Thermal wave imaging (TWI) was used to investigate damage processes in electron beam-physical vapor deposited (EB-PVD) ZrO 2 -8% wt.%Y 2 O 3 thermal barrier coatings (TBCs) during thermal cycling. TWI is a non-destructive and non-contact evaluation method. Small interfacial decohesions from manufacturing were captured by TWI after only two thermal cycles, but the decohesions did not grow with thermal cycling. No delamination was found in TBC specimens until failure, while surface temperature profiles of TWI indicated increases of thermal wave signal amplitude with thermal cycles. Local interfacial damage formed due to high thermal residual stresses during thermal cycling and induced thermal contact resistance in the TBCs, which resulted in the increases of thermal wave signal amplitude. The results of TWI confirmed the progressive damage evolution in TBCs observed by microscopy. The buckling propagation and spallation process were monitored by TWI.

Photo-stimulated luminescence piezo-spectroscopy (PLPS) is being developed as a non-destructive technique for thermal barrier coatings (TBC). In this study, the evolution of photo-stimulated luminescence with 1 hour at 1121 °C thermal cycling was systematically investigated from the thermally grown oxide (TGO) in a production TBC system which consists of electron beam physical vapor deposited 7wt% Y2O3-ZrO2, (Ni,Pt)Al bond coat and CMSX-4 superalloy substrate. The change of compressive stress in the TGO layer with thermal cycling was calculated from the wavelength shift of luminescence spectrum. It consists of an increase from 1.5 to 2.5 Gpa at as received state to 2.5 to 3.5 Gpa at 10 cycles, a gradual decrease to 1.0 to 2.0 Gpa until 500 cycles, then a plateau at this level until TBC spallation. Some other information, such as change of peak width and relative intensity of R1 and R2 peak with thermal cycling, was also analyzed. The Physical basis of the photo-stimulated luminescence evolution was examined using scanning electron microscopy (SEM).

Cr 3+ photoluminescence peizo-spectroscopy (CPLPS) is a technique that allows the measurement of the hydrostatic component of stress in α alumina oxides. This method is being developed in our laboratory as a non-destructive inspection technique for thermal barrier coating systems. It has been shown that the Cr 3+ concentration influences the magnitude of the shifts in the fluorescence frequencies, Δv of the R 1 and the R 2 peaks [1,2] and hence the apparent stress. Therefore, this work is an attempt to quantify the extent of this shift, consequent to the Cr 3+ concentration in the TBC systems and to study the variation of this effect with thermal cycles on the TBCs. In addition, we have developed a method of estimating Cr 3+ content based on the intensity of satellite peaks referred to as n-lines. The experiments are also designed to validate the n-line based method of estimating Cr 3+ content and its affect on the apparent stress value.

A major failure mechanism in plasma-sprayed thermal barrier coatings is spallation of the top coat due to the top/bond coat thermal expansion mismatch concomitant with deposition-induced interfacial roughness, oxide film growth and creep-induced normal stress reversal at the rough interface’s crest. Reduction of the thermal expansion mismatch through the use of heterogeneous bond coats has been suggested to increase coating durability. This approach is examined using the higher-order theory for functionally graded materials. Specifically, combined effects of a graded bond coat microstructure and oxide film thickness on the crack-tip stress field in the vicinity of a rough top/bond coat interface are investigated during furnace-type thermal cycling in the presence of a local horizontal delamination situated within the homogeneous top coat at the rough interface’s crest. The analysis, which accounts for the creep/relaxation effects within the individual constituents, is conducted in two distinct ways. In the first approach, the bond coat’s heterogeneous microstructure is fully taken into account while in the second approach the bond coat’s microstructure is homogenized. The feasibility of using graded bond coat microstructures to reduce horizontal delamination driving forces is critically examined and the limitations of the homogenization-based approach are highlighted.

Non-destructive determination of the remaining life of coatings of gas turbine parts is highly desirable. The present paper describes early attempts to prove the feasibility of doing this based on the optical measurement of the stress in the oxide that attaches the coating to the metal component. Both regression methods and neural network methods are compared and it was found that the neural network approach was superior for the case where multiple signal features were present. All methods provide useful predictions for the idealized case considered. Challenges presented by more complicated thermal cycles are discussed briefly.

There is a great deal interest in minimizing thermal and residual stresses at the interfaces of metals and ceramics. These stresses develop because of the large mismatch in thermally induced strains that exists between these two materials. The differences are significant enough to cause premature component failure in a variety of applications, including thermal barrier coatings (TBCs) for turbine blades and ceramic coatings for cutting tools. One approach to minimizing these stresses involves functionally grading the material distribution at the metal-ceramic interface. A significant amount of effort has been focused on the development of functionally graded nickel-alumina composites as a model system to determine the architectural features of the material gradient that will minimize these stresses. This effort has led to the development of an experimentally verified modeling approach based on thermomechanical, elastoplastic finite element analysis that can be used to predict the stress distributions in functionally graded metal-ceramic composite materials. This approach has been coupled with a mathematical optimization technique, known as a Genetic Algorithm (GA), to determine the optimal architecture for minimizing thermal and residual stresses at metal-ceramic interfaces.

Thermal protection systems based on thermal barrier coatings are widely used in turbine engines for propulsion and power generation. They commonly comprise oxide thermal barriers coatings (TBCs) deposited on an intermetallic bond coat (BC), and provide simultaneous thermal and oxidation protection. The benefit of these coatings resides in their ability to inhibit degradation of the underlying structural superalloy component by thermo-mechanical fatigue and oxidation. Existing commercial coatings are well-engineered with established durability and cost benefits. However, they lose adhesion and spall from the underlying metal with cyclic thermal exposure. Because coating failure occurs in a stochastic manner, with no assured cyclic life, the coatings cannot be used in a prime-reliant manner. Prime reliability is only achievable if a high level of basic understanding is gained about failure mechanisms, and material responses, that arise upon thermal cycling. Because of differing manufacturing approaches and operating scenarios, several specific mechanisms are involved. Present understanding of these phenomena has highlighted several nuances and challenges in developing thermal barrier coatings for use as prime-reliant components. This talk will review the current understanding of factors affecting coating durability and presents relationships between the durability, the governing material properties and the salient morphological features. The durability of thermal barrier coatings is governed by a sequence of crack nucleation, propagation and coalescence events that accumulate prior to final failure by large scale buckling and spalling.

Thermal spray coatings are used to protect surfaces against exposure to severe conditions. To insure a reliable protection, not only the structure and properties of the sprayed coatings must be optimized but also one needs to develop appropriate process control techniques to produce high performance coatings in a consistent manner, day after day. This is particularly important during plasma spraying as the wear of the electrodes affects significantly the plasma characteristics and consequently the coating properties. First, in this paper, the stability of plasma spray processes is investigated by monitoring in-flight particle characteristics and plasma fluctuations. Secondly, the possibility and advantages of controlling plasma spray processes by monitoring and regulating the condition of the sprayed particles are discussed. Finally, we will see how the properties of thermal barrier coatings and wear resistant coatings can be optimized by controlling the temperature and velocity of the sprayed particles both in the plasma spray and HVOF (high velocity oxy-fuel) processes.

Thermal barrier coatings (TBCs) are typically composed of a ceramic top coat, a thermally grown oxide, and an aluminide bond coat. These three layers each have specific roles in protecting super alloy substrates. State-of-the-art TBCs use Zirconia for the ceramic top coat and develop Alumina thermally grown oxide. Although the bond coats almost universally contain aluminides, their composition and processing vary greatly. In this work, a platinum aluminide bond coat system which was processed using an unactivated pack cementation process is studied. This bond coat system was formed on 1 inch diameter CMSX-4 super alloy disks.

In some oxide thin film systems, such thermal barrier coating systems (TBCs), thermal cycling leads to the development of geometric irregularities in the film. The evolution of these irregularities involves very large changes in aspect-ratio and often occurs rapidly over several hundreds of cycles. A key aspect of this behavior is the development of tensile stresses in the irregularity due to plastic yielding of the surrounding metal. These stresses can accelerate the elongation of the oxide (which translates into shape evolution of the irregularity) by various mechanisms, including enhanced oxide formation, inelastic stretching (creep) and failure of the oxide. An idealized analytical model consisting of a thin elastic shell embedded in an elastic-plastic matrix is used to explore the interactions between geometry, thermal strains, plasticity and oxide growth. Boundaries between purely elastic deformation, uni-directional yielding and reversed plasticity are shown to have a strong dependence on the size of the irregularity relative to the oxide thickness. For any given thermal strain, there is a critical aspect ratio of the irregularity that leads to maximum tensile stress in the oxide. The resulting closed-form solutions allow for quick and easy evaluations of various oxide-growth scenarios, including stress-dependent oxide formation. This talk will present the application of these models to TBC thermal cycling experiments, and discuss how stress-dependent oxide formation plays a role in the rapid evolution of these irregularities. A variety of oxide growth scenarios will be illustrated, and used to demonstrate that oxide failure and subsequent oxide formation in the cracked region is the most likely explanation for the rapid shape evolution seen in the experiments.

Yttria-stabilized Zirconia (YSZ) coatings deposited by electron beam physical vapor deposition on platinum aluminide and NiCoCrAlY bond coats on single crystal superalloy substrates have been oxidized at temperatures between 1000 and 1200°C in air. The cyclic oxidation lives of the systems with platinum aluminide bond coats were substantially longer than those with NiCoCrAlY bond coats. The thermally grown oxide (TGO) that develops between the bond coat and the TBC during oxidation, as well as the bond coat and the TBC adjacent to the TGO, have been examined in detail using optical metallography, scanning electron microscopy (SEM), and cross-sectional transmission electron microscopy (XTEM). The YSZ is observed to undergo significant amounts of sintering. The TGO grows by the inward diffusion of oxygen and the outward diffusion of aluminum. In some cases, the outward growth component incorporates some of the TBC into the TGO. The depletion of aluminum results in phase changes in the bond coats. Failure of the TBCs occurs after fixed amounts of oxidation which result in increasing amounts of elastic energy being stored in the TGO and YSZ as well as degradation of the TGO-bond coat interface. The fracture path changed as a function of exposure time and temperature with larger amounts of separation occurring at the TGO/BC interface for higher temperatures and longer exposures in dry air. Failure can be accelerated in the presence of water vapor, particularly if spinel formation is induced. Fracture occurs primarily in the oxides, in this case. The fracture surface for systems with platinum aluminide bond coats often contains precipitates, which are rich in refractory metals. These features do not appear to be prevalent with NiCoCrAlY bond coats.

Thermal protection systems based on ceramic thermal barrier coatings (TBCs) are used extensively to protect hot-section components in gas turbine engines. They comprise thermally insulating ceramic coatings, deposited on an aluminum-containing intermetallic bond coat (BC) that provides oxidation protection. A thin thermally-grown oxide (TGO layer forms between the TBC and BC during cyclic thermal exposure. Each of the system constituents evolves in service and all interact during thermal cycling to control the thermo-mechanical performance of the system. Exposed to thermal cycling conditions, TBC systems are susceptible to loss of adhesion and spalling failures. Multiple failure mechanisms exist, dependent upon differing thermal histoiy and processing approach for various coating systems. Coating failure is ultimately controlled by the large residual compression in the TGO and its role in amplifying the effects of imperfections in the vicinity of the TGO. The failure occurs through a process involving crack nucleation, propagation and coalescence events. For a particular commercial system, it is found that the TGO ‘ratchets’ into the bond coat with each thermal cycle, at an array of interfacial sites. The displacements induce strains in the superposed TBC that cause it to crack. The cracks extend laterally as the TGO ratcheting process proceeds, until the cracks from neighboring sites coalesce. Once this happens, the system fails by large scale buckling. It is shown that the displacements are ‘vectored’ by a lateral component of the growth strain in the TGO. The relative roles of bond coat visco-plasticity, initial interface morphology, and phase evolution are discuss. The behavior observed for this system is compared with predictions of a ratcheting model, as well as with the behavior observed for other commercial coating systems.

This paper presents the results of a systematic investigation of the effects of isothermal exposure on the microstructural evolution and the isothermal oxidation behavior of a thermal barrier coating with a platinum aluminide bond coat. Sintering of the columnar partially stabilized zirconia is shown to occur during isothermal exposure at 1150°C for durations up to 200 hours. This is accompanied by roughening of the TGO layers, the formation of hemi-spheroidal defects, parabolic coarsening of the TGO and final pull-out of the zirconia layers. The implications of the results are discussed for the analysis and design of TBCs for aeroengine applications.

The connection between processing and performance in thermal barrier coatings produced by electron-beam physical-vapor-deposition is not well understood. A significant component of the problem is the limited understanding of the role that microstructure (including defects) plays in this connection, and the relationships between microstructure and process parameters. This presentation will review recent work aimed at understanding these relationships. The focus will be on the evolution of porosity within the TBC, which is of paramount importance to both thermal conductivity and strain tolerance. Porosity is a result of shadowing processes in the presence of insufficient surface diffusion, and thus deposition temperature and vapor incidence pattern are major parameters. The presentation will highlight the interplay between vapor flux and surface topography, illustrating the relevant phenomena with experimental and modeling results. (Work sponsored by ONR under Grant N00014-99-1-0471 and the UC-LANL Collaborative Research Program.)

A major concern with thermal barrier coatings (TBCs) is their loss of adhesion during service, leading to spallation. In this research, an indentation test is used to quantify decreases in interfacial toughness of TBC systems as a function of the duration of isothermal or cyclic high-temperature exposures. A detailed description is given of the indentation test, which involves penetration of the TBC and the oxide layer below it, inducing plastic deformation in the underlying metal bond coat and superalloy substrate. This plastic deformation induces a compressive radial stress away from the indent, which drives an axisymmetric delamination of the TBC and oxide layers. Detailed elastic-plastic model results are presented which allow determination of the toughness of the oxide/bond coat interface from a measured radius of delamination.

Thermal Barrier Coatings have been widely used in modern turbine engines to protect the nickel based metal substrate from the high temperature service conditions, 1600–1800 K. In this study, some of the failure mechanisms of typical Air Plasma Sprayed Thermal Barrier Coatings (TBC) used in after-burner structures composed of three major layers: Inconel 718 substrate, NiCrAlY based metallic bond coat (BC) and Yttria Stabilized Zirconia (YSZ) based ceramic top coat (TC) are investigated. Investigation of the cracking mechanism of TBC in terms of design and performance is very important because the behavior of TBCs on ductile metallic substrates is brittle. To this end, four-point bending experiments conducted in Kütükoğlu (2015) is analyzed by using the Extended Finite Element Method (XFEM). All the analyses are conducted with the commercial finite element software ABAQUS. Three different models with varying TC and BC thicknesses are studied under four-point bending. It is observed that multiple vertical cracks are initiated in the TC. Cracks initiate at the top of YSZ and propagate through the whole TC. It is observed that the average crack spacing increases with the increasing thickness of the TC. Numerical results are found to be consistent with the experimental results. In other words, the average crack spacing for three different models are similar with the experimental results.

Thermal protection of components such as turbine blades is often done with thermal barrier coatings which are typically ceramic materials. Methods to manufacture ceramic coatings are being developed to create microstructures that optimize thermal protection without degrading mechanical properties of the coating. The coating requires sufficient mechanical properties to remain in place during loads associated with the operation of the component. The work presented in this paper is part of a broader effort that focuses on novel processing techniques. A fabrication method of interest is the inclusion of spherical micron-sized pores to scatter photons at high temperatures along with nano-sized grains to scatter phonons. Pores are sized and distributed so that mechanical strength is maintained. In the current work, yttria-stabilized zirconia (YSZ) is modeled. Three-dimensional microstructures representing YSZ are computationally generated. The defect sizes and orientations are generated to match an experimentally observed distribution. The defects are either randomly or regularly placed in the microstructural models. Stress-displacement analysis is used to determine effective bulk material properties. Comparisons are made to prior two-dimensional work and to experimental measurements available in the literature as appropriate. The influences that defect distributions and three dimensional effects have on the effective bulk material properties are quantified. This work is a preliminary step toward understanding the impacts that micron sized pores, voids and cracks have on thermal and mechanical characteristics. The goal is to facilitate optimizing the microstructure for thermal protection and strength retention.

Thermal barrier coatings are often used to protect a component by reducing temperature excursions. Such coatings are in use on engineered products such as turbine blades. The work presented is part of a broader effort that is focusing on new and novel processing techniques for thermal barrier coatings. Manufacturing methods are being developed to create microstructures that optimize thermal protection while not degrading the mechanical properties of the coating. Sufficient mechanical properties are necessary so the coatings do not fail as a result of loadings associated with the operation of the component. One fabrication method investigated is the inclusion of spherical micron-sized pores to reflect heat radiation at high temperatures along with nano-sized grains to reflect phonons thus providing thermal protection. Pores are sized and distributed so that sufficient mechanical strength is maintained. In the current work the model material used is a yttria-stabilized zirconia (YSZ). Two-dimensional microstructures representing YSZ are computationally generated. The size and distribution of defects that have been experimentally observed to develop during bulk processing are incorporated into the computationally generated microstructural models. Heat transfer and stress-displacement analyses are performed to determine effective bulk material properties. Comparisons are made to experimental measurements available in the literature as appropriate. The influence that defect dimensions and distributions have on the effective bulk material properties are quantified as a first step understanding the impacts that micron sized pores, voids and cracks have on thermal and mechanical characteristics which will facilitate optimizing the microstructure for thermal protection and strength retention.

Industries such as aerospace, automotive, oil and gas utilize various chemical, thermal, and mechanical techniques to improve the surface properties of engineering components. Deposition of metallic or nonmetallic materials on the surface of engineering components using different thermal spraying techniques is a common method to improve the mechanical properties of the surface of the components working at severe conditions. Thermal spraying techniques are capable of deposition of a coating layer with high corrosion, wear, erosion, and high temperature resistance. This technology can also be used for surface repair and treatment. Zirconia (ZrO 2 ) based coatings are excellent candidates to serve at high temperature due to their tribological and insulation properties, and also high stiffness. ZrO 2 -based coatings are usually used in aircraft and gas-turbine engines as thermal barrier coatings. However, the relatively low wear and erosion resistance of Zirconia-based coatings limits their application. Among all coating materials, Tungsten Carbide (WC) based materials are commonly used to improve wear and corrosion resistance of the surface. It is speculated that combination of ZrO 2 and WC follows by generating a coating with desirable thermal and mechanical properties, particularly at high temperature conditions. In the presented work, an innovative thermally sprayed coating material was proposed by depositing mixture of ZrO 2 -Y 2 O 3 and WC-Ni (YPSZ/WC-Ni) powders on a low carbon steel substrate using Atmospheric Plasma Spraying (APS). As thermomechanical properties of coatings are under the influence of the microstructural features such as porosity, micro cracks, voids, and possible oxides, in this study microstructure and phase consistency of the resultant coating was briefly evaluated. To this end, Optical Microscopy, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) were used. The results indicated that deposited coating was well-bonded to the substrate with minimum observed separation line. Porosity amount of APS deposited YPSZ/WC-Ni measured by image analysis of the cross-sectional area Moreover, mechanical properties including hardness and elastic modulus of the coating were evaluated. Since thermally sprayed coatings exhibit anisotropic behavior, the Knoop hardness in the longitudinal and transverse directions were analyzed in this study. Elastic modulus of the coating was also evaluated, based on the measurement of elastic recovery of Knoop indentation in both directions using Marshal analytical model. Wear resistance of the coating was also investigated by pin-on-disk method, at room temperature. The friction coefficient of the consecutive coating was calculated and had a value lower than that of reported for APS deposited YPSZ.

Thermal Barrier Coating (TBC) system has been increasingly applied to gas turbine engine because of its outstanding ability to effectively protect substrate materials against impinging hot gas. However, the durability issue has still existed due to the growth of oxidation layer called thermal growth oxidation (TGO) and depletion of aluminum. Therefore, more detailed investigation is required to understand the effects of these two factors on the durability of TBC. In this study, a finite element (FE) model was developed to calculate the stress state near TGO interfaces by considering both the morphology of TGO and aluminum depletion effect. For acquiring the mechanical properties at the depletion area of bond coating layer, indentation tests were carried out, particularly, to the TBC specimens isothermally exposed at 1000 degree Celsius for 100h. Based on the proposed FE model with specific parameters obtained from experiments, parametric studies were performed in a variety of conditions of mechanical properties and TGO thickness. Simulation results clearly showed the influence of aluminum depletion and TGO growth on the durability of TBC system. Finally, optimal design criteria can be suggested to minimize the stress of TBC system.