Isothermal Oxidation Behavior of Gd2Zr2O7/YSZ Multilayered Thermal Barrier Coatings

Efficiency of a gas turbine can be increased by increasing the operating temperature. Yttria‐stabilized zirconia (YSZ) is the standard thermal barrier coating (TBC) material used in gas turbine applications. However, above 1200°C, YSZ undergoes significant sintering and CMAS (calcium magnesium alumino silicate) infiltration. New ceramic materials of rare earth zirconate composition such as gadolinium zirconate (GZ) are promising candidates for thermal barrier coating applications (TBC) above 1200°C. Suspension plasma spray of single‐layer YSZ, double‐layer GZ/YSZ, and a triple‐layer TBC comprising denser GZ on top of GZ/YSZ TBC was attempted. The overall coating thickness in all three TBCs was kept the same. Isothermal oxidation performance of the three TBCs along with bare substrate and bond‐coated substrate was investigated for time intervals of 10 h, 50 h, and 100 h at 1150°C in air environment. Weight gain/loss analysis was carried out by sensitive weighing balance. Microstructural analysis was carried out using scanning electron microscopy (SEM). As‐sprayed single‐layer YSZ and double‐layer GZ/YSZ showed columnar microstructure, whereas the denser layer in the triple‐layer TBC was not columnar. Phase analysis of the top surface of as‐sprayed TBCs was carried out using XRD. Porosity measurements were made by water intrusion method. In the weight gain analysis and SEM analysis, multilayered TBCs showed lower weight gain and lower TGO thickness compared to single‐layer YSZ.     

Thermal cycling failure of the multilayer thermal barrier coatings based on LaMgAl11O19/YSZ

Thermal cycling failure of three multilayer TBCs based on LaMgAl₁₁O₁₉ (LaMA)/YSZ was comparatively investigated by using the burner-rig testing method in this work. Results indicate that through optimizing the weight ratio and thickness of the intermediate LaMA/YSZ composite layers, a five-layer TBC with much improved thermal cycling life of 11,749 cycles at 1372 °C surface and 1042 °C bond coat testing temperature has been realized. While, thermal cycling lifetimes of the tri- and six-layer TBCs were 7439 and 7804 cycles at surface/bond coat testing temperatures of 1378 °C/1065 °C and 1367 °C/1056 °C, respectively. Factors related to the 60 wt.% LaMA + 40 wt.% YSZ (60LaMA + 40YSZ) intermediate composite layer with the highest thermal expansion coefficient than other composite layers generating higher internal stress level to the tri- and six-layer TBCs, different bond coat temperature and TGO growth, as well as long-term stability of the LaMA coating during thermal cycling tests, were characterized and compared to understand the different thermal cycling lifetime and failure modes among such three multilayer 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.     

Experimental investigation of Al2O3/8YSZ and CeO2/8YSZ plasma sprayed thermal barrier coating on diesel engine

In this research work, aluminium oxide/yttria stabilized zirconia (20%Al₂O₃/80%8YSZ) and ceria/yttria stabilized zirconia (20%CeO₂/80%8YSZ) were coated through atmospheric plasma spray technique (APS) as thermal barrier coating (TBC) over CoNiCrAlY bond coat on aluminium alloy (Al-13%Si) substrate piston crown material and their thermal cycling behavior were studied experimentally. Thermal cycle test of both samples were conducted at 800?°C. Microstructural, phase and elemental analysis of the TBC coatings were experimentally investigated. The performance, combustion and emission characteristics of Al₂O₃/8YSZ, CeO₂/8YSZ TBC coated and uncoated standard diesel engine were experimentally investigated. The test results revealed that CeO₂/8YSZ based TBC has an excellent thermal cycling behavior in comparison to the Al₂O₃/8YSZ based TBC. The spallation of the Al₂O₃/8YSZ TBC occurred mainly due to the formation of thermally grown oxide (TGO), and growth of residual stresses at top coating and bond coating interface. The experimental results also revealed that the increase of brake thermal efficiency and reduction of specific fuel consumption for both TBC coated engine. Further reduction of HC, CO and smoke and increase of NOx emission were recorded for both TBC coated engine compared to the standard diesel engine.     

Thermal shock behavior of a 8YSZ/CoCrAlYTaSi thermal sprayed barrier coating on GH202 superalloy

The thermal shock behavior of a thermal barrier coating (TBC) prepared by plasma spraying at 1100 °C was investigated. The TBC consisted of a double layer structure of 8YSZ/CoCrAlYTaSi. The morphology, microstructure, phases and the elemental distribution of the TBCs were characterized using scanning electron microscopy (SEM), transmission electron microscope (TEM), scanning transmission electron microscope (STEM), X-ray diffraction (XRD) and electron probe micro-analysis (EPMA). The characterization results showed that the film consisted primarily of metastable tetragonal phases (t′), and a large number of micro-cracks were present in the 8YSZ crystals. Following eighty-six thermal shock cycles of the specimens a large areal spallation was observed on the 8YSZ coating. The decreased concentration of yttrium at the coating interfaces weakened the inhibition of crystal growth and the phase transition of the Al₂O₃. The growth of TGO (Thermal growth oxide) and the diffusion into the 8YSZ coating produced deformation and stress in the ceramic coating. Tantalum appeared to absorb the oxygen that diffused into the coatings and delayed the growth of TGO in the interface between the CoCrAlYTaSi and substrate, which was beneficial to prolonging the life of the TBC.     

Effect of functionally graded structure design on durability and thermal insulation capacity of plasma-sprayed thick thermal barrier coating

This paper aimed to design and optimize the structure of a thick thermal barrier coating by adding graded layers to achieve a balance between high thermal insulation capacity and durability. To this end, conventional TBC, conventional TTBC, and functionally graded TTBCs were deposited on the superalloy substrate by air plasma spraying. To determine the quality of the bond strength of the coatings, the bonding strength was measured. The durability of coatings was evaluated by isothermal oxidation and thermal shock tests. Then, at a temperature of 1000 °C, the thermal insulation capacity of the coatings was carried out. The microstructure of the coatings was characterized by a scanning electron microscope. The results showed that the thickness of the TGO layer formed on the bond coat in the conventional TBC and TTBC under the oxidation test at 1000 °C after 150 h was 2.79 and 2.11 μm, respectively, whereas, in the functionally graded TTBC samples, no continuous TGO layer was observed as a result of internal oxidation. The functionally graded TTBC presented higher durability than conventional TTBC due to improved bonding strength, thermal shock resistance, and the lack of a TGO layer at the bond/top coat interface. Also, the thermal insulation capacity of the functionally graded TTBC (with 1000 μm thickness of YSZ coating) was better than TTBC.     

Multilayer GZ/YSZ thermal barrier coating from suspension and solution precursor thermal spray

Rare-earth (RE) zirconates, such as gadolinium zirconate (GZ), gained much attraction to be used for the next generation TBC. A double-layer and triple-layer TBC were deposited using the suspension and solution precursor high velocity oxy fuel (HVOF) thermal spray. A dense solution precursor GZ layer was intended to minimise the crack propagation from underneath, thereby inhibiting the CMAS infiltration. In the furnace cycling test, the double- and triple-layer coatings had a comparable cyclic lifetime. For the CMAS test, both the double- and triple-layer coatings were exposed to CMAS at 1250 °C for 30 mins. The CMAS deposits melted and infiltrated both coatings through the dense vertical cracks (DVCs). Interestingly, the GZ reacted with the molten CMAS to form a gadolinium apatite phase (Ca₂Gd₈(SiO₄)₆O₂) that was detected in the double- and triple-layer TBC. Both the double- and triple-layer TBCs succeeded in reacting with CMAS.     

New Thermal Barrier Coatings Based on Pyrochlore/YSZ Double-Layer Systems

Pyrochlore materials La₂Zr₂O₇ and Gd₂Zr₂O₇ have been used to produce thermal barrier coating systems by atmospheric plasma spraying. The materials have been applied as single-layer coatings with only a topcoat made of pyrochlore material. In addition, double-layer systems with a first layer of yttria-stabilized zirconia (YSZ) and a top layer made of pyrochlore material were produced. These systems have been tested in thermal cycling test rigs at surface temperatures between 1200-1450°C and the results were compared to the behavior of YSZ coatings. Single-layer coatings had a rather poor thermal cycling performance. On the other hand, double-layer systems showed similar results to YSZ coatings at temperatures below about 1300°C. At higher temperatures the double-layer coatings produced from our own powders revealed excellent thermal cycling behavior. At the highest test conditions, lifetime was thereby orders of magnitude better than that of YSZ coatings. Results indicate that an increase of the maximum surface temperature in gas turbines by at least 100 K becomes possible with the new coatings. Coatings produced from commercial powders showed a somewhat reduced performance.     

Simulation of thermal barrier coating spallation induced by the initiation/growth/coalescence of internal crack and interfacial crack based on a real image model

In the furnace cycle test, the growth of oxide film leads to the propagation and coalescence of multiple cracks near the interface, which should be responsible for the spallation of thermal barrier coatings (TBCs). A TBC model with real interface morphology is created, and the near-interface large pore is retained. The purpose of this work is to clarify the mechanism of TBC spallation caused by successive initiation, propagation, and linkage of cracks near the interface during thermal cycle. The dynamic growth of thermally grown oxide (TGO) is carried out by applying a stress-free strain. The crack nucleation and arbitrary path propagation in YSZ and TGO are simulated by the extended finite element method (XFEM). The debonding along the YSZ/TGO/BC interface is evaluated using a surface-based cohesive behavior. The large-scale pore in YSZ near the interface can initiate a new crack. The ceramic crack can propagate to the YSZ/TGO interface, which will accelerate the interfacial damage and debonding. For the TGO/BC interface, the normal compressive stress and small shear stress at the valley hinder the further crack propagation. The growth of YSZ crack and the formation of through-TGO crack are the main causes of TBC delamination. The accelerated BC oxidation increases the lateral growth strain of TGO, which will promote crack propagation and coalescence. The optimization design proposed in this work can provide another option for developing TBC with high durability.     

Effects of TGO growth on the stress distribution and evolution of three-dimensional cylindrical thermal barrier coatings based on finite element simulations

Based on the ultrasonic C-scan results of 8YSZ coatings after thermal cycles, three-dimensional cylindrical numerical simulations of the physical geometry model of the thermal barrier coating (TBC) sinusoidal surfaces were conducted with finite elements to estimate the stress distribution and evolution law of the top coat (TC)/bond coat (BC) interface, including the centre and edge of the specimen affected by the dynamic growth of the thermally grown oxide (TGO). The results show that when a layer of TGO is grown on the TC/BC interface, compressive stress is uniformly distributed on the TGO interface, and the stress value decreases as a function of the TGO layer thickness. When the thickness of the TGO exceeds a certain value, the compressive stress of all parts of the interface gradually changes to tensile stress; meanwhile, the edges of the model affected by the crest and trough effects of the wave are reflected in the radial and circumferential directions, especially along the axial direction, with alternating concentrated tensile and compressive stresses. TGO growth imposes a minor influence on the magnitude and distributions of the radial and circumferential stresses at the BC interface. The linear elasticity, creep, fatigue, and stress accumulation effects of each layer of TBCs in each thermal cycle were fully considered in this model. The model not only interprets the crest and trough effects of the TC/BC surface interface during the growth of TGO, but also interprets the effects of the core and edge of the cylindrical model, further revealing the reason for which the core and edge of the TBC will most likely form cracks.     

Isothermal oxidation and TGO growth behaviors of YAG/YSZ double-ceramic-layer thermal barrier coatings

In this study, yttrium aluminum garnet/yttria-stabilized zirconia (YAG/YSZ) double-ceramic-layer thermal barrier coatings (DCL TBC) and yttria-stabilized zirconia (YSZ) single-ceramic-layer thermal barrier coatings (SCL TBC) were deposited by atmosphere plasma spray (APS) on the Inconel 738 alloy substrate, and isothermal oxidation tests were performed to investigate the formation and growth behavior of thermally grown oxide (TGO). Results showed that the Al₂O₃ TGO thickness of both TBC groups increased by increasing the isothermal oxidation time，and then slowly decreased with the appearance and growth of the adverse multilayer structure comprising CoCr₂O₄, (Ni,Co)Al₂O₄, NiCr₂O₄, and NiO mixed oxides. However, since the significant inhibition effect of the YAG coating to oxygen ionic diffusion, the mixed oxides appearance time and TGO growth behaviors were delayed in the DCL TBC. As a result, the TGO thickness of the DCL TBC was always smaller than that of the SCL TBC in the entire oxidation process. And the Al₂O₃ layer thickness proportion in the total TGO of the DCL TBC was greater than or equal to that of the SCL TBC after oxidation for the same period. The results of weight gain showed that compared with the SCL TBC, the parabolic oxidation rate of the DCL TBC was decreased approximately 35%. Consequently, the DCL TBC has better high-temperature oxidation resistance than the SCL TBC.     

Gradient thermal cycling behavior of a thermal barrier coating system constituted by NiCoCrAlY bond coat and pure metastable tetragonal nano-4YSZ top coat

Pure metastable tetragonal (t’) phase 4YSZ top coats with thickness of 100 and 200 μm were deposited on NiCoCrAlY-coated second generation single crystal superalloy by air plasma spray (APS). The two thermal barrier coatings were evaluated under gradient thermal cycling test using gas mixture of propane and oxygen. After flame shock test, the values of Young's modulus, hardness and degree of densification all exhibited a gradient distribution across YSZ thickness. In contrast to intensive sintering at surface of 200 μm 4YSZ coating, the TBC sample with 4YSZ layer of 100 μm underwent poor oxidation at interface of YSZ and bond coat, forming a duplex oxide scale: (Ni,Co)(Cr,Al)₂O₄ spinel over Al₂O₃, which promoted the delamination at the top-coat/bond-coat interface. The resistance against gradient thermal cycling, the phase stability of 4YSZ and the failure mechanism of the TBCs, were discussed correlating to the effects of YSZ thickness.     

Model construction and effect of thermally grown oxide dynamic growth on distribution of thermal barrier coatings

A physical geometric model of the dynamic growth of thermally grown oxide (TGO) was established based on an analysis of the TGO growth of 8YSZ thermal barrier coatings during thermal cycling. Finite-element simulation was used to simulate the evolution law between the coating residual stress and thermal cycling, and the linear elasticity, creep effect, and stress accumulation in each thermal cycle were studied. The interface between the top coat (TC) and the bond coat (BC) was covered with a TGO layer that grew vertically and slowly in a layer-like manner. The stress in the TGO was distributed with a “layer” zonal gradient, and the TGO/BC boundaries were distributed uniformly with a large compressive stress, which decreased the TGO layer thickening. With the longitudinal rapid random TGO growth, the boundaries were subjected to a tensile stress, and a high tensile stress concentration area developed at the boundaries. The internal stress consisted of an alternating and mixed distribution of concentrated compressive and tensile stresses. The concentration area of the maximum equivalent stress was distributed in the one-layer TGO near the TC/TGO interface. When a microcrack formed at the TGO/BC boundaries, the crack was subjected to a tensile stress of different size, with a higher tensile stress at both ends, which facilitated crack expansion. Thus, the 8YSZ thermal barrier coating was prone to crack formation and expansion at the TGO/BC boundaries and in the TGO layer near the TC/TGO boundaries.     

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.     

Thermal shock behavior of multilayer and functionally graded micro- and nano-structured topcoat APS TBCs

Performance of air plasma sprayed (APS) thermal barrier coatings (TBCs) with multilayer and functionally graded topcoat were investigated in thermal shock conditions. Ceria-yttria stabilized zirconia (CSZ) and micro- and nano-structured yttria stabilized zirconia (YSZ and YSZ-N) were used to produce coating samples. The samples were classified into four families, namely single-layer, double-layer, triple-layer and functionally graded (FG). To measure thermal shock resistance, the heating/water quenching cycles were repeated 70 times and 30% destruction of the coating was considered its functionality limit. Thus, cycles did not continue for those coatings that were destroyed more than 30%. At the end of each cycle, the surface and edge damage were determined from the photos of samples. Furthermore, scanning electron microscope (SEM) images and energy-dispersive spectrometer (EDS) analysis of samples’ cross-section were taken before and after the test. After collecting the experimental data, effects of various factors on outputs were investigated. The results showed that YSZ-N single-layer coating and triple-layer with CSZ as a top layer, has less thermally grown oxide (TGO) thickness and best performance in thermal shock conditions.     

Evaluation of stress distribution and failure mechanism in lanthanum–titanium–aluminum oxides thermal barrier coatings

LaTi₂Al₉O₁₉ (LTA) is one of the most promising materials for new thermal barrier coatings (TBCs) to fulfill the demand of advanced gas turbines owing to its high temperature stability and low thermal conductivity. In the present study, a finite element (FE) based numerical study has been carried out to investigate the stress distribution in LTA single layered coating system in comparison with traditional yttria stabilized zirconia (YSZ) TBC. Stresses in YSZ/LTA double ceramic layer TBC system are also determined and presented for comparative analysis. The thermal cycling effect is simulated by sequent increment in TGO thickness in a series of FE simulations. In-plane stresses (σ_xx), out-of-plane stresses (σ_yy) and shear stresses (σ_xy) are determined for all systems, and peak stress values are presented for quantitative comparison. Elastic strain energy stored in TGO of all systems is calculated from FE results for TBC structural integrity assessment. It has been found that maximum in-plane and shear stresses are lower in the double ceramic layer coating system than in the single layer ceramic coating system. However, peak axial tensile and compressive stresses in the double ceramic layer coating are very close or higher than those in the single layer topcoat. Calculation of elastic store energy shows that double ceramic layer TBC system may exhibit better stability as compared to single layer systems. Results are presented to explain the failure mechanism in LTA coatings.     

Stresses and Microstructural Development of Thermal Barrier Coatings Using AIP/D-gun Two-Step Processing

After depositing a Ni–32Co–20Cr–8Al–0.5Y–1Si–0.03B (wt%) bond coat on a Ni-base superalloy using arc ion plating (AIP), a ceramic top coat with hollow spherical powder of ZrO₂–8 wt%Y₂O₃ (HSP–YSZ) was deposited using detonation gun (D-gun) spraying. Thermal exposure behaviors of thermal barrier coatings (TBCs) were investigated at 1100°C. The thermal growth oxides (TGO) layer thickened and became more undulated during thermal exposure. Yttrium aluminum garnet (YAG) was observed within TGO, which produced thickness imperfections and thus aided to build up out-of-plane stresses. As a result, radial cracks initiated at the defects around TGO imperfections and separation developed through crack nucleation, propagation, and coalescence at the weaker TGO/bond coat interface. With further thermal exposure, coalescence of interfacial separations created a connected crack. The TBC detached and final failure occurred at the TGO/bond coat interface, leading to spallation of TBC when cooling to ambient. The stress distributions in the TGO layer with different thermal exposure times were also measured using luminescence spectroscopy. The stresses were independent of time after a transient period from θ-Al₂O₃ to α-Al₂O₃ scale. It is suggested that the lifetime of AIP/D-gun TBCs with an HSP–YSZ coat is controlled by the initiation and linking of a sub-critical interfacial crack.     

Evaluation of microstructural evolution in thermal barrier coatings during thermal cycling using impedance spectroscopy

Air plasma sprayed thermal barrier coatings are thermally cycled in air up to 1030 °C and evaluated using impedance spectroscopy in conjunction with scanning electron microscopy. When the number of cycles is less than 15, impedance measurements cannot be used to detect the thermally grown oxide (TGO, usually alumina) scale because it does not fully cover the top coat-bond coat interface and the YSZ (yttria-stabilised zirconia used as the top coat) is much more electrically conductive than the alumina, leading to most of the current passing through the YSZ rather than the alumina. After the specimens are subjected to 100 until 250 cycles, impedance measurements show that a continuous alumina scale is formed. In the impedance spectra, there are four relaxation processes, which correspond to the YSZ grains, the YSZ grain boundaries, the TGO, and the electrode effect. Impedance analyses demonstrate that the resistance of the alumina scale increases and the capacitance decreases with increasing cycling. When the specimen is subjected to 400 cycles, the impedance response to the continuous TGO vanishes due to the TGO degradation.     

Thermal shock resistance of thermal barrier coatings for nickel-based superalloy by supersonic plasma spraying

Double-layer thermal barrier coatings (TBCs), including a top ZrO₂ layer and an inner CoNiCrAlY layer, were deposited on nickel-based superalloy using supersonic atmospheric plasma spraying (SAPS). Thermal shock resistance of the TBCs between 1200 °C and room temperature was investigated. After thermal shock test, the adhesive strength of the coatings was evaluated through scratch test. The SAPS–TBCs present good thermal shock resistance, exhibiting only 0.26% mass gain up to 150-time thermal cycling. Before thermal cyclic treatment, SAPS–TBCs exhibited a strong adhesion with the absence of the thermally grown oxide (TGO) between out and inner layer. With the increasing of thermal cycles, the TGO layer was formed and its thickness firstly increased and then dropped down. The critical load fell down by about 32% for topcoat–bondcoat adhesion (up to 50 cycles) and 35% or so for TBCs–substrate adhesion (up to 150 cycles) compared to the counterpart of as-sprayed specimens. The strain introduced by the existence of TGO and mixed oxides resulted in a varied adhesion for TBCs on nickel-based alloy during thermal cycling.     

Fabrication of ceramic sealing coatings for shell bionic structures and their failure mechanism during thermal cycling

The porous ceramic coating as a "brick" layer sprayed by air plasma spraying(APS) and MK resin as a "mud" layer prepared by a high viscosity spray gun were characterized and tested. Three specifications of the "brick-mud" layered ceramic sealing coating were fabricated through the cyclic and orderly deposition of the "brick" layer and "mud" layer, and the thermal cycling performance and failure mechanism of the three new coatings were studied. The results showed that the agglomerated Y₂O₃ partially stabilized ZrO₂ (YSZ) particles had porous spherical structures and good sprayability, and the content of the YSZ phase in the prepared "brick" layer was 54.2%. The "mud" layer had good phase stability and was amorphous SiO₂ at and below 1100 °C. The fracture toughness of the pure YSZ coating was 2.295 ± 0.135 MPa?m^0.5, and which of the “mud” layer was reduced by 72.3%. The thermal cycling life of the conventional coating was only 67.3 times, which of A1, A2 and A3 coatings with 2, 3 and 6 "mud" layers were increased by 32.4%, 124.8% and 88.3%, respectively. In the thermal cycling process, the "weak" layer in the "brick-mud" layered coating led to the redistribution of internal stress and reduced the stress concentration in the top coating (TC)/TGO interface. Moreover, the initiation of microcracks in the "weak" layer, along with the "crack branching" effect and the "crack deflection" effect during the crack propagation process, could consume partial internal stress. Thus, the crack growth rates in the TC coating/TGO interface of the A1, A2 and A3coatings were lower than that of the conventional coating due to the above stress release mechanisms. In addition, the thermal cycling lives of the three new coatings with 2, 3 and 6 "mud" layers were improved to different degrees because of different stress effects.