Summary:

Automotive valves made out of silicon nitride were investigated by various nondestructive testing techniques. Fluorescent dye penetrant testing was used to detect surface defects. High frequency ultrasound (80 MHz) was employed to test the valve head and valve shaft regions for volume defects. Defects with dimensions down to 50 µm could be detected in the shaft. Leaky Rayleigh waves were used to find surface defects in the valve seat and at the surface of the shaft. Digital radiography was used to test the head region in a partly automated way. X-ray computed tomography in its 2D and 3D implementations could detect defects in the shaft, in the notch area and in the transition area between head and shaft, offering good spatial resolution and characterization. Resonant testing is a very fast testing technique, but probably not able to detect very small single defects.

Table of contents

• Introduction
• Dye penetrant testing
• High-frequency ultrasound testing
• X-ray testing
• Resonant testing
• Conclusion
• References

Introduction

Fig. 1: Automotive valve made out of silicon nitride

Ceramic components exhibit an attractive combination of advantageous properties, e. g. low density, good temperature and corrosion resistance. Therefore, they have a high potential for application in combustion engines. Valves made out of sintered silicon nitride are of primary interest (Fig. 1). A simple substitution of metallic valves is planned as a first step. The ceramic valves promise fuel savings and a reduction of noise emission. For a widespread use of such valves, mass production and quality control have to be mastered reliably. Here, nondestructive testing (NDT) can yield a significant contribution. In this article, a survey on application of various NDT techniques for testing volume and surface of valves is given. The work is based on public and automotive industry funded research projects and on round-robin tests performed within the German Ceramic Society.

Dye penetrant testing

Fluorescent dye penetrant testing allows to search nearly completely the surface of ceramic valves for open cracks. Indications for defects were found at many valves taken from production, e. g. on the valve heads, on the shafts and at the notch areas. Different surface finish in certain regions of the valve, in particular in the horn-like transition area between head and shaft (in the following called "transition region"), may easily cause background indications making detection of the defects more difficult. A disadvantage of dye penetrant testing is that cracks not open to the surface can not be detected.

High-frequency ultrasound testing

Fig. 2: Left side : C-scan of the head region of a Si3N4-ceramic valve showing a "natural" defect in the volume (arrow), image size 28 mm x 28 mm. Right side : C-scan of the head region of a test valve containing test defects with about 100 µm diameter (image size 32 mm x 32 mm)

For detection of critical defects with dimensions of 50-100 µm, a high-frequency ultrasound equipment for the frequency range 10-200 MHz is employed /1/. Using broad-band focusing polymer foil probes (80 MHz, Krautkrämer, Hürth), the short ultrasound pulses necessary are excited and detected. With this technique, the head region of the valves is tested for volume defects by C-scanning. Fig. 2 shows a defect in the head region of a valve which was found by this technique. It is difficult to set a time window for selection of the depth of interest because of the irregular shape of the valve head. A compromise between background signals from sound scattered at the inclined outer surfaces on one hand and unnecessarily reduced depth range on the other hand is necessary. A way out of this situation is a volume measurement by recording all backscattered signals, followed by analysis using a volume visualization system /2/. Reduction of detection sensitivity in depths beyond the focal point can be compensated by applying synthetic aperture focusing techniques /3/. As the smallest detectable defects are scattering in the Rayleigh regime, a clear statement on the size of the defect can not be easily obtained. Therefore, this point was investigated in more detail by using test valves (valve blanks). Diamond grains with known dimensions were mixed into the ceramic powder. During sintering, the diamonds were burning out partly, leaving defects of defined size /4/. For such valves it could be shown that using longitudinal waves in normal incidence, 200 µm and 100 µm defects were easily detected, and even 50 µm defects could be detected with somewhat more effort, in spite of relatively rough surfaces (Fig. 2, right image). Such defects can not be detected with higher signal amplitude when mode-converted transverse waves are used, even considering their more favorable ratio of wavelength to defect size /5/. Usually it is difficult to obtain information about the type of defect for the very small sizes /6/.

Volume testing of the shaft region is a challenge for ultrasound testing, as refraction of sound at the interfaces is very strong and anisotropic because of the high speed of sound (10.5 mm/µs longitudinal) and the low radii of the shaft. Up to now, only larger defects could be detected in the shaft.

Another application of high-frequency ultrasound is the detection of surface cracks by Rayleigh wave techniques. Using the same probes as for volume testing, the sample is insonified under the critical angle for excitation of a leaky Rayleigh wave. In spite of strong damping, the wave can be traced over a distance of a few mm. This technique is particularly applied to the valve seat surface (Fig. 3). A B-scan with the rotation angle around the long axis of the valve as the spatial coordinate is performed to test this critically loaded area of the valve. In the same way, the shaft surface can be scanned for surface defects. In contrast to dye penetrant testing, cracks not open to the surface can be detected.

Fig. 3: Left: Scheme of leaky Rayleigh wave testing of the valve seat. Right: Test result obtained on a valve. The rotation angle (360°) is the horizontal coordinate, ultrasound time-of-flight is the vertical coordinate. The surface defects appear as nearly vertical lines.

X-ray testing

Microfocus X-ray testing is particularly applied in regions, which are difficult to inspect by ultrasound because of their complicated geometry. Examples are the transition region and the notch area.

A first technique, which is used to inspect the valve head region, is digital radiography. The X-ray cone-beam, which is subject to different attenuation is first converted into visible light, then video-imaged and digitized. The raw image is evaluated by digital image processing. A typical image resolution is 512 x 512 pixels with a pitch of 10 µm. By subtraction and edge enhancement algorithms small defects are emphasized. In Fig. 4, an image with defects of 200 µm size in the head region of the test valves mentioned above is shown. A complete inspection of the plate requires to measure many of such images in an automated raster scan.

Other X-ray techniques are the 2D- and 3D-computed tomography (CT). They are particularly used for the shaft and the transition region. They also work with pixel sizes of 10 µm. A line detector or a detector array are employed /7/. Due to the large number of projections to be collected, the time consumption of these techniques is relatively high. Figs. 5 and 6 show reconstruction results from the shaft region and the transition region of valves. Other than for digital radiography, the defect position can be reconstructed in three spatial coordinates.

Fig. 4: Digital radiography showing two test defects in the valve head. Image size 5 mm x 5 mm

Fig. 5:2D-CT image of a defect in the valve shaft (10 µm pixel resolution in four planes with a spacing of 20 µm)

Fig. 6: Visualization of a pore detected by 3D-CT in the transition region between valve head and shaft. .

Compared with the other techniques described here, a good characterization of the defect is possible. Pores and metallic inclusions (the latter are often surrounded by reaction zones in ceramics) can be well distinguished. Density variations can be detected as well /8/.

Resonant testing

Mr Netzelmann's speech in Dresden about this chapter.
(in German 5 min, 300k, a streaming Real Audio file. Download the Player)

Acoustic resonant testing is a well known technique for fast quality control of components and has recently come again into the focus of interest /9,10/. Its basic principle is to induce a vibration of the component in one or several of its eigenmodes by using pulsed or periodic mechanical excitation. The resulting oscillation is analyzed. Defects in the component become visible as a change of the resonant spectrum (position of resonance lines, width and amplitude of the lines, line splitting for components with symmetries).

In the setup used for the present experiments, the valve is supported by piezoelectric transducers at two points (Fig. 7). One transducer is operated as transmitter, the other as receiver. Two approaches were followed for excitation and detection. In the first, the transmitter is excited by a sine wave and the acoustic signal transmitted through the valve is detected using a lock-in amplifier. The frequency of the sine wave is slowly sweeped, allowing to record amplitude and phase of the spectrum over a wide frequency range.

The second approach uses an excitation by a step-function. The resulting resonant signal is recorded over the full decay time using a transient digitizer with large storage depth. The signal is averaged over many repetitions of the experiment and then Fourier transformed over 50000 or more points (Fig. 8).

As expected, the result of both approaches was equivalent, but the time-domain technique turned out to be faster and more flexible. The experiments performed up to now suffered from the poor reproducibility of the mechanical coupling of the vibration to and from the component. In particular the low-frequency bending modes showed significant variations in the resonant frequencies.

Fig. 7: Setup used for resonant testing of ceramic automotive valves.

Fig. 8: Typical result from resonant testing of a valve (top: decay curve over 20 ms, bottom: resonance spectrum from 0 to 100 kHz, logarithmic amplitude scale).

The resonance lines of the valves inspected exhibit Q-values in the range of 1000 to 3000. Therefore, a high detection sensitivity can be expected. On the other hand, it is questionable, if defect sizes of 50 to 100 µm are detectable by this technique. Finite element calculations to study the influence of cracks in disk-shaped samples show, that crack lengths of about 10 % of the radius are required to cause relative frequency changes in the order of 10^-4 /11/. Such changes have to be detected with the background of tolerable variations of density, shape and mass already present in the production lot. Experimentally, by weighing a lot of ceramic valves a standard deviation of the mass of 37.4 mg was determined. If this mass is concentrated in a single point of the valve, it is represented by a pore of 2.8 mm in diameter, when one uses the density of silicon nitride.

Conclusion

Table 1 summarizes some major results obtained with the testing techniques discussed above. The testing times are referring to setups as they are realized at present in FhG-IzfP. They certainly could be reduced for testing in the production site.

The optimum technique, which would be able to detect smallest defects in all regions of the sample in a fast way and with highest sensitivity, does not exist. In some cases the numbers for detection limits are still unproved due to lack of suitable test defects.

The present investigations performed in the framework of research programs were aiming at an utmost complete testing for volume and surface defects. Such testing will not be feasible for series components due to economic limitations. From the viewpoint of NDT, an improved specification of the defect size to be detected as a function of the position in the component is necessary. This requires furthermore a close cooperation of developers in construction, production and application.

Table 1: Summary of main results obtained for ceramic valve testing
	ultrasound volume testing	ultrasound surface wave testing	resonant testing	X-ray digital radiography	X-ray computed tomography	dye penetrant testing
testing time	4 min	4 min	0,5 min	30 min	60 min	10 min
detection limit (valve head)	50-100 µm	20 µm	unknown	100 µm	150 µm	50 µm (open crack)
valve seat (surface)	-	30 µm (?)	unknown	100 µm	150 µm	50 µm (open crack)
detection limit shaft	100-200 µm (?)		cracks 1 mm (?)	100 µm	50 µm	50 µm (open crack)
defect characterization	moderate	moderate	poor	good	very good	moderate

References

1. S. Pangraz, H. Simon, R. Herzer, and W. Arnold, 'Non-destructive evaluation of engineering ceramics by high-frequency acoustic techniques', in: "Acoustical imaging", Vol. 18, ed. H. Lee and G. Wade (Plenum Press, New-York 1991), S. 189
2. W. Arnold, R. Herzer und U. Netzelmann, 'Visualisierung von Ultraschall-Volumendaten mit moderner Computergrafik', DGzfP Berichtsband 28, Jahrestagung 6.-8.5.1991, Luzern, (Deutsche Gesellschaft für zerstörungsfreie Prüfung, Berlin 1991), S. 287
3. M. Maisl, P. Kreier, W. Müller und U. Netzelmann, 'Rekonstruktion von Fehlstellen in Keramik mittels 3D-Röntgen CT und Hochfrequenz-Ultraschall - ein Vergleich', in: Zerstörungsfreie Materialprüfung, DGZfP Berichtsband 47, (DGZfP, Berlin 1995), S. 171
4. B. Caspers, J. Hennicke, H.-A. Lindner, M. Maisl, U. Netzelmann und H. Reiter, 'Zerstörungsfreie Prüfung von Maschinenbau-Komponenten aus Siliziumnitrid am Beispiel des Kfz-Ventils', Fortschrittsberichte der DKG 11, 29 (1996)
5. Y. Shi, J. Hennicke, and U. Netzelmann, 'High-frequency ultrasound detection of small defects in Si3N4 ceramic', Nondestr. Testing and Evaluation, accepted for publication
6. U. Netzelmann, H. Stolz, W. Arnold and G. Giunta, 'Defect sizing in ceramic materials by high-frequency ultrasound techniques', in: "Acoustical imaging", Vol. 20, Y. Wei, B. Gu (eds.), (Plenum Press, New-York 1993), S. 303
7. J. Buck, M. Maisl, H. Reiter, Čntwicklung eines schnellen Rekonstruktionsverfahrens für die 3D-Computertomographie', in: Zerstörungsfreie Materialprüfung, DGZfP Berichtsband 47.2, (DGZfP, Berlin 1995), S. 477
8. H. Reiter, 'Zerstörungsfreie Prüfung zur Qualitätssicherung keramischer Bauteile', Fortschrittsberichte der DKG 11, 9 (1996)
9. J. J. Schwarz, G. W. Rhodes, 'Resonance inspection for quality control', Rev. Progr. QNDE 15, D. O. Thompson and D. E. Chimenti (eds.), (Plenum Press, New-York 1996), S. 2265
10. U. Schlengermann, W. Hansen, 'Das Resonanzverfahren - die Antwort auf neue industrielle Forderungen an die Qualitätssicherung', DGZfP Jahrestagung Dresden 5.-7.5.1997, Vortrag 55
11. H. A. Lindner, J. Hennicke, B. Caspers, H. Feuer, I. Petzenhauser, 'Production-Oriented Non-Destructive Testing of Structural Ceramic Components: Vibration Analysis', cfi/Ber DKG 70, 294 (1993)

Authors

U. Netzelmann, H. Reiter, Y. Shi, J. Wang und M. Maisl
Y. Shi is guest scientist from the Beijing Institute of Aeronautical Materials, Beijing, P. R. China, J. Wang is guest scientist from the Jiao Tong University, Shanghai, P. R. China. The other authors are principal investigators at
Fraunhofer-Institute for Nondestructive Testing (FhG-IZFP),
University Bldg 37, D-66123 Saarbrücken
Phone: +49 681 302 3873
Fax: +49 681 39580
Email: NETZELMANN@izfp.fhg.de
Homepage http://mm.fhg.de/depts/izfp-e.html

The paper was presented on the DGZfP annual NDT confernce in Dresden in May '97
See Mr. Netzelmann during his presentation.