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Oxidation and crack nucleation/growth in an air-plasma-sprayed thermal barrier coating with NiCrAlY bond coat

Oxidation and crack nucleation/growth in an air-plasma-sprayed thermal barrier coating with NiCrAlY bond coat

Received 11 March 2004; accepted in revised form 14 June 2004 Available online 14 August 2004 Abstract The oxidation behavior of an air-plasma-sprayed thermal barrier coating (APS-TBC) system was investigated in both air and low-pressure oxygen environments. It was found that mixed oxides, in the form of (Cr,Al) 2 O 3 d Ni(Cr,Al) 2 O 4 d NiO, formed heterogeneously at a very early stage during oxidation in air, and in the meantime, a layer of predominantly Al 2 O 3 grew rather uniformly along the rest of the ceramic/bond coat interface. The mixed oxides were practically absent in the TBC system when exposed in the low-pressure oxygen environment, where the TBC had a longer life. Through comparison of the microstructures of the APS-TBC exposed in air and low-pressure oxygen environment, it was concluded that the mixed oxides played a detrimental role in causing crack nucleation and growth, reducing the life of the TBC in air. The crack nucleation and growth mechanism in the air-plasma-sprayed TBC is further elucidated with emphasis on the Ni(Cr,Al) 2 O 4 and NiO particles embedded in the chromia.
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A physics-based life prediction methodology for thermal barrier coating systems

A physics-based life prediction methodology for thermal barrier coating systems

5 . Conclusions A mechanism-based life prediction methodology for thermal barrier coating systems has been proposed. It relies on a combination of information about the TBC’s morphological characteristics, accumulated TBC damage inferred from non- destructive fluorescence measurements, and numerically predicted local TBC stresses responsible for the initiation of such damage. It incorporates the complex interaction between interfacial and microstructural features, local oxidation mechanisms and time-dependent processes. The methodology is applied to predict the life of an EB-PVD TBC with an MCrAlY bond coat. The results of parametric finite element studies using periodic unit cell techniques revealed the magnitudes of the local TBC stresses known to lead to the failure of this type of coating. The maximum TGO stresses responsible for microcrack nucleation were found to increase with oxidation time and TGO roughness.
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Effect of water vapor on the spallation of thermal barrier coating systems during laboratory cyclic oxidation testing

Effect of water vapor on the spallation of thermal barrier coating systems during laboratory cyclic oxidation testing

Introduction Thermal barrier coating (TBC) systems deposited on high-temperature blades in gas turbine applications are of high technological interest since they allow a higher combustion temperature. TBCs are known to fail because of a number of contributing factors such as stresses due to thermal expansion mismatch in this multilayered system upon cycling, stress concentration due to thermally grown oxide scales, impurities such as sulfur interfacial segregation leading to interfacial

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Damage assessment and fracture resistance of functionally graded advanced thermal barrier coating systems: experimental and analytical modeling approach

Damage assessment and fracture resistance of functionally graded advanced thermal barrier coating systems: experimental and analytical modeling approach

Abstract: Enhancement of stability, durability, and performance of thermal barrier coating (TBC) systems providing thermal insulation to aero-propulsion hot-section components is a pressing industrial need. An experimental program was undertaken with thermally cycled eight wt.% yttria stabilized zirconia (YSZ) TBC to examine the progressive and sequential physical damage and coating failure. A linear relation for parameterized thermally grown oxide (TGO) growth rate and crack length was evident when plotted against parameterized thermal cycling up to 430 cycles. An exponential function thereafter with the thermal cycling observed irrespective of coating processing. A phenomenological model for the TBC delamination is proposed based on TGO initiation, growth, and profile changes. An isostrain-based simplistic fracture mechanical model is presented and simulations carried out for functionally graded (FG) TBC systems to analyze the cracking instability and fracture resistance. A few realistic FG TBCs architectures were considered, exploiting the compositional, dimensional, and other parameters for simulations using the model. Normalized stress intensity factor, K 1 /K 0 as an effective design parameter in evaluating the fracture resistance of the interfaces is proposed. The elastic modulus difference between adjacent FG layers showed stronger influence on K 1 / K 0 than the layer thickness. Two advanced and promising TBC materials were also taken into consideration, namely gadolinium zirconate and lanthanum zirconate. Fracture resistance of both double layer and trilayer hybrid architectures were also simulated and analyzed.
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Thermal cycling behaviour of thermal barrier coating systems based on first- and fourth-generation Ni-based superalloys

Thermal cycling behaviour of thermal barrier coating systems based on first- and fourth-generation Ni-based superalloys

3. RESULTS 3.1 Spallation After thermal cycling at 1100  C, the AM1- and MCNG- based systems exhibited three different levels of spallation (Table 3). No spallation occured on three of the samples: A10, M10 and M50. Three samples show localised spalla- tion close to the edges: A50, M500 and M1000. As this phenomenon is mainly due to a geometrical effect – the ‘‘edge effect’’ – leading to stress concentrations, it is not representative of the intrinsic behaviour of the thermal barrier coating system. Therefore, those samples have been assimilated to the ones exhibiting no spallation at all. However, the sample M1000 spalled after cutting for sample preparation. Eventually, the AM1-based samples have totally spalled after 500 and 1000 cycles. Regular visual inspections during thermal cycling revealed that spallation occured at around 200 and 400 cycles, respec- tively. However, the choice was made to carry out thermal cycling to the duration initially planned. With respect to the previous comments regarding the ‘‘edge effect’’, it turned
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Self-healing thermal barrier coating systems fabricated by spark plasma sintering

Self-healing thermal barrier coating systems fabricated by spark plasma sintering

The present paper focuses on the Spark Plasma Sintering (SPS) manufacturing of a new type of self-healing ther- mal barrier coating (TBC) and a study of its thermal cycling behaviour. The ceramic coating consists on an Yttria Partially Stabilized Zirconia (YPSZ) matrix into which healing agents made of MoSi 2 -Al 2 O 3 core-shell particles are dispersed prior to sintering. The protocol used to sinter self-healing TBCs on MCrAlY (M: Ni or NiCo) pre-coated Ni-based superalloys is described and the reaction between the particles and the MCrAlY bond coating as well as the preventive solutions are determined. Thermal cycling experiments are performed on this complete multi- layer system to study the crack healing behaviour. Post-mortem observations highlighted local healing of cracks due to the formation of silica and the subsequent conversion to zircon at the rims of the cracks.
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Influence of Pt Addition and Manufacturing Process on the Failure Mechanisms of NiCoCrAlYTa-Base Thermal Barrier Coating Systems under Thermal Cycling Conditions

Influence of Pt Addition and Manufacturing Process on the Failure Mechanisms of NiCoCrAlYTa-Base Thermal Barrier Coating Systems under Thermal Cycling Conditions

Metals 2018, 8, 771 13 of 20 oxide scale. This last observation confirms that Pt favors Al selective oxidation, as demonstrated in one of our previous studies [ 23 ] and as already observed by others [ 19 , 20 ]. This is explained by the decrease in Al activity due to the presence of Pt, as shown by others [ 59 , 60 ], which led to an Al-uphill diffusion during the vacuum heat treatment. The AM3-Trib-Pt/1 TBC system failure was not due to heterogeneity of the TGO in terms of composition and thickness. Rather it was due to the presence of numerous corn kernel defects, characteristic of a surface exhibiting concave areas before thermal barrier deposition by EB-PVD. When the TGO/TBC interface is strong, the thermal barrier prevents the development of bond-coating surface undulations [ 20 , 61 – 63 ]. Because corn kernel defects are lightly bonded to the thermal barrier coating, the bond-coating surface deformation is easier in these specific areas. Furthermore, TGO undulations in the corn kernel areas lead to stress concentration and to fracture, according to Evans et al. [ 5 ]. In the present study, the presence of many corn kernels shows that the Pt electroplating process did not smooth the rough NiCoCrAlYTa bond-coating surface.
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High temperature durability of a bond-coatless plasma-sprayed thermal barrier coating system with laser textured Ni-based single crystal substrate

High temperature durability of a bond-coatless plasma-sprayed thermal barrier coating system with laser textured Ni-based single crystal substrate

Mechanical adhesion Creep A B S T R A C T Thermal barrier coating systems are usually build-up with bond coats to ensure a good adhesion of the ceramic top coat and to protect the substrate against oxidation and corrosion. Such system is often subjected to complex thermo-mechanical loading. Because of the very different damage processes encountered during service op- erations, a simpli fied system was investigated by removing the bond-coat. Recently adhesion bond strength was enhanced using laser surface texturing of the substrate in thermal spraying processes. Atmospheric plasma spray yttria-stabilized-zirconia thermal barrier coating system was deposited on the Ni-based AM1 single crystalline superalloy without bond coat. Adhesion bond strength was already increased compared to conventional pro- cessing method. Top coat durability was evaluated at high temperature and damage mechanisms were studied. Isothermal and cyclic oxidation tests showed durability of 1000 h and 400 cycles at 1100 °C. The oxidation mechanisms at the substrate/top coat interface changed due to fast solidi fication during the laser texturing process. Then, TBC system was studied under high temperature mechanical solicitation in tension creep. The textured interfaces were not damaged after 1% creep strain while top-coat/substrate interfacial cracking was observed for grit-blasted specimens. Moreover, no preferential crack development in the substrate was observed. Patterns provided an enhanced adhesion by changing the stress distribution near the interface.
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Cracking Behaviour of an Air-Plasma-Sprayed Thermal Barrier Coating

Cracking Behaviour of an Air-Plasma-Sprayed Thermal Barrier Coating

https://doi.org/10.1115/GT2009-59135 Access and use of this website and the material on it are subject to the Terms and Conditions set forth at Cracking Behaviour of an Air-Plasma-Sprayed Thermal Barrier Coating Chen, W. R.; Wu, X.; Marple, B. R.

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Self-healing thermal barrier coating systems fabricated by spark plasma sintering

Self-healing thermal barrier coating systems fabricated by spark plasma sintering

The present paper focuses on the Spark Plasma Sintering (SPS) manufacturing of a new type of self-healing ther- mal barrier coating (TBC) and a study of its thermal cycling behaviour. The ceramic coating consists on an Yttria Partially Stabilized Zirconia (YPSZ) matrix into which healing agents made of MoSi 2 -Al 2 O 3 core-shell particles are dispersed prior to sintering. The protocol used to sinter self-healing TBCs on MCrAlY (M: Ni or NiCo) pre-coated Ni-based superalloys is described and the reaction between the particles and the MCrAlY bond coating as well as the preventive solutions are determined. Thermal cycling experiments are performed on this complete multi- layer system to study the crack healing behaviour. Post-mortem observations highlighted local healing of cracks due to the formation of silica and the subsequent conversion to zircon at the rims of the cracks.
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Influence of Pt Addition and Manufacturing Process on the Failure Mechanisms of NiCoCrAlYTa-Base Thermal Barrier Coating Systems under Thermal Cycling Conditions

Influence of Pt Addition and Manufacturing Process on the Failure Mechanisms of NiCoCrAlYTa-Base Thermal Barrier Coating Systems under Thermal Cycling Conditions

Metals 2018, 8, 771 13 of 20 oxide scale. This last observation confirms that Pt favors Al selective oxidation, as demonstrated in one of our previous studies [ 23 ] and as already observed by others [ 19 , 20 ]. This is explained by the decrease in Al activity due to the presence of Pt, as shown by others [ 59 , 60 ], which led to an Al-uphill diffusion during the vacuum heat treatment. The AM3-Trib-Pt/1 TBC system failure was not due to heterogeneity of the TGO in terms of composition and thickness. Rather it was due to the presence of numerous corn kernel defects, characteristic of a surface exhibiting concave areas before thermal barrier deposition by EB-PVD. When the TGO/TBC interface is strong, the thermal barrier prevents the development of bond-coating surface undulations [ 20 , 61 – 63 ]. Because corn kernel defects are lightly bonded to the thermal barrier coating, the bond-coating surface deformation is easier in these specific areas. Furthermore, TGO undulations in the corn kernel areas lead to stress concentration and to fracture, according to Evans et al. [ 5 ]. In the present study, the presence of many corn kernels shows that the Pt electroplating process did not smooth the rough NiCoCrAlYTa bond-coating surface.
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Feasibility of luminescent multilayer sol-gel thermal barrier coating manufacturing for future applications in through-thickness temperature gradient sensing

Feasibility of luminescent multilayer sol-gel thermal barrier coating manufacturing for future applications in through-thickness temperature gradient sensing

From this perspective there has been a growing interest in the appli- cation of phosphor thermometry methods for the diagnostic of TBCs as the partial transparency of YSZ in the visible range of the spectrum allows to collect local information conveyed by the luminescence emis- sions from optically excited luminescent layers integrated throughout the depth of the TBC [5,6] . This functionalization can be obtained by the introduction of optically active components such as trivalent lantha- nide ions directly into the crystal structure of YSZ, thus without any detrimental alterations of the coating properties [6–8] . Such “sensor TBCs” has shown high potential for measuring substrate/TBC interface temperature [9,10] or investigate local interface delamination [11,12]
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Feasibility of luminescent multilayer sol-gel thermal barrier coating manufacturing for future applications in through-thickness temperature gradient sensing

Feasibility of luminescent multilayer sol-gel thermal barrier coating manufacturing for future applications in through-thickness temperature gradient sensing

provide a good balance between thermal insulation and thermo- mechanical strength [13,14,44] . The position and thickness of the different layers determined by EDX analysis are indicated in Fig. 8 .a. There were no signs of adherence loss or crack initiation and propagation at interfaces between layers or between the substrate and the deposit, indicating the satisfactory sintering of the coating as a morphologically uniform single layer well adherent to the substrate. The main concern regarding the design of TBC sensor is the individual thickness and thickness uniformity of the luminescent layers. Since TBCs are subject to large thermal gradients, uniform sensing layer thicknesses of about 10 μm of below are to be pre- ferred to avoid significant errors in the temperature readings [20] . There are in general little thickness variations within each layers, although the presence of some large particles (dimensions N15–30 μm) can locally disturb the layers' uniformity and density, either because of their large dimension or by contributing to the creation of large pores. However it should be noticed that the upper layers present a significantly larger thickness than the lower ones for the same deposition conditions, as the increase of the viscosity of the sol with time results in some notice- able film thickening with increasing number of dips. Nevertheless this effect can be easily controlled by adapting the number of dips for each layer or the withdrawal speed.
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Beneficial Effect of Pt and of Pre-Oxidation on the Oxidation Behaviour of an NiCoCrAlYTa Bond-Coating for Thermal Barrier Coating Systems

Beneficial Effect of Pt and of Pre-Oxidation on the Oxidation Behaviour of an NiCoCrAlYTa Bond-Coating for Thermal Barrier Coating Systems

After 300 h at 1100 °C, no more tantalum carbides were observed within the Pt-modified NiCoCrAlYTa on AM3 and only few were observed at the MC-NG/ Pt-modified NiCoCrAlYTa coating interface. During oxidation, aluminium was consumed at the bond-coating/TGO interface to form the oxide layer. Chemical elements also diffused from the bond-coating to the superalloy (such as Pt, Al) and from the superalloy towards the bond-coating (such as Ti, Ni). A previous study on Pt-modified NiCoCrAlYTa bond-coatings, coming from the same batch as the ones of the present work, showed that MC-NG-base systems were fabricated with a thinner Pt layer than the AM3-base systems [ 28 ]. It was proposed that the difference in this initial Pt thickness layer was responsible for the larger carbide volume fraction for MC-NG-base systems. Indeed, the idea was put forward that Pt could decrease tantalum activity and therefore destabilise tantalum carbides [ 28 ]. The fact that the tantalum carbide volume fraction was greatly decreased or even became nil after 300 h at 1100 °C (i.e. after Pt diffusion) supports this hypothesis. Based on the EDS spectral maps from [ 28 ], the average Pt concentration corresponding to the limit between the carbide-free zone and the carbide-rich zone within the Pt-modified NiCoCrAlYTa bond-coating could be determined after heat treatment. It was equal to respectively, 1.2 and 2.2 at.% with AM3 and MC-NG superalloys. After 300 h of oxidation at 1100 °C, the Pt concentration was also determined. This was done by an EDS analysis on one or two zones and not by EDS spectral maps. The Pt concentration just above the interface between the bond-coating and the superalloy was 4.0 at.% with AM3 and 2.5 at.% with MC-NG. So, if the Pt concentrations previously determined for the bond-coating after heat treatment are the limit concentration for tantalum carbide precipitation, it could be expected to observe complete dissolution of the carbides with AM3 superalloy. With MC-NG, the complete dissolution of carbides within the bond-coating is consistent with the limit in Pt concentration. Nevertheless, many carbides were still present at the bond- coating/superalloy interface. This could be due to the higher Ta concentration of MC-NG superalloy compared to AM3 superalloy (1.7 at.% for MC-NG against 1.3 at.% for AM3) which could lead to Ta diffusion from the superalloy towards the bond-coating.
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Influence of isothermal and cyclic oxidation on the apparent interfacial toughness in thermal barrier coating systems

Influence of isothermal and cyclic oxidation on the apparent interfacial toughness in thermal barrier coating systems

Fig. 3. SEM micrograph of indented samples oxidised 100 h at 1150 ◦ C (a) inden- tation charge 0.981 N (corresponding to 100 g); (b) indentation charge 2943 N (corresponding to 300 g). which in turn affects the mechanical properties. Strictly speak- ing, the mechanical response of the system to interfacial loading should depend on the elastic and plastic properties of all materi- als involved including that of the thermally grown oxide. However, a measurement of E and H of the growing oxide is not possible by means of standard micro and nano-indentation. The model detailed in [14] requires the knowledge of these characteristic parameters for the substrate and the coating. Accordingly, the TGO is assumed to play the role of a (three-dimensional) interface, thickening as temperature exposure increases and promoting, when loaded, spal- lation along the (two-dimensional) interface it shares with either the topcoat or the bond coat. Young modulus of the top coat E T and
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Influence of isothermal and cyclic oxidation on the apparent interfacial toughness in thermal barrier coating systems

Influence of isothermal and cyclic oxidation on the apparent interfacial toughness in thermal barrier coating systems

scale acting as diffusion barrier, the interfacial TGO is rather thin (less than 0.5 ␮m). Upon aging, the TGO layer grows according to a roughly parabolic kinetics. In all cases, two distinct interfaces formed between the bond coat and the TGO (inner interface), and the TGO and the top coat (outer interface), respectively, must be considered. Driven by the growth of the TGO layer, both inter- faces undergo morphological and roughness changes as the TGO thickens. The tortuosity of the interfaces, observed by SEM in cross- sections, is quantified by a folding or rumpling index estimated using image analysis. It is shown that both the oxide thickness and the folding index of the inner and outer interfaces, have a strong impact on the localization of the indentation-induced crack initia- tion, the path for propagation of crack once initiated and the ease or difficulty for crack to propagate. The apparent toughness deduced from interfacial indentation decreases as the 100-h isothermal- oxidation temperature increases from 1050 ◦ C to 1150 ◦ C indicating a progressive, thermally activated propensity for the degradation of TBC systems, as obviously expected. Apparent interfacial tough- ness controlled by interfacial roughness, TGO thickness and mostly by the temperature and time of isothermal or cyclic oxidation is a key parameter to address the mechanics and mechanisms of crack initiation and propagation prior to detrimental spallation of TBC systems. This is of course not the sole parameter entering in the implementation of possible models to predict TBC lifetime. Fur- ther improving the understanding of TBC behavior under severe oxidation exposure would require considering the fine variations of microstructural details and the evolution of the stored elastic strain energy, within each individual layer (Ni base single crys- tal, ␤-NiPtAl bond coat, Al 2 O 3 TGO and Yttria-Stabilised-Zirconia)
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Conformity assessment to air barrier system and air barrier material requirements of the NBC

Conformity assessment to air barrier system and air barrier material requirements of the NBC

This table differs from the table presented in the Part 5 Appendix and forms the basis of CCMC’s evaluation criteria for an air barrier system. To qualify, a minimum of three full-scale (i.e. 2.4 m x 2.4 m) wall specimens must be tested. One specimen must represent the air barrier system within the opaque insulated portion of the wall while the second and third specimens, to verify continuity, contain penetrations and joints (i.e. window, pipe, duct, concrete sill, etc.). These joints must be sealed by the accessories as part of the proprietary air barrier system. Before these specimens are measured for air leakage they are structurally aged to represent the structural wind loading to be experienced by the air barrier system in the field over an extended period of time. The structural wind loading consists of one-hour sustained loads, 2000 cyclic loads and one gust wind load.
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Improvement of barrier properties of a hybrid sol-gel coating by incorporation of synthetic talc-like phyllosilicates for corrosion protection of a carbon steel

Improvement of barrier properties of a hybrid sol-gel coating by incorporation of synthetic talc-like phyllosilicates for corrosion protection of a carbon steel

It must be underlined that the impedance diagram obtained for the coating containing natural talc (not reported here) was similar to that obtained on the bare carbon steel. The HF capacitive loop, characteristic of the coating was not observed and the polarisation re- sistance was low (about 400 Ω cm 2 ). This was attributed to the pres- ence of cracks in the coating which favoured the penetration of the electrolyte in the film. The incorporation of T160 decreased the barrier properties by comparison with the sol–gel without particles. T160 phyllosilicate contained a significant proportion of stevensite sheets and the shape of the aggregates was detrimental to the barrier effect of the coating. Moreover, the thermogravimetric analysis revealed a proportion of physisorbed water of 13.4% ( Fig. 4 ). Thus, the strong hydrophilic character allows easier penetration of the electrolyte through the coating due to the affinity of water of the particles. The barrier properties of the coating can be improved by decreasing the particle concentration. This was confirmed with a concentration of 1 g L − 1 . On the impedance diagrams (not shown), the film resistance was 4 times as high as the resistance obtained for the coating prepared with 10 g L − 1 of particles in the sol. The incorporation of particles
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Hydrodynamic coating of a fiber

Hydrodynamic coating of a fiber

Le probl`eme ´etudi´e a donc deux familles de solution : i a` basse vitesse, le d´epˆot r´esulte d’un compromis entre viscosit´e et capillarit´e, si bien qu’il est sensible `a la pr´esen[r]

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Coating of a textured solid

Coating of a textured solid

Key words: coating, interfacial flows (free surface), lubrication theory 1. Introduction Coating of solids is part of many industrial and daily-life processes such as painting, and it can be performed in various ways. One of the most common situations, the so-called dip-coating, consists of drawing the solid out of a bath. Ideally, the wetting liquid, of viscosity η and surface tension γ , is Newtonian, and the solid surface is flat and homogeneous. Withdrawing the surface at a constant speed V induces the deposition of a liquid film of constant thickness h d . As shown by Landau & Levich (1942) and Derjaguin (1943), the film thickness can be calculated by balancing viscous forces responsible for the coating with capillary forces, which oppose it. This yields the Landau–Levich–Derjaguin (LLD) equation:
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