Ultrasonic Surface Acoustic Waves Research Articles

Influence of ultrasonic surface acoustic waves. on local friction studied by lateral force microscopy

We studied dynamic friction phenomena introduced by ultrasonic surface acoustic waves using a scanning force microscope in the lateral force mode and a scanning acoustic force microscope. An effect of friction reduction was found when applying surface acoustic waves to the micro-mechanical tip-sample contact. Employing standing acoustic wave fields, the wave amplitude dependent friction variation can be visualized within a microscopic area. At higher wave amplitudes, a regime was found where friction vanishes completely. This behavior is explained by the mechanical diode effect, where the tip’s rest position is shifted away from the surface in response to ultrasonic waves.

純Ｔｉ／純Ａｌ摩擦圧接継手境界部の応力・ひずみ，音速および硬さ分布に関する検討

In order to clarify the stress/strain distribution across the joint interface of friction weld between pure titanium (Ti) /pure aluminium (Al), the characteristics of the joint interface have been investigated in view of ultrasonic leaky surface acoustic (Rayleigh) wave velocity (VR) and hardness distributions across it. Especially, one of the first efforts is focused on whether a mechanical scanning acoustic microscope (SAM) can be utilized to measure the VR distribution across the interface or not. The effect of tensile/compressive strains on VR of Ti and Al substrates, the relation of crystal direction versus VR for single crystAl pure Al substrate, and VR distribution across as-welded and post-weld heat-treated Ti/Al friction joints have been investigated by using a SAM.As for Ti and Al substrates, all direction (isotropic) VR (AVR) decreased when the applied tensile strain increased in elastic deformation region, however, they increased as increasing applied strain in plastic one. Compressive strain slightly affected the change of AVR versus in both elastic and plastic deformation regions. Uni direction (anisotropic) VR (UVR) increased as increasing tensile and compressive strains for Ti substrate. While UVR of Al substrate decreased with increasing of tensile strain, it increased for compressive strain. There was clear relation between UVR and crystal direction of single crystal pure Al substrate, and the highest UVR was obtained at the direction parallel to the most close-packed crystal direction of fcc metal ([101]).In case of as-welded Ti/Al friction joint, the scatter of AVR distribution was relatively high overall the measured region, and AVR of Al substrates adjacent to the joint interface was relatively high. It was thought that this high AVR was resulted from high plastic strain generated during friction weld operation. The scatter of AVR distribution was reduced and the high AVR was not observed when the joint was post-weld heat-treated. While axial direction UVR of Al substrate adjacent to the joint interface was extremely high in as-welded condition, that of radial direction was not high. Those of Ti side were not high in as-welded joint. The VR distributions were relatively similar characteristics to hardness and calculated stress/strain distributions.

Deposition and characterisation of carbon-containing tungsten coatings prepared by reactive magnetron sputtering

Carbon-containing tungsten coatings were deposited by magnetron sputtering onto AISI 316 stainless steel substrates in Ar C 2H 2 gas mixtures. Optical emission spectroscopy was used to control the C 2H 2 gas flow. The goal of this investigation was to study the influence of the carbon content in W 1− x C x coatings (with x being in the range of 0.05 to 0.19) on the microstructure, hardness, Young's modulus, adhesion and corrosion behaviour. The carbon concentration in the coatings was evaluated by glow discharge optical emission spectroscopy (GDOES) and wavelength-dispersive X-ray analysis (WDX) and was found to be linearly dependent on the relative optical intensity of the tungsten 400.8 nm line, used as a control signal for the reactive gas concentration at the vapour source. X-ray diffraction (XRD) was used for phase- and peak shape parameter analysis, whilst the coating morphology was evaluated by scanning electron microscopy. Knoop microhardness ( H k ), Young's modulus by ultrasonic surface acoustic waves (USAW) and scratch adhesion tests were performed to evaluate the mechanical properties of the coatings. The corrosion resistance was examined by potentiodynamic electrochemical testing. It was found that all coatings consist mainly of the b.c.c. αW phase. With increasing carbon content from W 0.95C 0.05 to W 0.81C 0.19 an expansion of the αW lattice occurs progressively, associated with an increase in the X-ray peak full width at half maximum (FWHM) and a decrease in intensity. The films density, hardness and corrosion values increased with increasing carbon content up to 15 at.% C, where the hardness reached maximum values of 4000 H k0.025 . The best adhesion results were observed for W 1− x C x coatings with x ⩽ 0.08.

Observation of wavelength-dependent generation efficiency of laser-induced ultrasonic surface acoustic waves on ceramic materials

In this work, the efficiency of laser generation of surface acoustic waves (SAWs) in ceramic materials is reported to be wavelength dependent. A pulsed Nd:YAG laser operating at wavelengths of 1064 or 532 nm was used to generate SAWs on ceramic (silicon nitride and silicon carbide) and metal substrates. It was observed that 1064 nm radiation is more efficient than 532 nm radiation for SAW generation on ceramics, whereas the opposite is the case for metals. While the wavelength dependence of SAW generation efficiency in metals is due to stronger optical absorption at the shorter wavelength, the dependence for the ceramics is attributed to a longer absorption length at the longer wavelength.

Dynamic relaxation of the elastic properties of hard carbon films

The effect of enhanced atomic mobility on the growth of hard carbon films was examined. Tetrahedrally bonded amorphous carbon films were deposited by condensing energetic carbon ions using an arc-discharge deposition method. The deposition temperature varied between 50 and 400 °C. The dependence of elastic properties on deposition temperature was examined by determining the frequency-dependent propagation velocity of ultrasonic surface acoustic waves induced by a laser. A remarkable decrease in elastic coefficient was revealed above the deposition temperature of 300 °C and complete relaxation was obtained at 400 °C. This observation was analyzed by using a simple model which was in turn supported by molecular dynamics simulations. The relaxation turns out to be a thermally activated, dynamic process with an activation energy of 0.57 eV. Possible relaxation mechanisms associated with the migration of atoms or defects on a growing surface are discussed.

Structural relaxation and surface diffusion of quench-condensed hydrogen films

We have studied the structural relaxation of quench-condensed films of molecular hydrogen with ultrasonic surface acoustic waves (SAW). Upon annealing, frequency-dependent scattering effects occur indicating a growth of hydrogen crystallites of sizes around 1 μm. The dominant scattering mechanism appears to be a resonant coupling of the SAW to some elastic eigenmodes of the crystallites. During deposition at 1.4 K and subsequent annealing we measured the change of attenuation and sound velocity of the SAW and are able to extract activation energies for the underlying surface diffusion. A strong temperature dependence below 2.5 K of the resonance frequency is reported in the case of H 2- and D 2-crystallites.

Ultrasonic technique to measure stiffness coefficients of CMC and its implications on characterizing material degradation

The main objectives of this research are to characterize the fiber content of composite materials using nondestructive methods, to characterize fiber, matrix and fiber-matrix interface degradation in a quantitative manner and to predict the elastic behavior of the composite material using ultrasonic techniques. We can achieve the above objectives if we can experimentally compute the stiffness matrix that can be derived by either destructive or nondestructive methods. We focus on nondestructive methods to generate the stiffness matrix of ceramic matrix composites (CMC) using ultrasonic techniques. The use of ultrasonic waves in measurement of the dynamic elastic moduli of solids is well known (Truell et al. (1969). Ultrasonic Methods in Solid State Physics, Academic Press). If the density and elastic anisotropy of a solid are specified then the elastic moduli can be deduced from wave speed measurements of shear and longitudinal waves propagating in particular directions in the solid. The relations between wave speed and moduli follow from the theory of small amplitude elastic wave propagating in an anisotropic solid (Musgrave (1970). Crystal Acoustics, Holden-Day). In this paper, we will discuss the experiments conducted on three CMC (CAS-Nicalon) unidirectional blocks with varying fiber fractions estimated at 31, 42 and 51%, using ultrasonic longitudinal, transverse and surface acoustic wave (SAW) velocities. Techniques to improve and automate data acquisition are discussed along with the experimental results.

A STUDY OF THE GROWTH OF SMALL FATIGUE CRACKS IN A HIGH STRENGTH STEEL USING A SURFACE ACOUSTIC WAVE TECHNIQUE

Abstract— An ultrasonic surface acoustic wave technique for studying the growth behaviour of small fatigue cracks is described. The technique allows crack depth and opening stress to be monitored continuously during the course of a fatigue test. Results are given for a 1740 MPa yield strength, silicon‐modified, AISI 4340 steel tested under zero‐to‐tension cyclic loading. Good agreement is shown between acoustically determined crack depth and that measured by post‐fracture optical microscopy. The monitoring of changing crack depth‐to‐surface length ratios during tests is also demonstrated. Acoustically determined crack opening stresses were found to be about 10% higher than values determined by measurements of crack tip opening displacements by scanning electron microscopy. Effects on crack growth of two different specimen surface preparations, electropolishing and diamond paste polishing, are also reported. Growth rates in electropolished specimens were as much as an order of magnitude higher than in diamond paste polished specimens which had a shallow but significant layer of compressive residual stress.

Ultrasonic approach to the integer and fractional quantum Hall effect

The integer and fractional quantum Hall effect is investigated using the acoustoelectric effect in a high mobility two-dimensional electron system. This effect is a result of the interaction of the coherent field of an ultrasonic surface acoustic wave and the completely quantized electron system in strong magnetic fields. The mutual interaction leads to both energy and momentum transfer of the wave to the electrons and is reflected in giant quantum oscillations in the sound-induced currents and voltages in the two-dimensional system. Unlike standard magnetotransport experiments the acoustoelectric effect probes the states of the bulk of the electron system rather than those of the edges and thus is suited to gain insight into the quantum Hall effect from a different point of view.

Ferroelectric Behavior and Microstructure of Calcium-Modified Lead Titanate Ceramics

AbstractCalcium-doped lead titanate ceramics exhibiting piezoelectric anisotropy were fabricated for applications in ultrasonic transducers, infrared detectors and surface acoustic wave devices. Transmission electron microscopy and X-ray diffraction techniques were used to characterize the development of the piezoelectric anisotropy and the dependence of the piezoelectric anisotropy on microstructure. Electron diffraction patterns indicated that most of the ferroelectric domains in samples with [CaA] up to 30 mole% were 90° domains. The size of the ferroelectric domains was not related to piezoelectric anisotropy. Ferroelectric domains could not be detected above this concentration. Measurements of the temperature dependence of dielectric properties at different frequencies did not show any ferroelectric relaxor behavior.

Piezoelectricity in the Field‐Induced Ferroelectric Phase of Lead Zirconate‐Based Antiferroelectrics

The piezoelectric characteristics were investigated in the field‐induced ferroelectric phase of the antiferroelectric Pb0.99 Nb0.02[(Zr0.6Sn0.4) 1 −yTiy]0.98O3 family. The electromechanical coupling factors kp and kt were measured, and the piezoelectric anisotropy was calculated. The piezoelectricity was controlled easily by the applied electric field pulse. The large piezoelectric anisotropy of these materials in comparison to that of conventional PZT piezoelectric materials makes them very applicable for ultrasonic probes and surface acoustic wave filters.

Ultrasonic surface acoustic wave investigation of thin films of superconducting NbN

Surface acoustic waves in the 700 MHz range were launched on two LiNbO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> substrates across thin films of sputter deposited NbN 0.1 microns thick using photodeposited interdigital electrodes. The attenuation monitored as a function of temperature in the 1.2 to 20 K range exhibited a very rapid change of 1 to 2 dB at T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> . However as the temperature was lowered further the attenuation leveled off and began a much more gradual decrease which lasted for the rest of the range. The four-wire resistance of the film monitored simultaneously with a 10 μA current dropped to zero within 1 K of T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> . The data are analyzed in four different ways in order to extract information about a possible mechanism that is producing these unexpected results.