The effects of surface treatment and microstructure, especially abnormal grain growth, on the strength of sintered SiC were studied. The surfaces of sintered SiC were treated with 400, 800 and 1200 grit diamond wheels. Grain growth was induced by increasing the sintering times at 2050 C. The beta to alpha transformation occurred during the sintering of beta-phase starting materials and was often accompanied by abnormal grain growth. The overall strength distributions were established using Weibull statistics. The strength of the sintered SiC is limited by extrinsic surface flaws in normal-sintered specimens. The finer the surface finish and grain size, the higher the strength. But the strength of abnormal sintering specimens is limited by the abnormally grown large tabular grains. The Weibull modulus increases with decreasing grain size and decreasing grit size for grinding

Kim, C. H.

Kim, Y. W.

Lee, J. G.

You, Y. H.

English

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NASA TECHNICAL MEMORANDUM . NASA TM-77871
THE EFFECTS OF SURFACE FINISH AND GRAIN SIZE
ON THE STRENGTH OF WINTERED SILICON CARBIDE
Young Hyuk You, Young Wook Kim, June Gunn
Lee and Chong Hee Kim
Translation from Yoop Hakhoechi, (Journal of the Korean"
Ceramic Society), Vol. 21, No. 1, 1984, Seoul South Korea,
pp. 27-32.
i (NASA-fM-77871) 1HE EFFEC1S OF SURFACE N86-21616
i F I N I S H AND G R A I N SIZE ON THE STRENGTH OF
SINTERED SILICCti C A R B I D E {National
Aeronautics and Space Administration) 12 p Unclas
HC A02/HF A01 CSCL 11D G3/24 05691
NATIONAL AERONAUTICS AND SPACE ADMINI
WASHINGTON, B . C . 205^6 OCTOBE"
https://ntrs.nasa.gov/search.jsp?R=19860012145 2020-03-20T14:59:55+00:00Z
ORIGINAL PAGE IS
OF POOR QUALITY
STAMDABO
NASA TM-77871
t. Tut* ««<< Juli.iU
THE EFFECTS OF SURFACE FINISH AND GRAIN
SIZE ON THE STRENGTH OF SINTERED
SILICON CARBIDE
7. **£>«•(•) , _
Young Hyuk You, Young Wook Kim, June
Gunn Lee and Chong Hee Aim
(You, Y.H. , Kim, Y.W. , Lee, J.G and Kim, C.H.)
9. P«<(o«w-nQ O«£on«to*tO« N«A« or>4 A^f«t«
Leo Kanner Associates
Redwood City, CA 9^063
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tration, Washington, D . C . 205^6
IS. Sc;; l«r- f M c » f No i« i
Translation from Yoop Hakhoechi (Journal
Ceramic Society), Vol. 21, No. 1, 1984, Seoul
pp. 27-32
3. R«po«i O«t«
October 19o5
4. Pufanolng 0>ganii«i<«« C«4«
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Translation
14. J j>on««f ln j A;«n<ir Coc!«
of the Korean
South Korea,
*i,..ot. The effects of surface treatment and microstructure,
esp. abnormal grain growth, on the strength of sintered SiC
were studied. The surfaces of sintered SiC were treated
with 400, 800 and 1200 grit diamond wheels. Grain growth was
induced by increasing the sintering times at 2050°. The beta
to alpha transformation occurred during the sintering of
beta-phase starting materials and was often accompanied by
abnormal grain growth. The overall strength distributions
were established using Weibull statistics. The strength of
the sintered SiC is limited by extrinsic surface flaws in
normal-sintered specimens. The finer the surface finish and
grain size, the higher the strength. But the strength of
abnormal sintering specimens is limited by the abnormally-
grown large tabular grains. The Weibull modulus increases
with decreasing grain size and decreasing grit size for
grinding.
17. t«T »o«ii (Sdcctca by Author(t)) It. Oliiilbuiion S«O»«O»M
Unclassified - Unlimited
IV. .
Unclassified Unclassified
31. 37.
THE EFFECTS OF SURFACE FINISH AND GRAIN SIZE
ON THE STRENGTH OF SINTERED SIC
Young-Hyuk You, Young-Wook Kim, June-Gunn Lee and Chong-Hee Kim
Division of Material Science and Engineering KAIST
A
.1. Introduction
During the last decade, there have been many studies on the
development and usefulness of the ceramics industry, especially on
ceramics engineering. Generally, the raw material for ceramics manu-
facture was recognized as SiC (silicon carbide) and SIN (silicon
nitride), and the study is naturally concentrated on the development
of both materials. Because these materials are recognized as superior
in their characteristics of heat-resistance, ^wear-resistance, and
others to metal raw materials, they are greatly appreciated as proper
materials for industrial ceramic uses. SiC was investigated by
Prochazka [1] as a—s-i-nteri-ng-eff-ect of carbon and boron, and there-
after research on the effects of'the sintering function of SiC
accelerated, mainly focusing on its mechanical clearance [2,3>4].
Nevertheless, SiC was not used as the raw material for engine
construction despite its superior characteristics compared with those
of metal, because of the corrosive characteristics of carbon com-
pounds. Therefore, it was necessary to show various data such as
K (stress intensity factor), the K,-V (crack velocity) diagram, m
(Weibull modulus) [5], the SPT (strength-probability-time) diagram,
besides its strength intensity. This research has focused on investi-
gating the relationship between m and K, using SiC which has a
theoretical density of 96% or greater. Also, research was done to
observe the influence of the microstructure of materials on m and
K, in the final stage.
Further, surface treatment of SiC was administered in step 5
and the influence of surface treatment results on the m value were
investigated. Therefore, as a result, this research has focused on
the influence of m value increase on the microstructure and surface
treatment of SiC.
II Experimental method
2-1 Raw materials
The raw materials for the synthesis of SiC are ultra-fine
powder of SiC (betarundum) of purity more than 99% and boron and
carbon which are used for the acceleration of sintering. The SiC
powder has a ^ -SiC crystal structure and the median diameter of the
powder is 0.26 urn and the powder particles are spherical. Carbon
2 w/o and boron 1 w/o are added to the SiC powder mix for 24 hours
with acetone in a WC ball-mill then dried and used for the sinter-
ing process.
2-2 Formation and Sintering
The primary formation was done in a 0.8 x 3.5 cm steel die under
2
a pressure of 250 kg/cm and then isostatic pressing was done under
2
1930 kg/cm pressure. At this point, the formation density gained
56-58% of the theoretical density. Sintering was done under 2050° C,
atmosphere Ar for 30 minutes and then extended sintering was done
for 1-1.5 hrs for the formation of particle sintering.
2-3 Strength experiment
We measured 4-point strength degree using a bar of material of
size of 0.6 x 0.4 x 2.85 cm. The inner span was 0.8 cm and the outer
span was 2.4 cm. The experimental bar was sintered and four different
types were prepared: one had an untreated surface and the others were
treated with diamond wheels of 400 grit, 800 grit, and 1200 each.
We polished the bar longitudinally, with a mark on the corner of the
bar. To see the blunting effects, the sintered bar of material was
polished with a 400 grit diamond wheel and heat treated at 2,000° C
Ar atmosphere for 20 minutes, then the strength intensity was tested.
2-4 Toughness Test
The toughness of sintered SiC was determined by the impact
technique (Vicker's impacter, 136 degrees) after polishing with
0.25 ym diamond paste. The equation used for the determination of
toughness and hardness was as follows:
K = 0.0726 ( F ) (1)
3^72-
H = P (2)
2 a 2
Here, P = applied load
C = Half of crack length
a = Half of the impacter cross-section
Lawn and Marshall [7] compared the character of ceramic materials
using toughness and hardness at the same time. They defined it as
brittleness and expressed it as follows:
H
B = (3)
Klc
2-5 Observation of microstructure and X-ray analysis
To observe the microstructure of the sintered SiC, the sintered
SiC specimen was polished with a 1200 grit diamond wheel and further
polished with 6 ym, 1 ym, 0.25 ym fine diamond paste. Etching was
done with boiling Murakami solution for the^-SiC particles and with
a KOH, KNO,, mixture solution for the a-SiC particles. The etched
specimen of sintered SiC was observed under electron microscopy.
The crystal structure of the sintered SiC was revealed under X-ray
analysis using a CuKa ray.
III. Experimental results and evaluation
3-1 Sintering
SiC powder which had added boron and carbon as the accelerator
was sintered for 30 minutes in an Ar atmosphere of 2,050° C, and
its length shrank 16$ and density more than 96%. The average particle
diameter was 2-3 ym by point counting, and there were partial abnor-
mal a-SiC particles. This is illustrated in Figure la. Figure Ib
had an extended sintering time of 1.5 hours at 2050° C and its fine
structure is illustrated. In this growing particle specimen, abnormal
growth of particles occurred, and the largest a-SiC particle was
measured at 150 ym. The abnormal particles are illustrated in Fig.2.
The clearest appearance in the specimen with extended sintering time
was the movement on a-SiC. a -SIC connects the particles of "3-SiC
around them and extends growth in the longitudinal direction,
increasing the aspect ratio. This phenonomenon particularly occurs
in the a-SiC which is in a better condition for growth or modifica-
tion. Table 1 shows the microstructure which changes according to
the sintering time.
3-2 Bending Strength
Figure 3 shows the results of investigation of the influence
on strength according to the surface treatment. In this figure, the
sintered specimen illustrates a strength increase of 40$ when treated
by the 1200 grit diamond wheel compared to the untreated specimen.
When treated by 400 grit, a 21% increase of strength was shown.
This phenomenon shows that bending strength is greatly influenced by
the surface condition. In Fig.3, there was no influence on surface
treatment above 400 grit. This phenomenon explains the fact that
surface treatment has influenced the remaining cohesive force of
the particles or breaking down of the particles.
Figure 4 illustrates the relationship between the strength of
the sintered specimen and the surface treatment. Even though the
surface treated specimen increases in strength compared to the
untreated specimen, the surface structure becomes fine, like the
half-hour sintered specimen, so that it can be said generally not to
increase in strength. Considering the fact that in Table 1 the
density is almost same, the strength seems greatly influenced by the
abnormal particles. Considering the specification of the MOR strength
test, it can be said that great conversion of strength occurs accord-
ing to the position of abnormal particles in the specimen which
was observed by the 4-point bending. This result shows the fact that
the strength connection which restricts the strength is not the same
as for the 0.5 hr sintered specimen. This is also the reason that
the value of the Weibull modulus(m) drops in Pig.1. In Fig.4, it can
be seen that abnormal growth of particles is more frequent in a speci-
men where micropolishing was done and the breaking strength is decreas-
ing. This is because the abnormal particles are a major function for
the cohesive force, and as we can see in Pig.l, the average particles
grow as obstacles per area compared with the 0.5 hr standard specimen.
In the specimen where abnormal growth has happened, a slight breakdown
of particles was seen.
3-3 Toughness Test
Figure 5 shows the difference in toughness and brittleness, and
hardness of SiC sintered according to the length of time. The hard-
ness shows the degree of resistence to material deformation, and the
toughness shows the degree of resistance to the breakdown of the
material. For the mechanical characteristics, hardness and brittleness
increase but toughness decreases. This is because as the sintering
time is extended, the average particle size grows and there is a
strong relationship with the microstructure of SiC. Therefore, abnor-
mal particles exhibit more sensitive responses to the breakdown stimulus
and reliability decreases with abnormal particles compared with normal
particles. This coincides with the fact of the experiment in Fig.7,
where the Weibull modulus value m decreases.
3-4 Weibull Analysis
Figure 6 shows the distribution pattern of bending strength.
The inclination was the Weibull modulus, m and more than 26 specimens
were used. The inclination m was measured by applying the LS(least
square) method. It was interpreted that the increase of the Weibull
modulus in the 400 grit polished specimen compared to the untreated
one is the result of increased uniformity. Also the reason why the
1200 grit polished specimen has more Weibull modulus compared to the
^00 grit polished one can be explained by the fact that diamond grit
size influences the surface deficit. Koepke et.al.[ll] defined the
relationship as follows:
Cs = agQ (0.2<a< 1) -------------------------- (4)
where, Cs = surface deficit on the surface of materials
g = grit size
Here, the value of a approaches 1 as the grit size becomes finer.
Figure 7 shows the comparison of bending strength after polish-
ing the 0.5 hr sintered specimen with a. 1200 grit diamond wheel for
the purpose of finding the effects of fine structure of the sintered
SiC on the Weibull modulus. The 1.5 hr sintered specimen showed
normal distribution of the strength. The following can explain the
reason for the low m value of the abnormal specimen compared to the
normal specimen. There could be a variation of distribution of the
strength, as the abnormal one restricts the strength.
As we can see in Fig. 5, the brittleness increases in the specimen
which has the abnormal particles, therefore, the breakdown rate of
this specimen also increases accordingly.
IV. Conclusion
1. SiC has a spherical format when sintered at 2050° C for 30 minutes,
and the size of the particles is 3-5 pm almost uniformly. When this
specimen is sintered with a 6 urn diamond wheel, a strength increase
can be visible up to 40$. This fact explains that the surface deficit
of SiC would cause the strength to vary.
2. Abnormal particles grow when SiC has been sintered for 1.5 hr,
and the abnormal particle has a larger aspect ratio. The size
reaches 150 ym.
3. The abnormal particle specimen exhibits less influence of surface
treatment on strength. This is because the abnormal particles are a
more important cause for strength variation.
p
4. The abnormal specimen showed an increase of strength from 14 GNm
to 21.4 GNm2, a decrease of toughness from 3-5 MNm -3/2 to 2.6 MNm -3/2,
This results in an increase of the brittleness and increase in the
breakdown rate of the sintered SIC.
5- Table 2 shows the Weibull modulus of a normal and abnormal sintered
specimen (au=0). Therefore, to use SiC for ceramic material it is
necessary to protect from fine surface treatment and abnormal particle
formation and to assist in increasing the Weibull modulus and break-
down strength.
REFERENCES
1. S. Prochazka, Sintering of silicon carbide, ceramics for
high performance applications, Ed. by Burke, Gorum and
Katz, Brook Hill, Mass., pp. 239-252.
2. S. Prochazka and R.M. Scanlan, "Effect of Boron and Carbon
on Sintering of SIC, J. Am. Ceram. Soc., 5_8 (1-2)72(1975).
3. Y. W. Kim, Reactive Sintering of SIC, .'M.S. Thesis, KAIST,
1983.
4. P.P. Lange and T.K. Gupta, "Sintering of SiC with Boron
Compounds", J. Am. Ceram. Soc., 59 (11-12), 537-538(1976).
5. W. Weibull, "Statistical Distribution Function of Wide
Applicability" J. App. Mech., 18 (9), 293-297 (1951)
6. G.R. Antis, P. Chantikul, B.RV Lawn and D B.Marshall,
"A Critical Evaluation of Indentation Techniques for
Measuring Fracture Toughness: I Direct Crack Measurements",
J. Am. Ceram. Soc., 6_4 (9, 533-538 (1981).
7. B.R. Lawn and D.B. Marshall, "Hardness, Toughness and
Brittleness: An Indentation Analysis", J. Am. Ceram. Soc.,
62 (7-8), 3^7-350 (1979).
8. N.W. Jepps and T.F. Page, "The Etching Behavior of SiC
Compacts", 124 pt3, 227-237 (1981).
9. D.G.S. Davis, "The statistical Approach to Engineering
Design in Ceramics", Proc. Brit. Ceram. Soc., 22 (6),
429-452 (1973).
10. S. Jayatilaka, Fracture of Engineering Brittle Materials.
chap. 5-6, Wiley Interscience, Applied Science Publishers
LTD, 1979-
11. B.G. Koepke and R.J. Stokes, J. Mat. Sci. 5, 240 (1970).
ORiGiMAL PAGE IS
OF POOR QUALFTY
Table I. Properties of specimens at various sintering times
x property
sintering time\
0.5 hr
1.0 hr
l .Shr
average grain
(largest grain
size) size
2-4 /an
(12 ^m)
8-10 ftm
(65 //m)
12-15 //m
(150 ^m)
microslructure
spherical shape and
uniform grain size
transient
large aspect ratio and
exaggerated grain growth
linear shrinkage
(56)
16.0
16 1
16 .2
sintered density
(% T. D.)
97
98
98
Table 2. Summary of Weibull modulus for various
materials
Hour
0.5 hr
1.5 r^-
Surface Finish
As Received
400 Grit Diamond
800 Grit Diamond
1200 Grit Diamond
m
5.14
5.99
7.15
5.30
"o
1356
3472
2187
1766
Fig.) Microstructurcs of sintered SiC(2000x). sinter-
ing conditions: (a) 2050°C, 0. 5br, (b) 2050°C,
1.5hr
Fig. 2 Typical microstruclurc of abnormal grain
growth (tOOOx). sintering conditions: 2050°C,
I.Shr
ORIGINAL PAGE SS
OF POOR QUALm
O JOOO
2
A I D E . 40O tOO I20O
Diamond Grit Number
Fig. 3 MOR as a function of surface finish in sin-
tered SK?. Above mark (A) indicated the
strength of specimens which were annealed
at 2000°C after surface treatment with 400
grit diamond. (AS. RE. = as received)
...... 101
4*0 «rtt •
• oe *it (AJ
040 vrtl ID)
OS IJJ"
- l / f( L o r g t J t G 'Q. f S.n)"'
Fig. 4 Strength-Grain Size-Surface Finish relations
in sintered SiC. (as rece. = as received)
E
2
O
— tt
OJ IO IS
Sinliring Time (hour)
Fig. 5 Microhardness(O), toughness(0) and
brittleness (A) as function of sintering
times in sintered silicon carbide.


The effects of surface finish and grain size on the strength of sintered silicon carbide

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