Characterization of acoustic Lamb wave propagation in polycrystalline diamond films by laser ultrasonics

The propagation of acoustic Lamb waves in free standing chemical vapor depo sited polycrystalline diamond has been studied using a laser ultrasonic tec hnique. The influence of film morphology, quality, and thickness on the wav es has been assessed. Acoustic waves with high velocities in the range 8700 -12 200 ms(-1) were observed; the lowest values were recorded for films wit h the lowest crystal quality and highest nondiamond content. High quality f ilms with differing crystal textures or thickness show little variation. Th e influence of temperature on the dispersion characteristics of Lamb wave p ropagating in a 50 mm diam polycrystalline diamond wafer were also investig ated. Little variation was apparent across the range studied (30-250 degree s C). Material parameters extracted from the dispersion chracteristics of t he acoustic signal together with scanning electron microscopy studies sugge sted that void, microcrack, and grain boundary density most influences the propagation of low frequency Lamb waves in free standing CVD diamond films. (C) 2000 American Institute of Physics. [S0021-8979(00)00713-1].