Lifetime Of Thermal Barrier Coatings Research Articles

Dominant effects of 2D pores on mechanical behaviors of plasma sprayed ceramic coatings during thermal exposure

Realizing engineering application of new candidate ceramic materials is highly required for the development of advanced thermal barrier coatings (TBCs). In addition to the optimization of intrinsic properties of ceramic materials, a feasible way is to tailor the structure of coatings based on the understanding of structure-properties evolution during thermal exposure. Herein, the unique sintering behavior and the consequent effect on spallation of plasma sprayed coatings made of a candidate material, i.e., La2Zr2O7 (LZO), were investigated by experiments and simulation. Experimental results showed that significant changes occurred for the porosity, 2D pore density and hardness during thermal exposure. The 2D pores are the essential microstructural characteristics and are dominantly responsible for the changes of mechanical property. Simulation results suggested that the healing of 2D pores increase the driving force for crack extension. This is the main cause for the spallation of ceramic top coat during thermal cyclic test. Finally, some structural designs towards long lifetime of TBCs were discussed. The dominant effects of 2D pores on mechanical properties and failure of TBCs provide fundamental understanding to the structural tailoring of advanced TBCs for future applications.

Experimental and computational study on sintering of ceramic coating layers with complex porous structures

AbstractThis study investigated the sintering behavior of an yttria‐stabilized zirconia coating for thermal barrier coatings (TBCs) with a complicated porous structure via both experiment and simulation using the finite element method for samples with only a coating (free coating) and samples with coating on a substrate (constrained coating). Sintering and grain growth proceeded from the bottom of the coating, and the coating bent convex upward in the free coating. In the constrained coating, sintering and grain growth proceeded in a manner similar to the free coating; however, the degrees of sintering and grain growth were small. Furthermore, sintering and grain growth were delayed because of substrate constraints. As a simulation result, the free coating was bent in a manner similar to the experiment. The experimental results could be reproduced in terms of time dependency and temperature dependency. The decrease in the porosity of the constrained coating was delayed compared with that in the free coating because of substrate constraints. This simulation result was able to reproduce the experimental results. Thus, the sintering behavior for the complex porous structures of TBCs can be predicted by experimental research and simulation, which could aid in the development of a prediction technology for the delamination of coatings (TBC lifetime).

Investigation of thermal shock resistant in three kinds thermal barrier cerium oxide coating (CeO2) with MCrAlY intermediate layer

The study aimed to compare the thermal shock behavior of three types of thermal barrier coatings (TBC) containing two and five layers. The substrate of the coatings, like as industrial samples, was selected from IN738LC superalloy. The first type was a two-layer TBC produced from CoNiCrAlY and CeO2 as a bond and top layers, respectively. The second type was a common five-layer TBC with a bond layer of CoNiCrAlY, a top layer of CeO2 and three intermediate layers composed of three mixing kinds of CeO2 + CoNiCrAlY. The third type was composed of a top layer of nano-structured CeO2, and the other four down layers were similar to the second type. To thermal shock test, the samples were kept at 1100 °C for 5 min and quenched in 20–25 °C water. The test was continued until all the samples were destructed. The sample was considered destructed when 20% of the coating surface was detached. To evaluate the microstructure of the samples, SEM and FESEM were used. Finally, the thermal shock lifetime of the five-layer TBC was 1.6 times higher than the two-layer TBC, and by making the top nano-structured layer in five-layer TBC, the lifetime was enhanced approximately 14%.

Design of high lifetime suspension plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) fabricated by suspension plasma spraying (SPS) have shown improved performance due to their low thermal conductivity and high durability along with relatively low production cost. Improvements in SPS TBCs that could further enhance their lifetime would lead to their widespread industrialisation. The objective of this study was to design a SPS TBC system with optimised topcoat microstructure and topcoat–bondcoat interface, combined with appropriate bondcoat microstructure and chemistry, which could exhibit high cyclic lifetime. Bondcoat deposition processes investigated in this study were high velocity air fuel (HVAF) spraying, high velocity oxy fuel spraying, vacuum plasma spraying, and diffusion process. Topcoat microstructure with high column density along with smooth topcoat–bondcoat interface and oxidation resistant bondcoat was shown as a favourable design for significant improvements in the lifetime of SPS TBCs. HVAF sprayed bondcoat treated by shot peening and grit blasting was shown to create this favourable design.

Time-/space-sensitive sintering kinetics of plasma sprayed multi-modal nanostructured coating

Sintering is considered to be one of the main reasons that accelerate performance degradation and shorten lifetime of thermal barrier coatings (TBCs). The anti-sintering design and underlying sintering kinetics are of great importance for optimizing the microstructure of TBCs. In this study, the competitive sintering of a novel multi-modal nanostructured coating is systematically studied by a burner-rig test. The results suggested that, due to the coexistence of unmelted nanozone, amorphous area and recrystallized region, the sintering behavior of nanostructured coating exhibited obvious time-/space-sensitive effects. Time-sensitive feature was initially associated with the coalescence growth of nano-particles and healing of micro-pores within the unmelted nanozone. Subsequently, the sintering was accompanied by the abnormal growth of grains in the unmelted nanozone and the formation of multi-contact neck in the recrystallized region. The diffusion-driven stiffening of recrystallized region promoted the stage-progressive enhancement of elastic modulus. The space-sensitive sintering kinetics caused the spontaneous formation and evolution of multi-scale cracks at the unmelted nanozone/recrystallized region interface.

Preparation and properties of LaMgAl11O19 thermal barrier coatings doped with Gd2O3

As a rare earth hexaaluminate, LaMgAl11O19 (LMA) has been one of the most promising materials used as thermal barrier coatings (TBCs). A large amount of amorphous phase, however, often exists in the plasma-sprayed LMA coating and significantly reduces the service lifetime of TBCs. In this study, La1-xGdxMgAl11O19 (x = 0, 0.2, 0.4, 0.6, and 0.8) ceramic powders are synthesised by solid-state reaction, and all of these powders are employed to prepare the corresponding coatings. The phase compositions and microstructures of samples are examined by X-ray diffraction and scanning electron microscopy, respectively. The linear thermal expansion behaviour and thermal cycling behaviour of the coatings are also analysed. The results show that the amorphous phase content is decreased and the thermal expansion behaviour is improved by doping the coatings with Gd2O3. The thermal cycling lifetime of the coating, however, basically remains unchanged.

Fracture behavior of TBCs with cooling hole structure under cyclic thermal loadings

The integrity of thermal barrier coatings (TBCs) plays an important role in the performance of turbine blades, nevertheless the effect of cooling hole on fracture of TBCs have been less studied. The present work aimed to study the stress evolution and fracture behavior of TBCs with cooling hole under cyclic thermal loadings. Cyclic thermal tests were conducted and the cracking behavior of TBCs around cooling hole was examined using scanning electron microscope (SEM). TBCs exhibited two types of cracks near the hole edge, i.e. surface crack in the top coat and interfacial crack, which could lead to local TBC spallation and shorten the TBC lifetime. To disclose the fracture mechanism, finite element (FE) analysis was also performed. The computed residual stress values were consistent with those measured by Raman spectroscopy tests. FE simulations indicated that the free-edge effect facilitated interfacial peeling and shear stresses near the cooling hole, and hence promoted the initiation of interfacial crack in TBCs. With the increase of thermal loading cycles, the interfacial crack propagated and then coalesced with the surface cracks in the top coat, leading to the final spallation of TBCs at the cooling hole.

Oxide Dispersion Strengthened Bond Coats with Higher Alumina Content: Oxidation Resistance and Influence on Thermal Barrier Coating Lifetime

The oxidation resistance of the bond coat in thermal barrier coating systems has significant influence on thermal cycling performance of the protective coating. In this study, the influence of varying the alumina content of plasma-sprayed oxide dispersion strengthened bond coats with CoNiCrAlY matrix material on the oxidation resistance was analysed by thermogravimetric analysis, SEM and TEM. Yttrium ions at the alumina scale grain boundaries and the grain size in the scale appear as major factors influencing oxidation properties. The ODS material with 2, 10 and 30 wt% alumina content was applied in TBC systems as an additional thin bond coat. The thermal cycling performance of those advanced TBC systems, in burner rig tests, was evaluated with respect to the ODS material properties. Thermal cycling behaviour is in good correlation with the isothermal oxidation resistance. All results indicate that TBC systems with 10 wt% alumina content in the ODS bond coat have a superior thermal cycling performance, as compared to ODS bond coats with lower or higher alumina content.

On the CMAS Problem in Thermal Barrier Coatings: Benchmarking Thermochemical Resistance of Oxides Alternative to YSZ Through a Microscopic Standpoint

This study focuses on experimental modelling of the failure of Thermal Barrier Coatings (TBCs) due to attack of CMAS (Calcia-Magnesia-Alumina-Silicate), which is often found in harsh environments, via glassy phase infiltration. Volcanic ash and dust, sand particles, and fly ash, which contain CMAS, are imminent threats impeding predictable lifetimes of TBCs. Such incurrence directly affects the geometry and clinging to bond coat, and intrinsic material properties such as thermal conductivity and crystal structure of TBC are modified after exposure to CMAS, which ultimately results in delamination, spallation and failure of the coating material. The scope of this work is to survey the reactivity of CMAS with various oxide systems, and evaluate possible oxide systems that can be replaced and/or used with Yttria-stabilized Zirconia (YSZ) by investigating the penetration depth and reactivity after sintering with CMAS. A cost-effective method to observe the reaction of candidate oxides with CMAS is suggested and administired; understanding the main mechanism that causes the failure of top coat in the wake of CMAS infiltration, and seeking solutions for the problem is performed by taking advantage of Scanning Electron Microscopy (SEM). Recently suggested ceramic oxide systems that form in pyrochlore structure, some perovskite structures in various compositions, monazite, mullite and YSZ are studied. The possible outcome consequent upon CMAS infiltration are concluded and course for designing novel material systems that are expected to withstand the CMAS attacks better than the state-of-the-art 4mole% YSZ is defined. 5% mole Yb-doped SrZrO3(5Yb-SZ) and favored pyrochlores such as Gd2r2O7 and GdYbZr2O7 are found to be better mitigating CMAS attacks.

Laser Thermal Gradient Testing and Fracture Mechanics Study of a Thermal Barrier Coating

It is critical for thermal barrier coating (TBC) development that a testing method be used to understand the potential and limitation of a coating’s durability and integrity under gas turbine engine operating conditions. To this end, a TBC-coated button is tested using a laser high-heat flux facility. The ceramic coating is ZrO2-8 wt.% Y2O3 applied via the air plasma spraying process on top of a NiCoCrAlY bond coating and an Inconel alloy 617 substrate button of 25.4 mm diameter. The coated button is subject to precisely controlled laser heating on the top side (1150 °C) and a temperature gradient of 63.9 °C/mm through the button overall thickness. The coated button lasts 160.9 h or 570 cycles of laser heating. The void fraction change before and after the test, the thermal conductivity change during the laser test and the failure assessment are presented. After the test, significant horizontal cracks exist in the top coating close to the thermally grown oxide (TGO) layer and near the button center. Based on the cracks and the TGO layer geometry, the stress intensity factor and strain energy release rate are computed. The combined experimental and computational approach can lead to a TBC lifetime model.

Architecture designs for extending thermal cycling lifetime of suspension plasma sprayed thermal barrier coatings

Suspension plasma spraying (SPS) as a relatively new spraying technology has great potential on depositing high performance thermal barrier coatings (TBCs). In some cases, however, columnar SPS TBCs show premature failure in thermal cycling test. To explain the reasons of such failure, a failure mechanism for columnar SPS TBCs was proposed in this work. The premature failure of TBCs might be related to the radial stresses in the vicinity of top coat/bond coat interface. These radial stresses were introduced by the thermal misfit and the roughness of bond coat. According to this mechanism, two architecture designs of SPS TBCs were applied to improve the thermal cycling lifetime. One was a double layered top coat design with a lamellar atmospheric plasma sprayed (APS) sub-layer and a columnar SPS top-layer. The other one was a low roughness bond coat design with a columnar SPS top coat deposited on a low roughness bond coat which was grinded before the spraying. With both designs, lifetimes of SPS TBCs were significantly extended. Especially, a lifetime even better than conventional APS TBCs was achieved with the double layered design.

Air Plasma Sprayed Bond Coat Oxidation Behavior and its Resistance to Isothermal and Thermal Shock Loading

An experimental investigation was conducted to find the effect of spraying method of coating of Thermal Barrier Coatings (TBCs) on their oxidation behaviour and resistance to various thermal loading. Isothermal and thermal shock tests were performed in order to study oxidation behaviour of air plasma sprayed Bond Coat (BC) and assess its effect on TBCs lifetime under the stated loadings. Specimens, after loading were investigated using Field Emission Scanning Electron Microscopy (FE-SEM). Results showed in spite of forming various oxide layers within the BC its oxidation in Bond Coat/Top Coat interface behave the same as samples coated by other methods and no failure was observed at substrate/BC interface or within the BC.

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.

Effect of water vapor on the failure behavior of thermal barrier coating with Hf-doped NiCoCrAlY bond coating

Abstract

Variation of Thermal Barrier Coating Lifetime Characteristics with Thermal Durability Evaluation Methods

The thermal durability of thermal barrier coating systems (TBCs) obtained using feedstock powders with different purity and phase content was investigated by thermal shock testing with different cycle times, including the effects on the sintering and phase transformation behaviors. Four 8 wt.% yttria-stabilized zirconia powders, with regular purity (TC1), high purity (TC2 and TC3), and without monoclinic phase (TC4), were employed to prepare the topcoat of TBC by atmospheric plasma spray on a NiCoCrAlY bondcoat deposited by high velocity oxy-fuel. The microstructure and phase stability of the topcoats affected the TBCs’ lifetime in the short-term (1 h) and long-term (24 h) furnace cyclic test (FCT) at 1100 °C and jet engine thermal shock (JETS) test. In the short-term FCT and JETS tests, in which coatings are severely subjected to thermal stress, the TBCs’ lifetime is most affected by the microstructure of the topcoat. The coating layer with the lowest monoclinic phase in the as-sprayed state showed the lowest phase-transformation characteristics in the isothermal oxidation test (1400 °C). These properties resulted in the best lifetime in the long-term FCT. Therefore, the coating material and evaluating methods of TBCs’ life should be selected depending on the usage environment.

Development of bondcoats for high lifetime suspension plasma sprayed thermal barrier coatings

Fabrication of thermal barrier coatings (TBCs) by suspension plasma spraying (SPS) seems to be a promising alternative for the industry as SPS TBCs have the potential to provide lower thermal conductivity and longer lifetime than state-of-the-art allowing higher engine efficiency. Further improvements in lifetime of SPS TBCs and fundamental understanding of failure mechanisms in SPS TBCs are necessary for their widespread commercialisation. In this study, the influence of varying topcoat-bondcoat interface topography and bondcoat microstructure on lifetime was investigated. The objective of this work was to gain fundamental understanding of relationships between topcoat-bondcoat interface topography, bondcoat microstructure, and failure mechanisms in SPS TBCs.Seven sets of samples were produced in this study by keeping same bondcoat chemistry but varying feedstock particle size distributions and bondcoat spray processes. The topcoat chemistry and spray parameters were kept identical in all samples. Three-dimensional surface measurements along with scanning electron microscopy images were used to characterise bondcoat surface topography. The effect of varying interface topography and bondcoat microstructure on thermally grown oxide formation, stresses and lifetime was discussed.The results showed that varying bondcoat powder size distribution and spray process can have a significant effect on lifetime of SPS TBCs. Smoother bondcoats seemed to enhance the lifetime in case of SPS TBCs in case of same bondcoat chemistry and similar bondcoat microstructures. When considering the samples investigated in this study, samples with high velocity air-fuel (HVAF) bondcoats resulted in higher lifetime than other samples indicating that HVAF could be a suitable process for bondcoat deposition in SPS TBCs.

Numerical study on the competitive cracking behavior in TC and interface for thermal barrier coatings under thermal cycle fatigue loading

Thermal barrier coatings (TBCs) have complex structures and service in the extremely high temperatures. The cracking phenomenon is usually observed at both the top coat and the interface between the bond coat and the thermally grown oxide layer under the thermal cyclic fatigue (TCF) loading. In this study, the competitive cracking behavior in top coat and interface for TBCs were numerical investigated by using a cohesive zone model (CZM) and extend finite element method (XFEM). The dominance stress controlling cracks initiation and propagation was analyzed. The initiation and propagation of TC cracks were controlled by tensile stress, while the interfacial cracking was led by tensile and shear stress commonly. Then, the results showed that the cracks in the TC layers could decrease the stress and delay the interfacial cracks propagation. In addition, this paper also analyzed the key factors affecting the cracking behaviors in TBCs. Changing of the thermal load waveform could lead different cracks propagation path in top coat and failure mode of TBCs. Moreover, the initiation location of TC cracks was independent on the interfacial roughness, but stress concentration of TBCs with the rougher interface was more evident. The thinner initial TGO layer could prolong the lifetime of TBCs. The results demonstrate that the load waveform should be designed to characterize the service environment.

The Effect of Coating Composition and Geometry on Thermal Barrier Coatings Lifetime

Several factors are being investigated that affect the performance of thermal barrier coatings (TBC) for use in land-based gas turbines where coatings are mainly thermally sprayed. This study examined high velocity oxygen fuel (HVOF), air plasma-sprayed (APS), and vacuum plasma-sprayed (VPS) MCrAlYHfSi bond coatings with APS YSZ top coatings at 900–1100 °C. For superalloy 247 substrates and VPS coatings tested in 1 h cycles at 1100 °C, removing 0.6 wt %Si had no effect on average lifetime in 1 h cycles at 1100 °C, but adding 0.3%Ti had a negative effect. Rod specimens were coated with APS, HVOF, and HVOF with an outer APS layer bond coating and tested in 100 h cycles in air + 10%H2O at 1100 °C. With an HVOF bond coating, initial results indicate that 12.5 mm diameter rod specimens have much shorter 100 h cycle lifetimes than disk specimens. Much longer lifetimes were obtained when the bond coating had an inner HVOF layer and outer APS layer.

Preoxidation of bond coat in IN-738LC/NiCrAlY/LaMgAl11O19 thermal barrier coating system

The low thickness of thermally grown oxide (TGO) layer and presence of amorphous phase in the as-sprayed LaMgAl11O19 (LaMA) coating reduce the thermal cycling lifetime of thermal barrier coatings (TBCs). In the present study, the as-sprayed Ni-22Cr-10Al-1.0Y bond coat was preoxidized at 1060 °C to produce a continuous oxide scale prior to subsequent deposition of the ceramic top coat. The optimum time of peroxidation treatment and thickness of the continuous aluminum oxide layer were estimated 15 h and 2 µm respectively. The oxidized layer due to the preoxidation treatment of bond coating reduces the amorphous phase in as-sprayed LaMA coating and increases the microhardness of LaMA coating from approximately 600 to 900HV. Also, preoxidation of the NiCrAlY bond coating increases adhesion strength of the LaMA top coating, even slightly more than the adhesion strength of the as-spray 8YSZ coating. The LaMA coatings have a lower hardness in compared with the 8YSZ coating (~ 1010Hv), which results a better elastic behavior.

Prolong the durability of La2Zr2O7/YSZ TBCs by decreasing the cracking driving force in ceramic coatings

Service lifetime and thermal insulation performance are both crucial for the application of thermal barrier coatings (TBCs). In this study, layered structure design under equivalent thermal insulation conception is introduced to lower the cracking driving force in TBCs, and with the goal of prolonging TBCs lifetime. Three groups of layered LZO/YSZ TBCs were designed with same thermal insulation of 500 μm YSZ, the LZO layers were deliberately designed with different initial elastic modulus. Virtual crack closure technique (VCCT) calculation result showed that the energy release rates at the crack tips are 28.2, 22, and 18.8 N/m corresponding to the initial elastic modulus of 70, 60, and 50 GPa. After gradient thermal cyclic tests with surface temperature of 1300 °C, TBCs with lowest initial elastic modulus showed the longest lifetime, and more than double of pure YSZ TBCs. This study provides a new option for the improvement of TBCs lifetime.