The Experts below are selected from a list of 306 Experts worldwide ranked by ideXlab platform
David R Clarke - One of the best experts on this subject based on the ideXlab platform.
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the application of photoluminescence Piezospectroscopy for residual stresses measurement in thermally sprayed tbcs
Surface & Coatings Technology, 2017Co-Authors: S Dosta, C R C Lima, J M Guilemany, David R ClarkeAbstract:Abstract Photoluminescence Piezospectroscopy (PLPS) was used as a non-destructive technique for the measurement of residual stresses within the thermally grown oxide (TGO) layer beneath plasma-spray thermal barrier coatings (TBC). The technique has proved to be very effective for such measurements in YSZ thermal barrier coatings applied by EB-PVD but its application to thermal sprayed coatings has been hindered by optical scattering in plasma sprayed coatings of usual thicknesses. PLPS experiments were performed on TBCs with cold sprayed bond coatings and several different ceramic layer thicknesses after thermal cycling. The results are discussed as a function of the coating characteristics like bond coat spraying process, thickness of both bond and top coat, microstructural features, and damage accumulation.
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refined crystal field model for the Piezospectroscopy analysis of stresses in polycrystalline alumina
Journal of Applied Physics, 2015Co-Authors: Samuel Margueron, David R ClarkeAbstract:Stresses produce systematic shifts in the R-line luminescence from Cr3+ ions in aluminium oxide. The two calculational models for determining the stress from observed R-line shifts are compared and combined with new measurements, made under a variety of different stress conditions, to determine refined crystal field model parameters for the piezospectroscopic shifts with stress. In addition to previous, well established results, it is found that the R2-luminescence line shifts linearly with the hydrostatic component of stress, (σ11 + σ22 + σ33), even under non-hydrostatic stresses. The separation of the R1 and R2 lines is not linear with the non-hydrostatic stress and increases from biaxial to uniaxial stress states. Excellent agreement with experimental data for polycrystalline materials under uniaxial and biaxial stresses was found when averaging the stresses using the Voigt approximation. However, the R2-R1 peak separation may be affected by the stress dependence of the spectral shape or stress relaxat...
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Piezospectroscopy strain induced photoluminescence changes
Meeting Abstracts, 2011Co-Authors: David R ClarkeAbstract:Piezospectrosopy refers to the relationship between local strain and spectroscopic properties, usually luminescence, of materials. The best known examples are changes in band-gap luminescence associated with changes in local lattice parameter, such as due to epitaxial mismatch strains. These effects are well documented and so this talk will be primarily devoted to Piezospectroscopy from luminescent dopants in crystalline and amorphous materials, such as phosphors and luminescence-based sensors.
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Stress-Driven Wrinkling oF α-Alumina Films Grown oN Fe-Cr-Al by Thermal Oxidation
MRS Proceedings, 2011Co-Authors: V K Tolpygo, David R ClarkeAbstract:AbstractA thin α-Al2O3 film forms on Fe-20w/oCr-5w/oAl alloys when they are heated in air to about 1000°C. This thermally grown oxide exhibits a wrinkled morphology within minutes and with an amplitude and characteristic wavelength that increases with oxidation time. Measurements of the residual strain in the oxide, using Cr3+ photoluminescence Piezospectroscopy, indicate that the wrinkling is associated with stress relaxation during the course of the thickening of the oxide. AFM and piezospectroscopic measurements reveal a strong time dependence of the growth stress and oxide wrinkling, as well as a marked dependence on the crystallographic orientation of the underlying Fe-Cr-Al substrate. Small additions (about 0.3 w/o) of yttrium to the metal completely suppress the wrinkling phenomenon although it is found not to lessen the growth stresses in the oxide film.
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Stress Distributions in Plasma-Sprayed Thermal Barrier Coatings Under Thermal Cycling in a Temperature Gradient
Journal of Applied Mechanics, 2010Co-Authors: Andi M. Limarga, Robert Vaßen, David R ClarkeAbstract:The residual stress distribution in plasma-sprayed zirconia thermal barrier coatings subjected to cyclic thermal gradient testing was evaluated using Raman Piezospectroscopy and finite element computation. The thermal gradient testing (approximately 440°C /mm at temperature), consisted of repeated front-side heating with a flame and constant cooling of the back-side of the substrate either with front-side radiative cooling only or with additional forced air cooling between the heating cycles. The coatings exhibited characteristic “mud-cracking” with the average crack spacing dependent on the cooling treatment. This is consistent with finite element calculations and Raman spectroscopy measurements in which the sudden drop in coating surface temperature on initial cooling leads to a large biaxial tension at the surface. The key to proper interpretation of the Raman shifts is that the stress-free Raman peaks need to be corrected for shifts associated with the evolution of the metastable tetragonal phase with aging. DOI: 10.1115/1.4002209
Giuseppe Pezzotti - One of the best experts on this subject based on the ideXlab platform.
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ball on ring test in ceramic materials revisited by means of fluorescence Piezospectroscopy
Journal of The European Ceramic Society, 2011Co-Authors: Alessandro Alan Porporati, Takahiro Miyatake, Kristina Schilcher, Giuseppe PezzottiAbstract:Abstract Ball-on-ring test is widely used to measure the biaxial strength of brittle materials by often making the approximation that the pressure applied by the ball in the central loading region is uniform. The purpose of the present study is to demonstrate the limits of such approximation by means of Piezospectroscopy and to substantiate the spectroscopic findings with calculations based on finite element modeling (FEM). In addition, we shall discuss in some detail the validity of theoretical expressions previously developed by Kirstein and Woolley for radial and tangential bending stresses in the case of a uniformly distributed concentric load applied at the center of the disk loading region. A comparison is also offered between the experimentally retrieved radius of such region and those computed by theoretical models. Errors in estimating the radius of the central loading region, which have led in the past to controversial discussions, have been shown to play a minor role in the analysis of the overall stress field.
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spatially resolved residual stress assessments of gan film on sapphire substrate by cathodoluminescence Piezospectroscopy
Journal of Applied Physics, 2008Co-Authors: Giuseppe Pezzotti, Alessandro Alan Porporati, Andrea LetoAbstract:Two cathodoluminescence piezospectroscopic (CL/PS) approaches for measuring the residual stress distribution in thin films are critically examined and compared using an intrinsic GaN film sample (2.5μm in thickness) grown on a (0001)-oriented sapphire substrate. The first approach invokes an analytical model to fit experimental stress distributions as retrieved in both film and substrate at the edge of an artificially created cross section of the sample. Such an edge-stress distribution takes into account both the thermal expansion mismatch between the film and substrate and the mechanistics of film growth process. In the second approach, we directly and nondestructively measure the bulk residual stress field from the sample top surface on the film side using an increase in electron beam voltage (maintaining a constant beam power) as a means for screening the film subsurface. In this latter case, the combined effects of self-absorption and misfit dislocations on the GaN spectrum severely affect the CL/PS ...
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spatially resolved crack tip stress analysis in semiconductor by cathodoluminescence Piezospectroscopy
Journal of Applied Physics, 2007Co-Authors: Alessandro Alan Porporati, Atsuo Matsutani, Nicola Lama, Giuseppe PezzottiAbstract:A spatially resolved cathodoluminescence piezospectroscopic analysis is attempted for the high-resolution evaluation of the stress field developed ahead of the tip of an equilibrium crack propagating in a semiconductor. GaN was selected for this assessment as a paradigm semiconductor material. Quantitative measurements of in-plane luminescence probe response function (PRF) were preliminarily performed at different acceleration voltages upon scanning across a straight and atomically sharp interface between GaN and gold metal. Then, based on the knowledge of PRF, the convoluting effect due to the finite size of the electron probe could be corrected and an improved plot of the crack-tip stress field could be retrieved by a computer-aided data restoration procedure. The crack-tip stress intensity factor KI obtained by the cathodoluminescence piezospectroscopic method was compared with that obtained on the same crack path according to high-resolution measurements of crack-tip opening displacement. This study n...
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Residual Stress Assessment in the Al2O3\Mullite Based Laminated System
Key Engineering Materials, 2007Co-Authors: Lorenzo Micele, Mylene Brach, Francis Chalvet, Goffredo De Portu, Giuseppe PezzottiAbstract:To improve mechanical properties of mullite, a mullite-Al2O3\mullite laminate composite was prepared. Lamination generates residual stresses within the structure, measured by Piezospectroscopy. A preliminary and complete piezo-spectroscopic characterization of the Al2O3\mullite system was carried out. A method to determine the concentration of Al2O3 in the composite by Raman spectrum was proposed and used to assess the composition of the laminated structure along the cross section. The experimental results evidenced a gradual change of composition and residual stress state between the two layer.
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Investigation of local orientation and stress analysis of PZT-based materials using micro-probe polarized raman spectroscopy
Journal of The European Ceramic Society, 2005Co-Authors: Marco Deluca, Tatsuo Sakashita, Carmen Galassi, Giuseppe PezzottiAbstract:Abstract 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.
Ping Xiao - One of the best experts on this subject based on the ideXlab platform.
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residual stress trend in thermal barrier coatings in through thickness direction measured by photoluminescence Piezospectroscopy
Advances in Applied Ceramics, 2010Co-Authors: C Chen, Xiaofeng Zhao, Ping Xiao, Ian P Shapiro, H Peng, S K GongAbstract:Abstract Photoluminescence Piezospectroscopy (PLPS) has been used to determine residual stresses in sapphire, alumina in the yttria stablised zirconia (YSZ)/Al2O3 composite and alumina in thermal barrier coatings (TBCs). The TBC of YSZ containing 0·5 wt-% alumina has been produced using electron beam physical vapour deposition. The stress profile through the TBC thickness was measured using a depth sensing method. Reasonable residual stress profiles have been obtained using PLPS with the confocal system for all three material systems. Measurements of TBCs suggest that stress distribution in a TBC system is not uniform in general. However, uniform stress distribution has been found in some positions where damage in TBCs might occur.
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focus effect of photoluminescence Piezospectroscopy and its influence on the stress measurement of thermally grown oxide in thermal barrier coatings
Scripta Materialia, 2007Co-Authors: Xiaofeng Zhao, Ping XiaoAbstract:The residual stresses in thermally grown oxide (TGO) on a Fecralloy substrate were measured by photoluminescence Piezospectroscopy (PLPS). It was found that the distance between the laser focus and the TGO surface has a remarkable effect on the peak shifts measured by PLPS. Further examinations on the TGO in thermal barrier coatings (TBCs) confirmed this effect, indicating that the presence of a TBC will affect the stress measurement in the TGO.
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residual stresses in thermal barrier coatings measured by photoluminescence Piezospectroscopy and indentation technique
Surface & Coatings Technology, 2006Co-Authors: Xiaofeng Zhao, Ping XiaoAbstract:Residual stresses in electron beam physical vapour deposition (EB-PVD) thermal barrier coatings (TBCs) have been measured by photoluminescence Piezospectroscopy (PLPS). It was found that the macro-stress, caused by the thermal mismatch between the substrate and the coating, decreases from the TBC/TGO interface to the top surface. The Young's modulus, measured on the cross-section of EB-PVD coating by nano-indentation, was also found to decrease from TBC/TGO interface to top surface. The variation of Young's modulus can be explained by change in the contact area of TBC under macro-stress. A model was employed to examine the contact area in relation to the stresses. And the stresses estimated based on results from nano-indentation measurements showed the same trend as that obtained from PLPS.
Alessandro Alan Porporati - One of the best experts on this subject based on the ideXlab platform.
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ball on ring test in ceramic materials revisited by means of fluorescence Piezospectroscopy
Journal of The European Ceramic Society, 2011Co-Authors: Alessandro Alan Porporati, Takahiro Miyatake, Kristina Schilcher, Giuseppe PezzottiAbstract:Abstract Ball-on-ring test is widely used to measure the biaxial strength of brittle materials by often making the approximation that the pressure applied by the ball in the central loading region is uniform. The purpose of the present study is to demonstrate the limits of such approximation by means of Piezospectroscopy and to substantiate the spectroscopic findings with calculations based on finite element modeling (FEM). In addition, we shall discuss in some detail the validity of theoretical expressions previously developed by Kirstein and Woolley for radial and tangential bending stresses in the case of a uniformly distributed concentric load applied at the center of the disk loading region. A comparison is also offered between the experimentally retrieved radius of such region and those computed by theoretical models. Errors in estimating the radius of the central loading region, which have led in the past to controversial discussions, have been shown to play a minor role in the analysis of the overall stress field.
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spatially resolved residual stress assessments of gan film on sapphire substrate by cathodoluminescence Piezospectroscopy
Journal of Applied Physics, 2008Co-Authors: Giuseppe Pezzotti, Alessandro Alan Porporati, Andrea LetoAbstract:Two cathodoluminescence piezospectroscopic (CL/PS) approaches for measuring the residual stress distribution in thin films are critically examined and compared using an intrinsic GaN film sample (2.5μm in thickness) grown on a (0001)-oriented sapphire substrate. The first approach invokes an analytical model to fit experimental stress distributions as retrieved in both film and substrate at the edge of an artificially created cross section of the sample. Such an edge-stress distribution takes into account both the thermal expansion mismatch between the film and substrate and the mechanistics of film growth process. In the second approach, we directly and nondestructively measure the bulk residual stress field from the sample top surface on the film side using an increase in electron beam voltage (maintaining a constant beam power) as a means for screening the film subsurface. In this latter case, the combined effects of self-absorption and misfit dislocations on the GaN spectrum severely affect the CL/PS ...
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spatially resolved crack tip stress analysis in semiconductor by cathodoluminescence Piezospectroscopy
Journal of Applied Physics, 2007Co-Authors: Alessandro Alan Porporati, Atsuo Matsutani, Nicola Lama, Giuseppe PezzottiAbstract:A spatially resolved cathodoluminescence piezospectroscopic analysis is attempted for the high-resolution evaluation of the stress field developed ahead of the tip of an equilibrium crack propagating in a semiconductor. GaN was selected for this assessment as a paradigm semiconductor material. Quantitative measurements of in-plane luminescence probe response function (PRF) were preliminarily performed at different acceleration voltages upon scanning across a straight and atomically sharp interface between GaN and gold metal. Then, based on the knowledge of PRF, the convoluting effect due to the finite size of the electron probe could be corrected and an improved plot of the crack-tip stress field could be retrieved by a computer-aided data restoration procedure. The crack-tip stress intensity factor KI obtained by the cathodoluminescence piezospectroscopic method was compared with that obtained on the same crack path according to high-resolution measurements of crack-tip opening displacement. This study n...
Xin Wang - One of the best experts on this subject based on the ideXlab platform.
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Evolution of stress and morphology in thermal barrier coatings
Surface and Coatings Technology, 2010Co-Authors: Xin Wang, L. Chirivi, A Atkinson, John R NichollsAbstract:Residual stress in the thermally grown oxide (TGO) in thermal barrier coatings (TBCs) was measured by photoluminescence Piezospectroscopy (PLPS) and stress maps created to track local stress changes as a function of thermal cycling. The local stress images were observed to be correlated with morphological features on the metal surface that were purposely introduced during specimen preparation. Local stress relaxation and morphological evolution with thermal cycling were studied using the stress maps combined by post-mortem SEM examination. It was found that the morphology in the specimen having an initial polished surface was quite stable, while that in the specimen with a rough surface was unstable. The average residual stress in the specimen with the unstable morphology decreased with thermal cycling and it eventually failed along TGO/YSZ interface. The specimen with stable morphology maintained a high TGO stress throughout the thermal cycling process and failed along TGO/bond coat interface. The rough surface was also found to give rise to the formation of transition alumina ([theta]-Al2O3) in the TGO which was correlated with a reduced TGO stress
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investigation of tbcs on turbine blades by photoluminescence Piezospectroscopy
Acta Materialia, 2009Co-Authors: Xin Wang, Alan AtkinsonAbstract:Abstract Thermal barrier coating (TBC) blade specimens with Pt diffusion bond coats were subjected to thermal cycling with periodic measurements of the residual stress in the thermally grown oxide (TGO) using photoluminescence Piezospectroscopy. Two distinct stress levels were generally found to coexist in the probed volume, i.e. a high stress (∼4 GPa) and a low stress (∼500 MPa) level. Both the high and low stress levels were independent of the curvature of the blade surface, in agreement with numerical modelling based on a composite cylinder stress model. The relative contributions of the two stress levels appear to be correlated with the θ -Al 2 O 3 content of the TGO, which was dependent on the position on the blade. The TBCs tended to fail along the TGO/bond coat interface in thermal cycling. This was modified by the presence of mixed transition metal oxides in the TGO. The results are interpreted in terms of a likely failure mechanism.