Piezospectroscopic Research Articles

Measurements for stress sensing of composites using tailored piezospectroscopic coatings

Chromium-doped α-alumina is naturally photo-luminescent with spectral properties that are characterized by R-lines with two distinct peaks known as R1 and R2. When the material is subjected to stress, shifts in the R-lines occur, which is known as the piezospectroscopic (PS) effect. Recent work has shown that improved sensitivity of the technique can be achieved through a configuration of nanoparticles within a polymer matrix, which can be applied to a structure as a stress-sensing coating. This study demonstrates the capability of PS coatings in mechanical tests and investigates the effect of nanoparticle volume fraction on sensing performance. Here, measurements of spectral shifts that capture variation in stress of the coating during mechanical testing and in the region of substrate damage showed that stress contours are more noticeable on a soft laminate than hard laminate. It was found that the 20 % volume fraction PS coating showed the most distinct features of all the coatings tested with the highest signal-to-noise ratio and volume fraction of α-alumina. Post failure assessment of the PS coatings verified that the coatings were intact and peak shifts observed during mechanical testing were due to the stress in the substrate. The results suggest the ability to design and tailor the “sensing” capability of these nanoparticles and correlate the measured stress variations with the presence of stress and damage in underlying structures. This study is relevant to nondestructive evaluation in the aerospace industry, where monitoring signs of damage is of significance for testing of new materials, quality control in manufacturing and inspections during maintenance.

Damage mapping with a degrading elastic modulus using piezospectroscopic coatings

The development of piezospectroscopic (PS) composites has enabled the creation of a non-destructive evaluation (NDE) technique which integrates piezospectroscopy, digital image correlation (DIC) and analytical multi-scale mechanics to map the elastic modulus of a coated material. The measured elastic modulus was represented as a normal distribution with a mean value (32.2GPa) which is within 8% of the conventionally recorded modulus (35GPa). Damage mechanics are applied to map elastic degradation in situ mechanical loading with an average uncertainty (~10GPa) that was sufficient in observing subsurface, progressive damage patterns which are unique to the coated material.

Multiscale mechanics to determine nanocomposite elastic properties with piezospectroscopy

The piezospectroscopic (PS) properties of chromium-doped alumina allow for embedded inclusion mechanics to be revisited with unique experimental setups that probe the particles’ state of stress when the composite is under applied load. These experimental investigations of particle mechanics will be compared to the Eshelby theory and a derivative theory. This work discovers that simple nanoparticle load transfer theories are adequate for predicting PS properties in the low to intermediate volume fraction range (⩽20%). By applying the multiscale mechanics to a PS response, the inverse problem was demonstrated to reveal the elastic modulus of the composite. The implications for this technique are damage monitoring through observation of reduced mechanical properties in addition to a method to assist with engineering nanomaterials.

Piezospectroscopic Measurements Capturing the Evolution of Plasma Spray-Coating Stresses with Substrate Loads

Plasma-spray coatings have a unique microstructure composed of various types of microcracks and weakly bonded interfaces which dictate their nonlinear mechanical properties. The intrinsic photo-luminescence (PL) characteristics of alpha-alumina (α-Al2O3) within these coatings offer a diagnostic functionality, enabling these properties to be probed experimentally at the microscale, under substrate loading. The piezospectroscopic (PS) measurements from the coatings are capable of revealing microstructural stress at high spatial resolution. Here, for the first time, the evolution of stresses within air plasma spray (APS) coatings under increasing substrate loads were captured using piezospectroscopy. With mechanical cycling of the substrate, the PS properties revealed anelastic and inelastic behavior and a relaxation of residual tensile stress within the APS coatings. With decreasing substrate thickness, the coating was observed to sustain more stress, as the substrate's influence on the mechanical behavior decreased. The findings provide an insight into the microstructural response that can serve as the basis for model validation and subsequently drive the design process for these coatings.

In-depth profiling of elastic residual stress and the in vivo wear mechanism of self-mating alumina hip joints

Residual (elastic) stress profiles developed in self-mating alumina heads upon long-term exposure in the human body have been quantitatively determined by means of three different piezo-spectroscopic (PS) approaches available to the alumina lattice. The PS measurements, collected at exactly the same area of the sample, included fluorescence spectroscopy (fs), Raman spectroscopy (rs) (in-depth resolutions both in the order of the tens of μm), and cathodoluminescence spectroscopy (cs) (in-depth resolution down to the tens of nm). The lateral spatial resolution was in the order of the tens of μm, of the single μm, and of the hundredths of nm for fs and rs and cs, respectively. Owing to the precise knowledge of the response functions in space of the three probes, deconvoluted stress profiles could be obtained in zone of different wear severity. The profiles revealed high stress gradients along the material subsurface, “cleaned” from averaging effects related to the finite probe volume. The subsurface stress field in heavily worn zones entailed a surface layer under strong compression, an abrupt compressive-to-tensile transition up to a similar stress magnitude, and a deeper-lying region of tension which ultimately relaxed in the unstressed bulk. The opposite trend was observed in mildly worn zones of the femoral head, with moderate tensile stresses in the immediate surface and weak counterbalancing stresses of compressive nature along the subsurface. A wear mechanism for self-mating alumina hip surfaces was then drawn consistently with the newly available residual stress profiles.

Raman analysis of three-dimensionally graded stress tensor components in sapphire

An improved method of confocal/polarized Raman spectroscopy is proposed, which enables the determination of the full set of tensor components in three dimensionally graded stress fields in sapphire single-crystal. From the experimental side, the proposed method employs a simple back-scattered configuration for the optical microprobe and relies on the experimental knowledge of the matrix of piezospectroscopic (PS) coefficients of sapphire (i.e., the proportionality constants relating the frequency shifts of individual Raman bands to the magnitude of an uniaxial stress applied along different crystallographic directions). From the theoretical side, the complete set of stress tensor components were generally expressed as a function of the PS coefficients for any unknown multiaxial stress state by means of the Euler transformation matrix, leading to a system of linear equations; such equations link each stress component to the experimentally observed spectral shifts of different Raman bands. The proposed PS method does not require a direct knowledge of the secular equation, but local crystallographic orientations should be retrieved from polarized Raman intensities. With probe averaging effects being systematically removed by probe deconvolutive routines, the PS method is shown to correctly resolve multiaxial stress tensor fields with steep gradients in space.

Contribution of Spatially and Spectrally Resolved Cathodoluminescence to Study Crack-Tip Phenomena in Silica Glass

A current controversy over the nature of fracture in silica glass is revisited in the light of additional experimental evidence obtained by cathodoluminescence (CL) spectroscopy in the immediate crack-tip zone of two different types of silica glass. The Letter describes the dual experimental output obtained by monitoring optically active oxygen defects in silica glass, as follows: (i) analysis of local crack-tip stoichiometry (i.e., lattice defect population); and, (ii) spatially resolved stress analysis ahead of the crack tip based on a piezo-spectroscopic (PS) approach. CL experiments provided us with direct access to crack-tip stress fields, thus unfolding some missing detail about the complex mechanochemical interactions occurring at the crack tip in silica glass.

Ex‐Situ Stress Measurements in Polycrystalline Ceramics Using Photo‐Stimulated Luminescence Spectroscopy and High‐Energy X‐Rays

The photo‐stimulated luminescence spectroscopy (PSLS) technique provides the means to establish stress dependencies of the well‐known R‐line peak positions in polycrystalline alumina. The uniaxial compression tests presented in this paper, which determine the coefficients describing this piezospectroscopic (PS) behavior, tackle two previously unexplored areas. Firstly, the vibronic band peaks in the emission spectrum are introduced here, and the PS coefficients of several peaks within these bands are established. These results set the foundation for the exploitation of vibronic band peakshifts, along with the R‐lines, in order to provide the non‐symmetric components of the stress tensor and therefore the measurement of the complete stress state in polycrystalline alumina. Secondly, high‐energy X‐rays serve as an ex‐situ stress measurement method upon which the optical fluorescence shifts are based, thereby advancing the accuracy of PS coefficient determination in a unique approach. The results of PSLS and synchrotron X‐ray experiments are presented here and used in conjunction to reveal new information on the PS behavior of polycrystalline alumina.

Microscopic scale evaluation of the anisotropic Young's modulus in PbWO4 single crystal using Raman microprobe spectroscopy

In this research, we attempted to calibrate Young's modulus of lead tungstate (PWO) single crystals by combing the piezo-spectroscopic (PS) method and the indentation technique. A series of Vickers indentation cracks were generated at the corners of the indenter print on the surface of a-plane and c-plane PWO single crystals, respectively. According to microscopic assessments of crack opening displacements (COD) for cracks propagated along different crystallographic directions, we first determined the stress intensity factor associated with the tensile stress field developed ahead of the indentation crack tip. Then, upon clarifying both the PS algorithm for spectral shift analysis and the convolution effect of the laser probe, the related plane Young's modulus could be retrieved from Raman line-scans ahead of the crack tip. Young's modulus values of 70 GPa and 89 GPa were determined for the a-plane and c-plane of PWO single crystal, respectively. The Raman PS technique applied to controlled microscopic stress fields represents a convenient method for assessing the mechanical properties in small portions of materials embedded in devices.

Spatially resolved piezo-spectroscopic characterizations for the validation of theoretical models of notch-root stress fields in ceramic materials

The main aim of this work is a precise experimental assessment of the local stress fields developed at the notch-root in a ruby crystal, selected as a paradigm brittle material, by means of photo- and electron-stimulated luminescence techniques. Our approach takes advantage of the piezo-spectroscopic (PS) effect, which consists of a spectral shift of the luminescence emitted by the material due to lattice strain. Highly spatially resolved stress maps were extensively collected at the notch-root and spectral shifts monitored for the chromophoric (R-lines) fluorescence observed in a single-crystalline ruby sample. Experimental data were analyzed and compared to the theoretical solutions of notch-root stress fields given by Filippi and by Creager-Paris. Due to its inherent simplifications, the Creager–Paris solution was found leading to underestimation of the maximum stress value piled up in the material, while the Filippi’s solution represented a more suitable approximation for the stress field developed at the notch-root.

Stress Dependence of Paramagnetic Point Defects in Amorphous Silicon Oxide

Room-temperature red cathodoluminescence (CL) emission (R band) arising from the paramagnetic point-defect population present in amorphous silicon oxide (SiOx) has been characterized with respect to its shift upon applied stress, according to a piezo-spectroscopic (PS) approach. The R band (found at around 630 nm) originates from nonbridging oxygen hole centers (NBOHC; Si-O*) generated in the presence of oxygen-excess sites. It is shown that reliable stress assessments can be obtained in silica glass with a relatively high spatial resolution, provided that appropriate spectroscopic procedures are developed to precisely extract from the CL spectrum the shift upon stress of the R band, isolated from other partly overlapping bands. Macroscopic and microscopic PS calibration procedures are shown to lead to consistent results on silica materials with different chemical characteristics and, thus, with different intrinsic defect populations. In addition, quantitative calibrations of both electron probe size and luminescence emission distribution within the electron probe are given. As an application of the PS technique, the magnitude of the residual stress piled up (mainly due to a thermal expansion mismatch) at a sharp silica/silicon interface has been characterized by taking into account the gradient in defect population developed as a function of distance from the interface. In the Results and Discussion section, brief comments are offered regarding the possible impact of highly spatially resolved stress assessments in silica glass upon the development of new materials and advanced electronic devices.

Visualization of microscopic stress fields in silica glass in the scanning electron microscope

Quantitative measurements were made in silica glass of highly graded stress fields, as they developed: (i) in the K-dominated zone ahead of the tip of a median-type indentation micro-crack; and, (ii) at a silicon-silica interface of a metal-oxide semiconductor (MOS) device. Stress fields could be visualized on a microscopic scale using a field-emission scanning electron microscope (FE-SEM) equipped with a spectrally resolved cathodoluminescence (CL) device, according to piezo-spectroscopic (PS) assessments. The peculiarity of this newly proposed PS assessment resides in the fact that the performed CL/PS analysis exploited a peculiar luminescence emitted by optically active oxygen point defects in silica glass.

High-resolution stress assessments of interconnect/dielectric electronic patterns using optically active point defects of silica glass as a stress sensor

A piezospectroscopic (PS) cathodoluminescence (CL) study has been carried out on a Cu-Ta∕SiOx (carbon-doped) model chip prepared on a Si substrate. The PS approach was applied to CL spectra arising from optically active point defects in dielectric silica. The red CL emission arising from nonbridging oxygen hole centers (NBOHC) in the carbon-doped SiOx dielectric layer was calibrated and used as a stress sensor. This approach enabled us to locate the trace of the residual stress tensor, as locally developed during manufacturing process in the dielectric interlayers between Cu-Ta interconnects. A minimally invasive electron beam allowed probing local residual stress fields with an improved spatial resolution as compared to more conventional photostimulated PS techniques applied to the Si substrate. In addition, a two-dimensional deconvolution procedure was attempted to retrieve the “true” residual stress distribution piled up between adjacent Cu-Ta lines, according to a theoretical model for embedded structural elements. As probed on the nanometer scale by the NBOHC sensor, the interfaces were found under a substantially enhanced residual stress, characteristic for low-temperature Si∕SiOx growth in the presence of metallic interconnects. CL/PS spectroscopy represents an improved tool to quantitatively monitor the residual stresses developed at SiOx∕metal interfaces, thus opening the possibility to systematically engineer the interface itself in search for high-reliability Si-based devices.

Stress Dependence of Sapphire Cathodoluminescence from Optically Active Oxygen Defects as a Function of Crystallographic Orientation

The cathodoluminescence (CL) spectrum of the synthetic sapphire single crystal has been studied with respect to the different crystallographic planes of the crystal in order to describe the tensorial stress dependence of the band related to oxygen defects. Experiments provide the link between CL spectral shift and stress, which is referred to as the piezo-spectroscopic (PS) relationship. Using the biaxial stress field developed at the tip of cracks generated from the corners of a Vickers indentation, we clearly detected stress dependence for the cumulative band arising from the F+-center transition of optically active oxygen vacancies. The matrix of PS coefficients along different crystallographic axes of single-crystal sapphire could be precisely determined. The shallow nature of the electron probe may enable the characterization of surface stress fields with a spatial resolution that may not be easily available by conventional laser probes. The PS calibration results collected on oxygen-defect bands allow the direct determination of unknown surface residual stress fields stored in sapphire substrates for electronics applications.

Quantitative Assessment of Crack-Tip Stress Field in Semiconductor GaN Using Electrostimulated Piezo-Spectroscopy

The piezo-spectroscopic (PS) effect, which may be defined as the shift in wavelength of a spectroscopic transition in a solid in response to an applied strain or stress, may occur both in crystalline and in amorphous structures, regardless of the particular spectroscopic transition involved (e.g., luminescence or Raman spectrum), and independent of the specific mechanism of luminescence emission (i.e., including spectra generated from substitutional impurities, optically active point defects, etc.). The PS effect can be monitored on electro-stimulated spectra when the scale on which the needed characterization lie is of a nanometer length. This effect, being a physical property of the studied material, should be calibrated case by case. The high scanning speed (and computer control) of the electron beam, which can be easily obtained with scan coils, is unsurpassed. Since the most recently developed optoelectronic devices have active areas of submicron dimensions and many of them less than 100 nm, no obvious choice is left but urgently developing an electro-stimulated probe for nano-scale residual stress assessments. In this paper, we show the feasibility of nano-scale stress assessments in the neighborhood of the tip of a crack propagating in GaN, selected as a paradigm semiconductor material.

Raman piezospectroscopic evaluation of intergrowth ferroelectric polycrystalline ceramic in biaxial bending configuration

The piezospectroscopic (PS) effect was studied in an intergrowth bismuth layer-structure ferroelectric ceramic Bi5TiNbWO15 according to a micro-Raman spectroscopic evaluation. By using a ball-on-ring flexure configuration, a biaxial stress was generated in a Bi5TiNbWO15 plate-like specimen and in situ collected Raman spectra were acquired and analyzed under several loading conditions. As the observed spectral line contained signals arising from the whole illuminated in-depth region, the laser probe information was deconvoluted (by means of an in-depth probe response function obtained according to the defocusing method) in order to deduce biaxial PS coefficients for the three Raman bands of Bi5TiNbWO15 located at 763, 857, and 886 cm−1, respectively. The biaxial PS coefficients of these bands were derived to be −1.74±0.16, −2.51±0.16, and -2.64±0.31 cm−1/GPa, respectively, and should be referred to the c axis of the Bi5TiNbWO15 crystal.

Cathodoluminescence Investigation of Residual Stress in Er3+:YAlO3Thin Films Grown on (110) SrTiO3Substrate by Metal-Organic Chemical Vapor Deposition

YAlO3 thin films doped with different amounts of Er3+ have been grown directly onto (110) SrTiO3 substrate using the metal-organic chemical vapor deposition method (MOCVD). X-ray diffraction patterns and the rocking curve of the (002) reflection point to the growth of <001>-oriented YAlO3 phase. Piezo-spectroscopic (PS) biaxial calibration was performed on two luminescence bands, related to transitions from the (4)S3/2 excited state, using a specially designed ball-on-ring loading jig. Such a PS calibration allowed us to retrieve the rate of wavelength shift with stress without separating the grown film from the substrate. The outcome of the PS calibration has been applied to build up in the field emission scanning electron microscope (FEG-SEM) high-resolution maps of the residual stress field developed in the film. Results indicate that the residual stress in Er3+:YAlO3 films were compressive in nature and increased with increasing Er3+ dopant concentration.

Stress dependence of F+-center cathodoluminescence of sapphire

The rate of spectral shift with applied biaxial stress [piezospectroscopic (PS) coefficient] was determined for the electron-stimulated F+ luminescence emitted from the c plane of sapphire (α-Al2O3) as Π=1.18±0.03nm∕GPa. The PS dependence could be determined to a degree of precision by applying a controlled biaxial stress field to a sapphire thin plate using a ball-on-ring biaxial bending jig and by measuring in situ the spectral shift of the emitted cathodoluminescence (CL) F+ band in a field-emission-gun scanning electron microscope. The ball-on-ring PS calibration results open the possibility of applying CL/PS assessments to directly determine unknown residual stress fields in sapphire-based devices using the optical activity of its oxygen vacancies.

Stress dependence of Raman vibrational bands of PbWO4 single crystals

AbstractA microscopic piezo‐spectroscopic (PS) investigation of the stress dependence of the Raman bands of lead tungstate was conducted, which used a ball‐on‐ring flexure method, aimed at adding controlled biaxial stresses on thin‐plate samples. Spectral shifts varying with applied loads and in‐depth positions were observed for a ‐ and c ‐plane PbWO4 single crystals, confirmed by polarized Raman experiments. Taking account of the laser probe function, theoretical convolutions were performed to determine precisely the PS coefficients, which were determined as Π 11 = Π 22 = –1.29 ± 0.20 cm–1/GPa and Π 33 = –0.87 ± 0.25 cm–1/ GPa for the 902 cm–1 A g species. Accordingly, the residual stresses generated by mechanical treatments were analysed based on the derived coefficients. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Raman Piezo-Spectroscopic Investigation of Microscopic Residual Stresses in Ni-MLCC Devices

Residual stresses in Ni-MLCC were investigated by Raman microprobe piezo-spectroscopic (PS) methods. The shape of the Raman spectrum of BaTiO3 in Ni-MLCC depended on the angle between the polarizing direction of the incident laser beam and the direction of the internal electrode. From a set of precise calibrations, we show that the orientation dependence arises from the interaction between internal stresses and the polycrystalline microstructure; by taking into account such an orientation dependence, we were able to establish a technique for the measurement of the distribution of residual stresses in Ni-MLCC.