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© 2003 – Ian A. Inman
5.3.8 Reversal of Sample and Counterface - Stellite 6 versus
Incoloy MA956 at 750°C and 0.314 m.s-1
© 2003 – Ian A. Inman
5.3.8
Reversal of Sample and Counterface - Stellite 6 versus Incoloy MA956 at
750°C and 0.314 m.s-1
5.3.8.1 Experimental Observations – Stellite 6 versus Incoloy MA956
When Stellite 6 was slid as the sample material against a Incoloy MA956 counterface,
weight changes remained extremely low (Figure 5.C1a).
Weight change and wear
rate values were near negligible, at -0.005(6) g (the small negative value indicating a
very slight weight loss) and 1.238 x 10-9 kg.m-1 respectively.
This indicated a very
slight removal of material from Stellite 6 as the sample material.
Coefficient of friction data (Figure 5.C2) indicated an extended unsettled run-in
period of approximately 100 minutes (1,884 m), with coefficient of friction reaching
as high as 1.73. This was followed by a settled ‘steady state’ period, with coefficient
of friction averaging at ~0.5 for Stellite 6 as the sample with a variability of
approximately 20%.
The longer run-in time with Stellite 6 as the sample is
indicative of a greater amount of sliding required to form stable, wear protective glaze
layers.
In comparison, with Incoloy MA956 as the sample slid against Stellite 6 as the
counterface, a virtually negative positive weight change was recorded at (0.000(7) g –
Figure 5.C1b).
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An extremely small negative value of wear rate of
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-0.155 x 10 kg.m
sample surface.
indicated a slight addition of material to the Incoloy MA956
An average maximum run-in value of 0.55 was obtained for
coefficient of friction during run-in with Incoloy MA956 as the sample, with a total
run-in period of no more than 5 minutes (94 m). Values of coefficient of friction for
steady state sliding average out at ~0.5, with a variability of no greater than ~12%.
The slight decrease in weight with Stellite 6 as the sample or increase in weight with
Incoloy MA956 as the sample indicated that at 0.314 m.s-1 and 750°C (load 7N,
4,522 m sliding distance), whether the Stellite 6 was the sample or the counterface
material, it underwent preferential wear to the Incoloy MA956. This is analogous to
the preferential wear observed when sliding Stellite 6 as the sample or counteface
versus Nimonic80A under the same conditions.
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© 2003 – Ian A. Inman
Figure 5.C1: Weight change versus sliding speed for Stellite 6 as the sample
material worn against a Incoloy MA956 counterface at 750°C
(load = 7N, sliding distance = 4,522 m)
a) Weight change
0.002
Weight change (g)
0.001
0
-0.0056
0.0007
-0.001
-0.002
-0.003
-0.004
-0.005
-0.006
Stellite 6 (sample) vs Incoloy MA956 (counterface)
Incoloy MA956 (sample) vs Stellite 6 (counterface)
b) Wear rate
1.4
Wear rate (x 10
-9
kg.m -1)
1.2
1
0.8
0.6
0.4
0.2
0
-0.155
1.238
-0.2
Stellite 6 (sample) vs Incoloy MA956 (counterface)
Incoloy MA956 (sample) vs Stellite 6 (counterface)
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© 2003 – Ian A. Inman
Figure 5.C2: Coefficient of friction versus sliding distance for Stellite 6 as a
sample versus Incoloy MA956 at 750°C without reciprocation
(load = 7N, sliding speed = 0.314 m.s-1, sliding distance = 4,522 m)
2.0
Coefficient of friction (µ)
1.8
Before formation
of glaze layers
1.6
After formation of
glaze layers
1.4
1.2
Beginning of glaze
formation (1,884 m)
1.0
0.8
0.6
0.4
0.2
0.0
0
500
1000
1500
2000
2500
3000
3500
4000
4500
Sliding distance (m)
5.3.8.2 Optical and SEM Microscopy – Stellite 6 versus Incoloy MA956
Reversal of sample and counterface did not result in any significant change in glaze
layer morphology (Figures 5.3.8.2 and 5.3.8.3).
With Stellite 6 as the sample slid
against an Incoloy MA956 counterface, an extremely thin (between 3 to 5 µm thick)
but comprehensive glaze layer formed across the surface of the wear scar that when
subject to both optical and SEM study, showed no difference to that formed with the
Incoloy MA956 / Stellite 6 (counterface) system (Figure 5.34).
However, the Stellite 6 as a sample (versus Incoloy MA956 as a counterface) did
show evidence of early enhanced removal of material, with the surface of the wear
scar at a level below the level of the unworn surface.
This is further evidence (in
addition to the weight change / wear rate data) that the Stellite 6 as the sample
material has undergone enhanced wear when slid against Incoloy MA956.
Backscatter (Figure 5.C5) indicated that the carbides did not protrude beyond the
surface of the Stellite 6 sample (and into the glaze – not visible), after sliding against
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© 2003 – Ian A. Inman
Incoloy MA956.
This indicates that the carbides did not take an active part in the
sliding process once the glaze layer formed.
The morphological observations made for the Stellite 6 / Incoloy MA956
(counterface) were very similar to those made for the Stellite 6 / Nimonic 80A
(counterface) system under the same sliding conditions (750°C, 0.314 m.s-1 sliding
speed, 7 N load, 4,522 m sliding distance), prior to the formation of the glaze layer.
5.3.8.3 EDX and XRD Analysis – Stellite 6 versus Incoloy MA956
EDX analysis of the oxide deposits on the Stellite 6 samples indicated that the
primary source of debris for the glaze layer was the Stellite 6 itself, though with a
high degree of mixing of material from the Incoloy MA956 counterface.
The
average composition obtained for the glaze on the Stellite 6 surface (~31% Cr, ~12%
Fe and ~45% Co) was not too far removed from that seen where Incoloy MA956 was
the sample and Stellite 6 was the counterface (~32% Cr, ~19% Fe and ~40% Co).
The higher cobalt and lower iron levels do indicate slightly more material has been
sourced from the Stellite 6 sample, the primary source of material for the glaze.
XRD (Figure 5.C6) confirmed that mixing of oxide material from the Stellite 6
sample and Incoloy MA956 counterface has occurred.
As well as the high cobalt-
chromium phase CoCr2O4, a diffraction pattern for the iron-chromium phase
Cr1.3Fe0.7O3 was also obtained. The only metallic phase detected was the Co in the
Stellite 6 sample – no iron-chromium metallic phases were detected, indicating no
metallic transfer from the Incoloy 800HT counterface.
Figure 5.C3: Compacted oxide produced on Stellite 6 samples slid against a
Incoloy MA956 counterface at 0.314 m.s-1 – optical images
(load = 7N, sliding distance = 4,522 m, temperature = 750°C)
A smooth glaze has
formed with little loose
debris covering surface of
wear scar
14 mm
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© 2003 – Ian A. Inman
Figure 5.C4: Compacted oxide produced on Stellite 6 samples slid against a
Incoloy MA956 counterface at 0.314 m.s-1 – SEM images
(load = 7N, sliding distance = 4,522 m, temperature = 750°C)
0.314 m.s-1
Smooth glaze with little
loose debris, mainly
Co-Cr based oxide
(CoCr2O4 / Co3O4), only
a small amount of
Fe-Cr based oxide
(Cr1.3Fe0.7O3)
Figure 5.C5: Backscatter image of the side profile of a Stellite 6 sample slid
against Incoloy MA956 at 0.314 m.s-1
(load = 7N, sliding distance = 4,522 m, temperature = 750°C)
Wear surface
Cobalt-chromium
matrix
Chromium carbide
grain boundary
precipitates
300 µm
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© 2003 – Ian A. Inman
Figure 5.C6: XRD for Stellite 6 versus Incoloy MA956 – 0.314 m.s-1
(load = 7N, sliding distance = 4,522 m, temperature = 750°C)
Glaze layer consists of CoCr2O4 (sourced from the Stellite 6 itself) and
Cr1.3Fe0.7O3 (transferred from the Incoloy MA956 counterface)
1700
1600
1]
1500
[1 1
1400
Co
1200
1]
[2 1
1]
-Cr
[2 1
Fe
Co
.3F
Cr 1
0]
[1 1
e0. Co [
7O 200
3 [0 ]
24]
.3F
e
0 .7
Co
O3
Cr2
[11
O4
6]
[51
1]
Co
Fe Cr2O
- Cr
4
[22 [440
]
0]
200
Cr 1
Co
300
Co
Cr2
400
.3F
O4
500
Cr1
600
e0.
7
]
700
O3
800
-Cr
900
Fe
1000
[01
2]
Cr2
O4
Cr1 [22
.3F 0]
e0.
7O
Co
Cr2
3 [1
O4
04]
[31
1]
1100
[11
1
Lin (Counts)
1300
100
0
11
20
30
40
50
60
70
80
2-Theta - Scale
MSA754Hb - File: MSA754Hb.RAW - Type: 2Th/Th locked - Start: 10.000 ° - End: 90.000 ° - Step: 0.020
Operations: Smooth 0.150 | Smooth 0.150 | Smooth 0.150 | Background 1.000,1.000 | Import
35-1112 (*) - Chromium Iron Oxide - Cr1.3Fe0.7O3 - Y: 14.75 % - d x by: 1. - WL: 1.5406
34-0396 (I) - Iron Chromium 434-L stainless steel - Fe-Cr - Y: 19.66 % - d x by: 1. - WL: 1.5406
22-1084 (*) - Cochromite, syn - CoCr2O4/CoO·Cr2O3 - Y: 17.52 % - d x by: 1. - WL: 1.5406
15-0806 (*) - Cobalt - Co - Y: 50.00 % - d x by: 1. - WL: 1.5406
5.3.8.4 Hardness Data at 0.314 m.s-1 – Stellite 6 versus Incoloy MA956
The presence of chromium carbides at the grain boundaries led to erratic values of
hardness for Stellite 6 worn against an Incoloy MA956 counterface during hardness
depth profiling (Figure 5.C7), in a manner similar to that observed for Stellite 6 worn
against a Nimonic 80A counterface (Figure 5.22).
Again, no clear pattern of
variation in hardness could be observed in the sub-surface layers that would indicate a
work-hardening effect, however, as with Stellite 6 versus Nimonic 80A, small
differences in reflectivity enabled estimation in differences in hardness between
matrix-only and carbide rich areas.
Matrix values for the Stellite 6 samples were in general between 4.4 GPa and 6 GPa.
Carbide values in comparison were between 5.8 GPa and 10.85 GPa, the highly
erratic nature of which can be accounted for by the difficulty in positioning the
hardness indenter over a carbide rich region (the thickness of such carbides was in
general between 10 and 15 µm).
In practice, carbide hardness was probably much
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© 2003 – Ian A. Inman
higher, as hinted by a value of 18.04 GPa being obtained for a region of carbides at a
depth of 0.84 µm into the material.
The glaze layers formed on the wear scar surfaces of the Stellite 6 samples were of
variable hardness, though values (Table 5.C1 - mean value 9.93 GPa) in general were
greater that those obtained for the Stellite 6 matrix (mean value 5.39 GPa) and
roughly equivalent to that of the carbides (mean value 9.42 GPa).
However, the
Vickers micro-indenter even with a 50 g load did more often than not penetrate the
extremely thin glaze layers (no more than 3 µm thick) into the Stellite 6 sample
material underneath. It is therefore probable that hardness values for the glaze layer
were greater than those actually obtained.
Figure 5.C7: Variation in hardness with increasing distance from the sliding
surface for both the cobalt rich matrix and the carbide precipitates
in Stellite 6 slid against an Incoloy MA956 counterface
(sliding speed = 0.314m.s-1, load = 7N, temperature = 750°C, sliding distance = 4,522 m,
hardness values in GPa, Vickers indenter, 50 g load)
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© 2003 – Ian A. Inman
Table 5.C1:
Hardness data for glaze, Stellite 6 versus Incoloy MA956
(sliding speed = 0.314m.s-1, load = 7N, temperature = 750°C, sliding distance = 4,522 m,
hardness values in GPa, Vickers indenter, 50 g load)
14.21
10.29
11.48
Mean hardness of glaze = 9.93 GPa
7.58
6.06
Figure 5.C8: Compacted oxide produced on Stellite 6 samples slid against a
Incoloy MA956 counterface at 0.314 m.s-1 – optical images
(load = 7N, sliding distance = 4,522 m, temperature = 750°C)
15 mm
Highly worn wear scar, with some transfer of metallic material to form asperities.
Glaze layers have formed primarily on the peaks of the asperities, with some
unincorporated loose debris collecting in the counterface wear track.
5.3.8.5 Incoloy MA956 Counterface – Stellite 6 versus Incoloy MA956
A highly worn wear scar was formed on the Incoloy MA956 counterface on sliding
against a Stellite 6 sample (Figure 5.C8).
Material up to a depth of 1.5 mm was
removed from the counterface early during the wear process, with some readhesion of
metallic debris taking place during the early run-in stage to form asperities in the wear
scar. Evidence of this readhesion is the height of the asperities, some of which rise
up to 1.5 mm above the original surface of the Incoloy MA956 counterface.
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© 2003 – Ian A. Inman
The later generation of glaze was restricted to the peaks of these asperities, where
they were in contact with the Stellite 6 sample surface.
Other loose oxide has also
collected within the Incoloy MA956 counterface wear track.
As for the compositions of the asperities and the glaze, it is unclear whether they were
formed from readhered Incoloy MA956 debris, transferred Stellite 6 material or a
mixture of both.
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