Acoustic Microscopy of Curved Surfaces

The Metrology and Imaging modes of the acoustic reflection microscope are applied to spherically shaped specimens. Metrology is usually practiced by translating the specimen along the acoustic beam axis. This operation yields a measurement of the local Rayleigh velocity at a single location in the specimen plane. Imaging is accomplished through raster-scanning in the plane transverse to the beam axis. The two modes are, in effect, simultaneously employed when a nonplanar surface of' known curvature is scanned. The resulting image reveals nearly concentric rings with radial, periodic brightness variation, if the surface is spherical in shape. Stainless steel bearing balls of the type used in gyros are used to demonstrate the technique. It is suggested that the obtained images represent a two-dimensional map of elastic properties applicable to convex (bearing ball) and concave (bearing raceway) surfaces. THE ESSENTIAL ELEMENTS OF THE REFLECTION ACOUSTIC MICROSCOPE (Figure 1) Images are obtained by raster scanning (in the XY plane) the highly converging sound beam across a planar object using a tiny drop of an "immersion" liquid, usually water. The object is usually situated in the acoustic focal plane and the reflected signals containing acoustic information are displayed on a synchronized cathode-ray tube monitor screen or storage oscilloscope. The focused acoustic beam is formed by a polished hemispherical depression in the sapphire rod shown in the figure. A piezoelectric transducer, cons isting of a sputter -deposited layer of zinc oxide, converts the acoustic energy to and from the electrical signals that are imaged on the television screen. The reflection acoustic microscope may be likened to a pulsed radar imaging system in which the target "flies" at constant altitude and speed. In this manner, images of surface as well as subsurface detail are obtained depending on the altitude (at or below the surface) at which the focal plane intersects the object (target). Range-gating (not shown in the diagram) is used to select the desired image pulse from a host of spurious signals generated within the sapphire rod. BEARING GEOMETRY (Figure 2) A typical rolling element bearing consists of bearing balls, outer and inner raceways, as is shown in Figure 2. The surface of the bearing ball is convex and spherical. The raceways exhibit both convex and concave surfaces of nearly cylindrical curvatures. Portions of a gyro bearing ball were sectioned and examined in the acoustic microscope, that operates in the pulsed reflection mode in the frequency range near 400 MHz. THE EXPERIMENT (Figure 3) Bearing balls were sectioned to provide a comparison of AMS results on both planar and spherical surfaces with different surface finish. The 3/32 inch diameter balls were of 52100 stain_less steel. The bearing ball sections were cemented to a fused quartz plate using a low temperature wax. A scanning electron micrograph of the mounted sections is shown in Figure 3. The ball sections with their planar faces up expose a Hughespolished 52100 stainless steel surface. The section denoted by the arrow is mounted with the spherical surface pointing up, and, therefore, presents a conventionally lapped bearing ball surface for diagnostic inspection. The arrow points to the apex of the sphere where the measurements were made. EXPERIMENTAL RESULTS -I (Figure 4) Acoustic material signatures (AMS) for these two cases are shown in Figure 4. The solid curve represents the measurement on the planar (Hughespolished) surface, while the dashed curve is for the spherical bearing surface. The AMS curves are the video-detected transducer output power variation with object translation along the lens axis z .. The AMS is the result of interference between two component waves that are reflected from the substrate (bearing) into the coupling liquid (deionized water, v = 1. 5 mm/f-Ls) and are vectorially summed in the piezoelectric transducer. As was shown previously, in the AMS mode Rayleigh waves are launched and detected coherently. From the physical model that was develo,ped to explain the acoustic material signature(! • 2), the AMS period ~zn is proportional to the square of the mean Rayleigh velocity in the plane of the substrate material. The measured period averaged over all periods in the figure yields the Rayleigh velocity vR directly as given by Equation 1: vR = (vQ . f . ~zN) 1/2 1/2 23. 56 (~zN) (1) where the frequency of 370 MHz has been assumed, and VQ is the velocity in water. Measured and derived results from Figure 4 are listed in the table. It can be seen that some difference exists between the two measurements on surfaces that differ both in shape as well as in their preparation.