Esteban P. Busso

A mechanistic study of oxidation-induced degradation in a plasma-sprayed thermal barrier coating system.: Part I: model formulation

The effect of the oxidation induced degradation of a typical plasma-sprayed thermal barrier coat- ing (PS-TBC) system on the local ceramic-metal interfacial stresses responsible for the nucleation of mesos- copic cracks is investigated. A coupled oxidation-constitutive approach is proposed to describe the effect of the phase transformations caused by local internal and external oxidation processes on the constitutive behav- iour of the metallic coating. The coupled constitutive framework is implemented into the finite element method and used in parametric studies employing periodic unit cell techniques. The effects of service, micro- structural and ceramic-metal interface parameters on the peak interfacial stresses during service and cooling to room temperature are quantified. The results of the parametric unit cell FE analyses revealed a strong dependency of the local stresses responsible for mesoscopic crack nucleation and growth on the local mor- phology of the oxidised interface, the sintering of the ceramic coating, stress relaxation effects due to creep, the thickness of the thermally grown oxide (TGO), and the applied mechanical loads. When no mechanical straining of the TBC system is considered, local tensile stresses normal to the coating surface within the ceramic top coating reach values of up to 330 MPa at room temperature for a critical TGO thickness of approx. 3 µm.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Multiscale modelling of nanomechanics and micromechanics: an overview

Recent advances in analytical and computational modelling frameworks to describe the mechanics of materials on scales ranging from the atomistic, through the microstructure or transitional, and up to the continuum are reviewed. It is shown that multiscale modelling of materials approaches relies on a systematic reduction in the degrees of freedom on the natural length scales that can be identified in the material. Connections between such scales are currently achieved either by a parametrization or by a ‘zoom-out’ or ‘coarse-graining’ procedure. Issues related to the links between the atomistic scale, nanoscale, microscale and macroscale are discussed, and the parameters required for the information to be transferred between one scale and an upper scale are identified. It is also shown that seamless coupling between length scales has not yet been achieved as a result of two main challenges: firstly, the computational complexity of seamlessly coupled simulations via the coarse-graining approach and, secondly, the inherent difficulty in dealing with system evolution stemming from time scaling, which does not permit coarse graining over temporal events. Starting from the Born–Oppenheimer adiabatic approximation, the problem of solving quantum mechanics equations of motion is first reviewed, with successful applications in the mechanics of nanosystems. Atomic simulation methods (e.g. molecular dynamics, Langevin dynamics and the kinetic Monte Carlo method) and their applications at the nanoscale are then discussed. The role played by dislocation dynamics and statistical mechanics methods in understanding microstructure self-organization, heterogeneous plastic deformation, material instabilities and failure phenomena is also discussed. Finally, we review the main continuum-mechanics-based framework used today to describe the nonlinear deformation behaviour of materials at the local (e.g. single phase or grain level) and macroscopic (e.g. polycrystal) scales. Emphasis is placed on recent progress made in crystal plasticity, strain gradient plasticity and homogenization techniques to link deformation phenomena simultaneously occurring at different scales in the material microstructure with its macroscopic behaviour. In view of this wide range of descriptions of material phenomena involved, the main theoretical and computational difficulties and challenges are critically assessed.

Use of scaling functions to determine mechanical properties of thin coatings from microindentation tests

The indentation of elastic‐plastic coatings deposited on elastic substrates is studied in this work. The functional expressions that relate the load‐indentation behaviour to coating and substrate material properties are derived using dimensional analysis in conjunction with finite element simulations. Based on these scaling functions, a method is proposed to determine the coating Young’s modulus, yield strength and strain hardening properties from the microindentation tests. A method to obtain improved estimates of the material hardness is also discussed. While the analyses are based on a conical (i.e. Rockwell) indenter, the indenter angle used in the simulations was selected so that the results can also be used to extract mechanical properties from measurements using the pyramidal indenters such as Vickers and Berkovich types. ” 2000 Elsevier Science Ltd. All rights reserved.

A physics-based life prediction methodology for thermal barrier coating systems

A novel mechanistic approach is proposed for the prediction of the life of thermal barrier coating (TBC) systems. The life prediction methodology is based on a criterion linked directly to the dominant failure mechanism. It relies on a statistical treatment of the TBC’s morphological characteristics, non-destructive stress measurements and on a continuum mechanics framework to quantify the stresses that promote the nucleation and growth of microcracks within the TBC. The last of these accounts for the effects of TBC constituents’ elasto-visco-plastic properties, the stiffening of the ceramic due to sintering and the oxidation at the interface between the thermally insulating yttria stabilized zirconia (YSZ) layer and the metallic bond coat. The mechanistic approach is used to investigate the effects on TBC life of the properties and morphology of the top YSZ coating, metallic low-pressure plasma sprayed bond coat and the thermally grown oxide. Its calibration is based on TBC damage inferred from non-destructive fluorescence measurements using piezo-spectroscopy and on the numerically predicted local TBC stresses responsible for the initiation of such damage. The potential applicability of the methodology to other types of TBC coatings and thermal loading conditions is also discussed.

A mechanistic study of oxidation-induced degradation in a plasma-sprayed thermal barrier coating system. - Part II: Life prediction model

A parametric study is conducted to quantify the effects of different microstructural variables and service conditions on the local stresses induced by oxidation, sintering processes and thermal cycling in a typical plasma sprayed thermal barrier coating (PS-TBC) system. The study relies on the numerical results obtained from a continuum mechanics-based mechanistic study of the oxidation-induced degradation of the PS-TBC system. Analytical expressions are presented for the peak out-of-plane stress component which promotes the nucleation and growth of mesoscopic cracks within the top zirconia-based ceramic coating in terms of thermal cycle parameters, and accumulated oxidation time. Based on the results of the parametric study, a damage mechanics-based life prediction methodology for the failure of the PS-TBC under thermal fatigue loading conditions is proposed. The model assumes that PS-TBC failure occurs by a cleavage-type mechanism within the top zirconia coating, in agreement with experimental evidence, and that the accumu- lation of damage with thermal cycling is linked to the gradual degradation of the intrinsic cleavage strength of the zirconia. The model is shown to be capable of predicting consistently a broad range of thermal fatigue data.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

A DISLOCATION MECHANICS-BASED CRYSTALLOGRAPHIC MODEL OF A B2-TYPE INTERMETALLIC ALLOY

A crystallographic slip based model for cubic oriented NiAl single crystals is derived from an idealization of the dislocation network observed in the active slip systems, viz. {110} 〈110〉. The crystallographic model successfully accounts for the cyclic steady-state behaviour of crystals subjected to strain histories within the range ϵ〈100〉 = ϵm ± 0.5%, for ϵm = 0 and 35%, at 750 and 850°C. It accurately predicts the flow stress dependence on temperature, strain rate and dislocation density arising from the lattice resistance to dislocation motion and from discrete obstacle resistance due to dislocation interactions. The kinematic and isotropic hardening modes associated with defect trails left behind by gliding dislocations and dislocation storage, respectively, are properly represented. The average distance that dislocations have to glide for their density to increase beyond the level needed to balance dynamic recovery processes was predicted to be approximately 260 times the random forest dislocation spacing. Measured dislocation densities at different mean strains were found to be consistent with the predictions of the theoretical model.

Self-consistent elastoplastic stress solutions for functionally graded material systems subjected to thermal transients

In this work, a self-consistent constitutive framework is proposed to describe the behaviour of a generic three-layered system containing a functionally graded material (FGM) layer subjected to thermal loading. Analytical and semi-analytical solutions are obtained to describe the thermo-elastic and thermo-elastoplastic behaviour of a three-layered system consisting of a metallic and a ceramic layer joined together by an FGM layer of arbitrary composition profile. Solutions for the stress distributions in a generic FGM system subjected to arbitrary temperature transient conditions are presented. The homogenisation of the local elastoplastic FGM behaviour in terms of the properties of its individual phases is performed using a self-consistent approach. In this work, power-law strain hardening behaviour is assumed for the FGM metallic phase. The stress distributions within the FGM systems are compared with accurate numerical solutions obtained from finite element analyses and good agreement is found throughout. Solutions are also given for the critical temperature transients required for the onset of plastic deformation within the three-layered systems.

Determination of the mechanical properties of metallic thin films and substrates from indentation tests

Abstract A procedure to obtain the elasto-plastic mechanical properties of strain-hardening materials from indentation tests, based on dimensional analysis and finite-element techniques, is proposed. The method is applicable to homogeneous materials and to coatings deposited on substrates of known mechanical properties. The Youngs modulus of the material is extracted from the initial slope of the unloading indentation curve and the yield strength and strain- hardening exponent are obtained from the maximum indentation load and the contact area after unloading. The method is used to obtain the properties of a high-alloy steel and Mo and AlSi coatings deposited on a steel substrate by plasma spraying. The sensitivity of the measurement to the depth of indentation is discussed.

Weibull stress solutions for 2-D cracks in elastic and elastic-plastic materials

The Weibull stress is widely used as a measure of the probability of cleavage failure. In this work analytical and semi-analytical expressions for the Weibull stress are developed in terms of the remote loading parameters, J or K, and material properties. Results are presented for sharp cracks and notches in elastic and elastic-plastic materials under plane stress and plane strain conditions. The proposed relations enable Weibull stress estimates to be obtained without the need for costly finite element analyses and provide insight into the use of the Weibull stress as a parameter for the prediction of cleavage failure of cracked bodies. The expressions have been verified using finite element techniques and good agreement has been found throughout. The results of the analyses have been used to interpret the mesh size dependence of Weibull stress values obtained from finite element calculations.

Numerical study of sliding wear caused by a loaded pin on a rotating disc

Abstract A computational approach is proposed to predict the sliding wear caused by a loaded spherical pin contacting a rotating disc, a condition typical of the so-called pin-on-disc test widely used in tribological studies. The proposed framework relies on the understanding that, when the pin contacts and slides on the disc, a predominantly plane strain region exists at the centre of the disc wear track. The wear rate in this plane strain region can therefore be determined from a two dimensional idealisation of the contact problem, reducing the need for computationally expensive three dimensional contact analyses. Periodic unit cell techniques are used in conjunction with a ratchetting-based failure criterion to predict the wear rate in the central plane strain region. The overall three dimensional wear rate of the disc is then determined by scaling the plane strain wear rate with a conversion factor related to the predicted shape of the wear track. The approach is used to predict pin-on-disc test data from an Al–Si coating using a tungsten carbide pin. The predicted results are found to be consistent with measured data.