CHARACTERISATION OF CERAMICS JOURNAL OF MATERIALS SCIENCE 39 (2 0 0 4 ) 6791 – 6805 Scanning electron acoustic microscopy (SEAM): A technique for the detection of contact-induced surface & sub-surface cracks T. F. PAGE, B. A. SHAW ∗ School of Chemical Engineering and Advanced Materials and ∗ Design Unit, School of Mechanical and Systems Engineering, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, UK E-mail: t.f.page@newcastle.ac.uk A variant of the scanning acoustic microscopy technique, scanning electron acoustic microscopy (SEAM), uses a pulsed electron beam in a conventional scanning electron microscope (SEM) to generate elastic waves near the surface of the sample. Conveniently for studies of surface damage, the contrast-generating processes are at a depth commensurate with the thickness of many thin hard ceramic coatings and the typical depths of fatigue-induced cracks in both gears and rolling element bearing systems. Using examples from our studies of contact damage induced in thin hard coated systems and gears, this paper will demonstrate the applicability of SEAM techniques to the study of near-surface damage in coated systems (coating fracture and debonding) and gears (fatigue damage). We show that clear contrast can arise from cracks oriented both parallel to and, sometimes, perpendicular to the surfaces of many samples, and show that useful information can be provided regarding the debonding of coatings. It has also been found possible to delineate sub-surface contact and contact fatigue cracks allowing some information regarding crack orientation and extent to be deduced without the need for either serial or vertical sectioning of the sample. C  2004 Kluwer Academic Publishers 1. Introduction In non-transparent solids, it can be difficult to detect either the presence of internal cracks or the sub-surface extent of surface-breaking cracks by conventional tech- niques, but sometimes these critical defects can be suc- cessfully imaged using acoustic techniques of varying resolutions (e.g., standard, bulk, ultrasonic, crack detec- tion methods (non-destructive evaluation (NDE) tech- niques), scanning acoustic microscopy (SAM) [e.g., 1– 3]). A less common variant of the acoustic technique, scanning electron acoustic microscopy (SEAM), uses a pulsed electron beam in a conventional scanning elec- tron microscope (SEM) to generate elastic waves near the surface of the sample. The depth below the surface at which acoustic waves are generated, and from which the majority of image contrast arises, depends on both the electron beam energy and the properties of the sam- ple, but is typically in the range 0 → 5 μm for beam energies of 0 → 30 kV–a figure attractively commen- surate with the thickness of many thin hard coatings and the typical depths of fatigue-induced cracks in both gears and rolling element bearing systems. While SEAM has been available for nearly two decades, and a number of useful applications demon- strated, it appears that the technique has found little ap- plicability in one of the branches of the research com- munity to whom it may be most useful. The purpose of this paper is therefore to demonstrate the potential applicability of the SEAM technique in the areas of contact damage and surface engineering and to encour- age others to explore this powerful and promising tech- nique. As a host for this technique, scanning electron mi- croscopy (SEM) offers a powerful means of charac- terising materials by utilising the wide range of sig- nals resulting from the interaction of solid samples with a scanning beam of energetic electrons. In this way, a wide range of topographic (e.g., detailed frac- ture surface shapes), functional (e.g., the dimensions of magnetic domains), microstructural (e.g., the size and shape distributions of phases) and microchemi- cal (e.g., segregation profiles) information can be re- trieved, often with sub-micron spatial resolutions [e.g., 4]. An obvious attraction of SEM techniques is the ability to form readily-accessible visual images from the various signals emanating from the sample—for example, surface topography and shape (secondary electron imaging), the mapping of atomic number (z ) (backscattered electron imaging), the determination of local crystallographic orientation using either selected area channelling patterns or backscattered electron pat- terns (backscattered electron imaging), and the de- termination of local chemical composition (including the detection of light elements down to carbon) using 0022–2461 C  2004 Kluwer Academic Publishers 6791