Characterization of materials properties is critical for the understanding of materials behavior and performance under operating conditions. Tailoring materials properties, which are functions of the materials states, is essential for advanced product design. The need to characterize materials for a myriad of applications has spurred the development of many new methods and instruments. Unfortunately many of these characterization tools require destructive sectioning. Also many characterization techniques do not provide key information about material parameters in their operating environments. An ideal characterization tool would provide data about the material properties that are related to micro-and macrostructure without destructive sectioning. Such data can only be obtained using nondestructive-evaluation (NDE) methodologies. Therefore NDE is essential for almost any industrial product. Nondestructive evaluation has become an integral part of materials research because it enables the determination of material parameters (such as micro- and macrostructure, stress, physical properties, and defects) at nearly any point, line, surface, or volume element of interest and at nearly any state during the life of the material. Nondestructive evaluation is based on many different methods that rely on elastic waves, penetrating radiation, light, electric and magnetic fields, chemical sensing, etc. The large number of potential methods makes NDE not a single field but a synergism of many scientific and engineering disciplines. Since it would be impractical here to present all the new NDE methodologies with application to materials research, this issue of MRS Bulletin focuses exclusively on those ultrasonic techniques that are increasingly important in materials characterization.

Ultrasonic waves have been used extensively for material characterization and for sensing in material process control. The waves produce small-amplitude mechanical vibrations and, depending on the mode being used, may induce both longitudinal and shear stresses in the solid. Information on the structural properties of a substance can be obtained by measuring both the velocity and the attenuation of the ultrasonic wave. The phase velocity of the wave depends on the elastic constants and density of the body while attenuation depends on microstructure and crystalline defects.

In an isotropic solid medium, which has only two independent elastic moduli, there exist two elastic waves: the longitudinal and the shear. Three kinds of bulk elastic waves may propagate in an anisotropic solid: a quasilongitudinal and two quasitransverse waves, differing in polarization and velocity. To determine the set of elastic constants, one must measure the phase velocity in several different directions relative to the crystallographic axes.

The attenuation of an ultrasonic wave is associated with absorption of elastic waves (inelastic effect) and the scattering of elastic waves by structural inhomogeneities. Scattering may be the governing attenuation mechanism in polycrystalline, composite, and ceramic materials. As a result of scattering, elastic energy is lost by the prime ultrasonic beam in the form of a stochastically scattered field, which is gradually absorbed in the material. The latter is associated with conversion of elastic into thermal energy as a result of various inelastic effects termed internal friction.

Acoustic microscopy is useful for characterizing with high spatial resolution the elastic structure and properties of an object. A range of techniques is now available for doing this, which enables the user to select the method and instrument that is most appropriate for a particular requirement. For imaging the interior of structures such as electronic-component packaging, an acoustic microscope operating at a relatively modest frequency can provide advanced nondestructive testing. For characterizing surface coatings and layers that may be only a fraction of a micrometer thick, higher frequency quantitative techniques are needed. For a given application, three questions should be asked at the outset: (1) What depth of material do I wish to include in my inspection? (2) Do I wish to image structures and/or defects, or do I wish to characterize elastic properties? (3) What is the minimum size of a defect or inhomogeneity that I wish to resolve or characterize (at a given depth) during my inspection? Selection of the appropriate technique will depend on the answers.

As an imaging method of elastic properties and subsurface features on the microscopic scale, the scanning acoustic microscope (SAM) provides spatial resolution comparable or superior to that of optical microscopes. Nondestructive evaluation methods of defects and elastic properties on the microscopic scale were developed by using the SAM, and they have been widely applied to various fields in science and technology. One major problem in acoustic microscopy is resolution. The best resolution of SAM with water as the coupling fluid has been 240 nm at a frequency of 4.4 GHz. At a more conventional frequency of 1 GHz, the resolution is about 1 μm. Therefore the resolution of SAM is not always sufficient for examining nanoscale defects and advanced micro/nanodevices.

For materials characterization on the nanometer scale, atomic force microscopy (AFM) was developed and extended in order to observe elastic properties in force-modulation mode. In the force-modulation mode, the sample is vibrated, and the resultant cantilever-deflection vibration is measured and used to produce elasticity images of objects. The lateral force-modulation AFM can evaluate the friction force or the shear elasticity in real time. However in the force-modulation mode, it is difficult to analyze stiff objects such as metals and ceramics.

When the sample is vertically vibrated at ultrasonic frequencies much higher than the cantilever resonance frequency, the tip cannot vibrate due to the inertia of the cantilever. However by modulating the amplitude of the ultrasonic vibration, deflection vibration of the cantilever at the modulation frequency is excited due to the rectifier effect of the nonlinear force curves. Based on the tip-sample indentation during ultrasonic vibration, we developed ultrasonic force microscopy (UFM) for contact elasticity and subsurface imaging of rigid objects using a soft cantilever with a stiffness of the order of 0.1 N/m.

Characterization of materials usually requires microscopy techniques. Some of the most useful are based on a scanning microscope and involve scanning the sample surface with a focused beam (e.g., photons, electrons, ions, etc.). For example, photoacoustic microscopy uses a laser beam, acoustic microscopy uses an ultrasound beam, and scanning electron microscopy uses an electron beam. The interaction between the material and the beam produces a signal that can be used to generate a two-dimensional image.

In scanning photoacoustic microscopy (SPAM), an intensity-modulated light beam is used to produce oscillations in the surface temperature of the sample. These oscillations induce changes in the pressure of a fluid in the photoacoustic cell as a consequence of the periodic heat conduction from the surface to the cell fluid. Subsequently many material-characterization methods have employed the same philosophy as SPAM, using a modulated beam as an excitation probe. The breadth of such techniques is due to the large number of possible excitation sources and signal detectors that have been proposed to probe the specimen response. In particular, scanning electron-acoustic microscopy (SEAM), also referred to as thermal wave microscopy, is a technique based on the utilization of a scanning electron microscope developed in 1980 and applied in recent years to material characterization. It can be considered an additional mode of scanning electron microscopy (SEM), which uses the generation of acoustic waves in the sample. Most reviews have concentrated on the application of SEAM to metals and semiconductors. However many other possibilities exist.

Photoacoustic spectroscopy is a technique in which the absorption of periodically intensity-modulated light is detected by the sound that it produces at the (acoustic) modulation frequency in a closed volume of gas or liquid in thermal contact with the material absorbing the light. The spectroscopic aspect of the technique relies on the ability to scan the wavelength of the light that stimulates the sound. Thus one can determine the absorption as a function of wavelength through the conversion of absorbed energy to heat and thence to sound. The acoustic detection is carried out synchronously with respect to the imposed intensity-modulation frequency on the light, making it possible to use narrow-band noise reduction. The existence of this technique for detecting sound generated by the absorption of light led Wong and co-workers to investigate the possibility of using the same techniques for microscopy.

A noncontact inspection system that uses lasers to both generate and detect ultrasonic waves has been developed at the Rockwell Science Center for the purpose of nondestructively inspecting metallic and composite structures. The laser-based ultrasound (LBU) system was initially developed to inspect large-area composite airframe structures, including the automated inspection of integrally stiffened and complexly curved composite structures. Furthermore the benefits offered by LBU, such as its noncontacting nature and ability to operate at elevated temperatures, have extended its usefulness to a wide range of applications—for example, in situ process monitoring where the capability to monitor a fabrication process or possibly inspect a part while still in a mold or autoclave could lead to significant cost benefits. Two manufacturing techniques that have shown promise and are presently under investigation are resin-transfer molding and compression molding. Since both of these processes typically occur at elevated temperatures, the noncontacting nature of LBU makes it an ideal candidate for an in situ sensor. More recently, measurement of the case depth in ground vehicle components has been investigated. This article describes current research that uses the LBU technique to determine surface hardness in steel-axle shafts.