Ultrasonic Evaluation of High-Density Silicon Carbide Ceramics

Ultrasonic Evaluation of High-Density Silicon CarbideCeramics

Raymond Brennan,* Richard Haber, and Dale Niesz

Department of Materials Science and Engineering, Rutgers University, Piscataway, New Jersey08854-8065

James McCauley

US Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005-5066

Nondestructive ultrasound testing has been evaluated as a technique for analyzing isolated bulk defects and microstruc-tural inhomogeneities in silicon carbide (SiC). Three SiC samples of varying thickness, two of which were fabricated by hotpressing and a third that was fabricated by chemical vapor deposition (CVD), were characterized using pulse–echo ultrasoundcharacterization at a frequency of 75 MHz. Point analysis techniques were utilized to measure variations in time-of-flight(TOF), or ultrasound travel time through each sample, for calculation of regional differences in material velocity and elasticproperties. C-scan imaging was used to evaluate differences in both TOF and reflected signal amplitude over the area of eachsample. Area-under-the-curve (AUTC) and full-width at half-maximum (FWHM) data were obtained from normalized his-tograms to establish trends for direct sample comparison. It was determined that lower AUTC and FWHM values correlated tohigher density samples with fewer inhomogeneities. However, the histogram tail area and distribution were also importantfeatures, providing information about specific inhomogeneities and their distributions.

Introduction

Nondestructive evaluation techniques such asultrasound testing have been commonly used in themedical field for noninvasive imaging of internal fea-tures. Ultrasound C-scan images are collected by utiliz-ing ultrasonic transducers, which transmit and receive

acoustic waves, typically at low frequencies between200 kHz and 10 MHz, for evaluating millimeter-sizeand larger features. Acoustic impedance (Z) is a mate-rial property defined as the product of its density (r)and acoustic velocity (c). Variations in acoustic imped-ance between the bulk and any inhomogeneities causereflection of the waves, with large mismatches leading tosignificant acoustic changes.1 As the impedance ratio oftwo dissimilar materials increases, the amount of soundcoupled through the interface decreases.1 The reflectionscaused by acoustic impedance mismatch occur at acous-tic boundaries and can aid in the detection of materialinhomogeneities such as pores and inclusions. Reflected

Int. J. Appl. Ceram. Technol., 5 [2] 210–218 (2008)DOI:10.1111/j.1744-7402.2008.02208.x

Ceramic Product Development and Commercialization

This work was financially supported by the U.S. Army Research Laboratory’s Material

Center of Excellence—Lightweight Materials for Vehicle Protection Program, Cooperative

Agreement Number W911NF-06-2-0007 and the Ceramic and Composite Materials Pro-

gram, an NSF I/UCRC Program, Agreement Number EEC-0436504.

*[email protected]

r 2008 The American Ceramic Society

signals are received either by the same or a second ul-trasonic transducer, amplified, and converted into elec-trical pulses, which can be plotted as a function ofvoltage versus time for further analysis. As opposed tomost medical field applications in which millimeter-sizeand larger features are of interest, micrometer-scale fea-tures in ceramic materials can be detected and resolvedby utilizing higher frequency ultrasound transducers.While studies have been performed using ultrasoundtesting to identify variations in time-of-flight (TOF),material velocity, and material attenuation in ceramicmaterials,2–8 this study uses similar techniques forhigh-frequency evaluation and unique quantitativeanalysis of high-performance silicon carbide (SiC) struc-tural ceramics.

Ultrasound Testing

Three SiC samples (A, B, and C) were evaluatedusing ultrasonic methods. Sample A was a high-densityhot-pressed SiC armor-grade plate with a length of101.60 mm, a width of 101.60 mm, and an averagethickness of 19.07 mm. The density of the sample wasmeasured as 3.22570.005 g/cm3 using Archimedes’method, which was comparable to the density of ahot-pressed SiC sample of 3.220 g/cm3 found in theliterature.9 Sample B was a high-density hot-pressed SiCarmor plate with a length of 101.70 mm, a width of101.70 mm, an average thickness of 50.92 mm, and adensity of 3.22470.005 g/cm3. Sample C was a SiC

sample fabricated by chemical vapor deposition (CVD)with a diameter of 177.80 mm, an average thickness of4.05 mm, and a density of 3.19770.005 g/cm3.

Two types of ultrasound testing were utilized toexamine the three SiC samples. The first method waspoint analysis in which longitudinal and shear trans-ducers were placed in direct contact with each samplefor determining material velocity and elastic propertydifferences over specified regions. For this purpose, lon-gitudinal contact 50 MHz (VSP-50)10 and shear contact25 MHz (SFD18-25) transducers10 were placed at ninedifferent locations to cover a wide area for each sample.Figure 1 provides examples of point locations that weremeasured for specimens A and B (Fig. 1a) and specimenC (Fig. 1b). Reflected ultrasound signals from the topand bottom surface were obtained on a voltage (x-axis)versus time (y-axis) plot in the form of an amplitudescan, or A-scan, as shown in Fig. 2. The second methodof ultrasound testing was C-scan imaging. The trans-ducer in this case was a longitudinal immersion 75 MHz(MDS-75) transducer,10 which was mounted to an x–yscanning frame for mechanical collection of data pointsby rastering over selected regions. Images of the changesin TOF over each sample area were collected by gating,or selecting, both the bottom surface- and the top sur-face-reflected signals and determining the difference be-tween them to obtain the transit time, or TOF, inmicroseconds (ms). Images of the changes in reflectedsignal amplitude were collected by gating the bottomsurface-reflected signal and measuring the peak intensityin milliVolts at each point to identify signal loss, or

1

2

4 5 6

7 8

9

25 mm

12 3

4 6

7 9

10

12

11

25 mm

6

3

5

8

(a) (b)

Fig. 1. Positions for point analysis of (a) samples A and B and (b) sample C.

www.ceramics.org/ACT Ultrasonic Evaluation of High-Density SiC Ceramics 211

attenuation, over each sample. As opposed to pointanalysis in which the data were collected for only ninedifferent points, C-scan imaging was used to collect databetween approximately 200,000 and 250,000 datapoints from a 4 in.� 4 in. sample. The receiver gainwas optimized when conducting C-scan imaging to en-sure a sufficient signal-to-noise ratio for resolving thebottom surface signal of each SiC sample. This was es-pecially important for thicker specimens like sample B,which was expected to encounter a higher degree of at-tenuation due to a longer travel path through the sam-ple. A receiver gain of 30 dB was utilized to maintainsufficient signal intensity for resolving both the top- andbottom-reflected signals for all three samples.

Results and Discussion

Point Analysis

The SiC samples were first tested using 50 MHzlongitudinal and 25 MHz shear contact transducers toconduct point analysis at nine locations specified inFig. 1a and b. After designating the point positions and

measuring thickness values, A-scans were collected asshown in Fig. 2 for sample A. Longitudinal TOF valueswere measured as the difference between the top andbottom surface-reflected signals in microseconds. ShearTOF values were measured as the difference between thetop surface-reflected signal and the shear-reflected sig-nal, which was reported in the literature as approxi-mately 1.5–1.7 times the longitudinal TOF, alsomeasured in microseconds.7 The data collected for sam-ples A, B, and C can be found in Table I, with averagevalues over the original nine points calculated for eachproperty.

Sample A showed consistent values over all ninepositions, as the thickness was measured as 19.07 mm,the longitudinal TOF was measured between 3.096 and3.105 ms, and the shear TOF was measured between4.919 and 4.927 ms. With average longitudinal andshear TOF values of 3.103 and 4.923 ms, the averagelongitudinal and shear velocities were calculated as12,290 and 7750 m/s, respectively. Based on the sampledensity measured by Archimedes’ method, the average Zvalue was calculated as 39.6070.06� 105 g/cm2 and theaverage n was 0.17070.007. The elastic property average

Fig. 2. Amplitude scan of top- and bottom-surface reflections from sample A.

Table I. Average Point Analysis Measurements and Values for Samples A, B, and C

q (g/cm3)t

(mm)TOFl

(ls)TOFs

(ls)cl

(m/s)cs

(m/s)Z (� 105

g/cm2s) nE

(GPa)G

(GPa)K

(GPa)

SD 70.005 70.01 70.020 70.040 718 76 70.06 70.007 71 70.3 71A 3.224 19.07 3.103 4.923 12,290 7750 39.60 0.170 450 190 230B 3.225 50.92 8.322 13.115 12,240 7770 39.40 0.160 450 190 220C 3.197 4.05 0.663 1.081 12,210 7490 39.10 0.200 430 180 240

TOF, time-of-flight; SD, standard deviation.

212 International Journal of Applied Ceramic Technology—Brennan, et al. Vol. 5, No. 2, 2008

values were consistent at all nine points, with anE of 45071 GPa, a G of 19070.03 GPa, and aK of 23071 GPa. These measured values comparedfavorably with the reported hot-pressed SiC valuesof n5 0.17, E 5 430 GPa, G 5 180 GPa, and K 5

220 Gpa.9,11 Sample B had the highest average thick-ness of the three samples at 50.92 mm and also showedconsistent values over the nine points. The average TOFvalues of 8.322 ms for longitudinal and 13.115 ms forshear resulted in average velocity values of 12,240 m/sfor longitudinal and 7770 m/s for shear. The longitu-dinal velocity was slightly lower than that for sample A,which may have been attributed to the fact that sampleB was nearly 2.7 times thicker, creating a longer ultra-sonic path through the sample. The elastic properties ofsample B, which were 45071, 19070.03, and22071 GPa for E, G, and K, respectively, were com-parable to those of sample A, both of which were hot-pressed materials. Sample C was the only SiC samplemade by an alternate fabrication technique (CVD) andwas also the thinnest sample with the largest area andlowest density. The average TOF values of 0.663 ms forlongitudinal and 1.081 ms for shear resulted in velocityvalues of 12,210 m/s for longitudinal and 7490 m/s forshear. The range of material property values over thenine points was greater than that for samples A and B, asthe longitudinal velocities varied from 12,160 m/s atpositions one and three to 12,310 m/s at position nine.The elastic properties were 43071, 18070.03, and24071 GPa for E, G, and K, respectively, which wereslightly lower in E and G than for the hot-pressed sam-ples. After conducting point analysis to determine re-gional elastic property trends for the SiC armor samples,a full collection of C-scan data was obtained for imaging.

TOF C-Scan Imaging

C-scan imaging was performed using the 75 MHzlongitudinal immersion transducer to collect TOF dataover the entire area of each SiC sample. The data werecollected in addition to x and y positions at each pointand the ranges were chosen based on a representation ofapproximately 95% of the maximum TOF value in mi-croseconds for comparison purposes. The TOF C-scanimages for each of the SiC samples are shown in Fig.3a–c. The minimum, maximum, average, and standarddeviation TOF values are shown in Table II. A smallnumber of data points in the C-scan images that ap-peared dark blue in color were scattered throughouteach sample due to the presence of noise from electricalinterference. After analyzing the corresponding A-scandata at the aforementioned locations, it was confirmedthat the data represented noise and the points were re-moved from quantitative evaluation because they werenot representative of the samples.

Sample A appeared to be the most homogeneous ofthe three samples, with the lowest standard deviationTOF value of 0.004 ms. Visually, the C-scan image wasalso consistent, with no detectable variation over therange from 2.902 to 3.108 ms. The majority of SampleB also appeared to show little variation. However, a longtail region was identified on the left side of the sample,extending the curve to much higher TOF values. Whenanalyzed separately, the homogeneous region of thesample had an average TOF of 8.320 ms with a stan-dard deviation of 0.007 ms, while the higher TOF regionhad an average TOF of 8.390 ms with a standard devi-ation of 0.021 ms. This region increased the overall TOFof the sample to 8.323 ms and a standard deviation of

Fig. 3. Time-of-flight C-scan images for (a) sample A, (b) sample B, and (c) sample C.


0.020 ms. The higher TOF region further impeded thevelocity of ultrasound wave travel through the sampleand was consistent with the idea that the thicker hot-pressed sample experienced a higher degree of inhomo-geneity due to the increased difficulty of fabricating athicker fully dense sample. Sample C showed the high-est TOF standard deviation of 0.054 ms due to an in-homogeneous range of TOF values, most notably on theupper right edge of the sample in which there was a re-gion of lower TOF. The average TOF value over thesample was 0.651 ms, and the majority of the samplemaintained a consistent thickness between 4.06 and4.08 mm. However, the lower TOF region, which fellwithin the area of position one, position three, and po-sition six (Fig. 1b), was significantly thinner, with valuesbetween 3.99 and 4.03 mm. In addition to this thick-ness difference, which lowered the overall TOF datavalues, there were also distinct points of high TOFwithin the center of the sample, which were believed tobe bulk inhomogeneities. These circular regions dem-onstrated higher TOF and therefore lower material ve-locity for areas of the same thickness. These featuresmay have been the result of carbon inclusions depositedduring CVD processing because carbon has a lower ve-locity than the bulk material and is commonly found asa defect in SiC.12 Although the extended tail region insample B increased its standard deviation as comparedwith sample A, sample C showed the highest standarddeviation due to a large overall TOF difference fromthickness variations and individual features of higherTOF.

TOF C-Scan Data Analysis

Data points collected from the C-scan images wereanalyzed further to look for distribution trends in eachsample that could be used for comparison of material

integrity. Because the specimens were all different interms of thickness, the TOF values and homogeneityalso varied, with values of t B4.05 mm and TOFB0.663 ms for sample C, t B19.07 mm and TOFB3.103 ms for sample A, and t B50.92 mm andTOF B8.322 ms for sample B. For this reason, thenumber of TOF occurrences and the TOF range werenormalized, and the plot is shown in Fig. 4. The area-under-the-curve (AUTC) and full-width at half-maxi-mum (FWHM) values were calculated for further quan-titative comparison. In the normalized plot, sample Ashowed the most narrow distribution, which was ex-pected because of the homogeneous nature of theC-scan image and point analysis data. The AUTCvalue was 11.1 and the FWHM 0.2 for this sample.Sample B also showed a narrow distribution, but thehigher TOF region in the sample appeared as a tail onthe right side of the normalized histogram curve. TheAUTC value of 21.3 was higher than that for sample A

Normalized TOF Range

94 95 96 97 98 99 100

No

rmal

ized

TO

F O

ccu

rren

ces

0

20

40

60

80

100 Sample ASample BSample C

AreaA: 11.1B: 21.3C: 165.2

FWHMA: 0.2B: 0.3C: 1.6

Fig. 4. Normalized time-of-flight histograms with area-under-the-curve (AUTC) and full-width at half-maximum (FWHM)values for sample A (red), B (green), and C (blue).

Table II. Average C-Scan Image TOF and AMP Values for Samples A, B, and C

Sample Type Minimum Maximum Average Standard Deviation

A TOF 2.902 ms 3.108 ms 3.103 ms 0.004 msB TOF 7.818 ms 8.446 ms 8.323 ms 0.020 msC TOF 0.336 ms 0.664 ms 0.651 ms 0.054 msA AMP 196 mV 266 mV 227 mV 12.93 mVB AMP 34 mV 233 mV 186 mV 29.44 mVC AMP 256 mV 384 mV 346 mV 13.39 mV

TOF, time-of-flight; AMP, amplitude.


due to the additional tail region, but the FWHM, whichwas not affected by the tail, was very similar to sampleA, with a value of 0.3. Sample C showed a much broad-er distribution than the other two samples, with anAUTC value of 165.2 and a FWHM value of 1.6. Thedistribution of the CVD-fabricated sample was muchbroader than the hot-pressed samples due mostly to thelower TOF regions caused by thickness variations alongthe sample edge. This normalized histogram plot dem-onstrated the importance of several quantitative fea-tures. The tail was useful for quantifying isolatedinhomogeneities and regions that showed uncharacter-istic differences from the bulk of the sample. Separateanalysis of the tail provided a distribution representativeof the cumulative inhomogeneities within the sample.The AUTC was favorable for comparison of the histo-grams to one another because it took the entire sampleinto account. The FWHM, on the other hand, focusedon the distribution and did not take into account thefeatures of minimum or maximum TOF values repre-sented in the tail regions. This was evident in the com-parison of samples A and B, which were almost identicalin FWHM but quite different in AUTC.

Reflected Signal Amplitude C-Scan Imaging

The second C-scan imaging technique that wasused to evaluate the specimens was based on monitor-ing reflected signal amplitude changes. The intensity ofthe bottom surface-reflected signals contained informa-tion about the bulk of the sample because the soundwaves had fully passed through the sample. Any degreeof loss caused by inhomogeneities or microstructuraldifferences could be attributed to a lower reflected signal

amplitude and, therefore, a higher attenuation. The am-plitude data were collected in addition to x and y po-sitions for each point and plotted over a rangedetermined by the minimum and maximum values foreach scan. The reflected signal amplitude C-scan imagesare shown in Fig. 5a–c. The ranges were chosen basedon a representation from zero to the maximum ampli-tude value for comparison purposes. The minimum,maximum, average, and standard deviation amplitudevalues are shown in Table II. Because the samples variedin thickness, the average amplitude values for each sam-ple were significantly different. The thicker samples hada higher degree of attenuation, or loss of ultrasound en-ergy, on average due to the longer ultrasound path ofacoustic wave travel. This resulted in lower average re-flected signal amplitude values for the thicker samplesbecause they were all run under equal conditions at areceiver gain of 30 dB, with sample B B186 mV, sam-ple A B227 mV, and sample C B346 mV.

When observing the reflected signal amplitudeC-scan images in Fig. 5a–c, the same features detectedin the TOF images were identified while new featuresthat had not been previously detected were also found.The reflected signal amplitude images were more sensi-tive to isolated microstructural inhomogeneities and lesssensitive to thickness variations than the TOF images, assample C appeared to be much more homogeneous.Sample A remained the most homogeneous of the three,with the lowest standard deviation of 12.93 mV, butminor variations were evident due to slight differencesin the intensity of the bottom surface-reflected signal. Insample B, which represented the highest standard devi-ation of 29.44 mV, the area of interest was the low-amplitude region on the left side of the sample. In this

Fig. 5. Reflected signal amplitude C-scan images of (a) sample A, (b) sample B, and (c) sample C.


region, there was a high degree of loss where the am-plitude dropped to B34 mV. Compared with an aver-age of B186 mV for the overall sample, the reflectedsignal amplitude in this region dropped by B152 mV,or an equivalent attenuation loss of B44 dB. This re-gion was more pronounced in the amplitude C-scanimage due to a higher degree of signal loss. Sample Cshowed some degree of overall reflected signal ampli-tude variation, with a standard deviation of 13.39 mV,but the most noted features were the small circular re-gions of lower amplitude that appeared throughout thesample. More of these features, believed to be carboninclusions with significantly different acoustic imped-ance than the bulk SiC material, were observed in thereflected signal amplitude C-scan image. The thicknessvariation along the edge of sample C, which was a majorfactor in the TOF distribution data, was not significantin the reflected signal amplitude data. This was also anexample of why it was important to use both reflectedsignal amplitude and TOF C-scan imaging as part ofthe evaluation process. While individual features wereovershadowed by broad TOF changes in the previousimages, the reflected signal amplitude C-scans picked upminor variations from isolated inhomogeneities thatwere previously undetected. With minimal influencefrom thickness variations in the reflected signal ampli-tude scans, the standard deviation trends also changed,as shown in Table II. While sample A still showed themost homogenous amplitude distribution and loweststandard deviation of 12.93 mV, sample C was similarat 13.39 mV, and sample B had the highest standarddeviation of 29.44 mV. This showed that the TOF datawas heavily influenced by thickness variations along theedge of sample C and that the most significant bulkvariation affecting the standard deviation was evident insample B.

Reflected Signal Amplitude Data Analysis

A more quantitative evaluation of the reflected sig-nal amplitude data was achieved using reflected signalamplitude histograms in which the normalized ampli-tude occurrences were plotted on the y-axis against thenormalized amplitude range on the x-axis, as shown inFig. 6. For sample A, which had an AUTC value of 680and a FWHM value of 7, the distribution was narrowerthan for the other samples, which was consistent withthe TOF results. For sample B, there was a very similardistribution of amplitude values as compared with sam-

ple A, but again, there was a long tail representative oflower reflected signal amplitude data from the region onthe left side of the sample. The AUTC value for sampleB was 782 and the FWHM value was 7. While theAUTC difference was due in large part to the tail, theFWHM was identical to that of sample A. The distri-bution was very similar for the two hot-pressed samples,with the exception of the low-amplitude tail region.Sample C showed the highest AUTC value of 922 andthe highest FWHM value of 10. These higher valueswere the result of a significant number of regions con-taining isolated lower amplitude inhomogeneities,which were more apparent in the amplitude scansthan the TOF scans. Despite the fact that sample Bhad a higher standard deviation than sample C becauseof a broader distribution of values extending into the tailregion, the tail did not significantly increase the AUTCand did not affect the FWHM. This showed that themost significant bulk variations affecting the AUTC andFWHM were reflected signal amplitude inhomogene-ities evident in sample C.

Further Analysis of Sample B

In response to the TOF and reflected signal ampli-tude C-scan image trends observed in sample B, pointanalysis was conducted for three additional points in theregion that was detected as a low-TOF/high-amplitudearea. The point locations, designated as 10, 11, and 12,are shown in Fig. 1a. The data for these points areshown in Table III. While the same thickness valueswere again confirmed as 50.92 and 50.93 mm, the TOF

Normalized Amplitude Range

20 40 60 80 100

No

rmal

ized

Am

plit

ud

e O

ccu

rren

ces

0

20

40

60

80

100 Sample ASample BSample C

AreaA: 680B: 782C: 922

FWHMA: 7B: 7C: 10

Fig. 6. Normalized reflected signal amplitude histograms witharea-under-the-curve (AUTC) and full-width at half-maximum(FWHM) values for sample A (red), B (green), and C (blue).


values increased and the velocity values decreased atthese additional points. The TOF values increased froman average of 8.322 ms for the previously measured ninepoints to 8.364, 8.418, and 8.437 ms, while the velocityvalues decreased from an average of 12,240 m/s to12,180, 12,100, and 12,070 m/s for points 10, 11,and 12, respectively. These changes in TOF and veloc-ity led to a slight decrease in elastic properties, with theaverage bulk values dropping in n and K from 0.16 and220 GPa to 0.15 and 210 GPa for points 10, 11, and12, respectively.

Another test was conducted for analysis of both thebulk and high-TOF regions from sample B in terms ofdensity differences. For the hot-pressed materials, sam-ple A had a density of 3.22570.005 g/cm3 and sampleB had a density of 3.22470.005 g/cm3. This additionalstudy was conducted to determine the impact of thehigher TOF region in terms of altering the overall den-sity of sample B. The regions were isolated and evalu-ated based on separate TOF ranges, with the bulk regionrepresented by 8.300–8.330 ms and the high-TOF re-gion represented by 8.340–8.460 ms, as shown in Fig. 7.

The bulk region, which covered approximately 91.6%of the sample, had an average TOF value of 8.319 mswhile the high-TOF region, which covered approxi-mately 8.4%, had an average TOF value of 8.396 ms. Byusing the thickness values measured at each point, thematerial velocities were calculated as 12,240 m/s for thebulk region and 12,130 m/s for the high-TOF region.By utilizing the Z values from point analysis studies,which were 39.4070.06� 105 g/cm2 for the bulk re-gion and 39.0070.06� 105 g/cm2 for the high-TOFregion, densities were calculated as 3.2190 and3.2152 g/cm3, respectively. This corresponded to a den-sity difference of 0.118% between the two regions. Be-cause the high-TOF region covered approximately 8.4%of the sample, the overall sample density was calculatedas 3.2187 g/cm3. The TOF difference, which appearedto be quite significant in the C-scan image, related toonly a minor overall change in density. This test dem-onstrated the sensitivity of ultrasound testing for de-tecting small variations in density and also showed thatthe differences between the two regions in sample Bwere within a conceivable range to account for the den-sity differences between the two hot-pressed samples.

Summary and Conclusions

Ultrasound point analysis and C-scan imaging wereused for comparison of SiC samples of different thick-ness that were fabricated by either hot-pressing or CVD.Point analysis was conducted to establish broad regionalmaterial velocity and elastic property trends. While theelastic properties were similar for the three samples, theydecreased slightly from the higher density hot-pressedsamples to the CVD sample. TOF C-scan imaging wasperformed to increase the data set from nine in the pointanalysis method to approximately 200,000–250,000data points in order to more comprehensively studyproperty trends. TOF data were normalized and plotted

Table III. Additional Point Analysis Measurements for Sample B Region of Interest

#q

(g/cm3)t

(mm)TOFl

(ls)TOFs

(ls)cl

(m/s)cs

(m/s)Z

(� 105 g/cm2s) nE

(GPa)G

(GPa)K

(GPa)

10 3.225 50.93 8.364 13.127 12,180 7760 39.20 0.160 450 190 22011 3.225 50.92 8.418 13.146 12,100 7750 39.00 0.150 440 190 21012 3.225 50.92 8.437 13.164 12,070 7740 38.90 0.150 440 190 210Avg 3.225 50.92 8.406 13.146 12,120 7750 39.00 0.150 440 190 210

TOF, time-of-flight; Avg, average.

Fig. 7. Time-of-flight range analysis of 8.300–8.330 ms and8.340–8.460ms areas.


as histograms. Reflected signal amplitude C-scan imag-ing was also performed to obtain a dataset based onamplitude, or attenuation, variations over each samplearea. Normalized amplitude histograms were obtainedfor sample comparison. The full set of ultrasound datawas consistent in establishing sample A as the most ho-mogeneous sample, with the most narrow distributionand the lowest AUTC and FWHM values. Sample Bshowed a similar distribution over the bulk of the sam-ple, but a region in the bottom left corner was observedwith uncharacteristically higher TOF and lower ampli-tude. When point analysis was conducted within thisregion, the material velocity and elastic property valueswere found to be lower. TOF C-scan imaging of sampleC identified a nonparallel region on the edge of thesample due to inhomogeneous polishing that produceda region of lower thickness. Reflected signal amplitudeC-scan imaging identified a significant number of iso-lated circular features with higher attenuation thanthe bulk.

The TOF and reflected signal amplitude C-scanimages were both useful for demonstrating similartrends among the three SiC samples, although each in-dividual technique was successful in focusing on specificfeatures that the other could not. This was most appar-ent for sample C in which the TOF data highlightedsignificant differences due to uneven polishing that werenot a factor in the amplitude data. On the other hand,the reflected signal amplitude C-scans for sample Cshowed more frequent occurrences of isolated features,which were not detected using TOF C-scan imaging.This emphasized the importance of utilizing both re-flected signal amplitude and TOF C-scan imaging for

obtaining a full range of ultrasound data for samplecomparison.

Acknowledgments

The authors thank Dr. Mahesh Bhardwaj andThe Ultran Group for providing their assistance andtechnical support.

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Ultrasonic Evaluation of High-Density Silicon Carbide Ceramics

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