Behavior Of Thermal Barrier Coatings Research Articles

Interfacial failure behavior of Thermal Barrier Coatings (TBCs) at high temperatures: An in-situ indentation study based on X-ray imaging

Interfacial failure behavior is a major concern during the service of thermal barrier coatings (TBCs). Quantitative study on interfacial failure behavior of TBCs at high temperatures remains a great challenge. In this study, an in-situ high-temperature cross-sectional indentation (HCSI) method based on X-ray imaging was developed. The out-of-plane displacement of TBCs was measured by in-situ digital radiography (DR) technology. The morphology of the interfacial crack was characterized by an in-situ computer tomography technology. It was found that the interfacial fracture region tended to be semi-elliptical shaped. The strain energy during HCSI was quantified and analyzed by finite element analysis (FEA). The composite stress intensity factor (SIF) and the mixed modal angles at high temperatures were obtained by analyzing the stress field at the crack tip. Interfacial toughness of TBCs at RT, 400 °C, and 800 °C were obtained. The interfacial toughness changes little ranging from RT to 400 °C, while it sharply decreases ranging from 400 °C to 800 °C. A prediction model for the evolution of interfacial toughness with temperature was obtained. The competition mechanism of interfacial cracks at different temperatures was discovered. The relationship between the change in interfacial toughness and the failure mode evolution was discussed.

Impact of bond coat types on calcium-magnesium-alumina-silicate and hot corrosion behavior in thermal barrier coatings

This study examines the corrosion behavior of thermal barrier coatings (TBCs) with bond coats (BCs) fabricated using Air plasma spraying (APS) and High velocity oxygen fuel (HVOF) against molten salt and Calcium-magnesium-alumina-silicate (CMAS) corrosion. Results show that TBCs with the APS-BC have higher resistance and experience fewer failures than those with the HVOF-BC due to the further growth of the aluminum-rich oxide layer in APS-BC coatings, causing the salt to be neutralized during the reaction with BC. The products of the reaction between CMAS and TGO lead to changes in coating interface morphology and degradation in coatings with HVOF BC.

Characteristics and performance of suspension plasma sprayed thermal barrier coatings on additively manufactured superalloy substrates

The complex-shaped hot-section parts of new-generation turbine engines demand unique design solutions. Additive Manufacturing (AM) is an emergent production method that can produce metallic parts with complex geometries and minimal material wastage. In this work, the characteristics and performance behavior of Thermal Barrier Coatings (TBCs) deposited on forged and AM-built HAYNES®282® superalloy substrates were studied and compared. The bond coats were produced by High-Velocity Air-Fuel (HVAF) spraying using NiCoCrAlY powder feedstock and TBC top-coats by Suspension Plasma Spraying (SPS) using water- and ethanol-based suspensions of Yttria-Stabilized Zirconia (YSZ). The microstructural features, adhesion, Thermal Cycling Fatigue (TCF) lifetime, and thermal shock lifetimes of the TBCs were comprehensively investigated. The results showed that the deposition of bond coats reduced the roughness and asperities of the AM-built substrates. Depending on the type of suspension used and the spray parameters employed, the TBCs exhibited vertically cracked and columnar microstructures. However, no significant differences in TCF and thermal shock lifetimes of TBCs on AM and forged substrates were observed. It is demonstrated that TBC systems can be produced on AM-built metallic substrates, and the resulting TBCs can have similar microstructures and properties as TBCs deposited on conventional substrates.

Role of Re in NiAl bond coating on isothermal oxidation behavior of a thermal barrier coating system at 1100 ℃

Role of Re in NiAl bond coating on intermittent isothermal oxidation behavior of thermal barrier coatings (TBCs) was researched in this study. Isothermal oxidation behaviors of NiAl-YSZ coatings with and without Re were investigated at 1100 ℃. Results showed that NiAlRe-YSZ coating revealed lower level of β to γ′ phase transformation, and less severe delamination of YSZ was observed compared with NiAl-YSZ coating after 300 h oxidation. Moreover, the synergistic effect of Re and Zr/Y jointly improved delamination resistance of NiAlRe-YSZ coating during oxidation. Severe internal oxidation and rumpling prompted the delamination of YSZ for NiAl-YSZ coating without Re.

Role of numerical modeling in predicting the oxidation behavior of thermal barrier coatings

Role of numerical modeling in predicting the oxidation behavior of thermal barrier coatings

Microstructure evolution and thermal cyclic failure behavior of thermal barrier coatings with interface modification

Thermal barrier coatings (TBCs) interface between bond coat and ceramic top coat is critical since most of degradation presented in this region as TBCs failure analysis. Shot peening, as a valuable surface engineering, has been applied for TBCs interface modification. The interface modification by shot peening process to the morphology evolution and degradation optimization still need to be studied. In this work, three different shot peening parameters have been selected for interface modification of as-prepared TBCs. The divergent column microstructure has been reduced and surface roughness improved with the shot peening parameters changing. Thermal cyclic failure behavior has been summarized as two mode, thermally grown oxide (TGO) growth induced spalling and interface premature delamination. TGO growth induced spalling dominated failure at roughly interface condition, while interface delamination dominated failure at smooth interface condition. This failure mode has explained the thermal cyclic life tendency, climbing up and then declined, of TBCs.

Research on damage behavior of thermal barrier coatings based on acoustic emission technology

ABSTRACT Thermal barrier coatings (TBCs) is a typical brittle, heterogeneous, multilayer structure material. The heat-loading effect during service will lead to premature peeling failure of the coating. The main failure modes are cracking and interface spalling failure of ceramic coating. The failure of the TBCs is mainly caused by the initiation, propagation and connection of microcracks. In this study, acoustic emission technique (AE) combined with microscopic morphology are used to study the failure process of TBCs under tensile load, and fast Fourier transform (FFT) and wavelet transform are used to identify the crack damage mode of TBCs. The results show that the tensile failure process of the TBCs is that the crack firstly sprouts on the surface of the ceramic coating and then spreads to the interface of the ceramic coating/bond coating. After reaching the interface, the crack will grow and propagate along with the interface, eventually leading to ceramic coating is peeled off; The failure process of the TBCs is divided into four steps. Meanwhile, the spectrum analysis shows that the frequency component of the substrate is around .020 MHz, the frequency component of the surface crack is .20–.25 MHz, and the interface crack is .15–.20 MHz.

Thermo-structural behavior of thermal barrier coatings for thrust chamber applications

The investigation of the failure mechanism is essential for the application of thermal barrier coating in the thrust chamber of liquid rocket engines. In this work, a four-layer thermal barrier coating system in a typical regeneratively-cooled thrust chamber was simulated by finite element method, and the interface morphology between layers was represented by an ideal cosinusoidal curve. A thermo-structural analysis, the boundary conditions of which were calculated by a fluid-thermal coupling method, was carried out to study the influence of the interface roughness and thermally grown oxide on the temperature and stress distributions under thermal cycles. Elasto-plastic deformation and creep behavior were also taken into consideration. The computed results reveal that the roughness parameters including interface amplitude and wavelength as well as the thermally grown oxide thickness have limited effects on the temperature distribution in the thrust chamber wall, but a significant impact on the residual stress distribution in thermal barrier coatings. As these parameters grow, the magnitude of residual stress generally increases, meanwhile, the dangerous regions transform in a complicated way. Much attention should be paid to the interface of thermal barrier coatings in thrust chambers.

Test atmospheres affecting voids distribution on MCrAlY-bond coats for TBCs at 1050 °C

The oxidation behaviour of thermal barrier coatings with a NiCoCrAlY bond coat at 1050 °C in different atmospheres was investigated. Water vapour contributed to fewer defects and lesser internal growth of Al2O3 in the bond coat. Additionally, the presence of excess water vapour in the air + water vapour atmosphere suppressed the local inward oxide growth. Electron backscatter diffraction and transmission electron microscopy analyses were performed to investigate the influence of water vapour on the grain growth of alumina. The grain size of Al2O3 depended on the water vapour content. The driving forces for the morphological distribution of voids in various atmospheres were discussed.

Experimental and Simulation Analysis of the Evolution of Residual Stress Due to Expansion via CMAS Infiltration in Thermal Barrier Coatings

The failure behavior of thermal barrier coatings (TBCs) involves multilayered systems infiltrated with calcium–magnesium–alumino-silicates (CMAS). The metastable tetragonal phase is mainly composed of 7YSZ (7 mol.% Y2O3-stabilized ZrO2), and it destabilizes into the Y-lean tetragonal phase, which may be induced by CMAS infiltration, and transforms into a monoclinic phase during cooling. The phase transformation leads to volume expansion around the CMAS-rich layer. Furthermore, it is shown that the spalling of the coating system emerges when the surface of the coating system is subjected to significant residual stress. In this study, a double-cantilever beam model is established to describe the macroscopic phenomenon of thermal buckling induced via CMAS. The result of the buckle height is used to demonstrate the consistency of the experiment and finite element simulation. The experimental parameters are imported into a multilayer cantilever beam model to analyze the interfacial stresses due to CMAS infiltration. The finite element results indicate that the phase transformation leads to damage in the coating system wherein the interfacial stresses due to phase transformation are 27% higher than those in the model without phase transformation.

Multidisciplinary approach for investigating the hot corrosion behavior of thermal barrier coatings

Herein, we propose a new multidisciplinary approach for investigating the corrosion behavior of thermal barrier coatings, combining the thermal-gradient mechanical fatigue method with hot corrosion tests. Corrosive salts (Na2SO4; V2O5) of varying concentrations (10−20 mg/cm2) were deposited on the surface of yttria-stabilized zirconia (YSZ) coatings to evaluate the microstructural changes occurring during the reaction, via thermal graded mechanical fatigue tests. The tests were conducted at a surface temperature of 1150 °C in 10 min cycles and underwent applied uniaxial tensile loads. The corrosion tests show that the ZrV2O7 plays an important role as an intermediate in the collapse of the lamellar structures during the reaction, which results from the repetitive precipitation of V2O5 and m-ZrO2 from the ZrV2O7. The microstructure of corroded YSZ coatings exhibited a different degradation mechanism under a thermomechanical environment, compared with the testing under isothermal atmospheric conditions.

Thermal cycling of EB-PVD TBCs based on YSZ ceramic coat and diffusion aluminide bond coat

The objective of this research was to optimize the surface morphology and element compositional distribution of the bond coat by adjusting the hydrogen flow, and to fabricate the 6–8 wt% yttria partially stabilized zirconia (YSZ) ceramic topcoat via EB-PVD. The interfacial stability and spalling failure behavior of thermal barrier coatings (TBCs) were evaluated by cyclic oxidation test at 1373 K. The results show that with the increment of hydrogen flow, the X-ray diffraction peak belonging to the β phase tends to be obvious and the high temperature oxidation resistance becomes correspondingly excellent. There are no microcracks, spalling, peeling, edge warping and cloud spatter on the ceramic coating surface. The cross section of the ceramic layer exhibits a typical columnar grain microstructure. The columnar crystal grows evenly perpendicular to the interface between the ceramic and bonding layers. Although it has been exposed to long-term thermal exposure, the ceramic coat has densified and sintered locally, and the grain boundary between the columnar grains is basically disappeared, the ceramic coat surface still has the typical columnar crystal “cauliflower” shape. Lots of transverse cracks and voids have been found at the interface of the bond coat and TGO layer, and within a dozen micrometers approach the bond coat surface. Internal fracture has occurred in the bonding layer and bridging structures have been developed due to internal oxidation in the bond coat.

A cumulative oxide growth model considering the deterioration history of thermal barrier coatings

In this study, we developed a modified oxidation model to explain the oxidation behavior of thermal barrier coatings (TBCs) of gas turbines and predicted accumulated oxides under complex temperature conditions. The applicability of the method was verified using a deterioration test between 1100 and 1200 °C. In addition, the oxidation model was simulated for large temperature changes over time, and the results were compared with the experimental results. Consequently, the oxidation of TBCs could be predicted using the deterioration history, and the oxidative damage could be compared between tests in different deterioration cycles.

Failure behaviour of Y2O3-ZrO2 thermal barrier coatings under the combined effects of hot corrosion and thermal shock

ABSTRACT Y2O3-ZrO2 thermal barrier coatings on a GH2132 alloy were fabricated by atmospheric plasma spraying method. Failure behaviours of the thermal barrier coatings under the combined condition of hot corrosion and thermal shock were investigated. Surface and cross-sectional morphologies of thermal barrier coatings before and after exposures were assessed by using scanning electron microscopy. It was established that the main failure mode was the formation of net-like cracks in coatings and penetrating cracks to the bond coat/substrate interface. By detailed consideration to the analysis of X-ray diffraction and morphologies, the effect of hot corrosion on the failure of thermal barrier coatings was inferred to be due to the induced stress during the cooling stage of the combined condition, which was prone to induce the crack formation.

Formation and Growth Behaviour of Thermally Grown Oxide Layer in Thermal Barrier Coatings with HVOF Sprayed Nickel-Chromium Bond Coats

In this study, the oxidation behavior of thermal barrier coatings (TBCs) with a nickel–chromium (NiCr) bond coat was investigated under high-temperature atmospheric conditions. Nickel–chromium bond...

Research on strain distribution and damage behavior of thermal barrier coatings based on digital image correlation

AbstractDue to the complicated structure and serving environments, thermal barrier coatings (TBCs) usually encounter failure in the form of surface coating cracking and interface spalling without warning. At present, although many experimental techniques and equipment were developed to predict their service life, the transfer process of stress between different layers and the strain characteristics of ceramic surface are not clearly explained. In this paper, the nondestructive digital image correlation method was used to observe the character of surface strains of supersonic plasma‐sprayed TBCs systems in tensile failure processes. Also, mathematical model was established basing on the principle of minimum function to calculate interface stress and coating strain expressions. The results show the characteristics of strain change in the whole tensile stage can be divided into four stages. At first, strain concentration occurs in the range of 9%‐27% of the effective distance between one end of the tensile specimen, second, a certain number of strain fringes are formed and distributed at a certain distance, and then the first crack appears in the initial strain concentration area; as the load continues, more and more cracks on the coating surface reach saturation and finally fail. In the microlinear elasticity stage, the shear strain in the coating and the interface shear stress are in a linear relationship. As the thickness of the single‐layer coating increasing, the strain value of the surface strain of the coating decreases, the surface strain value of the single‐layer coating is about six times larger than that of the double‐layer coating.

Effect of coating thickness on contact fatigue and wear behavior of thermal barrier coatings

Effect of coating thickness on contact fatigue and wear behavior of thermal barrier coatings

Computational investigation of porosity effects on fracture behavior of thermal barrier coatings

The influence of microstructural pore defects on fracture behaviour of Thermal Barrier Coatings (TBC) is analysed using finite element analysis involving cohesive elements. A concurrent multiscale approach is utilised whereby the microstructural features of the TBC are explicitly resolved within a unit cell embedded in a larger domain. Within the unit cell, a random distribution of pores is modelled along with three different layers in a TBC system, namely, the Top Coat (TC), the Bond Coat (BC) and the Thermally Grown Oxide (TGO). The TC/TGO and the TGO/BC interfaces are assumed to be sinusoidal of specified amplitude and frequency extracted from experimental observations reported in the literature. To simulate fracture in the TBC, cohesive elements are inserted throughout the inter-element boundaries in order to enable arbitrary crack initiation and propagation. A bilinear traction-separation relation with specified fracture properties for each layer is used to model the constitutive behaviour of the cohesive elements. Parametric studies are conducted for various pore geometrical features, porosity, fracture properties of Top Coat layer and Thermally Grown Oxide layer thicknesses. The results are quantified in terms of crack initiation and evolution. It is found that the presence of pores has a beneficial effect on the fracture behavior up to a certain value of porosity after which the pores become detrimental to the overall performance. Insights derived from the numerical results can help in understanding the failure behavior of practical TBC systems and further aid in engineering the TBC microstructure for a desired fracture behavior.

Effect of TGO on the tensile failure behavior of thermal barrier coatings

Thermally grown oxide (TGO) may be generated in thermal barrier coatings (TBCs) after high-temperature oxidation. TGO increases the internal stress of the coatings, leading to the spalling of the coatings. Scanning electron microscopy and energy-dispersive spectroscopy were used to investigate the growth characteristics, microstructure, and composition of TGO after high-temperature oxidation for 0, 10, 30, and 50 h, and the results were systematically compared. Acoustic emission (AE) signals and the strain on the coating surface under static load were measured with AE technology and digital image correlation. Results showed that TGO gradually grew and thickened with the increase in oxidation time. The thickened TGO had preferential multi-cracks at the interface of TGO and the bond layer and delayed the strain on the surface of the coating under tensile load. TGO growth resulted in the generation of pores at the interface between the TGO and bond layer. The pores produced by TGO under tensile load delayed the generation of surface cracks and thus prolonged the failure time of TBCs.

Modelling and analysis of the oxide growth coupling behaviour of thermal barrier coatings

A chemo-transport-mechanics model is developed to study the growth of thermally grown oxide (TGO) and its impact on deformation and stress in air plasma-sprayed thermal barrier coatings (TBCs). As the driving force for oxygen transport, the chemical potential consists of contributions from both species concentration and hydrostatic pressure. The model suggests that both the concentration boundary condition and the transport process of the oxygen are affected by hydrostatic stress. Since oxygen has smaller diffusion coefficient in TGO than in BC, the retarding effect of the formed TGO on oxygen transport is considered and clarified by the coupled model. The competition between geometrical imperfection (i.e. concave morphology) and the chemo-mechanics coupling to influence the transport of oxygen is also identified numerically. The geometrical imperfection can introduce additional oxygen transport at the margin of the concave imperfection due to the horizontal component of the gradient of the chemical potential of the oxygen, which plays a dominant role in the TGO growth kinetics for the studied TBCs. Consequently, there is a limited effect of the chemo-mechanics coupling on the growth kinetics of a concaved TGO. The amplitude change of the concave portion is found to be up to 0.36 µm after 600-h exposure at 1150 °C, which leads to large tensile stress above the concave portion potentially causing micro-cracks.