Observation of a sub-surface defect in sapphire by Rayleigh wave reflection in the scanning acoustic microscope

Abstract

Acoustic microscopy is a novel technique for nondestructive examination (NDE) that is rapidly finding increasing application in many areas of materials science. The principle of operation of the reflection scanning acoustic microscope (SAM) is well known and is described elsewhere [1,2]. Contrast in the images is a function of the elastic properties of the specimen.An important contribution to the contrast comes from the Rayleigh waves which are generated when an acoustic lens of sufficiently wide aperture is moved from the focus position towards the surface of the specimen. The Rayleigh waves propagate in the surface layers of the specimen, and re-radiate part of their energy back into the coupling medium. This is responsible, through the so-called V(z) effect [3], for the enhanced contrast seen in SAM images of cracks [4,5], grains and grain boundaries [6] and other material features.The Rayleigh waves may also be reflected from discontinuities such as cracks, edges, and similar features in the sample surface, giving rise to the characteristic fringes often seen in SAM images. In this letter we report the observation of a sub-surface feature detected in the SAM not by the production of a separate echo, or by its modification of V(z), but by its effect on the interference fringes produced by Rayleigh wave reflection.Single crystal sapphire was chosen for this study because it is transparent, therefore enabling subsurface features to be detected optically; it is acoustically and elastically well characterized, and the polycrystalline form, alumina, is in widespread use as an industrial material. The surface of the sample was oriented 15 ° off the (000 1) plane and was polished using diamond paste and silica suspension to a good optical finish. An impression was left in the surface of the sample by a Vickers pyramid indenter applied with a force of 4.9 N.The specimen was examined in a conventional reflection SAM operating at 0.8 GHz with a lens of half angle 57 °. Fig. 1 shows the SAM image obtained with the sample placed at the focus of the lens. The impression left by the indenting tool is clearly visible, as are the radial cracks, and a bifurcation near the end of the right-hand crack. The series of bright and dark fringes visible on the sides of the impression are due to interference and can be used to estimate the depth of the impression, given a knowledge of the wavelength. Although the faint contrast from around the indentation impression may possibly be attributed to the presence of lateral vents (a type of submerged crack) [7], there is little sub-surface information in the image of Fig. 1.An indication of the practical resolution of the SAM is given by Fig. 2 which shows a backscattered SEM image of the upper branch of the bifurcated crack. From Fig. 2 the crack width may be estimated to be ~ 75 mm. This crack branch is clearly visible in the SAM image. Thus the detection limit for surfacebreaking cracks in the SAM operating at 0.8 GHz is equal to or better than ~ 75 nm. The SEM image (Fig. 2) also reve...

Response of surface acoustic wave imaging systems to cylindrical inhomogeneities

“…If the characterization is of primary concern rather than just detection of faults, an imaging system may be utilized. The scanning acoustic microscope has been used successfully, because of its ability to excite surface waves [2,3]. The disadvantage of scanning acoustic microscope is that all such modes are excited simultaneously and each with a low efficiency.…”