The strength and toughness of two composite and two monolithic silicon nitrides were measured from 25 to 1400 C. The monolithic and composite materials were made from similar starting powders. Both of the composite materials contained 30 vol percent silicon carbide whiskers. All measurements were made by four point flexure in surrounding air and humidity. The composite and monolithic materials exhibited similar fast fracture properties as a function of temperature

Salem, Jonathan A.

English

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NASA Technical Memorandum 102423
Strength and Toughness of Monolithic
and Composite Silicon Nitrides
Jonathan A. Salem
Lewis Research Center
Cleveland,, Ohio
January 1990
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STRENGTH AND TOUGHNESS OF MONOLITHIC AND COMPOSITE
SILICON NITRIDES*
Jonathan A. Salem
National Aeronautlcs and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
SUMMARY
The strength and toughness of two composite and two monolithic silicon
nitrldes were measured from 25 to 1400 °C. The monolithic and composite mate-
rials were made from similar starting powders. Both of the composite materi-
als contained 30 vol % silicon carbide whiskers. All measurements were made
by four-polnt flexure in surrounding alr and humidity. The composite and mono-
llthlc materials exhibited similar fast fracture properties as a function of
temperature.
INTRODUCTION
The need for more fuel efficient transportation and lower engine emls-
sions has made ceramics increasingly important structural materials. Ceramics
offer many beneficial properties, such as high-temperature strength, low
density, and low thermal conductivity. However, they exhibit low toughness
and low rellablllty.
One approach that may improve the toughness, crack growth resistance and
reliability of monolithic silicon nltride is the addltlon of whiskers or fib-
ers. Whiskers may bridge developing cracks and thereby Impart toughness,
crack growth and creep reslstance. The objective of this work was to deter-
mine properties and structural behavior of developmental s11icon carbide
whlsker reinforced silicon nitrldes. Similar monolithic materials were also
studied to provide a general base of comparison.
MATERIALS PROCESSING AND MECHANICAL PROPERTIES
Materials Processing
The materials used in this study were based on Garrett's GN-}O and a Nor-
ton monolithic silicon nltrlde. At the tlme of this study, composite versions
were not commercially available, however, limited quantities had been
processed.
Garrett Company GN-IO silicon nitride powder composition was slip cast
into 50 mm diameter, 75 mm helght billets, glass encapsulated by the ASEA I
method and HIPed to produce monolithic material. Part of the same powder
*Prepared for Oak Ridge National Laboratory's Ceramic Technology for
Advanced Heat Engines Project, Interagency Agreement DE-AIO5-870R21749, W.B.S.
Element 3.2.1.7.
IABB Autoclave Systems, Columbus, Ohlo.
batch was blended wlth 30 vol % SIC whiskers by ACMC2 and processed wlth the
sameprocedures as the monolithic. Densities of the monolithic and composite
materla]s were 3.31 and 3.27 g/cm3, respectively.
The Norton Companybased materials were processed by mixing silicon pow-
der wlth 4 wt %Y203 and then adding 30 vol % Tateho T44 SiC whiskers to a por-
tion of the powders. The powder mixtures were cast as a slurry into 50 mm
square by 6 mmthick p]ates, cold isostatically pressed, nitrided, and HIPed
by the ASEAglass encapsulation method. Densitles of the monolithic and com-
posite materials were 3.24 and 3.23 g/cm3, respectlvely.
Machining and Nondestructlve Evaluation
Strength and fracture toughness specimens were machined from billets of
Garrett material with the longitudinal axis perpendicular to the billet diame-
ter. Specimensfrom the Norton materials were machined wlth the 3 mmdimen-
sion paral]el to the hot pressing direction of the plates.
Test specimenswere inspected using radiography, optical microscopy and
ultrasonic evaluation. Specimenswere radiographed through the thickness and
width at several intensities and times to produce optimum conditions. Optical
inspection was performed with a machinists scope from lOx to 50x. Precislon
ultrasonic measurementswere performed with an automated system at 50 MHz(ref. I).
Mechanica] Testing
Four-polnt bend strength was determined in accordance with MIL Standard
1942 (ref. 2) at temperatures from 25 to 1400 °C. Betweenthree and ten speci-
menswere tested per temperature. The speclmensmeasured3 by 4 mmIn thick-
ness and width and were tested with spans of 20 and 40 mm(size B). The nature
and location of failure origins was determlned with optical microscopy or scan-
nlng electron microscopy.
Fracture toughness was determined by the chevron-notch method (ref. 3).
Becauseof slight varlatlons In the notch geometry, the stress
Intenslty factor coefflclents were determined with the stralght-through crack
assumptlon (ref. 3). Specimensmeasured3 by 6 mmin thickness and width,
with 20 and 40 mmspans. Three to six specimenswere tested per temperature.
The specimenswere tested at a stroke rate of 0.05 mm/mln. The low stroke
rate used for chevron-notch testing was required to insure stable crack
extension.
RESULTS AND DISCUSSION
Nondestructive Evaluation
The Garrett monolithic material exhibited a coarse cellular structure
that was visible to the unaided eye and in radiographs. The composite
2Advanced Composite Mater|als Corp., Greer, South Carollna.
occasionally exhlbited the samemacro-cellular structure and a banding or
chemlcal layering, flgure I, in the radial direction, slmllar to the billet
geometry (a cylinder). Thls mayhave resulted from shrinkage during the slip
casting process. Such radiographic observations have been related to density
variations (ref. 4). However, ultrasonic velocity measurements,which are sen-
sltlve to density variations, were similar for regions of radiographic dlsslml-
larity, Indlcating chemlcal, rather than density varlations. Attempts to
determine composltional differences vla energy dispersive analysis and elec-
tron mlcrophobe scans were fruitless. However, dye penetrant tests indicated
the cell boundaries to contain fine disconnected porosity.
Very little anisotropy (less than 1 percent) in composite or monolithic
billets of Garrett material could be detected with velocity measurementsin
longitudinal and transverse directions, as shown in table I. Velocity measure-
ments on samplesmachined from different billets also showedlittle difference
(table II). The velocity as a function of longitudinal position was also
determined to be similar for the center 2 cm of two billets (table III).
Radlography of the Norton material indicated the composite to be homogene-
ous. However, the monolithic material had slgnlficant density or compositional
variations on one 50 mmface of the plates (fig. 2). Acoustic microscopy was
not performed on the Norton materials. Electron microprobe scans indicated
someconcentrations of yttrium and a few regions of tungsten concentration.
No slgnlflcant edge-to-edge chemical differences were determined.
Bend Strength
A summaryof bend strength and fracture toughness test results is given
In table IV. Strength of the Garrett composite and monolithic materials were
similar at each test temperature (fig. 3).
In contrast, the Norton composite exhlblted higher strength than the cor-
respondlng monolithic at 25, 800, and 1200 °C, and similar strength at lO00
and 1400 °C (flg. 3). The low room temperature strength of the monolithlc was
attributed to the aforementioned density variations.
Fracture Toughness
Fracture toughnesses of the Garrett composlte and monolithlc materials
were comparable as a function of temperature (fig. 4). Fracture toughness,
calculated from maxlmumload and minimumstress intensity coefficient,
decreased with temperature to 1200 °C. However, at 1400 °C the observed tough-
ness Increased substantially. Evidently the combination of slow stroke rate
and high temperature resulted in creep deformation during crack extension.
For such temperatures, toughness measurementsdetermined with chevron-notches
can be muchhigher than those madewith precracked beams(ref. 5). Thus,
chevron-notches maybe inappropriate for such high temperatures.
The Norton composite and monolithic materlals exhibited similar tough-
nesses as a function of temperature; toughness Increased from a minimum at
room temperature to a maximum at lO00 °C, and decreased through 1400 °C
(fig. 4). This brittle room temperature behavior, followed by Intermediate
temperature toughening, was attributed to softenlng of glass In the grain
boundary phase. Continued glass softenlng resulted In decreasing strength and
toughness above lO00 °C.
Fractography and Microscopy
Typical microstructures are illustrated in figures 5 and 6. The whisker
distribution in the Garrett composite was somewhat heterogeneous, with the
whiskers distributed randomly in some reglons and orlented in others. Whisk-
ers, shown in figure 7, were revealed with a modified plasma etching procedure
(ref. 6). The procedure did not work well on the Norton material because of
the short whisker lengths.
TEM observations of the monolithic and composite Norton materlals indi-
cated graln-boundarles with significant crystallizatlon. The crystallized
grain boundaries were Y-rich, but the phases could not be identified. Glassy
regions remained, some of which were phase separated. These regions were low
in yttrium content, but had accumulated many impurities, e.g., K, Ca, CI, Na,
P, S, etc. thought to be from abrasion of a rubber mill seal during process-
Ing. The glassy regions also contained small, spherical H-rich particles, pre-
sumably from mllling media. Many SI3N 4 grains contained dislocations. The
composite material contained large SiC grains, which were debris added with
the SiC whiskers (Norton is taking steps to remove large chunks from whiskers).
Many of the whiskers were hollow and contained Y-rich amorphous phases and
small SI3N4 crystals. The SiC whiskers were bonded to the matrix by a glassy
phase, which explained the lack of signlflcant whisker pull-out observed on
room temperature fracture surfaces, shown In figure 8.
Fracture surfaces of both composite materials did exhibit whiskers and
their impressions. The size of exposed whlskers and impressions were typlcally
on the same order as the grains and agglomerates. However, large extensions
of fibers were occasionally observed (fig. 8). To attain toughening via whis-
ker additlons, the fibers may have to act over dlmenslons greater than the
grain and agglomerate slze.
The Garrett materials failed from porous regions, coarse grained regions,
and agglomerates (fig. 9). The failure initiating flaws in the Norton mono-
lithic were subsurface porous areas 30 to 60 _m across (fig. lO).
The Norton composite falled from SiC chunks of approximately 50 pm
length. The size of the failure orlglns were much greater than whisker
lengths, and strength improvements can only be expected If the fibers are capa-
ble of brldglng such flaws, or stabilizing cracks that extend from them.
SUMMARY OF RESULTS
The addition of 30 vol % SiC whiskers to monolithic silicon nitrides d_d
not substantially improve room or elevated temperature strength or toughness.
Sillcon carbide chunks occaslonally acted as failure origins in both composite
materials. Whiskers and remnant impressions on the order of graln size where
typIcally vlsible, however, well extended flbers were only occasionally visi-
ble. Either different whiskers, different bonding, or improved microstruc-
tures may be required to improve the fracture properties via whisker additions.
ACKNOLEDGEMENTS
The author would like to thank ORNLfor fundlng, and T. Leonhardt,
D. Stang, S. KIima, and Dr.'s E. Generazlo, S. Levlne, and J. Gyekenyesl for
assistance that madethls report possible.
REFERENCES
]. Generazlo, E.R.; Roth, D.O.; and Baaklini, G.Y.: Acoustic Imaging of Sub-
tle Porosity Variations In Ceramics. Mater. Eva]., vo]. 46, no. lO, Sept.
1988, pp. 1338-1343.
2. High Performance Ceramlcs at Ambient Temperature. MIL-STD-1942, Nov. 21,
1983.
3. Munz, D.; Bubsey, R.T.; and Shannon, J.L., Jr.: Fracture Toughness Deter-
mlnatlon of AI203 Using Four-Point-Bend Specimens with Straight-Through
and Chevron Notches. J. Am. Ceram. Soc., vol. 63, no. 5-6, May-June 1980,
pp. 300-305.
4. Generazlo, E.R.; Stang, D.B.; and Roth, D.O.: Dynamic Poroslty Variatlons
In Ceramics. NASA TM-IOI340, 1988.
5. Salem, J.A., et al.: Elevated Temperature Strength, Toughness, and Elas-
tlc Modulus of a Silicon Nltrlde Combustor. NASA TM- , to be pub-
llshed, 1990.
6. Siebein, K.N.; and Lovlngton, W.M.: Plasma Etching of SI3N 4. Microstruc-
tural Science, Vol. 16, Metallography of Advanced Materials, J.H. Cialone,
et al., eds., ASM International, Metals Park, OH, 1988, pp. 319-329.
TABLE I. - ULTRASONIC SHEARWAVE VELOCITIES IN
THE TRANSVERSE AND LONGITUDINAL DIRECTIONS
OF MOR BARS OF GARRETT MATERIALS
Billet or
specimen
C-2103
C-3103
C-11105
C-12/04
M-II01
Velocity,
cm/_s
(longitudinal)
0.628
.624
.631
.627
.601
Velocity
(transverse)
a0.626
a.624
a.632
a.628
b.601
aComposite.
bMonolithic.
TABLE II. - ULTRASONIC VELOCITIES IN GARRETT BILLETI
Billet
(composite)
C-12
C-11
C-8
C-3
C-2
Velocity,
cm/_s
1.082
1.092
1.083
1.075
1.082
Billet
(monolithic)
M-I
M-2
M-3
M-C
Velocity,
cm/_s
1.081
1.082
1. 088
1.082
TABLE III. - VARIATION OF ULTRASONIC
VELOCITY ALONG THE LENGTH OF
MONOLITHIC AND COMPOSITE
GARRETT BILLETS
Distance from
center,
cm
-I .0
-0.5
0.0
+0.5
+I .0
Velocity,
cm/_s
Monolithic
1.084
1.083
1.081
1.077
1.077
Composite
1.082
1.083
1.082
1.082
1.082
TABLEIV. - STRENGTHANDTOUGHNESS WITH STANDARD DEVIATION
Material TemPerature, °C
25 I 800 I000 1200
&
1400
Strength MPa
Garrett
Si3N4 732±61
SiC/Si3N 4 698±85
Norton
Si3N4 550±82
SiC/Si3N 4 758±66
715±52
671±45
531±67
648±95
679±15
702±14
585±40
562±362
617±35
628±44
535±109
693±31
405±68
344±27
336±36
370±6
Toughness MPa. m I/2
Garrett
Si3N4
SiC/Si3N 4
Norton
Si3N4
SiC/Si3N 4
6.5±.3
7.1±.5
4.3±.3
4.9±.2
6.5±.4
5.6±,2
4.4±.2
5.4±I
5.7±.5
6.2±.I
6.5±.7
6.2±. 1
5.8±.4
5.7±.4
6.3±. 1
5.g±. 1
10.1±1.3
I0.7±.4
5.g±.l
5.7±1.4
7
(a)sic_ Js_N4
(b) Si3N 4_
Figure 1. - X-ray radiographs of MOR bars machined from cylindrical billets of Garret_
materials. Dark zones imply higher density or constituents with greater x-ray absorption.
ORIGINAL PAGE
BI_ACK AND W_ITE pwOTr_A, PF1
(a) SiC w/Si 3 N 4.
(b) Si,3 N 4.
Figure 2. - X-ray radiographs of MOR bars machined from plates of Norton materials.
Dark zones Imply higher density or oonstituents with greater x-ray absorption.
800.
700.
._ 600,
5O0.
400.
,Z 300.
rr
200.
100.
[] MONOLITHIC
[] COMPOSITE
25 800 1000 1200 1400 25 800 1000 1200 1400
TEMPERATURE, °C TEMPERATURE, +C
(a) Garrett. (b) Norton.
Figure 3. - Four-point bend strength of Si3N 4 and SiCw/Si3N 4 materials•
ORIGINAL PAGE
BLACK AND WHITE PHOTOgrAPH
1 [] COMPOSITE
"-E
0 I I [ I I l l I l
25 800 1000 1200 1400 2S 8OO 1000 1200 1400
TEMPERATURE, °C TEMPERATURE, °C
(a) Garrett, (b) Norton.
Figure 4, - Fracture toughness of Si 3N4 and SiOw/Si3N 4 materials.
(a)S5N4.
(b) SiCw/Si3N 4.
Figure 5. - Microstructure of Garrett materials: where darkest regions are Si3N 4, grey regions are SiC
whiskers, and light regions are tntergranular phase.
10 ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
(a) Si3N 4.
U --,Ul
10 i.Lm
(b) SiCw/Si3N 4.
Figure 6. - Microslructure of Norton materials: where darkest regions are SiaN 4, grey regions are SIC
whiskers, and light regions are interangular phase.
11
Figure 7. - Whiskers in Garrett SiCw/Si3N 4 exposed via plasma etching.
ORIGINAL PAGE
BLACK AND WH[,TE PHOTOGRAPH
12
ORIGINAL PAGE
BLACK AND WHITE F_OTO, CRApH
(a) Garrett.
2O
[
2o p.m
(b) Norton,
Figure 8. - Fracture topography of SiCw;Si3N 4 fracture toughness specimens (25 °C).
13
(c) Whiskerpulloutarounda failure originin Garrettcomposite.
Figure8. - Concluded.
ORIGINA, E PAGE
BLACK AND WHITE PHOTOGRAPH
14
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
(a) Coarse grained region (monolithic),
(b) Agglomerate (monolithic).
Figure 9. - Failure origins in the Garrett materials.
_" 15
(c) Porous region (composite).
(d) Agglomerate (composite).
Figure 9. - Concluded.
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH-
16
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
(a) Porous regions (monolithic).
(b) Porous regions (monolithic).
Figure 10. - Failure origins In the Norton materials.
17
(c) SiC chunk (composite).
(d) Agglomerate (composite).
Figure 10.- Concluded.
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPN
18
National Aeronautics and Report Documentation Page
Space Administration
1. Report No. 2. Government Accession No.
NASA TM- 102423
4. Title and Subtitle
Strength and Toughness of Monolithic and Composite Silicon Nitrides
7. Author(s)
Jonathan A. Salem
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
3. Recipient's Catalog No.
5. Report Date
January 1990
6. Performing Organization Code
8. Performing Organization Report No.
E-5188
10. Work Unit No.
505-63-IM
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared for Oak Ridge National Laboratory's Ceramic Technology for Advanced Heat Engines Project.
Interagency Agreement DE-AI05-87OR21749, W.B.S. Element 3.2.1.7.
16. Abstract
The strength and toughness of two composite and two monolithic silicon nitrides were measured from 25 to
1400 *C. The monolithic and composite materials were made from similar starting powders. Both of the
composite materials contained 30 vol % silicon carbide whiskers. All measurements were made by four-point
flexure in surrounding air and humidity. The composite and monolithic materials exhibited similar fast fracture
properties as a function of temperature.
17. Key Words (Suggested by Author(s))
Strength; Fracture toughness; Silicon nitride;
Composites; Elevated temperature
18. Distribution Statement
Unclassified - Unlimited
Subject Category 27
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages
Unclassified Unclassified 20
NASA FORM 1626 OCT 86 *For sale by the Nat ona Technical Information Service, Springfield, Virginia 22161
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Strength and toughness of monolithic and composite silicon nitrides

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