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An AES and SIMS study of a surface reactional film induced during ball bearing lubrication
Applications of Surface Science 8 (1981) 343—352 North-Holland Publishing Company
AN AES AND SIMS STUDY OF A SURFACE REACTIONAL FILM INDUCED DURING BALL BEARING LUBRICATION A. GAUTHIER, C. BLANC, H. MONTES, J. BRISSOT Laboratoire de Technologie des Surfaces, Ecole Centrale de Lyon, 69310 Ecully, France
and M. DAOUST Laboratoire de Recherches Balistiques et Aerodynamiqu es, 27200 Vernon, France Received 11 July 1980 Revised manuscript received 15 June 1981
The real interest of the use of surface analytical tools such as AES or SIMS in the study of precision miniature ball bearings has been largely displayed in this paper. So an electron microbeam allows to obtain the superficial repartition of the constituant on the raceway. When tncresylphosphate (TCP) is used in an elastohydrodynamic lubricating system, a film presence is revealed. This film, similar to the one observed in boundary lubrication, is essentially constituted of iron phosphate and iron oxide.
1. Introduction Many authors are studying today antiwear additives to clarify their acting mechanism. Many publications report investigations on organophosphate esters additives, and some kind of explanation has been proposed: chemical or mechanical degradation of additive to form iron phosphide [1] or iron phosphate [2,3], polymer formation or the influence of impurities [4]. However, a common conclusion resulting from a large number of investigations in this field is that the additive efficiency is due to a reactional film formation on the surface [1,2,5,61. In order to understand the additive/steel surface interaction, the chemical nature of such a film must be achieved. Because of its very low thickness, surface techniques such as Auger electron spectroscopy (AES) or secondary ion mass spectrometry (SIMS) prove to be useful tools for such experiments. Until now, the surface films studied have been obtained in boundary lubrication conditions [7—9] or by soaking in pure additives [10,11]. In this paper, we present some analytical results concerning the presence and nature of a reactional film on the roffing track of ball bearing. The film is induced during run under elastohydrodynamic operating conditions, lubricated with an additivated base oil containing tnicresyiphosphate (TCP). 0 378-5963/81/0000—0000/s 02.75 © 1981 North-Holland
344
A. Gau thier eta!.
/ Ball bearing lubrication
2. Experimental 2.1. Tribological test The bearing studied is a precision miniature bearing of R3 class made of AISI 52100 steel. The bearing, subsequently examined with Auger and SIMS analysis, has been tested in the Laboratoire de Recherches Balistiques et Aerodynamiques with a high speed simulator [12]. Operating conditions were the following: elastohydrodynaniic range; rotation speed 24000 RPM, load 12 N, temperature 80°C,duration 100 h. The oil used as lubricant consists in a superrelmed paraffinic base oil, additivated with tricresylphosphate (1 wt.%). Sample, before entering the vacuum system, was only subjected to a n-hexane ultrasonic cleaning in order to remove any oil trace. 2.2. Apparatus Experiments were performed inside an UHV chamber that could be evacuated at pressure < l0~ Pa. At such a low pressure, the main residual gas consisted in hydrogen. The samples studied were mounted on a manipulator so that they could be successively positioned in front of AES and SIMS optics. The Auger spectrometer was a cylindrical mirror analyzer with a coaxial electron gun. The accelerating voltage for primary electron beam was set at 3 kV; the spot size 2. was jim wide and the current density (Faraday cup measurement) wassource. 150 A/rn The 30 Auger spectrometer was associated with a 3 keV argon ion sputter During proffles, an ion current density of 2 X 10~A/rn2 was used. The SIMS equipment consists in a 5 keV ion gun coupled with an electrostatic quadrupole mass analyzer. For all measurements, the ~ ion beam current density was kept at 1 A/rn2 and accelerating voltage at 3 kV. The beams used for AES and SIMS could be rastered over an area of about (5 X 5) mm2 in order to obtain an image of the analyzed specimen on a scope. This possibility available was necessary to localize the raceway (about 250 jim wide). 3. Experiments and results 3.1. Substrate analysis A depth profile was performed outside the rolling track by alternating Auger analysis and ion beam etching. Results are presented in fig. 1 where atomic concentration for each element detected is plotted versus sputter time. Atomic concentrations have been calculated using the sensitivity factor method [14]. Thus, atomic concentration is given by:
A. Gauthier eta!. / Ball bearing lubrication
cj
50
0
~
•
50
100 sputter
150
200
345
Carbide
250
time (mn.)
Fig. 1. Auger depth profile on a 52100 steel surface (outside the roffing track).
C~= where h, is the peak-to-peak height of element i and S7 the sensitivity factor given in Palmberg’s handbook [14]. Such an analysis is very typical and can be generally obtained when analyzing 52100 steel surface. Four layers can be observed during the profile: (1) The rapid decrease of the carbon intensity is significant of the removal of a superficial contamination layer. (2) The constant levels observed for oxygen and iron reveal an oxide layer. (3) Afterwards during a transition layer the oxygen signal decrease shows the penetration of the oxide layer. At this step, the carbon signal exhibits a typical change of shape: the carbon contamination feature is replaced by a carbide type signal. (4) Finally the substrate analysis is obtained as revealed by the new constant levels reached by the iron, oxygen and carbon. 3.2. Demonstration ofa film presence The optical examination of the bearing shows a rolling track about 250 jim wide. Its greatest part is covered by a coloured, inhomogeneous, solid and adherent film. Referring to fig. 2, we can see a photograph of a typical aspect of this film, i.e., alternate brown and blue bands spread on all the raceway. It is to be noted that this film resulting from an elastohydrodynamic test is very alike with those obtained by a pin on flat test in a boundary lubrication range when TCP is used. In the boundary lubrication case, we have demonstrated [7—9] that these typical coloured film are due to the TCP use and phosphorus presence, while runs in the base alone lead to a surface free of color. A similar relation can be ob-
346
A. Gauthier et al.
/ Ball bearing lubrication
~:i. —.,-
Brown film
~
~‘!rI
.1
.
~.
. -
Rotation
-
—~
Blue film
—*~-
Brown
m
~
Fig. 2. Typical aspect of an elastohydrodynamic induced film.
served in ball bearing lubrication: the film presence is correlated with the axial distribution of phosphorus along the bearing generatnix (see fig. 3). The signal is ohtamed from AES analysis by taking the peak-to-peak height of the 110 eV phosphorus LMM transition. We can note that the width of the curve is approximately
~ER
RACE
Fig. 3. Phosphorus axial distribution along a bearing generatrix.
A. Gauthier et al.
/ Ball bearing lubrication
347
equal to the rolling track width (half-height width of 300 jim as compared with the optical width of 250 jim). 3.3. Determination of the nature of the film 3.3.1. AES analysis An Auger analysis (see fig. 4a) was performed on the film after a slight ionic etching to remove the hydrocarbon contamination over-layers. Detected elements were oxygen, carbon, iron and phosphorus. No contaminants as chlorine, sulfur or sodium were observed. Low energy FeMMM and ~LMM spectra presented charactenistical features. Phosphorus signal showed three minima located at 84, 94 and 110 eV. No line shape variation has been observed under the ion or electron beam impact. These values
dN(E)
1~hf1k~’~
0
100
500 E (electron volts)
E (electron
950
volts)
Fig. 4. AES analysis of surface film: (a) elastohydrodynamic induced film on ball bearing, (b)
iron soaked in H3P04, (c) TCP treatment on 52100 steel.
348
A. Gauthier eta!.
/ Ball bearing lubrication
were consistent with data published for ~LMM transition in a phosphate anion (P04)3 [13]. Similar features have been obtained by analyzing two other reactional films on surface: A pure iron surface pre-soaked in phosphoric acid presenting a thin iron phosphate film (fig. 4b), and a steel surface soaked in TCP (TCP treatment induces a surface phosphate layer [10, 11] (fig. 4c). Thus, the very likeness of the three phosphorus spectra urges us to think that the analyzed film is partly composed of phosphate. The FeMMM behaviour is not obvious. As it does not present a metallic iron lineshape with only one minimum located at about 47 eV, the observed two peaks (see fig. 4a) can be attributed to a Fe—U or Fe—P bond. 3.3.2. Auger depth profile A depth profile is carried out on the rolling track. Results are presented in fig. 5. Three successive layers are observed. The first one is due to contamination overlayers and the carbon atomic concentration rapidly drops. Then the film is revealed by a simultaneous stabilization of iron, oxygen and phosphorus; phosphorus reaches a 4% atomic concentration. Afterwards, phosphorus and carbon are slowly decreasing while iron reaches a second level. An oxide sublayer appears when both oxygen and iron remain constant. 3.3.3. SIMS analysis Surface examination with secondary ion mass spectrometry reveals the P+, P0+, POH~(fig. 6a) and PO~,FeP~,FePO~,FePO~(fig. 6b) atoms associations. Analysis also shows Na+, Al+, Ca+ and K+ which are not to be interpreted as major constituents; the high intensity of their peaks are only due to their very high sensitivity in SIMS. The observed ionic groupments are characteristic of a phosphate compound and have been previously detected when analyzing an iron phosphate [7].
xFe
.0
+C
•P
I_ 0
100 sputter
200 time (mn.)
300
Fig. 5. Depth profile of EHD induced film on bail bearing.
A. Gauthier etal. /Ball bearing lubrication
349
il JJ~ J
Na’Al’ a.
K’ Ca’
Fe’
O’POH P. a Cr’
.1111
_____
.b. FeO’ P0
PO~’ 3’
xiJ
I I
FeP’
FePO’ Fe2’
I
Fe20’ FeP0~’
m ~e Fig. 6. Mass spectra of
main
elements detected on the surface.
4. Discussion The relation between the optical examination (fig. 2) and the phosphorus axial distribution by AES (fig. 3) clearly show that the coloured film is partly formed by phosphorus compounds. Fig. 4 reveals that the phosphorus present in the rolling track is chemically bounded with oxygen under a phosphate form. Further details can be obtained by looking at the in-depth distribution of elements. Referring to fig. 5, a sequence of three layers has been shown. Another interesting representation of this depth profile is given in fig. 7 where logarithm of atomic concentration is plotted versus sputter time. These curves allow an easier explanation for the different steps involved in depth profiling. The three stages are clearly observed, each being characterized by a straight line. Thus, the carbon curve shows two slope breaks (fig. 7) due to sputter rate changes for the different layers. We can observe a high residual carbon concentration all along the profile, no plateau is found when passing through the phosphate layer and no change in the carbon lineshape can be distinguished. This indicates the carbon to be a contamination component not directly bound to phosphorus. Phosphorus in the phosphate layer reaches a 4% maximum atomic concentration. Such a low value forces us to assume that surface is composed by a kind of iron oxide and phosphate mixture. This interpretation can be confirmed by the high heterogeneity of the real surface (see fig. 2): blue and brown spots,scratches, blank areas... A schematic representation of such a surface can be seen in fig. 8. So from the beginning of sputtering non-covered areas, composed of iron oxide, exist. Their pres-
350
A. Gauthier et al.
100
0
IRON
~
560 sputter time (mn)
100
~-‘
OXYGEN
1ÔO
3O0 - 500 sputter time (mn) -
/ Ball bearing lubrication 10
0
PHOSPHORUS
~oo
3Ô0 - 5OO sputter time (mn)
100
0
CARBON
ióo
-
3OO
sputter
-
5OO
time (mn)
Fig. 7. Semi-logarithmic graph of Auger depth profile on the EHD film.
ence reduces the phosphorus concentration from 17% (value required for an iron phosphate layer) to the 4% measured. Phosphorus decrease and iron increase, during the phosphate abrasion, are due to a gradual increase of oxide areas on the analyzed surface (see fig. 8). Oxygen behavior is in good agreement with the mixture assumption; its concentration reaches a constant level before the iron and phosphorus evolution is achieved (see fig. 7). This can be explaitied by the low concentration variation expected when passing from oxide/phosphate mixture to iron oxide (for example 60 at.% in oxide as compared to 70 at.% in phosphate). Oxide nature can be stated by looking at spectra obtained after the phosphate layer removal. The atomic concentration ratio of oxygen to iron is found to be 0/Fe = 0.91. During the analysis of the substrate (fig. 1) the ratio 0/Fe is 0.95. Such low values are not typical of any iron oxide and can be partly explained. The OXIDE
FILM
~RATE~~
OXIDE
FILM
Fig. 8. Schematic representation of the surface: (a) before etching, (b) during etching.
A. Gauthier eta!.
/ Ball bearing lubrication
351
~W~e
E (electron
volts)
Fig. 9. FeMMM transition lines in the oxide sublayer.
sensitivity factors used in quantitative analysis are pure material ones and variations can be expected when analyzing compounds. Besides, ion beam impact during abrasion induces a substoichiometric layer formation on the surface [16]. Some precision on oxide nature can be obtained by looking at the FeMMM transitions. They are presented in fig. 9 where two lines are observed, one at 46 eV and one at 52 eV. Literature data [15] drive us to conclude to an a-Fe2 03 presence. Another interesting point concerning the oxide layer is the larger thickness obtained on the tested surface. So we can remark that a run in our conditions increases the oxide layer thickness.
5. Conclusion The presence of a coloured film has been observed on the raceway of precision ball bearings (R3 class) after running in a high speed simulator. Surface analysis methods such as AES and SIMS are powerful tools to study the nature of these films. A good correlation between the optical trace and the presence of phosphorus on the raceway has been pointed out thanks to the use of an electron microbeam (30 jim). The peculiar shapes of phosphorus Auger lines are typical of a phosphate. After study of the depth repartition of constituants, we assume that the superficial film is composed of both iron phosphate and iron oxide. Since phosphorus could only originate from TCP, the film formation involves a degradation process for this additive.
Acknowledgement The sponsorship by the French D.R.E.T. is here acknowledged.
A. Gauthier eta!. / Ball bearing lubrication
352
References [1]0. Beeck, J.W. Givens and E.C. Williams, in: Proc. Roy. Soc. (London) Al17 (1940) 103. [2] D. Godfrey, Trans. ASLE 8 (1965) 1. [3] FT. Barcroft and S.G. Daniel, ASME Tr. J. Basic Eng. 870 (1965) 761. [4] E.E. Klaus and 1-I.E. Bieber, Trans. ASLE 8 (1965) 12. [5] J.M. Georges, J.M. Martin, T. Mathia, Ph. Kapsa, G. Meille and H. Montès, Wear 53 (1979) 9.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
M.J. Furey, Trans. ASLE 6 (1969) 48. H. Montbs, These d’Etat, no. 7915, Lyon (1979). A. Gauthier, These de Docteur Ingénieur, no. 412, Lyon (1981). H. Montès, J.M. Martin and P. Bouchaib, in: Proc. 7th Intern. Vacuum Congress and 3rd Intern. Conf. Solid Surfaces, Vienna (1977) 1757. C. Blanc, These de troisiême cycle, no. 780, Lyon (1978). E.G. Shafnin and J.S. Murday, Trans. ASLE 21(1978) 329. M. Daoust and M. Auzolle, Journées d’Ecully (September, 1978). M.K. Bernett, J.S. Murday and N.H. Turner, J. Electron Spectrosc. Relat. Phenom. 12 (1977) 375. P.W. Palmberg, L.E. Davis, N.C. MacDonald, G.E. Riach and RE. Weber, Handbook of Auger Electron Spectroscopy (Phys. Electronics Ind., Eden Prairie, MN, 1972). M. Seo,J.B. Lumsden and R.W. Staehle, Surface Sci. 50(1975)541. R. Kelly, Nuclear Instr. Methods 149 (1978) 553.
AN AES AND SIMS STUDY OF A SURFACE REACTIONAL FILM INDUCED DURING BALL BEARING LUBRICATION A. GAUTHIER, C. BLANC, H. MONTES, J. BRISSOT Laboratoire de Technologie des Surfaces, Ecole Centrale de Lyon, 69310 Ecully, France
and M. DAOUST Laboratoire de Recherches Balistiques et Aerodynamiqu es, 27200 Vernon, France Received 11 July 1980 Revised manuscript received 15 June 1981
The real interest of the use of surface analytical tools such as AES or SIMS in the study of precision miniature ball bearings has been largely displayed in this paper. So an electron microbeam allows to obtain the superficial repartition of the constituant on the raceway. When tncresylphosphate (TCP) is used in an elastohydrodynamic lubricating system, a film presence is revealed. This film, similar to the one observed in boundary lubrication, is essentially constituted of iron phosphate and iron oxide.
1. Introduction Many authors are studying today antiwear additives to clarify their acting mechanism. Many publications report investigations on organophosphate esters additives, and some kind of explanation has been proposed: chemical or mechanical degradation of additive to form iron phosphide [1] or iron phosphate [2,3], polymer formation or the influence of impurities [4]. However, a common conclusion resulting from a large number of investigations in this field is that the additive efficiency is due to a reactional film formation on the surface [1,2,5,61. In order to understand the additive/steel surface interaction, the chemical nature of such a film must be achieved. Because of its very low thickness, surface techniques such as Auger electron spectroscopy (AES) or secondary ion mass spectrometry (SIMS) prove to be useful tools for such experiments. Until now, the surface films studied have been obtained in boundary lubrication conditions [7—9] or by soaking in pure additives [10,11]. In this paper, we present some analytical results concerning the presence and nature of a reactional film on the roffing track of ball bearing. The film is induced during run under elastohydrodynamic operating conditions, lubricated with an additivated base oil containing tnicresyiphosphate (TCP). 0 378-5963/81/0000—0000/s 02.75 © 1981 North-Holland
344
A. Gau thier eta!.
/ Ball bearing lubrication
2. Experimental 2.1. Tribological test The bearing studied is a precision miniature bearing of R3 class made of AISI 52100 steel. The bearing, subsequently examined with Auger and SIMS analysis, has been tested in the Laboratoire de Recherches Balistiques et Aerodynamiques with a high speed simulator [12]. Operating conditions were the following: elastohydrodynaniic range; rotation speed 24000 RPM, load 12 N, temperature 80°C,duration 100 h. The oil used as lubricant consists in a superrelmed paraffinic base oil, additivated with tricresylphosphate (1 wt.%). Sample, before entering the vacuum system, was only subjected to a n-hexane ultrasonic cleaning in order to remove any oil trace. 2.2. Apparatus Experiments were performed inside an UHV chamber that could be evacuated at pressure < l0~ Pa. At such a low pressure, the main residual gas consisted in hydrogen. The samples studied were mounted on a manipulator so that they could be successively positioned in front of AES and SIMS optics. The Auger spectrometer was a cylindrical mirror analyzer with a coaxial electron gun. The accelerating voltage for primary electron beam was set at 3 kV; the spot size 2. was jim wide and the current density (Faraday cup measurement) wassource. 150 A/rn The 30 Auger spectrometer was associated with a 3 keV argon ion sputter During proffles, an ion current density of 2 X 10~A/rn2 was used. The SIMS equipment consists in a 5 keV ion gun coupled with an electrostatic quadrupole mass analyzer. For all measurements, the ~ ion beam current density was kept at 1 A/rn2 and accelerating voltage at 3 kV. The beams used for AES and SIMS could be rastered over an area of about (5 X 5) mm2 in order to obtain an image of the analyzed specimen on a scope. This possibility available was necessary to localize the raceway (about 250 jim wide). 3. Experiments and results 3.1. Substrate analysis A depth profile was performed outside the rolling track by alternating Auger analysis and ion beam etching. Results are presented in fig. 1 where atomic concentration for each element detected is plotted versus sputter time. Atomic concentrations have been calculated using the sensitivity factor method [14]. Thus, atomic concentration is given by:
A. Gauthier eta!. / Ball bearing lubrication
cj
50
0
~
•
50
100 sputter
150
200
345
Carbide
250
time (mn.)
Fig. 1. Auger depth profile on a 52100 steel surface (outside the roffing track).
C~= where h, is the peak-to-peak height of element i and S7 the sensitivity factor given in Palmberg’s handbook [14]. Such an analysis is very typical and can be generally obtained when analyzing 52100 steel surface. Four layers can be observed during the profile: (1) The rapid decrease of the carbon intensity is significant of the removal of a superficial contamination layer. (2) The constant levels observed for oxygen and iron reveal an oxide layer. (3) Afterwards during a transition layer the oxygen signal decrease shows the penetration of the oxide layer. At this step, the carbon signal exhibits a typical change of shape: the carbon contamination feature is replaced by a carbide type signal. (4) Finally the substrate analysis is obtained as revealed by the new constant levels reached by the iron, oxygen and carbon. 3.2. Demonstration ofa film presence The optical examination of the bearing shows a rolling track about 250 jim wide. Its greatest part is covered by a coloured, inhomogeneous, solid and adherent film. Referring to fig. 2, we can see a photograph of a typical aspect of this film, i.e., alternate brown and blue bands spread on all the raceway. It is to be noted that this film resulting from an elastohydrodynamic test is very alike with those obtained by a pin on flat test in a boundary lubrication range when TCP is used. In the boundary lubrication case, we have demonstrated [7—9] that these typical coloured film are due to the TCP use and phosphorus presence, while runs in the base alone lead to a surface free of color. A similar relation can be ob-
346
A. Gauthier et al.
/ Ball bearing lubrication
~:i. —.,-
Brown film
~
~‘!rI
.1
.
~.
. -
Rotation
-
—~
Blue film
—*~-
Brown
m
~
Fig. 2. Typical aspect of an elastohydrodynamic induced film.
served in ball bearing lubrication: the film presence is correlated with the axial distribution of phosphorus along the bearing generatnix (see fig. 3). The signal is ohtamed from AES analysis by taking the peak-to-peak height of the 110 eV phosphorus LMM transition. We can note that the width of the curve is approximately
~ER
RACE
Fig. 3. Phosphorus axial distribution along a bearing generatrix.
A. Gauthier et al.
/ Ball bearing lubrication
347
equal to the rolling track width (half-height width of 300 jim as compared with the optical width of 250 jim). 3.3. Determination of the nature of the film 3.3.1. AES analysis An Auger analysis (see fig. 4a) was performed on the film after a slight ionic etching to remove the hydrocarbon contamination over-layers. Detected elements were oxygen, carbon, iron and phosphorus. No contaminants as chlorine, sulfur or sodium were observed. Low energy FeMMM and ~LMM spectra presented charactenistical features. Phosphorus signal showed three minima located at 84, 94 and 110 eV. No line shape variation has been observed under the ion or electron beam impact. These values
dN(E)
1~hf1k~’~
0
100
500 E (electron volts)
E (electron
950
volts)
Fig. 4. AES analysis of surface film: (a) elastohydrodynamic induced film on ball bearing, (b)
iron soaked in H3P04, (c) TCP treatment on 52100 steel.
348
A. Gauthier eta!.
/ Ball bearing lubrication
were consistent with data published for ~LMM transition in a phosphate anion (P04)3 [13]. Similar features have been obtained by analyzing two other reactional films on surface: A pure iron surface pre-soaked in phosphoric acid presenting a thin iron phosphate film (fig. 4b), and a steel surface soaked in TCP (TCP treatment induces a surface phosphate layer [10, 11] (fig. 4c). Thus, the very likeness of the three phosphorus spectra urges us to think that the analyzed film is partly composed of phosphate. The FeMMM behaviour is not obvious. As it does not present a metallic iron lineshape with only one minimum located at about 47 eV, the observed two peaks (see fig. 4a) can be attributed to a Fe—U or Fe—P bond. 3.3.2. Auger depth profile A depth profile is carried out on the rolling track. Results are presented in fig. 5. Three successive layers are observed. The first one is due to contamination overlayers and the carbon atomic concentration rapidly drops. Then the film is revealed by a simultaneous stabilization of iron, oxygen and phosphorus; phosphorus reaches a 4% atomic concentration. Afterwards, phosphorus and carbon are slowly decreasing while iron reaches a second level. An oxide sublayer appears when both oxygen and iron remain constant. 3.3.3. SIMS analysis Surface examination with secondary ion mass spectrometry reveals the P+, P0+, POH~(fig. 6a) and PO~,FeP~,FePO~,FePO~(fig. 6b) atoms associations. Analysis also shows Na+, Al+, Ca+ and K+ which are not to be interpreted as major constituents; the high intensity of their peaks are only due to their very high sensitivity in SIMS. The observed ionic groupments are characteristic of a phosphate compound and have been previously detected when analyzing an iron phosphate [7].
xFe
.0
+C
•P
I_ 0
100 sputter
200 time (mn.)
300
Fig. 5. Depth profile of EHD induced film on bail bearing.
A. Gauthier etal. /Ball bearing lubrication
349
il JJ~ J
Na’Al’ a.
K’ Ca’
Fe’
O’POH P. a Cr’
.1111
_____
.b. FeO’ P0
PO~’ 3’
xiJ
I I
FeP’
FePO’ Fe2’
I
Fe20’ FeP0~’
m ~e Fig. 6. Mass spectra of
main
elements detected on the surface.
4. Discussion The relation between the optical examination (fig. 2) and the phosphorus axial distribution by AES (fig. 3) clearly show that the coloured film is partly formed by phosphorus compounds. Fig. 4 reveals that the phosphorus present in the rolling track is chemically bounded with oxygen under a phosphate form. Further details can be obtained by looking at the in-depth distribution of elements. Referring to fig. 5, a sequence of three layers has been shown. Another interesting representation of this depth profile is given in fig. 7 where logarithm of atomic concentration is plotted versus sputter time. These curves allow an easier explanation for the different steps involved in depth profiling. The three stages are clearly observed, each being characterized by a straight line. Thus, the carbon curve shows two slope breaks (fig. 7) due to sputter rate changes for the different layers. We can observe a high residual carbon concentration all along the profile, no plateau is found when passing through the phosphate layer and no change in the carbon lineshape can be distinguished. This indicates the carbon to be a contamination component not directly bound to phosphorus. Phosphorus in the phosphate layer reaches a 4% maximum atomic concentration. Such a low value forces us to assume that surface is composed by a kind of iron oxide and phosphate mixture. This interpretation can be confirmed by the high heterogeneity of the real surface (see fig. 2): blue and brown spots,scratches, blank areas... A schematic representation of such a surface can be seen in fig. 8. So from the beginning of sputtering non-covered areas, composed of iron oxide, exist. Their pres-
350
A. Gauthier et al.
100
0
IRON
~
560 sputter time (mn)
100
~-‘
OXYGEN
1ÔO
3O0 - 500 sputter time (mn) -
/ Ball bearing lubrication 10
0
PHOSPHORUS
~oo
3Ô0 - 5OO sputter time (mn)
100
0
CARBON
ióo
-
3OO
sputter
-
5OO
time (mn)
Fig. 7. Semi-logarithmic graph of Auger depth profile on the EHD film.
ence reduces the phosphorus concentration from 17% (value required for an iron phosphate layer) to the 4% measured. Phosphorus decrease and iron increase, during the phosphate abrasion, are due to a gradual increase of oxide areas on the analyzed surface (see fig. 8). Oxygen behavior is in good agreement with the mixture assumption; its concentration reaches a constant level before the iron and phosphorus evolution is achieved (see fig. 7). This can be explaitied by the low concentration variation expected when passing from oxide/phosphate mixture to iron oxide (for example 60 at.% in oxide as compared to 70 at.% in phosphate). Oxide nature can be stated by looking at spectra obtained after the phosphate layer removal. The atomic concentration ratio of oxygen to iron is found to be 0/Fe = 0.91. During the analysis of the substrate (fig. 1) the ratio 0/Fe is 0.95. Such low values are not typical of any iron oxide and can be partly explained. The OXIDE
FILM
~RATE~~
OXIDE
FILM
Fig. 8. Schematic representation of the surface: (a) before etching, (b) during etching.
A. Gauthier eta!.
/ Ball bearing lubrication
351
~W~e
E (electron
volts)
Fig. 9. FeMMM transition lines in the oxide sublayer.
sensitivity factors used in quantitative analysis are pure material ones and variations can be expected when analyzing compounds. Besides, ion beam impact during abrasion induces a substoichiometric layer formation on the surface [16]. Some precision on oxide nature can be obtained by looking at the FeMMM transitions. They are presented in fig. 9 where two lines are observed, one at 46 eV and one at 52 eV. Literature data [15] drive us to conclude to an a-Fe2 03 presence. Another interesting point concerning the oxide layer is the larger thickness obtained on the tested surface. So we can remark that a run in our conditions increases the oxide layer thickness.
5. Conclusion The presence of a coloured film has been observed on the raceway of precision ball bearings (R3 class) after running in a high speed simulator. Surface analysis methods such as AES and SIMS are powerful tools to study the nature of these films. A good correlation between the optical trace and the presence of phosphorus on the raceway has been pointed out thanks to the use of an electron microbeam (30 jim). The peculiar shapes of phosphorus Auger lines are typical of a phosphate. After study of the depth repartition of constituants, we assume that the superficial film is composed of both iron phosphate and iron oxide. Since phosphorus could only originate from TCP, the film formation involves a degradation process for this additive.
Acknowledgement The sponsorship by the French D.R.E.T. is here acknowledged.
A. Gauthier eta!. / Ball bearing lubrication
352
References [1]0. Beeck, J.W. Givens and E.C. Williams, in: Proc. Roy. Soc. (London) Al17 (1940) 103. [2] D. Godfrey, Trans. ASLE 8 (1965) 1. [3] FT. Barcroft and S.G. Daniel, ASME Tr. J. Basic Eng. 870 (1965) 761. [4] E.E. Klaus and 1-I.E. Bieber, Trans. ASLE 8 (1965) 12. [5] J.M. Georges, J.M. Martin, T. Mathia, Ph. Kapsa, G. Meille and H. Montès, Wear 53 (1979) 9.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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