6 as an base oil. The suspensions in oil/water were prepared by suspending 10ml of oil with 1 wt % surfactant using an Ultrasonication. 1 wt% surfactants was added into PAO6 oil/water and colloidal suspensions were prepared which is further ultrasonic treated for 15–min in order to break agglomerates. The
2
frequency and power of the ultrasound were constant 20 kHz and 100 W, respectively. Properties of these surfactants with base oil are listed in Table 1.
3. Results and Discussion Figure 1 a, b represents the average friction values of vanadis 23 and orvar steel substrates while slid against steel balls with and without surfactant (PAO6 alone) in oil medium. In both the cases PAO6 base oil alone gives higher results compared to surfactant based lubricant. Oil based coolant add lubricity to the contact zone which reduces friction and furthermore dissipated heat from the contacting zone. PAO6 oil alone gives higher coefficient of friction (COF) values. Addition of ZDDP which is used as engine oil additive reduces this value close to 0.14 which gives significant change in the COF value. For ZDDP tribo-films amorphous polyphosphate base layers are presumed to dominate the mechanical properties. Outer layer is surrounded by a sulphide which is better served to control the friction properties of the film. On other hand SDS outperform among all with the least COF value of 0.12. SDS possess soapiness behaviour at the metallic surface and due to this a slipping phenomenon developed rather than sticking at the surface. COF values for C-TAB and SHMP relatively higher than ZDDP and SDS for both the substrates. In case of orvar, C-TAB value reaches at its maximum of 0.21and 0.20 for SHMP. Whereas, for vanadis these values are of lower ones with 0.16 for SHMP and 0.17 for C-TAB. C-TAB, a cationic surfactant can be used as stabiliser in aqueous media. It prevents agglomeration to yield highly stable, well-dispersed metal nanoparticles and restricted the further growth of the suspended particle [19]. In case of water based lubricant (Figure 1 c, d), compared to oil medium COF values are measured relatively higher in case of C-TAB and attained 0.35, 0.40 for vanadis and orvar steel substrates respectively. SDS and ZDDP based lubricant gives varying COF values. In case of vanadis substrate COF values of 0.20 and 0.23 were measured for SDS and ZDDP. This value goes beyond in case of orvar and attained 0.22 and 0.27 for SDS and ZDDP based water lubricants. Whereas, COF values 0.26 and 0.28 were measured for SHMP in case of vanadis and orvar substrates. Although the measured friction values are of higher side compared to oil medium but still water based lubricants in the contact zone reduces friction and remove excessive generated heat which gives prolong operational life to the equipment where dry sliding action required. These lubricants are efficient cooler and environmentally friendly in nature which excessively increases its usefulness. Solubility of these surfactants depends on hydrophilic-lipophilic balance (HLB), lower HLB values represents lipophilic surfactants whereas, higher HLB ascribed to hydrophilic surfactants. Among these tested surfactants, SDS possesses higher HLB value of 40 which reflects hydrophobic in nature. In order to wet the surface
3
completely contact angle must be below 90°. SDS spreads well over the surface and making the contact angle below 90°. In a liquid medium each molecule are surrounded by another molecules and cohesion forces keeps the molecules close to each other. Each surface molecule directed towards inward due to strong attraction in perpendicular direction which is normal to the surface and causes the surface under state of lateral tension [20]. Surfactants favours temperature dependent solubility and it increases with temperature. After certain point rise in further temperature causes sudden change in solubility. For an industrial application of surfactants lower temperature variation recommended.
The SEM analysis for both steel substrates under oil and water lubricated contact is depicted in Figure 2 a-d. Figure 2a, c shows oil treated samples of vanadis and orvar steel respectively. The micro thermal cracks observed for Figure 2a and Figure 2b. Surfactants at coating interface as a coolant produces thermal shock due to its low thermal expansion [21]. Figure 2c and Figure 2d shows the adhesion of materials on the exposed surface of coated steel substrates during sliding test under water/surfactant lubricated contact. Figure 3a and Figure 4a shows an Energy Dispersive X-Ray Spectroscopy analysis to determine the chemical composition with their relative proportions of layer coating. The EDX analysis on the smooth surface revealed mainly W, N2 for vanadis 23 (Figure 3a) and W, O2, C, Fe for orvar (Figure 3b) steel substrate. High percentage of Fe content at orvar steel substrate indicates the material transfer between substrate and ball thus this material transfer leads to plastic deformation of coating. Lack of iron content at vanadis steel surface indicates materials resistance to plastic deformation/coating delamination. The cross section images of vanadis 23 (Figure 3b) and orvar supreme substrate (Figure 4b) depicted in order to obtained the cumulative wt.% of the coating content. The maximum deposited tungsten content of 98% and lower of 96% were obtained for vanadis23, orvar supreme steel substrate respectively. At certain instant lower friction values achieved with higher nitrogen wt.% with their lower compressive strength [18].
XRD pattern for tungsten nitride [18, 22] coatings are presented in Figure 5 and three main zones i.e. α-W, β-W and β-W/W2N revealed during the study. First, Low nitrogen content structure zone α-W with (110) preferential orientation at 2ϴ = 51̊. Secondly, structural zone β-W with intermediate nitrogen content represents portion of α-W and β-W2N zone. In this zone tungsten film shows a peak with lattice parameter which corresponds to (210) preferential orientation at 2ϴ = 47.5̊. Addition of nitrogen resulted in the formation of (200) preferential orientation at 2ϴ = 36.5̊. Further addition of nitrogen content leads formation of third
4
structural zone β-W2N with (111) preferential orientation at 2ϴ = 31̊ along with the presence of β-W2N (200) peak at 2ϴ = 45̊. XRD peak displacement and the peak broadening were used to measure the internal stresses and the particle size whereas, β-W2N (210) is considered for the calculations. At lower nitrogen content [18] (≤ 20%) internal stress is about 6GPa whereas, increase in nitrogen content (≥25%) further decreases the internal stresses and attains the value of 4GPa.
4. Discussion The coefficient of friction reduces and it depends on the type of surfactant dispersed in oil or water medium. In this study used surfactants have some unique characteristic which makes it excellent candidate of lubricants like high chemical and thermal stability and low melting point. Dry sliding friction behaviour of orvar, vanadis substrates without surfactant are shown in Figure 6. It is clear that with out surfactant under dry sliding condition friction values for vandis and orvar steel reached its maximum value of 0.24 and 0.255 respectively, and found to be higher compared to oil medium. During tribological contact between the rubbing surfaces lower friction values achieved due to thread like micelles form on the metallic surfaces. A thin monolayer surfactant film is responsible for the reduction of friction coefficient and it does not depend on the overall layer thickness. A proper concentration (preferably ≥1wt.%) of the surfactants would be advisable because low concentration have a negative effect on the friction and antiwear properties. One of the basic criteria to reduce friction is polarity of these surfactants with their fluid drag properties changing with time [23]. Surfactants have ability to repair mechanically degraded surfaces and it consists of amphiphiles with hydrophilic heads and hydrophobic tails which aggregates in water above a critical micelle concentration [24]. If chemically absorbed films on the substrate are in crystalline or semi-crystalline [25,26] form the material contact protected from severe wear-out. The Water is heavier than oil, a thin film of water always covers a hydrophilic steel surface tightly than oil and further it requires strong force of attraction to contact with steel. Although, friction response in case of SDS is rather complex due to change in layer structure and composition by the combined action of load and shear; nevertheless, SDS surfactant in the oil/water medium shows strong attractive interaction behaviour towards steel substrates with lower interfacial tension and hence gives lesser coefficient of friction values among the all tested samples.
5
5. Conclusion Friction properties of four surfactants were measured under oil and water lubricated contact. SDS gives less friction values among all tested surfactants whereas, C-TAB gives worst results in the both cases. An EDX analysis were performed to measure the chemical composition and we found iron content only for orvar supreme steel; for that only material transfer took place between ball and disk. Dry sliding friction values of both substrates are higher compared to oil lubricated ones due to the fact that during dry contact of both surfaces asperities rub against each other and this lead to detachment of material from one to another. The major part of the material transfer took place from the softer one due to the ploughing action. In case of water lubricants friction values are higher than dry condition due to solubility behaviour of these surfactants.
Acknowledgement Author gratefully acknowledges the support from the faculty members CTU-Prague and ISM-Dhanbad.
References 1.
Z. Wang, Y. Xia, Z. Liu, Z. Wen, Tribology Letters 46 (2012) 33–42.
2.
F. J. Carrion, S. Martınez, G. Nicolas, P. Iglesias, J. Sanes, M. D. Bermudez, International Journal of Molecular Sciences 10 (2009) 4102–4115.
3.
N. Gungor, A. Alemdar, O. Atici , I.O. Ece, Materials Letters 51 (2001) 250–254.
4.
I. Minami, Molecules 14 (2009) 2286–2305.
5.
A. E. Somers, P. C. Howlett, J. Sun, D. R. MacFarlane, M. Forsyth, Tribology Letters 40 (2010) 279– 284.
6.
A. Hernandez Battez, R.Gonzalez, J.L.Viesca, A. Fernandez-Gonzalez, M.Hadfield, Tribology International 58 (2013) 71–78.
7.
A. E. Jimenez,M. D. Bermudez, Tribology Letters 40 (2010) 237–246.
8.
D. Li, M. Cai, D. Feng, F. Zhou, W. Liu. Tribology International 44 (2011) 1111–1117.
9.
Z. Guoa, J. Xiong, M. Yang, S. Xiong, J. Chen, Y. Wu, H. Fan, L. Sun, J. Wang, H. Wang, Journal of Alloys and Compounds 493 (2010) 362–367.
10.
M. Halder, D. N. Nath, Chemical Physics Letters 387 (2004) 81–84.
11.
Z. Komeily-Nia, M. Montazer, M. Latifi, Colloids and Surfaces A: Physicochem. Eng. Aspects 439 (2013) 167– 175.
6
12.
J. Pang, G. Xu, Chemical Physics Letters 537 (2012) 118–125.
13.
F. Kılıç, H. Gül, S. Aslan, A. Alp, H. Akbulut, Colloids and Surfaces A: Physicochem. Eng. Aspects 419 (2013) 53– 60
14.
V. G. Rao, U. Brahmachari, S. Mandal, S. Ghosh, C. Banerjee, Chemical Physics Letters 552 (2012) 38–43.
15.
S. Park, G. R. Choi, J. H. Kim, J. C. Lee, Journal of the Korean Physical Society 61 (2012)1400-1403.
16.
X. Wang, L. Chen, E. Bonaccurso, J. Venzmer, Langmuir 29 (2013) 14855−14864.
17.
R. K. Upadhyay, L. A. Kumaraswamidhas, Materials at high Temperatures, 2014, DOI: 1878641314Y.0000000003 (in press).
18.
R.
K.
Upadhyay,
L.
A.
Kumaraswamidhas,
Surface
Engineering,
2014,
DOI:
1743294414Y.0000000260 (in press). 19.
M. Bicker, I. Sisman, Powder Technology 198 (2010) 279–284.
20.
H. Y. Erbil, Surface Chemistry of Solid and Liquid Interfaces, Blackwell publishing, 2006, India.
21.
J. Gu, S. Tung, G. Barber, Wear Process Manufacturing, ASTM STP1362 (1999) 31–47.
22.
S. Guruvenket, G. M. Rao, Material Science and Engineering B, 106 (2004) 172-176.
23.
G. aguilar, K. Gasljevic, E. F. Matthys, International Journal of Heat and Mass Transfer 44 (2001) 2835-2843.
24.
Y. Qi, E. Kesselman, D. J. Hart, Y. Talmon, Anthony Mateo, Jacques L. Zakin, Journal of Colloid and Interface Science 354 (2011) 691–699.
25.
D. Kumar, J. Daniel, S.K. Biswas, Journal of Colloid and Interface Science 345 (2010) 307–315.
26.
C.M. Mate, Tribology on the Small Scale: A Bottom up Approach to Friction, Lubrication and Wear, Oxford Univ. Press, New York, 2008.
7
Table 1 Structure and properties of surfactants and base oil Surfactant SDS
IUPAC/Other name Sodium lauryl sulfate
Cationic/Anionic Anionic
C-TAB
Hexadecyl-trimethylammonium bromide
Cationic
SHMP
Hexasodium metaphosphate
Anionic
ZDDP
ZnDTP
Nonionic
Structure
Density (15°C) = 0.826 g/cm3 PAO 6 Oil kinematic viscosity (40°C) = 30.5 cSt
8
a
b
c
d
Figure 1 COF values of (a): vanadis 23, (b): orvar steel substrates for oil based lubricants and COF values of (c): vanadis 23 (d): orvar steel substrates for water based lubricants.
9
ab
cd
b
d
Figure 2 SEM morphology of (a) vanadis 23, (b) orvar supreme under oil lubricated contact; and (c) vanadis 23, (d) orvar supreme under water lubricated contact
10
a
11
b
Figure 3 EDX spectrm of (a) vanadis 23 and (b) cross-section cumulative wt.% of content measured on substrate
12
a
13
b
Figure 4 EDX spectrm of (a) orvar supreme and (b) cross-section cumulative wt.% of content measured on substrate
14
Figure 5 XRD pattern for W/W2N coatings
15
Figure 6 Friction performance of vanadis 23 and orvar supreme steel substrate under dry sliding condition
16
Highlights Vanadis 23, Orvar Supreme samples were tested under wet and dry condition. SDS, C-TAB, SHMP and ZDDP are used as surfactant in oil/water medium. SDS outperforms whereas, C-Tab gives worst result among all surfactants.
18
Graphical Abstract
Dry sliding friction behaviour of vanadis 23, orvar supreme substrates without surfactant shows its maximum value of 0.24 and 0.255 respectively, and found to be higher compared to oil medium. These values are of higher side due to the fact that during dry contact of both surfaces asperities rub against each other and this lead to detachment of material from one to another. The major part of the material transfer took place from the softer one due to the ploughing action.
19