A high frequency 250 MHz A-scan system has been used for flaw detection. We have been able to detect 25-500 ~m defects of different types (C, Si, SIC, BN, Fe, WC) in a Si3N4 plate. Since it is difficult to determine the defect type and size from the amplitude of the backscattered signal , we have carried out Fourier transforms of the backscattered signal to obtain reflectivity as a function of frequency, and used that information to characterize the size and type of defect. Our ea~ly experiments have been with voids in glass and Si 3N4 and we are able to predict the size of the defects we detect

Khuri-Yakub, B. T.

Kino, Gordon S.

Tittmann, Bernard R.

English

A high frequency 250 MHz A-scan system has been used for flaw detection. We have been able to detect 25-500 ~m defects of different types (C, Si, SIC, BN, Fe, WC) in a Si3N4 plate. Since it is difficult to determine the defect type and size from the amplitude of the backscattered signal , we have carried out Fourier transforms of the backscattered signal to obtain reflectivity as a function of frequency, and used that information to characterize the size and type of defect. Our ea~ly experiments have been with voids in glass and Si 3N4 and we are able to predict the size of the defects we detect.</p

Kino, Gordon

Khuri-Yakub, B.

Tittmann, Bernard

Digital Repository @ Iowa State University (ISU)

HIGH FREQUENCY ULTRASONICS G. S. Kino, B. T. Khuri-Yakub Stanford University Stanford, California 94305 and B. R. Ti ttmann Science Center, Rockwell International Thousand Oaks, California 91360 ABSTRACT A high frequency 250 MHz A-scan system has been used for flaw detection. We have been able to detect 25-500 ~m defects of different types (C, Si, SIC, BN, Fe, WC) in a Si3N4 plate. Since it is difficult to determine the defect type and size from the amplitude of the backscattered signal , we have carried out Fourier transforms of the backscattered signal to obtain reflectivity as a function of frequency, and used that information to characterize the size and type of defect. Our ea~ly experiments have been with voids in glass and Si 3N4 and we are able to predict the size of the defects we detect. The objective of ultrasonic nondestructive evaluation of ceramics is the detection of defects in the 10-100 ~m range. The aim is to detect the defects and determine their location, size, and possibly the type of inclusions within these flaws. Prior to this program of very high frequency testing, no technique existed where such an evaluation could be made. The longitudinal wave velocity in ceramics Is typically of the order of 10,000 m/sec. So, in order to obtc~in a maximum reflection· fr'lr .. defects, we have decided to work in the frequency ra~ge of 150 MHz to 500 MHz, or with acoustic wavelengths In the range from 70-20 I'm. For this purpose, we have constructed the A-scan system shown in Fig. 1. This system operates at frequencies 5-10 times higher than those that have been used before in the NOT field. SELF• AliGNING JIG SAPPIIIR[ 8UffEA AOO Figure 1. Schematic of A-scc1n system. The system consists basically of a piezo-electric transducer on a sapphire buffer rod. This Is placed in contact with the ceramic under study. The piezoelectric transducer consists of an 8 l'rn rf sputtered zinc oxide film on a 2000 A gold film that is used as a back contact of the transducer. The top contact is formed by normal photolithographic techniques; typically, we use a top contact that is .75 mn In diameter. Such a transducer resonates at a center frequency of 250 MHz, and has a useful bandwidth of 300 MHz. We use a 1 em long saphhire buffer rod which is tapered to a diameter of 2 mm at its lower end in order to ease the problem of contacting to the ceramic. A 25 I'm gold foil is used iS the con-tacting material between the sapphire and the ceramic. Gold is soft and has an acoustic imped-ance comparable to those of the sapphire and the ceramic; so it makes a good contacting material. A self-aligning jig is used to help make good reproducible contact. The system is used in a pulsed echo mode. A 20 volt, 2 nsec pulse excites the transducer, and we look for reflected pulses from defects in the time range between the echoes corresponding to the front and back surfaces of the ceramic under study. The acoustic pulses obtained are of the order of 2 ns wide between 3 dB points, as shown in Fig. 2. Figure 2. A demonstration of the depth of resol-ution obtained by measuring the thick-ness of a 134 um glass slide. 237 The theoretical two-way insertion loss of the transducer is 26 dB, which includes 6 dB of diffrac-tion loss in the sapphire buffer rod. Typically, we measure a round trip insertion and propagation loss of the order of 28-30 dBs. We calculate that 90~of the power incident on the gold foil passes through it to the ceramic; experimentally, we measure a reflection coefficient at the fold foil of 0. 1. Typically, we measure the propagation loss in the ceramic to be of the order of 4 dB/em at 200 MHz for the fully dense hot pressed silicon nitride . A theory hal been developed to predict the propagation loss; this is in excellent agree-ment with the experimental measurements on which it has been checked. The depth resolution of the system is demon-strated, as shown in Fig. 2. We measure the thick-ness of a nominally 134 pm glass sl ide. Our experimental measurement yields a thickness of 132 pm. Notice that we can easily resolve the thickness of the gold foil, which is 25 Pm thick. We can then determine the location of a defect to an accuracy of a couple of microns. In order to test for the sensitivity of the system, we have looked for defects in the seeded sample shown in Fig. 3. This s13114 plate had six different types of supposedly spheric.al in-clusions (Si, SiC, Fe, WC, BN, and C) of four different sizes (500 pm, 250 pm, 125 pm, 25 pm). We have been able to detect all the defects in the sample with ease, even SiC that has material con-stants very close to those of Si N . Typical reflections from two different inclusions are shown in Fig. 4. Notice that a 180 degree phase shift of the reflected pulse indicates whether the inclusion has a higher or lower acoustic impedance than the host material. We haYe also measured the amplitude of the echoes coming back from these defects and compared them to the echo from the back side of the ceramic. Thest measure-ments agree well with a simple theory which pre-dicts that the reflected power is proportional to the square of the diameter of the defect, at least for the 125 pm, 250 ~m. and 500 pm defects , but gives larger scattering than we would expect from the smallest defects (25 pm) as seen in Tabl e I. This is due to the fact that the defect aaterials were actually sprinkled into the sample. So, we could have been irradiating more than one 25 pm defect. WC Fe 8N SiC Sl C .•..••........ .... .•..•...•... ...... ..••.•............ ... N o; 2!10,. ::; ... ... ... 0 1251' ~ i 0 z Figure 3. Schematic of Si3N4 seeded plate. 238 {a) (b) Figure 4. (a) The signal obtained from a 125 ~m C defect; (b) the signal obtained from a 25 ~·we defect. Table I. Experimental and Theoretical Scattering from Boron Nitride Defects in Silicon Nitride Diameter 500 Pm 250 Pm 125 Pm 25 Pm Experillenta 1 Return Signa 1 Amplitude -18.4 dS -26.7 dB -30.8 dB -36.0 dB Theoretical Return Signal Amplitude 2 -1g.8 + 10 log10 7 2 -25.8 + 10 log10 72 -31.8 + 10 log10 72 -45.8 + 10 log10 7 In addition. the value of the reflected power from the defects, compared to that from the back side of the ceramic, was higher than expected. The discrepancy between theory and experiment made it clear that it is Y!TY difficult to use the echo from the back side of the ceramic as a reference signal. This is because misalignment that results in a 6 Pm ti l t across the transducer at 200 MHz results in large changes of the signal level coming from the back of the ceramic. We demonstra-ted this point by looking at the echoes coming back from hemispherical voids polished in the back side 01 a ceramic piece. Two hemispherical voids 200 pm and 400 ~m in diaNeter gave echoes differing by 6 dBs, an excellent agreement with theory. The alignment problem becomes non-critical as the reflecting surface is spherical. In order to tackle the problem of recognizing the type of inclusion within the defect, we turn to the frequency domain and look at the response of the defect as a function of frequency. The idea here is that different defects have different signatures, as seen in the theoretical calculations of Fig. 5. Notice the difference responses of the void and the we inclusion in Si3N4. Our first prob 1 em was to be ib 1 e to read the nanosecond pul ses involved into a digital computer to carry out the Fast Fourier Transformation (FFT) required. This was done using a sampling oscilloscope to give a slowed down version of the pulse in the set up shown in Fig. 6. A typical FFT of the zinc oxide transducer response is shown in Fig. 7. The useful bandwidth of the transducer is 300 MHz. The wide bandwidth and the extremely symmetric trans-ducer response is a result of the good acoustic match of the piezoelectric ZnO transducer into sapphire. In order to obtain the frequency response of a scatterer, we divide the FFT of the pulse echo obtained from the flaws by that of a transducer, and thus eliminate the effect of the response of the transducer. To check our exper-iaental procedures, we first carried out such an analysis on the reflection fraa a gold foil; the result is shown in Fig. 8. This result agrees very well with theory. By this technique , we measure the frequency minima, which yield a gold foil thickness of 22 ~m for a nominally 25 ~m foil; the decrease in the thickness of the foil is due to the pressure applied to it. We have also carried out Fourier analysis on some unknown defects in a SiC sample. Typical results are shown in Fig. 9. We cannot, as yet, infer the type of inclusion that we are looking at , because of incomplete theoretical information at the present time. However, we know that different defects have different signatures. d8 ~r------------------------------, •1• ka Figure 5. Theoretical backscattered spectra of a spherical void and • we inclusion in Si3N4. Figure 6. Schematic of setup used to digitize high frequency pulses. 239 :1 :::> "' .. ... w ... "' "' w t ~ I ... 0 w N ..J .. liE "' 0 z Figure 7. "' 2 w ~ f:s :::> oa: .., .. I NU =-~ ;., '"' 100 500 I MHZ Power spectrum of Zn0/AI1203 high fre-quency transducer. ~ 0~o----~.o~o~--~2~oo=---~so~o~--~4~00~--~5oo I MHZ Figure 8. Measured reflection power spectrum of a 25 ~m Au foil . "' 2 w ~ G. liE :::> oa: wo-1 Nu ~~ ~ (I)~ OL__~•·z o--loo zoo 300 400 sao I MHZ 500 I MHZ Figure 9. Experimental backscattered pawer spec-tra for unknown inclusions ia SIC.  We have shown that, using high frequency ultrasonics, it is possible to detect defects in the 10-100 ~m range in ceramics. The location of the defect is determined with high accuracy. We are presently working on several techniques to speed up and characterize the observation defects. We are making a 16 element 8-scan system that is designed to speed the process of defect detection. We are establishing a catalog of defect responses. We measure the defect response , then, by sectioning, we characterize the defect; thus associating it with its response. We are also carrying out simplified scattering to see whether or not more information can be obtained from the response of the defect in the time domain or the frequency domain. Acknowledgement This research was sponsored by the Center for Advanced NDE operated by the Science Center, Rock-well International, for the Advanced Research Projects Agency and the Air Force Materials Lab-oratory under Contract F33615-74-C-5180. References 1. A. G. Evans, G. S. Kino, B. T. Khuri-Yakub, and B. R. Tittman, "Failure Prediction in Structural Ceramics," Materials Evaluation, 35, No. 4, pp. 85-95, April lg77. 240 

High Frequency Ultrasonics

Khuri-Yakub, B

https://dr.lib.iastate.edu/handle/20.500.12876/15927

High Frequency Ultrasonics

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