Degradation Of Thermal Barrier Coatings Research Articles

Sintering of electron beam physical vapor deposited thermal barrier coatings under flame shock

Abstract Flame shock treatment of electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) was carried out to investigate the degradation of TBCs exposed to a temperature gradient. The flame treatment resulted in temperature gradients both in the surface plane and through the thickness of the TBCs. TBC densification occurred where the flame was centered and resulted in a fine crack network at the surface of the TBC. The TBCs with addition of 0.5 wt% alumina were made, in which alumina was used as stress sensors for stress measurements by photoluminescence piezo-spectroscopy (PLPS). This allowed stress measurements to be made through the TBC thickness using a depth sensing method. Such stress measurements help us to better understand the densification of the TBCs.

Evaluation of Thermal Barrier Coatings Exposed to Hot Corrosion Environment by Impedance Spectroscopy

In this investigation, impedance spectroscopy (IS) is used, as a non-destructive tool, to examine the hot corrosion behavior of thermal barrier coatings (TBCs) exposed to a mixture of 25wt% NaCl and 75wt% Na2SO4. The results show that the thermally grown oxide (TGO) formed along the top coat/bond coat interface is a mixed oxide layer, i.e. Cr2O3, (Ni, Co)(Cr, Al)2O4 spinel and NiO. The growth of TGO layer appears to follow the parabolic law. The resistance of TGO increases due to the increase of thickness when the number of cycles is less than 110. While for more than 110 cycles, the resistance of TGO decreases with the increase of porosity of TGO layer, even though the thickness of TGO layer increases. The nucleation and propagation of cracks within top coat increase the electrical resistance of top coat. The parameters in equivalent circuit could be used to characterize the degradation of TBCs.

Thermal-gradient testing of thermal barrier coatings under simultaneous attack by molten glassy deposits and its mitigation

Degradation of thermal barrier coatings (TBCs) in gas-turbine engines by molten calcium–magnesium–aluminosilicate (CMAS) glassy deposits is becoming a pressing issue, as engines are required to operate under increasingly harsh conditions. While new approaches for mitigating CMAS attack of TBCs are starting to emerge, there is a need for appropriate tests for evaluating the efficacy of those approaches. To that end, we present here a new thermal-cycling test for the evaluation of TBC performance, where a thermal gradient is applied across the TBC, with simultaneous injection of CMAS. The conditions simulated in this new test are closer to actual conditions in an engine, as compared to the conventional furnace test without thermal gradient. This new test is applied to a TBC with a new composition designed for mitigating CMAS attack, where the mitigation mechanisms are observed to be similar to those found in the conventional furnace test. However, based on a previously established mechanics model it is shown here that thermal-gradient cyclic testing is essential for evaluating thermomechanical performance of TBCs under attack by CMAS, and that the use of just the conventional furnace test can lead to the underestimation of the TBC performance.

Degradation of Thermal Barrier Coatings by Fuel Impurities and CMAS: Thermochemical Interactions and Mitigation Approaches

Degradation of free-standing yttria-stabilized zirconia (YSZ) and CoNiCrAlY coatings (300 μm) due to V2O5 and a laboratory-synthesized CMAS was investigated at temperatures up to 1400 °C. Reactions, phase transformations, and microstructural development in coatings were examined by using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The molten deposits destabilized the YSZ and reacted with the thermally grown oxide with various phase transformations and reaction product formation. A dense, continuous environmental barrier overlay, based on oxides, applied by electrophoretic deposition was effective in mitigating the molten deposit attack. Enriching CMAS composition with Al promoted the crystallization of anorthite platelets and MgAl2O4 spinel, and mitigated CMAS ingression. EPD MgO overlay was also effective in protection against V2O5 melt by formation of magnesium vanadates. EPD alumina overlay deposited on thermal barrier coatings with APS 8YSZ and bond-coated IN939 superalloy retained its adhesion and structural integrity after prolonged furnace thermal cycle test at 1100 °C.

Delamination Behavior of Thermal Barrier Coatings under Thermo-Mechanical Fatigue

It has become common to apply thermal barrier coatings (TBC) to hot gas path parts in gas turbines to increase thermal efficiency. However, some uncertainty about the delamination life prediction method for TBC still remains. The influences of mechanical parameters on delamination behavior were investigated through thermo-mechanical fatigue (TMF) tests to improve TBC delamination life prediction. The parameters investigated were loading pattern, strain range, ceramic coating thickness, and high temperature exposure processing prior to TMF tests.Delamination behavior with a simulated loading pattern seemed to better correspond to behavior under out-of-phase (OP) loading than that with in-phase (IP) loading. The OP loading pattern was better for evaluating delamination lives because of its shorter test period. The delamination lives became shorter under larger strain ranges. Of the specimens tested, the specimen with a 0.9-mm-thick ceramic coating had the shortest delamination life. The specimen with a 0.6-mm-thick coating had almost the same or longer life than the specimen with a 0.3-mm-thick coating. These results strongly suggest that an optimum ceramic coating thickness keeps the cooling performance consistent with delamination strength. Thermally grown oxides (TGO) are generally thought to be the main cause of TBC degradation in long-term operated gas turbine blades. The growth behavior of TGO was observed by exposing specimens to a high-temperature environment and it was suggested that the Larson-Miller parameter (LMP) could predict TGO thickness well. In addition, some specimens were exposed to high-temperature environments prior to TMF tests to evaluate the effect of TGO thickness on delamination lives. This high temperature pre-exposure seemed to accelerate the delamination growth rate due to sintering and TGO.

Degradation of Yttria Stabilized Zirconia Thermal Barrier Coatings by Molten CMAS (CaO-MgO-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub>) Deposits

In advanced gas turbine engines that operate in a dust-laden environment causing ingestion of siliceous debris into engines, thermal barrier coatings (TBCs) are highly susceptible to degradation by molten CMAS (calcium-magnesium alumino silicate) deposits. In this study, the degradation mechanisms other than the commonly reported thermomechanical damage are investigated with an emphasis on the thermochemical aspects of molten CMAS induced degradation of TBCs. Free-standing yttria stabilized zirconia (8YSZ) TBC specimens in contact with a model CMAS composition were subjected to isothermal heat treatment in air at temperatures ranging from 1200°C to 1350°C. Phase transformations and microstructural development were examined by using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Starting at 1250°C, the molten CMAS readily infiltrated and dissolved the YSZ coating followed by reprecipitation of zirconia with a different morphology and composition that depends on the local melt chemistry. Significant amount of Y2O3 depleted monoclinic ZrO2 phase evolved from CMAS melt that dissolved ť-ZrO2 was evident. Thus the mechanism of dissolution and reprecipitation due to molten CMAS damage resulted in destabilization of the YSZ with disruptive phase transformation (t’ f + m).

Regularities of degradation of thermal barrier coatings of the MCrAlY/ZrO2 system under sulfide-oxide corrosion conditions

The results from investigation of the service life of thermal barrier coatings under the conditions of sulfide-oxide corrosion at 800, 850, and 900°C are considered. The effect of the ceramic layer thickness, the aluminum and chromium concentration in the metallic layer, and the formation of the oxide film in the boundary region on degradation and corrosion longevity of coatings is determined.

Microstructural Evolution of Durable Thermal Barrier Coatings with Hf and/or Y Modified CMSX-4<sup>TM</sup> Superalloy Substrates

Thermal cyclic lifetime and microstructural degradation of thermal barrier coatings (TBCs) with (Ni,Pt)Al bond coat and Hf- and/or Y-modified CSMX-4 superalloy substrates were examined. Thermal cyclic lifetime of TBCs was measured using a furnace thermal cycle test that consisted of 10-minute heat-up, 50-minute dwell at 1135°C, and 10-minute forced-air-quench. TBC lifetime was observed to improve from 600 cycles to over 3200 cycles with appropriate Hf- and/or Y-alloying of CMSX-4 superalloys. This significant improvement in TBC lifetime is the highest reported lifetime in literature with similar testing parameters. Cross-sectional microstructure of TBC specimens were examined by scanning electron microscopy (SEM) after the spallation failure. While undulation of TGO/bond coat interface (e.g., rumpling and racheting) was observed to be main damage mechanisms for TBCs on baseline CMSX-4, the same interface remained relatively flat for durable TBCs on Hf- and/or Y-modified CSMX-4. The parabolic growth constant of the TGO scale was slightly lower for TBCs with Hfand/ or Y-modified CSMX-4.

2420 Development of High-Sensitive Non-Destructive Evaluation Technique of Degraded Thermal Barrier Coatings by Using Microwave

Thermal barrier coatings (TBCs) have been widely used in gas turbine engines in order to protect the substrate metal alloy against high temperature and to enhance turbine efficiency. After service at high temperature, delamination or micro-cracks will initiate in TBC, leading to TBC separation from substrate and ultimately in unexpected gas turbine destruction. TBC failure is primarily attributed to the. growth of TGO and partly to TBC degradation. Currently, there are no reliable nondestructive techniques available to monitor TBC integrity over lifetime of the coating. Thus, to detect TBC and TGO thicknesses, a microwave nondestructive technique that utilizes a rectangular waveguide was developed. The phase of the reflection coefficient at the interface of TBC and waveguide varies for different TGO and TBC thicknesses. Therefore, measuring the phase of the reflection coefficient enables us to accurately calculate TGO and TBC thicknesses. Finally, a theoretical analysis was used to evaluate the reliability of the experimental results.

Emission spectroscopy analysis for the non-destructive evaluation of the health of thermal barrier coatings

Lithium oxide was selected as an emission spectroscopic marker in yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). The spectral response of excited lithium atoms from dip-coated YSZ containing 5, 3, 1, and 0.3 wt.% lithium oxide and plasma-sprayed YSZ containing 1 wt% lithium oxide was examined under an oxy-acetylene flame. Results showed that the intensity of lithium emission spectrum is a function of the concentration of lithium oxide in the YSZ, the flame temperature, and the degree of TBC degradation. It indicates that an emission spectroscopy can be used to monitor the degradation of TBCs.

Impedance spectroscopy of thermal barrier coatings as non-destructive evaluation tool for failure detection

Abstract Since thermal barrier coatings (TBCs) for turbine blades suffer from bond coat oxidation and sintering of the ceramic top coat during service, quantification of TBC degradation by non destructive evaluation methods (NDE) is essential. IN617 substrates with standard NiCoCrAlY/PYSZ EBPVD TBCs were annealed at 1100 °C and measured by impedance spectroscopy. Parameters such as phase angle, total impedance, real and imaginary part were analyzed. To clearly separate bond coat oxidation from aging of the zirconia, freestanding ceramic top coats and ceramic free samples were analyzed as well. A straight correlation between the changes in the impedance spectra and the measured thickness of the thermally grown oxide was found.

Study of the Sulfur Segregation for a TBC System

The degradation of thermal barrier coatings is closely linked to their spalling resistance, which depends on the stability of the protective oxide scale produced by oxidation of the bond coat. In this context, sulfur is well known to reduce the adherence of the thermally grown oxide on the boundary layer in the TBC systems. So, the aim of the present work is to report experimental results concerning the chemical changes at Al2O3/bond coat layer and Al2O3/superalloy interfaces, namely the effect of interfacial sulfur segregation as a function of the oxidation time, the initial S content and the presence of voids at the alumina/ bond coat interface. Sulfur segregations were identified by SIMS depth profiles and the effect of these segregations discussed according to the types of alloys and heat treatments. SIMS depth profiles and TEM results allowed us to localize the S segregation at the alumina/bond-coat interface and in the bulk of the bond-coat and to differentiate sulfur coming from the superalloy and sulfur trapped in the NiAlPt bond-coat

Degradation of thermal barrier coatings due to thermal cycling up to 1150°C

The degradation of thermal barrier coatings (TBCs) due to thermal cycling up to 1150°C has been studied. During thermal cycling, the bond coat in the TBCs was oxidised to form an alumina and a mixed oxide layer between the top coat of yttria stabilised zirconia (YSZ) and the bond coat of MCrAlY alloy. The mixed oxide layer mainly consists of α-Cr2O3 and (Ni,Co)(Cr,Al)2O4 spinel phases, which are formed above the α-alumina layer. Interestingly, the alumina layer gradually disappeared during the oxidation while the content of chromium in the mixed oxide increased with increasing oxidation time. As the oxidation accelerated after the disappearance of the alumina layer, cracks initiated and propagated in the mixed oxide layer near the YSZ. Eventually, the crack propagation induced the spallation of some YSZ top coatings after the 2000 h oxidation.

Evaluation of degradation of thermal barrier coatings using impedance spectroscopy

In this work, impedance spectroscopy (IS) has been used to evaluate the degradation of thermal barrier coatings (TBCs) due to oxidation at 1150oC. The spallation of TBCs was found to be induced by the development of thermally grown oxides (TGO) produced from the oxidation of TBCs. The change in the electrical properties of the TGOs was found to be related to the change in the microstructure and microchemistry of the TGO. The resistivity of the TGO due to oxidation from 10 to 1000 h decreased rapidly, which corresponded to the increase in porosity in the TGO and the compositional change of the TGO from α-Al2O3 to a mixture of α-Cr2O3 and (Ni,Co)(Cr,Al)2O4 spinel. The slow decrease in the resistivity of the TGO from oxidation for 1000–2000 h indicated that there was little change in the composition of the mixed oxides, although the growth rate of the TGOs was relatively fast during this oxidation period. The disappearance of α-Al2O3 in the TGO caused a rapid oxide growth during oxidation and led to the spallation of TBCs.

Thermal cycling of EB-PVD/MCrAlY thermal barrier coatings: II. Evolution of photo-stimulated luminescence

Degradation of thermal barrier coatings (TBCs) was investigated as a function of thermal cycles by examining the structural integrity of the thermally grown oxide (TGO) and TBC/TGO/bond coat interfaces using photo-stimulated luminescence spectroscopy (PSLS). The disk-shaped specimens of a production TBC employed in this study consist of: an electron beam physical vapor deposited (EB-PVD) ZrO 2–7 wt.% Y 2O 3 (YSZ); thermally grown oxide (TGO); MCrAlY bond coat; and IN-738 polycrystalline superalloy substrate. The furnace thermal cycling of the TBC consisted of a 10-min heat-up, a 40-min hold at 1121°C (2050°F) and a 10-min forced air quench to ambient temperature. Various characteristics of the photo-stimulated luminescence spectra from the TGO, including peak shifts (i.e. residual stress) and relative intensities of the R 1–R 2 photo-luminescence, were examined as a function of thermal cycles and related to the spallation failure of the TBC, which occurred at approximately 400 cycles. A gradual increase in the compressive residual stress in the TGO was observed with thermal cycling up to 200 cycles. The magnitude of compressive stress in the TGO remained constant (∼4.5 GPa) from 200 to 350 cycles, then decreased sharply near 400 cycles, associated with spallation of the TBC. Photo-stimulated, bimodal luminescence spectra, consisting of stressed and stress-free luminescence peaks were frequently observed for TBCs. Q 1 and Q 2 luminescence from metastable θ-Al 2O 3 and N-luminescence (in addition to R 1–R 2 luminescence) arising from Cr 3+ concentration in the α-Al 2O 3 scale were also observed. The physical basis for the evolution of these photo-stimulated luminescence spectra is discussed and related to the progressive damage and the final spallation of the TBCs.

Study of the coating stability of a TBC system

The degradation of thermal barrier coatings is closely linked to their spalling resistance which depends on the stability of the protective oxide scale produced by oxidation of the bond-coat. This study deals with a characterisation of the oxide layer, its interfaces with the bond-coat and the ceramic and their evolution during isothermal oxidation at 1,100°C. The microstructural evolution of the bond-coat, the outer part of the superalloy and the ceramic layer was also studied.

Study of the coating stability of a TBC system

The degradation of thermal barrier coatings is closely linked to their spalling resistance which depends on the stability of the protective oxide scale produced by oxidation of the bond-coat. This study deals with a characterisation of the oxide layer, its interfaces with the bond-coat and the ceramic and their evolution during isothermal oxidation at 1,100°C. The microstructural evolution of the bond-coat, the outer part of the superalloy and the ceramic layer was also studied.

Non-Destructive Characterisation of Thermal Barrier Coatings Using Impedance Spectroscopy

ABSTRACTImpedance spectroscopy (IS) has been used to characterise the degradation of thermal barrier coatings (TBCs) due to thermal treatments at 1100°C for a period up to 200 hrs. The growth of the oxide layer at the yttria stabilised zirconia (YSZ)/bond coat interface in TBCs can be examined by measuring the impedance diagrams. From the analysis of the impedance diagrams of TBCs, the electrical properties of YSZ were found to be nearly constant during the thermal treatments, indicating there was little change in the microstructure and composition of YSZ. However, there was a clear change in the electrical properties of the oxide layer in the TBCs after thermal treatments, suggesting both microstructural and composition changes occurred in the oxide layer. These studies indicate that the IS is a very useful method in non-destructive characterisation of the degradation of TBCs.

Degradation of Thermal-Barrier Coatings at Very High Temperatures

Thermal-barrier coatings are finding increasing use in engineering applications, particularly in gas turbines. Such coatings, consisting of ceramic insulating layers bonded to the superalloy substrate by oxidation-resistant alloy coatings, are deposited onto components to reduce heat flow through the cooled substrate and to limit operating temperature. They have been used effectively on static components such as combustor cans, flare heads, hot gas seal segments, fuel evaporators, and deflector plates, giving considerable improvements in component life. They have been used successfully on vane platforms. In recent years, the emphasis has shifted toward the development of coatings for high-risk areas, such as turbine blades.A ceramic thermal-barrier coating needs to be refractory and chemically inert, and to have low thermal conductivity. However, it also needs to possess a high thermal expansion coefficient of ~11 × 10−6 K−1, to match the nickel-base superalloy substrate. The latter specification has focused attention on ZrO2. However, ZrO2 is polymorphic and undergoes two phase changes, cubic to tetragonal at 2350°C and tetragonal to monoclinic at 1170°C. The latter transformation is accompanied by a 5% volume increase which means that ZrO2 has to be alloyed to stabilize one of the high-temperature phases. Early systems in the 1970s consisted of ZrO2 stabilized with MgO, but this has been shown to be a metastable system. Present-day commercial thermal-barrier coatings consist of a plasma-sprayed yttria- or magnesiastabilized zirconia layer on top of an M-Cr-Al-Y bond coat. The latter plays a very important role by helping to key the ceramic to the alloy substrate and to accommodate the mechanical strains arising because of differences in thermal expansion coefficients and elastic moduli between the ceramic and the substrate.

ChemInform Abstract: Degradation Mechanisms of ZrO2‐8 wt.% Y2O3/Ni‐22Cr‐10Al‐1Y Thermal Barrier Coatings.

AbstractPlasma spraying duplex ZrO2‐8 wt.% Y2O3/Ni‐22Cr‐10Al‐1Y thermal barrier coatings (TBCs) on Mar‐M247 superalloy are tested under various operating conditions within the temp. range 1000‐1150 °C. From the results it is concluded that oxidation of bond coatings is the dominant TBC degradation mechanism, whereas the operationally induced stresses exert a conjugate effect.