Comprehensive dynamic failure mechanism of thermal barrier coatings based on a novel crack propagation and TGO growth coupling model

Comprehensive understanding of failure mechanism of thermal barrier coatings (TBCs) is essential to develop the next generation advanced TBCs with longer lifetime. In this study, a novel numerical model coupling crack propagation and thermally grown oxide (TGO) growth is developed. The residual stresses induced in the top coat (TC) and in the TGO are calculated during thermal cycling. The stresses in the TC are used to calculate strain energy release rates (SERRs) for in-plane cracking above the valley of undulation. The overall dynamic failure process, including successive crack propagation, coalescence and spalling, is examined using extended finite element method (XFEM). The results show that the tensile stress in the TC increases continuously with an increase in an undulation amplitude. The SERRs for TC cracks accumulate with cycling, resulting in the propagation of crack toward the TC/TGO interface. The TGO cracks nucleate at the peak of the TGO/bond coat (BC) interface and propagate toward the flank region of the TC/TGO interface. Both TC cracks and TGO cracks successively propagate and finally linkup leading to coating spallation. The propagation and coalescence behavior of cracks predicted by this model are in accordance with the experiment observations. Therefore, this study proposed coating optimization methods towards advanced TBCs with prolonged thermal cyclic lifetime.     

Fatigue life prediction of thermal barrier coatings using a simplified crack growth model

Models that can predict the life of thermal barrier coatings (TBCs) during thermal cycling fatigue (TCF) tests are highly desirable. The present work focuses on developing and validating a simplified model based on the relation between the energy release rate and the TCF cycles to failure. The model accounts for stresses due to thermal mismatch, influence of sintering, and the growth of TGO (alumina and other non-protective oxides). The experimental investigation of TBCs included; 1) TCF tests at maximum temperatures of 1050 °C, 1100 °C, 1150 °C and a minimum temperature of 100 °C with 1 h and 5 h (1100 °C) hold times. 2) Isothermal oxidation tests at 900, 1000 and 1100 °C for times up to 8000 h. The model was calibrated and validated with the experimental results. It has been shown that the model is able to predict the TCF life and effect of hold time with good accuracy.     

Effect of material properties on residual stress distribution in thermal barrier coatings

Residual stress has a significant influence on the crack nucleation and propagation in thermal barrier coatings (TBC) system. In this work, the residual stress in the air plasma spraying (APS) TBC system during cooling process was numerically studied, and the influence of the material properties of each layer on the residual stress was investigated. The morphologies of the interface were described by a piecewise cosine function, and the amplitude for each segment gradually increases. The elasticity, plasticity and creep of top coat (TC), thermally grown oxide (TGO) layer and bond coat (BC) were considered and the elasticity and creep of the substrate layer were taken into account. The material properties of all layers vary with temperature. The results show that the material properties have complex influence on the residual stress during cooling. The effect of the material properties of TC and BC on the residual stress at the interface is relatively large, and that of TGO and substrate is relatively small. These results provide important insight into the failure mechanism of air plasma spraying thermal barrier coatings, and important guidance for the optimization of thermal barrier coating interfaces.     

Microstructure analyses and thermophysical properties of nanostructured thermal barrier coatings

Nanostructured 8 wt% yttria partially stabilized zirconia coatings were deposited by air plasma spraying. Transmission electron microscopy, scanning electron microscopy, and X-ray diffraction were carried out to analyze the as-sprayed coatings and powders. Mercury intrusion porosimetry was applied to analyze the pore size distribution. Laser flash technique and differential scanning calorimetry were used to examine the thermophysical properties of the nanostructured coatings. The results demonstrate that the as-sprayed nanostructured zirconia coatings consist of the nonequilibrium tetragonal phase. The microstructure of the nanostructured coatings includes the initial nanostructure of powder and columnar grains. Moreover, micron-sized equiaxed grains were also exhibited in the nanostructured coatings. Their evolution mechanisms are discussed. The as-sprayed nanostructured zirconia coating shows a bimodal pore size distribution, and has a lower value of thermal conductivity than the conventional coating.     

Outstanding durability of sol-gel thermal barrier coatings reinforced by YSZ-fibers

Thermal barrier coatings (TBC) were fabricated with commercial powders of yttria stabilized zirconia with spherical and fiber-like morphologies. The influence of fiber percentage and sintering temperature on the thermomechanical behavior was studied. TBCs with 60%–80% fibers content had the best lifetime in cyclic oxidation with less than 10% of coating spallation after 1000 cycles, with very good reproducibility. They reached lifetimes higher than industrial TBCs made by EB-PVD. The enhancement of durability is believed to be due to an increase in the thermomechanical constraints accommodation thanks to higher porosity and higher tenacity due to the presence of well anchored fibers, indeed deviation of the cracks were observed. Moreover, the morphology of the thermally grown oxide (TGO) layer is also favorable as it includes anchorage points of the TGO with fibers. This increased the adherence at the substrate interface and improved lifetime.     

Effect of scandia content on the thermal shock behavior of SYSZ thermal sprayed barrier coatings

Nanostructured 4SYSZ (scandia (3.5 mol%) yttria (0.5 mol%) stabilized zirconia) and 5.5 SYSZ (5 mol% scandia and 0.5 mol% yttria) thermal barrier coatings (TBCs) were deposited on nickel-based superalloy using NiCrAlY as the bond coat by plasma spraying process. The thermal shock response of both as-sprayed TBCs was investigated at 1000 °C. Experimental results indicated that the nanostructured 5.5SYSZ TBCs have better thermal shock performance in contrast to 4SYSZ TBCs due to their higher tetragonal phase content and higher fracture toughness of this coating     

Influence of interface morphology on erosion failure of thermal barrier coatings

The thermal barrier coating system (TBCs) has complex structure and works in severe service environment. Erosion is one of the main factors causing the failure of TBCs. In the present study, the particle erosion process of atmospheric plasma sprayed (APS) thermal barrier coatings at elevated temperature was simulated by the finite element method. The effects of interface morphology on the penetration depth, particle ricochet velocity and interface stress state were studied, and the key parameters such as particle size, initial velocity and erosion position were also considered. The cosine curve with constant wavelength and varying amplitude was used to represent different interface roughness of TBCs. The results show that the interface morphology has little effect on the penetration depth of top coat (TC) and the particle ricochet velocity. The influence of particle erosion position related to the interface morphology is obvious. Basically, the greater the interface roughness is, the more violent the interfacial stress fluctuation is. During the erosion process, the stress in the middle of the interface is significantly higher than that at other positions. These results facilitate understanding of the particle erosion failure mechanism of APS TBCs. The influence of interface morphology should be considered in erosion research.     

Modelling of catastrophic stress development due to mixed oxide growth in thermal barrier coatings

In thermal barrier coatings (TBCs) of heavy-duty gas turbines, thermally grown oxide (TGO) develops in two stages, i.e. firstly, a thin layer of dense protective α-Al₂O₃ forms slowly, and then, a layer of porous detrimental mixed oxide (MO) between top coat (TC) and α-Al₂O₃ appears. During long-term isothermal oxidation at high temperature, the failure of TBCs usually occurs when a critical thickness of MO is reached, but the exact failure mechanism is still largely unclear, let alone the related stress development. In this paper, we analyze the stress evolution and the resultant failure modes due to the whole-layer growth of uniform MO. The results show that it is MO, rather than α-Al₂O₃, that is mainly responsible for the micro-cracking and/or delamination in TBCs. The fast growth of expansive MO induces catastrophic stresses, which leads to micro-cracking in the α-Al₂O₃ layer. The cracking of α-Al₂O₃ layer reduces the oxidation resistance and further accelerates the MO growth. Our theoretical analysis provides a reasonable explanation of the experimental results.     

Laser surface modification of plasma sprayed CYSZ thermal barrier coatings

In this study, Inconel 738 LC superalloy coupons were first sprayed with a NiCoCrAlY bond coat and then with a ceria and yttria stabilized zirconia (CYSZ) top coat by air plasma spraying (APS). After that, the plasma sprayed CYSZ thermal barrier coatings (TBCs) were treated using a Nd:YAG pulsed laser. The effect of laser glazing on the microstructure of the coatings was investigated. The microstructures and surface topographies of both as-sprayed and laser glazed samples were investigated using field emission scanning electron microscope (FESEM) and atomic force microscope (AFM). The phases of the coatings were analyzed with X-ray diffractometry (XRD). The microstructural analysis results revealed that laser surface glazing of ceramic top coat reduced the surface roughness considerably, eliminated the surface porosities and produced a network of continuous cracks perpendicular to the surface. XRD patterns also showed that both as-sprayed and laser glazed top coats consisted of nonequibrium tetragonal (T′) phase.     

Stress-dependent sintering behavior of porous thermal barrier coatings

Thermal barrier coatings (TBCs) are subjected to high temperature and complex stress fields during service in gas turbines. In this process, densification and hardening take place as the result of sintering, which is sensitive to boundary condition/external load. The stress-dependent sintering behaviors of porous TBCs were investigated in this work using a customized four-point bending method. Furthermore, stress-dependent sintering model was developed and implemented in finite element analysis to elucidate sintering mechanisms. It was found that stress gradient induced nonlinear differential sintering behavior, due to the accelerating and retarding effects of compressive and tensile stresses, respectively. In addition, microstructure-mechanical property relation was determined following the exponential law and high-throughput method was proposed for the characterization of stress dependence. The in-depth understanding of stress-dependent sintering behavior could provide guidance to the design and failure analysis of TBCs applied on complex shaped components in the hot section of gas turbines.     

Lanthanum cerate thermal barrier coatings generated from thermal plasma synthesized powders

Lanthanum cerate (La₂Ce₂O₇, LC) is one of the promising advanced thermal barrier coating (TBC) materials due to its high melting point, no phase transformation between room temperature and operating temperature, low thermal conductivity, comparable coefficient of thermal expansion (CTE) with metallic substrate. The present study investigates plasma transferred arc synthesis of LC powder, its subsequent spheroidization in a thermal plasma jet and plasma spray deposition. The PTA-synthesized LC powder, spheroidized as well as the plasma sprayed coatings was found to possess excellent phase stability; the single phase cubic fluorite structure of LC was found to be retained even after prolonged arc-melting, corroborating that the material was stable from room temperature up to its melting point. It was observed that PTA melting for longer duration resulted in small deviation from stoichiometry, although the phase structure of LC was retained. Spheroidization efficiency was found to increase with the input power of the torch. Very good adherent LC coatings could be deposited on nickel super alloy with reasonably good deposition efficiency.     

Preparation of long-lifetime thermal barrier coatings and toughening mechanism by using hierarchy structured zirconia-based microspheres

The thermal cycling lifetime of thermal barrier coatings was doubled when deposited by electro-sprayed (ESP) microspheres instead of by commercial hollow spherical powders. It was believed that partial-molten nodules with featured microstructures inherited from the feedstock microspheres were the main contributor for prolonged thermal cycling durability due to improved fracture toughness and strain tolerance. The maximum lifetime was observed on samples with 20?30 vol.% of partial-molten microspheres. The hierarchy pores may both slow down the crack propagation by triggering multi-deflecting and promote cracking by reducing the tendency of interfacial deflection, the net effect depends on situation. The ESP coatings exhibited bimodal Weibull moduli upon indentation, which was regarded as originated from the hierarchy porous structure. Finally, the criterion was verified by micro-indentation and residual stain-stress evaluation by Raman spectroscopy.     

Substrate-constrained effect on the stiffening behavior of lamellar thermal barrier coatings

Thermal exposure would compromises the compliance and thermal insulating performance of thermal barrier coatings (TBCs). However, most publications were based on free-standing coatings in which the stress resulting from substrate is essentially different from TBCs on superalloy substrate. In this paper, the constrained effect of substrate on the ceramic top-coat of plasma sprayed lamellar TBCs was investigated. Results showed that the structural changes evolve from micro-scale to macro-scale during thermal exposure. In a relatively shorter thermal exposure stage, the inter-splat pores became narrowed, whereas the intra-splat cracks became widened. Consequently, the healing kinetics of inter-splat pores was much faster than that of the intra-splat cracks. In a relatively longer thermal exposure stage, some macroscale cracks appeared in coating surface owing to the gradually stiffening coatings. As a result, the microscale intra-splat cracks near the macroscale cracks were healed rapidly. In brief, the substrate constraint induced structural changes were stage sensitive.     

Residual stress trend in thermal barrier coatings in through thickness direction measured by photoluminescence piezospectroscopy

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.     

CMAS corrosion resistance,thermal shock resistance and numerical simulation of novel surface micromesh thermal barrier coatings

Thermal barrier coatings (TBCs) are widely used as insulating layers to protect the underlying metallic structure of gas turbine blades. However, the thermal cycling performance of TBCs is affected by their complex working environments, which may shorten their service life. Previous studies have shown that preparing a mesh structure in the bonding layer can relieve thermal stress and improve the bonding strength, thereby prolonging the service life of TBCs. In this paper, a micromesh structure was prepared on the surface of the bonding layer via wet etching. The microstructure and failure mechanism of the micromesh TBCs after CMAS (CaO-MgO-Al₂O₃-SiO₂) thermal erosion were investigated. Numerical simulation was combined with thermal shock experiments to study the stress distribution of the micromesh-structured TBCs. The results showed that the circular convex structure can effectively improve the CMAS corrosion resistance and thermal shock resistance of TBCs.     

Thermal shock resistance of air plasma sprayed thermal barrier coatings

The spallation resistance of an air plasma sprayed (APS) thermal barrier coating (TBC) to cool-down/reheat is evaluated for a pre-existing delamination crack. The delamination emanates from a vertical crack through the coating and resides at the interface between coating and underlying thermally grown oxide layer (TGO). The coating progressively sinters during engine operation, and this leads to a depth-dependent increase in modulus. Following high temperature exposure, the coating is subjected to a cooling/reheating cycle representative of engine shut-down and start-up. The interfacial stress intensity factors are calculated for the delamination crack over this thermal cycle and are compared with the mode-dependent fracture toughness of the interface between sintered APS and TGO. The study reveals the role played by microstructural evolution during sintering in dictating the spallation life of the thermal barrier coating, and also describes a test method for the measurement of delamination toughness of a thin coating.     

Scale-progressive healing mechanism dominating the ultrafast initial sintering kinetics of plasma-sprayed thermal barrier coatings

The thermal insulating performance of plasma-sprayed thermal barrier coatings (PS-TBCs) depends dominantly on inter-splat pores, which would be inevitably healed during high temperature exposure. The sintering kinetics of TBCs appears to be highly stage-sensitive. However, the ultrafast sintering kinetics during the initial sintering stage is not yet well understood. In this study, the sintering behavior of PS-TBCs was investigated in a scale-progressive (from nano- to micro-scale) way. Moreover, a novel healing mechanism suitable for lamellar TBCs was proposed based on a combined-effect of material and pore structure. Regarding the changing behavior of material, nano-scale roughening can be found at the as-deposited smooth pore surface after thermal exposure. Regarding the 2D featured inter-splat pores, the roughening behavior facilitates multiple contacts between the counter-surfaces of inter-splat pores. As a result, micro-scale bridge-connection can be observed at the healed parts. This multiple contacts mechanism caused by scale-progressive healing behavior significantly accelerated the matter transfer, resulting in ultrafast sintering kinetics at the initial sintering stage.     

Local residual stress evolution of highly irregular thermally grown oxide layer in thermal barrier coatings

Local residual stress in thermally grown oxide (TGO) layers is the primary cause of failure of thermal barrier coating (TBC) systems, especially TBCs prepared by air plasma spray (APS) with a highly irregular TGO. Herein, the distribution of residual stress and the evolution of the irregular TGO layer in APS TBCs were investigated as a function of oxidation time. The stress was measured from cross-sectional micrographs and converted to the actual stress inside the coatings before sectioning. The TGO exhibited significant inhomogeneity at different locations. Stress conversion occurred across the TGO thickness; the layer near the yttria-stabilised zirconia (YSZ) component exhibited compressive stress, whereas that along the bond coat was under tensile stress. The evolution of the compressive stress is also discussed. These analyses may provide a better understanding of the mechanism of APS TBCs.     

Tailoring the EB-PVD columnar microstructure to mitigate the infiltration of CMAS in 7YSZ thermal barrier coatings

The CMAS associated degradation of 7YSZ TBC layers is one of the serious problems in the aero engines that operate in dusty environments. CMAS infiltrates into TBC at high temperatures and stiffens the TBC which ultimately loses its strain tolerance and gets delaminated. The EB-PVD technique is used to coat TBCs exhibiting a columnar microstructure on parts such as blades and on vanes. By varying the EB-PVD process parameters, columnar morphology and porosity of the 7YSZ coating is changed and its effect on the CMAS infiltration behaviour is studied in detail. Two different TBC pore geometries were created and infiltration experiments were carried out at 1250 °C and 1225 °C for different time intervals. The 7YSZ coating with more ‘feathery’ features has resulted in higher CMAS resistance by at least by a factor of 2 than its less ‘feathery’ counterpart. These results are explained on the basis of a proposed physical model.     

Influence of SiC fiber on thermal cycling lifetime of SiC fibers /YSZ thermal barrier coatings by atmospheric plasma spraying

The initiation and propagation of cracks under thermal stresses easily is one of the problems limiting the thermal cycling lifetime of thermal barrier coatings (TBCs). In order to improve the thermal cycling lifetime, SiC fibers were introduced to yttria stabilized zirconia (YSZ) coating deposited on In738LC substrate by atmospheric plasma spray (APS). Phase composition, thermal cycling behaviors and fiber toughening mechanisms of coatings were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal cycling test. Results showed that the thermal cycling lifetime and fracture toughness of SiC fibers/YSZ coatings could reach 442?±?13 and 1.54?±?0.19?MPa $\sqrt{m}$ respectively, which were 1.6 times and 1.3 times higher than that of conventional TBCs. There are two stages of fiber reinforced during thermal cycling, and the first is crack deflection and termination, the second is fiber debonding, pull-out, breakage and bridging. Meanwhile, SiC fibers could prevent the stress-activated ZrO₂ martensitic transformation by reducing the stress in the lattice.