Radiography and several acoustic and thermoacoustic microscopy techniques are investigated for application to structural ceramics for advanced heat engines. A comparison is made of the results obtained from the use of scanning acoustic microscopy (SAM), scanning laser acoustic microscopy (SLAM), and thermoacoustic microscopy (TAM). These techniques are evaluated on research samples of green and sintered monolithic silicon nitrides and silicon carbides in the form of modulus-of-rupture (MOR) bars containing deliberately introduced flaws. Strengths and limitations of the techniques are described, with the emphasis being on statistics of detectability of flaws that constitute potential fracture origins. Further, it is shown that radiographic evaluation and guidance helped develop uniform high-density Si3N4 MOR bars with improved four-point flexural strength (875, 544, and 462 MPa at room temperature, 1200 C, 1370 C, respectively) and reduced scatter in bend strength

Baaklini, George Y.

Klima, Stanley J.

Sanders, William A.

English

NASA Technical Reports Server

N88-22416
NONDESTRUCTIVE EVALUATION OF SINTERED CERAMICS
George Y. Baaklini, Stanley J. Klima,
and William A. Sanders*
Structural Integrity Branch
NASA Lewis Research Center
ABSTRACT
Radiography and several acoustic and thermoacoustic microscopy techniques are
investigated for application to structural ceramics for advanced heat engines.
A comparison is made of the results obtained from the use of scanning acoustic
microscopy (SAM), scanning laser acoustic microscopy (SLAM), and thermoacoustic
microscopy (TAM). These techniques are evaluated on research samples of green
and sintered monolithic silicon nitrides and silicon carbides in the form of
modulus-of-rupture (MOR) bars containing deliberately introduced flaws.
Strengths and limitations of the techniques are described, with the emphasis
being on statistics of detectability of flaws that constitute potential fracture
origins. Further, it is shown that radiographic evaluation and guidance helped
develop uniform high-density Si3N 4 MOR bars with improved four-point flexural
strength (875, 544, and 462 MPa at room temperature, 1200 °C, and 1370 °C,
respectively) and reduced scatter in bend strength.
*Materials Division.
3-123
https://ntrs.nasa.gov/search.jsp?R=19880013032 2020-03-20T06:31:28+00:00Z
SAM uses a single transducer to generate and receive ultrasonic energy 
(Nikoonahad, 1 9 8 4 ) .  Good resolution and sensitivity are achieved by focusing 
moderately high-frequency ultrasonic energy (30 to 100 MHz) on a small spot, 
raster-scanning the lens with respect to the sample, and then time-gate sam- 
pling the reflected ultrasonic pulse amplitude. 
acoustic impedance mismatch within the sample, or that produce a change in 
acoustic impedance at the specimen surface, can cause detectable variations in 
the digitized and stored spatial map of reflected signal amplitude. Images of 
microflaws are generally sharp. Depth location of flaws is easy to determine. 
Access is needed to only one side of the test sample. Further, SAM can be 
adapted to handle curved surfaces. Detection and resolution of microflaws are 
affected by (1) surface roughness ( 2 )  ultrasound depth of penetration, and 
( 3 )  the aperture size of the lens. SAM images are not generally displayed in 
real time because computer time is needed to process the entire block of data 
used to produce the image. 
Any features that produce an 
SAM IMAGE OF VOIDS IN A SILICON NITRIDE MOR BAR 
SEEDED VOIDS (WHITE SPOTS) 20 pm DIAMETER, 1 mm DEEP 
DIAMOND GROUND SURFACE, 2 pm FINISH 
ULTRASONIC REFLECTION MODE 
50 MHz TRANSDUCER, SPHERICALLY FOCUSED 
CD-88-32329 
3-1 24 
DETECTION OF VOIDS WITH ACOUSTIC MICROSCOPY
Scanning laser acoustic microscopy (SLAM) uses I00 MHz ultrasonic waves that
are transmitted through the specimen and are modulated by material surface and
internal characteristics. The relative intensity and phase of the waves are
detected by a laser beam that is raster-scanned over an area approximately
2 mm 2. Features such as cracks, voids, density variations, etc. are displayed
in real time on a video monitor at xl00. SLAM images are affected by the sur-
face roughness of the test object. Unlike SAM, SLAM has a limited capability
for handling complex shaped objects, and also requires access to opposite sides
of the sample. The plot below summarizes SLAM probability of detection (POD)
data obtained for internal voids in specimens with diamond-ground surfaces
(Roth and Baaklini, 1986). The boundaries of the bar graphs indicate the mini-
mum void sizes and maximum depths (from the laser-scanned surface) at which the
0.90/0.95 POD/confidence level was achieved. Also shown is a single data point
representing preliminary results obtained with the SAM technique. All of the
38 voids (nominal diameter 20 pm) were detected I mm below the surface of
silicon nitride samples, yielding a POD better than 0.90/0.95 (Klima et al.,
1986).
90% PROBABILITYOF DETECTIONAT 95% CONFIDENCELIMIT
VOID
DEPTH,
pm
1400
1200
1000
8OO
6OO
4OO
2O0
,:.:.:,:
...../.
.:,:,:.:
-:.:.:.:
".L'.'.
ii:i!iil
,-SAM iiiiiiii
ili!iiil
.:.:.:-:
SLAM_ !iiiiili
\ !iiiiiiill ii_i_i_i
iiNiiiiiiiiiiiiii!iiiiiiiiiiiii:
iiiiiiiiiiiiiiiiiiii!iiiiiiiiiiiii
iiiiiiS_L,CON:;;:iil
!i;iilN TRIBE ;iiii!i
1DO 2oo 500 d00
VOIDDIAMEIER,pm
SINIERED
SILICON
CARBIDE
I I
5O0 6_)
CD-88- 32330
3-125
POSTER PRESENTATION 
THERMOACOUSTIC MICROSCOPY 
Thermoacoustic microscopy (TAM) measures relative differences in surface and 
near-surface thermal properties of the material being evaluated (Rosencwaig, 
1980). 
a laser or an electron beam), focused at any point on the sample, gives rise to 
localized cyclic heating and cooling that, in turn, generates elastic waves at 
the modulation frequency. The amplitude and phase of these waves can be meas- 
ured either at another location on the specimen surface by a piezoelectric 
crystal in contact with the specimen, or in the surrounding gas medium by a non- 
contacting method using a sensitive microphone. A sensitive, high-resolution 
image representing varying thermal characteristics of the sample can be gen- 
erated point-by-point using this technique. Its applicability to engine parts 
can be limited by vacuum requirements, coating requirements, and shape com- 
plexity. By utilizing the electron beam method, TAM proved to be useful for 
detecting tight surface cracks and pits on research samples, as shown below 
(Klima et al., 1986). 
The absorption of intensity-modulated electromagnetic radiation (usually 
CD-88-32331 
3-126 
DETECTABILITY OF FRACTURE-CAUSING TYPE OF FLAWS VIA
RADIOGRAPHY AND ACOUSTIC MICROSCOPY
A comparison is made between acoustic and radiographic techniques for detecting
fracture-causing defects such as voids, cracks, and inclusions. In general,
acoustic microscopy is not applicable to green ceramics because either (I) the
sample is damaged by the presence of a liquid-coupling medium or (2) the sur-
face of the sample is damaged by the intensity of the laser or electron beam
energy sources. However, acoustic microscopy can be successfully used to char-
acterize small areas of sintered ceramic parts. Radiographic techniques are
suitable for characterizing both green and sintered ceramics.
TECHNIQUE DEFECT TYPE
SLAM CRACKS
VOIDS
INCLUSIONS
SAM CRACKS
VOIDS
INCLUSIONS
TAM VOIDS
(LASER)
INCLUSIONS
TAM CRACKS
(ELECTRON
BEAM) VOIDS
INCLUSIONS
MICROFOCUS CRACKS
X RAY
VOIDS
INCLUSIONS
GRADIENTS
CT CRACKS
X RAY
VOIDS
INCLUSIONS
GRADIENTS
RESOLUTION LIMITATIONS COMPONENT APPLiCABiLiTY
UNDEFINED
25 pm
25/,m
UNDEFINED
20 am
20/*m
25 am
25 #m
1 am WIDE
10 am
10 _m
ORIENTATION DEPENDENT
1-2%
2%
ORIENTATION DEPENDENT
<<1%
< < .5%
< <2%
SURFACE ROUGHNESS.
MATERIAL DEPENDENT.
NEAR SURFACE
SURFACE ROUGHNESS
SURFACE CHEMISTRY.
ABSOItPTIVITY.
NEAR SURFACE
SURFACE ROUGHNESS.
REQUIRES VACUUM.
REQUIRES COATING.
NEAR SURFACE
SMALL ABSORPTIVITY
DIFFERENCES
SLOW
THIN SECTION.
SIMPLE SHAPES
SMALL AREAS
SMALL AREAS
SIMPLE SHAPES.
SMALL AREAS
FILM-UNIFORM
THICKNESSES
REAL TIME-COMPLEX
GEOMETRIES. REDUCED
SENSITIVITY
COMPLEX GEOMETRIES
CD-88-32332
3-127
RADIOGRAPHIC EVALUATION OF NASA 6Y Si3N 4
In mechanical properties of Si3N 4, scatter is generally attributed to defects
and inhomogeneities occurring during powder processing and/or during fab-
rication of parts (Bowen, 1980, and Evans, 1982). Based on preliminary
x-radiographic characterization work on Si3N 4 by Klima (1986), a program was
undertaken to systematically investigate density-gradient, flexural-strength
relationships as affected by sintering and powder-processing variables for sin-
tered Si3N 4. All batches were radiographically evaluated at all stages of fab-
rication. Test bars were radiographed in the (W,L) and (T,L) modes where x rays
are transmitted through the thickness and the width of the bar, respectively.
FEEDBACK TO -
POWDER
PROCESSING
SINTERING
PROCEDURES
J SOPRESSING _ JfiST BARS
I
-_ X-RADI?RAPHY J
_TTE;I_OF
I
X-RADIOGRAPHY l]
I I iGORRE T,O  OF.
I X'RAD'O_ RAPHY I'-- D_NS',TEYTS
_.ET_UR_E_!_;_OCSFRE_FH
FRACTOGRAPHY (SEM): FLAW CLASSIFICA"
MEFALLOGRAPHY IlO_ MICRO-
J (LIGHT AND TEM) _'J STRUCTURE
e"
-_.--._-_ ANODE
CATHODE /!_
W_ iV /
/" /
(W, L) RADIOGRAPHIC IMAGE 1 cm IF.L)RADIOGRAPHIC IMAGE
CD-88-32333
3-128
COMBINED EFFECT OF MODIFIED PROCESSING/SlNTERING PROCEDURES
ON THE FLEXURAL STRENGTH OF NASA 6Y Si3N 4
The sintering variables were (i) temperature (2) nitrogen pressure (PN) (3)
time (t s) (4) spacer contact area (BN), and (5) furnace position. The powder-
processing variables were (i) grinding time tg and (2) inclusion or exclusion
of powder wet-sieving procedures. The cumulative positive effects of all the
variables on flexural strength are shown below. (For more information, see
Sanders and Baaklini (1988).) In processing from batch to batch (baseline to
28 to 29 to 31), the room-temperature strength continually increased, with an
overall improvement of 56 percent, and more than a threefold reduction in the
standard deviation. The summary of individual parameter effects on room-
temperature strength is shown in the table below. Strength improved 28 and 21
percent for 1200 and 1370 °C, respectively. All successive improvements in the
mechanical properties were guided by x-radiographic characterization.
900 I
F_XURAL 5_--
STRENGTH,
MPa . 400 --
300--
200--
1_ --
tg, h
ts, h
PN'MPa
BN
SINTER
HT. ADJ.
POWDER
WET SIEVE
ROOM TEMPERATURE 1200°C 1370°C
m
29 :
28
BASE
LINE
I17 14 108
_.I2 3. 21 3. 2_
24 lO0 lO0
l 2 2
2.5 5.0 5.0
max. rain. rain.
,/ ,4
,/
i
3l
36
3. 24I
30O
2
5.0
min.
,/
,/
_,TANDARD
DEVIA-IBASE
TION, LINE
MPa 53
l
DENSITY.
glcm3
24
I
2.5
max.
BATCH
NUMBER
129
55
J
lO0
2
5.0
rain.
,/
,/
31
33
3OO
2
5.0
min.
,,/
,/
LINEi
3O I
J
24
l
2.5
max.
m
29
43
100
2
5.0
min.
4
,/
m
31
59
i
3OO
2
5.0
rnin.
4
,,/
CD-88-32335
3-129
INDIVIDUAL PARAMETEREFFECTSON
ROOM-TEMPERATURESTRENGTH
VARIABLE
BN SPACER CONTACT
SINTERING TIME
SINTERING TEMPERATURE
SIEVING
GRINDING TIME
LEVELS
MAX--MIN
1 ---1,25 HR
1 --1.5 HR
1 --2 HR
2050--2140 °C
NO --YES
2_ --100 HR
2q _300 HR
ROOM-TEMPERATURE
STRENGTH CHANGES,
MPA
+197. +157
+129
+87
+145
-117
+60
-61, -13
+41, +54
SINTERING HEIGHT
NITROGEN PRESSURE
100 _300 HR
LOW--HIGH
2.5 _3.5 MPA
3.5 --5.0 MPA
2.5 --5.0 MPA
+102, +67, +62, +125
+24
-205
+45, ,29, +24
-181
CONCLUSIONS
EXTREMELY HELPFUL
VERY HELPFUL
VERY HELPFUL
HELPFUL
VERY HELPFUL
SLIGHTLY HELPFUL
_INCONCI_USIVE
CD- 88-32334
3-130
D+?-T,>?T.y * \r "-7 .?? 
QT j.l-,l- . '-1.  ? 
NASA 6Y SINTERED SILICON NITRIDE IMPROVED BY 
RADIOGRAPHICALLY-GUIDED PROCESSING CHANGES 
The successful use of conventional x-radiography in guiding the fabrication 
process resulted in denser and more uniform Si3N4 over the baseline materials. 
Previously dominant failure-causing voids were replaced by large grains, which 
are less detrimental to strength properties. The improved structure can be 
attributed to (1) increased powder fineness that improves sinterability and uni- 
formity (2 )  powder wet sieving that results in reduction of agglomerates and 
certain impurity particles ( 3 )  minimizing BN spacer contact that results in more 
uniform densification as a consequence of more uniform heating ( 4 )  an increase 
in sintering time from 1 to 2 hr, thus improving density and uniformity, and 
(5) raising sinter cup height to reduce the top bar to bottom bar temperature 
gradient. 
CD-88-32336 
3-131 
SUMMARY
• ACOUSTIC MICROSCOPY CAN BE USED TO CHARACTERIZE SMALL AREAS OF SINTERED
CERAMIC PARTS, BUT IT IS NOT APPLICABLE TO GREEN PARTS
• RADIOGRAPHY IS SUITABLE FOR CHARACTERIZING BOTH GREEN AND SINTERED
CERAMIC PARTS
• RADIOGRAPHY IS VERY BENEFICIALIN GUIDING POWDER PROCESSING AND SINTERING
PARAMETER CHANGES FOR IMPROVED Si3N4
--IMPROVED Si3N4 EXHIBITED HIGHER DENSITY, LOWER DENSITY GRADIENT, AND
BETTER BAR-TO-BAR UNIFORMITY
--IMPROVED Si3N4 POSSESSES LESS SCATTER IN BEND STRENGTH, IMPROVED
STRENGTH, AND LESS CRITICAL DOMINANT FLAW
CD-88-32337
3-132
REFERENCES
Bowen, H.K., 1980, "Basic ResearchNeeds on High Temperature Ceramics for
Energy Applications," Mater. Sci. Eng., Vol. 44, pp. 1-56.
Evans, A.G., 1982, "Consideration of Inhomogeneity Effects in Sintering," J.
Am. Ceram. Soc., Vol. 65, No. i0, pp. 497-501.
Klima, S.J., 1986, "NDEof AdvancedCeramics," Mater. Eval., Vol. 44, No. 5,
pp. 571-576.
Klima, S.J., Baaklini, G.Y., and Abel, P.B., 1986, "Nondestructive Evaluation
of Structural Ceramics," NASATM-88978, presented at the 24th Automotive
Technology DevelopmentContractors' Coordination Meeting, Dearborn, MI,
Oct. 27-30.
Nikoonahad, M., 1984, "Reflection Acoustic Microscopy for Industrial NDE," in
Research Techniques in Nondestructive Testing, R.S. Sharpe (ed.), Academic
Press, London, Vol. 7, pp. 217-257.
Rosencwaig, A., 1980, Photoacoustics and Photoacoustic Spectroscopy, John
Wiley and Sons, New York.
Roth, D.J., and Baaklini, G.Y., 1986, "Reliability of Scanning Laser Acoustic
Microscopy for Detecting Internal Voids in Structural Ceramics," Adv.
Ceram. Mater., Vol. I, No. 3, pp. 252-258.
Sanders, W.A., and Baaklini, G.Y., 1988, "Correlation of Processing and Sinter-
ing Variables with the Strength and Radiography of Silicon Nitride," Adv.
Ceram. Mater., Vol. 3, No. i, pp. 88-94.
3-133


Nondestructive evaluation of sintered ceramics

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