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ESCA investigation of a failure case in brass coated steel cords used in radial tyres
390
Applications of Surface Science 17(1984) 390—400 North-Holland, Amsterdam
ESCA INVESTIGATION OF A FAILURE CASE IN BRASS COATED STEEL CORDS USED IN RADIAL TYRES G. MARLET1TA and S. PIGNATARO Istituto Dipartimentale di Chimica e Chimica Industriale de/l’Università di Catania, Via/c A. Doria 6, 1-95125 Catania, Italy
and G. SANCISI Pirelli SpA., Fig/me Valdarno, Via Petrarca 104, Firenze, Italy
Received 26 September 1983; accepted for publication 13 February 1984
The ESCA spectra of two commercial sets of brass coated steel cords giving “good” and “bad” adhesion levels to a commercial rubber used in radial tyres have been obtained. Their interpretation suggests that the brass surface of the “bad” became more rich in /~phase in some step of the production process. This might be connected to an overtemperature suffered by the cord in the production line and subsequent different oxidation processes of the two samples. This phenomenon is found to influence also the chemical situation of the subsurface as it may be investigated by argon ion sputtering, as well as the chemical evolution of the two systems for prolonged ambient atmosphere exposure. In particular the two subsurfaces tend to become chemically similar for aging, thus justifying the observation of an averaging of the adhesion level of the two types of cords in aging.
1. Introduction The importance of the steel cords covered by a brass layer is mainly connected to their use into the radial tyre industries. This industrial importance suggested to study the adhesion of brass to rubber with the new tools for surface and interface analysis. This has been extensively done by Van Ooij and co-workers [1—5]. More recently some papers dealing with the ESCA study of oxidation of brass appeared in the literature [6,7]. These last works prompted us to report on a failure case observed in the production of radial tyres connected to differences in the production lines of the brass coated steel cords. All the above studies [1—7]were performed in model systems. However, it is becoming more and more clear that it is also very important to study real
0378-5963/84/$03.OO © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
G. Marietta et al.
/ Brass
coated steel cords in radial tyres
391
samples from production lines [8,9]. This because the complexity of the production processes involves several possibilities for the failure of a manufacture. This is particularly true of the processes involved in the wire production [10], so that any study of failure case with surface tools should be helpful in assessing a more scientific picture of wire manufacturing. The paper resports on the ESCA analysis of a failure case observed in the production of brass coated steel cords. The failure refers to the lowering of the adhesion level (about 50%) of some of those cords to a given commercial rubber used in radial tyres.
2. Experimental The ESCA spectra were taken with a KRATOS ES-300 electron spectrometer by using Al Ka radiation. The brass coated steel cords were produced by electrochemical deposition of brass on steel threads and successive drawing. Two different types of drawing machine and two different cooling processes were used for these samples giving “good” and “bad” adhesion to rubber respectively. Their adhesion indexes were 100% and 47% respectively. The cords analyzed had a diameter of 1.75—1.8 mm and were made by 4 x 0.25 mm threads. The plating weight was 5 g/kg and the copper content 70%. The rubber compound had a 6% sulfur level and was without cobalt containing promoters.
3. Results and discussion The ESCA spectra of the two samples reported in fig. 1 show that the Cu/Zn ratio is lower in the surface of the “bad” sample (compare Zn 2p and Cu 2p signals). Moreover, the same spectra show that in the surface region there is a concentration gradient. This conclusion can be reached considering that the Cu 3p/Zn 3p intensity ratio, which gives information on a thicker layer of surface, is higher than the corresponding Cu 2p/Zn 2p ratios. This compositional situation may, alone, justify the observed failure according to the brass-to-rubber adhesion model proposed by Van Ooij et al. [1—5]: according to this model bad adhesion has to be expected for the sample having too much Zn in the outermost layers, since it is preparared to form a weak boundary layer in the vulcanization step. The various signals of the ESCA spectra in expanded scale show that there are differences not only in the elemental but also in the chemical composition of the two samples. An analysis of these differences is useful in the understand-
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cis zn
2p
cu2~
GOOD
cis
BAD
I
250
500
I
I
I
750
1000
1250
K E (eV)
Fig. 1. ESCA spectra of two samples of brass coated steel cords giving “good” and “bad” adhesion to a commercial rubber. Cu2p312
CuLMM
Cu° BAD
364.3
eV
Cu°
GOOD
2P
cuox
364.3 eV
Fig. 2. Cu 3/2 and Cu L3M45M4,5 Auger bands of the two samples whose ESCA spectra have been reported in fig. 1.
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ing of the observed failure. Figs. 2 and 3 report the Cu 2p, Cu LMM, Zn 2p and Zn LMM bands with the assignment made on the base of the BE and Auger parameters. The strong contribution of the “oxides” to the Zn LMM band has to be noted. The assignment of the high BE component of this band to Zn(OH)2 is a tentative one. The comparison of the situation in the two samples shows that the “bad” sample contains a large amount of Zn “oxides” with respect to metallic Zn, while its Cu0~(/CuOratio is lower than the corresponding ratio in the “good” sample. These findings in the light of the recent result [6] on the oxidation of brass suggest that the brass surface of the “bad” sample became more rich in /3 phase in some step of the production process. It is known that the steel cords have to be covered with a phase brass and the /3 phase content in the bulk should not exceed 20—25% in order to be able to perform an acceptable drawing process of the cords. The “bad” and the “good” samples gave acceptable bulk a and /3 phase ratio. However, a temperature increase of an a phase brass induces [6] Zn to accumulate in the surface creating a “surface” /3-like brass. When this occurs the oxidation is globally slower. In particular, the Zn is more oxidized, thus forming a protective layer which preserves Cu to be further oxidized [6,7]. Zn2p312
ZnLMM Zn(OH)2
ZnO
BAD
522.6 eV
Zn (OH)2 GOOD
522.6 eV
2P3/2 and Zn L
Fig. 3. Zn been reported in fig. 1.
3M45M45 Auger bands of the two samples whose ESCA spectra have
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Brass coated steel cords in radial tyres
This leads to a situation very close to that observed here for the “bad” sample; so that the failure observed seems connected to an overtemperature suffered by the cord in the production line, probably during the drawing process. A less efficient cooling of the cords or a higher oxidation atomosphere suffered by the cords while still hot after the drawing process might also interpret the observed failure through a mechanism very similar to that suggested above.
Cu LMM BAD
Cu°
GOOD
1200
300”
Fig. 4. Cu L
3M45M45 Auger bands versus sputtering time for the two samples whose ESCA spectra have been reported in fig. 1.
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The effects which produce the differences above described on the surfaces of the two samples are reflected also in the situation of the subsurface as may be investigated by sputtering. Figs. 4 and 5 show the ESCA Cu and Zn Auger signals and table 1 collects the Cu/Zn ratios of the 2p and 3p band intensities before and after successive sputtering treatments (Are, 2 keV, 10 ~tAof beam current). The data in table I show that the Cu content increases in the subsurface for both “bad” and “good” samples, although for a given depth the “bad”
Zn LMM
BAD
0” Zn°
120”
300”
Fig. 5. Zn L
3M45M45 Auger bands versus sputtering time for the two samples whose ESCA spectra have been reported in fig. 1.
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Table 1 Cu/Zn intensity ratio versus sputtering time ~ 2p peaks
3p peaks
Good
Bad
1.03 (1.04) — (1.59) 2.8 (2.47) 3.29 (2.66)
0.86 1.34 2.25 2.52
0
(0.82) (1.36) (2.20) (2.48)
-
Sputtering
Good
Bad
1.36 (1.08) — (1.4) 2.5 (1.9) 2.7 (2.3)
1.0 1.6 2.16 2.2
time (s) (0.73) (1.42) (2.1) (2.4)
0 30 120 300
The numbers in parentheses refer to the values found after 8 months of exposure of the cords to
atmosphere.
samples is always more rich in Zn than the corresponding “good” one. This effect has to be considered only in a qualitative way since a preferential sputtering of Zn [11,12] may lead to the observed trend, this being especially true of the data on the 2p bands. Brass subsurfaces richer in a phase are to be expected for oxidized brass, in agreement with the previous study on the oxidation of brass [6,7], so that the trends reported in table I confirm the above arguments on the failure phenomenum. Fig. 4 shows that the CuO/Cuox ratio increases for the good material with sputtering time; while such ratio is less sensitive to the sputtering treatments for the bad material. This effect is in agreement with the above reported bad and good surface situation. Since the surface of good brass is richer in a phase, the sputtering removes the outermost surface layers richer in oxidized Cu [6,7] causing the observed CuO/Cu0~~ increase. By contrast in the /3-like surface of bad, Cu is more protected against the oxidation, so that the sputtering works on a surface layer where Cu°and Cu0” are present in more comparable amounts. Thus, no important alteration of the CuO/Cu0~(ratio is observed. Fig. 5 shows that the Zn°/Zn°”increase with sputtering time for both bad and good. This effect is again in agreement with the above discussion. The increase of the metal components should not be mainly due to ion induced reduction of the oxides since ZnO sputters congruently [12,131 and CuO should reduce to Cu 20 which is also sputtered congruently [13]. Further information on the failure phenomenum can be obtained by analyzing the surface situation of the two samples after 8 months of exposure to ambient atmosphere. It is found that the different chemical composition of the surfaces (bad and good) is reflected in a different chemistry of the two surfaces themselves. In other words, the chemical evolution of the two surfaces is found to be different. In particular the comparison of the Cu/Zn ratio for the 2p electrons reported in table 1 shows that the most external layers are only slightly modified with the exposure to atmosphere. By contrast, the subsurfaces monitored by the 3p signals (or for higher depth by the signals obtained after
G. Marietta et at
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coated steel cords in radial tyres
397
sputtering) appear to be Cu depleted. This result is in agreement with the oxidation growth model connected to a cationic migration of the Cu oxides [7,14]. According to this model, the Cu cations follow the oxygens initially bonded to Zn and diffusing into the bulk through the ZnO layer. Also, the evolution of the chemical composition of the two subsurfaces is different and their study is informative of the failure observed. Figs. 6 and 7 show that: (1) The distribution of the Zn compounds changes with the prolonged exposure to atmosphere becoming similar for good and Zn L MM
~
~.
30”
120”
Zn~X Zn° Zn~ Zn° 300”
Fig. 6. Zn L
3M45M45 Auger bands versus sputtering time for the “good” and “bad” samples after eight months of ambient atmosphere exposure.
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Brass coated steel cords in radial tyres
bad. Such similarity between good and bad is maintained also in the deeper layers explored through the sputtering treatments. (2) The distribution of the Cu compounds seems to change less with the exposure to the atmosphere. The differences found on the surfaces of the two samples do not disappear with aging. On the contrary, Cu-in bad material is more resistant to oxidation, while the CuO/Cu05 in good material seems to increase with aging.
Cu L M M
BAD
X3
—\
cuox Cu°
cP~ Cu° 0”
30”
GOOD
X3
120”
300”
Fig. 7. Cu L 3M45M45 Auger bands versus sputtering time for the “good” and “bad” samples after eight months of ambient atmosphere exposure.
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399
The situation changes for the subsurfaces where the copper chemistry tends to become similar for bad and good after aging. Thus, at the depth sampled by 200” of sputtering (Art, 2 keV, 10 isA), bad and good are very similar for copper and zinc. This holds for the elemental composition as well as for the distribution of the Zn and Cu compounds. The above findings are in agreement with the observation that bad and good after aging give similar adhesion strength with good deteriorating its adhesion characteristic more than bad.
4. Conclusions The failure observed seems to be connected with the formation of a /3-like brass surface due to an overtemperature suffered by the cords during some step in the production process (probably the drawing). This surface modification is shown to produce different oxidation kinetics in terms of both total amount and type of oxides and hydroxides. Another possibility is that the two kinds of cords have suffered two different oxidative processes with consequent different Zn diffusion and formation of two surfaces different in both elemental composition and total amount and type of oxides and hydroxides. In any case, whatever is the cause of observed differences, the surface and the near surface of the “bad” is prepared to form a weak boundary layer in the vulcanization process. This layer may be formed by the Zn oxides and hydroxides themselves which prevent the formation of the CuS layer and the related crosslinking step. An alternative possibility which cannot be ruled out is that the oxides and hydroxides of Zn migrate into the rubber, leaving copper-rich layer which, reacting vigorously [3] with rubber, gives again a weak boundary layer containing an excess of brittle cuprous sulfide. Apart from the above conclusion on the mechanism of the failure observed, the study confirms the complexity of the problems encountered in producing goods involving surface preparations. The surface tools appear essential in the study of those real samples. The characterization of their surfaces has to be made to ensure good characteristics of the end products through the check of optimized chemical composition of the surface samples. In particular, the paper suggests that care has to be taken in avoiding the overtemperature or the oxidation during the production processes of the brass-coated steel cords. Moreover, it suggests a systematic surface study of the effect of the drawing process, lubrification, chemical etching and other treatments “suffered” by these cords in the production lines.
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Brass coated steel cords in radial tyres
Acknowledgement The work has been supported by CNR “Progetto Finalizzato Chimica Fine e Secondaria”.
References [1]
W.J.
van Ooij, Surface Sci. 68 (1977) 1.
[2] W.J. van Ooij, Rubber Chem. Technol. 51(1978) 52. 131 W.J. van Ooij, A. Kleinhesselink and S.R. Leyenaar, Surface Sci. 89 (1979) 165. 141 W.J. van Ooij, Rubber Chem. Technol. 52 (1979) 605; W.J. van Ooij, W. Weemng and P.F. Murray, Rubber Chem. Technol. 54 (1981) 227. [51W.J. van Ooij and A. Kleinhesselink, Appl. Surface Sci. 4 (1980) 324. [6] S. Maroie, R. Caudano and J. Verbist, Surface Sci. 100 (1980) 1; S. Maroie, PA. Thiry, R. Caudano and J.J. Verbist, Surface Sci. 127 (1983) 200. [7] T.L. Barr and J.J. Hackenberg, AppI. Surface Sci. 10 (1982) 523. [81 A. Torrisi, S. Pignataro and G. Noceririo, AppI. Surface Sci. 13 (1982) 389. [9] A. Tornsi, G. Marietta, 0. Puglisi and S. Pignataro, Surface Interface Anal. 5 (1983) 161. [10] D.A. Stout, AppI. Surface Sci. 15 (1983) 166. [11] R. Hoim and S. Storp, Phys. Scripta 16 (1977) 442. (12] R. Kelly, Surface Sci, 100 (1980) 85. [13] K.S. Kim, W.E. Baitinger, J.W. Amy and N.J. Winograd, J. Electron Spectrosc. 5 (1974) 351. [141T.L. Barr and J.J. Hackenberg, J. Am. Chem. Soc. 104 (1982) 5390.
Applications of Surface Science 17(1984) 390—400 North-Holland, Amsterdam
ESCA INVESTIGATION OF A FAILURE CASE IN BRASS COATED STEEL CORDS USED IN RADIAL TYRES G. MARLET1TA and S. PIGNATARO Istituto Dipartimentale di Chimica e Chimica Industriale de/l’Università di Catania, Via/c A. Doria 6, 1-95125 Catania, Italy
and G. SANCISI Pirelli SpA., Fig/me Valdarno, Via Petrarca 104, Firenze, Italy
Received 26 September 1983; accepted for publication 13 February 1984
The ESCA spectra of two commercial sets of brass coated steel cords giving “good” and “bad” adhesion levels to a commercial rubber used in radial tyres have been obtained. Their interpretation suggests that the brass surface of the “bad” became more rich in /~phase in some step of the production process. This might be connected to an overtemperature suffered by the cord in the production line and subsequent different oxidation processes of the two samples. This phenomenon is found to influence also the chemical situation of the subsurface as it may be investigated by argon ion sputtering, as well as the chemical evolution of the two systems for prolonged ambient atmosphere exposure. In particular the two subsurfaces tend to become chemically similar for aging, thus justifying the observation of an averaging of the adhesion level of the two types of cords in aging.
1. Introduction The importance of the steel cords covered by a brass layer is mainly connected to their use into the radial tyre industries. This industrial importance suggested to study the adhesion of brass to rubber with the new tools for surface and interface analysis. This has been extensively done by Van Ooij and co-workers [1—5]. More recently some papers dealing with the ESCA study of oxidation of brass appeared in the literature [6,7]. These last works prompted us to report on a failure case observed in the production of radial tyres connected to differences in the production lines of the brass coated steel cords. All the above studies [1—7]were performed in model systems. However, it is becoming more and more clear that it is also very important to study real
0378-5963/84/$03.OO © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
G. Marietta et al.
/ Brass
coated steel cords in radial tyres
391
samples from production lines [8,9]. This because the complexity of the production processes involves several possibilities for the failure of a manufacture. This is particularly true of the processes involved in the wire production [10], so that any study of failure case with surface tools should be helpful in assessing a more scientific picture of wire manufacturing. The paper resports on the ESCA analysis of a failure case observed in the production of brass coated steel cords. The failure refers to the lowering of the adhesion level (about 50%) of some of those cords to a given commercial rubber used in radial tyres.
2. Experimental The ESCA spectra were taken with a KRATOS ES-300 electron spectrometer by using Al Ka radiation. The brass coated steel cords were produced by electrochemical deposition of brass on steel threads and successive drawing. Two different types of drawing machine and two different cooling processes were used for these samples giving “good” and “bad” adhesion to rubber respectively. Their adhesion indexes were 100% and 47% respectively. The cords analyzed had a diameter of 1.75—1.8 mm and were made by 4 x 0.25 mm threads. The plating weight was 5 g/kg and the copper content 70%. The rubber compound had a 6% sulfur level and was without cobalt containing promoters.
3. Results and discussion The ESCA spectra of the two samples reported in fig. 1 show that the Cu/Zn ratio is lower in the surface of the “bad” sample (compare Zn 2p and Cu 2p signals). Moreover, the same spectra show that in the surface region there is a concentration gradient. This conclusion can be reached considering that the Cu 3p/Zn 3p intensity ratio, which gives information on a thicker layer of surface, is higher than the corresponding Cu 2p/Zn 2p ratios. This compositional situation may, alone, justify the observed failure according to the brass-to-rubber adhesion model proposed by Van Ooij et al. [1—5]: according to this model bad adhesion has to be expected for the sample having too much Zn in the outermost layers, since it is preparared to form a weak boundary layer in the vulcanization step. The various signals of the ESCA spectra in expanded scale show that there are differences not only in the elemental but also in the chemical composition of the two samples. An analysis of these differences is useful in the understand-
392
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G. Marietta et at
Brass coated steel cords in radial tyres
cis zn
2p
cu2~
GOOD
cis
BAD
I
250
500
I
I
I
750
1000
1250
K E (eV)
Fig. 1. ESCA spectra of two samples of brass coated steel cords giving “good” and “bad” adhesion to a commercial rubber. Cu2p312
CuLMM
Cu° BAD
364.3
eV
Cu°
GOOD
2P
cuox
364.3 eV
Fig. 2. Cu 3/2 and Cu L3M45M4,5 Auger bands of the two samples whose ESCA spectra have been reported in fig. 1.
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/
Brass coated steel cords in radial tyres
393
ing of the observed failure. Figs. 2 and 3 report the Cu 2p, Cu LMM, Zn 2p and Zn LMM bands with the assignment made on the base of the BE and Auger parameters. The strong contribution of the “oxides” to the Zn LMM band has to be noted. The assignment of the high BE component of this band to Zn(OH)2 is a tentative one. The comparison of the situation in the two samples shows that the “bad” sample contains a large amount of Zn “oxides” with respect to metallic Zn, while its Cu0~(/CuOratio is lower than the corresponding ratio in the “good” sample. These findings in the light of the recent result [6] on the oxidation of brass suggest that the brass surface of the “bad” sample became more rich in /3 phase in some step of the production process. It is known that the steel cords have to be covered with a phase brass and the /3 phase content in the bulk should not exceed 20—25% in order to be able to perform an acceptable drawing process of the cords. The “bad” and the “good” samples gave acceptable bulk a and /3 phase ratio. However, a temperature increase of an a phase brass induces [6] Zn to accumulate in the surface creating a “surface” /3-like brass. When this occurs the oxidation is globally slower. In particular, the Zn is more oxidized, thus forming a protective layer which preserves Cu to be further oxidized [6,7]. Zn2p312
ZnLMM Zn(OH)2
ZnO
BAD
522.6 eV
Zn (OH)2 GOOD
522.6 eV
2P3/2 and Zn L
Fig. 3. Zn been reported in fig. 1.
3M45M45 Auger bands of the two samples whose ESCA spectra have
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Brass coated steel cords in radial tyres
This leads to a situation very close to that observed here for the “bad” sample; so that the failure observed seems connected to an overtemperature suffered by the cord in the production line, probably during the drawing process. A less efficient cooling of the cords or a higher oxidation atomosphere suffered by the cords while still hot after the drawing process might also interpret the observed failure through a mechanism very similar to that suggested above.
Cu LMM BAD
Cu°
GOOD
1200
300”
Fig. 4. Cu L
3M45M45 Auger bands versus sputtering time for the two samples whose ESCA spectra have been reported in fig. 1.
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The effects which produce the differences above described on the surfaces of the two samples are reflected also in the situation of the subsurface as may be investigated by sputtering. Figs. 4 and 5 show the ESCA Cu and Zn Auger signals and table 1 collects the Cu/Zn ratios of the 2p and 3p band intensities before and after successive sputtering treatments (Are, 2 keV, 10 ~tAof beam current). The data in table I show that the Cu content increases in the subsurface for both “bad” and “good” samples, although for a given depth the “bad”
Zn LMM
BAD
0” Zn°
120”
300”
Fig. 5. Zn L
3M45M45 Auger bands versus sputtering time for the two samples whose ESCA spectra have been reported in fig. 1.
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Brass coated steel cords in radial tyres
Table 1 Cu/Zn intensity ratio versus sputtering time ~ 2p peaks
3p peaks
Good
Bad
1.03 (1.04) — (1.59) 2.8 (2.47) 3.29 (2.66)
0.86 1.34 2.25 2.52
0
(0.82) (1.36) (2.20) (2.48)
-
Sputtering
Good
Bad
1.36 (1.08) — (1.4) 2.5 (1.9) 2.7 (2.3)
1.0 1.6 2.16 2.2
time (s) (0.73) (1.42) (2.1) (2.4)
0 30 120 300
The numbers in parentheses refer to the values found after 8 months of exposure of the cords to
atmosphere.
samples is always more rich in Zn than the corresponding “good” one. This effect has to be considered only in a qualitative way since a preferential sputtering of Zn [11,12] may lead to the observed trend, this being especially true of the data on the 2p bands. Brass subsurfaces richer in a phase are to be expected for oxidized brass, in agreement with the previous study on the oxidation of brass [6,7], so that the trends reported in table I confirm the above arguments on the failure phenomenum. Fig. 4 shows that the CuO/Cuox ratio increases for the good material with sputtering time; while such ratio is less sensitive to the sputtering treatments for the bad material. This effect is in agreement with the above reported bad and good surface situation. Since the surface of good brass is richer in a phase, the sputtering removes the outermost surface layers richer in oxidized Cu [6,7] causing the observed CuO/Cu0~~ increase. By contrast in the /3-like surface of bad, Cu is more protected against the oxidation, so that the sputtering works on a surface layer where Cu°and Cu0” are present in more comparable amounts. Thus, no important alteration of the CuO/Cu0~(ratio is observed. Fig. 5 shows that the Zn°/Zn°”increase with sputtering time for both bad and good. This effect is again in agreement with the above discussion. The increase of the metal components should not be mainly due to ion induced reduction of the oxides since ZnO sputters congruently [12,131 and CuO should reduce to Cu 20 which is also sputtered congruently [13]. Further information on the failure phenomenum can be obtained by analyzing the surface situation of the two samples after 8 months of exposure to ambient atmosphere. It is found that the different chemical composition of the surfaces (bad and good) is reflected in a different chemistry of the two surfaces themselves. In other words, the chemical evolution of the two surfaces is found to be different. In particular the comparison of the Cu/Zn ratio for the 2p electrons reported in table 1 shows that the most external layers are only slightly modified with the exposure to atmosphere. By contrast, the subsurfaces monitored by the 3p signals (or for higher depth by the signals obtained after
G. Marietta et at
/ Brass
coated steel cords in radial tyres
397
sputtering) appear to be Cu depleted. This result is in agreement with the oxidation growth model connected to a cationic migration of the Cu oxides [7,14]. According to this model, the Cu cations follow the oxygens initially bonded to Zn and diffusing into the bulk through the ZnO layer. Also, the evolution of the chemical composition of the two subsurfaces is different and their study is informative of the failure observed. Figs. 6 and 7 show that: (1) The distribution of the Zn compounds changes with the prolonged exposure to atmosphere becoming similar for good and Zn L MM
~
~.
30”
120”
Zn~X Zn° Zn~ Zn° 300”
Fig. 6. Zn L
3M45M45 Auger bands versus sputtering time for the “good” and “bad” samples after eight months of ambient atmosphere exposure.
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Brass coated steel cords in radial tyres
bad. Such similarity between good and bad is maintained also in the deeper layers explored through the sputtering treatments. (2) The distribution of the Cu compounds seems to change less with the exposure to the atmosphere. The differences found on the surfaces of the two samples do not disappear with aging. On the contrary, Cu-in bad material is more resistant to oxidation, while the CuO/Cu05 in good material seems to increase with aging.
Cu L M M
BAD
X3
—\
cuox Cu°
cP~ Cu° 0”
30”
GOOD
X3
120”
300”
Fig. 7. Cu L 3M45M45 Auger bands versus sputtering time for the “good” and “bad” samples after eight months of ambient atmosphere exposure.
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The situation changes for the subsurfaces where the copper chemistry tends to become similar for bad and good after aging. Thus, at the depth sampled by 200” of sputtering (Art, 2 keV, 10 isA), bad and good are very similar for copper and zinc. This holds for the elemental composition as well as for the distribution of the Zn and Cu compounds. The above findings are in agreement with the observation that bad and good after aging give similar adhesion strength with good deteriorating its adhesion characteristic more than bad.
4. Conclusions The failure observed seems to be connected with the formation of a /3-like brass surface due to an overtemperature suffered by the cords during some step in the production process (probably the drawing). This surface modification is shown to produce different oxidation kinetics in terms of both total amount and type of oxides and hydroxides. Another possibility is that the two kinds of cords have suffered two different oxidative processes with consequent different Zn diffusion and formation of two surfaces different in both elemental composition and total amount and type of oxides and hydroxides. In any case, whatever is the cause of observed differences, the surface and the near surface of the “bad” is prepared to form a weak boundary layer in the vulcanization process. This layer may be formed by the Zn oxides and hydroxides themselves which prevent the formation of the CuS layer and the related crosslinking step. An alternative possibility which cannot be ruled out is that the oxides and hydroxides of Zn migrate into the rubber, leaving copper-rich layer which, reacting vigorously [3] with rubber, gives again a weak boundary layer containing an excess of brittle cuprous sulfide. Apart from the above conclusion on the mechanism of the failure observed, the study confirms the complexity of the problems encountered in producing goods involving surface preparations. The surface tools appear essential in the study of those real samples. The characterization of their surfaces has to be made to ensure good characteristics of the end products through the check of optimized chemical composition of the surface samples. In particular, the paper suggests that care has to be taken in avoiding the overtemperature or the oxidation during the production processes of the brass-coated steel cords. Moreover, it suggests a systematic surface study of the effect of the drawing process, lubrification, chemical etching and other treatments “suffered” by these cords in the production lines.
400
C. Marietta et at
/
Brass coated steel cords in radial tyres
Acknowledgement The work has been supported by CNR “Progetto Finalizzato Chimica Fine e Secondaria”.
References [1]
W.J.
van Ooij, Surface Sci. 68 (1977) 1.
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