Quantitative Mapping of Pore Fraction Variations in Silicon Nitride Using an Ultrasonic Contact Scan Technique

Abstract

An ultrasonic scan procedure using the pulse-echo contact configuration was employed to obtain maps of pore fraction variations in sintered silicon nitride samples in terms of ultrasonic material properties. Ultrasonic velocity, attenuation coefficient, and reflection coefficient images were obtained simultaneously over a broad band of frequencies (e.g., 30 to 110 MHz) by using spectroscopic analysis. Liquid and membrane (dry) coupling techniques and longitudinal and shear-wave energies were used. The major results include the following: Ultrasonic velocity (longitudinal and shear wave) images revealed and correlated with the extent of average through-thickness pore fraction variations in the silicon nitride disks. Attenuation coefficient images revealed pore fraction nonuniformity due to the scattering that occurred at boundaries between regions of high and low pore fraction. Velocity and attenuation coefficient images were each nearly identical for machined and polished disks, making the method readily applicable to machined materials. Velocity images were similar for wet and membrane coupling. Maps of apparent Poisson's ratio constructed from longitudinal and shear-wave velocities quantified Poisson's ratio variations across a silicon nitride disk. Thermal wave images of a disk indicated transient thermal behavior variations that correlated with observed variations in pore fraction and velocity and attenuation coefficients.

Ultrasonic Attenuation of Ceramic and Inorganic Materials Using the Through-Transmission Method

“…It was concluded that the buffer-rod method [30][31][32] is not practical to determine material attenuation under normal laboratory conditions. While it may have worked in situations where advanced sample preparation facilities and precision alignments were available [33][34][35][36][37], their results still showed unacceptably large differences among them, as discussed in Section 1. It appears that the refraction occurring at the buffer-sample interface needs to be examined as the source of the difficulties encountered.…”

“…However, for Si 3 N 4 , Evans et al [33] obtained attenuation coefficients ranging from 150 to 5300 dB/m at 11 to 53 MHz. In contrast, Generazio [34] reported 235 dB/m at 60 MHz and Roth et al [36] reported 130 to 1020 dB/m at 30 to 110 MHz (with 480 or 696 dB/m at 50 or 60 MHz). These results from two distinguished laboratories revealed large discrepancies of more than a factor of ten, highlighting the difficulty of the buffer-rod method.…”

“…Evans et al [33] used this method for several advanced ceramics, SiC, Si 3 N 4 , MgO, ZnS and lead zirconatetitanate (PZT). Generazio and others further developed the method for the ASTM C1332 standard document [34][35][36]. Additionally, Papadakis [32] provided attenuation data for graphite (ATJ grade) and ASTM C1332 document included cobalt-bonded WC cermet data [30].…”

“…Thus, no report of its usage can be found in the literature. Additionally, the validity of this method is questionable since there existed vast differences in the attenuation values reported for SiC and Si 3 N 4 in [33] and [34][35][36]. Both groups used the same bufferrod method, and the values were expected to be comparable.…”

Nondestructive assessment of pore size in foam-based hybrid composite material

“…Simple theory was found useful in predicting the velocity change for low porous material. 1,2 For fluid statured porous foam, two types of ultrasonic waves: slow and fast waves can be generated. 3–6 This phenomenon has been predicted by Biot.…”