Experimental Determination of the Raman Sampling Depth in Zirconia Ceramics

Abstract

This study provides experimentally determined values for the actual micro-Raman spectroscopy sampling depth in zirconia ceramics (ZrO2) via line scans on a wedge-shaped sample. Common instrumental settings with metallurgical objective lenses in dry air, argon-ion, and helium-neon laser radiation of approximately 10 mW were chosen. Under those conditions effective sampling depths, defined as the depth at which 99% of the information is recorded, range from 20 to more than 50 microm, depending on the numerical aperture of the lens and the laser wavelength. These results elucidate the pitfalls of the investigation of surface phenomena in zirconia ceramics such as low-temperature degradation or mechanically induced phase transformations by Raman spectroscopy.

The role of beam dispersion in Raman and photo-stimulated luminescence piezo-spectroscopy of yttria-stabilized zirconia in multi-layered coatings

“…The optical absorption coefficients (1/B) in Eq. (2) gave ∼0.010 µm −1 (514 and 632.8 nm) for APS-YSZ and ∼0.012 µm −1 (514 and 632.8 nm) for EBPVD-YSZ, which are of a similar magnitude to those previously determined for dense YSZ (0.05-0.19 µm −1 ) [18] and [43]. In particular, Eldridge et al [44] and [45] have determined experimentally the scattering and absorption coefficients of freestanding 8 wt.% APS-YSZ (11% porosity) using normalincidence directional-hemispherical reflectance and transmittance spectra over the range of 250 nm up to 15 µm (data for the range 0.8-15 µm can be found in Refs.…”

“…4). The absorption coefficient of the APS-YSZ samples tested in this paper is smaller than that given by Presser et al [18] because the sample adopted allows the collection of signals through a thickness of ∼250 µm rather than being limited to 40 µm. The difference can be accounted for by the complex microstructure of the two YSZ coatings used in this study which has the potential to reduce the absorption coefficient.…”

“…A commonly encountered method for measuring the sampling depth of a Raman signal in a material is to create a double-layer system by applying the target material, for example, a wedge of YSZ, on top of a highly Raman active acryl varnish coating [18]. The intensity decay in the Raman spectra of the acryl varnish is then monitored as a function of thickness.…”

“…The assumption generally made for stress measurements on air plasma sprayed APS-YSZ is that the Raman signal acquired is limited to the near surface but without specifying the range of depth [17]. Recent experimental studies have shown that the sampling depth can reach 20-50 µm even in dense YSZ [18]. As sampling depth increases the resolution decreases due to beam dispersion.…”

“…In bulk material, the size of the cone-like excitation volume depends on the numerical aperture of the objective lens and the difference in refractive index between the material in which the objective is immersed and the material being investigated [18], [23] and [26]. For materials with porous microstructures, such as thermally sprayed YSZ, the lateral resolution can be further reduced by the scattering of the laser beam by pores and unmelted particles [27] and by ray guiding [28] when the incident beam strikes the internal splat boundaries at an angle greater than the critical refraction angle [29].…”