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Chemical analysis of boundary lubrication film formed on metal nitride coatings with ethanol by means of TOF-SIMS
Wear 268 (2010) 1225–1229
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Chemical analysis of boundary lubrication film formed on metal nitride coatings with ethanol by means of TOF-SIMS Tomoo Kubo a , Hidetaka Nanao a , Shigeyuki Mori a,∗ , Yuji Enomoto b,1 , Hao Nie b , Hiroo Nomura c a b c
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan Matsuyama Giken Co. Ltd., 1050 Nagase, Maruko, Ueda 386-0407, Japan
a r t i c l e
i n f o
Article history: Received 24 May 2008 Received in revised form 15 December 2009 Accepted 17 December 2009 Available online 14 January 2010 Keywords: TOF-SIMS Boundary lubrication film Alkoxide Metal nitride Tribochemical reaction
a b s t r a c t Tribochemistry of metal nitride coatings in the presence of alcohol was investigated by using a stable isotopic tracer. The tribological properties were evaluated by a ball-on-disk type tribo-tester with steel ball and metal nitride coated disk under reciprocating motion. It was found that the friction coefficient of CrN was approximately 60% lower than that of TiN. On the other hand, TiN afforded lower wear than CrN. Considerable amount of deposit was observed on TiN. The rubbed surfaces were chemically analyzed by means of time-of-flight secondary ion mass spectroscopy (TOF-SIMS). In order to eliminate the undesirable fragment ions originated from organic contaminations in TOF-SIMS analysis, pentadeuterioethanol was used as an isotopic tracer. It was found that CrN reacted with alcohol to provide chromium ethoxide as boundary film. On the other hand, transferred Fe ion from the ball was the major contents on rubbed surfaces of TiN coatings. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Surface reaction products formed during lubrication tests affect tribological properties in boundary lubrication. Since the amount of the products is expected to be very small, the most sensitive techniques have been applied to detect the products. A radioactivetracer technique is one of the sensitive methods to detect the components of boundary films and wear debris. For example, quite small wear-rate of engine piston rings has been measured [1] and also wear of UHMWPE hip-joint liners has been detected in vitro [2] by using radioactive tracers. Tribochemical reaction mechanism has also been investigated using radioactive compounds of additives such as 14 C and 35 S for an oiliness agent [3] and EP additives [4], respectively. Stable isotopes instead of radioactive tracers can be applied for the investigation of tribochemical reactions using mass spectroscopy. We have reported tribochemical decomposition of benzene on nascent nickel surfaces using deuterated benzene (C6 D6 ) [5], and tribochemical decomposition of formic acid using deuterated formic acid (DCOOD) [6]. Recently, TOF-SIMS has been employed for the investigation of tribochemistry, and it is powerful to detect lubricant components and tribochemical reaction products because of its high sensitivity and high mass resolution. ∗ Corresponding author. Fax: +81 19 621 6335. 1 Present address: Toyama Industrial Technology Center, 150 Futagami, Takaoka 933-0981, Japan. 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.12.035
In order to realize a reduction in the usage of petroleum resources, ethanol generated from biomass resources has recently received attention as a new energy source, especially for the usage of fuels for automobile engines. The engine system involves such sliding components as a fuel injection plunger and a piston ring, which are subjected to sliding under the fuel substance. TiN and CrN are candidate tribo-coatings utilized for the engine components to reduce friction and wear. It is well known that the friction and wear of hard coatings composed of metal nitrides are often dramatically improved by water [7,8] or alcohol [9,10]. The importance of tribochemistry was pointed out in this literature. Since less study has been conducted on the tribochemical reaction between TiN and CrN with ethanol, we aimed to clarify the reaction scheme by means of TOF-SIMS method in this paper. To this end, we used C2 D5 OH (referred as ethanold5 ) instead of C2 H5 OH as lubricant fluid in order to eliminate the organic contaminations which can be detected undesirably by TOF-SIMS. The utility of stable isotopic tracers in studying tribochemistry had been reported in our previous reports [11,12]. 2. Experimental details TiN and CrN are synthesized on a steel disk of 20 mm in diameter and 5 mm thick by means of an arc ion plating method. The thickness of both films is 3 m. The hardness of TiN is 2300 kg/mm2 and that of CrN is 1800 kg/mm2 .
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After the tribo-test, the disk was immersed in ethanol-d5 in a glass container until it was subjected to a TOF-SIMS analysis. 3.2. Optical micrograph of rubbed surface Morphology of wear tracks was studied by an optical microscope, as shown in Fig. 2. TiN provided a narrower wear scar width than CrN under these conditions. A clear deposit was found on TiN surfaces, whereas those on CrN were not obvious. It was suggested that the higher friction coefficient of TiN disk was due to the formation of the deposits. Therefore, TOF-SIMS analysis was conducted to understand the chemical structure of the boundary lubrication film on wear tracks. Fig. 1. Effect of materials on friction coefficient obtained in ethanol-d5 (load: 3.92 N, sliding speed: 10 mm/s).
In the friction experiments, reciprocated sliding was made on the coated disk against a 10 mm diameter stainless steel (SUS 304) ball, using a ball-on-disk type tribo-tester. During sliding, a small amount of ethanol-d5 was supplied into sliding interface using a micro-dispenser, so that a meniscus of the fluid was always formed in the sliding contact area. Since the present study aims to clarify the tribochemical reaction at the sliding interface, the typical sliding condition is selected as follows: the applied load is 3.92 N, sliding speed is 10 mm/s. In order to analyze the chemical structure of boundary lubrication films formed on the rubbed surface, TOF-SIMS (time-of-flight secondary ion mass spectroscopy) was applied, with Ga+ as primary ion source. Other conditions of measurement were shown in detail in our previous work [11–14]. We analyzed both the rubbed surface (4 positions), the outside (4 positions) and the boundary (1 position) to identify the different chemical species existing on these areas. 3. Results and discussion 3.1. Friction tests The average friction coefficient of TiN was 0.62, while that of CrN was 0.26 during the period of the sliding time of 10 min (Fig. 1).
3.3. TOF-SIMS analyses In order to analyze the chemical structure of the boundary lubrication film, chemical images and mass spectra were obtained by TOF-SIMS with a high spatial or a high mass resolution. These results are discussed as follows. 3.3.1. Chemical images by TOF-SIMS CrN disk: the intensity of positive fragment ion at m/z 52 (Cr+ ) derived from CrN coating on the rubbed surfaces was lower than outside (Fig. 3a). On the other hand, the intensities of m/z 56 (Fe+ ) and 2 (D+ ) on the rubbed surface were higher than those outside. These results can be explained by the transfer of iron from ball surface onto CrN coating and the tribochemical reaction of ethanold5 , which results in a boundary film. TiN disk: similar to the result of CrN coating, m/z 48 (Ti+ ) and 56 + (Fe ) were found on outside and inside of the wear tracks, respectively (Fig. 3b). However, the intensity of m/z 2 (D+ ) was too weak to obtain the chemical image by TOF-SIMS. 3.3.2. TOF-SIMS spectra and relative intensity of the key fragment ions The chemical image of D+ ion by TOF-SIMS in Fig. 3 suggests the formation of a D-containing film on the rubbed surface of CrN. Then careful studies of TOF-SIMS spectra were carried out in order to identify the chemical structure of the boundary lubrication film containing deuterium. Among several D-containing fragment ions, we paid attention to the quasi-molecular ion at m/z 50.07 (C2 D5 O− ), which can be generated by the migration of hydrogen from C2 D5 OH.
Fig. 2. Optical micrographs of the wear track and the positions at TOF-SIMS analysis.
T. Kubo et al. / Wear 268 (2010) 1225–1229
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Fig. 3. TOF-SIMS images at boundary area (high spatial resolution mode, 180 m2 ).
Fig. 4 shows the negative ion spectra focused at m/z 48–55 on the boundary area which covers both wear track and outside. Quasimolecular ion at m/z 50.07 (C2 D5 O− ) near m/z 50.02 (C4 H2 − ) was found on CrN disk, whereas it was not found on TiN disk in the
least. This result indicates that the boundary film is composed of C2 D5 O moiety. In other words, metal ethoxide was generated on the rubbed CrN surface. The structure of the boundary film is discussed in detail in following section.
Fig. 4. Negative ion spectra at boundary area by TOF-SIMS (m/z 48–55, high mass resolution mode, 180 m2 ).
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Fig. 5. Relative intensity of quasi-molecular ion on CrN disk (C2 D5 O− : M-1, high mass resolution mode, 180 m2 ). Fig. 7. Ion fragmentation of chromium ethoxide.
Intensity of the negative quasi-molecular ion of ethanol-d5 was measured. Fig. 5 shows a relative intensity of the quasi-molecular ion which exists on CrN surface, normalized by the intensity of total ions. The relative intensity of C2 D5 O− on rubbed surface was in the range of 0.00025 ± 0.00003, whereas that on outside rubbed surface was 0.00010 ± 0.00003. The much higher intensity of C2 D5 O− ion on rubbed surface indicates that the tribological process promoted the interaction of ethanol-d5 with CrN surfaces. The ethoxide ion (C2 D5 O− ) was not observed on rubbed TiN surface. There are two possible causes for the result: (1) titanium does not form alkoxides and (2) the reaction of alcohol with TiN is prevented by a coating on rubbed TiN surface. Since titanium alkoxides are known in literature [17], the reaction may be prevented by surface coating, as mentioned below. In our previous work on lubricity of alcohols under boundary conditions, we found the effects of chemical structure of alcohol on the tribological properties [9,10,15,16]. We also proposed therein the formation of a boundary film composed of alkoxides. Perry and coworkers have also been reported that ethanol reacts with vanadium carbide ceramic-material producing a thin chemisorbed layer [18]. In this study, our attention was also focused on the formation of boundary film composed of alkoxides. Fig. 6 shows the relative intensity of fragment ion at m/z 67.94 (CrO− ) normalized by Cr− ion on CrN disk. The source of CrO− ion can be considered as the fragment ion of chromium ethoxide (Fig. 7) and/or chromium oxide. The much higher intensity of CrO− ion in the rubbed surface suggests that the CrO species were generated tribochemically. On the other hand, the intensity of titanium oxide at m/z 64 (TiO− )
Fig. 8. Relative intensities of Fe+ ion on wear track normalized by Cr+ or Ti+ ion.
was very low on the rubbed surface of TiN. Fig. 8 shows the relative intensity of Fe+ ion on CrN and TiN disks normalized by Cr+ ion and Ti+ ion, respectively. These results indicate that transfer of iron from the steel ball onto TiN disk occurred easily in comparison with the iron–CrN contact. In other words, the transferred iron has prevented the formation of titanium oxide and/or ethoxide. Another possible reason for the absence of ethoxide on iron surface is decomposition of chemisorbed ethanol by nascent iron surface. We have reported that even hydrocarbon oil could be decomposed tribochemically on nascent steel surface [19]. 4. Conclusions In this study, the tribological properties of ethanol at steel/CrNcoating and steel/TiN-coating contacts were investigated by ball-on-disk type tribo-tester and optical micrographs. Then tribochemistry of the system was studied by a TOF-SIMS analysis using a stable isotopic tracer.
Fig. 6. Relative intensity of CrO− ion on CrN disk (high mass resolution mode, 180 m2 ).
1. Friction coefficient of CrN was lower than that of TiN. 2. However the wear width of CrN was larger than that of TiN. TiN provided considerable deposits during the tribo-test. 3. By means of stable isotopic tracers, we effectively eliminate the unfavorable noise caused by organic contaminations in TOFSIMS analysis. Clear fragment ions containing deuterium were found on the wear track of CrN. 4. Chromium ethoxide film formed by a tribochemical reaction was found on CrN surface. However the titanium ethoxide was not
T. Kubo et al. / Wear 268 (2010) 1225–1229
observed on TiN surface. It is concluded that a lot of transferred iron has prevented the formation of titanium ethoxide on TiN surface. 5. We wish to propose that the tribological properties of metal nitride coatings can be attributed to the tribochemistry. References [1] M.B. Treuhaft, Iddings, F.A. Boyd, A. Glen, S.R. Sprague, The use of radioactive tracer technology in studying lubricant chemistry to enhance bearing and ring wear control in an operating engine, SAE SP 1055 (1994) 75–88. [2] C.M. Wall, D.C. Eberle, M.B. Treuhaft, J.H. Arps, Technique for high-sensitivity “in vitro” wear measurement of UHMWPE hip joint liners using radioactive tracer technology, Wear 259 (2005) 964–971. [3] H.A. Smith, T. Fort Jr., Some properties of surface films formed by adsorption of n-decanoic acid on mechanically activated metal surfaces, Journal of Physical Chemistry 62 (1958) 519–527. [4] T. Sakurai, H. Okabe, Y. Takahashi, Kinetic study of the reaction of labeled sulfur compounds in binary additive systems during boundary lubrication, ASLE Transaction 10 (1967) 91–101. [5] S. Mori, M. Yoshida, Decomposition of aromatic compounds on cut nickel surface, ASLE Transaction 31 (1988) 128–132. [6] S. Mori, T. Kawada, W.C. Wu, Tribochemical decomposition of formic acid on the nascent surfaces of steel formed by scratching, Applied Surface Science 108 (1997) 391–397. [7] T.E. Fischer, H. Tomizawa, Interaction of tribochemistry and microfracture in the friction and wear of silicon nitride, Wear 105 (1985) 29–45. [8] H. Tomizawa, T.E. Fischer, Friction and wear of silicon nitride and silicon carbide in water: hydrodynamic lubrication at low sliding speed obtained by tribochemical wear, ASLE Transaction 30 (1987) 41–46.
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[9] Y. Hibi, Y. Enomoto, Chemical analyses of mechanochemical reaction products of ␣-Si3 N4 in ethanol and other lower alcohols, Journal of Materials Science Letters 16 (1997) 316–319. [10] Y. Hibi, S. Sasaki, Y. Enomoto, Tribological behavior of titanium nitride–titanium composites in short-chain alcohols, Journal of American Ceramic Society 88 (2005) 470–472. [11] I. Minami, T. Kubo, S. Fujiwara, Y. Ogasawara, H. Nanao, S. Mori, Investigation of tribo-chemistry by means of stable isotopic tracers: TOF-SIMS analysis of Langmuir–Blodgett films and examination of their tribological properties, Tribology Letters 20 (2005) 287–297. [12] I. Minami, T. Kubo, H. Nanao, S. Mori, S. Okuda, T. Sagawa, Investigation of tribochemistry by means of stable isotopic tracers, Part 2: lubrication mechanism of friction modifiers on diamond-like carbon, Tribology Transaction 50 (2008) 477–487. [13] T. Kubo, S. Fujiwara, H. Nanao, I. Minami, S. Mori, TOF-SIMS analysis of boundary films derived from calcium sulfonates, Tribology Letters 23 (2006) 171–176. [14] T. Kubo, S. Fujiwara, H. Nanao, I. Minami, S. Mori, Boundary film formation from overbased calcium sulfonate additives during running-in process of steel–DLC contact, WEAR 265 (2008) 461–467. [15] Y. Hibi, Y. Enomoto, Mechanochemical reaction and relationship to tribological response of silicon nitride in n-alcohol, Wear 231 (1999) 185–194. [16] S. Igari, S. Mori, Y. Takikawa, Effects of molecular structure of aliphatic diols and polyalkylene glycol as lubricants on the wear of aluminum, Wear 244 (2000) 180–184. [17] E.A. Jeffery, T. Mole, Organoaluminium Compounds, Elsevier Publishing Company, Amsterdam, 1972. [18] B.L. Kim, S. Lee, R. Guenard, L.C. Fernandez, S.S. Perry, P. Franz, S.V. Didziulis, Chemical modification of the interfacial frictional properties of vanadium carbide through ethanol adsorption, Surface Science 481 (2001) 185–197. [19] R. Lu, S. Mori, T. Kubo, H. Nanao, Effect of sulfur-containing additive on the decomposition of multialkylated cyclopentane oil on the nascent steel surface, Wear 267 (2009) 1430–1435.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Chemical analysis of boundary lubrication film formed on metal nitride coatings with ethanol by means of TOF-SIMS Tomoo Kubo a , Hidetaka Nanao a , Shigeyuki Mori a,∗ , Yuji Enomoto b,1 , Hao Nie b , Hiroo Nomura c a b c
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan Matsuyama Giken Co. Ltd., 1050 Nagase, Maruko, Ueda 386-0407, Japan
a r t i c l e
i n f o
Article history: Received 24 May 2008 Received in revised form 15 December 2009 Accepted 17 December 2009 Available online 14 January 2010 Keywords: TOF-SIMS Boundary lubrication film Alkoxide Metal nitride Tribochemical reaction
a b s t r a c t Tribochemistry of metal nitride coatings in the presence of alcohol was investigated by using a stable isotopic tracer. The tribological properties were evaluated by a ball-on-disk type tribo-tester with steel ball and metal nitride coated disk under reciprocating motion. It was found that the friction coefficient of CrN was approximately 60% lower than that of TiN. On the other hand, TiN afforded lower wear than CrN. Considerable amount of deposit was observed on TiN. The rubbed surfaces were chemically analyzed by means of time-of-flight secondary ion mass spectroscopy (TOF-SIMS). In order to eliminate the undesirable fragment ions originated from organic contaminations in TOF-SIMS analysis, pentadeuterioethanol was used as an isotopic tracer. It was found that CrN reacted with alcohol to provide chromium ethoxide as boundary film. On the other hand, transferred Fe ion from the ball was the major contents on rubbed surfaces of TiN coatings. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Surface reaction products formed during lubrication tests affect tribological properties in boundary lubrication. Since the amount of the products is expected to be very small, the most sensitive techniques have been applied to detect the products. A radioactivetracer technique is one of the sensitive methods to detect the components of boundary films and wear debris. For example, quite small wear-rate of engine piston rings has been measured [1] and also wear of UHMWPE hip-joint liners has been detected in vitro [2] by using radioactive tracers. Tribochemical reaction mechanism has also been investigated using radioactive compounds of additives such as 14 C and 35 S for an oiliness agent [3] and EP additives [4], respectively. Stable isotopes instead of radioactive tracers can be applied for the investigation of tribochemical reactions using mass spectroscopy. We have reported tribochemical decomposition of benzene on nascent nickel surfaces using deuterated benzene (C6 D6 ) [5], and tribochemical decomposition of formic acid using deuterated formic acid (DCOOD) [6]. Recently, TOF-SIMS has been employed for the investigation of tribochemistry, and it is powerful to detect lubricant components and tribochemical reaction products because of its high sensitivity and high mass resolution. ∗ Corresponding author. Fax: +81 19 621 6335. 1 Present address: Toyama Industrial Technology Center, 150 Futagami, Takaoka 933-0981, Japan. 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.12.035
In order to realize a reduction in the usage of petroleum resources, ethanol generated from biomass resources has recently received attention as a new energy source, especially for the usage of fuels for automobile engines. The engine system involves such sliding components as a fuel injection plunger and a piston ring, which are subjected to sliding under the fuel substance. TiN and CrN are candidate tribo-coatings utilized for the engine components to reduce friction and wear. It is well known that the friction and wear of hard coatings composed of metal nitrides are often dramatically improved by water [7,8] or alcohol [9,10]. The importance of tribochemistry was pointed out in this literature. Since less study has been conducted on the tribochemical reaction between TiN and CrN with ethanol, we aimed to clarify the reaction scheme by means of TOF-SIMS method in this paper. To this end, we used C2 D5 OH (referred as ethanold5 ) instead of C2 H5 OH as lubricant fluid in order to eliminate the organic contaminations which can be detected undesirably by TOF-SIMS. The utility of stable isotopic tracers in studying tribochemistry had been reported in our previous reports [11,12]. 2. Experimental details TiN and CrN are synthesized on a steel disk of 20 mm in diameter and 5 mm thick by means of an arc ion plating method. The thickness of both films is 3 m. The hardness of TiN is 2300 kg/mm2 and that of CrN is 1800 kg/mm2 .
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After the tribo-test, the disk was immersed in ethanol-d5 in a glass container until it was subjected to a TOF-SIMS analysis. 3.2. Optical micrograph of rubbed surface Morphology of wear tracks was studied by an optical microscope, as shown in Fig. 2. TiN provided a narrower wear scar width than CrN under these conditions. A clear deposit was found on TiN surfaces, whereas those on CrN were not obvious. It was suggested that the higher friction coefficient of TiN disk was due to the formation of the deposits. Therefore, TOF-SIMS analysis was conducted to understand the chemical structure of the boundary lubrication film on wear tracks. Fig. 1. Effect of materials on friction coefficient obtained in ethanol-d5 (load: 3.92 N, sliding speed: 10 mm/s).
In the friction experiments, reciprocated sliding was made on the coated disk against a 10 mm diameter stainless steel (SUS 304) ball, using a ball-on-disk type tribo-tester. During sliding, a small amount of ethanol-d5 was supplied into sliding interface using a micro-dispenser, so that a meniscus of the fluid was always formed in the sliding contact area. Since the present study aims to clarify the tribochemical reaction at the sliding interface, the typical sliding condition is selected as follows: the applied load is 3.92 N, sliding speed is 10 mm/s. In order to analyze the chemical structure of boundary lubrication films formed on the rubbed surface, TOF-SIMS (time-of-flight secondary ion mass spectroscopy) was applied, with Ga+ as primary ion source. Other conditions of measurement were shown in detail in our previous work [11–14]. We analyzed both the rubbed surface (4 positions), the outside (4 positions) and the boundary (1 position) to identify the different chemical species existing on these areas. 3. Results and discussion 3.1. Friction tests The average friction coefficient of TiN was 0.62, while that of CrN was 0.26 during the period of the sliding time of 10 min (Fig. 1).
3.3. TOF-SIMS analyses In order to analyze the chemical structure of the boundary lubrication film, chemical images and mass spectra were obtained by TOF-SIMS with a high spatial or a high mass resolution. These results are discussed as follows. 3.3.1. Chemical images by TOF-SIMS CrN disk: the intensity of positive fragment ion at m/z 52 (Cr+ ) derived from CrN coating on the rubbed surfaces was lower than outside (Fig. 3a). On the other hand, the intensities of m/z 56 (Fe+ ) and 2 (D+ ) on the rubbed surface were higher than those outside. These results can be explained by the transfer of iron from ball surface onto CrN coating and the tribochemical reaction of ethanold5 , which results in a boundary film. TiN disk: similar to the result of CrN coating, m/z 48 (Ti+ ) and 56 + (Fe ) were found on outside and inside of the wear tracks, respectively (Fig. 3b). However, the intensity of m/z 2 (D+ ) was too weak to obtain the chemical image by TOF-SIMS. 3.3.2. TOF-SIMS spectra and relative intensity of the key fragment ions The chemical image of D+ ion by TOF-SIMS in Fig. 3 suggests the formation of a D-containing film on the rubbed surface of CrN. Then careful studies of TOF-SIMS spectra were carried out in order to identify the chemical structure of the boundary lubrication film containing deuterium. Among several D-containing fragment ions, we paid attention to the quasi-molecular ion at m/z 50.07 (C2 D5 O− ), which can be generated by the migration of hydrogen from C2 D5 OH.
Fig. 2. Optical micrographs of the wear track and the positions at TOF-SIMS analysis.
T. Kubo et al. / Wear 268 (2010) 1225–1229
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Fig. 3. TOF-SIMS images at boundary area (high spatial resolution mode, 180 m2 ).
Fig. 4 shows the negative ion spectra focused at m/z 48–55 on the boundary area which covers both wear track and outside. Quasimolecular ion at m/z 50.07 (C2 D5 O− ) near m/z 50.02 (C4 H2 − ) was found on CrN disk, whereas it was not found on TiN disk in the
least. This result indicates that the boundary film is composed of C2 D5 O moiety. In other words, metal ethoxide was generated on the rubbed CrN surface. The structure of the boundary film is discussed in detail in following section.
Fig. 4. Negative ion spectra at boundary area by TOF-SIMS (m/z 48–55, high mass resolution mode, 180 m2 ).
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Fig. 5. Relative intensity of quasi-molecular ion on CrN disk (C2 D5 O− : M-1, high mass resolution mode, 180 m2 ). Fig. 7. Ion fragmentation of chromium ethoxide.
Intensity of the negative quasi-molecular ion of ethanol-d5 was measured. Fig. 5 shows a relative intensity of the quasi-molecular ion which exists on CrN surface, normalized by the intensity of total ions. The relative intensity of C2 D5 O− on rubbed surface was in the range of 0.00025 ± 0.00003, whereas that on outside rubbed surface was 0.00010 ± 0.00003. The much higher intensity of C2 D5 O− ion on rubbed surface indicates that the tribological process promoted the interaction of ethanol-d5 with CrN surfaces. The ethoxide ion (C2 D5 O− ) was not observed on rubbed TiN surface. There are two possible causes for the result: (1) titanium does not form alkoxides and (2) the reaction of alcohol with TiN is prevented by a coating on rubbed TiN surface. Since titanium alkoxides are known in literature [17], the reaction may be prevented by surface coating, as mentioned below. In our previous work on lubricity of alcohols under boundary conditions, we found the effects of chemical structure of alcohol on the tribological properties [9,10,15,16]. We also proposed therein the formation of a boundary film composed of alkoxides. Perry and coworkers have also been reported that ethanol reacts with vanadium carbide ceramic-material producing a thin chemisorbed layer [18]. In this study, our attention was also focused on the formation of boundary film composed of alkoxides. Fig. 6 shows the relative intensity of fragment ion at m/z 67.94 (CrO− ) normalized by Cr− ion on CrN disk. The source of CrO− ion can be considered as the fragment ion of chromium ethoxide (Fig. 7) and/or chromium oxide. The much higher intensity of CrO− ion in the rubbed surface suggests that the CrO species were generated tribochemically. On the other hand, the intensity of titanium oxide at m/z 64 (TiO− )
Fig. 8. Relative intensities of Fe+ ion on wear track normalized by Cr+ or Ti+ ion.
was very low on the rubbed surface of TiN. Fig. 8 shows the relative intensity of Fe+ ion on CrN and TiN disks normalized by Cr+ ion and Ti+ ion, respectively. These results indicate that transfer of iron from the steel ball onto TiN disk occurred easily in comparison with the iron–CrN contact. In other words, the transferred iron has prevented the formation of titanium oxide and/or ethoxide. Another possible reason for the absence of ethoxide on iron surface is decomposition of chemisorbed ethanol by nascent iron surface. We have reported that even hydrocarbon oil could be decomposed tribochemically on nascent steel surface [19]. 4. Conclusions In this study, the tribological properties of ethanol at steel/CrNcoating and steel/TiN-coating contacts were investigated by ball-on-disk type tribo-tester and optical micrographs. Then tribochemistry of the system was studied by a TOF-SIMS analysis using a stable isotopic tracer.
Fig. 6. Relative intensity of CrO− ion on CrN disk (high mass resolution mode, 180 m2 ).
1. Friction coefficient of CrN was lower than that of TiN. 2. However the wear width of CrN was larger than that of TiN. TiN provided considerable deposits during the tribo-test. 3. By means of stable isotopic tracers, we effectively eliminate the unfavorable noise caused by organic contaminations in TOFSIMS analysis. Clear fragment ions containing deuterium were found on the wear track of CrN. 4. Chromium ethoxide film formed by a tribochemical reaction was found on CrN surface. However the titanium ethoxide was not
T. Kubo et al. / Wear 268 (2010) 1225–1229
observed on TiN surface. It is concluded that a lot of transferred iron has prevented the formation of titanium ethoxide on TiN surface. 5. We wish to propose that the tribological properties of metal nitride coatings can be attributed to the tribochemistry. References [1] M.B. Treuhaft, Iddings, F.A. Boyd, A. Glen, S.R. Sprague, The use of radioactive tracer technology in studying lubricant chemistry to enhance bearing and ring wear control in an operating engine, SAE SP 1055 (1994) 75–88. [2] C.M. Wall, D.C. Eberle, M.B. Treuhaft, J.H. Arps, Technique for high-sensitivity “in vitro” wear measurement of UHMWPE hip joint liners using radioactive tracer technology, Wear 259 (2005) 964–971. [3] H.A. Smith, T. Fort Jr., Some properties of surface films formed by adsorption of n-decanoic acid on mechanically activated metal surfaces, Journal of Physical Chemistry 62 (1958) 519–527. [4] T. Sakurai, H. Okabe, Y. Takahashi, Kinetic study of the reaction of labeled sulfur compounds in binary additive systems during boundary lubrication, ASLE Transaction 10 (1967) 91–101. [5] S. Mori, M. Yoshida, Decomposition of aromatic compounds on cut nickel surface, ASLE Transaction 31 (1988) 128–132. [6] S. Mori, T. Kawada, W.C. Wu, Tribochemical decomposition of formic acid on the nascent surfaces of steel formed by scratching, Applied Surface Science 108 (1997) 391–397. [7] T.E. Fischer, H. Tomizawa, Interaction of tribochemistry and microfracture in the friction and wear of silicon nitride, Wear 105 (1985) 29–45. [8] H. Tomizawa, T.E. Fischer, Friction and wear of silicon nitride and silicon carbide in water: hydrodynamic lubrication at low sliding speed obtained by tribochemical wear, ASLE Transaction 30 (1987) 41–46.
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