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Cryotechnology for the structural analysis of lubricants in relation to their friction properties
Wear,61 (1980) 191 - 195 0 Elsevier Sequoia &A., Lausanne - Printed in the Netherlands
191
Short Communication
Cryotechnology for the structural analysis of lubricants in relation to their friction properties
PH. KAPSA, J. M. MARTIN and J. F. CHARRION Ecole Centrale de Lyon, Laboratoire de Technologie 69130 Ecully (France)
des Surfaces, 36 route de Dardilly,
(Received July 20,1979)
Lubricants are subjected to many physical and chemical transformations during lubricated friction tests and between contacting surfaces. This topic has been extensively studied during the last decade and is of great interest. During friction, two situations can be considered. (1) The thickness of the interface is large compared with the size of the structural units of the lubricant. In this case the viscosity of the interfacial liquid is an important parameter. The behavior of the interfacial lubricant film is governed by elastohydrodynamic (EHD) equations and various conditions such as shearing stress, contact pressure and contact surface temperature have been found to be important parameters. Another view of the system is to consider the physicochemical behaviour of the lubricant during its passage through the interface in relation to its structural modification or more generally to its physical transformation. Recently it has been shown that the high pressures which can exist in an EHD contact may cause the vitrification of certain lubricant types during transit and their rheological behavior can be affected [ 1,2] . (2) The thickness of the interface is of the same order as the size of the structural units of the lubricant. This is known as boundary lubrication and the molecules or associations of molecules of the lubricant can play an important role in its chemical function and its structural arrangement. In both cases the molecular structure of the lubricant is an important parameter in lubrication and hence requires detailed study. Cryotechnics has been extensively used and developed to study the ultra-molecular structure of materials in biological applications [ 31. Several specific methods such as freeze-etching and cryofracturing have been described. The principle of this technique is to freeze the specimen (generally in liquid nitrogen) and then to create a new surface by fracturing or microtoming from which a replica can be obtained for examination in a high resolution transmission electron microscope. A micrograph of the instantaneous morphology of the liquid state can thus be obtained and examined.
192
To obtain an artefact-free replica of the specimen the freezing rate and the fracturing conditions must be carefully controlled. If the freezing rate is greater than 10 000 K s-i vitrification of the specimen is obtained without any adverse effects on the initial morphology of the system. It has been found that if the specimen is subjected to high hydrostatic pressure during freezing vitrification occurs at lower freezing rates. However, Moor and Hoechli [4] have shown that the shorter the exposure to pressure, the greater is the pressure that can be tolerated. It is interesting to note a certain analogy between the vitrification of a liquid material as in cryotechnology and the instantaneous vitrification of a lubricant in an EHD contact as described by Winer and Sanborn [ 21. For example the results seem to be identical, but the manner of achieving them is quite different in the pressure-temperature diagram. Cryofracturing of a lubricant has been applied to the study of its frictional properties. Boundary lubrication was obtained with a classical pin on a flat friction machine. A round-headed cylinder was applied to a plane surface under a normal load of 1 daN. The sliding speed was 1 mm s-’ and the movement was rectilinear and alternating (length of stroke, 10 mm). The apparent pressure corresponds to 40 daN mm - 2. Under such conditions the lubrication regime is mainly governed by the boundary conditions. The pin was pure chromium and the plane surface was AISI 52100 steel. The specimens were polished to obtain a value of RTmaX of 0.02 pm. Details of the experimental procedure are published elsewhere [ 51. A complex ester of polyglycol was used as the lubricant additive because of its good frictional properties f6] and its relatively large size (molecular weight about 8000). Pure ndodecane was used as a base. The lubricants were mixtures of the ester in dodecane at different concentrations. The friction coefficient was measured with a device using a quartz crystal during each test; the electrical contact resistance was measured and recorded simultaneously. Figure 1 shows the results obtained with several solutions containing different ester concentrations in the n-dodecane base. For ester concen~tions higher than 0.1% the friction coefficient is reduced to well-stabilized values of 0.06 - 0.07, and the average value of the electrical contact resistance (ECR) indicates that a dielectric interface film exists in the contact. For higher concentrations the friction coefficient is appro~ma~iy constant, and the ECR reaches values in excess of 10 MR. The friction coefficient for the pure ester is low, having a value of 0.03 which is significantly different from that of the solutions previously used. Optical examination of the worn surfaces revealed no coloured film as is the case with conventions additives [ 5] . Specimens for analysis were frozen in liquid nitrogen, partially solidified at 123 K (freezing rate of the order of several thousands K s-r) and then fractured under ult~igh vacuum conditions at a temperature of
193
f t
FRICTION COEFFICIENT
XJt 09
1M
1oK
Fig. 1. Lubricated friction test in the boundary regime: simultaneous measurement of the friction coefficient fand the ECR as a function of the lubricant composition (log(C%), concentration of the pure complex ester in dodecane).
158 K to reduce contamination. Replicas were prepared by metal shadowing and carbon evaporation according to the classical method used in freezeetching techniques [ 31. The replicas were washed several times with pure nhexane and doubledistilled water, and then examined in a Hitachi HU 12 transmission electron microscope. Figure 2 shows micrographs of two lubricants, the pure ester (Fig. 2(a)) and the 10% solution used to obtain the friction results given in Fig. 1. The electron micrographs show the instantaneous ultramolecular structure of the liquid used, and the differences between the two lubricants can easily be seen. The pure ester of molecular weight 8000 shows structural features in the form of multiple fibers of diameter approximately 100 nm which appear to be dispersed at the fracture surface. Since the ester molecule is relatively complex the exact significance of such a pattern and the manner in which it represents a micellar arrangement of the molecules of the substance are not understood. The 10% solution of the ester in dodecane shows a very different molecular arrangement where striations are clearly visible; considering the shadowing parameters (45” angle) and the multiplication factor, the thicknesses of the lamellae are of the order of 10 nm. In summary, observation of replicas of the two frozen lubricants in an electron microscope show that there are fundamental differences in their structures.
(b) Fig. 2. Transmission electron micrographs of cryofractured lubricant formulations: The (a) pure polyglycol complex ester (M = 8000); (b) 10 wt.% ester in n-dodecane. arrow shows the direction of shadow casting.
During boundary lubrication the molecular structure of the lubricant influences the frictional properties. Furey [6] has demonstrated the efficiency of polyglycol complex esters as antiwear additives and the way in which the polymer state is a fundamental aspect of their mechanism of action. It has now been shown that changes in the friction coefficient during boundary lubrication appear to be associated with modifications in the internal structure of the liquid. It appears that the optimum lubricant effectiveness is obtained with ester concentrations in dodecane near the critical micellar concentrations. A more comprehensive investigation is in progress to confirm this postulation. It is considered that, as in cryotechnics, lubricant freezing techniques are a potentially attractive method of studying lubricant behaviour in dynamic contacts. The authors thank J. Denis, Institut Francais du P&role, and J. P. Eudeline, Produits Chimiques Ugine Kuhlmann, for their permission to publish this work.
195 1 W. Hirst and A. J. Moore, Proc. R. Sot. London, Ser. A, 365 (1979) 537 - 565. 2 W. 0. Winer and D. M. Sanbom, NASA Contract. Rep. CR 2837, May 1977. 3 E. L. Benedetti and P. Favard, Freeze Etching Techniques and Applications, Soci&.e Francake de Microscopic Electronique, Paris, 1973. 4 H. Moor and M. Hoechli, Proc. 7th Znt. Con@. on Electron Microscopy, Vol. 1, Societk Francake de Microscopic Electronique, Paris, pp. 449 - 460. 5 J. M. Georges, J. M. Martin, T. Mathia, Ph. Kapsa, G. Meille and H. Montes, Wear, 53 (1979) 9-34. 6 J. M. Furey, Wear, 26 (1973) 369 - 392.
191
Short Communication
Cryotechnology for the structural analysis of lubricants in relation to their friction properties
PH. KAPSA, J. M. MARTIN and J. F. CHARRION Ecole Centrale de Lyon, Laboratoire de Technologie 69130 Ecully (France)
des Surfaces, 36 route de Dardilly,
(Received July 20,1979)
Lubricants are subjected to many physical and chemical transformations during lubricated friction tests and between contacting surfaces. This topic has been extensively studied during the last decade and is of great interest. During friction, two situations can be considered. (1) The thickness of the interface is large compared with the size of the structural units of the lubricant. In this case the viscosity of the interfacial liquid is an important parameter. The behavior of the interfacial lubricant film is governed by elastohydrodynamic (EHD) equations and various conditions such as shearing stress, contact pressure and contact surface temperature have been found to be important parameters. Another view of the system is to consider the physicochemical behaviour of the lubricant during its passage through the interface in relation to its structural modification or more generally to its physical transformation. Recently it has been shown that the high pressures which can exist in an EHD contact may cause the vitrification of certain lubricant types during transit and their rheological behavior can be affected [ 1,2] . (2) The thickness of the interface is of the same order as the size of the structural units of the lubricant. This is known as boundary lubrication and the molecules or associations of molecules of the lubricant can play an important role in its chemical function and its structural arrangement. In both cases the molecular structure of the lubricant is an important parameter in lubrication and hence requires detailed study. Cryotechnics has been extensively used and developed to study the ultra-molecular structure of materials in biological applications [ 31. Several specific methods such as freeze-etching and cryofracturing have been described. The principle of this technique is to freeze the specimen (generally in liquid nitrogen) and then to create a new surface by fracturing or microtoming from which a replica can be obtained for examination in a high resolution transmission electron microscope. A micrograph of the instantaneous morphology of the liquid state can thus be obtained and examined.
192
To obtain an artefact-free replica of the specimen the freezing rate and the fracturing conditions must be carefully controlled. If the freezing rate is greater than 10 000 K s-i vitrification of the specimen is obtained without any adverse effects on the initial morphology of the system. It has been found that if the specimen is subjected to high hydrostatic pressure during freezing vitrification occurs at lower freezing rates. However, Moor and Hoechli [4] have shown that the shorter the exposure to pressure, the greater is the pressure that can be tolerated. It is interesting to note a certain analogy between the vitrification of a liquid material as in cryotechnology and the instantaneous vitrification of a lubricant in an EHD contact as described by Winer and Sanborn [ 21. For example the results seem to be identical, but the manner of achieving them is quite different in the pressure-temperature diagram. Cryofracturing of a lubricant has been applied to the study of its frictional properties. Boundary lubrication was obtained with a classical pin on a flat friction machine. A round-headed cylinder was applied to a plane surface under a normal load of 1 daN. The sliding speed was 1 mm s-’ and the movement was rectilinear and alternating (length of stroke, 10 mm). The apparent pressure corresponds to 40 daN mm - 2. Under such conditions the lubrication regime is mainly governed by the boundary conditions. The pin was pure chromium and the plane surface was AISI 52100 steel. The specimens were polished to obtain a value of RTmaX of 0.02 pm. Details of the experimental procedure are published elsewhere [ 51. A complex ester of polyglycol was used as the lubricant additive because of its good frictional properties f6] and its relatively large size (molecular weight about 8000). Pure ndodecane was used as a base. The lubricants were mixtures of the ester in dodecane at different concentrations. The friction coefficient was measured with a device using a quartz crystal during each test; the electrical contact resistance was measured and recorded simultaneously. Figure 1 shows the results obtained with several solutions containing different ester concentrations in the n-dodecane base. For ester concen~tions higher than 0.1% the friction coefficient is reduced to well-stabilized values of 0.06 - 0.07, and the average value of the electrical contact resistance (ECR) indicates that a dielectric interface film exists in the contact. For higher concentrations the friction coefficient is appro~ma~iy constant, and the ECR reaches values in excess of 10 MR. The friction coefficient for the pure ester is low, having a value of 0.03 which is significantly different from that of the solutions previously used. Optical examination of the worn surfaces revealed no coloured film as is the case with conventions additives [ 5] . Specimens for analysis were frozen in liquid nitrogen, partially solidified at 123 K (freezing rate of the order of several thousands K s-r) and then fractured under ult~igh vacuum conditions at a temperature of
193
f t
FRICTION COEFFICIENT
XJt 09
1M
1oK
Fig. 1. Lubricated friction test in the boundary regime: simultaneous measurement of the friction coefficient fand the ECR as a function of the lubricant composition (log(C%), concentration of the pure complex ester in dodecane).
158 K to reduce contamination. Replicas were prepared by metal shadowing and carbon evaporation according to the classical method used in freezeetching techniques [ 31. The replicas were washed several times with pure nhexane and doubledistilled water, and then examined in a Hitachi HU 12 transmission electron microscope. Figure 2 shows micrographs of two lubricants, the pure ester (Fig. 2(a)) and the 10% solution used to obtain the friction results given in Fig. 1. The electron micrographs show the instantaneous ultramolecular structure of the liquid used, and the differences between the two lubricants can easily be seen. The pure ester of molecular weight 8000 shows structural features in the form of multiple fibers of diameter approximately 100 nm which appear to be dispersed at the fracture surface. Since the ester molecule is relatively complex the exact significance of such a pattern and the manner in which it represents a micellar arrangement of the molecules of the substance are not understood. The 10% solution of the ester in dodecane shows a very different molecular arrangement where striations are clearly visible; considering the shadowing parameters (45” angle) and the multiplication factor, the thicknesses of the lamellae are of the order of 10 nm. In summary, observation of replicas of the two frozen lubricants in an electron microscope show that there are fundamental differences in their structures.
(b) Fig. 2. Transmission electron micrographs of cryofractured lubricant formulations: The (a) pure polyglycol complex ester (M = 8000); (b) 10 wt.% ester in n-dodecane. arrow shows the direction of shadow casting.
During boundary lubrication the molecular structure of the lubricant influences the frictional properties. Furey [6] has demonstrated the efficiency of polyglycol complex esters as antiwear additives and the way in which the polymer state is a fundamental aspect of their mechanism of action. It has now been shown that changes in the friction coefficient during boundary lubrication appear to be associated with modifications in the internal structure of the liquid. It appears that the optimum lubricant effectiveness is obtained with ester concentrations in dodecane near the critical micellar concentrations. A more comprehensive investigation is in progress to confirm this postulation. It is considered that, as in cryotechnics, lubricant freezing techniques are a potentially attractive method of studying lubricant behaviour in dynamic contacts. The authors thank J. Denis, Institut Francais du P&role, and J. P. Eudeline, Produits Chimiques Ugine Kuhlmann, for their permission to publish this work.
195 1 W. Hirst and A. J. Moore, Proc. R. Sot. London, Ser. A, 365 (1979) 537 - 565. 2 W. 0. Winer and D. M. Sanbom, NASA Contract. Rep. CR 2837, May 1977. 3 E. L. Benedetti and P. Favard, Freeze Etching Techniques and Applications, Soci&.e Francake de Microscopic Electronique, Paris, 1973. 4 H. Moor and M. Hoechli, Proc. 7th Znt. Con@. on Electron Microscopy, Vol. 1, Societk Francake de Microscopic Electronique, Paris, pp. 449 - 460. 5 J. M. Georges, J. M. Martin, T. Mathia, Ph. Kapsa, G. Meille and H. Montes, Wear, 53 (1979) 9-34. 6 J. M. Furey, Wear, 26 (1973) 369 - 392.