- Email: [email protected]
Characterization of wear scar surfaces using combined three-dimensional topographic analysis and contact resistance measurements
Tribology
Intermrionnl All
ELSEVIER SCIENCE:
PII:
Jiang”,
F. H. Stott
rights
30, No. 7, pp. 517-526. 1997 0 1997 Elsevier Science Ltd reserved. Printed in Great Britain 0301-679X/97/$17.00 + 0.00
80301-679X(96)00074-6
aracterization of wear surfaces using combined dimensional topographic and contact resistance measurements Jiaren
Vol.
scar threeanalysis
and M. M. Stack
In this paper, a technique for the quantitative characterization of wear scar surfaces, using combined three-dimensional topcgraphical analysis and contact resistance measurements, is introduced. Parameters for the characterization of wear surfaces, developed during sliding of pin-on-disk specimens in oxygen at high temperature, such as wear volume, roughness, average wear depth on the disk specimen, surface coverage by wear-protective oxide layers and their distributions over the wear surface, are presented and calculated. Such analyses provide more effective data for the analysis of wear processes and wear mechanisms. This method has been applied to the analysis of dry reciprocating sliding wear of a nickel-base alloy, N80A, at temperatures to 600°C. It was found that there was usually a difference between the wear rates of the pin and the disk. This difference increased with increase in temperature, the wear of the pin being much less than that of the disk at the higher temperatures. Although the total wear of both the pin and the disk decreased cons derably with increase in temperature, the damage to the disk, judged by the wear depth of the scar, was much higher at elevated temperatures than at low temperatures. The roughnesses of th? wear surfaces generally increased with increase in temperature. Less than 50% coverage of the scar surfaces by wear-protective oxide layers was sufficient for the severe-to-mild wear transition. However, the distribution of the wear-protective layers over the wear surfaces was non-uniform. Most of them were concentrated near the centre of the scar, along the sliding direction, under the present conditions. These features of the wear scar surfaces were mainly related to the adhesion and compaction of wear debris particles onto the wear surfaces, leading to development of the wear-protective layers at the various temperatures. 0 1997 Elsevier Science Ltd. Keywords:
sliding
wear,
wear
surface
characterization,
*Present address: Aeronuutical and Mechanical Etlgineering nwzt, Uviwrsity of Salford, Sa(ford M5 4WT, UK Corrosiosl md Protection Centre, Universi2y of Manchester qf Science and Technology, Manchester, U.K.
DepartInstitute
Tribology
wear
transitions
Introduction In investigations of sliding wear, surface examination using optical and scanning electron microscopy l-3 plays an important role since it supplies information about the mechanisms of wear. To obtain better descriptions and understanding of the wear processes,
International
Volume
30 Number
7 1997
517
Characterization
of wear
scar surfaces:
J. Jiang
et al.
more and more attention has been paid, in recent years, to topographic examination of wear scar surfaces that, in addition to providing a means of measuring wear under certain conditions, provides further information about the wear processes and mechanisms. The relevant techniques include roughness profile (two-dimensional) 4-7 three-dimensional measurements quantification methods 6,xm12,optical confocal microscopy l3 and laser scanning confocal microscopy 14. Whitenton and Blau l5 conducted a comparison of the methods for determining wear volumes and surface parameters of spherically tipped sliders. It was concluded that the three-dimensional technique requires fewer assumptions and can yield a large number of qualitative parameters. Obviously, the more information that is obtained from a given surface, the better is the understanding of the wear processes that can be achieved. During sliding wear of metals, especially at elevated temperatures, a transition from severe wear to mild wear usually occurs r6, which has been shown to be a result of the development of wear-protective layers 17m23. These layers contain high concentrations of oxides and are, therefore, electrically resistant. To investigate the mechanjsms and kinetics of the wear transition, it is desirable to determine the surface coverage by the wear-protective layers as a function of experimental conditions and time. Although weight loss is commonly used for measuring wear, in the presence of oxidation, as encountered in high-temperature sliding wear, the measurement of volume change is a more effective method because the development of oxide on the non-contacting specimen surfaces would strongly influence the measured weight change due to the high relative area of such surfaces compared to the wear scar surface. The main purpose of this paper is to introduce a technique for wear surface characterization involving combined three-dimensional topographic examination and contact resistance measurements and its application to the characterization of wear surfaces produced during dry sliding wear of a high-temperature alloy, N8OA, at temperatures to 600°C.
Wear
tests
and
The sliding wear tests were performed on a pin-ondisk reciprocating wear rig in the like-on-like mode. The stroke of sliding was 5 mm and the reciprocating frequency was 500 cycles per minute, giving an average sliding speed of 83 mm s - ‘. The pin specimens have a domed end with a radius of curvature of 12.5 mm. To eliminate the effect of environmental humidity fluctuations on the tribological behaviour of the alloy, the tests were performed in pure dry oxygen. This environment should produce oxidation conditions for the alloy similar to those in dry air. The normal load was 15 N. The temperatures were in the range of 20 to 600°C. Further details of the test rig and experimental procedures are given elsewhere 23. Tribology
Instrument surfaces
and
characterization
for the examination
Three-Dimensional
topographic
of wear
of wear
examinations
Fig 1 shows a schematic diagram of the apparatus used for the examination of wear surfaces in this study - an adapted three-dimensional Talysurf. Basically, it consists of four major units: (i) the specimen stage and its control unit by which relative movement of the specimen in the two orthogonal (X and Y) directions in a plane horizontal to the transducer is achieved; (ii) the transducer unit used for measurement of the height variations across the scar surfaces; (iii) the electronic driving circuits; and (iv) the controlling and data acquisition unit. The specimen stage is driven by two stepper motors for the X- and the Y-direction movements; these are controlled by a computer via a Digital Output Register. The displacement signals, reflecting the topography of the surface picked up by the transducer via the stylus contacting the specimen surface, are amplified and then converted into digital numbers and recorded onto the computer. In order to examine the development of oxide-containing wearprotective layers on the wear surfaces, a contact resistance measurement circuit was introduced into the system, details of which are given later. During the examinations, a rectangular area of the specimen surface was scanned; this included both the complete scar and part of the surrounding unworn surface. The topographic data from the unworn area were fitted to a reference curve using a ‘segmentedspline-fitting’ method and subtracted from the measured topographic profile so that a profile representing wear of the specimen was obtained. This treatment was applied to both pin and disk specimens, using the same program. The subsequent calculations for characterization of the wear surfaces were the same for both pins and disks, irrespective of the shapes of the specimens, as long as they were regular.
specimens
The alloy was a commercial nickel-base alloy, Nimonic 80A (N8OA), having a nominal composition of 18.021.0% Cr, 1.8-2.7% Ti: l.O-1.8% Al, balance Ni.
518
Examination scar surfaces
International
Volume
30 Number
Measurement of the surface
of electrical layers
contact
resistance,
R,,
In order to examine the distribution of wear-protective oxide layers on the wear surfaces, the contact resistance measurement technique used to monitor wear transitions during sliding 23 was introduced. An electropolished high-speed steel stylus was used for the Talysurf measurements. The specimen and the stylus were electrically isolated from the rest of the equipment. The contact resistance between the metal stylus and the specimen surface was measured using the measurement circuit shown in Fig 2; it is similar to that described elsewhere “. The contact resistance to be measured, R,, was first compared with the resistances, R5, R, and R7, and the voltage drop proportional to the resistance value of R, was amplified by a logarithmic operational amplifier. The output from the amplifier was proportional to the logarithmic value of the contact resist7 1997
Characterization
PICK-UP
J
I SPECIMEN
I
X- STEPEMOTOR
I
J. Jiang
et al.
:
I+---1
2 $
’ STEPPER
scar surfaces:
r------
UNIT
I
STAGE
of wear
+-
I
MOTOR L___--‘I_------------------
L-----------------I SPECIMEN
STAGE
AND
Fig. 3 Schematic diagram
R4
1 kohm 338 ohm 1.4 kohm 390 ohm
R, % R, % R, R ,,
10 68 30 470 R,, 2.2
Rl
R* R,
ohm ohm ohm kohm l-10 kohm
Fig. 2 The electronic
ITS MOVEMENT
ELECTRONIC
of the three-dimensional
R 18 R 19 R 20 R 21 R 22 R23
R 24 R 25 Mohm(decade)
circuit for contuct
R26
CONTROL
DRIVING
topographic
220 2.2 2.2 IO 2.2
ohm kohm kohm kohm kohm
22 47 120 100
kohm kohm ohm ohm
examination
pot
Cl c2 c3 Tl
ICl IC2 Sl
resistance
LED IN53396 IN4148 47 UF
Dl
D3
pot
DATA
ACQUISITION
equipment.
D2
pot
AND
0.1 UF 0.1 UF BFY51 uA741 uA741 1 pole
Zener
diode
transistor op amp op amp 12 way switch
measurement. Tribology
International
Volume
30 Number
7 1997
519
Characterization
of wear
scar surfaces:
J. Jiang
et al.
ante, In(&), and was further amplified by a linear operational amplifier and fed into the computer. Calibration of the system was performed by switching in the logarithmic cascade of resistances (Fig 2), R9 to R,e, successively and measuring the output so that a calibration curve between the measured count values by the computer and the actual contact resistance values was obtained; this relationship was used in the wear surface characterization measurements to convert the data into contact resistance using the computer. Figs 3(a) and (b) show examples of the measured topography and the distribution of contact resistance over a wear surface, plotted in the three-dimensional mode. Characterization
of wear
(9
The coverage of a wear scar surface by highresistance wear-protective oxide layers, CO,, which was defined as co, =
(ii)
Area covered by high resistance layers Projected area of the scar
Cl?
The relative area of the scar surface, R,, which, similar to Wehbi et nl. ‘, was defined as the ratio between the total rough corrugated surface area of the scar, A,,,, to the project scar surface area, Aprj, R = A,,,(Total surface area of the wear scar) A A&Projected area of the wear scar)
surfaces
From the data obtained in the Talysurf examinations of the wear surfaces, the following parameters were defined and calculated:
(iii)
(iv>
h=
Data
(21
This parameter was used to reflect the relative roughness of a wear scar; the higher the value of R,, the rougher the surface. It was used rather than a normal roughness parameter for metrology because the latter could not be obtained for a wear scar surface formed after severe wear when the scar was deep and the fluctuations in topography of the wear surface were superimposed on a curved profile, representing the deep scar. The calculations of A,,, and A,, are given in Equations (41, (6) and (7) in the following section. The wear volumes, V, of the pin specimen and the disk specimen are the volumes of the wear scars on the respective specimens, determined from the Talysurf profiles. Under some conditions, it was found that similar wear volumes could result from a wide but shallow scar and a small but deep scar. To reflect such characteristics, the average depth of the wear scar on the disk specimen, h, was calculated by dividing the wear volume by the project area of the wear scar on a disk, wear volume projected area of the scar
17,
treatment
Determination wear scar
of the
outline
boundaries
of a
To calculate the projected area of a wear scar, its outline boundaries must first be determined. This was achieved by the application of a ‘valley detection’ technique, the principles of which are as follows (Fig 4). In a topographic profile, the part representing the Xi
Fig. 3 Examples qf three-dimensional plots for (a) SUYface topography and (b) distribution of contact resistance obtained III the Talys~~rf analysis. 520
Tribology
International
Fig. 4 Schematic diagram showitlg the ‘valle)~-detectiori ’ technique used in the determination of wear scar bounduries.
Volume 30 Number 7 1997
Characterization scar area is essentially a ‘valley’. The ‘valley-seeking’ procedure was based on a comparison of the slopes of the lines, starting from a given point, x,, to five successive points in the profile (Fig 4). If the slopes of the five lines (a, b, c, etc...) were all negative and the highest coordinate relative to the base-line (the original unworn surface of the specimen), H,,,, was greater than some critical value, then the given starting point, xi, was taken as the start of the boundary for the ‘valley’ - the scar boundary. The critical value, H,,,, for the judgement of a ‘valley’ was introduced because surface fluctuations owing to a local asperity of the surface roughness, waviness of the surface and/or system errors may also cause five successive negative slopes locally. By choosing an appropriate value for H,,,. the probability for a local surface feature in the unworn surface area to satisfy both the height criterion and the slope criterion is reduced to a minimum. The value of H,,, is a function of surface roughness and step size between data points in the profiles. The surface roughness of specimens used in this study was (R,) 2.5 pm; this gives an average peak-to-valley height of 5 pm and produces quite significant local fluctuations in the surface profiles. According to experimental experience, a value of 7 pm for H,,, was found to be appropriate and was used in this study. The ‘valley-seeking’ was performed twice for each profile, commencing from the two ends of the profile, respectively, and moving towards the centre of the scar to find the two boundaries. The projected area of a scar, A,,, was thus obtained by integrating the areas within the scar boundaries: c
A,,, = (within Calculation
Ax Ay
(4)
scar boundary)
of the wear
volume,
V
Suppcse that the Cartesian coordinate system for a wear scar surface is as shown in Fig 5(a). The total wear volume of the specimen, V, was calculated by summing all the volume elements within the scar area:
+ 2 i C ZiO + ‘x \ j=, j= li r
-In-l
,
zoj + -2 1
Z,,J) /
i=l ny
-
I nx
-
1 ,
\
of wear
Calculation RA
scar surfaces:
of the relative
J. Jiang
area of the wear
et a/. scar,
The element surface area, AA,, covering a volume element is equal to the sum of the areas of the two triangles on top of this volume element, S, and S, (Fig 5(b)): AA,=&
+S,=$.xAy + (Zij + 1 -
)”
Zij12+
CAyI +
(Zi+
lj
-
Zi+
Ijt
(6)
CAY)”
The total scar surface area, A,,,, is thus obtained from (7) (within
scar boundary)
and the relative surface area of a wear scar, R,, is calculated by substituting Equations (4), (6) and (7) into Equation (2). Mapping and evaluation by surface wear-protective oxide layers
coverage
of
The data for contact resistance over the scar surface can be used to map the distribution of the highresistance wear-protective oxide layers and calculate their coverage. It was observed in the experiments that the variation in contact resistance at boundaries near regions of high-resistance layers was very steep. Therefore, for the purpose of simplicity in the data treatment, a criterion was set to distinguish between ‘high’ and ‘low’ resistance areas; this criterion was set as 1000 R in this study. When the contact resistance at a given point is higher than 1000 0, this element area is regarded as a high resistance area and its position is recorded for mapping. Otherwise, it is a ‘low’ contact resistance area. The total area of the wear scar that is covered by high contact resistance layers can thus be calculated and the surface coverage of such layers, C,,,, is obtained according to Equation (1). At the same time, the plot at points where the contact resistance is high shows the distribution of such high resistance areas.
‘I
Results where II, is the number of datum points in one profile within the scar area; n? is the number of profiles along the Y-direction over the whole scanned wear scar area; zmin and zopp are the minimum z value and the corresponding z value at its opposite comer coordinate, respectively, in the four z values constituting each volume element (Fig 5(b)); zij is the z value at the position, (xi, .v,), on the specimen surface; zoo, zio, zoj, etc, are the z values at the corresponding positions of (i = 0. j = 0), (i = i, j = 0), (i = 0, j = j), etc, respectively; and AK and Ay are the step lengths in the X- and the Y-directions, respectively. Tribology
and
Wear behaviour temperatures
discussions of the alloy
at the various
The variations in wear volume as a function of sliding time at temperatures of 20°C 250°C and 600°C are shown in Fig 6. Here, wear was expressed as the sum of the wear volumes of the pin and of the disk. The differences in wear in the very early stages of sliding at the various temperatures were small. However, they became more significant after longer sliding times. At the low temperature, 20°C the wear volume continued to increase in size with sliding time; however, at temperatures above 250°C it remained relatively conInternational
Volume
30 Number
7 1997
521
Fig. 5 Diagram
showing
(a) the coordinate
system for the analysis
of wear scar surfaces
and (b) calculatiorl
qf the
wear
volume.
Z OPP
8. . L
Characterization
of wear
scar
surfaces:
J. Jiang
et al,
c--J 5.00< E
4.00-
; 3 5 z 2 $
3.00-
ti; :: Y g
2.00.
; %
302
‘.OO-
5
20-
%
lo-
‘3 00
6050s 40-
0
50
100
150
200 time,
*2OC
250
300
350
400
2g z m
min
01 O
+250C+6OOC
50
100
*2OC
Fig. 6 Total wear of pin and disk as a function sliding time at the various temperatures.
150
200 time, min
250
300
350
4
+250C-+6OOC
of
Fig. 8 The variations in average depth of disk wear scar, h, with sliding time at the various temperatures.
stant and further wear became negligible after some time of sliding. The wear volume after a given time of sliding generally decreased with increase in temperature.
above 250°C were considerably deeper than those formed below 250°C. It is important to notice that, although the wear of the pins and the total wear of the pins and the disks generally decreased with increase in temperature (Fig 6), as has been widely reported 20,27-31,the wear damage to the disks, expressed by the penetration depth into the metal, h, increased with increase in temperature.
According to the wear volume measurements, a clear difference between wear of the pin and of the disk was usually observed, especially at the higher temperatures. Fig 7 shows the variations in the ratio of pin wear to disk wear with sliding time at the various temperatures. Apart from the two points at 20°C and 250°C in the very initial stages of sliding, the wear volunres for the pins and the disks at the low temperature, 20°C were quite similar, with the ratio of pin wear to disk wear being greater than, but close to, one. However, when the temperature was above 250°C the wear of the pin was much less than that of the disk. This feature can be related with the wear processes pertaining at the various temperatures, as discussed in the following section. The variations scars at the
in average depth various temperatures
of disk
wear
Fig 8 shows the variations in average depth of the wear scars on the disks with sliding time at the various temperatures. The wear scars formed at temperatures
~ __,_. ______.
50
100
--.----cc-
150
200 time,
*20c
-------------
250
300
350
400
min
t25Oc-+-60Oc
Fig. 7 ‘The d@erences in wear rates between pin wear and disk wear (expressed as the ratio of pin wear to disk wear) as a function of sliding time at the various temperatures. Tribology
This was possibly due to two reasons. The first is the fact that the lengths of the wear scars noticeably decreased with increase in temperature, from an average length of 8.7 mm at 20°C to 6.1 mm at 600°C; this was presumably due to the decreased friction at elevated temperatures which, in turn, resulted in a decrease in dynamical vibration of the pin holder in the sliding direction, leading to decreased actual reciprocating stroke. The second factor is related to the wear processes occurring at the various temperatures. At low temperatures, the wear-protective layers developed from compact wear debris particles were not very compact 23,25.The removal of the loose wear debris particles from the rubbing interface caused some abrasive wear to the rubbing surfaces and broadened the contact area between them. On the other hand, the wear-protective layers developed at higher temperatures were very compact and had a higher load-bearing capacity. The development of these layers resulted from the accumulation and compaction of wear debris particles. The higher the temperature, the easier it is for the particles to adhere to the surface and be compacted under the applied load. Due to the geometry of the specimens and the sliding mode, at the higher temperatures, the pin could pick up more particles 25 and form the compact wear-protective layers earlier than the disk in a given experiment. Thus, the hard wear-protective layers formed on the pin could cause significant wear of the disk before the development of effective wear-protective layers on the disk scar, resulting in more severe damage to the disk than to the pin. At low temperatures, wear damage was evenly distributed over a wide area of the scar surface. As a result, the depths of the disk scars formed at the higher temperatures were significantly greater than those at the lower temperatures. For the same reasons, wear of the pins was considerably lower than that of the disks at the higher temperatures (Fig 7). This is consistent International
Volume
30 Number
7 1997
523
Characterization
of wear
scar surfaces:
J. Jiang
et al.
with the observations by Mishina 26 who noticed that, in reactive gases where adhesion was reduced due to the formation of an adsorbed layer of the gases on the wear surfaces and wear debris particles were more easily ejected from the rubbing surfaces, the difference in wear between the pin and the disk was reduced.
Granville 32. However, the wear-protective layers are not uniformly distributed over the whole scar surface but are mainly concentrated near the central line of the scar, along the sliding direction, where wear debris particles are more easily entrapped and compacted, as shown in Fig 10.
The development of wear-protective layers wear surfaces at the various temperatures
Variations in surface and temperature
on
It has been shown 23,25 that the wear and friction transitions with sliding time for metals were accompanied by the development of high-resistance load-bearing layers. Fig 9 shows the variations in average surface coverage by such layers on the pin and disk wear scars with sliding time at the various temperatures. Generally, the coverage by high-resistance layers increased with increase in sliding time. However, at 25O”C, the value decreased at a time near the wear transition time, following an initial increase. This presumably resulted from the fact that the highresistance layers developed in the early stages of sliding at the intermediate temperatures were not very stable 13, and, subsequently, broke down, causing the observed decrease in coverage. The variations in surface coverage by wear-protective oxide layers with temperature were more complicated. At temperatures below 250°C, the value after 6 hours’ sliding decreased with increase in temperature. When the temperature was increased above 25O”C, the surface coverage of the wear scars increased again with further increase in temperature. The considerably higher surface coverage by high resistance layers at 600°C was probably a result of the significant general oxidation of the wear scar surfaces, as was observed during the course of the tests. It should be appreciated that the surface coverage of the wear scars by wear-protective layers after the wear transition was generally less than 50%, except at the high temperature, 6OO”C, where considerable general oxidation of the wear surfaces had occurred. This is consistent with the theoretical estimation and visual observation of 50% coverage during sliding of Fe9%Cr at temperatures of 290-500°C by Sullivan and
1.0 I0.6
I$,, with
time
The variations in relative area of the wear scar surfaces, RA, with sliding time at the various temperatures are shown in Fig 11(a). No clear pattern can be definitely identified. At 20°C, the R, values were invariant with time except that a higher roughness value was obtained in the very early stage of sliding. At temperatures above 250°C, there was an increase in the R, value in the early stages of sliding, which then either decreased slightly or remained almost unchanged with further sliding. On the other hand, the differences in the R, values at the various temperatures was significant, as shown in Fig 1 l(b). The roughness of the wear scar surfaces, RA, increased steadily with increase in temperature. This is in agreement with Skinner’s 33 observations on stainless steels measured using a conventional profilometer; he reported a general increase in RA values for wear scars with increase in temperature, although, as the author pointed out in his paper, the use of RA values was not really adequate, owing to the crescent-shaped surfaces of the scars. The reason for the increase in roughness of the wear surfaces with increase in temperature is probably as follows. With the increase in temperature, the adhesion forces between the solid wear debris particles and between the wear debris particles and the wear scar surface increase 25,34.Thus, larger wear debris particles can become entrapped within the rubbing interface and compacted to form wear-protective layers. As a result, the ‘islands’ of the wear-protective layers become more elevated with respect to the surrounding areas and the average surface of the scars becomes rougher at the higher temperatures even though the surface of the layers themselves are very smooth at high temperatures. This trend was indeed observed visually during the experiments. Conclusions
----+I
1.
time, --H--- R.T,
-*-.
250C
min -
600C
2.
Fig. 9 The variations in sur$ace coverage of wear scars by high- resistance layers with sliding time at the various temperatures. The data were the average values for the pin and the disk specimens. 524
roughness,
Tribology
International
Volume
30 Number
7 1997
A new technique for the quantitative characterization of wear scar surfaces using combined threedimensional topographical analysis and contact resistance measurements has been developed and applied to the analysis of dry wear of pin-on-disk specimens of a nickel-base alloy, NXOA, under reciprocating sliding conditions at temperatures to 600°C. It provides additional information for the analysis of wear processes and wear mechanisms. Under the present experimental conditions, there was usually a difference between the wear rates of the pin and the disk. This difference increased with increase in temperature, the wear of the pin being much less than that of the disk at the higher temperatures.
Clnaracterization
of wear
scar
surfaces:
et a/.
J. Jiang
L06d
I
lmm L__)
Fig. :O An example
qf mapping the distributiorl of wear-protective layers on a wear scar. The scar boundary is
also rhc7wn.
l.lOS# 0
50
100
150
200
time, j+ZOC
250
300
350
4 400
5.
min
+250C-+-6OOC
/
1
’ 030’ (b)
ment of wear-protective oxide layers that had a high contact resistance. Less than 50% of the coverage of the scar surfaces by such layers was sufficient for the wear transition. However, the distribution of the wear-protective layers over the wear surface was non-uniform; they were concentrated near the centre of the scar, along the sliding direction under the present conditions. The above features of the wear scar surfaces developed during sliding wear at elevated temperatures can be explained according to the model for sliding wear of metals at elevated temperatures, as presented earlier 25, i.e. they were related to the adhesion and compaction of wear debris particles onto the wear surfaces to develop wear-protective layers at the various temperatures.
References I. Miyoshi K. and 82973. National pp. l-25.
1 .:I05
G
100
200
300
400
500
The kwintions in relnth~e area of wear scat suryfkx~s, R,,: (a) with sliding time at the various temperQrtre.s; (1,) nd temperature (average values for specimens sliding c$ter the wear transition time, which was qzproximately 50 min (Fig 6)).
4.
2. Miyoshi National
K. and Buckley D. H., NASA Techr~icnl Paper 1991. Aeronautics and Space Administration, 1982, pp. 1-I 1.
3. Miyoshi National
K. and Buckley D. H.. NASA Technical Paper 21-10. Aeronautics and Space Administration. 1983, pp. i-19.
3. Torrance
A. A. Wear
5. Hurricks
P. L. Weal- 1978;
1995.
181-183,
397404
600
Fig. li
3.
Buckley D. H.. NASA Techniml Men~onmdmr Aeronautics and Space Admmistration, 1982.
Aiihough the total wear of the pin and the disk decreased considerably with increase in temperature: the damage to the disk, determined by the average depth of the wear scar, was higher at the higher temperatures than at the lower temperatures. The transition from severe wear to mild wear at the various temperatures resulted from the developTribology
47. 335-358
0. Engei
P. A. and
Millis
D. B. Wecw 1982,
7. Yost
F. G. Wear
1983.
92, 135-142
8. Wehbi D.. Clerc M. A., Roques-Carmes Wear 1986, 107. 2633-278 9. Bengtsson
A. and
W. P. and
Riinnberg Stout
C. and Barquins
A. Wear
K. J. Wear
75. 433-442
i986,
1995.
M.
109, 329-342
10.
Dong
181-183.
700-716
11
Stout K. J., Sullivan P. J., Dong W. P., Mainsah E., Luo N.. Mathia T. and Zahyouani H.. The developmer~r qf methods ,%r the characterixuion of roughness in three dimensions. Commission of the European Communities, Brussels, 1993.
12. Stout K. J.. Three-dirnensiorzni Swfizce ment, Itzterpretntion and Applications. London. 1994.
Topogmp/z’h?;; Measureed. K. J. Stout. Fenton.
N. J. 13. Gee M. G. and McCormick Applied Plqvics 1992, 25, A230-A235
./ou~rza/
International
Volume
30 Number
of P1z~sic.s D:
7 1997
525
Characterization
of wear
14. Anamalay R. V., Kirk 183. 771-776 15. Whitenton
scar surfaces:
T. B. and Panzera
E. P. and
Blau
P. J. Wear
D. Wear 1988,
of
16. Archard J. F. and Hirst W. Proceedings of Lmdon 1956. A236, 397410 17. Stott Wear
F. H., Lin D. S.. Wood 1976, 36, 147-174
18. Smith
A. F. Trihology
G.
Intenzatior~al
19. Wilson J. E., Stott F. H. and Wood Royal Society of London 1980. A369, 20. Stott F. H., 311-324 21. Newman
Glascott
P. T. and
J. and Skinner
J. Jiang
C. and 1986,
1995,
G.
J. Wear
181-
Stevenson
1986,
21. Goldman
C. W.
28. Appeldoorn Trunsacfions
Stott
F.
H.
and
Stack
M.
M.
qf
198.5,
the
101,
112, 291-325
Wear
24. Stott F. H., Glascott J. and Wood G. C. Journal D: Applied Physics 1985, 18, 541-556
526
Tribology
International
Volume
Transcrcrions
F. H. and
H. Wear I.
B.,
1994, of
176,
Physics
13,
M.
M.
Wear
1995,
181-183,
152, 99-110 J.
K.
and
Tao
F.
F.
ASLE
I.
B.
and
Tao
F.
F.
ASLE
29-38
J. K., Goldman 1969, 12, 140-150 F. H. and
Wood
G. C. Oxidation
of Metals
30. Stott F. H. and Glascott J., The sliding wear of in-situ formed oxides on iron- and nickel-base alloys. In Corrosion-Erosion oj’ Materials at Elevated Tempemfuws. ed. A. V. Levy. NACE, 1987. pp. 263-277. Published by The National Association of Corrosion Engineers, Houston, Texas. 31. Stott F. H., Stevenson C. W. and Wood 6. C. Friction and wear properties of Stellite 3 1 at temperatures from 293 to 1073 K, Metals Technology 1977, February, 66-74 32. Sullivan J. L. and 1984, 17, 63-71
Granville
N.
W.
Trihology
33. Skinner .I., Friction and wear of an austenitic temperature carbon dioxide. high RDIB15220N81. 1981. 34. Uamamoto
30 Number
1992,
Stack
Appeldoorn
1970,
29. Glascott J., Stott 1985. 24, 99-114
22. Stott F. H. and Wood G. C., The influence of oxides in high temperature wear. In High Temperature Corrosion. NACE-6, 1983. pp. 4066412. Published by The National Association of Corrosion Engineers, Houston, Texas. 23. Jiang J., 185-194
J., Stott
Sock8
19, 65-71
C. Wear
25. Jiang 20-31 26. Mishina
124. 291-309 the Royul
G. C. Proceedings 557-574
Wood
et al.
7 1997
M.
and
Nakajima
K. Wear
1981,
lnterrmtionnl
stainless CEGB
steel in Report,
70. 321-327
Intermrionnl All
ELSEVIER SCIENCE:
PII:
Jiang”,
F. H. Stott
rights
30, No. 7, pp. 517-526. 1997 0 1997 Elsevier Science Ltd reserved. Printed in Great Britain 0301-679X/97/$17.00 + 0.00
80301-679X(96)00074-6
aracterization of wear surfaces using combined dimensional topographic and contact resistance measurements Jiaren
Vol.
scar threeanalysis
and M. M. Stack
In this paper, a technique for the quantitative characterization of wear scar surfaces, using combined three-dimensional topcgraphical analysis and contact resistance measurements, is introduced. Parameters for the characterization of wear surfaces, developed during sliding of pin-on-disk specimens in oxygen at high temperature, such as wear volume, roughness, average wear depth on the disk specimen, surface coverage by wear-protective oxide layers and their distributions over the wear surface, are presented and calculated. Such analyses provide more effective data for the analysis of wear processes and wear mechanisms. This method has been applied to the analysis of dry reciprocating sliding wear of a nickel-base alloy, N80A, at temperatures to 600°C. It was found that there was usually a difference between the wear rates of the pin and the disk. This difference increased with increase in temperature, the wear of the pin being much less than that of the disk at the higher temperatures. Although the total wear of both the pin and the disk decreased cons derably with increase in temperature, the damage to the disk, judged by the wear depth of the scar, was much higher at elevated temperatures than at low temperatures. The roughnesses of th? wear surfaces generally increased with increase in temperature. Less than 50% coverage of the scar surfaces by wear-protective oxide layers was sufficient for the severe-to-mild wear transition. However, the distribution of the wear-protective layers over the wear surfaces was non-uniform. Most of them were concentrated near the centre of the scar, along the sliding direction, under the present conditions. These features of the wear scar surfaces were mainly related to the adhesion and compaction of wear debris particles onto the wear surfaces, leading to development of the wear-protective layers at the various temperatures. 0 1997 Elsevier Science Ltd. Keywords:
sliding
wear,
wear
surface
characterization,
*Present address: Aeronuutical and Mechanical Etlgineering nwzt, Uviwrsity of Salford, Sa(ford M5 4WT, UK Corrosiosl md Protection Centre, Universi2y of Manchester qf Science and Technology, Manchester, U.K.
DepartInstitute
Tribology
wear
transitions
Introduction In investigations of sliding wear, surface examination using optical and scanning electron microscopy l-3 plays an important role since it supplies information about the mechanisms of wear. To obtain better descriptions and understanding of the wear processes,
International
Volume
30 Number
7 1997
517
Characterization
of wear
scar surfaces:
J. Jiang
et al.
more and more attention has been paid, in recent years, to topographic examination of wear scar surfaces that, in addition to providing a means of measuring wear under certain conditions, provides further information about the wear processes and mechanisms. The relevant techniques include roughness profile (two-dimensional) 4-7 three-dimensional measurements quantification methods 6,xm12,optical confocal microscopy l3 and laser scanning confocal microscopy 14. Whitenton and Blau l5 conducted a comparison of the methods for determining wear volumes and surface parameters of spherically tipped sliders. It was concluded that the three-dimensional technique requires fewer assumptions and can yield a large number of qualitative parameters. Obviously, the more information that is obtained from a given surface, the better is the understanding of the wear processes that can be achieved. During sliding wear of metals, especially at elevated temperatures, a transition from severe wear to mild wear usually occurs r6, which has been shown to be a result of the development of wear-protective layers 17m23. These layers contain high concentrations of oxides and are, therefore, electrically resistant. To investigate the mechanjsms and kinetics of the wear transition, it is desirable to determine the surface coverage by the wear-protective layers as a function of experimental conditions and time. Although weight loss is commonly used for measuring wear, in the presence of oxidation, as encountered in high-temperature sliding wear, the measurement of volume change is a more effective method because the development of oxide on the non-contacting specimen surfaces would strongly influence the measured weight change due to the high relative area of such surfaces compared to the wear scar surface. The main purpose of this paper is to introduce a technique for wear surface characterization involving combined three-dimensional topographic examination and contact resistance measurements and its application to the characterization of wear surfaces produced during dry sliding wear of a high-temperature alloy, N8OA, at temperatures to 600°C.
Wear
tests
and
The sliding wear tests were performed on a pin-ondisk reciprocating wear rig in the like-on-like mode. The stroke of sliding was 5 mm and the reciprocating frequency was 500 cycles per minute, giving an average sliding speed of 83 mm s - ‘. The pin specimens have a domed end with a radius of curvature of 12.5 mm. To eliminate the effect of environmental humidity fluctuations on the tribological behaviour of the alloy, the tests were performed in pure dry oxygen. This environment should produce oxidation conditions for the alloy similar to those in dry air. The normal load was 15 N. The temperatures were in the range of 20 to 600°C. Further details of the test rig and experimental procedures are given elsewhere 23. Tribology
Instrument surfaces
and
characterization
for the examination
Three-Dimensional
topographic
of wear
of wear
examinations
Fig 1 shows a schematic diagram of the apparatus used for the examination of wear surfaces in this study - an adapted three-dimensional Talysurf. Basically, it consists of four major units: (i) the specimen stage and its control unit by which relative movement of the specimen in the two orthogonal (X and Y) directions in a plane horizontal to the transducer is achieved; (ii) the transducer unit used for measurement of the height variations across the scar surfaces; (iii) the electronic driving circuits; and (iv) the controlling and data acquisition unit. The specimen stage is driven by two stepper motors for the X- and the Y-direction movements; these are controlled by a computer via a Digital Output Register. The displacement signals, reflecting the topography of the surface picked up by the transducer via the stylus contacting the specimen surface, are amplified and then converted into digital numbers and recorded onto the computer. In order to examine the development of oxide-containing wearprotective layers on the wear surfaces, a contact resistance measurement circuit was introduced into the system, details of which are given later. During the examinations, a rectangular area of the specimen surface was scanned; this included both the complete scar and part of the surrounding unworn surface. The topographic data from the unworn area were fitted to a reference curve using a ‘segmentedspline-fitting’ method and subtracted from the measured topographic profile so that a profile representing wear of the specimen was obtained. This treatment was applied to both pin and disk specimens, using the same program. The subsequent calculations for characterization of the wear surfaces were the same for both pins and disks, irrespective of the shapes of the specimens, as long as they were regular.
specimens
The alloy was a commercial nickel-base alloy, Nimonic 80A (N8OA), having a nominal composition of 18.021.0% Cr, 1.8-2.7% Ti: l.O-1.8% Al, balance Ni.
518
Examination scar surfaces
International
Volume
30 Number
Measurement of the surface
of electrical layers
contact
resistance,
R,,
In order to examine the distribution of wear-protective oxide layers on the wear surfaces, the contact resistance measurement technique used to monitor wear transitions during sliding 23 was introduced. An electropolished high-speed steel stylus was used for the Talysurf measurements. The specimen and the stylus were electrically isolated from the rest of the equipment. The contact resistance between the metal stylus and the specimen surface was measured using the measurement circuit shown in Fig 2; it is similar to that described elsewhere “. The contact resistance to be measured, R,, was first compared with the resistances, R5, R, and R7, and the voltage drop proportional to the resistance value of R, was amplified by a logarithmic operational amplifier. The output from the amplifier was proportional to the logarithmic value of the contact resist7 1997
Characterization
PICK-UP
J
I SPECIMEN
I
X- STEPEMOTOR
I
J. Jiang
et al.
:
I+---1
2 $
’ STEPPER
scar surfaces:
r------
UNIT
I
STAGE
of wear
+-
I
MOTOR L___--‘I_------------------
L-----------------I SPECIMEN
STAGE
AND
Fig. 3 Schematic diagram
R4
1 kohm 338 ohm 1.4 kohm 390 ohm
R, % R, % R, R ,,
10 68 30 470 R,, 2.2
Rl
R* R,
ohm ohm ohm kohm l-10 kohm
Fig. 2 The electronic
ITS MOVEMENT
ELECTRONIC
of the three-dimensional
R 18 R 19 R 20 R 21 R 22 R23
R 24 R 25 Mohm(decade)
circuit for contuct
R26
CONTROL
DRIVING
topographic
220 2.2 2.2 IO 2.2
ohm kohm kohm kohm kohm
22 47 120 100
kohm kohm ohm ohm
examination
pot
Cl c2 c3 Tl
ICl IC2 Sl
resistance
LED IN53396 IN4148 47 UF
Dl
D3
pot
DATA
ACQUISITION
equipment.
D2
pot
AND
0.1 UF 0.1 UF BFY51 uA741 uA741 1 pole
Zener
diode
transistor op amp op amp 12 way switch
measurement. Tribology
International
Volume
30 Number
7 1997
519
Characterization
of wear
scar surfaces:
J. Jiang
et al.
ante, In(&), and was further amplified by a linear operational amplifier and fed into the computer. Calibration of the system was performed by switching in the logarithmic cascade of resistances (Fig 2), R9 to R,e, successively and measuring the output so that a calibration curve between the measured count values by the computer and the actual contact resistance values was obtained; this relationship was used in the wear surface characterization measurements to convert the data into contact resistance using the computer. Figs 3(a) and (b) show examples of the measured topography and the distribution of contact resistance over a wear surface, plotted in the three-dimensional mode. Characterization
of wear
(9
The coverage of a wear scar surface by highresistance wear-protective oxide layers, CO,, which was defined as co, =
(ii)
Area covered by high resistance layers Projected area of the scar
Cl?
The relative area of the scar surface, R,, which, similar to Wehbi et nl. ‘, was defined as the ratio between the total rough corrugated surface area of the scar, A,,,, to the project scar surface area, Aprj, R = A,,,(Total surface area of the wear scar) A A&Projected area of the wear scar)
surfaces
From the data obtained in the Talysurf examinations of the wear surfaces, the following parameters were defined and calculated:
(iii)
(iv>
h=
Data
(21
This parameter was used to reflect the relative roughness of a wear scar; the higher the value of R,, the rougher the surface. It was used rather than a normal roughness parameter for metrology because the latter could not be obtained for a wear scar surface formed after severe wear when the scar was deep and the fluctuations in topography of the wear surface were superimposed on a curved profile, representing the deep scar. The calculations of A,,, and A,, are given in Equations (41, (6) and (7) in the following section. The wear volumes, V, of the pin specimen and the disk specimen are the volumes of the wear scars on the respective specimens, determined from the Talysurf profiles. Under some conditions, it was found that similar wear volumes could result from a wide but shallow scar and a small but deep scar. To reflect such characteristics, the average depth of the wear scar on the disk specimen, h, was calculated by dividing the wear volume by the project area of the wear scar on a disk, wear volume projected area of the scar
17,
treatment
Determination wear scar
of the
outline
boundaries
of a
To calculate the projected area of a wear scar, its outline boundaries must first be determined. This was achieved by the application of a ‘valley detection’ technique, the principles of which are as follows (Fig 4). In a topographic profile, the part representing the Xi
Fig. 3 Examples qf three-dimensional plots for (a) SUYface topography and (b) distribution of contact resistance obtained III the Talys~~rf analysis. 520
Tribology
International
Fig. 4 Schematic diagram showitlg the ‘valle)~-detectiori ’ technique used in the determination of wear scar bounduries.
Volume 30 Number 7 1997
Characterization scar area is essentially a ‘valley’. The ‘valley-seeking’ procedure was based on a comparison of the slopes of the lines, starting from a given point, x,, to five successive points in the profile (Fig 4). If the slopes of the five lines (a, b, c, etc...) were all negative and the highest coordinate relative to the base-line (the original unworn surface of the specimen), H,,,, was greater than some critical value, then the given starting point, xi, was taken as the start of the boundary for the ‘valley’ - the scar boundary. The critical value, H,,,, for the judgement of a ‘valley’ was introduced because surface fluctuations owing to a local asperity of the surface roughness, waviness of the surface and/or system errors may also cause five successive negative slopes locally. By choosing an appropriate value for H,,,. the probability for a local surface feature in the unworn surface area to satisfy both the height criterion and the slope criterion is reduced to a minimum. The value of H,,, is a function of surface roughness and step size between data points in the profiles. The surface roughness of specimens used in this study was (R,) 2.5 pm; this gives an average peak-to-valley height of 5 pm and produces quite significant local fluctuations in the surface profiles. According to experimental experience, a value of 7 pm for H,,, was found to be appropriate and was used in this study. The ‘valley-seeking’ was performed twice for each profile, commencing from the two ends of the profile, respectively, and moving towards the centre of the scar to find the two boundaries. The projected area of a scar, A,,, was thus obtained by integrating the areas within the scar boundaries: c
A,,, = (within Calculation
Ax Ay
(4)
scar boundary)
of the wear
volume,
V
Suppcse that the Cartesian coordinate system for a wear scar surface is as shown in Fig 5(a). The total wear volume of the specimen, V, was calculated by summing all the volume elements within the scar area:
+ 2 i C ZiO + ‘x \ j=, j= li r
-In-l
,
zoj + -2 1
Z,,J) /
i=l ny
-
I nx
-
1 ,
\
of wear
Calculation RA
scar surfaces:
of the relative
J. Jiang
area of the wear
et a/. scar,
The element surface area, AA,, covering a volume element is equal to the sum of the areas of the two triangles on top of this volume element, S, and S, (Fig 5(b)): AA,=&
+S,=$.xAy + (Zij + 1 -
)”
Zij12+
CAyI +
(Zi+
lj
-
Zi+
Ijt
(6)
CAY)”
The total scar surface area, A,,,, is thus obtained from (7) (within
scar boundary)
and the relative surface area of a wear scar, R,, is calculated by substituting Equations (4), (6) and (7) into Equation (2). Mapping and evaluation by surface wear-protective oxide layers
coverage
of
The data for contact resistance over the scar surface can be used to map the distribution of the highresistance wear-protective oxide layers and calculate their coverage. It was observed in the experiments that the variation in contact resistance at boundaries near regions of high-resistance layers was very steep. Therefore, for the purpose of simplicity in the data treatment, a criterion was set to distinguish between ‘high’ and ‘low’ resistance areas; this criterion was set as 1000 R in this study. When the contact resistance at a given point is higher than 1000 0, this element area is regarded as a high resistance area and its position is recorded for mapping. Otherwise, it is a ‘low’ contact resistance area. The total area of the wear scar that is covered by high contact resistance layers can thus be calculated and the surface coverage of such layers, C,,,, is obtained according to Equation (1). At the same time, the plot at points where the contact resistance is high shows the distribution of such high resistance areas.
‘I
Results where II, is the number of datum points in one profile within the scar area; n? is the number of profiles along the Y-direction over the whole scanned wear scar area; zmin and zopp are the minimum z value and the corresponding z value at its opposite comer coordinate, respectively, in the four z values constituting each volume element (Fig 5(b)); zij is the z value at the position, (xi, .v,), on the specimen surface; zoo, zio, zoj, etc, are the z values at the corresponding positions of (i = 0. j = 0), (i = i, j = 0), (i = 0, j = j), etc, respectively; and AK and Ay are the step lengths in the X- and the Y-directions, respectively. Tribology
and
Wear behaviour temperatures
discussions of the alloy
at the various
The variations in wear volume as a function of sliding time at temperatures of 20°C 250°C and 600°C are shown in Fig 6. Here, wear was expressed as the sum of the wear volumes of the pin and of the disk. The differences in wear in the very early stages of sliding at the various temperatures were small. However, they became more significant after longer sliding times. At the low temperature, 20°C the wear volume continued to increase in size with sliding time; however, at temperatures above 250°C it remained relatively conInternational
Volume
30 Number
7 1997
521
Fig. 5 Diagram
showing
(a) the coordinate
system for the analysis
of wear scar surfaces
and (b) calculatiorl
qf the
wear
volume.
Z OPP
8. . L
Characterization
of wear
scar
surfaces:
J. Jiang
et al,
c--J 5.00< E
4.00-
; 3 5 z 2 $
3.00-
ti; :: Y g
2.00.
; %
302
‘.OO-
5
20-
%
lo-
‘3 00
6050s 40-
0
50
100
150
200 time,
*2OC
250
300
350
400
2g z m
min
01 O
+250C+6OOC
50
100
*2OC
Fig. 6 Total wear of pin and disk as a function sliding time at the various temperatures.
150
200 time, min
250
300
350
4
+250C-+6OOC
of
Fig. 8 The variations in average depth of disk wear scar, h, with sliding time at the various temperatures.
stant and further wear became negligible after some time of sliding. The wear volume after a given time of sliding generally decreased with increase in temperature.
above 250°C were considerably deeper than those formed below 250°C. It is important to notice that, although the wear of the pins and the total wear of the pins and the disks generally decreased with increase in temperature (Fig 6), as has been widely reported 20,27-31,the wear damage to the disks, expressed by the penetration depth into the metal, h, increased with increase in temperature.
According to the wear volume measurements, a clear difference between wear of the pin and of the disk was usually observed, especially at the higher temperatures. Fig 7 shows the variations in the ratio of pin wear to disk wear with sliding time at the various temperatures. Apart from the two points at 20°C and 250°C in the very initial stages of sliding, the wear volunres for the pins and the disks at the low temperature, 20°C were quite similar, with the ratio of pin wear to disk wear being greater than, but close to, one. However, when the temperature was above 250°C the wear of the pin was much less than that of the disk. This feature can be related with the wear processes pertaining at the various temperatures, as discussed in the following section. The variations scars at the
in average depth various temperatures
of disk
wear
Fig 8 shows the variations in average depth of the wear scars on the disks with sliding time at the various temperatures. The wear scars formed at temperatures
~ __,_. ______.
50
100
--.----cc-
150
200 time,
*20c
-------------
250
300
350
400
min
t25Oc-+-60Oc
Fig. 7 ‘The d@erences in wear rates between pin wear and disk wear (expressed as the ratio of pin wear to disk wear) as a function of sliding time at the various temperatures. Tribology
This was possibly due to two reasons. The first is the fact that the lengths of the wear scars noticeably decreased with increase in temperature, from an average length of 8.7 mm at 20°C to 6.1 mm at 600°C; this was presumably due to the decreased friction at elevated temperatures which, in turn, resulted in a decrease in dynamical vibration of the pin holder in the sliding direction, leading to decreased actual reciprocating stroke. The second factor is related to the wear processes occurring at the various temperatures. At low temperatures, the wear-protective layers developed from compact wear debris particles were not very compact 23,25.The removal of the loose wear debris particles from the rubbing interface caused some abrasive wear to the rubbing surfaces and broadened the contact area between them. On the other hand, the wear-protective layers developed at higher temperatures were very compact and had a higher load-bearing capacity. The development of these layers resulted from the accumulation and compaction of wear debris particles. The higher the temperature, the easier it is for the particles to adhere to the surface and be compacted under the applied load. Due to the geometry of the specimens and the sliding mode, at the higher temperatures, the pin could pick up more particles 25 and form the compact wear-protective layers earlier than the disk in a given experiment. Thus, the hard wear-protective layers formed on the pin could cause significant wear of the disk before the development of effective wear-protective layers on the disk scar, resulting in more severe damage to the disk than to the pin. At low temperatures, wear damage was evenly distributed over a wide area of the scar surface. As a result, the depths of the disk scars formed at the higher temperatures were significantly greater than those at the lower temperatures. For the same reasons, wear of the pins was considerably lower than that of the disks at the higher temperatures (Fig 7). This is consistent International
Volume
30 Number
7 1997
523
Characterization
of wear
scar surfaces:
J. Jiang
et al.
with the observations by Mishina 26 who noticed that, in reactive gases where adhesion was reduced due to the formation of an adsorbed layer of the gases on the wear surfaces and wear debris particles were more easily ejected from the rubbing surfaces, the difference in wear between the pin and the disk was reduced.
Granville 32. However, the wear-protective layers are not uniformly distributed over the whole scar surface but are mainly concentrated near the central line of the scar, along the sliding direction, where wear debris particles are more easily entrapped and compacted, as shown in Fig 10.
The development of wear-protective layers wear surfaces at the various temperatures
Variations in surface and temperature
on
It has been shown 23,25 that the wear and friction transitions with sliding time for metals were accompanied by the development of high-resistance load-bearing layers. Fig 9 shows the variations in average surface coverage by such layers on the pin and disk wear scars with sliding time at the various temperatures. Generally, the coverage by high-resistance layers increased with increase in sliding time. However, at 25O”C, the value decreased at a time near the wear transition time, following an initial increase. This presumably resulted from the fact that the highresistance layers developed in the early stages of sliding at the intermediate temperatures were not very stable 13, and, subsequently, broke down, causing the observed decrease in coverage. The variations in surface coverage by wear-protective oxide layers with temperature were more complicated. At temperatures below 250°C, the value after 6 hours’ sliding decreased with increase in temperature. When the temperature was increased above 25O”C, the surface coverage of the wear scars increased again with further increase in temperature. The considerably higher surface coverage by high resistance layers at 600°C was probably a result of the significant general oxidation of the wear scar surfaces, as was observed during the course of the tests. It should be appreciated that the surface coverage of the wear scars by wear-protective layers after the wear transition was generally less than 50%, except at the high temperature, 6OO”C, where considerable general oxidation of the wear surfaces had occurred. This is consistent with the theoretical estimation and visual observation of 50% coverage during sliding of Fe9%Cr at temperatures of 290-500°C by Sullivan and
1.0 I0.6
I$,, with
time
The variations in relative area of the wear scar surfaces, RA, with sliding time at the various temperatures are shown in Fig 11(a). No clear pattern can be definitely identified. At 20°C, the R, values were invariant with time except that a higher roughness value was obtained in the very early stage of sliding. At temperatures above 250°C, there was an increase in the R, value in the early stages of sliding, which then either decreased slightly or remained almost unchanged with further sliding. On the other hand, the differences in the R, values at the various temperatures was significant, as shown in Fig 1 l(b). The roughness of the wear scar surfaces, RA, increased steadily with increase in temperature. This is in agreement with Skinner’s 33 observations on stainless steels measured using a conventional profilometer; he reported a general increase in RA values for wear scars with increase in temperature, although, as the author pointed out in his paper, the use of RA values was not really adequate, owing to the crescent-shaped surfaces of the scars. The reason for the increase in roughness of the wear surfaces with increase in temperature is probably as follows. With the increase in temperature, the adhesion forces between the solid wear debris particles and between the wear debris particles and the wear scar surface increase 25,34.Thus, larger wear debris particles can become entrapped within the rubbing interface and compacted to form wear-protective layers. As a result, the ‘islands’ of the wear-protective layers become more elevated with respect to the surrounding areas and the average surface of the scars becomes rougher at the higher temperatures even though the surface of the layers themselves are very smooth at high temperatures. This trend was indeed observed visually during the experiments. Conclusions
----+I
1.
time, --H--- R.T,
-*-.
250C
min -
600C
2.
Fig. 9 The variations in sur$ace coverage of wear scars by high- resistance layers with sliding time at the various temperatures. The data were the average values for the pin and the disk specimens. 524
roughness,
Tribology
International
Volume
30 Number
7 1997
A new technique for the quantitative characterization of wear scar surfaces using combined threedimensional topographical analysis and contact resistance measurements has been developed and applied to the analysis of dry wear of pin-on-disk specimens of a nickel-base alloy, NXOA, under reciprocating sliding conditions at temperatures to 600°C. It provides additional information for the analysis of wear processes and wear mechanisms. Under the present experimental conditions, there was usually a difference between the wear rates of the pin and the disk. This difference increased with increase in temperature, the wear of the pin being much less than that of the disk at the higher temperatures.
Clnaracterization
of wear
scar
surfaces:
et a/.
J. Jiang
L06d
I
lmm L__)
Fig. :O An example
qf mapping the distributiorl of wear-protective layers on a wear scar. The scar boundary is
also rhc7wn.
l.lOS# 0
50
100
150
200
time, j+ZOC
250
300
350
4 400
5.
min
+250C-+-6OOC
/
1
’ 030’ (b)
ment of wear-protective oxide layers that had a high contact resistance. Less than 50% of the coverage of the scar surfaces by such layers was sufficient for the wear transition. However, the distribution of the wear-protective layers over the wear surface was non-uniform; they were concentrated near the centre of the scar, along the sliding direction under the present conditions. The above features of the wear scar surfaces developed during sliding wear at elevated temperatures can be explained according to the model for sliding wear of metals at elevated temperatures, as presented earlier 25, i.e. they were related to the adhesion and compaction of wear debris particles onto the wear surfaces to develop wear-protective layers at the various temperatures.
References I. Miyoshi K. and 82973. National pp. l-25.
1 .:I05
G
100
200
300
400
500
The kwintions in relnth~e area of wear scat suryfkx~s, R,,: (a) with sliding time at the various temperQrtre.s; (1,) nd temperature (average values for specimens sliding c$ter the wear transition time, which was qzproximately 50 min (Fig 6)).
4.
2. Miyoshi National
K. and Buckley D. H., NASA Techr~icnl Paper 1991. Aeronautics and Space Administration, 1982, pp. 1-I 1.
3. Miyoshi National
K. and Buckley D. H.. NASA Technical Paper 21-10. Aeronautics and Space Administration. 1983, pp. i-19.
3. Torrance
A. A. Wear
5. Hurricks
P. L. Weal- 1978;
1995.
181-183,
397404
600
Fig. li
3.
Buckley D. H.. NASA Techniml Men~onmdmr Aeronautics and Space Admmistration, 1982.
Aiihough the total wear of the pin and the disk decreased considerably with increase in temperature: the damage to the disk, determined by the average depth of the wear scar, was higher at the higher temperatures than at the lower temperatures. The transition from severe wear to mild wear at the various temperatures resulted from the developTribology
47. 335-358
0. Engei
P. A. and
Millis
D. B. Wecw 1982,
7. Yost
F. G. Wear
1983.
92, 135-142
8. Wehbi D.. Clerc M. A., Roques-Carmes Wear 1986, 107. 2633-278 9. Bengtsson
A. and
W. P. and
Riinnberg Stout
C. and Barquins
A. Wear
K. J. Wear
75. 433-442
i986,
1995.
M.
109, 329-342
10.
Dong
181-183.
700-716
11
Stout K. J., Sullivan P. J., Dong W. P., Mainsah E., Luo N.. Mathia T. and Zahyouani H.. The developmer~r qf methods ,%r the characterixuion of roughness in three dimensions. Commission of the European Communities, Brussels, 1993.
12. Stout K. J.. Three-dirnensiorzni Swfizce ment, Itzterpretntion and Applications. London. 1994.
Topogmp/z’h?;; Measureed. K. J. Stout. Fenton.
N. J. 13. Gee M. G. and McCormick Applied Plqvics 1992, 25, A230-A235
./ou~rza/
International
Volume
30 Number
of P1z~sic.s D:
7 1997
525
Characterization
of wear
14. Anamalay R. V., Kirk 183. 771-776 15. Whitenton
scar surfaces:
T. B. and Panzera
E. P. and
Blau
P. J. Wear
D. Wear 1988,
of
16. Archard J. F. and Hirst W. Proceedings of Lmdon 1956. A236, 397410 17. Stott Wear
F. H., Lin D. S.. Wood 1976, 36, 147-174
18. Smith
A. F. Trihology
G.
Intenzatior~al
19. Wilson J. E., Stott F. H. and Wood Royal Society of London 1980. A369, 20. Stott F. H., 311-324 21. Newman
Glascott
P. T. and
J. and Skinner
J. Jiang
C. and 1986,
1995,
G.
J. Wear
181-
Stevenson
1986,
21. Goldman
C. W.
28. Appeldoorn Trunsacfions
Stott
F.
H.
and
Stack
M.
M.
qf
198.5,
the
101,
112, 291-325
Wear
24. Stott F. H., Glascott J. and Wood G. C. Journal D: Applied Physics 1985, 18, 541-556
526
Tribology
International
Volume
Transcrcrions
F. H. and
H. Wear I.
B.,
1994, of
176,
Physics
13,
M.
M.
Wear
1995,
181-183,
152, 99-110 J.
K.
and
Tao
F.
F.
ASLE
I.
B.
and
Tao
F.
F.
ASLE
29-38
J. K., Goldman 1969, 12, 140-150 F. H. and
Wood
G. C. Oxidation
of Metals
30. Stott F. H. and Glascott J., The sliding wear of in-situ formed oxides on iron- and nickel-base alloys. In Corrosion-Erosion oj’ Materials at Elevated Tempemfuws. ed. A. V. Levy. NACE, 1987. pp. 263-277. Published by The National Association of Corrosion Engineers, Houston, Texas. 31. Stott F. H., Stevenson C. W. and Wood 6. C. Friction and wear properties of Stellite 3 1 at temperatures from 293 to 1073 K, Metals Technology 1977, February, 66-74 32. Sullivan J. L. and 1984, 17, 63-71
Granville
N.
W.
Trihology
33. Skinner .I., Friction and wear of an austenitic temperature carbon dioxide. high RDIB15220N81. 1981. 34. Uamamoto
30 Number
1992,
Stack
Appeldoorn
1970,
29. Glascott J., Stott 1985. 24, 99-114
22. Stott F. H. and Wood G. C., The influence of oxides in high temperature wear. In High Temperature Corrosion. NACE-6, 1983. pp. 4066412. Published by The National Association of Corrosion Engineers, Houston, Texas. 23. Jiang J., 185-194
J., Stott
Sock8
19, 65-71
C. Wear
25. Jiang 20-31 26. Mishina
124. 291-309 the Royul
G. C. Proceedings 557-574
Wood
et al.
7 1997
M.
and
Nakajima
K. Wear
1981,
lnterrmtionnl
stainless CEGB
steel in Report,
70. 321-327