Precipitation phases in the nickel-based superalloy DZ 125 with YSZ/CoCrAlY thermal barrier coating

Interdiffusion behavior of the thermal barrier coating (TBC) with the CoCrAlY bond coat (BC) and directionally solidified Ni-based superalloy DZ 125 was investigated. Severe inward-diffusion of Al, Co and Cr from the BC to the superalloy and outward diffusion of Ni and refractory elements such as W from the superalloy occur during annealing at 1050 °C in air. After 100 h annealing, a ∼30 μm thick inter-diffusion zone (IDZ) forms between the BC and superalloy, and a ∼35 μm thick secondary reaction zone (SRZ) forms beneath the IDZ. The IDZ mainly consists of β phase and γ matrix. Besides, small amount of Ta and Hf containing carbides are also observed in the IDZ. Needle and fine granular topologically close-packed (TCP) phases, characterized as rhombohedral μ phases, are abundant in the SRZ. The formation of SRZ is mainly due to the precipitation of refractory elements such as W and Mo from the γ matrix and β phase. The formation mechanism of SRZ and μ-TCP phase is discussed.     

Effects of annealing on the microstructure and the mechanical properties of EB-PVD thermal barrier coatings

The effects of thermal annealing at 1000 °C in air on the microstructure and the mechanical properties (Young's modulus and hardness) of thermal barrier coatings consisting of a 4 mol% Y₂O₃ partially stabilized ZrO₂ top coat and a NiCoCrAlY bond coat, deposited by electron beam physical vapour deposition on nickel-based superalloy IN 625, have been investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), image analysis and nanoindentation. During annealing, the ceramic top coat undergoes sintering and recrystallization. These processes lead to stress relaxation, an increase of the intra-columnar porosity and the number of large pores as measured by image analysis of SEM micrographs. An increase of the grain size of the γ-phase in the bond coat, accompanied by changes in the morphology of γ-grains with annealing time, is also observed. Correlations between these microstructural changes in the top coat and the bond coat and their mechanical properties are established and discussed.     

Improving stability of thermal barrier coatings on magnesium alloy with electroless plated Ni–P interlayer

Thermal barrier coatings (TBCs) of zirconia stabilized by 8 wt.% yttria (8YSZ) on MB26 rare earth–magnesium alloy with MCrAlY as bond coat were prepared by air plasma spraying (APS). In order to improve the thermal shock resistance of the coatings, an interlayer of Ni–P alloy between the substrate and bond coat was prepared by electroless plating. The preparation, microstructure, bond strength and thermal shock resistance of the coatings were investigated. The results indicate that Ni–P interlayer not only has favorable effects on the protection of Mg alloy substrate from thermal oxidation during thermal spraying, but also significantly improves the bond strength of TBCs. The thermal shock life of TBCs was enhanced from 5 cycles to longer than 130 cycles with the application of Ni–P interlayer. The failure of TBCs in thermal shock test was mainly induced by the corrosion of Mg alloy substrate.     

Thermal cycling behavior of EBPVD TBC systems deposited on doped Pt-rich γ–γ′ bond coatings made by Spark Plasma Sintering (SPS)

In the last decade, an increasing interest was given to Pt-rich γ–γ′ alloys and coatings as they have shown good oxidation and corrosion properties. In our previous work, Spark Plasma Sintering (SPS) has been proved to be a fast and efficient tool to fabricate coatings on superalloys including entire thermal barrier coating systems (TBC). In the present study, this technique was used to fabricate doped Pt-rich γ–γ′ bond coatings on AM1® superalloy substrate. The doping elements were reactive elements such as Hf, Y or Zr, Si and metallic additions of Ag. These samples were then coated by electron beam physical vapour deposition (EBPVD) with an yttria partially stabilized zirconia (YPSZ) thermal barrier coating. Such TBC systems with SPS Pt rich γ–γ′ bond coatings were compared to conventional TBC system composed of a β-(Ni,Pt)Al bond coating. Thermal cycling tests were performed during 1000-1 h cycles at 1100 °C under laboratory air. Spalling areas were monitored during this oxidation test. Most of the Pt rich γ–γ′ samples exhibited a better adherence of the ceramic layer than the β-samples. After the whole cyclic oxidation test, cross sections were prepared to characterize the thickness and the composition of the oxide scales by using scanning-electron microscopy. In particular, the influence of the doping elements on the oxide scale formation, the metal/oxide roughness, the TBC adherence and the remaining Al and Pt under the oxide scale were monitored. It was shown that RE-doping did not improve the oxidation kinetics of the studied Pt rich γ–γ′ bond coatings, nevertheless most of the compositions were superior to “classic” β-(Ni,Pt)Al bond coatings in terms of ceramic top coat adherence, due to lower rumpling kinetics and better oxide scale adherence of the γ–γ′-based systems.     

Spinel formation in thermal barrier systems with a Pt-enriched γ-Ni + γ′-Ni3Al bond coat

The oxidation of electron beam physical vapour deposited thermal barrier coatings with a Pt-enriched γ-Ni + γ′-Ni₃Al bond coat was investigated. Due to the growth of the thermally grown oxide (TGO), γ-Ni formed underneath the TGO as a result of Al depletion. Phase characterisation by X-ray diffraction, as well as microstructural observations, indicated that a NiAl₂O₄ spinel phase formed at the TGO/bond coat interface after prolonged oxidation. It is proposed that the formation of spinel occurs when local cracks present at the interface and the underlying bond coat is Al-depleted. The cracks provide a direct path for oxygen and nickel oxide forms at the bond coat surface. With further oxidation, the spinel forms at the interface through solid state reaction between the TGO and nickel oxide.     

On the development of plasma-sprayed thermal barrier coatings

Various zirconate coatings were prepared on bare Nimonic-75 and on CoCrAlY bond coat by plasma spraying. The cyclic oxidation and hotcorrosion resistance of these coatings have been evluated. The wide difference in the properties of these coatings has been rationalized in terms of the thermal expansion mismatch between the coating and the substrate. On continued thermal cycling and in presence of molten salt, the life-limiting factor has been identified to be oxidation of the bond coat. The interconnected porosity in the ceramic coating is mainly responsible for such oxidation, and controlling this porosity would lead to life improvement.     

Tensile fracture behavior of thermal barrier coatings on superalloy

Tensile fracture behavior of thermal barrier coatings (TBCs) on superalloy was investigated in air at room temperature (RT), 650 °C and 850 °C. The bond coat NiCrAlY was fabricated by either high velocity oxygen fuel (HVOF) or air plasma spraying (APS), and the top coat 7%Y₂O₃-ZrO₂ was deposited by APS. Thus two kinds of the TBC system were formed. It was shown that the coating had little effect on tensile stress-strain curves of the substrate and similar tensile strength was obtained in two kinds of the TBC system. However, the cracking behavior in the two kinds of TBC system at RT was different, which was also different from that at 650 °C and 850 °C by scanning electron microscopy. The interface fracture toughness of the two kinds of TBC system was evaluated by the Suo-Hutchinson model and the stress distribution in the coating and substrate was analyzed by the shear lag model.     

Synthesis and oxidation behavior of platinum-enriched γ + γ′ bond coatings on Ni-based superalloys

Simple Pt-enriched γ + γ′ coatings were synthesized on René 142 and René N5 Ni-based superalloys by electroplating a thin layer of Pt followed by a diffusion treatment at 1150-1175 °C. The Al content in the resulting γ + γ′ coating was in the range of 16-19 at.% on superalloys with 13-14 at.% Al. After oxidation testing, alumina scale adherence to these γ + γ′ coatings was not as uniform as to the β-(Ni,Pt)Al coatings on the same superalloy substrates. To better understand the effect of Al, Pt and Hf concentrations on coating oxidation resistance, a number of Ni-Pt-Al cast alloys with γ + γ′ or β phase were cyclically oxidized at 1100 °C. The Hf-containing γ + γ′ alloys with 22 at.% Al and 10-30 at.% Pt exhibited similar oxidation resistance to the β alloys with 50 at.% Al. An initial effort was made to increase the Al content in the Pt-enriched γ + γ′ coatings by introducing a short-term aluminizing process via chemical vapor deposition or pack cementation. However, too much Al was deposited, leading to the formation of β or martensitic phase on the coating surface.     

Reaction, transformation and delamination of samarium zirconate thermal barrier coatings

The rare earth zirconates have attracted interest for thermal barrier coatings (TBCs) because they have very low intrinsic thermal conductivities, are stable above 1200 °C and are more resistant to sintering than yttria-stabilized zirconia (YSZ). Samarium zirconate (SZO) has the lowest thermal conductivity of the rare earth zirconates and its pyrochore structure is stable to 2200 °C but little is known about its response to thermal cycling. Here, columnar morphology SZO coatings have been deposited on bond coated superalloy substrates using a directed vapor deposition method that facilitated the incorporation of pore volume fractions of 25 to 45%. The as-deposited coatings had a fluorite structure which transformed to the pyrochlore phase upon thermal cycling between 100 and 1100 °C. This cycling eventually led to delamination of the coatings, with failure occurring at the interface between the TGO and a “mixed zone” that formed between the thermally grown alumina oxide (TGO) and the SZO. While the delamination lifetime increased with coating porosity (reduction in coating modulus), it was significantly less than that of similar YSZ coatings applied to the same substrates. The reduced life resulted from a reaction between the rare earth zirconate and the alumina-rich bond coat TGO, leading to the formation of a mixed zone consisting of SZO and SmAlO₃. Thermal strain energy calculations show that the delamination driving force increases with TGO and mixed layer thicknesses and with coating modulus. The placement of a 10 μm thick YSZ layer between the TGO and SZO layers eliminated the mixed zone and restored the thermal cyclic life to that of YSZ structures.     

On the Degradation Modes and Oxidation Behavior of Platinum Aluminide Bond Coats in Thermal Barrier Coating Used as Surface Protection System for a Turbine Blade Superalloy

Adhesion of thermally grown oxide (TGO) to the bond coat is known to limit the useful life of thermal barrier coatings used in gas turbine blade applications. This is determined by the structure and composition of the bond coat as well as its thermal stability and in turn, its ability to develop and maintain a protective oxide. In this study, the degradation modes of platinum aluminides of the β-(Ni,Pt)Al- and PtAl₂ + β-(Ni,Pt)Al-types used as bond coats in thermal barrier coatings deposited on Ni-base superalloy and utilizing zirconia-7 wt% yttria and as top coat have been examined. Thermal exposure tests have been carried out at 1150 °C with cycling to room temperature every 24 h. Various electron-optical techniques have been used to characterize the microstructures of the bond coats and TGO. Particular emphasis has been placed upon the susceptibility of the bond coat to degradation by interdiffusion, oxidation, rumpling and formation of internal cavities. It is shown that the oxidation behavior and thermal stability characteristics are functions of the exact distribution of Pt in the bond coats. The β-(Ni,Pt)Al-type bond coat is found to have higher thermal stability and oxidize at a slower rate in comparison with the PtAl₂ + β-(Ni,Pt)Al₂-type. However, both bond coats are observed to exhibit a similar behavior in that the Al-rich and Pt-modified β-phase is progressively transformed into the Al-depleted γ′- and γ-phases with continued thermal exposure but at a slower rate in the β-(Ni,Pt)Al bond coat. Under the test conditions used in the study, there has been no evidence for rumpling, however, internal cavities are observed near the surface of each bond coat during the later stages of thermal exposure showing that rumpling is not necessarily a prerequisite. Failure of the respective thermal barrier coating systems is found to occur by loss of adhesion between the TGO and bond coat whose composition has approached that of the superalloy substrate by interdiffusion.     

Surface rumpling of a (Ni,Pt)Al bond coat induced by cyclic oxidation

The surface of an initially flat, platinum-modified nickel aluminide bond coat formed on a single crystal superalloy is shown to progressively roughen (“rumple”) with thermal cycling in air. Far less surface roughening occurs after isothermal oxidation or after the same number of thermal cycles but with a shorter high-temperature exposure in each cycle. Mechanisms of the observed rumpling and the implications of the bond coat surface evolution leading to the failure of thermal barrier coatings are discussed. It is concluded that local volume changes in the bond coat, caused by aluminum depletion and subsequent decomposition of the β-(Ni, Pt)Al phase, are responsible for the observed rumpling.     

Improved cyclic oxidation resistance of electron beam physical vapor deposited nano-oxide dispersed β-NiAl coatings for Hf-containing superalloy

Oxide dispersed (OD) β-NiAl coatings and OD-free β-NiAl coatings were deposited onto a Hf-containing Ni-based superalloy by electron beam physical vapor deposition (EB-PVD). Excessive enrichment of Hf was found in the TGO on the OD-free coating due to outward diffusion of Hf from the superalloy, causing accelerated TGO thickening and spalling. The OD-coating effectively prevented Hf from outward diffusion. Only small amount of Hf diffused to the coating surface and improved the TGO adherence by virtue of the reactive element effect. The OD-coating exhibited an improved oxidation resistance as compared to the OD-free coating.     

Thermally grown oxide growth behavior and spallation lives of solution precursor plasma spray thermal barrier coatings

The effects of thermally grown oxide (TGO) growth rate and bond coat oxidation behavior on the spallation lives of thermal barrier coatings (TBCs) have been investigated. Yttria partially stabilized zirconia (7YSZ) coatings have been applied to various bond coat/superalloy substrate combinations using the Solution Precursor Plasma Spray (SPPS) process. The coatings have been furnace thermal cycled at 1121 °C, using one hour cycles. A large variation in the spallation lives, from 125 to 1230 cycles, has been observed and are attributed to (a) the spatially averaged TGO growth rate, (b) the maximum localized TGO thickness, (c) the formation of non-alumina oxides with weak interfaces, and (d) the formation of yttrium aluminate stringers in low pressure plasma spray (LPPS) processed bond coat. Of these four factors, the average TGO thickness is the most important. Surprisingly vacuum plasma sprayed bond coated samples consistently had shorter cyclic live compared to air plasma sprayed bond coated samples.     

Oxidation and diffusion barrier behaviors of double-layer NiCoCrAlY coatings produced by plasma activated EB-PVD

A double-layer (DL) NiCoCrAlY coating was coated onto a Hf-containing Ni-based superalloy DZ125. The bottom layer produced by plasma activated electron beam-physical vapor deposition (PA EB-PVD) was featured by coarse-grained equiaxed microstructure, which is different from the columnar microstructure of the top layer produced by EB-PVD. Oxidation resistance of the DL coating and a conventional single-layer (SL) NiCoCrAlY coating produced by EB-PVD was investigated at 1373 K. The DL coating exhibited an improved oxidation resistance as compared with the SL coating. The bottom layer in the DL coating effectively inhibited outward diffusion of refractory elements, such as Hf and W from the superalloy substrate. Oxidation and diffusion barrier mechanisms of the DL coating were also discussed.     

The effect of silicon on thermal shock performance of aluminide-thermal barrier coatings

The failure of air-plasma-sprayed thermal barrier coatings (APS TBCs) with conventional pack aluminide and slurry Si-modified aluminide bond coats on superalloy In-738LC was investigated during a thermal-shock test. Thermal shock experiments consisted of rapid thermal cycling between 1100 °C and 300 °C for 120 times. It was found that the lifetime of APS TBCs on aluminide bond coats can be extended by introducing silicon into aluminide structure. Silicon improved the bond coat oxidation resistance as well as the stability of β-NiAl phase, which is critical to the coating life enhancement.     

Effect of superalloy substrate and bond coating on TBC lifetime

Several different single-crystal superalloys were coated with different bond coatings to study the effect of composition on the cyclic oxidation lifetime of an yttria-stabilized zirconia (YSZ) top coating deposited by electron beam physical vapor deposition from a commercial source. Three different superalloys were coated with a 7 μm Pt layer that was diffused into the surface prior to YSZ deposition. One of the superalloys, N5, was coated with a low activity, Pt-modified aluminide coating and Pt-diffusion coatings with 3 and 7 μm of Pt. Three coatings of each type were furnace cycled to failure in 1 h cycles at 1150 °C to assess average coating lifetime. The 7 μm Pt diffusion coating on N5 had an average YSZ coating lifetime > 50% higher than a Pt-modified aluminide coating on N5. Without a YSZ coating, the Pt-modified aluminide coating on N5 showed the typical surface deformation during cycling, however, the deformation was greatly reduced when constrained by the YSZ coating. The 3 μm Pt diffusion coating had a similar average lifetime as the Pt-modified aluminide coating but a much wider scatter. The Pt diffusion bond coating on superalloy X4 containing Ti exhibited the shortest YSZ coating lifetime, this alloy-coating combination also showed the worst alumina scale adhesion without a YSZ coating. The third generation superalloy N6 exhibited the longest coating lifetime with a 7 μm Pt diffusion coating.     

Structure and durability evaluation of YSZ + Al2O3 composite TBCs with APS and HVOF bond coats under thermal cycling conditions

Plasma sprayed thermal barrier coatings (TBCs) are applied to gas turbine components for providing thermal insulation and oxidation resistance. The TBC systems currently in use on superalloy substates typically consists of a metallic MCrAlY based bond coat and an insulating Y₂O₃ partially stabilized ZrO₂ as a ceramic top coat (ZrO₂ 7–8 wt.% Y₂O₃). The oxidation of bond coat underlying yttria stabilized zirconia (YSZ) is a significant factor in controlling the failure of TBCs. The oxidation of bond coat induces to the formation of a thermally grown oxide (TGO) layer at the bond coat/YSZ interface. The thickening of the TGO layer increases the stresses and leads to the spallation of TBCs. If the TGO were composed of a continuous scale of Al₂O₃, it would act as a diffusion barrier to suppress the formation of other detrimental mixed oxides during the extended thermal exposure in service, thus helping to protect the substrate from further oxidation and improving the durability. The TBC layers are usually coated onto the superalloy substrate using the APS (Atmospheric plasma spray) process because of economic and practical considerations. As well as, HVOF (High velocity oxygen fuel) bond coat provides a good microstructure and better adhesion compared with the APS process. Therefore, there is a need to understand the cycling oxidation characteristic and failure mode in TBC systems having bond coat prepared using different processes. In the present investigation, the growth of TGO layers was studied to evaluate the cyclic oxidation behavior of YSZ/Al₂O₃ composite TBC systems with APS-NiCrAlY and HVOF-NiCrAlY bond coats. Interface morphology is significantly effective factor in occurrence of the oxide layer. Oxide layer thickening rate is slower in APS bond coated TBCs than HVOF bond coated systems under thermal cycle conditions at 1200 °C. The YSZ/Al₂O₃ particle composite systems with APS bond coat have a higher thermal cycle life time than with the HVOF bond coating.     

Hot corrosion behaviour of plasma sprayed YSZ/Al2O3 dispersed NiCrAlY coatings on Inconel-718 superalloy

Oxide dispersed NiCrAlY bond coatings have been developed for enhancing thermal life cycles of thermal barrier coatings (TBCs). However, the role of dispersed oxides on high temperature corrosion, in particular hot corrosion, has not been sufficiently studied. Therefore, the present study aims to improve the understanding of the effect of YSZ dispersion on the hot corrosion behaviour of NiCrAlY bond coat. For this, NiCrAlY, NiCrAlY + 25 wt.% YSZ, NiCrAlY + 50 wt.% YSZ and NiCrAlY + 75 wt.% YSZ were deposited onto Inconel-718 using the air plasma spraying (APS) process. Hot corrosion studies were conducted at 800 °C on these coatings after covering them with a 1:1 weight ratio of Na₂SO₄ and V₂O₅ salt film. Hot corrosion kinetics were determined by measuring the weight gain of the specimens at regular intervals for a duration of 51 h. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques were used to determine the nature of phases formed, examine the surface attack and to carry out microanalysis of the hot corroded coatings respectively. The results show that YSZ dispersion causes enhanced hot corrosion of the NiCrAlY coating. Leaching of yttria leads not only to the formation of the YVO₄ phase but also the destabilization of the YSZ by hot corrosion. For the sake of comparison, the hot corrosion behaviour of a NiCrAlY + 25 wt.% Al₂O₃ coating was also examined. The study shows that the alumina dispersed NiCrAlY bond coat offers better hot corrosion resistance than the YSZ dispersed NiCrAlY bond coat, although it is also inferior compared to the plain NiCrAlY bond coat.     

Simultaneous synthesis by spark plasma sintering of a thermal barrier coating system with a NiCrAlY bond coat

As-fabricated thermal barrier coating (TBC) systems generally consist of a superalloy substrate, a MCrAlY bond coat (M = Ni, Co, Fe), and a ceramic (usually partially stabilized zirconia) top coat. The conventional methods for producing the two coating layers generally derive from thermal spray and physical vapor deposition techniques. Thermal exposure leads to the formation of an additional layer: the thermally grown oxide (TGO) between the bond coat and top coat. In the present work, a TBC system is synthesized through the application of spark plasma sintering (SPS), which provides not only the opportunity to synthesize all three layers at once, but the process is quite rapid and can produce dense layers. More specifically, this paper describes the application of this method to an yttria-stabilized ZrO₂ (YSZ) top coat and a NiCrAlY bond coat on a Ni-base Hastelloy X substrate. A one-micron thick Al₂O₃ TGO layer is also created from the reaction between an Al foil layer inserted in the stack prior to sintering and the ZrO₂ in the top coat. The effects of select process conditions are considered. The resulting multi-layer system is characterized with optical microscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray analysis (EDAX) and X-ray diffraction (XRD). Differential thermal analysis (DTA) is used to investigate the reaction between the Al foil and the YSZ top coat.     

EB-PVD热障涂层热循环过程中粘结层的氧化和相结构

采用磁控溅射方法在镍基单晶高温合金基体上沉积Ni-30Cr-12Al-0.3Y（质量分数，％）粘结层，采用电子束物理气相沉积方法（EB－VPD）沉积7％Y2O3（质量分数）－ZrO2陶瓷顶层，结果表明，在热循环过程中，非平衡相t′-ZrO2中的Y2O3含量逐渐减少，t′－ZrO2相逐渐分解成平衡相t-ZrO2（冷却时变转变成斜相）和立方组ZrO2,1050℃循环200次，粘结层氧化物（Al2O3)厚度约为3μm，表明Ni-Cr-Al-Y达宜作粘结层，继续热循环，陶瓷层中出现单斜阳，粘结层中Al贫化，氧化层中出现NiO及尖晶石等，引起应力集中，导致涂层失效。