Durability Of Thermal Barrier Coatings Research Articles

Toward Durable Thermal Barrier Coating with Composite Phases and Low Thermal Conductivity

High-efficiency turbine engines highly rely on the further improvement of the novel technologies related to combustion, cooling and thermal barrier coating (TBC) with the increase in inlet temperatures. Thermal barrier coatings with higher thermal stability and lower thermal conductivity (low-k) than current 8YSZ TBC had attracted a lot of academia and industries’ attentions and activities. The present work aimed to focus on exploring a path toward a durable TBC with better thermal durability and low-k capability by overcoming the technical, practical and economic barriers for current low-k TBC development and applications. The concept of composite phase ceramics was proposed for low-k TBC material design, in an effort to combine the desirable attributes of unique phase constitution, low conductivity k, high fracture toughness and good process economy. Further, thermal spray process was optimized for the topcoats of the low-k ceramics by controlling and measuring the effect of key process parameters on porous coating architecture, deposition rate and process efficiency. To evaluate the performance of the low-k TBCs, both an isothermal oxidation test and a thermal cycling test were conducted. The test results of the composite phase ceramics exhibited promising for a durable low-k TBC measured by several desirable property attributes.

On the Dependence of Durability of Thermal Barrier Coatings on the Oxidation Resistance of Composite Substrate Systems Produced by Diffusing Platinum into Selected Ni-Based Superalloys

Platinum is diffused into the single-crystal superalloys CMSX-4 and CMSX-10 to integrate a surface protection layer consisting of a mixture of γ′- and γ-phases. It is shown that the Pt/CMSX-10 system is characterized by higher thermal stability and resistance to oxidation in comparison with the Pt/CMSX-4 system. Such difference is correlated with the initial microstructure and composition of the two systems and is reflected on their performance in thermal barrier coatings utilizing yttria-stabilized zirconia as top coating. Based upon cyclic oxidation tests, the Pt/CMSX-10 system is found to provide about threefold increase in useful life. However, both coatings are found to fail by spallation of the top coating due to decohesion of the thermally grown oxide developed by each system. It is concluded that the higher performance provided by the Pt/CMSX-10 stems from decelerated kinetics of interdiffusion and oxidation.

Durability of Gadolinium Zirconate/YSZ Double-Layered Thermal Barrier Coatings under Different Thermal Cyclic Test Conditions.

Higher durability in thermal barrier coatings (TBCs) is constantly sought to enhance the service life of gas turbine engine components such as blades and vanes. In this study, three double layered gadolinium zirconate (GZ)-on-yttria stabilized zirconia (YSZ) TBC variants with varying individual layer thickness but identical total thickness produced by suspension plasma spray (SPS) process were evaluated. The objective was to investigate the role of YSZ layer thickness on the durability of GZ/YSZ double-layered TBCs under different thermal cyclic test conditions i.e., thermal cyclic fatigue (TCF) at 1100 °C and a burner rig test (BRT) at a surface temperature of 1400 °C, respectively. Microstructural characterization was performed using SEM (Scanning Electron Microscopy) and porosity content was measured using image analysis technique. Results reveal that the durability of double-layered TBCs decreased with YSZ thickness under both TCF and BRT test conditions. The TBCs were analyzed by SEM to investigate microstructural evolution as well as failure modes during TCF and BRT test conditions. It was observed that the failure modes varied with test conditions, with all the three double-layered TBC variants showing failure in the TGO (thermally grown oxide) during the TCF test and in the ceramic GZ top coat close to the GZ/YSZ interface during BRT. Furthermore, porosity analysis of the as-sprayed and TCF failed TBCs revealed differences in sintering behavior for GZ and YSZ. The findings from this work provide new insights into the mechanisms responsible for failure of SPS processed double-layered TBCs under different thermal cyclic test conditions.

Non-destructive evaluation of thermally grown oxides in thermal barrier coatings using impedance spectroscopy

The growth of thermally grown oxide (TGO) layers in thermal barrier coatings (TBCs) due to the oxidation of the bondcoat alloy is a critical factor affecting the durability of TBCs. In the present study, diverse TBC specimens were subjected to long-term oxidation at various temperatures. The TGO growth mechanism was investigated according to cross-sectional images of the oxidized specimens. Impedance spectroscopy (IS) was performed to measure the electrical properties of the integrated TBCs non-destructively. Considering the influence of the TGO composition, the derived TGO electrical capacitance was found to have a good correspondence to the observed TGO thickness over a wide range 0–18.3 μm, regardless of the diverse specimens and oxidation conditions. The error was less than ±2.0 μm. With a certain design of the electrode size, IS is generalized and is recommended as an accurate and practical non-destructive evaluation method for the determination of TGO thickness within a very wide range in TBC systems under real operating conditions.

Extend the thermal cyclic lifetime of La2Zr2O7/YSZ DCL TBCs by reducing modulus design on a toughening ceramic surface

La2Zr2O7 (LZO), a potential thermal barrier coating material, is extensively used for components bearing remarkably higher temperatures due to its phase stability up to 2300 °C, but the lifetime of the pure LZO coatings is generally limited by the low toughness due to the cracking driving force. In this study, to decrease the driving force facilitating the failure of LZO layer and then extend the durability of thermal barrier coatings (TBCs), reducing modulus design on a toughening ceramic surface is introduced into the system. Three groups of LZO/YSZ double ceramic layers (DCL) TBCs with different low/high modulus (E) combination are used to satisfy the design. The crack driving force is evaluated by the strain energy release rate (SERR) at the tip of the LZO/YSZ interface crack. The total SERR G decreases from 28.2 N/m to 20.6 N/m when the thickness of low E LZO layer increases from 0 to 2 units (1 unit = 48 μm low E LZO), which reduces the cracking driving force of TBCs. After subjected to the gradient thermal cyclic tests with the surface temperature of 1300 °C. The TBCs system with 2 low E LZO shows the longest lifetime, which is 50% higher than that of pure YSZ TBCs. Thereby, reducing modulus design on a toughening ceramic surface may provide an alternative approach to improve the durability of TBCs.

Durable TBCs with self-enhanced thermal insulation based on co-design on macro- and microstructure

During thermal exposure of thermal barrier coatings (TBCs), sintering causes degradation of the thermal barrier performance, as well as spallation. However, it is difficult to simultaneously enhance the thermal insulation and extend the lifetime, because the dominant factors (e.g., micropores and the macroscopic thickness) may have opposite effects on these two performances. In this study, a co-design of the macro- and microstructure is proposed to enhance multiple performance characteristics of TBCs. The microstructural design is achieved by using a co-spraying method, to form a hybrid layered coating. During thermal exposure, new pores are formed because the contractions of dense splats and loose porous nanoheaps are reversed. Consequently, the thermal barrier performance is self-enhanced by 40% with respect to that of mono-layered coatings. The macrostructural design is realized by tailoring the thickness according to the equivalent thermal insulation. As a result, the lifetime is extended by 35%. The mechanisms responsible for the simultaneously enhanced performance characteristics are discussed. This study is expected to contribute to structural tailoring of advanced TBCs with both high thermal insulation and long lifetimes.

Achieving self-enhanced thermal barrier performance through a novel hybrid-layered coating design

The thermal insulation and durability of thermal barrier coatings (TBCs) are mainly affected by sintering-induced healing of 2D micropores, which is inevitable under high temperature conditions. In this study, we designed and prepared novel hybrid-layered TBCs. During thermal exposure, the degree of degradation in thermal conductivity is observed to decrease from 80 to 100% for conventional coatings to ~20% for the novel coatings. For a detailed understanding, the evolution of the hybrid-layered TBCs can be divided into two stages: during stage I (0−10 h), ultrafast healing of 2D micropores occurs, mainly caused by the multiple contacts between the counter-surface. At this stage, the thermal and mechanical properties also increase sharply. During stage II (after 10 h), some new 2D mesopores are formed. Compared with the 2D micropores, the newly formed 2D mesopores have a much larger aspect ratio that increases the ratio of the effective area for thermal insulation from 10 to 30% to 60%, which accounts for the ~50% self-enhancement in the thermal barrier performance. This self-enhancing behavior is expected to prolong the lifetime and increase the performance of the TBCs, which is the main objective of using advanced TBCs in next-generation applications.

Thermal Durability of Thermal Barrier Coatings in Furnace Cyclic Thermal Fatigue Test: Effects of Purity and Monoclinic Phase in Feedstock Powder

Thermal Durability of Thermal Barrier Coatings in Furnace Cyclic Thermal Fatigue Test: Effects of Purity and Monoclinic Phase in Feedstock Powder

Coupled mechanical-oxidation modeling during oxidation of thermal barrier coatings

The durability of thermal barrier coatings (TBCs) is controlled by the thermally grown oxide (TGO) growth, which is sustained by continuous diffusion of oxygen in TBCs. At the same time, stresses are induced due to volumetric change when TBCs are oxidized. Such stress may in return affect the diffusion of oxygen in the TGO layer, thus changing the TGO growth kinetics. In the present research, a continuum thermodynamic model is developed to account for such stress–diffusion interaction in the oxidation of TBCs. Then we numerically implement our chemo-mechanically-coupled constitutive theory into the widely used finite element software to simulate the oxidation behavior of TBCs. The results demonstrate that the maximum tensile stresses locate at the peak regions of bond coating (BC) layer and the valley region of top coating, respectively. It implies that the failure of the TBCs may occur at the peak of BC/TGO and the valley of TC/TGO interface, which is consistent with the experimental observations. It is also found the stress significantly slows down the rate of oxidation. Consequently, the TGO growth kinetic is not strictly parabolic.

Advanced Cooling in Gas Turbines 2016 Max Jakob Memorial Award Paper

Gas turbines have been extensively used for aircraft engine propulsion, land-based power generation, and industrial applications. Power output and thermal efficiency of gas turbines increase with increasing turbine rotor inlet temperatures (RIT). Currently, advanced gas turbines operate at turbine RIT around 1700 °C far higher than the yielding point of the blade material temperature about 1200 °C. Therefore, turbine rotor blades need to be cooled by 3–5% of high-pressure compressor air around 700 °C. To design an efficient turbine blade cooling system, it is critical to have a thorough understanding of gas turbine heat transfer characteristics within complex three-dimensional (3D) unsteady high-turbulence flow conditions. Moreover, recent research trend focuses on aircraft gas turbines that operate at even higher RIT up to 2000 °C with a limited amount of cooling air, and land-based power generation gas turbines (including 300–400 MW combined cycles with 60% efficiency) burn alternative syngas fuels with higher heat load to turbine components. It is important to understand gas turbine heat transfer problems with efficient cooling strategies under new harsh working environments. Advanced cooling technology and durable thermal barrier coatings (TBCs) play most critical roles for development of new-generation high-efficiency gas turbines with near-zero emissions for safe and long-life operation. This paper reviews basic gas turbine heat transfer issues with advanced cooling technologies and documents important relevant papers for future research references.

Influence of Lamellar Interface Morphology on Cracking Resistance of Plasma-Sprayed YSZ Coatings

Splat morphology is an important factor that influences the mechanical properties and durability of thermal barrier coatings (TBCs). In this study, yttria-stabilized zirconia (YSZ) coatings with different lamellar interface morphologies were deposited by atmospheric plasma spraying (APS) using feedstocks with different particle sizes. The influence of lamellar interface roughness on the cracking resistance of the coatings was investigated. Furthermore, the thermal shock and erosion resistance of coatings deposited by two different powders was evaluated. It was found that the particle size of the feedstock powder affects the stacking morphology of the splat that forms the coating. Coatings fabricated from coarse YSZ powders (45–60 μm) show a relatively rough inter-lamellar surface, with a roughness about 3 times greater than those faricated from fine powders (15–25 μm). Coatings prepared with fine powders tend to form large cracks parallel to the substrate direction under indentation, while no cracking phenomena were found in coatings prepared with coarse powders. Due to the higher cracking resistance, coatings prepared with coarse powders show better thermal shock and erosion resistances than those with fine powders. The results of this study provide a reference for the design and optimization of the microstructure of TBCs.

Development of Highly Durable Thermal Barrier Coating by Suppression of Thermally Grown Oxide

Durability of thermal barrier coating (TBC) systems is important because of recent rising of turbine inlet temperature (TIT) for improved efficiency of industrial gas turbine engines. However, high-temperature environment accelerates the degradation of the TBC as well as causes spalling of the top coat. Spalling of the top coat may be attributed to several factors, but evidently the growth of thermally grown oxide (TGO) should be considered as an important factor. One method for reducing the growth rate of TGO is to provide a dense α-Al2O3 layer at the boundary of the bond coat and top coat. This α-Al2O3 layer will suppress the diffusion of oxygen to the bond coat and consumption of aluminum of the bond coat is suppressed. In this study, we focused on thermal pre-oxidation of the bond coat as a means for forming an α-Al2O3 barrier layer that would be effective at reducing the growth rate of TGO, and we studied the suitable pre-oxidation conditions. In the primary stage, we analyzed the oxidation behavior of the bond coat surface during pre-oxidation heat treatment by means of in situ synchrotron X-ray diffraction (XRD) analysis. As a result, we learned that during oxidation in ambient air environment, in the initial stage of oxidation metastable alumina is produced in addition to α-Al2O3, but if the thermal treatment is conducted under some specific low oxygen partial pressure condition, unlike in the ambient air environment, only α-Al2O3 is formed with suppressing formation of metastable alumina. We also conducted transmission electron microscope (TEM) and XRD analysis of oxide scale formed after pre-oxidation heat treatment of the bond coat. As a result, we learned that if pre-oxidation is performed under specific oxygen partial pressure conditions, a monolithic α-Al2O3 layer is formed on the bond coat. We performed a durability evaluation test of TBC with the monolithic α-Al2O3 layer formed by pre-oxidation of the bond coat. An isothermal oxidation test confirmed that the growth of TGO in the TBC that had undergone pre-oxidation was suppressed more thoroughly than that in the TBC that had not undergone pre-oxidation. Cyclic thermal shock test by hydrogen burner rig was also carried out. TBC with the monolithic α-Al2O3 layer has resistance to >2000 cycle thermal shock at a load equivalent to that of actual gas turbine.

Synthesis of alumina ceramic encapsulation for self-healing materials on thermal barrier coating

Durability of Thermal Barrier Coating or TBC can be optimized by inducing Self-Healing capabilities with intermetallic materials MoSi2. Nevertheless, high temperature operation causes the self-healing materials to become oxidized and lose its healing capabilities. Therefore, a method to introduce ceramic encapsulation for MoSi2 is needed to protect it from early oxidation. The encapsulation process is synthesized through a simple precipitation method with colloidal aluminum hydroxide as precursor and variations on calcination process. Semi-quantitative analysis on the synthesized sample is done by using X-ray diffraction (XRD) method. Meanwhile, qualitative analysis on the morphology of the encapsulation was carried out by using Scanning Electron Microscope (SEM) and Field Emission Scanning Electron Microscope (FESEM) equipped with dual Focus Ion Beam (FIB). The result of the experiment shows that calcination process significantly affects the final characteristic of encapsulation. The optimum encapsulation process was synthesized by colloidal aluminum hydroxide as a precursor, with a double step calcination process in low pressure until 900 °C.

Higher Temperature Thermal Barrier Coatings with the Combined Use of Yttrium Aluminum Garnet and the Solution Precursor Plasma Spray Process

Gas-turbine engines are widely used in transportation, energy and defense industries. The increasing demand for more efficient gas turbines requires higher turbine operating temperatures. For more than 40 years, yttria-stabilized zirconia (YSZ) has been the dominant thermal barrier coating (TBC) due to its outstanding material properties. However, the practical use of YSZ-based TBCs is limited to approximately 1200 °C. Developing new, higher temperature TBCs has proven challenging to satisfy the multiple property requirements of a durable TBC. In this study, an advanced TBC has been developed by using the solution precursor plasma spray (SPPS) process that generates unique engineered microstructures with the higher temperature yttrium aluminum garnet (YAG) to produce a TBC that can meet and exceed the major performance standards of state-of-the-art air plasma sprayed YSZ, including: phase stability, sintering resistance, CMAS resistance, thermal cycle durability, thermal conductivity and erosion resistance. The temperature improvement for hot section gas turbine materials (superalloys & TBCs) has been at the rate of about 50 °C per decade over the last 50 years. In contrast, SPPS YAG TBCs offer the near-term potential of a > 200 °C improvement in temperature capability.

Tailoring columnar microstructure of axial suspension plasma sprayed TBCs for superior thermal shock performance

This paper investigates the thermal shock behavior of thermal barrier coatings (TBCs) produced by axial suspension plasma spraying (ASPS). TBCs with different columnar microstructures were subjected to cyclic thermal shock testing in a burner rig. Failure analysis of these TBCs revealed a clear relationship between lifetime and porosity. However, tailoring the microstructure of these TBCs for enhanced durability is challenging due to their inherently wide pore size distribution (ranging from few nanometers up to few tens of micrometers). This study reveals that pores with different length scales play varying roles in influencing TBC durability. Fracture toughness shows a strong correlation with the lifetime of various ASPS TBCs and is found to be the prominent life determining factor. Based on the results, an understanding-based design philosophy for tailoring of the columnar microstructure of ASPS TBCs for enhanced durability under cyclic thermal shock loading is proposed.

Mechanisms governing the failure modes of dense vertically cracked thermal barrier coatings

The introduction of dense vertical cracks in ceramic top coat has been recognized as an efficient way to improve the strain tolerance and service durability of thermal barrier coatings (TBCs). However, further growth of these pre-existing cracks may cause the delamination of the coatings to form various failure modes. The motivation of this work is to numerically study the mechanisms that govern the formation of failure modes in dense vertically cracked TBCs. The effects of vertical crack density and material properties on the fracture behaviors are discussed. An energy-based criterion is employed to deal with the surface crack deflection/penetration behavior at a bimaterial interface, and an interaction integral method for discontinuous materials is utilized to evaluate the fracture parameters. The failure maps of TBCs are constructed by examining the continuous variations of crack tip driving forces over the whole fracture process. It is verified that the interfacial fracture resistance could be enhanced by introducing high-density vertical cracks. At the same time, these cracks tend to penetrate into the bond coat that results in debonding at bond coat/substrate interface or at both interfaces. In addition to benefit in alleviating the interfacial mismatch stress, it is found that the high-density vertical cracks give toughening increases to the coating system by releasing more energy from multiple paths. Moreover, the effects of mixed-mode interface toughness and initially residual stress on the formations of failure mode are discussed.

Characteristics of thick columnar YSZ coatings fabricated by plasma spray-physical vapor deposition

Columnar-structure is favourable to the durability of thermal barrier coatings (TBCs), and the efficacy of TBCs increases with the thickness of the TBCs. In this study, thick columnar YSZ coatings (∼1.5mm) were deposited by multiple PS-PVD processes. The phase compositions, microstructures and mechanical properties were examined with various techniques including X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and nanoindentation. Results show that there are only tetragonal phase on the top surface while both monoclinic and tetragonal zirconia are present on the bottom surface. The first layer of the coatings is composed of randomly oriented grains. Immediately after the quasi-equiaxed zone, columns start to grow. The top coatings exhibit looser microstructure compared with the middle and bottom regions, and thus have lower values of hardness and Young’s modulus.

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.

CMAS (CaO–MgO–Al2O3–SiO2) resistance of Y2O3-stabilized ZrO2 thermal barrier coatings with Pt layers

Calcium–magnesium–alumina–silicate (CMAS) corrosion significantly affects the durability of thermal barrier coatings (TBCs). In this study, Y2O3 partially stabilized ZrO2 (YSZ) TBCs are produced by electron beam-physical vapor deposition, followed by deposition of a Pt layer on the coating surfaces to improve the CMAS resistance. After exposure to 1250°C for 2h, the YSZ TBCs were severely attacked by molten CMAS, whereas the Pt-covered coatings exhibited improved CMAS resistance. However, the Pt layers seemed to be easily destroyed by the molten CMAS. With increased heat duration, the Pt layers became thinner. After CMAS attack at 1250°C for 8h, only a small amount of Pt remained on the coating surfaces, leading to accelerated degradation of the coatings. To fully exploit the protectiveness of the Pt layers against CMAS attack, it is necessary to improve the thermal compatibility between the Pt layers and molten CMAS.

Evaluation of Thermal Durability of Thermal Barrier Coating and Change in Mechanical Behavior

This study investigates changes in the mechanical behavior, such as changes in indentation load-displacement curve, wear resistance and contact fatigue resistance of thermal barrier coatings (TBCs) by thermal cycling test and thermal shock test. Relatively dense and porous TBCs on nickel-based bondcoat/superalloy are prepared; the highest temperature applied during thermal durability test is 1350℃. The results indicate that the porous TBCs have relatively longer lifetime during thermal cycling and thermal shock tests, while denser TBCs have relatively higher wear and contact fatigue resistance. The mechanical behavior is influenced by sintering of the TBCs by exposure to high temperature during tests.