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On the critical thickness of protective films at sliding interfaces
Wear, 95 (1984)
165
165 - 175
ON THE CRITICAL THICKNESS SLIDING INTERFACES
OF PROTKCTIV~
FILMS AT
R. Y. LEE and Z. ELIEZER department of mechanical Engi~ee~ng and material Science The Unive~it~ of Texas at Austin, Austin, TX 7871.2 (U.S.A.)
and ~nginee~ng
Program,
(Received February 20, 1984; accepted March 15, 1984)
Summary Friction and wear experiments were conducted on couples consisting of Invar and Fe-3%Si steel pins sliding against a tool steel disk in a mild vacuum (0.1 mmHg) at room temperature. At loads below a critical value, protective films, identified as compacted oxides, were observed on the sliding surfaces. The resulting friction and wear values were very low. A critical film thickness -was observed that was thinner for Invar (6 pm) than for Fe-3%Si steel (22 pm), presumably because of a larger difference between the thermal expansion coefficients of oxide and metal for Invar than for Fe-3%Si steel. This critical thickness was found to be independent of sliding speed or applied load. However, at higher loads, the critical thickness was reached at lower sliding times, probably as a result of a higher flash temperature.
1. Introduction The fo~ation of oxide films on metal surfaces during sliding is known to lead to reduced friction and wear [ 11. Rabinowicz [ 2 ] argues that for an oxide film to act as a solid lubricant it must be at least 10v6 cm thick under specified conditions. If the films were less thick than this critical value, they were said to break down and metal-to-metal contact ensued. However, to be beneficial, it has been found [3] that the mech~ic~ properties of the oxide and its substrate materials must be such that the oxide is adherent to the surface. Peterson [3] showed that considerable benefit can be gained by using alloys which form a soft continuous type of oxide on the surface. Development of compacted oxides, the so-called glazes, on sliding metal and alloy surfaces can result in a si~ific~t decrease in the friction and wear rate. Stott et al. [4, 51 show that the glazes consist of very fine compacted crystalline particles of almost any oxide. The tribological properties of the glazes are associated with their low shear strength and the high strength of the underlying alloy substrate [ 61. @ Elsevier Sequoia/~inted
in The Netherlands
166
There has been considerable discussion on the effect of oxides and other surface films in the literature. At the present time, the oxidation processes occurring during sliding wear are not completely understood. The investigation reported in this paper was aimed at a better understanding of the formation and the effect of protective oxide films at sliding interfaces.
2. Experimental procedure The friction and wear experiments were conducted in a mild vacuum (0.1 mmHg) at room temperature on a pin-on-disk type of machine. A strain ring and a linear variable-displacement transducer (LVDT) were used to measure the friction force and the vertical displacement (i.e. the pin wear) respectively. The flat-faced pins, 3.5 mm in diameter, were made of Invar (Fe36%Ni) and of Fe-3%Si steel. The disk counterface (37.5 mm wear track diameter) was made of SAE 01 tool steel hardened to 58 HRC. An Apple computer, which was linked through an analog-to-digital (A/D) converter to a Hewlett Packard 321 dual-channel recorder, was used to monitor the friction force and LVDT position during tests. The computer was programmed to monitor the data as they were generated. The automatic collection and reduction of data permits a more complex analysis to be performed. In particular, the analyzing capacity for small wear rates was much improved by using the computer set-up.
L 0
I
4.45
8.9
13.5
17.8
22.3
LOAOIN)
Fig. 1. The shift in critical load with sliding speed for Invar: 0, 120 rev min-’ ; A, 340 rev min-’ ; X, 780 rev min-‘. The magnitude of the critical load increases when the velocity is decreased. The discontinuity in the vertical scale should be noted.
167
3. Results and discussion 3.1. Friction and wear A plot of the wear rate uersus the applied load for Invar tested in mild vacuum is shown in Fig. 1. The wear rates are very low in a certain load range. There is a discontinuity in the wear rate-load relationship, and two distinct regimes of wear are obtained. The transition between these two regimes is represented by a broken line. This is a transition from mild to severe wear. It should be noted that a discontinuous vertical scale is used on this plot. Such a discontinuity is also observed for Fe-3%Si steel. At loads higher than the transition load, the wear mode is predominantly adhesive. For example, at a load of 17.8 N, the wear rate is 0.28 mm3 km-’ for the Invar specimen. Using suitable values of L = 1.8 kgf
(4
(b)
Fig. 2. (a) Scanning electron micrograph .of the worn surface of an Invar pin (load, 2.23 N; sliding speed, 770 rev min- 1 (1.52 m s-l); (b) plate-like metallic wear debris (experimental conditions as for (a)). (Magnifications: (a) 16.5~ ; (b) 150x.)
(a)
(b)
Fig. 3. Scanning electron micrographs showing the severely damaged surfaces at loads higher than the critical load: (a) the pile-up of deformed surface layers on an Invar pin; (b) the breaking off of welded junctions on the surface of an Fe-3%Si pin. (Magnifications: (a) 400x; (b) 750x.).
168
(17.8 N) and H = 200 kgf mm- 2 (about 96 HRB), the wear coefficient is found to be of the order of 10P4, which is a typical value for adhesive wear between unlike clean metals [ 71. The appearance of the surface after sliding at loads above the transition leaves little doubt that intimate metallic contact has occurred (Fig. 2). The wear debris are metallic in nature and have the shape of sheets or plates, which result from the breakage of the deformed layer. The severely damaged surface at this high load is shown in Fig. 3; layers of deformed material form on top of each other. Cold welding and shearing at junctions can be clearly seen at high magnifications. The formation of a protective oxide film on the surface seems to be the main reason for the mild wear observed below the transition load. A scanning electron photograph of an Invar specimen after 25 h of sliding at a load of 2.23 N and 120 rev min-’ 1s shown in Fig. 4. This film has a glassy appearance. It is very smooth, has a dark color and cannot be easily
(b)
(a) Fig. 4. Scanning electron pin (load, 2.23 N; sliding (b) 150x.)
Fig. 5. Protective tion, 165x.)
micrographs showing the protective film formed speed, 120 rev mm’ -l (0.24 m s-l)). (Magnifications:
film transferred
from
on an Invar (a) 16.5~;
the Invar pin to the tool steel disc. (Magnifica-
169
0.8
0.6
0
fa f
0.c
p-0
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i
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0.4
0.4
I
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4 I
Y
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:
,’
:
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+/
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/’
:
: :
Y
0 ,
I
0.2 a2
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4.45
8.9
13.5
LO AD (N)
17.8
22.3
I
0
1
4.45
1
9.9
13.5
lZ9
22.5
LOADiN,
Fig. 6. The variation in friction coefficient with applied load for an Fe-3%Si against a tool steel disc at 340 rev min-l (0.67 m s-l).
pin sliding
Fig. 7. Friction coefficient us. load at sliding speeds of 120 rev min-’ (0) and 340 rev min-’ (A). A reduction in the speed increases the magnitude of the critical load.
peeled off. A similar protective film is found on the wear track of the disc surface (Fig. 5). Plots of the friction coefficient uersus the load are shown in Fig. 6 and Fig. 7 for Fe-3%Si and Invar respectively. The friction-load relationship is very consistent with the wear rate plot. The experimental results show that the friction of an Fe-3%Si or Invar slider on a tool steel disc depends on the normal load applied to the sliding surface. Amonton’s law holds at both high and low loads, but with different values of the coefficient of friction. The broken lines represent the transition from a low value of the friction to a high value. This transition occurs at the same load as that in the wear rate. It is evident that a surface film is protecting the naked metal at low loads. In previous publications [8, 91 it has been shown that this film consists of a mixture of silicon oxides and iron oxides, with the silicon oxides being more prevalent near the interface while the iron oxides predominate at the subsurface. The low values of the friction correspond to sliding on this film, not on the metal. The low shear strength of the film combined with the high hardness of the substrate are thought to be the main reason for the observed low friction. 3.2. Pro tee tive film formation The various stages in the formation of the protective visualized by plotting the LVDT displacement uers’sus the
film can be sliding time.
170
At loads above the critical load, the height of the pin continuously decreases (constant wear rate) with sliding time. However, it is found that the height of the pin initially increases instead of decreasing with sliding time at loads below the transition load (Fig. 8). Positive wear actually does not occur until the oxide reaches a critical thickness. The increase in pin height, i.e. the negative wear, lasts for 2 - 10 h depending on the load. The probable reason for the negative wear is that the oxide films produced during sliding occupy a larger volume than the metal from which they formed. An Auger analysis [ 91 seems to support this interpretation. When the thickness of the protective film reaches a critical value, it begins to break down, thus resulting in positive wear. The critical thickness was measured with the LVDT together with the A/D converter. This method is very sensitive and can measure displacements to an accuracy of about 1 pm. The measured critical value for the Invar-steel couple was about 6 pm. It was also found that the critical thickness was nearly independent of the load at loads below the transition value. In general the time period for the negative wear increases with decreasing load at a constant speed (Fig. 9). For example, it lasts for 2.5 h and 4 h at loads of 4.45 N and 2.23 N respectively. Another interesting pattern in the LVDT displacement and friction traces is observed in the transition region at loads just above the critical load. Figure 10 shows the changes in LVDT displacement and friction coefficient with sliding time at an 8.9 N load and a sliding speed of 120 rev min-’ (0.22 m s-l) for Invar. Under these experimental conditions, a high wear rate (0.18 mm3 km-‘) and a high friction coefficient (0.72) are observed for about 18 h. After this time a discontinuity in the graph can be seen, i.e. the coefficient of friction and wear rate decreased drastically to
\
L
0
(4
o’2t--_ 0
4
Sliding
Time tHr;
8
4
Sliding
TimerHr
)
(b)
Fig. 8. Traces of (a) LVDT displacement and (b) friction vs. sliding time. The initial decrease in displacement indicated by the LVDT displacement signifies negative wear.
171
1
I
0
8
Sliding4 Tims(Hrk Fig. 9. The critical thickness load (L, > Lz).
(6 pm) of the protective
film is independent
of the applied
0.28 and 2.4 X 10v3 mm3 km-’ respectively. This discontinuity also can be associated with the formation of a protective film during sliding. The plot of the LVDT displacement against the sliding time for Fe3%Si is similar to that for Invar. Here, again, negative wear occurred for a certain time and was followed by positive wear. However, the thickness of this protective film for Fe-3%Si steel was found to be 22 brn, compared with 6 pm for Invar. Several investigators [4 - 61 reported that the development of compacted oxides, particularly the so-called glares, on sliding metal and alloy surfaces can result in a significant decrease in the friction coefficient and wear rates at elevated temperatures. They proposed a model for the glaze formation. This model involves compaction and sintering of oxide debris during sliding. On initiation of sliding, the very thin preformed oxide film is rapidly removed and the virgin alloy is exposed to oxygen. Transient oxidation ~media~ly takes place on the exposed alloy. As sliding continues, frictional heating increases at the point of contact, thus increasing the amount of transient oxidation. Although oxide is removed on each traverse, it is rapidly re-formed owing to the high localized temperature until eventually the build-up of oxide is sufficient to form a stable oxide layer on the load-bearing areas. The localized temperatures are sufficiently high to cause thermal softening, giving rise to the thin physically homogeneous glaze. The negative wear observed in the present experiments corresponds to these steps. Thus a protective film or glaze is formed during this period. It is interesting to realize that such a protective glaze film can be formed at room temperature and at reduced oxygen pressure. The actual thickne~es (6 pm for Invar and 22 pm for Fe-3%Si steel) of the protective fihn formed in these experiments are somewhat higher
172
I
0 0
3
18
18
Sliding (a)
Time,
hour
ib)
Fig. 10. The change in (a) depth of wear and (b) friction coefficient with sliding time for Invar at a load just above the critical value (load, 8.9 N; sliding speed, 120 rev min I (0.24 m s I)).
than the reported values af 0.5 - 5 pm for nickel-base alloys [4]. The measured thickness in the present experiments is the thickness of the glaze plus the iron oxides beneath it [ 91.
3.3. Protective film removal The breakdown of a proactive film is associated with the existence of a critical stress in the surface layer. The fact that high stresses may exist in the surface layer of a material has been pointed out for gaseous oxidation by several investigators. These internal stresses may range in magnitude from lo2 to lo4 MPa. Several models for stress generation during oxidation were proposed. Pilling and Bedworth [lo] suggested that the net volume expansion would yield a compressive stress in the oxide. Another model relating the stress in the oxide to the oxygen concentration gradient was proposed by Pawell
[ill. Deadmore and Lowell [12 ] suggested that oxide spalling resulted from the coefficient of thermal expansion rn~rna~h between oxide and metal. The thermal expansion mismatch stress in the oxide is given by [12 J o ox = E‘,, AT (cy,, - CY~)
(1)
where u,, is the thermal stress at the interface between oxide and metal, E,, is the elastic modulus of oxide, AT is the difference between the oxidizing temperature and the temperature at which the oxide is cooled and of thermal expansion of oxide and metal cynx and (Y, are the coefficients respectively. The flaking off of a protective layer is shown in Fig. 11. It is thought that, when the stress within the layer exceeds a critical value, the protective film will break down. The nominal stress generated in the oxide increases as the oxide grows [13] and, when the stress level reaches the critical stress, the protective film is destroyed.
173
Fig. 11. Scanning electron micrograph of a flake resulting from a protective film that reached its critical thickness. (Magnification, 150x.)
After breaking off at the critical thickness, the protective oxide film will be re-formed during subsequent sliding. In turn, the repetition of this sequence results in positive wear. It is difficult to describe quantitatively the stresses generated in the oxide layer during sliding motion in the presence of external tangential stresses. If these stresses were known, the critical film thickness could eventually be estimated. Nevertheless, the difference in the critical thicknesses for Fe-3%Si and Invar specimens can be explained qualitatively. From eqn. (1) a higher stress can be expected for the material which has a higher thermal mismatch between the oxide and the bulk metal. The thermal expansion values are 9.9 X 1O-6 K-‘, 10.3 X 10m6 K-l, 10.8 X 10m6 K-’ and 0.13 X 10V6 K-’ for FezOs, SiOz, Fe-3%Si and Invar respectively at room temperature [ 141. Therefore, at the same AT, the thermal mismatch stress is higher for Invar because of the higher value of ACY.This higher stress for the Invar specimen might result in a smaller value of the critical thickness (6 pm) compared with that (22 pm) for Fe-3%Si steel. Critical Load x
I
x
0
4.45
, 8.9
135
17.8
Load (N) Fig. 12. The variation in surface roughness with load for an Fe-3%Si 340 rev min-’ (0.67 m s-l)).
pin (sliding speed,
174
3.4. Surface roughness The initial roughnesses are 0.33 - 0.45 pm and 0.25 - 0.3 ym for the pins and discs respectively after polishing with emery papers. The measured roughness after tests in mild vacuum and at a sliding speed of 340 rev min‘ ’ for Fe-3%Si steel are plotted against the applied loads in Fig. 12. It is clear that the pin surfaces are severely damaged at loads above the critical load. However, below the critical load, the surfaces do not show any s~n~i~ant damage. The protective film at these lower loads prevents the penetration of asperities or decreases the metal-to-metal contacts. Thus the worn surface is nearly as smooth as the original surface.
4. Conclusions (1) In mild vacuum, at loads below a critical value, the wear rates and the friction coefficients exhibit very low values, owing mainly to the formation of protective surface oxide films. (2) At loads above the critical values, the wear rate begins to increase sharply. There is no protective film on the surfaces at these high loads and the surfaces are severely damaged. (3) It has been found that there exists a critical thickness for protective films. Negative wear occurs until the thickness reaches the critical value. At a thickness higher than the critical value the film breaks down, resulting in positive wear thereafter. The negative wear, i.e. the increase in pin height, is caused by the volume expansion due to the formation of oxide on the surface. The negative wear lasts for 2 - 10 h depending on the load; it takes less time at higher loads owing to rapid oxidation. (4) In our investigation the critical film thickness was found to be 6 ,um for Invar (Fe--36%Ni) and 22 pm for Fe-3%Si steel. The smaller thickness for Invar is caused by the larger difference in thermal expansion between the oxide and the bulk metal.
References 1 M. G. Hayler and S. W. E, Earles, An investigation of the unlubricated sliding process between NT5 and EnlA, Wear, 18 (1971) 393 - 402. 2 E. Rabinowicz, Lubrication of metal surfaces by oxide films, ASLE Trans., tO (1967) 400 - 407. 3 M. B. Peterson, Sliding characteristics of metals at high temperature, ASLE Trans., 3 (1960) 101 - 109. 4 F. H. Stott, D. S. Lin and G. C. Wood, The structure and mechanism of formation of the giaze oxide layers produced on Ni-based alloys during wear at high temperature, Corros. Sci., 13 (1973) 449 - 469. 5 F. H. Stott and G. C. Wood, The influence of oxides on the friction and wear of alloys, Tribology, 11 (4) (August 1978) 213 - 262. 6 D. S. Lin, F, N. Stott and G. C. Wood, The effects of elevated ambient temperatures on the friction and wear behavior of some eormnerciat Ni-base alloys, ASLE Trans., 17 (1974) 251 - 262.
175 7 E. Rabinowicz, Friction and Wear of Materials, Wiley, New York, 1965. 8 R. Y. Lee and Z. Eiiezer, Silicon segregation in sliding wear, Wear, 87 (1983) 227 233. 9 R. Y. Lee, H. S. Luftman and Z. Ehezer, Silicon segregation in sliding wear: an ESCA-AES investigation. In R. Kossowsky and S. C!. Singhal (eds.), Proc. of the North
Atlantic Treaty Organization Advanced Study Institute on Surface Engineering, Les Arcs, July 1983, Nijhoff, to be published. 10 N. B, Pilling and R. E. Bedworth, J. Inst. Met., 29 (1923) 523. 11 R. E. Pawell, Stress generation in tantalum during oxidation, J. Electrochem. Sot., 110 (1963) 551- 557. 12 D. L. Deadmore and C. E. Lowell, The effect of AT on oxide spalling, Oxid. Met., 11 (1977) 91 - 106. 13 H. E. Evans, D. J. Norfolk and T. Swan, Perturbation of parabolic kinetics resulting from the accumulation of stress in protective oxide layers, J. Electrochem. Sot., 125 (1978) 1180 - 1185.
14 Thermophysicai
Properties of Matter - Thermophys~~
Vols. 12 and 13, Thermal Expansion, IF&Plenum,
Research Center Data Series,
New York, 1975.
165
165 - 175
ON THE CRITICAL THICKNESS SLIDING INTERFACES
OF PROTKCTIV~
FILMS AT
R. Y. LEE and Z. ELIEZER department of mechanical Engi~ee~ng and material Science The Unive~it~ of Texas at Austin, Austin, TX 7871.2 (U.S.A.)
and ~nginee~ng
Program,
(Received February 20, 1984; accepted March 15, 1984)
Summary Friction and wear experiments were conducted on couples consisting of Invar and Fe-3%Si steel pins sliding against a tool steel disk in a mild vacuum (0.1 mmHg) at room temperature. At loads below a critical value, protective films, identified as compacted oxides, were observed on the sliding surfaces. The resulting friction and wear values were very low. A critical film thickness -was observed that was thinner for Invar (6 pm) than for Fe-3%Si steel (22 pm), presumably because of a larger difference between the thermal expansion coefficients of oxide and metal for Invar than for Fe-3%Si steel. This critical thickness was found to be independent of sliding speed or applied load. However, at higher loads, the critical thickness was reached at lower sliding times, probably as a result of a higher flash temperature.
1. Introduction The fo~ation of oxide films on metal surfaces during sliding is known to lead to reduced friction and wear [ 11. Rabinowicz [ 2 ] argues that for an oxide film to act as a solid lubricant it must be at least 10v6 cm thick under specified conditions. If the films were less thick than this critical value, they were said to break down and metal-to-metal contact ensued. However, to be beneficial, it has been found [3] that the mech~ic~ properties of the oxide and its substrate materials must be such that the oxide is adherent to the surface. Peterson [3] showed that considerable benefit can be gained by using alloys which form a soft continuous type of oxide on the surface. Development of compacted oxides, the so-called glazes, on sliding metal and alloy surfaces can result in a si~ific~t decrease in the friction and wear rate. Stott et al. [4, 51 show that the glazes consist of very fine compacted crystalline particles of almost any oxide. The tribological properties of the glazes are associated with their low shear strength and the high strength of the underlying alloy substrate [ 61. @ Elsevier Sequoia/~inted
in The Netherlands
166
There has been considerable discussion on the effect of oxides and other surface films in the literature. At the present time, the oxidation processes occurring during sliding wear are not completely understood. The investigation reported in this paper was aimed at a better understanding of the formation and the effect of protective oxide films at sliding interfaces.
2. Experimental procedure The friction and wear experiments were conducted in a mild vacuum (0.1 mmHg) at room temperature on a pin-on-disk type of machine. A strain ring and a linear variable-displacement transducer (LVDT) were used to measure the friction force and the vertical displacement (i.e. the pin wear) respectively. The flat-faced pins, 3.5 mm in diameter, were made of Invar (Fe36%Ni) and of Fe-3%Si steel. The disk counterface (37.5 mm wear track diameter) was made of SAE 01 tool steel hardened to 58 HRC. An Apple computer, which was linked through an analog-to-digital (A/D) converter to a Hewlett Packard 321 dual-channel recorder, was used to monitor the friction force and LVDT position during tests. The computer was programmed to monitor the data as they were generated. The automatic collection and reduction of data permits a more complex analysis to be performed. In particular, the analyzing capacity for small wear rates was much improved by using the computer set-up.
L 0
I
4.45
8.9
13.5
17.8
22.3
LOAOIN)
Fig. 1. The shift in critical load with sliding speed for Invar: 0, 120 rev min-’ ; A, 340 rev min-’ ; X, 780 rev min-‘. The magnitude of the critical load increases when the velocity is decreased. The discontinuity in the vertical scale should be noted.
167
3. Results and discussion 3.1. Friction and wear A plot of the wear rate uersus the applied load for Invar tested in mild vacuum is shown in Fig. 1. The wear rates are very low in a certain load range. There is a discontinuity in the wear rate-load relationship, and two distinct regimes of wear are obtained. The transition between these two regimes is represented by a broken line. This is a transition from mild to severe wear. It should be noted that a discontinuous vertical scale is used on this plot. Such a discontinuity is also observed for Fe-3%Si steel. At loads higher than the transition load, the wear mode is predominantly adhesive. For example, at a load of 17.8 N, the wear rate is 0.28 mm3 km-’ for the Invar specimen. Using suitable values of L = 1.8 kgf
(4
(b)
Fig. 2. (a) Scanning electron micrograph .of the worn surface of an Invar pin (load, 2.23 N; sliding speed, 770 rev min- 1 (1.52 m s-l); (b) plate-like metallic wear debris (experimental conditions as for (a)). (Magnifications: (a) 16.5~ ; (b) 150x.)
(a)
(b)
Fig. 3. Scanning electron micrographs showing the severely damaged surfaces at loads higher than the critical load: (a) the pile-up of deformed surface layers on an Invar pin; (b) the breaking off of welded junctions on the surface of an Fe-3%Si pin. (Magnifications: (a) 400x; (b) 750x.).
168
(17.8 N) and H = 200 kgf mm- 2 (about 96 HRB), the wear coefficient is found to be of the order of 10P4, which is a typical value for adhesive wear between unlike clean metals [ 71. The appearance of the surface after sliding at loads above the transition leaves little doubt that intimate metallic contact has occurred (Fig. 2). The wear debris are metallic in nature and have the shape of sheets or plates, which result from the breakage of the deformed layer. The severely damaged surface at this high load is shown in Fig. 3; layers of deformed material form on top of each other. Cold welding and shearing at junctions can be clearly seen at high magnifications. The formation of a protective oxide film on the surface seems to be the main reason for the mild wear observed below the transition load. A scanning electron photograph of an Invar specimen after 25 h of sliding at a load of 2.23 N and 120 rev min-’ 1s shown in Fig. 4. This film has a glassy appearance. It is very smooth, has a dark color and cannot be easily
(b)
(a) Fig. 4. Scanning electron pin (load, 2.23 N; sliding (b) 150x.)
Fig. 5. Protective tion, 165x.)
micrographs showing the protective film formed speed, 120 rev mm’ -l (0.24 m s-l)). (Magnifications:
film transferred
from
on an Invar (a) 16.5~;
the Invar pin to the tool steel disc. (Magnifica-
169
0.8
0.6
0
fa f
0.c
p-0
h
0
I
i
I
0.4
0.4
I
I
I
:
4 I
Y
I
:
,’
:
&
+/
0
/’
:
: :
Y
0 ,
I
0.2 a2
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4.45
8.9
13.5
LO AD (N)
17.8
22.3
I
0
1
4.45
1
9.9
13.5
lZ9
22.5
LOADiN,
Fig. 6. The variation in friction coefficient with applied load for an Fe-3%Si against a tool steel disc at 340 rev min-l (0.67 m s-l).
pin sliding
Fig. 7. Friction coefficient us. load at sliding speeds of 120 rev min-’ (0) and 340 rev min-’ (A). A reduction in the speed increases the magnitude of the critical load.
peeled off. A similar protective film is found on the wear track of the disc surface (Fig. 5). Plots of the friction coefficient uersus the load are shown in Fig. 6 and Fig. 7 for Fe-3%Si and Invar respectively. The friction-load relationship is very consistent with the wear rate plot. The experimental results show that the friction of an Fe-3%Si or Invar slider on a tool steel disc depends on the normal load applied to the sliding surface. Amonton’s law holds at both high and low loads, but with different values of the coefficient of friction. The broken lines represent the transition from a low value of the friction to a high value. This transition occurs at the same load as that in the wear rate. It is evident that a surface film is protecting the naked metal at low loads. In previous publications [8, 91 it has been shown that this film consists of a mixture of silicon oxides and iron oxides, with the silicon oxides being more prevalent near the interface while the iron oxides predominate at the subsurface. The low values of the friction correspond to sliding on this film, not on the metal. The low shear strength of the film combined with the high hardness of the substrate are thought to be the main reason for the observed low friction. 3.2. Pro tee tive film formation The various stages in the formation of the protective visualized by plotting the LVDT displacement uers’sus the
film can be sliding time.
170
At loads above the critical load, the height of the pin continuously decreases (constant wear rate) with sliding time. However, it is found that the height of the pin initially increases instead of decreasing with sliding time at loads below the transition load (Fig. 8). Positive wear actually does not occur until the oxide reaches a critical thickness. The increase in pin height, i.e. the negative wear, lasts for 2 - 10 h depending on the load. The probable reason for the negative wear is that the oxide films produced during sliding occupy a larger volume than the metal from which they formed. An Auger analysis [ 91 seems to support this interpretation. When the thickness of the protective film reaches a critical value, it begins to break down, thus resulting in positive wear. The critical thickness was measured with the LVDT together with the A/D converter. This method is very sensitive and can measure displacements to an accuracy of about 1 pm. The measured critical value for the Invar-steel couple was about 6 pm. It was also found that the critical thickness was nearly independent of the load at loads below the transition value. In general the time period for the negative wear increases with decreasing load at a constant speed (Fig. 9). For example, it lasts for 2.5 h and 4 h at loads of 4.45 N and 2.23 N respectively. Another interesting pattern in the LVDT displacement and friction traces is observed in the transition region at loads just above the critical load. Figure 10 shows the changes in LVDT displacement and friction coefficient with sliding time at an 8.9 N load and a sliding speed of 120 rev min-’ (0.22 m s-l) for Invar. Under these experimental conditions, a high wear rate (0.18 mm3 km-‘) and a high friction coefficient (0.72) are observed for about 18 h. After this time a discontinuity in the graph can be seen, i.e. the coefficient of friction and wear rate decreased drastically to
\
L
0
(4
o’2t--_ 0
4
Sliding
Time tHr;
8
4
Sliding
TimerHr
)
(b)
Fig. 8. Traces of (a) LVDT displacement and (b) friction vs. sliding time. The initial decrease in displacement indicated by the LVDT displacement signifies negative wear.
171
1
I
0
8
Sliding4 Tims(Hrk Fig. 9. The critical thickness load (L, > Lz).
(6 pm) of the protective
film is independent
of the applied
0.28 and 2.4 X 10v3 mm3 km-’ respectively. This discontinuity also can be associated with the formation of a protective film during sliding. The plot of the LVDT displacement against the sliding time for Fe3%Si is similar to that for Invar. Here, again, negative wear occurred for a certain time and was followed by positive wear. However, the thickness of this protective film for Fe-3%Si steel was found to be 22 brn, compared with 6 pm for Invar. Several investigators [4 - 61 reported that the development of compacted oxides, particularly the so-called glares, on sliding metal and alloy surfaces can result in a significant decrease in the friction coefficient and wear rates at elevated temperatures. They proposed a model for the glaze formation. This model involves compaction and sintering of oxide debris during sliding. On initiation of sliding, the very thin preformed oxide film is rapidly removed and the virgin alloy is exposed to oxygen. Transient oxidation ~media~ly takes place on the exposed alloy. As sliding continues, frictional heating increases at the point of contact, thus increasing the amount of transient oxidation. Although oxide is removed on each traverse, it is rapidly re-formed owing to the high localized temperature until eventually the build-up of oxide is sufficient to form a stable oxide layer on the load-bearing areas. The localized temperatures are sufficiently high to cause thermal softening, giving rise to the thin physically homogeneous glaze. The negative wear observed in the present experiments corresponds to these steps. Thus a protective film or glaze is formed during this period. It is interesting to realize that such a protective glaze film can be formed at room temperature and at reduced oxygen pressure. The actual thickne~es (6 pm for Invar and 22 pm for Fe-3%Si steel) of the protective fihn formed in these experiments are somewhat higher
172
I
0 0
3
18
18
Sliding (a)
Time,
hour
ib)
Fig. 10. The change in (a) depth of wear and (b) friction coefficient with sliding time for Invar at a load just above the critical value (load, 8.9 N; sliding speed, 120 rev min I (0.24 m s I)).
than the reported values af 0.5 - 5 pm for nickel-base alloys [4]. The measured thickness in the present experiments is the thickness of the glaze plus the iron oxides beneath it [ 91.
3.3. Protective film removal The breakdown of a proactive film is associated with the existence of a critical stress in the surface layer. The fact that high stresses may exist in the surface layer of a material has been pointed out for gaseous oxidation by several investigators. These internal stresses may range in magnitude from lo2 to lo4 MPa. Several models for stress generation during oxidation were proposed. Pilling and Bedworth [lo] suggested that the net volume expansion would yield a compressive stress in the oxide. Another model relating the stress in the oxide to the oxygen concentration gradient was proposed by Pawell
[ill. Deadmore and Lowell [12 ] suggested that oxide spalling resulted from the coefficient of thermal expansion rn~rna~h between oxide and metal. The thermal expansion mismatch stress in the oxide is given by [12 J o ox = E‘,, AT (cy,, - CY~)
(1)
where u,, is the thermal stress at the interface between oxide and metal, E,, is the elastic modulus of oxide, AT is the difference between the oxidizing temperature and the temperature at which the oxide is cooled and of thermal expansion of oxide and metal cynx and (Y, are the coefficients respectively. The flaking off of a protective layer is shown in Fig. 11. It is thought that, when the stress within the layer exceeds a critical value, the protective film will break down. The nominal stress generated in the oxide increases as the oxide grows [13] and, when the stress level reaches the critical stress, the protective film is destroyed.
173
Fig. 11. Scanning electron micrograph of a flake resulting from a protective film that reached its critical thickness. (Magnification, 150x.)
After breaking off at the critical thickness, the protective oxide film will be re-formed during subsequent sliding. In turn, the repetition of this sequence results in positive wear. It is difficult to describe quantitatively the stresses generated in the oxide layer during sliding motion in the presence of external tangential stresses. If these stresses were known, the critical film thickness could eventually be estimated. Nevertheless, the difference in the critical thicknesses for Fe-3%Si and Invar specimens can be explained qualitatively. From eqn. (1) a higher stress can be expected for the material which has a higher thermal mismatch between the oxide and the bulk metal. The thermal expansion values are 9.9 X 1O-6 K-‘, 10.3 X 10m6 K-l, 10.8 X 10m6 K-’ and 0.13 X 10V6 K-’ for FezOs, SiOz, Fe-3%Si and Invar respectively at room temperature [ 141. Therefore, at the same AT, the thermal mismatch stress is higher for Invar because of the higher value of ACY.This higher stress for the Invar specimen might result in a smaller value of the critical thickness (6 pm) compared with that (22 pm) for Fe-3%Si steel. Critical Load x
I
x
0
4.45
, 8.9
135
17.8
Load (N) Fig. 12. The variation in surface roughness with load for an Fe-3%Si 340 rev min-’ (0.67 m s-l)).
pin (sliding speed,
174
3.4. Surface roughness The initial roughnesses are 0.33 - 0.45 pm and 0.25 - 0.3 ym for the pins and discs respectively after polishing with emery papers. The measured roughness after tests in mild vacuum and at a sliding speed of 340 rev min‘ ’ for Fe-3%Si steel are plotted against the applied loads in Fig. 12. It is clear that the pin surfaces are severely damaged at loads above the critical load. However, below the critical load, the surfaces do not show any s~n~i~ant damage. The protective film at these lower loads prevents the penetration of asperities or decreases the metal-to-metal contacts. Thus the worn surface is nearly as smooth as the original surface.
4. Conclusions (1) In mild vacuum, at loads below a critical value, the wear rates and the friction coefficients exhibit very low values, owing mainly to the formation of protective surface oxide films. (2) At loads above the critical values, the wear rate begins to increase sharply. There is no protective film on the surfaces at these high loads and the surfaces are severely damaged. (3) It has been found that there exists a critical thickness for protective films. Negative wear occurs until the thickness reaches the critical value. At a thickness higher than the critical value the film breaks down, resulting in positive wear thereafter. The negative wear, i.e. the increase in pin height, is caused by the volume expansion due to the formation of oxide on the surface. The negative wear lasts for 2 - 10 h depending on the load; it takes less time at higher loads owing to rapid oxidation. (4) In our investigation the critical film thickness was found to be 6 ,um for Invar (Fe--36%Ni) and 22 pm for Fe-3%Si steel. The smaller thickness for Invar is caused by the larger difference in thermal expansion between the oxide and the bulk metal.
References 1 M. G. Hayler and S. W. E, Earles, An investigation of the unlubricated sliding process between NT5 and EnlA, Wear, 18 (1971) 393 - 402. 2 E. Rabinowicz, Lubrication of metal surfaces by oxide films, ASLE Trans., tO (1967) 400 - 407. 3 M. B. Peterson, Sliding characteristics of metals at high temperature, ASLE Trans., 3 (1960) 101 - 109. 4 F. H. Stott, D. S. Lin and G. C. Wood, The structure and mechanism of formation of the giaze oxide layers produced on Ni-based alloys during wear at high temperature, Corros. Sci., 13 (1973) 449 - 469. 5 F. H. Stott and G. C. Wood, The influence of oxides on the friction and wear of alloys, Tribology, 11 (4) (August 1978) 213 - 262. 6 D. S. Lin, F, N. Stott and G. C. Wood, The effects of elevated ambient temperatures on the friction and wear behavior of some eormnerciat Ni-base alloys, ASLE Trans., 17 (1974) 251 - 262.
175 7 E. Rabinowicz, Friction and Wear of Materials, Wiley, New York, 1965. 8 R. Y. Lee and Z. Eiiezer, Silicon segregation in sliding wear, Wear, 87 (1983) 227 233. 9 R. Y. Lee, H. S. Luftman and Z. Ehezer, Silicon segregation in sliding wear: an ESCA-AES investigation. In R. Kossowsky and S. C!. Singhal (eds.), Proc. of the North
Atlantic Treaty Organization Advanced Study Institute on Surface Engineering, Les Arcs, July 1983, Nijhoff, to be published. 10 N. B, Pilling and R. E. Bedworth, J. Inst. Met., 29 (1923) 523. 11 R. E. Pawell, Stress generation in tantalum during oxidation, J. Electrochem. Sot., 110 (1963) 551- 557. 12 D. L. Deadmore and C. E. Lowell, The effect of AT on oxide spalling, Oxid. Met., 11 (1977) 91 - 106. 13 H. E. Evans, D. J. Norfolk and T. Swan, Perturbation of parabolic kinetics resulting from the accumulation of stress in protective oxide layers, J. Electrochem. Sot., 125 (1978) 1180 - 1185.
14 Thermophysicai
Properties of Matter - Thermophys~~
Vols. 12 and 13, Thermal Expansion, IF&Plenum,
Research Center Data Series,
New York, 1975.