High toughness silicon nitride ceramics were processed with the addition of small quantities of beta-Si3N4 whiskers in a commercially available alpha-Si3N4 powder. These whiskers grew preferentially during sintering resulting in large, elongated beta-grains, which acted to toughen the matrix by crack deflection and grain pullout. The fracture toughness of these samples seeded with beta-Si3N4 whiskers ranged from 8.7 to 9.5 MPa m(exp 0.5) depending on the sintering additives

Choi, Sung R.

Sanders, William A.

Tikare, Veena

English

NASA Technical Reports Server

NASA Contractor Report 191190 
Processing and Testing of High Toughness 
Silicon Nitride Ceramics 
Veena Tikare 
Case Western Reserve University 
Cleveland, Ohio 
William A. Sanders 
Analex Corporation 
Brook Park, Ohio 
and 
Sung R. Choi 
Cleveland State University 
Cleveland, Ohio 
Prepared for the 
Pacific Rim Meeting of the American Ceramic Society 
sponsored by the American Ceramic Society 
Honolulu, Hawaii, November 7-10, 1993 
NI\S/\ 
National Aeronautics and 
Space Administ ration 
'---- - - -------- -----.-- --
https://ntrs.nasa.gov/search.jsp?R=19940010070 2020-06-16T19:45:00+00:00Z
PROCESSING AND TESTING OF HIGH TOUGHNESS SILICON 
NITRIDE CERAMICS 
* Veena Tikare 
Case Western Reserve University 
Cleveland, Ohio 44106 
William A. Sanders 
Analex Corporation 
3001 Aerospace Parkway 
Brook Park, Ohio 44142 
* Sung R. Choi 
Cleveland State University 
Cleveland, Ohio 44115 
Abstract 
High toughness silicon nitride ceramics were processed with the addition of 
small quantities of ,8-Si3N 4 whiskers in a commercially available a-SigN 4 
powder. These whiskers grew preferentially during sintering resulting in 
large, elongated ,B-grains, which acted to toughen the matrix by crack deflec-
tion and grain pullout. The fracture toughness of these samples seeded with 
,8-Si3N4 whiskers ranged from 8.7 to 9.5 MPav'iii depending on the sintering 
additives. 
* Resident Research Associate at NASA Lewis Research Center, 
Cleveland, OR 44135. Please address all correspondence to NASA. 
1 
INTRODUCTION 
Silicon nitride ceramics are candidate materials for high temperature struc-
tural applications such as advanced automobile and aircraft engines, heat 
exchangers, etc. However, like all ceramics, they are brittle and flaw intoler-
ant. These limitations have lead to a great deal of interest in toughening sili-
con nitride ceramics. In-situ toughened silicon nitride ceramics, also called 
self-reinforced silicon nitrides , have received much attention in recent years 
as they are easier to process than whisker or fiber reinforced silicon nitrides. 
The mechanical properties of these in-situ toughened silicon nitride ceramics 
are attractive. They are among the toughest of monolithic ceramic materials, 
and some silicon nitrides retain good mechanical properties to temperatures 
as high as 1350 0 C depending on the composition and amount of the grain 
boundary phase. 
The disadvantage of in-situ toughening is that prolonged heat treatments at 
high temperature are required to promoted ,B-Si3N 4 grain growth. These heat 
treatments are not only expensive, but also lead to decomposition of Si3N4 
to its constitutive elements, Si and N, resulting in lower density and poor 
mechanical properties. In this paper, we present a simple, inexpensive tech-
nique which facilitates the processing of in-situ toughened silicon nitrides in 
almost any silicon nitride ceramic which is processed by sintering, hot-
pressing or HIP'ing with small amounts of the grain boundary phase. Small 
quantities of ,B-Si3N 4 whiskers were used to seed the growth of the large 
,B-Si3N4o grains rather than using prolonged heat treatments to obtain grain 
growth. The fracture toughness of these silicon nitride ceramics will be pre-
sented and contrasted with traditional in-situ toughened ceramics. These 
results will be discussed in terms of microstructural evolution during liquid 
phase sintering. 
MATERIAL PROCESSING AND TESTING PROCEDURE 
Material Processing 
Silicon nitride ceramics of two different compositions with and without 
,B-Si3N 4 whiskers were prepared. One composition contained 6 wt% Y 20 3 and 
2 wt% Si02 as sintering aids, and the other 8.6 wt% Sc20 3. They are desig-
nated as 6Y and 8Sc, respectively. A commercially available silicon nitride 
** . powder was milled in ethanol with the appropriate sintering aids and 
UKemanord Engineering Ceramics, Ljungaverk, Sweden. 
2 
1 wt% stearic acid in a l L sintered, reaction-bonded silicon nitride mill. The 
milling media were hot-pressed silicon nitride cylinders (1 cm long by 1 cm 
in diameter). The milling time was 300 h. The samples with whiskers are 
designated as 6Y-W and 8Sc-W. The samples without whiskers were pre-
pared by drying the slurry in an apparatus employing a hot water bath and 
vacuum t o remove the ethanol, followed by drying in an oven at 110 ° C. 
This dried powder contained some soft agglomerates which were crushed us-
ing hot-pressed silicon nitride hardware and sieved through a 150-JLm screen. 
The samples with the whiskers were prepared by mixing commercially avail-
*** able ,8-Si3N 4 whiskers with the slurry by ultrasonic agitation after mill-
ing. T his mixture was dried as before. Again, the dried powder was gently 
crushed but not sieved to prevent segregation of the whiskers and powder. 
Both types of samples were pressed using st andard powder pressing tech-
niques. The powders were uniaxially pressed at 25 MPa to fabricate preform 
bars (57 by 7.5 by 5 mm) . These bars were vacuum sealed in latex tubing 
and isostatically cold-pressed at 414 MPa. T he pressed bars were sintered in 
groups of 15 in a tungsten resist ance furnace for 4 hat 2140 °c and 2.5 MPa 
nitrogen overpressure. Test bars were machined by grinding along the longi-
tudinal axis of sint ered bars with a 400-grit diamond grinding wheel. 
Testing Procedure 
Fracture toughness of these ceramics was measured by the single-edge-
precrack-beam (SEPB) method.! The prospective tensile surface of a flexural 
beam was indented with a Vicker's indenter under an indentation load of 
294 N. The indented sample was loaded in a double anvil type geometry; and 
at some critical load the indentation crack would pop-in to form a sharp, sin-
gle ed~e precrack. A detailed description of this technique is given else-
where. The precracked samples were loaded in a four-point bend geometry. 
The fracture toughness values were calculated as: 
K 1C = 
3PY(1o - IJ /::: 
2Wn 2 
(1) 
where P is the fracture load, 10 and Ii are the outer and inner spans, respec-
tively (10 = 18 mm and Ii = 10 mm), D is the sample depth, and W is the 
sample width, and Y is the geometric factor given by:3 
*** UBE-SNWB, UBE Industries Ltd. , Ube City , Japan. 
3 
(2) 
where a is the crack length. Nine specimens of each composition were tested. 
RESULTS 
Particle size analysis of the starting SigN 4 powder revealed that 90 percent 
of the powder was under 0.9 fJ,m and 50 percent was under 0.45 fJ,m. The 
,B-SigN4 whiskers ranged from 5 to 60 fJ,m in length and 0.2 to 3 pm in diam-
eter as shown in Fig. 1. For a complete description of the .8-SigN4 whiskers 
the reader is directed to Homeny et al.4 The whiskers are considerably larger 
t han the starting powder size. Good dispersion of the whiskers in the starting 
powder was essential. Poor dispersion resulted in clumping of the whiskers 
which in turn lead to lower density, lower strength and lower fracture tough-
ness. Damage to the whiskers during green processing must be kept to a min-
imum. Milling or crushing of the whiskers into more equiaxed particles 
resulted in properties similar to those of the samples without whiskers. The 
shrinkage during sintering was uniform in width, length and height for sam-
ples with and without whiskers. The densities of all four compositions were 
the same, 3.25 g/cm3. The presence of inert particles, especially rod-shaped 
particles similar to the whiskers used in this investigation, hinder densifi-
cation by applying "drag stresses" to the surrounding material during sinter-
ing. By dispersing small quantities of .8-SigN4 whiskers, we were able to keep 
these back stresses low and achieve high densities even in samples with 
whiskers. 
Figure 1 SEM micrograph of .8-SigN 4 whiskers used to seed 
the starting powder. 
4 
The fracture toughnesses of the silicon nitride ceramics with and without 
whisker additions obtained by SEPB are given in Fig. 2. The fracture tough-
ness of the 6Y-Si3N4 with whiskers (6Y-W), B.7 MPav'ni, was almost double 
that of the 6Y-Si3N4 without whiskers (6Y) , 4.5 MPav'ni. Similarly, the frac-
ture toughness of the BSc-W was higher than that of BSc although the in-
crease was not as dramatic as that between the 6Y and 6Y-W silicon nitride. 
10 
9 
~8 
cO 
~ 7 ~ 
rn 6 Vl (I) 
c 5 .I:: 
OJ 
:J 
.9 4 
~ 3 :J 
t> 
cO 2 u: 
1 
0 
6Y 6Y-W 8Se SSc-W 
Figure 2 Average fracture toughness of the silicon nitride ceramics 
tested by SEPB. The error bar represents one standard 
deviation. 
Scanning electron microscope (SEM) analysis of the fracture surfaces (shown 
in Figs. 3(a) to (d)) revealed two obvious differences between samples with 
and without whiskers. The samples with whisker additions had a few elon-
gated Si3N4 grains which were much larger than the surrounding grains. 
Samples without whisker additions had a much more uniform grain size dis-
tribution. The second difference was that samples with whiskers had a much 
more tortuous fracture surface with extensive pullout of the large, elongated 
,B-grains. 
Crack trajectories of indentation cracks at the indent surface were examined 
to determine the toughening mechanisms. Figures 4(a) and (d) show that 
crack deflection and bridging are the toughening mechanisms in both the 
samples with and without whiskers added. However, more effective crack 
deflection is achieved by the large, elongated grains in the samples with 
whiskers as a result of their ability to deflect the crack further away from the 
original crack plane than that of the smaller grains. 
5 
Figure 3 SEM micrographs of the fractured surfaces of 
a. 6Y, h. 6Y-W, c. 8Sc, and d. 8Sc-W. 
Figure 4 SEM micrographs of indentation cracks showing crack 
trajectories of a . 6Y, h. 6Y-W, c. 8Sc, and d. 8Sc-W. 
6 
DISCUSSION 
The fact that large, elongated ,B-SisN 4 grains were seen only in the samples 
with whisker additions but not in the samples without whisker additions indi-
cated that the large, elongated grains grew from the ,B-Si3N 4 whiskers. It is 
now . well est ablished that grain growth in silicon nitride occurs by an 
Ostwald ripening mechanism. Smaller silicon nitride grains dissolve in the 
liquid silicate phase and reprecipitate onto larger grains leading to grain 
growth. Initially, the a-SisN4 grains or particles dissolve and precipitate onto 
the ,B-Si3N 4 part icles which are invariably present in the starting composi-
tion. Later, when all the SisN4 has transformed to the fi-phase, Ostwald 
ripening occurs. In the materials with whiskers added, initially the a-phase 
and later, the smaller ,a-grains dissolve and reprecipitate onto the ,8-whiskers 
preferentially to give exaggerated grain growth of the whiskers. 
Examination of these large, elongated grains from random two-dimensional 
cross-sections of as-sintered samples and from fracture surfaces revealed that 
the lengths of the large grains were typically 30 to 60 /-lm and the diameters 
10 to 12 /-lm. Comparison of the large grain sizes to the whiskers (see Fig. 1) 
suggests that the whiskers grew much faster in diameter than they did in 
length. This is in agreement with the finding of Lai and Tien.5 
It is not clear why the presence of a few large grains increased the fracture 
t oughness of the 6Y -W composition over that of the 6Y composition so 
dramatically; whereas , only a modest increase in toughness was realized for 
the 8Sc-W composition over that of the 8Sc composition. Although, the grain 
sizes of the 6Y and 8Sc were similar, the cr ack deflection characteristics were 
very different as shown in Fig. 4. The 8Sc silicon nitride experienced 
extensive crack deflection; while, 6Y experienced modest crack deflection. 
Crack deflection was enhanced tremendously by the large, elongated fi-Si3N 4 
grains in 6Y-W. The large fi-grains in 8Sc-W increased crack deflection, but 
extensive crack deflect ion was already occurring in 8Sc. Thus the increase 
between 8Se and 8Sc-W was not dramatic. 
An explanation for the difference in fracture toughness of the 6Y and 8Sc 
materials in spite of their similar grain size may be due to differences in their 
grain boundary phases. X-ray diffraction analysis of6Y and 8Sc revealed that 
the grain boundary phase in 8Sc had devitrified to Sc2Si20 7. In contrast , no 
crystalline silicate phase was detectable in 6Y. Perhaps crystallization of the 
grain boundary phase in 8Sc created some residual stresses which enhanced 
crack deflection in the 8Sc silicon nitride ceramic. 
7 
----. ---
It has been known for some time that only a few elongated grains are neces-
sary for toughening by crack deflection. Faber and Evans6 have shown that 
fracture toughness does not increase significantly with increasing volume 
fraction of rod-shaped particles above 20 vol%. Li and Yamanis7 have shown 
that a few large, elongated SisN 4 grains increase toughness by increasing 
crack deflection. Yet in-situ toughening of silicon nitride ceramics has been 
achieved primarily through coarsening of all the grains. Prolonged heat treat-
ments at temperatures exceeding 1850 °C in N2 overpressure environments 
or prolonged heat treatment under HIP'ing conditions with glass-
encapsulated samples are primarily the techniques used to achieve grain 
growth in silicon nitride ceramics. The data given in this paper suggests that 
seeding the starting powder with a small amounts of well dispersed ,B-SisN4 
whiskers and sintering with conventional techniques yields the desirable 
microstructure (a few large, highly elongated grains surrounded by smaller 
grains) and high toughness without the prolonged heat treatments. 
We have only used this seeding technique with cold pressed compacts that 
were sintered in a N 2 overpressure furnace. However, it should also work with 
any silicon nitride which relies on the presence of a liquid phase for densifica-
tion and grain growth. Silicon nitride ceramics made by colloidal techniques, 
injections molding, cold compaction, etc:. can be easily modified to used 
seeded compositions. These can then be hot pressed, HIP'ed with glass-
encapsulation or other encapsulation methods or sintered in N2 overpressure 
atmospheres at temperatures where liquid phase sintering occurs. 
CONCLUSIONS 
High toughness silicon nitride ceramics can be processed by seeding the start-
ing powder with small quantities of ,B-SisN4 whiskers. The ,a-whiskers grow 
faster than the surrounding grains to give a microstructure consisting of a 
few large, highly elongated grains surrounded by much smaller elongated 
grains. The large grains enhanced crack deflection, grain pullout and crack 
bridging leading to higher toughness. Future work will determine if the . 
strength and creep properties of these materials are compromised via the 
seeding approach. 
Acknowledgments 
The authors wish to thank R. Pawlik for the toughness testing presented here 
and D. Bors for the SEM analysis of fracture surfaces. 
8 
-.------ -----
_J 
References 
1. T. Nose and T. Fujii, "Evaluation of Fracture Toughness for 
Ceramic Materials by a Single-Edge-Precracked-Beam Method," J. Am. 
Ceram. Soc., 71 [5J 328-333 (1988). 
2. S.R. Choi, A. Chulya, and J .A. Salem, "Analysis of Precracking 
Parameters for Ceramic Single-Edge-Precracked-Beam Specimens," 
pp. 73-88, Fracture Mechanics of Ceramics, Vol. 10, edited by R. C. Bradt 
et al., Plenum Press, New York (1992). 
3. Stress Intensity Factors Handbook, Vol. 1, pp. 349-360, edited by 
Y. Murakami, Pergamon Press, New York (1987). 
4. J. Homeny, L.J. Neergaard, K.R. Karasek, J.T. Donner, and 
S.A. Bradley, "Characterization of ,a-Silicon Nitride Whiskers", J. Am. 
Ceram. Soc., 73 [1]102-105 (1990). 
5. K.R. Lai and T.Y. Tien, "Kinetics of ,B-SisN4 Grain Growth in 
SisN4 Ceramics Sintered Under High Nitrogen Pressure," J. Am. Ceram. 
Soc., 76 [1] 91-96 (1993). 
6. K.T. Faber and A.G. Evans, "Crack Deflection Processes-
1. Theory," Acta Metal!. 31 [4J 565-576 (1983). 
7. C.W. Li and J. Yamanis, "Super-Tough Silicon Nitride with 
R-Curve Behavior," Ceram. Eng., Sci. Prod. 10 [7-8] pp. 632-645 (1989). 
9 
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Processing and Testing of High Toughness Silicon Nitride Ceramics 
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Veena Tlkare, William A. Sanders, and Sung R. Choi 
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Prepared for the Pacific Rim Meeting of the American Ceramic Society sponsored by the American Ceramic Society, Honolulu, Hawaii, 
November 7-10, 1993. Veena Tikare, Case Western Reserve University; William A. Sanders, Analex Corporation, 3001 Aerospace Parkway, 
BrookPark, Ohio 44142; and Sung R. Choi, Cleveland Stale University, Qeveland, Ohio 44115. Project Manager, Stanley Levine, Materials 
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13. ABSTRACT (Maximum 200 words) 
High toughness silicon nitride ceramics were processed with the addition of small quantities of ~-Si3N4 whiskers in a 
commercially available a -Si3N4 powder. These whiskers grew preferentially during sintering resulting in large, 
elongated 13-grains, which acted to toughen the matrix by crack deflection and grain pullout. The fracture toughness 
of these samples seeded with ~-Si3N4 whiskers ranged from 8.7 to 9.5 MPaYm depending on the sintering additives. 
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Grain growth; Liquid phase sintering 16. PRICE CODE 
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Processing and testing of high toughness silicon nitride ceramics

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