Ultrasonic Surface Waves Research Articles

Reactively r.f. magnetron sputtered carbon nitride films

Carbon nitride films have been prepared on silicon (100) substrates by reactive r.f. magnetron sputtering of graphite in pure nitrogen to investigate the influence of the ion bombardment on film characteristics. Deposition was carried out at frequencies of 4.00 MHz and 13.56 MHz, respectively, for various discharge conditions in the r.f. power range between 50 W and 1000 W at process gas pressures of 0.3 Pa to 8 Pa and at substrate temperatures up to 500°C. The films were analyzed as to their bonding structure, chemical composition, density and Young's modulus using Fourier transform IR and Raman spectroscopy, electron probe microanalysis and elastic recoil detection analysis. X-ray diffraction and analysis of ultrasonic surface waves. The results indicate that both the energy flux density of the ion bombardment and the bombardment energy per condensing atom of carbon and nitrogen are fundamental internal process parameters for carbon nitride formation. The discharge frequency did not have a remarkable influence on the film features. All CN x films deposited in this way are found to be amorphous. At increasing energy flux density the bonding structure was found to change from a more graphite-like structure with mainly C≡N triple bonds to a more disordered sp 2 bonded carbon phase with predominantly C=N double bonds incorporated. The overall N/C concentration ratio of the films has a maximum of about 1.0 at the lowest energy flux density of the ion bombardment and runs through a minimum of below 0.4 with increasing energy flux density attended by increasing Young's moduli of the carbon nitride films. This is attributed to desorption of volatile species and nitrogen atoms from the surface region of the films. The density of the films increases with the bombardment energy per condensing atom of carbon and nitrogen, but remains below that of graphite.

Elastic modulus: a suitable quantity for characterization of thin films

Young's modulus is often underestimated in its relevance for thin film characterization. It represents the stiffness of the material that correlates with the atomic structure. The microstructure of thin films may distinctly deviate from that of the bulk materials and may depend on the special deposition conditions. For instance, Young's modulus of refractory carbide and nitride films depends on the stoichiometry. For carbon films the modulus can vary in the wide range from < 100 GPa up to about 1000 GPa. Boron nitride films have similar properties. Their Young's modulus may be in the range between 35 and 600 GPa. Furthermore, for hard coatings the elastic modulus often correlates with the hardness so that is difficult to measure reliably for submicrometre thick films. This suggests Young's modulus to be an excellent indicator for testing, classifying and optimizing thin films. A photoacoustic measuring technique was developed to measure Young's modulus of thin films with thickness down to 100 nm. It uses short laser pulses for generating wide-band ultrasonic surface waves. The measurement is performed on a plane surface with minimum dimensions of 5 mm × 10 mm. No special specimen preparation is required. Results obtained for diamond-like carbon and boron nitride films are presented, which show the film modulus to depend on deposition conditions such as ion energy, angle of incidence of the plasma plume, substrate temperature or atom:ion ratio. This reveals the method to be very suitable for optimizing the deposition technology. The subplantation model is applied to explain the variation in the properties of the diamondlike carbon films.

DLC and metallic nanometer multilayers deposited by laser-arc

Abstract The method of laser-induced vacuum arc (laser-arc) combines the good controllability of pulsed laser deposition with the high efficiency of a vacuum arc technique. One advantage of this technique is the essential reduction of droplets allowing the deposition of high-quality amorphous carbon films. These hydrogen-free films with very high hardness up to the superhard range exhibit excellent wear resistance and low friction. In the present paper, another advantage of the laser-arc is demonstrated, i.e. the possibility of depositing multilayer coatings down to the nanometer level of each individual layer thickness with high efficiency and high accuracy. These possibilities open new ways to overcome the principal problem of hard PVD coatings, i.e. the high internal stress which restricts the film thickness. Multilayer systems of Al–C and Ti–C with systematic variations of single layer thickness and thickness relationship were analysed by electron microscopy and Auger electron spectroscopy. The Young's moduli were measured by the non-destructive ultrasonic surface wave method (US–SAW). The alternating hard and ductile layers allowed a remarkable relaxation of the internal stresses. Furthermore, the growth of the particle induced defects (droplets) could be strongly reduced.

Laser ultrasonic chirp sources for low damage and high detectability without loss of temporal resolution

Linear frequency-modulated (chirped) acoustic signals have been generated using a pulsed laser spatially modulated by an absorption mask at the surface of a test material. By distributing the laser energy over an area, instead of focusing it to a point or line source, the peak power density of the laser source can be kept below the damage threshold of the material. The corresponding chirped ultrasonic surface wave packet produced by the source, although extended in time, is detected and processed using a matched filtering technique which compresses the packet into a pulse, thus preserving temporal resolution for accurate time-of-flight measurements. Matched filter processing of the chirped wave packet has been compared with the same processing applied to a narrow-band tone burst wave packet. Processing of the chirped signal permits easy separation of overlapped return echoes which could not be resolved when narrow-band signals were used. Finally, by compressing the energy within a chirped signal to a single detection spike, an apparent 15-fold enhancement in signal-to-noise ratio is observed.

Detection of deterioration within and beneath concrete pavements with sonic and ultrasonic surface waves : Nazarian, S. Proceedings of Nondestructive Evaluation of Civil Structures and Materials Conference, Boulder, Colorado (United States), 15–17 Oct. 1990. pp. 371–391. PB94-121720. Edited by B.A. Suprenant, S. Sture, J.L. Noland and M.P. Schuller. National Technical Information Service (1990)

Detection of deterioration within and beneath concrete pavements with sonic and ultrasonic surface waves : Nazarian, S. Proceedings of Nondestructive Evaluation of Civil Structures and Materials Conference, Boulder, Colorado (United States), 15–17 Oct. 1990. pp. 371–391. PB94-121720. Edited by B.A. Suprenant, S. Sture, J.L. Noland and M.P. Schuller. National Technical Information Service (1990)

Bond strength measurement by ultrasonic guided waves

A quasi-static model is proposed for ultrasonic guided waves interaction with imperfect interfaces. Incomplete bonding and the effect of this condition on the velocity is comprehensively discussed. The theoretical model, serves as reference for extracting ultrasonic wave parameters from experimental data. These parameters are correlated to adhesion strength. This work presents the results of a series of experiments with bonds of varying quality and thickness. Ultrasonic surface waves have been generated by a piezoelectric transducer and the detection was performed using an optical interferometer. The optical sensing of the ultrasonic surface waves is used to provide information about the ultrasonic guided waves which propagates along the boundaries between solids. Specifically, the normal component of the ultrasonic wave has been measured for providing information about the bond strength. The challenge of discriminating imperfect bonds was achieved.

Intermediate regime of light diffraction by surface ultrasonic waves in planar gyrotropic optical waveguides

The intermediate regime of optoacoustic interaction, similar to the Bragg one, with Rayleigh surface ultrasonic waves in gyrotropic planar waveguides is studied. A system of equations of bound waves is presented that describes planar optoacoustic interaction in the intermediate, Raman-Natoh, and Bragg regimes of light diffraction by surface ultrasonic waves. It is shown that the optical activity of a wavequide film favors the incident light energy pumping to diffracted light for a TE-polarization wave and decelerates this process for an incident TM-polarization wave.

Non-destructive characterization and evaluation of thin films by laser-induced ultrasonic surface waves

Ultrasonic surface waves induced by a pulsed laser act as high-frequency elastic oscillations propagating along the target materials surface. The velocity of these propagating waves is effected by the elasticity and density of the material. These surface waves have a unique property in that their depth of penetration in the material decreases with an increase in their frequency. At higher frequencies, the velocity of the surface waves are effected if the film or near-surface region composition differs from the substrate material. Generating surface waves using short laser pulses has allowed for non-destructive measurements to be performed over a wide frequency range. This in turn results in a spectrum of velocity frequencies which, when compared to known film and density characteristics using a fit program, can be used to characterize surface composition and thin films. This velocity spectrum can be used to determine Young's modulus, density, and thickness of the film. The ultrasonic thin film analyzer has been developed for use in industrial test laboratories and research institutes.

Elastic properties and microstructure of LPCVD polysilicon films

This paper investigates the influence of film thickness, high-temperature annealing and doping by ion implantation on Young's modulus and the residual stress of LPCVD polysilicon. Films with thicknesses between 100 nm and 800 nm were deposited at with a pressure of 100 mTorr. For annealing investigations, films were annealed in a nitrogen atmosphere for 2 hours at temperatures between 600 and . The implantation doses of boron and phosphorus varied between and . This corresponds to a doping concentration of and . Young's modulus and the residual stress were determined by load-deflection measurements with suspended membranes and by the use of ultrasonic surface waves. The microstructure of the film and grain size was studied by TEM analyses and texture variations were investigated by x-ray deflection. Although Young's modulus was found to be very stable, it showed a small dependence on film thickness and annealing temperature. It varied between 151 GPa and 166 GPa. The residual stress could be strongly influenced by film thickness (-420 MPa to -295 MPa), annealing temperature (-350 MPa to -20 MPa) and ion implantation (-560 MPa to +30 MPa). The as-deposited film always showed compressive stress, a pronounced texture and a grain size of around 55 nm. Strong correlations between the variations of the elastic properties and the variations in the film thickness, annealing temperature, grain size, mass density and refractive index were found. These correlations and the observed microstructure are used to develop a model for the origin of the compressive stress and for the mechanism of stress variation. A theoretical value for Young's modulus of textured polySi was calculated and corresponds well with the measured values.

Elastic modulus as a measure of diamond likeness and hardness of amorphous carbon films

The elastic modulus directly reflects the bonding strength and structure of a solid state. Usually this factor can only be modified slightly for a given material, but in amorphous carbon films large variations in Young's modulus (from less than 100 GPa up to more than 500 GPa) are found. This can be attributed mainly to the varying fraction of sp 3 and sp 2 bonds. Even for hard amorphous carbon films of diamond-like nature, the modulus achieves only 30%–50% of the value of crystalline diamond. Young's modulus was measured by a method based on the determination of the frequency-dependent propagation velocity of ultrasonic surface waves and its mathematical evaluation. The potential of this non-destructive method has been demonstrated for amorphous carbon films down to a thickness of 100 nm. In addition to reference films deposited by various methods, films prepared by pulsed arc processes were investigated systematically. The influence of the technological key parameters (ion flux, substrate temperature and hydrogen content) has been stressed. The changes in elastic modulus reflect directly the diamond likeness of the carbon-carbon network. Hence, Young's modulus represents a suitable parameter for the quantitative characterization and classification of amorphous carbon materials. Apart from certain contributions by compressive internal stresses, the hardness can be estimated to be about one-tenth of the elastic modulus.

Quality Control of Portland Cement Concrete Slabs with Wave Propagation Techniques

Coring is normally done to monitor the thickness and quality of portland cement concrete (PCC) slabs during construction. Because this procedure requires a considerable amount of time, it is done at widely spaced intervals. As a result, the most critical points, in terms of strength or thickness, are sometimes not tested. Their repeatability and extreme sensitivity to the properties of surface layer enable wave propagation techniques to be used for quality control. The main advantage of these techniques is that they are nondestructive. Fortunately, these techniques have been automated in the last few years. Two seismic devices (seismic pavement analyzer and a portable version of it called the Lunch Box) have been used extensively for quality control. With them, slabs can be tested at closely spaced points and at a fraction of the cost and time of coring. The main tests used are the impact echo for determining the thickness of the slab, the ultrasonic body wave for determining the modulus, and the ultrasonic surface wave (an offshoot of the spectral analysis of surface waves method) also for determining the modulus. On the basis of extensive field testing on many types of base and subgrade, the techniques in general—and the two devices in particular—are suitable for many quality-control projects. It was found that the most robust method for determining the modulus is the ultrasonic surface wave. The impact echo also works well, as long as enough contrast exists between the properties of the PCC and the underlying materials.

Dünne Schichten zerstörungsfrei prüfen

The feasible characterization of layer materials is decisive for optimizing the surface properties and for quality assurance. The module of elasticity and/or density of thin and very thin layers down to 100 nanometres can be determined quickly and reliably by means of high-frequency ultrasonic surface waves excited by short laser pulses.

Non-destructive evaluation of the mechanical behaviour of TiN-coated steels by laser-induced ultrasonic surface waves

Measuring the propagation velocity of ultrasonic surface waves has been used to determine the Young's modulus of TiN films on steel. Wide-band surface-wave pulses are generated by a pulsed nitrogen laser and detected by a piezoelectric PVDF-foil transducer. A spectrum of the surface-wave phase velocity is obtained from the Fourier transformation of the surface-wave signals. The TiN films were deposited after different presputtering times with argon ions to vary the adhesion on the steel substrate. The adhesion behaviour of the films was investigated with the scratch tester. A correlation was found between the Young's modulus of the film material and the acoustic emission activity in the scratch test, which has been shown to be a suitable measure for adhesion.

Non-destructive evaluation of the mechanical behaviour of TiN-coated steels by laser-induced ultrasonic surface waves

Non-destructive evaluation of the mechanical behaviour of TiN-coated steels by laser-induced ultrasonic surface waves

Assessment of Damage in Ceramics and Ceramic Matrix Composites Using Ultrasonic Techniques

This paper addresses the application of ultrasonic methods to damage assessment in ceramics and ceramic matrix composites. It focuses on damage caused by thermal shock and oxidation at elevated temperatures. The damage-induced changes in elastic constants and elastic anisotropy are determined by measuring the velocities of ultrasonic waves in different propagation directions within the sample. Thermal shock damage measurement is performed in ceramic samples of reaction bonded silicon nitride (RBSN) and aluminum oxide. Thermal shock treatment from different temperatures up to 1000°C is applied to produce the microcracks. Both surface and bulk ultrasonic wave methods are used to correlate the change of elastic constants to microstructural degradation and to determine the change in elastic anisotropy induced by microcrack damage. Oxidation damage is studied in silicon carbide fiber/reaction bonded silicon nitride matrix (SCS-6/RBSN) composites. The oxidation is done by exposing the samples in a flowing oxygen environment at elevated temperatures, up to 1400°C, for 100 hours. Significant changes of ultrasonic velocities were observed for composites before and after oxidation. The elastic constants of the composites were determined from the measured velocity data. The Young’s modulus in the fiber direction as obtained from ultrasonic measurements decrases significantly at 600°C but retains its original value at temperatures above 1200°C. This agrees well with the results of destructive tests by other authors. The transverse longitudinal and shear moduli obtained from ultrasonic measurements decrease continually until 1200°C. The results of this work show that the damage-induced anisotropy in both ceramics and ceramic matrix composites can be determined successfully by ultrasonic methods. This suggests the possibility of assessing damage severity using ultrasonic techniques.

Variations in Young's modulus and intrinsic stress of LPCVD-polysilicon due to high-temperature annealing

The effect of high-temperature annealing on Young's modulus E and the intrinsic stress sigma of thin films made of LPCVD-polysilicon was investigated. The films were annealed for 2 hours in a nitrogen atmosphere at temperatures between 600 degrees C and 1100 degrees C. Then Young's modulus and the intrinsic stress were determined by the membrane deflection method. An extended analytical theory for the membrane deflection was developed and the results correspond well with FEM analysis of Pan J.Y. et al. (1990 Technical Digest, IEEE Solid-State Sensor and Actuator Workshop, Hilton Head Island, SC, USA p 70). LPCVD-polysilicon was produced with a SiH4 flow rate of 70 sccm and a total pressure of 100 mTorr at 620 degrees C. The film thickness was 460 nm. For the as deposited films the method of membrane deflection yields a Young's modulus of 151+or-6 GPa and an intrinsic stress of -350+or-12 MPa. After annealing at temperatures higher than the deposition temperature the compressive stress started to decrease with increasing annealing temperature. It relaxed nearly completely after annealing at 1100 degrees C. Young's modulus seems to increase a little with increasing annealing temperature up to 162+or-8 GPa at 1100 degrees C. The values for E and sigma obtained with the membrane deflection method were compared with the values obtained by the method of ultrasonic surface waves. The method of ultrasonic surface waves yields systematically higher values for E. The discrepancy can be explained by the uncertainty of Poisson's ratio of polysilicon.

Characterization of laser hardened steels by laser induced ultrasonic surface waves

Ultrasonic surface waves are suitable for the characterization of surface hardened materials. This is shown on laser hardened turbine blades. The martensitic microstructure within the surface layer of surface hardened steels has a lower surface wave propagation velocity than the annealed or normalized substrate material. Because the propagation velocity depends on the ratio of layer thickness to wavelengthd/λ, its measurement allows the determination of the hardening depth. If the surface wave frequency is high enough, the surface wave propagates mainly within the hardened layer. A correlation of the surface wave velocity to the surface hardness has been found. Because the variation of the surface velocity in hardened steels is small, a high measurement accuracy is necessary to obtain the interesting hardening parameters with sufficient certainty. Therefore, a measuring arrangement has been developed where laser pulses, guided by optical fibers to the surface hardened structure, generate simultaneously surface wave pulses at two different positions. The two ultrasonic pulses are received by a piezoelectric transducer. The surface wave velocity is obtained from the time delay between these pulses which is determined by the cross-correlation method. To evaluate simultaneously surface waves with different penetration depths from the same signal acquisition, digital filtering has been used in connection with the cross-correlation.

In-situ ultrasonic surface wave assessment of mechanical fatigue damage accumulation in metal matrix composites

This study demonstrates that an in-situ nondestructive, ultrasonic surface wave technique can successfully detect the onset and extent of matrix cracking fatigue damage in a titanium metal matrix composite (MMC). A quasi-isotropic [0/±45/90] sSCS-6/Timetal® 21S MMC material was used for room temperature fatigue tests and the resultant matrix cracking damage was ultrasonically monitored in situ as a function of cycle count. Damage accumulation in the material was successfully correlated with decreases in ultrasonic pitch catch amplitude and verified through the use of immersion ultrasonic C-scans and metallographic techniques. Damage initiation and progression was tracked through the use of complementary nondestructive and destructive techniques. The in-situ surface wave data show that the higher the fatigue stress level, the more quickly damage occurs; conversely, the lower the stress level, the slower the damage initiation. The in-situ surface wave technique proved to be more sensitive to the accumulating damage than standard load-displacement modulus measurements. The surface wave technique also indicated a change in material properties after only one fatigue cycle. The data acquired show that a better understanding of damage initiation and accumulation can be gained using the in-situ surface wave technique in comparison to current load-displacement modulus measurements.

Ultrasonic surface waves for studying the properties of thin films

The change of the spreading velocity of ultrasonic surface waves by thin film deposition is described by an approximation of the nonlinear equations of motion. The effect of residual stress in deposited films on the spreading velocity is considered. As an example for practical applications, the velocity change of surface waves after TiN film deposition on steel bars is investigated.

Elastic modulus of diamond-like carbon films prepared by pulsed vacuum arc

Amorphous carbon films have been prepared by special pulsed vacuum arc deposition methods allowing high currents up to 1 kA and more. The Young's modulus of these films has been determined with respect to various technological parameters. For these measurements a method based on the propagation of ultrasonic surface waves has been applied which has been specially designed for the investigation of thin films below one micrometer. Large changes of the elastic modulus, depending on the technology, have been observed, contrasting with the common understanding of invariability of the elastic behaviour. Reflecting the large structural variations possible in amorphous carbon, the elastic modulus represents a suitable parameter for characterizing the carbon—carbon network. Furthermore, the elastic modulus of amorphous carbon films may be used for a first estimation of film hardness because of the strong correlation of these two quantities.