The 4-point bending strength of 4 grades of hot-pressed SiC was determined at different temperatures. With a transgranular mode of fracture the values for bending strength are retained up to high temperatures. For intergranular fracture the decrease of strength is governed by subcritical crack growth. The intergranular fracture is caused by a high content of silicate glassy phase at the grain boundaries of hot-pressed SiC

Kriegesmann, J.

English

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https://ntrs.nasa.gov/search.jsp?R=19850004061 2020-03-20T19:56:51+00:00Z
JNASA TECHNICAL MEMORANDUM	 NASA TM-77512
(NASA-Td-77512) BiLNDING S2fEhG1ki SIUJIES ON 	 b85-1,069
HCT-PEESSEL SILICON CARLIOE (Eatiouai
Aeronautics and Space AdmiriSttation) 13 P
HC e02/81 X101
	
CSCL 20K	 Uuclas
G3/39 24431
BENDING STRENGTH STUDIES ON HOT-PRESSED SILICON
CARBIDE
J. Kriegesmann
Translation of "Biegefestigkeitsuntersuchungen
an heissgepresstein Siliciumcarbid," Berichte der
Deutschen Keramischen Gesellschaft, Vol. 55,
No. 8, 1978, pp. 391-394.
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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON D.C. 20546
	 AUGUST 1984
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BENDING STRENGTH STUDIES ON HOT-PRESSED
SILICON CARBIDE
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Translation of "Biegefestigkeitsuntersuchungen an
heissge?resstem Siliciumcarbid," Berichte der Deutschen Keram-
ischen ^esellschaft, Vol. 55, No. 8, 1978, pp. 391-394.
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The 4-point bending strength of 4 grades of hot-pressed FiC
was determined at different temperatures.
	
With a transgranular
mode of fracture the values for bending strength are retained
up to high temperatures.
	 For intergranular fracture the
decrease of strength is governed by subcritical crack growth..
The intergranular fracture is caused by a high content
of silicate glassy phase at the grain boundaries of hot-
pressed SiC.
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BENDING STRENGTH STUDIES ON HOT-PRESSED SILICON CARBIDE **
Dr. Jochen Kriegesmann***
SUMMARY The 4-point bending strength of four
different grades of hot-pressed SiC was measured
at different temperatures. Using a scanning
electron microscope it was possible to demon-
strate that with a transgranular mode of
fracture the values for bending strength are
retained up to high temperatures. In the case of
intergranular fracture the decrease in strength
is governed by subcritical crack growth. The
reason for intergranular fracture is assumed to
be a high content of silicate glassy phase at
the grain boundaries of hot-pressed SiC.
1 INTRODUCTION
To date, mechanical engineering has used almost exclusively 	 /391*
metallic materials. Because of their deficient thermal
resistance, in most cases the use of metals and alloys under
oxidizing conditions is limited to temperatures below 1000°C.
For this reason, ceramic construction materials are usually
*Numbers in margin indicate foreign pagination
**Paper presented at the DKG [Deutsche Keramische Gesellschaft
= German Ceramics Society] discussion meeting "High temperature
characteristics of ceramic materials" in Baden-Baden. on 02-17-78
***Sintering Section, Electric Smelting Plant Kempten, Inc.,
P.O. Box 15 26, D-8960 Kempten, FRG
1
considered at high temperatures. Among the possible candidates,
the ceramic materials most suitable for high temperature
mechanical engineering are silicon nitride (S1 3N4 ) and silicon
carbide (SiC), because in addition to their high hardness they
exhibit excellent corrosion resistance and unusually good
temperature change resistance, in comparison to other ceramics.
In this paper we shall examine the bending strength of
hot-pressed silicon carbide. The high temperature hardness
characteristics will be discussed jointly with the fracture
behavior and the structure arrangement. We shall show that for
this material high room temperature hardness does not
necessarily entail an elevated high-temperature hardness.
2 SAMPLE PREPARATION
Four SiC powders of 3 pm grain size or finer, which differed in
the kind and quantity of sintering-promoting additive used, were
pressure-sintered into discs in a Degussa hot press VSPI 25/15
at 2100°C and a pressure of 400bar. The discs' diameter was 72
mm and their height 6 ram. Using diamond tools, 4-point bending
strength specimens were cut from these formed pieces, with the
dimensions 3.15 mm x 4.1 mm x 35 mm, and smoothed longitudinal-
ly. The average density of the little rods was 3.19 mg/cm 3 (>99%
of the theoretical density of SiC).
3 TEST DESIGN AND TEST METHOD
The bending specimens were tested on edge, such that the
2
hot-pressing direction coincided with the direction in which the
pressure is applied.
The tests were performed using a 4-point bending apparatus of 	 's
our own manufacture. The load is transmitted according to the
lever principle and applied through an overflowing water
container. The apparatus',core - the bending device - is located
in the heating zone of a silicon carbide tube furnace. The
individual parts of the bending device are made of hot-pressed
(HP) silicon carbide.
The pressure transmission to the bending device and the specimen
occurs in the furnace via two corundum tubes that are water-
cooled at the upper and lower ends. The temperature is measured
with a PtRh/Pt thermoelement whose sensor is directly under the
specimen. Another thermoelement, next to the specimen, is used
for controlling the furnace. The furnace itself can be displaced
upwards or downwards by means of a chain drive, to insert the
specimens. The furnace's heating rate is 100 K/min. The bending
strain's stressing rate is 3MN/m's.
/392
4 TEST RESULTS
The test results for the four kinds of HP-SiC examined are shown
graphically in Figure 1, below.
of the four kinds examined, HP-SiC No. 1 had the highest'
strength at room temperature, but it decreased most rapidly with
temperature, such that at 1500°C it was less than 1/5 of the
value attained at room temperature. While in specimen No. 2 the
strength also decreased with increasing temperature, it fell
much less rapidly than for No. 1: at 1500°C the material had
	 s
3
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`.'
-------	
.....	 Nr 20
Nr)
400- rN60-
--0
200-
0
0-+_ -
20 750	 1000	 1200	 1370 1500
— --	 ►.mp.r0fwr I • C I
Figure 1. H.)t bending strength of HP-SiC as a function of
temperature
lost only approximately half its strength. Specimen No. 3 had
the lowest strength of the four, at room temperature, but iLs
strength proved to be substantially independent of temperature.
In fact, it seems to improve with temperature, such that at 15000
C it had the highest strength of any of these four materials.
While at room temperature the strength of No. 4 is below that of
No. 1 and No. 2 1 it is substantially higher than that of No. 3.
It decreases slightly near 1000°C, but at 1200 0C rises again,
nearly to the original :-clue, to decrease again only at 1500°C.
5 CERAMOGRAPHIC EXAMINATION
The characterization of the four types of SiC was performed on
4
'	 a	 5
a)
it
. 9	 'i.. Y'' 7.
r	 Q
LA^
tr
fi
Figure 2. SEM iaia(^e of etched, ;round and polished surfaces. a)
HP-SiC No. 1, b) HP-SiC No. 2, c) HP-SiC No. 3, d) HP-SiC No. 4
the etched and polished structural images of specimens, obtained
with a type Super II International Scientific Instruments
scanning alectron ^^icroscope (SEM). The etching agent used was
boiling Mur.akami solution (alkaline ferricyanicie solution); the
etching time was al)proximatel.y 3 mi,iutes. With this method it is
	 j
5
b) t1
kfrf-
f 1v
5Nm +
11
Figure 3 SEM, images of fracture planes, broken at room
temperature. a) HP-SiC No. 1, b) HP-SiC No. 2, c) HP-SiC No. 3
possible to attain grain surface etching on the polished SiC
specimen.
The polished surface images in Figure 2, page 5, clearly show
that the ;rains in TIP- S iC Mos. 1, 2 and 4 are equiaxial, while
that of No. 3 represents long, racial crystals. In No. 3, grain
growth occurred during sinterizaticn under pressure.
Figures 3 (ah ove) and 4 (page 7) show the SEM images of the
fracture planes. For types No. 1, 2 and 3 there are no
significant differences in the fracture images at different test
temperatures. Hence the representation at room temperature
(Figure 3) is adequate to characterize the fracture behavior.
But AP-SiC No. 4 exhibits different fracture behaviors at the
different temperatures and hence Figure 4 shows a fracture plane
image for each temperature, for this type. The high temperature
^I
	
	 fracture planes were etched with HF, because the original
fracture surfaces became cuvered with an oxide film after the
to i
	 high temperature strength test.
Types Nos. 1 and 2 exhibit intergranular fracture. The fracture
6
PP V A 
s	 ^'
w
5Nm
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'21
5Nm
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i
j; 5Nm
.ms
s
Or
 
as'
571
SF
a! c)
-.-^--P
- . I _- -
d)	 e)
Figure 4. SE14 images of HP-SiC No. 4 fracture planes, broken at
a) Roo,(i temperature, h) 1000°C, c) 1200'C, d) 1370°C, e) 1500°C;
b) - e) etct:ed with concentrated HF
_n No. 3 is transgranular throughout the temperature range
tested. While No. 4 exibits transgranular fracture at room
temperature, at 1000°C the fracture is intergranular; at 1200°C
the transgranular portion increases again, even though it is not
comparable to the fracture image at room temperature.
If the images of the fracture surfaces are compared to the
course of the bending strengths, it becomes apparent that an
intergronular fracture is associated with a decrease in strength
at increasing temperature (HP-SiC Nos. 1 and 2). In the case of
1	 i	 5`—~	 S
7
transgranular fractures, the strength remains constant as the
temperature is increased (HP-SiC No. 3), provided the
transgranular mode of fracture is retained over the entire
temperature range.
In HP-SiC No. 4, the transition to intergranular fracture
becomes apparent, at 1000°C, by the decrease in strength. The
transgranular portion increases in this case, at higher
temperatures, so that an improvement in strength is noted also
here. Beyond 1370°C both the strength and the fraction of
transgranular fractures decrease again.
6 DISCUSSION
In many materials the crack length hardly changes, when a strain
is imposed on the specimen at low temperatures; at high
temperatures, in contrast, most ceramic materials will
experience "subcritical crack propagation" [2]. In this case the
material's strength becomes dependent on the stressing rates the
higher it is, the higher becomes the mean measured strength.
Charles [3,43 has found an analytical expression for this
phenomenon, which he confirmed for glasses. Lange and Iskoe [5],
as well as Evans and Lange [6] performed strength measurements
as a function of stressing rate with SiC. They showed that at
room temperature the subcritical crack propagation is low, while
it becomes quite evident at high temperatures. Hence subcritical
crack propagation can be considered the cause for the decrease
in strength, with increasing temperature, at higher
temperatures. Two models were suggested for the slow crack
propagation in ceramic materials [2,7]. In materials with high
NOW
Primary c
Secondary cracks
Figure 5 . Model of subcritical crack propagation by the grain
boundary slippage mechanism, from [7]
dislocation mobility a dislocation mechanism is assumed; in
cases of low dislocation mobility - which are found among
covalent solids- grain boundary slippage is assumed (Figure 5,
above). The model for this was described in detail by Evans [7].
According to the grain boundary slippage model, subcritical
crack propagation is only possible for intercrystalline fracture:.
If trans-crystalline fracture occurs, either another model is
valid or slow crack growth barely takes place. For transcrystal-
line fracture we found either no strength decrease or a slow
strength decrease with temperature in the materials investigated.
Apparently the influence of subcritical crack propagation is
small in these materials which show transcrystalline fracture.
We can ask which material prcperties have a decisive effect on
determining which fracture mode occurs (trans- or inter-
crystal'.ine). HP-SiC no. 1 and 2 were doped with aluminium,
just like the HP-SiC material of Lange [5.6j. No. 1 had about
twice as much aluminium as no. 2. Perhaps alumosilicate glass"
was formed at the grain boundary, which would be present at a
higher concentration on the grain boundary for a larger aluminium
content. This would explain the clear strength decrease of
no. 1 with temperature. HP-SiC No. 4 had & much lower aluminium
9
content, so that there is hardly any glass phase. Some
transcrystalline fracture is present in this material.
HP-SiC no. 3 was doped with boron. We can assume that the
grain boundary phase is B 4C. A protective layer is formed
by oxidation, which leads to crack healing on the surface.
This explains the slight strength increase as a function of
temperature. In a less compressed, boron-containing HP-SiC
series, the author [L] found a particularly clear improvement
of the strength as a function of temperature, which could
also be attributed to this curing process.
7 CONCLUSIONS
Hot-pressing of SiC powders makes it possible to manufacture
materials in which subcritical crack propagation barely takes
place, during stressing at high temperatures. Such materials
could be used as high temperature construction materials
wherever long stress periods are required, since they are
especially resistant to creeping processes.
8 REFERENCES
1 Lange, F.F., Origin and use of fracture mechanics, in: R.C.
Bradt, D.P.H. Hasselman and F.F. Lange (Eds.), "Fracture
mechanics of ceramics", Vol. 1: Concepts, flaws and
fractography, pp. 3-15. Plenum Press, New York/London 1974
2 Evans, A.G., High temperature slow crack growth in ceramic
materials, in: J.J. Burke, A.E. Gorum and R.N. Katz
(Eds.), "Ceramics for high-performance applications", pp.
3'73-396. Brook Hill Publishing Co., Chestnut Hill MA 1974.
10
3 Charles, R.J., Static: fatigue of glass, J. appl. Phys. 29,
1554-1560 (1950)	 x
4 Charles, R.J., Dynamic fatigue of glass, J. appl. Phys. 29,	 T4
1657-1662 (1958)
5 Lange, F.F. and Iskoe, J.? . ., Strength of hot-pressed Si3N4
and SiC: Effect of impurities; in: J.J. Burke, A.E. Gorum
and R.N. Katz (Eds.), "Ceramics for high-performance
applications", pp. 223-238. Brook Hill Publishing Co.,
Chestnut Hill MA 1974
6 Evans, A.G. and Lange, F.F., Crack propagation and fracture
in silicon carbide, J. Mat. Sci. 10, 1659-1664 (1975)
7 Evans, A.G. and Langdom, T.G., Structural c-!ramie::;, in: B.
Chalmers, J.W. Christian and T.B. Massalski (Edsz. :
"Progress in materials sc..ence", Vol. 21, Pergamon Press,
Oxford/New York 1976
8 Kriegsmann, J. ; Unpublished measurement results
These investigations are part of the research program sponsored
by the Federal Ministry for Research and Technology. The author
wishes to express his gratitude to Dr. A. Lipp, Dr. K. Reinmuth
and Dr. K. Schwetz for their helpful contributions during the
discussion.
11


Bending strength studies on hot-pressed silicon carbide

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