Kazushi Yamanaka

Ultrasonic force microscopy for nanometer resolution subsurface imaging

We present a novel method for nanometer resolution subsurface imaging. When a sample of atomic force microscope (AFM) is vertically vibrated at ultrasonic frequencies much higher than the cantilever resonance, the tip cannot vibrate but it is cyclically indented into the sample. By modulating the amplitude of ultrasonic vibration, subsurface features are imaged from the cantilever deflection vibration at the modulation frequency. By adding low‐frequency lateral vibration to the ultrasonic vibration, subsurface features with different shear rigidity are imaged from the torsional vibration of cantilever. Thus controlling the direction of vibration forces, we can discriminate subsurface features of different elastic properties.

Nonlinear Detection of Ultrasonic Vibrations in an Atomic Force Microscope

A new method is proposed to detect ultrasonic vibration of the samples in the Atomic Force Microscope (AFM) using nonlinearity in the tip-sample interaction force curve F(z). Small amplitude ultrasonic vibration less than 0.2 nm is detected as an average displacement of a cantilever. This Ultrasonic Force Mode (UFM) of operation is advantageous in detecting ultrasonic vibration with frequencies up to the GHz range, using an AFM cantilever with a resonant frequency below 100 kHz. It was found that a strong repulsive force is acting after an ultrasonic amplitude threshold of the is crossed, with the amplitude of this threshold depending upon the average force applied to the tip.

Ultrasonic Atomic Force Microscope with Overtone Excitation of Cantilever

We propose a novel atomic force microscope (AFM) combined with ultrasonic frequency vibration of a cantilever excited at its support. This method enables both topography and elasticity imaging of stiff samples such as metals and ceramics, without a need for bonding a transducer to the sample. When the sample surface is contacted with a tip attached to the cantilever, the cantilever vibration mode is changed according to the sample properties. It is theoretically predicted that the amplitude and resonant frequency of vibration at higher-order modes are useful parameters for elasticity evaluation of stiff samples. A preliminary experimental verification of this principle is presented using a glass-fiber-reinforced plastic sample. Clear elastic contrast was successfully obtained using a soft cantilever only when it was vibrated at MHz frequency higher-order modes.

Quantitative material characterization by ultrasonic AFM

In an atomic force microscope equipped with a micromachined cantilever tip, the cantilever vibration spectra in contact with the sample were found to be strongly dependent on the excitation power. However, if the excitation power is small enough, the resonance peak width decreases and the peak frequency increases to a certain limiting value. In this condition the tip-sample contact is kept linear, and satisfactory agreement between the measured and calculated frequency is obtained, assuming a constant contact stiffness; the agreement is further improved by taking into account the lateral stiffness. More quantitative information on the elasticity of the sample is obtained from the contact load dependence of the frequency, where contact stiffness of a non-spherical tip shape is derived from the Sneddon-Maugis formulation, and the tip shape index is estimated by an inverse analysis of the load-frequency relation. A further advantage of evaluating not only the vertical but also the lateral stiffness is demonstrated on a ground silicon wafer by simultaneous measurement of deflection and torsional vibration.

Resonance frequency and Q factor mapping by ultrasonic atomic force microscopy

We developed an improved ultrasonic atomic force microscopy (UAFM) for mapping resonance frequency and Q factor of a cantilever where the tip is in linear contact with the sample. Since the vibration amplitude at resonance is linearly proportional to the Q factor, the resonance frequency and Q factor are measured in the resonance tracking mode by scanning the sample in the constant force mode. This method enables much faster mapping of the resonance frequency and Q factor than the previous one using a network analyzer. In this letter, we describe the principle and instrumentation of the UAFM and show images of carbon-fiber-reinforced plastic composites.

Observation of Surface Cracks with Scanning Acoustic Microscope

Surface cracks introduced in soda lime glass and MgO single crystal have been examined with a 200‐ and 420‐MHz scanning acoustic microscope. A fringe pattern was observed around the cracking line on the surface which could not be observed with an optical microscope. A simplified model is proposed in which fringe formation is due to the interference between the leaky surface wave reflected by the crack and the specularly reflected wave at the surface or the leaky surface wave that did not reach the crack. By monitoring these fringes, a Hertzian crack in soda lime glass with a depth as small as 25 μm was detected. The direction of crack orientation relative to the surface normal was also obtained from the asymmetrical contrast distribution of the fringes.

Imaging of closed cracks using nonlinear response of elastic waves at subharmonic frequency

The authors constructed a novel apparatus based on subharmonic ultrasound for the accurate imaging of closed cracks. Linear and nonlinear responses not only from the tip but also from other parts of cracks were observed in fundamental and subharmonic images, which were changed with varying closure stress. The subharmonic images always gave an accurate length of partially closed cracks, in contrast to the fundamental images in which the crack length was underestimated. Significant similarities in generation and resonance phenomena of subharmonic waves, acoustic emission, and the vibration of microbubbles are discussed.

Ultrasonic Evaluation of Closed Cracks Using Subharmonic Phased Array

We developed a novel imaging method, subharmonic phased array for crack evaluation (SPACE) based on subharmonic waves and a phased array algorithm, to measure closed-crack depth in the thickness direction. This implementation of SPACE used a LiNbO3 single-crystal transmitter to generate the intense ultrasound required for subharmonic generation and a phased array sensor as a receiver for focusing using delay laws. We applied SPACE to closed fatigue and stress corrosion cracks and found that the measurement error of SPACE in measuring crack depths was approximately 1 mm, while that in the conventional method was 20 mm in an extreme case. To establish the basis of SPACE, we propose the concept of localized subharmonic resonances (LSRs) and explain SPACE images as accumulations of LSRs. As an example of LSR, the Rayleigh-mode resonance of a crack is described. The similarities and differences between subharmonic waves at closed cracks and at microbubbles are also discussed.

Observation by ultrasonic atomic force microscopy of reversible displacement of subsurface dislocations in highly oriented pyrolytic graphite

Graphite is an important material for use as a solid lubricant that works even at high temperatures. Generally, the reason for the lubricity is that carbon layers easily slide against each other due to the layered structure with weak interlayer interaction. However, the atomic nature of the interlayer interaction is still not fully understood. To improve this understanding, we applied ultrasonic atomic force microscopy to highly oriented pyrolytic graphite and observed edge dislocations accompanied by extra half-planes. Through observation of the dislocation behaviour under different loads, we found that the dislocation moved laterally by 20 nm as the load increased by 80 nN, and it returned to the original position as the load decreased. To explain this result, we propose a model for the lateral motion of the dislocation, which includes a spring and pinning point. This finding of the large lateral motion confirms the extraordinarily easy sliding between carbon layers, which is relevant to the performance as a solid lubricant. It may also be relevant to the significant material transport in graphite intercalation compounds and in carbon nanotubes.

Evaluation of Closed Cracks by Model Analysis of Subharmonic Ultrasound

Cracks in solids can be detected by ultrasound if they are open. However, their detection is not easy when they are closed with a closure stress, and thus it is a fundamental problem in ultrasonic testing. Subharmonics with half the input frequency is potentially useful in the detection and evaluation of such cracks, although quantitative analysis has not been established. In this work, we develop analytical and numerical theories accounting for the crack parameters, such as closure stress and crack surface conditions, for the first time. We proved their validity by comparison with experiments on a well-defined fatigue crack in aluminum alloy, finding reasonable agreements. Based on these theories, it will be possible to estimate important parameters of partially closed cracks by fitting measured waveforms to theoretical predictions, which solves the fundamental problem in ultrasonic testing of cracks.