Shigemi Tochino

Quantitative evaluation of probe response functions for Raman and fluorescence bands of single-crystalline and polycrystalline Al2O3

The probe response functions of Raman and fluorescence bands of single-crystalline sapphire and polycrystalline Al2O3 with different grain sizes were systematically evaluated and compared. As a general issue, it was found that the probe response function strongly depended on the selected signal and, more importantly, on grain size. From a comparison between the probe characteristics of Raman and fluorescence bands, the following characteristics could be found: (i) the probe depth of the Raman bands of sapphire single-crystal (the main band at 417?cm?1 was selected as a paradigm Raman sensor) was about a half that of the Cr3+ fluorescence doublet (a direct comparison was made with the R1 band located at 14?405?cm?1); (ii) the probe depth in polycrystalline Al2O3 decreased with decreasing grain size. In this latter case, the probe depth of the R1 band could be up to >10 times larger than that of the 417?cm?1 Raman band, the difference in depth increasing with decreasing grain size. These large differences in probe geometry need to be taken into consideration when applying spectroscopy methods as, for example, piezo-spectroscopic stress assessments. Nevertheless, the results of this investigation suggest that a noticeable improvement in the in-depth spatial resolution can be obtained using Raman rather than fluorescence bands as stress probes for assessing high property gradients present in the materials.

Spatially Resolved Raman Spectroscopy for In-Depth Non-Destructive Residual Stress and Phase Transformation Assessments in 3Y-TZP Bioceramics

A non-destructive assessment of phase transformation and residual stress is presented for a 3 mol.% Y2O3 added ZrO2 ceramic using Raman microprobe spectroscopy. Low CIP pressure has been selected in the sample procedure to increase a potential to transform ZrO2. Aging tests were made and the transformation depth and residual stresses caused by transformation were evaluated by Raman spectroscopy A Raman microprobe technique using a visible wavelength laser coupled with a confocal optical device may enable one to retrieve spatially resolved information along the material subsurface. To demonstrate the potentiality of the confocal technique, aging of a ZrO2 sample has been made in autoclave and phase transformation gradually promoted from the surface towards the sub-surface of the sample (up to ～60 µm, in a sample autoclaved 168 h). Then, a quantitative spatially resolved assessment was attempted on these samples from their surface. The confocal information from the subsurface was compared with results of Raman spectroscopy collected from a cross-section. Accordingly, a quantitative equation was proposed, which allows the quantitative assessment of the thickness of the surface layer, which underwent phase transformation in ZrO2 ceramics, according to in-depth non-destructive assessments.

Estimation of Residual Stress in a Ceramic Coating Layer

We attempted to characterize by Raman piezo-spectroscopy residual stresses as they develop in chemical vapor deposition (CVD) Al2O3 coatings on Si3N4 ceramic substrates. According to a piezo-spectroscopic procedure coupled with a confocal configuration of the optical probe used, two-dimensional stress maps could be collected at various depths along the thickness of the coating. By comparing Al2O3 coatings produced on Si3N4 substrates at different CVD temperatures, a tensile residual stress field has been detected in the coating, whose magnitude increased with increasing the CVD temperature. As for the three-dimensional distribution of tensile residual stress within the Al2O3 coating, it was found that the stress value was minimum at the coating external surface, while it gradually increased to reach a maximum near the coating/substrate interface. Similarly, the compressive stress within the Si3N4 substrate was maximized near the coating/substrate interface and decreased with proceeding towards the substrate material bulk. It could be concluded that confocal Raman piezo-spectroscopy is a very suitable tool for three-dimensional stress characterization of ceramic coating materials.

Raman Analysis of Microscopic Residual Stresses Stored in the Secondary Phase of Sc2O3-Doped Si3N4

Residual stress studies were performed on the intergranular phase of a Sc2O3-doped Si3N4 polycrystal. Sc2O3 additive represents an ideal substance for residual stress analysis because of the intense Raman spectrum of its related silicate (intergranular) phases, which form during sintering. Crystallization of Sc2O3 to Sc2SiO7 during the cooling process after sintering causes a negative volume change at triple pockets of polycrystalline Si3N4 which overlaps mismatches in thermal expansion coefficients between secondary phase and the Si3N4 matrix. Conventional piezo-spectroscopic (PS) stress analyses have usually been limited to measurements in the Si3N4 matrix. In this paper, we show for the first time measurements of residual stresses stored within the secondary phase of Si3N4, using the Raman spectrum of Sc2O3. A change in crack propagation mode from intergranular to transgranular was found after annealing the Sc2O3-doped Si3N4 material, such a difference leading to an embrittlement of the material. Residual stress analysis of the annealed sample revealed that the tensile residual stress stored within the secondary phase was removed, thus explaining why the rising R-curve behavior of the material was substantially suppressed as compared to the as-sintered sample.