Scanning electron acoustic microscopy (SEAM): A technique for the detection of contact-induced surface & sub-surface cracks

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.