Failure Mechanism Of Thermal Barrier Coatings Research Articles

Mechanical Loading Effect on Stress States and Failure Behavior in Thermal Barrier Coatings

Under service conditions, apart from the harsh temperature environment, mechanical loading also seriously affects the life of thermal barrier coatings (TBCs). A comprehensive understanding of the combined effects of thermo-mechanical loads can help to further reveal the failure mechanism of TBCs. In this work, a portion of a thin-walled circular pipe is intercepted for numerical analysis, and the interface is simplified as a sinusoidal curve. The dynamic growth of thermally grown oxide (TGO) is included into the model. A cohesive model is used for interfacial cracking analysis. The results show that the effects of tensile and compressive loads on the normal stress of the coating are not significant, while the effect on the shear stress of the coating is more obvious. In addition, the in-phase load will delay the occurrence of interfacial failure behavior, while an out-of-phase load can promote the failure. These results will help to better understand the effects of the coupling of mechanical and temperature loads and to provide support and guidance for the design of new TBCs structures in the future.

The relationship between the film hole distribution and thermal cycle performance of TBCs by APS and HVOF

Thermal barrier coatings (TBCs) play an important role in protecting hot-end components of aero-engines from high-temperature oxidation and corrosion. The peeling of the TBCs around film holes has emerged as a major source of failure in thermal cycle oxidation. This study examines the failure behavior of TBCs on floating tiles in the combustion chamber of an aero-engine. Ninety thermal cycles were performed on two types of floating tiles with different film hole distribution. Results showed that the surface TBCs of sample No.1 peeled off by approximately 40 % of the total area, while 90 % of the surface TBCs of sample No.2 floating tile remained intact. A thermal cycling experiment of the TBCs on the surface of the floating tiles was performed to determine the stress distribution of the film holes by finite element (FE) simulation. The results indicated that there was a significant stress gradient on the TBCs surface around the film holes after thermal cycles, with 86.09 MPa and 62.84 MPa higher stress gradient around the film holes of sample No.1 floating tile compared to the film hole of sample No.2 floating tile on the surface and back side, respectively. This difference in the stress gradient and the film hole location on the TBCs led to spalling of the TBCs on the surface of both floating tiles. This study elucidates the failure mechanism of TBCs around film holes under thermal cycling through experiments and FE simulations, offering insights to improve the lifetime of TBCs.

Solid Particle Erosion Behavior of La2Ce2O7/YSZ Double-Ceramic-Layer and Traditional YSZ Thermal Barrier Coatings at High Temperature

Thermal barrier coatings (TBC) used for turbine blades are indispensable for the most advanced aero-engines due to their excellent thermal insulation performance. Solid particle erosion (SPE) at high temperatures is one of the most critical factors in TBC failure. The high-temperature SPE failure behavior of TBC on circular sheets and turbine blades was investigated in this paper at erosion angles 60° and 90°. The high-temperature thermal shock behavior of TBC was also studied as the control group. The SPE failure mechanism of TBC is attributed to the spallation and thickness decrease of TBC. The formation of thermally grown oxide is the main reason for the TBC spallation, while the thickness decrease of TBC is due to the impaction of solid particles by near-surface cracking. The erosion angle is critical to the failure behavior of TBC, and TBC is more susceptible to SPE at an erosion angle of 60° than that at 90° because of the additional shear stress. Furthermore, a La2Ce2O7/YSZ double-ceramic-layer TBC was designed and deposited on turbine blades. The experimental results indicate that this type of double-layer TBC has more excellent performance under SPE than traditional YSZ TBC.

Failure Mechanism of EB-PVD Thermal Barrier Coatings under the Synergistic Effect of Thermal Shock and CMAS Corrosion

Thermal barrier coatings (TBCs) suffer from the thermo-chemo-mechanical coupling action of thermal shock and calcium–magnesium–alumina–silicate (CMAS) corrosion. However, the failure mechanism of TBCs under the synergistic effect of thermal shock and CMAS corrosion is still unclear due to a lack of an environmental simulator. Herein, an 8YSZ ceramic coating is deposited on a PtAl bond coating/DD419 nickel-based single crystal superalloy substrate using the electron beam physical vapor deposition (EB-PVD) method. The thermo-chemo-mechanical coupling effect of TBCs is achieved in a self-developed environmental simulator. The interaction of volume expansion induced by the phase transition of ZrO2, structural degradation and thermal fatigue further increases the out-of-plane tensile stress and in-plane shear stress in the ceramic coating, which accelerates the initiation and propagation of surface vertical cracks and horizontal cracks. As multiple surface vertical cracks propagate to the interface and merge with interfacial cracks, the ceramic coating spalls from the substrate.

Thermal shock performance and failure behavior of Zr6Ta2O17-8YSZ double-ceramic-layer thermal barrier coatings prepared by atmospheric plasma spraying

Zr 6 Ta 2 O 17 has higher fracture toughness, better phase stability, thermal insulation performance and calcium-magnesium-alumino-silicates (CMAS) attack resistance than yttria-stabilized zirconia (8 YSZ, 7–8 wt%) at temperatures above 1200 °C. However, the thermal expansion coefficients between Zr 6 Ta 2 O 17 coating and bond coating do not match well. A double-ceramic-layer design is applied to alleviate the thermal stress mismatch. The Zr 6 Ta 2 O 17 /8 YSZ double-ceramic-layer thermal barrier coatings (TBCs) are prepared by atmospheric plasma spraying (APS). During the thermal shock test, Zr 6 Ta 2 O 17 /8 YSZ double-ceramic-layer TBCs exhibit a better thermal shock resistance than 8 YSZ and Zr 6 Ta 2 O 17 single-layer TBCs. The thermal shock performance and failure mechanism of TBCs in the thermal shock test are investigated and discussed in detail.

LaNdZrO thermal barrier coatings by electron beam physical vapor deposition: Morphology, thermal property and failure mechanism

Thermal barrier coatings (TBCs) have been widely studied over the last decade. Lanthanum zirconate (LaZrO) is known for high melting point and good thermal stability in aircraft engine environment. The thermal property and failure mechanism of these advanced TBCs still remain a challenge. This study focuses on phase structure, thermal expansion coefficient, thermal conductivity, morphology, composition, thermal durability, and failure behaviors in the LaNdZrO/YSZ/NiCoCrAlYHf TBCs. The LaNdZrO and YSZ ceramic coatings have been deposited by EB-PVD, and NiCoCrAlYHf metallic coating has been deposited by AIP-PVD. The as-deposited TBCs exhibit good thermal cycling life and thermal shock life at 1100 °C due to the improvement of TEC and the formation of feathery microstructure. In the thermal durability test, the locally broken region of the YSZ/TGO interface, YSZ coating and further the LaNdZrO coating might facilitate propagation of the instability. The element diffusion and cracks evolution might be two critical factors for the failure mechanism of TBCs. This study would further explain the failure of other advanced TBCs.

Real-time detection of damage evolution and failure of EB-PVD thermal barrier coatings using an environmental simulator with high-temperature and high-speed rotation

Thermal barrier coatings (TBCs) are key to improving aero-engine performance. The lifetime assessment of TBCs in a simulation environment is an essential consideration for successful application. Environmental simulators for rotor blade TBCs are still scarce, largely owing to the difficulty in obtaining real-time TBC failure data in extreme environments. In this work, an environmental simulator with high-speed rotation and gas thermal shock characteristics was developed and tested. High-temperature resistant strain gauges and thermocouples were fixed onto the surface of turbine blades by a flame-sprayed Al2O3 layer. The damage evolution and failure mechanism of TBCs were analysed by the real-time detection of strain damage and temperature field. The results reveal only a little fluctuation in the temperature distribution of the TBC surface (122 °C) under the synergistic effect of high-speed rotation and thermal shock, and the strain at each measuring point is tensile. During thermal cycling tests of the TBCs, the strain at a specific point varied from 0.21% to 0.79%. The final thermal shock life was determined to be 156 cycles, thus proving that the coating enables good thermal shock resistance. The spalled area was mainly distributed from the suction surface to the leading edge, and TBC failure could be attributed to crack initiation and propagation caused by the impact of the high-temperature flame and accumulation of fatigue load. The present work paves the way for coating preparation process optimisation and real service performance evaluation.

Effects of shot peening on microstructure, thermal performance, and failure mechanism of thermal barrier coatings

AbstractShot peening might be a potential technology to optimize the interface microstructure, plays a critical role on failure behaviors, of thermal barrier coatings (TBCs). It remains a significant challenge to understand the influence of shot peening on microstructure, oxidation resistance, and thermal shock life. In this work, the Y2O3‐stabilized ZrO2 TBCs have been deposited by EB‐PVD. The phase, microstructure, thermal performance, and failure mechanism of TBCs have been systemically investigated after shot peening. The shot peening process can improve the planeness of interface and reduce the formation of the cauliflower‐liked microstructure in TBCs. After shot peening, the TBC coatings exhibit relatively good isothermal oxidation resistance and high thermal shock life due to the optimization of TGO growth and the thermal stability. The phase transformation, TGO growth, and cracks extension might give rise to the failure of TBCs. This work might guide the investigation of the improvement of interface microstructure and failure behaviors.

CMAS Issues in Jet Engines: The Less, the Better

Jet engines are susceptible to contamination by environmental debris (volcanic ash, sand, and dust, called CMAS). When CMAS ash is ingested into gas turbines, it melts and can attach to hot components of the jet engines that may clog engine parts and damage protective ceramic thermal barrier coatings (TBCs). The engine’s performance may be impeded and can even cause catastrophic failure when this contamination is in excess. This research presents the current understanding of these CMAS challenges in nature and deposit-induced failure mechanisms of TBCs. The strategy mitigation of new functional TBCs to stop the wetting of CMAS explained the details of experimental and theoretical analysis of the melting and impacting processes of CMAS ash in jet engines.

Durability of plasma sprayed Thermal Barrier Coatings with controlled properties part II: Effects of geometrical curvature

The present investigation elucidates the effects of substrate curvature on the durability of Air Plasma Sprayed (APS) Thermal Barrier Coatings (TBCs). Traditionally, planar disk specimens are utilized in Furnace Cycle Testing (FCT). In most cases, delamination is initiated at the disk's free edge and then propagates along the TBC-bond coat interface. However, in turbine components (e.g., blades/vanes), they lack significant free edges and the coatings are deposited on non-flat surfaces of varying radii of curvature. These geometrical discrepancies imply significant differences in the stress states as compared to planar disk specimens. Therefore, the part geometry inevitably affects the TBC failure mechanisms in the turbine system, and consequently influences TBC durability. Nevertheless, these effects have not been fully explored in the past, despite the anecdotal knowledge which suggests the leading edge of turbine blades to be one of the most common failure/spallation locations. In this study, representative TBC systems were deposited onto superalloy disks with flat substrate and rods with curved substrate. The experimental results from FCT suggest TBC durability on rods is significantly lower than when sprayed on disks. Furthermore, the durability of TBCs on rods appears to increase with higher porosity, which is contradictory to reported trends for TBCs on disks. In addition, the effects of various bond coats are not consistent with those observed in disks. To clarify the underlying effects of curved geometries, detailed stress analyses were performed. They revealed a state of thermal stresses which is unique to curved geometries. It was found that TBC on rods experience tensile radial stresses as well as tensile hoop stresses during the cooling. These stresses result in a complex interplay in failure processes in rods, which explains the different observed trends in the durability from those obtained with disks.

The morphology, thermal property, and failure mechanism of GdNdZrO thermal barrier coatings by EB‐PVD

AbstractNowadays, the Gd2Zr2O7 thermal barrier coatings (TBCs) have been evaluated as a promising alternative to yttria‐stabilized zirconia (YSZ). Thus, this investigation focuses on the thermal property, morphology, and failure mechanism of double ceramic layers (DCLs) GdNdZrO/YSZ advanced TBCs. The GdNdZrO coatings with columnar morphology have been deposited on NiCoCrAlYHf bond coating using an electron beam physical vapor deposition method. Material characterizations mainly include X‐ray diffraction, scanning electron microscope, and transmission electron microscopy. The thermal conductivity of GdNdZrO ceramic material is 0.494 W/mK at 1200°C. The thermal shock life of GdNdZrO/YSZ TBCs shows an average shock life of 5235 cycles. The TBC degradation occurs on the crack area within thermally grown oxide layer leading to the interface instability. The interface broken might play an important role in the failure mechanism of TBCs.

Microstructure evolution of Al-modified 7YSZ PS-PVD TBCs in thermal cycle

The PS-PVD method was used to prepare 7YSZ thermal barrier coatings (TBCs) and NiCrAlY bond coatings on a DZ40 M substrate. To prevent oxidation of the coating, magnetron sputtering was used to modify the surface of TBCs with an Al film. To explore the stability of TBCs during thermal cycling, water quenching was performed at 1100 °C, and ultralong air cooling for 16,000 cycles was performed. The results showed that before water quenching and air cooling, the top surface structure of the 7YSZ TBCs changed. After water quenching, the surface of the Al film was scoured and broken, the surface peeled off layer-by-layer, and cracks formed at the interface between the thermally grown oxide and NiCrAlY. During air cooling of the thermal cycle, the Al film reacted with O2 in the air to form a dense Al2O3 top layer that coated the cauliflower-like 7YSZ surface and maintained the feather-like shape. At the same time, the TGO layer between 7YSZ and NiCrAlY grew and cracked. The two thermal cycles of water quenching and air cooling led to different failure mechanisms of TBCs. Water quenching failure was caused by layer-by-layer failure of the 7YSZ top coat, while air cooling failure occurred due to the internal cracking of the TGO layer at the 7YSZ/NiCrAlY interface and the failure of the TGO/NiCrAlY interface.

Improving the Hot Corrosion Resistance of Nanostructured ZrO2-7wt.%Y2O3 Thermal Barrier Coatings Fabricated by Plasma Spraying

Conventional and nanostructured ZrO2-7wt.%Y2O3 thermal barrier coatings (TBCs) were prepared by plasma spraying technology. Microstructure and hot corrosion resistance of two TBCs in 75%Na2SO4+25%NaCl (mass fraction) mixed molten salt under 850 °C were compared. Meanwhile, hot corrosion failure mechanisms of TBCs were investigated. Results demonstrated that both conventional and nanostructured TBCs were typical lamellar stacking characteristic. However, compared with conventional TBC, the nanostructured TBC presented the base phase which is gained from recrystalization of fully melted nano-particles as well as the embedding phase of partially melted nano-particles. The nanostructured TBC has lower porosity and higher density. For both conventional and nanostructured TBCs, hot corrosion failure is caused by partial transformation of tetragonal phase (t-ZrO2) into monoclinic-phase (m-ZrO2) martensite as a response to the consumption of the stabilizer Y2O3. However, the nanostructured TBC generally has lower degree of hot corrosion and better protection to the metal bonding layer and matrix compared with conventional TBC.

Study on Sol–Gel Synthesized IN800 Thermal Barrier Coatings Subjected to Thermal Cyclic Loading: Effect of Metallic Substrates

In this work, the effect of sol–gel deposited top-coat on thermal fatigue resistance of thermal barrier coatings (TBCs) subjected to thermal fatigue loading is evaluated experimentally. To obtain non-conventional sol–gel thermal barrier coatings (SGTBC), coated samples underwent thermal fatigue loading at 1100 °C for 10 min heating and cooling. The tested sol–gel thermal barrier coatings were then compared to conventional air plasma sprayed (APS) thermal barrier coatings as well. The life of samples was investigated as a function of number of sustaining thermal cycles to times. Furthermore, the performed experiment was analyzed using scanning electron microscope, energy dispersive spectroscopy and X-ray diffractometer. The obtained results indicated that the non-conventional sol–gel thermal barrier coatings exhibited 1.46 times better thermal fatigue life in IN800SGTBC against 1.31 times thermal fatigue life of IN718SGTBC but overall thermal fatigue life was found to be better in IN718 SGTBC, signified effects of metallic substrates in thermal fatigue life determination. However, the nanostructured SGTBC had higher thermal cyclic resistance than conventional APS TBC, resulted in improved lifetime indicating the increased adherence at the substrate interface. Results also showed that the dominant failure mechanism of TBCs was destabilization of top-coat (YSZ), resulting composition of (Al, Cr)2O3 and spinel as reaction products for depleting Y2O3 producing from yttria stabilized zirconia (YSZ). Furthermore, the results showed that the amount of the porosity percent in the sol–gel TBCs was 2.3% higher than the conventional TBCs. Figure: Material Processing, and thereafter testing analysis for present work.

Research Progress of Failure Mechanism of Thermal Barrier Coatings at High Temperature via Finite Element Method

In the past decades, the durability of thermal barrier coatings (TBCs) has been extensively studied. The majority of researches emphasized the problem of oxidation, corrosion, and erosion induced by foreign object damage (FOD). TBCs with low thermal conductivity are usually coated on the hot-section components of the aircraft engine. The main composition of the TBCs is top-coat, which is usually regarded as a wear-resistant and heat-insulating layer, and it will significantly improve the working temperature of the hot-section components of the aircraft engine. The application of TBCs are serviced under a complex and rigid environment. The external parts of the TBCs are subjected to high-temperature and high-pressure loading, and the inner parts of the TBCs have a large thermal stress due to the different physical properties between the adjacent layers of the TBCs. To improve the heat efficiency of the hot-section components of aircraft engines, the working temperature of the TBCs should be improved further, which will result in the failure mechanism becoming more and more complicated for TBCs; thus, the current study is focusing on reviewing the failure mechanism of the TBCs when they are serviced under the actual high temperature conditions. Finite element simulation is an important method to study the failure mechanism of the TBCs, especially under some extremely rigid environments, which the experimental method cannot realize. In this paper, the research progress of the failure mechanism of TBCs at high temperature via finite element modeling is systematically reviewed.

Real-time Detection of CMAS Corrosion Failure in APS Thermal Barrier Coatings Under Thermal Shock

Calcium-magnesium-alumina-silicate (CMAS) corrosion has been regarded as the most important factor that leads to the degradation of thermal barrier coatings (TBCs). The failure mechanism of TBCs attacked by CMAS corrosion in the actual service conditions is still not clear due to the lack of an environmental simulator and nondestructive testing techniques. To solve the above problems, a real-time acoustic emission method combined with infrared thermography are developed to investigate the failure mechanism of TBCs attacked by CMAS corrosion. The results show that the acoustic emission signal spectrum only depends on the failure mode of the TBCs, and five failure modes are identified: surface vertical cracks, sliding interfacial cracks, opening interfacial cracks, substrate deformation and noise. The lifetime of TBCs attacked by CMAS corrosion is 40 thermal shock cycles, which is nearly six times lower than that of TBCs without CMAS corrosion (350 cycles). Conclusions: The failure mechanism of the former is interlaminar cracking and delamination in the ceramic coating; while that for the latter is interfacial delamination in the vicinity of thermal growth oxide.

Gradient stiffening induced interfacial cracking and strain tolerant design in thermal barrier coatings

Strain tolerant structures endow thermal barrier coatings (TBCs) with exceptional capability to bear various strains generated during service. However, the strain tolerance of TBCs inevitably degrades when stiffening occurs at high temperatures. Herein, we tailored a strain tolerant structure to resist degradation based on an understanding of the unique stiffening behavior and consequent failure mechanism of TBCs. The degree of gradient stiffening across the thickness of TBCs is caused by temperature-dependent sintering kinetics of ceramic coatings. As a result, vertical and in-plane cracks are formed in a scale-progressive way. Simulation results reveal that the differential degree of stiffening is a main cause of interfacial cracking. Subsequently, we proposed a strain tolerant structure that is tailored by lowering the stiffening rate of regions exposed to higher temperatures. Due to the weakened differential stiffening effect, the driving force that extends the interfacial cracks was significantly lowered. Thus, this strain tolerant TBC is expected to be able to resist degradation caused by sintering. These results will guide advanced design of TBCs for future applications.

Stress states and crack behavior in plasma sprayed TBCs based on a novel lamellar structure model with real interface morphology

Abstract To ascertain the crack growth behavior and coalescence mechanism in thermal barrier coatings (TBCs) is beneficial for understanding the failure of TBCs and proposing the probable optimization methods. In this work, a novel lamellar structure model with real interface morphology is developed to explore the crack growth behavior and the failure mechanism of TBCs during thermal cycling. Three typical defects which include pore, inter-splat crack, and intra-splat are incorporated in the model. To simulate the oxidation process of the bond coat (BC) realistically, The oxidation growth process is simulated via changing the BC properties to thermally grown oxide (TGO) properties layer by layer. The effects of the lateral growth strain distribution through TGO thickness on the stress states are executed. Moreover, the influences of BC creep on the crack growth and coating lifetime are further elaborated. The results show that the larger the lateral growth strain gradient, the smaller the residual tensile stress. The irregular interface morphology results in the redistribution of residual stresses. Although the pores and cracks can alleviate the tensile stress near the valley, large stress concentration will occur near them. At the early phase of thermal cycling, the cracks grow steadily. After more cycles, the cracks propagate rapidly and merge with others. The simulated delamination path is in agreement with the experiment results. Not only does BC creep change the crack coalescence mechanism, it also decreases the thermal cyclic lifetime of TBCs. The coating optimization method proposed in this study provides another option for developing advanced TBCs with longer lifetime.

Finite Element Modelling of TBC Failure Mechanisms by Using XFEM and CZM

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, 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 reported in Kütükoğlu (2015) are analyzed by using the Extended Finite Element Method (XFEM) and the Cohesive Zone Method (CZM). All the analyses are conducted with the commercial finite element software ABAQUS. Three different models with varying TC and BC thicknesses are studied. 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 until they reach the interface between the TC and the BC. Then, delaminations at the interface between the TC and the BC start. It is observed that the average spacing of cracks in TC increases with the increasing thickness of the TC and the delamination becomes prominent with the increasing TC thickness. Numerical results are found to be consistent with the experimental results.

Fracture behavior and thermal durability of lanthanum zirconate-based thermal barrier coatings with buffer layer in thermally graded mechanical fatigue environments

The effects of buffer layer on the fracture behavior and lifetime performance of lanthanum zirconate (La2Zr2O7; LZO)-based thermal barrier coatings (TBCs) were investigated through thermally graded mechanical fatigue (TGMF) tests, which are designed to simulate the operating conditions of rotating parts in gas turbines. To improve the thermal durability of LZO-based TBCs, composite coats consisting of two feedstock powders of LZO and 8wt% yttria-doped stabilized zirconia (8YSZ) were prepared by mixing different volume ratios (50:50 and 25:75, respectively). The composite coat of 50:50 volume ratio was employed as the top coat, and two types of buffer layers were introduced (25:75 volume ratio in LZO and 8YSZ, and 8YSZ only). These TBC systems were compared with a reference TBC system of 8YSZ. The TGMF tests with a tensile load of 60MPa were performed for 1000cycles, at a surface temperature of 1100°C and a dwell time of 10min, and then the samples were cooled at room temperature for 10min in each cycle. For the single-layer TBCs, the composite top coat showed similar results as for the reference TBC system. The triple-layer coating (TLC) showed the best thermal cycle performance among all samples, suggesting that the buffer layer was efficient in improving lifetime performance. Failure modes were different for the TBC systems. Delamination and/or cracks were created at the interface between the bond and top coats or above the interface in the single-layer TBCs, but the TBCs with the buffer layer were delaminated and/or cracked at the interface between the buffer layer and the top coat, independent of buffer layer species. This study allows further understanding of the LZO-based TBC failure mechanisms in operating conditions, especially in combined thermal and mechanical environments, in order to design reliable TBC systems.