Piezospectroscopic Research Articles

Methods of piezo-spectroscopic calibration of thin film materials: II. Tensile stress field at indentation crack tip

A new method is reported for determining the piezo-spectroscopic (PS) coefficient of brittle materials using the stress field generated at the tip of a Vickers indentation crack. The method requires a minimum amount of material (e.g., small areas of devices or thin films) and can be applied to both Raman and fluorescence spectra. Applications are shown for sapphire and silicon single-crystalline thin plates as paradigm examples of highly transparent and opaque materials, respectively. The crack opening displacement was preliminarily measured in a field emission scanning electron microscope, and fitted by a recently proposed theory to obtain an in situ estimate of the crack stress intensity factor. Taking advantage of an automated travelling jig, capable of submicrometric lateral displacements, spectral shifts typical of the K-dominated zone (i.e., the zone along the axis of crack propagation) were recorded as a function of distance from the crack tip. To assess the reliability of the indentation method for PS calibration, the interaction between the sample and laser probe was also analysed in detail. A confocal configuration was required in highly transparent materials to reduce the probe depth and convolution error as well. This study confirms the possibility of extending Raman and fluorescence piezo-spectroscopy to the high-resolution quantitative stress analysis of transparent materials.

Microscopic Stress Analysis of BaTiO3 Using Optical Activity of Its Point Defects

Microscopic stress analysis of BaTiO3 thin films was performed on the basis of the optical activity of their point defects by means of piezo-spectroscopic (PS) method applied to cathodoluminescence (CL) measurements. Accordingly, the interfacial residual stress that accumulates during manufacturing between a BaTiO3 film and its sapphire substrate could be estimated. Taking advantage of the dependence of the penetration depth of the electron beam in CL measurement on accelerating voltage, CL spectra collected as a function of accelerating voltage could be considered to represent an in-depth profile. In this context, an approximately 2 nm CL spectral shift between the film surface and interface was observed. The accelerating voltage that corresponded to the CL spectrum in the vicinity of the interface was chosen on the basis of the appearance of the sapphire F-band at approximately 330 nm. A PS coefficient was measured using CL spectra from a BaTiO3 single crystal, and a three-dimensional PS coefficient, Π=21.8 nm/GPa, could be retrieved. Because of the biaxial nature of interfacial stress, an averaged biaxial PS coefficient (2/3Π=14.5 nm/GPa) was employed for calculating interfacial stress from the observed spectral shift. This procedure allowed us to measure a maximum tensile stress of 140 MPa near the film/substrate interface in the BaTiO3 film. FEM simulation was also performed, and the results were compared with those of CL measurements.

Investigation of local orientation and stress analysis of PZT-based materials using micro-probe polarized Raman spectroscopy

Investigation of crystallographic orientation and the related residual stress fields stored in piezoelectric sensor/actuator devices would improve reliability of industrial products and help to optimize the entire production process. Micro-probe Raman piezospectroscopy is a very attractive technique, yet not applied to lead zirconate titanate (PZT)-based materials, as the response is convoluted into contributions arising not only from stress, but also from crystallographic orientation. In this work, we have attempted to evaluate the correlation between Raman spectra and orientation in two differently doped lead zirconate titanate materials (PZT–LN and PZT–NSY), finding in the “softer” PZT a correlation between crystal orientation and Raman shift. We performed also some calibration experiments, and a piezospectroscopic (PS) coefficient has been retrieved in the “harder” PZT.

Raman Analysis of Microscopic Residual Stresses Stored in the Secondary Phase of Sc<sub>2</sub>O<sub>3</sub>-Doped Si<sub>3</sub>N<sub>4</sub>

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.

Indirect Determination of the Piezospectroscopic Coefficients of Ceria‐Stabilized Tetragonal Zirconia Polycrystals

A method is proposed for the indirect determination of the stress dependence (expressed as piezospectroscopic (PS) coefficients) of spectroscopic bands of ceramic materials/phases. This method is based on the intimate mixture (intimate at the microstructural, grain‐size level) of two phases/materials when the stress in one is independently known; it is used to determine the PS coefficients of the most intense Raman bands in ceria‐stabilized tetragonal zirconia polycrystals (Ce‐TZP). Different amounts of Ce‐TZP were mixed with alumina and the composite pellets sintered; subsequently, the stress in alumina was determined through the PS coefficient of its R2 luminescence band and the stress in Ce‐TZP derived from the static equilibrium condition. The frequency shifts of each Raman band of Ce‐TZP have been plotted against the stress and the slopes provide the PS coefficients. The method has the advantage of not requiring any type of loading device (i.e., diamond anvil‐cell, bending jig). Finally, the limits are also discussed, the most important one being the requirement of immiscibility of the two materials/phases.

Pressure Dependence of Laser-Induced Fluorescence of Sm3+ in Al2O3—Y2O3 System Compounds

The piezospectroscopic (PS) coefficients relating frequency shift to pressure are evaluated for Sm3+ in the Al2O3–Y2O3 system compounds, which include Al2O3, Y3Al5O12, YAlO3, Y4Al2O9, and Y2O3. The fluorescence peaks are inherently narrow for all the synthesized compounds, on the basis of the f–f transition nature of Sm3+ ion. Furthermore, the PS coefficients are ranged from −2.2 to −8.2 cm−1/GPa, which are similar to the reported value for the R1 line of ruby and suitable for the PS stress evaluation. PS stress distribution measurements are demonstrated with sintered Y3Al5O12 ceramics as a sample, with 50 μm intervals, under a microscope.

Mapping of residual stresses around an indentation in β-Si3N4 using Raman spectroscopy

The stress dependence of the Raman bands of silicon nitride (β-Si3N4) have been investigated and applied to indentation experiments. Seven high-frequency bands have been found to have linear and negative stress dependencies. On the other hand, low frequency bands (namely 183, 205 and 226 cm-1 bands) showed small positive correlations with the stress. The piezospectroscopic (PS) coefficients of all the observed Raman bands have been determined. As an application, one of the PS coefficients has been used to determine the stress distribution around a triangular indentation.