A commercially used cast cobalt base alloy was investigated as a turbine shroud material which revealed a surface enriched with tungsten and carbon suggesting a surface layer of tungsten carbide. Adhesion and friction of this segregated surface layer are higher than for the bulk cobalt base alloy composition. Auger spectroscopy analysis of the segregation of tungsten in the alloy indicates that it occurs between 850 and 1000 C

Buckley, D. H.

English

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NASA TECHNICAL NASATMX-71609
MEMORANDUM
C:)
10*
S (NASA-TM-X-71609) ADHESION, FRICTION AND N74-32965AUGER SPECTROSCOPY ANALYSIS OF A
CCNHMERCIAL COBALT BASE AIRCRAFT TURBINE
SHROUD ALLOY (NASA) 14 p HC $3.00 Unclas
CSCL 11/6 G3/17 48759
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ADHESION, FRICTION AND AUGER SPECTROSCOPY ANALYSIS OF A
COMMERCIAL COBALT BASE AIRCRAFT TURBINE SHROUD ALLOY
Donald H. Buckley
Lewis Research Center
Cleveland, Ohio
Nat
c A 1K
https://ntrs.nasa.gov/search.jsp?R=19740024852 2020-03-23T05:17:39+00:00Z
ADHESION, FRICTION AND AUGER SPECTROSCOPY
ANALYSIS OF A COMMERCIAL COBALT BASE
AIRCRAFT TURBINE SHROUD ALLOY
By
Donald H. Buckley
NASA Lewis Research Cqnter
ABSTRACT
Adhesion, friction and Auger spectroscopy analysis of a commer-
cially used cast cobalt base alloy as a turbine shroud material has
revealed a surface enriched with tungsten and carbon suggesting a
surface layer of tungsten carbide. Adhesion and friction of this
segregated surface layer are higher than for the bulk cobalt base
alloy composition. Auger spectroscopy analysis of the segregation
C\)
0 of tungsten in the alloy indicates that it occurs between 850 and
10000C.
1
INTRODUCTION
The wear of turbine blade tips by shrouds Is a problem in currently used
aircraft engines. This wear results from contact of the turbine
blades with the shroud of the engine as a result of thermal and
mechanical growth and excursions. Such "rubs" result in adhesion,
friction, wear to the blade and the need to replace or rebuild blade tips.
An alloy used for a shroud material is a high temperature cobalt
base super alloy containing chromium, tungsten and nickel. It is
this shroud material and not the blades that should wear. With the
shroud rather than the blade tip wearing the turbine performance losses
from wear are minimized.
The objective of this study was to examine the composition of the
shroud material in the surficial layers using Auger emission spectro-
scopy to clarify the wear process and suggest means to reduce wear
of the blades. The adhesion and friction behavior of the alloy were
also examined while conducting Auger spectroscopy depth profiling.
2
Material
The alloy investigated in this study was a cast cobalt base alloy
used as an aircraft turbine shroud material. The composition of the
alloy is presented in Table I. It is important to note that the alloy
contains seven percent tungsten. The alloy was examined in the as
received condition after removal from an engine. Thus, it had been
exposed to high temperatures.
Apparatus
The apparatus used in this study was an adhesion and friction
device with built-in LEED (low energy electron diffraction) and Auger
emission spectroscopy capabilities. The specimen configuration for
adhesion and friction studies was that of a 2 mm radius pin contacting
a flat disk 8 mm in diameter and 2 mm thick. Loads applied ranged
from I to 10 grams. Strain gages mounted at right angles to each
other on a beam containing the rider were used to measure load,
adhesion and friction forces.
The chamber containing the specimens was capable of achieving
-10
vacuum pressures to 1010 torr. Sputtering of the disk specimen to
remove surface layers was accomplished by back filling the chamber
with argon gas to a pressure of 10 torr.
Experimental Procedure
The as received shroud material of the composition presented in
Table I was electric discharge machined to produce a tablet size
specimen 8 mm in diameter. The specimen was mounted in a holder and
inserted into the vacuum chamber. The system was evacuated and
baked-out at 250* C. Auger spectra, adhesion forces and friction
3
forces were measured as follows: 1) on the as received sample, 2)
at five intervals, one after each one hour sputtering cycle, 3) after
abrading the surface with 600 grit paper, 4) after heating the
abraded surface to 850 0 C and 5) after heating the abraded surface to
10000C.
Sputtering was accomplished with argon ions at a pressure of
10 torr, 1000 volts, 3 milliamperes and for a period of one hour.
4
RESULTS AND DISCUSSION
The Auger spectrum for the as received commercial cobalt base
alloy is presented in Figure 1. The principal surface species present
are carbon and oxygen. The oxygen has its origin in the surface
oxides and the carbon, as will be shown, with surface carbides.
In order to probe the alloys surface chemistry profile, sputter-
ing of the alloy surface was conducted in one hour intervals for a
total period of five hours. A typical Auger spectra after just one
hour of sputter removal of surface layers is presented in Figure 2.
Tungsten and carbon are the principal peaks in the spectrum.
Carbon and tungsten Auger peaks have been observed together in
attempts to achieve clean tungsten surfaces (ref. 1). The alloy
contains only seven percent tungsten yet this is the only metallic
.element seen in the Auger spectrum of Figure 2.
Continued sputtering of the alloy surface for four additional
hours beyond that employed in obtaining the Auger spectrum of Figure
2 resulted in the Auger trace of Figure 3. The tungsten peaks are
now the principal peaks with the carbon peak decreasing considerably
from that seen in Figure 2. In the spectrum of Figure 3 there is
still an absence of the other metallic elements present in the bulk
alloy, particularly cobalt and chromium (see Table I).
The relative Auger peak intensities for tungsten to oxygen and
tungsten to carbon with sputtering are presented in Figure 4. The
data of Figure 4 indicate a relatively gradual diminishing of the
oxygen as surface layers are stripped away. This is as might be
anticipated.
5
With the carbon there is a rather abrupt decrease in the carbon
Auger peak intensity between four and five hours. The surficial
concentration of carbon together with tungsten would suggest the
presence of a tungsten carbide surface layer (ref. 2). The presence
of hard tungsten carbide on the surface of the shroud alloy con-
tributed to abrasive wear of the turbine blades. Tungsten carbide
is hard and can act to abrade the blade material.
Adhesion and friction data were obtained on the alloy within
successive sputtering intervals and the results obtained are presented
in Figure 5. The data of Figure 5 indicate an increase in both
adhesion and friction with progressive removal of surface layers.
Thus., as less and less oxygen and carbon are seen in the Auger ,
spectra leaving principally tungsten as the surface layer (Figure 4)
both adhesion and friction are seen to increase.
In order to examine the bulk alloy composition the specimen was
removed from the apparatus and abraded on 600 grit metallurgical
paper and then placed back into the apparatus. Adhesion and friction
results obtained on the abraded specimen are presented in Figure 5.
Abrasive removal of the surface layers resulted in an Auger spectrum
containing oxygen, carbon, chromium and cobalt but absent was
tungsten. Both adhesion and friction were markedly less for the bulk
alloy composition as indicated in the data of Figure 5.
The abraded alloy was heated to 850 0 C for forty-five minutes to
determine if the tungsten would segregate to the surface. The Auger
spectrum from heating is presented in Figure 6. Cobalt, chromium,
carbon and sulfur peaks were detected in the spectrum but tungsten
was not observed. The specimen was then heated to 1000 0 C and the
6
results of Figure 7 were obtained. In Figure 7 cobalt, chromium,
carbon and tungsten were now observed at the surface. These results
indicate that between 850 and 100OC the tungsten segregated from
the bulk to the alloy surface.
CONCLUSIONS
Based upon the adhesion, friction and Auger spectroscopy data
obtained in this study on a commercial cobalt base alloy used as an
aircraft turbine shroud material the following conclusions are
drawn.
1. Between 850 and 1000OC tungsten in the cobalt base alloy
segregates to the surface of the alloy. Carbon is also present and
the compound tungsten carbide is suggested. This material could
contribute to abrasive wear of turbine blades.
2. The surface tungsten rich layer has markedly higher adhesion
and friction characteristics than the bulk alloy composition. The
bulk alloy friction behavior is comparable to that measured for
cobalt.
3. Sputtering of the alloy surface revealed oxygen diffused
into the surficial layers with a gradual diminution of the same.
With carbon, however, there was a rather abrupt decrease in its
intensity with surface layer removal.
4. A replacement alloy should be sought that does not contain
tungsten or other carbide forming constituents with surface segre-
gation tendencies. The new alloy should be evaluated as a potential
shroud material.
7
REFERENCES
1. Soyner, R. W.; Rickman, J.; and Roberts, M. W.: "Preparation
of Atomically Clean Tungsten Surfaces," Sur. Sci. 39,
pp. 445-449 (1973).
2. Ollis, D. F.; and Boudart, M.: "Surface and Bulk Carburization
of Tungsten Single Crystals," Sur. Sci. 23, pp. 320-346
(1970).
TABLE I. - CONPOSITION OF CAST COBALT ALLOY
Element Percent
Chromium 23.4
Nickel 10.0
Tungsten 7.0
Carbon 0.6
Titanium 0.2
Tantalum and
Zirconium Additions
Cobalt Balance
8
ORIGIN P P
... : .. 7- . ....
- "-. - - i-
Ffgure 1. -Auger spectrum for a commercial cobalt base alloy in the
as received condition.
-711 1-------...... -.- .-. I-
, ....... -. -.---- .. .. ... ... ........
i I t
_ 
ii..
.I .ii1I -. u lI-
Figure 2. - Auger spectrum for a commercial cobalt base alloy
after 1 hour of sputtering. Sputtering conditions 1000 volts,
3 mA, argon and a pressure of 10 torr.
-I ---- - - -
•~~~~~.. ... ... . :.. :I
Figure 3. - Auger spectrum for a commercial cobalt base alloy
after 5 hours of sputtering. Sputtering conditions 1000 volts,
3 mA, argon and a pressure of 10 torr.
/[
a) /
Sputter time, hr
Figure 4. - Auger spectroscopy peak intensity ratios for tungsten
to carbon and tungsten to oxygen as a function of surface sputter
removal for a commercial cobalt base alloy. Sputter gas argon,
1000 volts, 3*0 mA and a pressure of 10 torr.
REPRODUCIBILITY OF THE
ORIGINAL PAGE IS POOR
0
.O
o
0E
0
0
l/ 600 Grit
surface
Sputter time, hr abrasion
Figure 5. - Adhesion and friction coefficients for a commercial cobalt
base alloy as a function of argon sputter removal of surface material.
Sputter gas, argon, 1000 volts, 3 mA and 10 torr. Sliding velocity 1. 5
cm/min, load 1.0 Cm, 10 -10torr and 230 C.
...... . . .
... . --- --- ..-
:j.7-----C 
-
.........
Figure 7. -Auger spectrum for commercial cobalt base alloy sputtered,
abraded and heated for 45 minutes at 10000 C.


Adhesion, friction and Auger spectroscopy analysis of a commercial cobalt base aircraft turbine shroud alloy

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