EB-PVD TBC Research Articles

Comparative study on microstructure evolution and failure mechanisms of ordinary and refurbished EB-PVD TBC under cyclic oxidation

Comparative study on microstructure evolution and failure mechanisms of ordinary and refurbished EB-PVD TBC under cyclic oxidation

The Correlation Between the Cyclic Oxidation Behavior of EB-PVD TBC and Refurbishment Process

The Correlation Between the Cyclic Oxidation Behavior of EB-PVD TBC and Refurbishment Process

Stress models for electron beam-physical vapor deposition thermal barrier coatings using temperature-process-dependent model parameters

This article presents a new stress model for EB-PVD TBC to provide insight into TBC failure mechanisms. Cycling-induced and temperature-process dependent model parameters are incorporated into stress analysis of EB-PVD TBC and then used to simulate the variation of mechanical, thermal and inelastic behaviour from metallic bond coat to thermally grown oxide (TGO). This gives a smooth evolution of residual stresses and is more realistic than prior finite element (FE) work. Two types of interfacial roughness approximate profiles are presented and implemented in the FE model. Geometrical parameters are used to model the interface roughness, and residual stresses are then evaluated at specific positions within TBCs. This article's stress analysis establishes a link between stress distribution and the evolution of interfacial roughness during thermal cycles. Consequently, the results are expected to provide insight into the failure modes related to localized interfacial roughness evolution.

The Effect of Interfacial Roughness on Residual Stresses in Electron Beam-Physical Vapor Deposition of Thermal Barrier Coatings

Residual stresses play an essential role in determining the failure mechanisms and life of an electron beam-physical vapour deposition thermal barrier coating (EB-PVD TBC) system. In this paper, a new transitional roughness model was proposed and applied to describe the interfacial roughness profile during thermal cycles. Finite element models were implemented to calculate residual stresses at specific positions close to the interface of TBCs using temperature process-dependent model parameters. Combining stresses evaluated at valleys of the topcoat (TC) and excessive sharp tip roughness profiles, positions where the maximum out-of-plane residual stresses occur were identified and used to explain possible cracking routes of EB-PVD TBCs as interfacial roughness evolves during thermal cycling.

Buckling and interface strength analyses of thermal barrier coatings combining Laser Shock Adhesion Test to thermal cycling

Laser Shock Adhesion Test (LASAT) is a non-contacting technique that can be applied to evaluate the interfacial adherence of ceramic layer on metallic substrate. This study aims at combining the laser-induced delamination and blistering when applied to a conventional EB-PVD TBC submitted to thermal cycling. Besides, the use of white spot diagnostic as a NDT technique and 3D profilometer measurement along thermal cycling were fully consistent. The LASAT analysis of as-deposited samples with various initial substrate roughness is discussed regarding the corresponding life to top-coat spallation under air thermal cycling. Mixing thermal cycling with LASAT has enabled to show that the adherence was sensitive to process condition consistently with the performance measured by conventional thermal cycling tests. Moreover, the evolution of the adherence has also been be assessed by LASAT applied to TBC samples with different interrupted thermal cycling conditions. According to this methodology, LASAT could also be used to introduce an interfacial defect known in location, size of debonded area, height of the blister and number of cycles. After further interrupted thermal cycling, the crack propagation and the buckling were monitored. Such measurements allowed to derive the interfacial toughness corresponding to these various aging conditions.

Life and FCT Failure of Yttria- and Ceria-Stabilized EBPVD TBC Systems on Ni-Base Substrates

The lifetime at 1100 °C of furnace oxidation tested TBC samples having YSZ (ZrO2-7 wt.% Y2O3) and CeSZ (ZrO2-25 wt.% CeO2-2.5 wt.% Y2O3) topcoats on four Ni-base alloys including a no-bond coat NiCrAl alloy were studied. The thermally grown oxide (TGO) was investigated by energy-dispersive spectroscopy. The outer zone of the TGO revealed preferred enrichment of Zr and of rare earth elements Ce and Y in dependence of the stage of life. The oxygen potential gradients in the TGO have been rated; they are stronger for the YSZ TBC systems than for the CeSZ systems and cause a more intensive interaction of the outer zone alias mixed zone with the metal substrate than with the ceramic topcoat. When comparing the chemical composition data of the mixed zone as a function of the test period, the average lifetime as well as the particular composition of the two topcoats and the refractory element content of the substrates allowed conclusions to be drawn about the mechanisms which influence the lifetime.

Microstructure and lifetime of Hf or Zr doped sputtered NiAlCr bond coat/7YSZ EB-PVD TBC systems

NiAlCr-X overlay coatings deposited by magnetron sputtering on IN100 and CMSX-4 superalloys and thereafter top-coated with 7YSZ thermal barrier coatings by EB-PVD were investigated with emphasis on the effects of superalloy type and Hf or Zr doping. The sputtered films in the “as coated” condition (after EB-PVD deposition) showed strong diffusion effects of different elements from the superalloys. The measured phase content of β and γ′ was in good agreement with calculated data obtained from experimental compositions normalized to a Ni-Al-Cr system following the guidelines of site preference of ternary alloying elements. Microstructure evolution and failure characteristics after furnace cyclic tests at 1100°C were investigated as well. The lifetime of the coatings is equivalent to standard MCrAlY TBCs systems and is compared to PtAl and NiCoCrAlY coatings from literature. The effects of microstructure and elements diffusing from the substrates into the bond coat (Ti, Co, Mo and Ta) in combination with the effect of Hf- or Zr-doping is presented and discussed.

Alumina Growth and Interface Strengthening Mechanisms of Pt on the Surface of Bond Coats in EB-PVD TBC System Based on First-Principles

Oxygen adsorption, aluminium segregation and interface adhesion on the surface of NiPtAl and MCrAlY bond coats (BC) in EB-PVD TBC system were investigated using first-principles calculations within the density functional theory (DFT). Examination of oxygen adsorption and aluminium segregation indicated that the addition of Pt always obstructed the growth of alumina. In addition, NiPtAl, as bond coats in EB-PVD TBC System, had less lattice variation and stronger interface adhesion than MCrAlY when alumina was produced. It is found that Pt is an important factor that affects the Al2O3 growth in thermal barrier coating. It is proved that Pt improves the bonding performance of Al2O3 and lifetime of thermal barrier coating. This offsets the high cost of Pt in industry application.

Influence of temperature on phase stability and thermal conductivity of single- and double-ceramic-layer EB–PVD TBC top coats consisting of 7YSZ, Gd2Zr2O7 and La2Zr2O7

Yttria Stabilized Zirconia (YSZ) is usually used as ceramic top coat for gas turbine blades and vanes. Investigations have shown the accelerated phase transformation and the intensified sinter effects of the YSZ top coat at temperatures between 1200°C and 1300°C, leading to changes of microstructure. Such modifications of the microstructure lead to higher thermal stresses and consequently reduce the lifetime. Furthermore, thermal conductivity λ of the top coat increases. For this reason lanthanum zirconate (La2Zr2O7) and gadolinium zirconate (Gd2Zr2O7) as top coat get into focus because of their high phase stability up to their melting points and the lower thermal conductivity compared to YSZ.Within this work single- (SCL) and double-ceramic-layer (DCL) top coats consisting of 7wt.% yttria stabilized zirconia (7YSZ), La2Zr2O7 or Gd2Zr2O7 are deposited by means of Electron Beam–Physical Vapor Deposition (EB–PVD). Aim of this work is on the one hand the investigation of temperature-dependent phase behavior and change of thermal conductivity of SCL and DCL top coats. On the other hand the influence of different top coat materials and architectures on the growth of thermally grown oxide (TGO) is of key interest. In a first step morphology and coating thickness were determined using scanning electron microscopy (SEM). The SCL and DCL systems show a columnar microstructure with a coating thickness of about 150μm. In a second step thermal conductivity of SCL and DCL systems was measured between 400°C and 1300°C by means of laser flash technique. The third step contained X-ray diffraction measurements of SCL and DCL systems after atmospherical isothermal oxidation at 1300°C. Finally, the TGO at the interface was analyzed. The TGO composition was determined by means of energy dispersive X-ray spectroscopy (EDX). The TGO phase was identified by using X-ray diffraction. Thus, by a correlation between morphology, architecture, coating material and TGO behavior information about oxygen diffusion processes have been obtained.

Effect of Pt Content on TGO Growth in EB-PVD TBC Systems with Niptal Bondcoats

The oxidation behavior of Pt modified aluminide coating on the CMSX-4 Ni-base alloy plays major role to the EB-PVD TBC failure. The thermally growth oxide (TGO) is one of the most important factors to affect TBC lifetime. Two different Pt-content NiPtAl coatings in EB-PVD TBC systems were studied at 1100°C in air. The results indicated that cross-sections of oxide layer on the NiPtAl coatings within TBC in air were similar for the both bondcoats. The cracks could be found on the TBC/TGO/BC interfaces for the two bondcoats. The TGO morphologies of the low and high-Pt bondcoats on the side without TBC showed great different due to small PtAl particles size within high-Pt bondcoats. The irregular alumina on the both bondcoats was also showed on the sides with TBC compared to ones without TBC due to absence of the TBC. The TGO growth on the high-Pt bondcoats was faster than the low-Pt coatings during initial oxidation time. With the time increasing, the high-Pt content could suppress the TGO growth rate. Thinner TGO thickness could be obeserved on the both NiPtAl coatings due to the stress in TGO accumulation and oxide spallation.

Effect of Thermal Cycling on Lifetime and Failure Mechanism of EB-PVD TBC with NiCoCrAlYZr Bondcoats

EB-PVD TBC with conventional MCrAlY bondcoats was widely used within the high temperature environments. The effect of oxidation frequency on the lifetime of TBC with NiCoCrAlYZr and the failure mechanism was investigated in this paper. The TBC systems with Zr-doped MCrAlY bondocats presented a longer lifetime after discontinuous oxidation than cyclic oxidation. Formation of thick TGO in the TBC-system with Zr-containing bondcoat did not result in an immediate failure. The crack propagation in the case of the Zr-doped NiCoCrAlY bondcoat at the TGO/bondcoat interface was hindered due to the inhomogeneous TGO morphology. The inner oxidation and pores hindered the above small cracks propagation and then result a longer lifetime. However, the lifetime of TBC-system with NiCoCrAlY+Zr bondcoat is significantly shorter in the cyclic than in the discontinuous test due to the rapid propagation of cracks under high frequency thermal cycling conditions.

Assessment of Cyclic Lifetime of NiCoCrAlY/ZrO2-Based EB-PVD TBC Systems via Reactive Element Enrichment in the Mixed Zone of the TGO Scale

The chemical composition of the alumina-zirconia mixed zone (MZ) of an electron beam physical vapor deposited thermal barrier coating (EB-PVD TBC) system is affected by service conditions and by the interdiffusion of elements from the substrate alloy below and the zirconia top coat. Three NiCoCrAlY bond-coated Ni-base substrates with YPSZ or CeSZ EB-PVD TBCs were subjected to a cyclic furnace oxidation test (FCT) at 1373 K (1100 °C) in order to provide experimental evidence of a link between chemistry of the MZ, the substrate alloy, the ceramic top coat, and the time in the FCT. Energy dispersive spectroscopy of the MZ revealed preferred accumulation of Cr, Zr, Y, and Ce. The concentration of the reactive elements (RE = Ce + Y + Zr) was related to the respective average lifetimes of the TBC systems at 1373 K (1100 °C). The RE content in the MZ turned out to be a life-limiting parameter for YPSZ and CeSZ TBC systems which can be utilized to predict their relative lifetimes on the individual substrates. Conversely, the TBC failure mechanisms of YPSZ and CeSZ TBC systems are dissimilar.

Degradation Mechanisms of an Advanced Jet Engine Service-Retired TBC Component

Current use of TBCs is subjected to premature spallation failure mainly due to the formation of thermally grown oxides (TGOs). Although extensive research has been carried out to gain better understanding of the thermo - mechanical and -chemical characteristics of TBCs, laboratory-scale studies and simulation tests are often carried out in conditions significantly differed from the complex and extreme environment typically of a modern gas-turbine engine, thus, failed to truly model service conditions. In particular, the difference in oxygen partial pressure and the effects of contaminants present in the engine compartment have often been neglected. In this respect, an investigation is carried out to study the in-service degradation of an EB-PVD TBC coated nozzle-guide vane. Several modes of degradation were observed due to three factors: 1) presence of residual stresses induced by the thermal-expansion mismatches, 2) evolution of bond coat microstructure and subsequent formation of oxide spinels, 3) deposition of CMAS on the surface of TBC.

Delamination toughness of electron beam physical vapor deposition (EB-PVD) Y2O3–ZrO2 thermal barrier coatings by the pushout method: Effect of thermal cycling temperature

A pushout test method was used to quantify effect of thermal cycling temperatures on the delamination toughness of an electron beam physical vapor deposited thermal barrier coating (EB-PVD TBC). The delamination toughness, Γi, was related to the maximum thermal cycling temperature, Th, equal to 1000, 1025, 1050, and 1100 °C. The measured delamination toughness varied from 9 to 95 J/m2. At Th = 1000 °C, Γi attained a maximum value, larger than that of the as-deposited sample and decreasing with increased Th. During the thermal cycling tests, the thermally grown oxide (TGO) was formed between the TBC and the bond coat deposited onto the superalloy substrate. Inside the TGO layer, mixture of Al2O3 and ZrO2 oxides was observed close to the TBC side with nearly pure Al2O3 phases close to the bond-coat side. During the pushout test, delamination occurred at the interface of the mixture and pure Al2O3 layer with an exception for Th = 1100 °C specimens where delamination also occurred at the interface between the TGO and bond-coat layers. The effect of thermal cycling temperatures on the delamination toughness is discussed in terms of the microstructural change and delamination behavior.

Compatibility of mixed zone constituents (YAG, YAP, YCrO 3) with a chromia-enriched TGO phase during the late stage of TBC lifetime

The effects of chromia accumulation in the TGO scale of late-stage MCrAlY-based EB-PVD TBC systems are addressed by comparing mixed zone microstructures from isothermal as well as cyclic oxidation tests at 1100 °C with phase relationships derived from the 1100 °C isothermal section of the ternary system Al 2O 3–Cr 2O 3–Y 2O 3. For this approach the c-ZrO 2 phase zirconia coexisting with mixed zone constituents is considered an excess component. Compacted powder pellets were processed via co-decomposition of nitric salt solutions. The reaction products were characterized by X-ray diffraction and Raman spectroscopy. Characteristics of the ternary Al 2O 3–Cr 2O 3–Y 2O 3 relevant for the mixed zone include (i) a continuous solid solution series between the orthorhombic phases YAlO 3 (YAP) and YCrO 3, (ii) a limited Cr vs. Al substitution (16 mol%) in Cr–YAG Y 3(Al 1 − x Cr x ) 5O 12 and (iii) extended phase fields involving Cr-YAG and sesquioxide phases which represent typical late-stage mixed zones in the excess-zirconia model scenario. The YCrO 3 phase was found to be limited to very chromia-rich environments only. A high chromium enrichment of the mixed zone may be established upon extensive cycling through the miscibility gap in the alumina–chromia system. Later in TBC lifetime the thickness of the mixed zone is increasing due to outward growth of the Al 2 − x Cr x O 3 phase into the bottom TBC layer which results in a morphological instability of the TGO/TBC interface.

Parameters affecting TGO growth and adherence on MCrAlY-bond coats for TBC's

Electron Beam–Physical Vapor Desposited Thermal Barrier Coatings (EB–PVD TBC) on the Ni-base superalloy IN738LC were tested in respect to non-isothermal and cyclic oxidation resistance at 1100 °C. Two types of MCrAlY's ( M = Ni, Co), a Co-base and a Ni-base, were used as bond coats (BC) for the TBC's. Additionally, free standing MCrAlY specimens of 2 mm thickness were manufactured by vacuum plasma spraying. The results of the present studies strongly indicate, that an important lifetime governing factor of the TBC is the yttrium incorporation into the alumina based Thermal Grown Oxide (TGO) which results in an increase of the TGO growth rate and in parallel in a decrease of the yttrium concentration in the coating. If the yttrium concentration has been decreased beneath a critical level, its positive effect on TGO adherence is lost, resulting in TGO spallation. The time required for yttrium exhaustion will not only depend on the initial yttrium content but also on the yttrium reservoir, which is directly proportional to the BC thickness. The transport of yttrium into the TGO seems to occur slower in a Co-based than in a Ni-based coating, resulting in a longer life time for the TBC on the CoNiCrAlY-BC.

Nucleation and Growth of Oxide Constituents on NiCoCrAlY Bond Coats during the Different Stages of EB-PVD TBC Deposition and Upon Thermal Loading

Nucleation and Growth of Oxide Constituents on NiCoCrAlY Bond Coats during the Different Stages of EB-PVD TBC Deposition and Upon Thermal Loading

Effect of thermal exposure on stress distribution in TGO layer of EB-PVD TBC

The stress distributions in thermally grown oxide (TGO) layer of electron beam enhanced physical vapor deposited thermal barrier coating (EB-PVD TBC) before and after thermal exposure are measured by photo-stimulated luminescence spectrum. It is found that the stress in the TGO layer in original state is an order of ~3 GPa. It increases from 3.0–3.9 GPa with the increase of heat exposure time from 0–100 h. The stress distribution in the direction of thickness in the TGO layer is not uniform. The stress near ceramic top coat is smaller than that close to metal bond coat. The stress at thickness imperfections in the TGO near the bond coat is smaller than that in the regular TGO.

Influence of substrate material on oxidation behavior and cyclic lifetime of EB-PVD TBC systems

EB-PVD NiCoCrAlY/P-YSZ TBCs on several polycrystalline, directionally solidified, and single crystalline (SX) substrate alloys were thermally cycled at 1100°C. TBC spallation does not correlate solely to TGO thickness, but depends also very much on the substrate alloy. The longest lifetimes are achieved on Hf-containing alloys while SX alloys suffer from early TBC spallation. The formation of the thermally grown oxide was investigated in detail by TEM. A mixed layer of alumina and zirconia exists in the as-coated condition. After initial slight thickening, the thickness of this mixed layer remains constant over a long period of time. During thermal exposure, a continuous layer of pure α-alumina forms and grows underneath the mixed zone by oxygen inward diffusion.

Two-source jumping beam evaporation for advanced EB-PVD TBC systems

The continuous increase of the turbine inlet temperature in gas turbines necessitates new TBCs with a temperature capability beyond the current partially yttria stabilized material coatings. The present paper focuses on two-source jumping EB-PVD processed novel candidate layers for future TBC applications. It is shown that mixtures of oxides with widely different vapor pressures can be manufactured by this technique. The microstructure of the layers depends strongly on deposition conditions and on materials properties as well. Partial yttria stabilized zirconia coatings show no differences in microstructure and phase formation if deposited either by two-source jumping-beam or by one-source one-beam evaporation, while for ceria stabilized zirconia coatings large differences mainly in chemistry are found; depending on the jumping frequency multilayers are formed. In mixed silica zirconia coatings, crystalline zircon (ZrSiO 4) was formed neither in the as coated condition nor after annealing. Finally, jumping beam experiments allowed a detailed understanding of the growth kinetics of EB-PVD TBCs and the formation of a feather-like structure within the columns.