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 Shock Resistance of APS 8YSZ Thermal Barrier Coatings on Titanium Alloy

Thermal barrier coatings (TBCs) with a typical 8YSZ ceramic top coat and CoNiCrAlY bond coat were deposited on titanium alloy substrate (Ti-6Al-4V in wt.%) by air plasma spraying. Thermal insulation and thermal shock resistance of the TBCs at different temperatures as well as their failure behavior were investigated. The results showed that the test temperature had a significant effect on thermal shock life of the TBCs. Failure of the TBCs systems was caused by the formation of crack, bond coat oxidation and elemental diffusion. The vertical cracks induced by thermal shock cycles were probably responsible for the enhancement in thermal shock resistance of the TBCs. Furthermore, elemental diffusion had a great effect on the acceleration of the TBCs failure. The TBCs could provide a good thermal protection for the titanium alloy substrate.     

等离子喷涂热障涂层高温风洞热震行为

采用等离子喷涂工艺制备ZrO2-8%Y2O3(质量分数,下同)陶瓷层,冷喷涂制备CoNiCrAlY粘结层,在高温燃气风洞条件下测试热障涂层的热震性能,并研究了高温氧化处理对试样热震性能的影响.结果表明,等离子喷涂热障涂层具有较好的抗热震性能,经过100次热震循环后,涂层与基体结合良好,涂层较为完整,未出现大面积的剥落;经过氧化处理后的试样抗热震性较差.     

Behavior of plasma-sprayed thermal barrier coatings during thermal cycling and the effect of a preoxidized NiCrAlY bond coat

The failure mechanisms of thermal barrier coatings (TBCs) subjected to a thermal load are still not entirely understood. Thermal stresses and/or oxidation cause the coating to fail and hence must be minimized. During the present investigation, TBCs up to 1.0 mm were sprayed and withstood high thermal stresses during thermal testing. Owing to the substantial thickness, the temperature at the top coat/bond coat interface was relatively low, resulting in a low oxidation rate. Furthermore, bond coats were preoxidized before applying a top coat. The bond strength and the behavior during three different thermal loads of the preoxidized TBCs were compared with a standard duplex TBC. Finite-element model (FEM) calculations that took account of bond coat preoxidation and interface roughness were made to calculate the stresses occurring during thermal shock. It is concluded that the thick TBCs applied during this research exhibit excellent thermal shock resistance and that a preoxidizing treatment of the bond coat increases the lifetime during thermal loading, where oxidation is the main cause of failure. The FEM analysis gives a first impression of the stress conditions on the interface undulations during thermal loading, but further development is required.     

Laser cycling and thermal cycling exposure of thermal barrier coatings on copper substrates

Thermal barrier coatings (TBCs) are successfully applied in turbines and could also protect combustion chambers in rocket engines. Apart from different loading conditions, the main difference between these applications is the substrate material, which is nickel-based for turbines and copper-based for rocket engines. To optimize the coating system, more knowledge of possible failure modes is necessary.In this work a standard coating system was applied by atmospheric plasma spraying to copper specimens. These specimens were exposed to thermal cycling with different cooling rates and to laser shock testing. A laser-cycling set-up was developed to qualify different coating systems. This set-up consists of a high-power diode laser (3 kW) which provides high heating rates to up to 1500 °C. Laser shock testing has proven to be a suitable alternative to burner rig testing.The results were different to the common failure modes for TBCs on nickel substrates as the coatings system does not fail at the interface between top coat and bond coat, but at the interface between substrate and bond coat. Two failure modes were observed: copper oxide was undermining the coatings at the substrate/bond coat-interface in the case of thermal cycling experiments, and complete delamination occurred at the same interface in the case of laser shock testing. Consequently, this interface is critical in the investigated material system.     

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.     

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.     

Effects of bond coat preoxidation on the properties of ZrO2-8wt.% Y2O3/Ni-22Cr-10Al-1Y thermal-barrier coatings

Plasma-sprayed thermal barrier coatings (TBCs) consist of an intermediate MCrAlY bond coat to protect the substrate superalloy from oxidation/hot corrosion and a thermal insulating zirconia-based ceramic top coat. This system is developed for advanced turbine-engine, hot-section components. In this study the as-sprayed Ni-22Cr-10Al-1Y bond coat was subjected to preoxidation treatment at 1000° C for 1, 50, 100, and 200 hr, also at 1100°C, 1200°C and 1300°C for 1 hr to form an oxide scale before subsequent deposition of a ZrO₂-8wt.% Y₂O₃ top coat. The oxidation kinetics were measured, and the phase constitution and morphology of pregrown oxides on the Ni-22Cr-1Y bond coat were analyzed by x-ray diffractometer and SEM/EDS to elucidate the improvement and degradation mechanisms of the new system. The results of the experiments in this study showed that the tentative ZrO₂-8wt.% Y₂O₃ TBC specimens with preoxidized Ni-22Cr-10Al-1Y bond coat, when properly processed, exhibited lower oxidation rates and generally longer lifetime compared with traditional TBC specimens.     

Effects of defects on the thermal fatigue behavior of detonation-gun sprayed thermal barrier coatings

The effects of coating defects, such as pores and cracks, on the thermal fatigue behavior of zirconia based thermal barrier coatings (TBCs) have been investigated. Duplex TBCs, which are composed of an 8 wt.% Y₂O₃ stabilized ZrO₂ (YSZ) layer on top of a NiCrAlY bond layer were produced by detonation gun spraying. Thermal fatigue tests were conducted on three different TBC specimens, the YSZ layers of which were varied in terms of porosity and crack morphology, and failure analyses were subsequently carried out on the tested specimens. From these results, the roles of the defects on the thermal and mechanical degradation behavior of the TBCs were investigated.     

Oxidation and Degradation of EB–PVD Thermal–Barrier Coatings

Thermal–barrier coatings (TBCs) consist of a magnetron-sputtered Ni–30Cr–12Al–0.3Y (wt.%) bond coat to protect the substrate superalloy from oxidation/hot corrosion and an electron-beam physical-vapor deposited (EB–PVD) 7 wt.% yttria partially stabilized zirconia (YPSZ) top coat. The thermal cyclic life of the TBC system was assessed by furnace cycling at 1050°C. The oxidation kinetics were evaluated by thermogravimetric analysis (TGA) at 900, 1000, and 1100°C for up to 100 hr. The results showed that the weight gain of the specimens at 1100°C was the smallest in the initial 20 hr, and the oxide scale formed on the sputtered Ni–Cr–Al–Y bond coat is only Al₂O₃ at the early stage of oxidation. With aluminum depletion in the bond coat, NiO, Ni(Cr,Al)₂O₄, and other spinel formed near the bond coat. During thermal cycling, microcracks were initiated preferentially in the YPSZ top coat along columnar grain boundaries and then extended through and along the top coat. The growth stress of TGO added to the thermal stress imposed by cycling, lead to the separation at the bond coat–TGO interface. The ceramic top coat spalled with the oxide scale still adhering to the YPSZ after specimens had been cycled at 1050°C for 300 cycles. The failure mode of the EB–PVD ZrO₂–7 wt.% Y₂O₃ sputtered Ni–Cr–Al–Y thermal-barrier coating was spallation at the bond coat–TGO interface.     

Influence of thermal shock on insulation effect of nano-multilayer thermal barrier coatings

Thermal barrier coatings (TBCs) with nano-multilayer structure were investigated by thermal shock test. The change of insulation effect during thermal shock test was studied by in-situ temperature monitor with a thermal couple set into the substrate. Microstructure and electrical properties of TBCs were characterized by SEM and Impedance Spectroscopy, respectively. Initial increase in insulation effect was observed and related to the formation and growth of perpendicular microcracks in top coat and transversal microcracks in TGO. With thermal shock, the insulation effect decreased due to the further growth of microcracks in top coat and TGO which induced the failure of TBCs.     

Investigation of interfacial properties of atmospheric plasma sprayed thermal barrier coatings with four-point bending and computed tomography technique

A modified four-point bending test has been employed to investigate the interfacial toughness of atmospheric plasma sprayed (APS) yttria stabilised zirconia (YSZ) thermal barrier coatings (TBCs) after isothermal heat treatments at 1150 °C. The delamination of the TBCs occurred mainly within the TBC, several to tens of microns above the interface between the TBC and bond coat. X-ray diffraction analysis revealed that the TBC was mainly tetragonal in structure with a small amount of the monoclinic phase. The calculated energy release rate increased from ~ 50 J/m^− 2 for as-sprayed TBCs to ~ 120 J/m^− 2 for the TBCs exposed at 1150 °C for 200 h with a loading phase angle about 42°. This may be attributed to the sintering of the TBC. X-ray micro-tomography was used to track in 3D the evolution of the TBC microstructure non-destructively at a single location as a function of thermal exposure time. This revealed how various types of imperfections develop near the interface after exposure. The 3D interface was reconstructed and showed no significant change in the interfacial roughness after thermal exposure.     

Effect of pre-aluminization on the properties of ZrO2-8wt.% Y2O3/Co-29Cr-6Al-1Y thermal-barrier coatings

Specimens of investment-cast Mar-M247 superalloy were vacuum-plasma sprayed with Co-29Cr-6Al-1Y bond coat, and part of the specimens were further pre-aluminized at 980°C for 2, 4, 6, 8, and 10 hours. All the specimens were then deposited with ZrO₂-8 wt.% Y₂O₃ thermal-barrier coatings (TBCs) and thermally cycled at 1050°C to evaluate the effect of time of the prealuminizing treatment on the performance and failure mechanism of the modified system. Results showed that TBC specimens with pre-aluminized bond coatings exhibited lower oxidation rates and significantly higher cyclic life when compared with unaluminized specimens. The failure of bond-coat pre-aluminized TBC specimens was observed to propagate mainly along the lamellar splats of the top coat, whereas the failure of conventional TBC specimens occurred mainly along the top-coat/spinel oxides interface.     

Microstructural observations of as-prepared and thermal cycled NiCoCrAlY bond coats

Microstructures of as-prepared and 1100 °C/100 h isothermally annealed NiCoCrAlY bond coat specimens as well as a bond coat obtained from an end of life turbine blade were characterized with TEM. In all specimens the γ grains were observed to consist of fine γ′ precipitates, which form during cooling and are unstable at the higher operating temperatures. The β grains present in the as-prepared specimens were observed to transform to L1₀ martensite in the 1100 °C/100 h isothermally annealed specimen. As a result of substrate-bond coat interdiffusion the M_s temperature increases during thermal cycling due to an increase in Ni and decrease in the Cr concentrations of the β-phase. The turbine blade bond coat was also found to contain Cr and Co-rich σ-phase precipitates.     

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.     

Experimental and numerical life prediction of thermally cycled thermal barrier coatings

This article addresses the predominant degradation modes and life prediction of a plasma-sprayed thermal barrier coating (TBC). The studied TBC system consists of an air-plasma-sprayed bond coat and an air-plasma-sprayed, yttria partially stabilized zirconia top layer on a conventional Hastelloy X substrate. Thermal shock tests of as-sprayed TBC and pre-oxidized TBC specimens were conducted under different burner flame conditions at Volvo Aero Corporation (Trollhättan, Sweden). Finite element models were used to simulate the thermal shock tests. Transient temperature distributions and thermal mismatch stresses in different layers of the coatings during thermal cycling were calculated. The roughness of the interface between the ceramic top coat and the bond coat was modeled through an ideally sinusoidal wavy surface. Bond coat oxidation was simulated through adding an aluminum oxide layer between the ceramic top coat and the bond coat. The calculated stresses indicated that interfacial delamination cracks, initiated in the ceramic top coat at the peak of the asperity of the interface, together with surface cracking, are the main reasons for coating failure. A phenomenological life prediction model for the coating was proposed. This model is accurate within a factor of 3.     

Comparison of thermal shock behaviors between plasma-sprayed nanostructured and conventional zirconia thermal barrier coatings

NiCoCrAlTaY bond coat was deposited on pure nickel substrate by low pressure plasma spraying(LPPS), and ZrO2-8%Y2O3 (mass fraction) nanostructured and ZrO2-7%Y2O3 (mass fraction) conventional thermal barrier coatings(TBCs) were deposited by air plasma spraying(APS). The thermal shock behaviors of the nanostructured and conventional TBCs were investigated by quenching the coating samples in cold water from 1 150, 1 200 and 1 250 ℃, respectively. Scanning electron microscopy(SEM) was used to examine the microstructures of the samples after thermal shock testing. Energy dispersive analysis of X-ray(EDAX) was used to analyze the interface diffusion behavior of the bond coat elements. X-ray diffractometry(XRD) was used to analyze the constituent phases of the samples. Experimental results indicate that the nanostructured TBC is superior to the conventional TBC in thermal shock performance. Both the nanostructured and conventional TBCs fail along the bond coat/substrate interface. The constituent phase of the as-sprayed conventional TBC is diffusionless-transformed tetragonal(t′). However, the constituent phase of the as-sprayed nanostructured TBC is cubic(c). There is a difference in the crystal size at the spalled surfaces of the nanostructured and conventional TBCs. The constituent phases of the spalled surfaces are mainly composed of Ni2.88Cr1.12 and oxides of bond coat elements.     

The effect of pre-oxidation atmosphere on the durability of EB-PVD thermal barrier coatings with CoNiCrAlY bond coats

The effects of pre-oxidation heat treatment on oxidation behavior and thermal cycle life of electron-beam physical deposited (EB-PVD) thermal barrier coatings (TBCs) with CoNiCrAlY bond coats were investigated as a function of the pO₂ of the pre-oxidation atmosphere. The pO₂ of the pre-oxidation atmosphere was controlled by using a solid-state electrochemical oxygen pump system. The purity and microstructure of the continuous Al₂O₃ layer formed on the bond coat during pre-oxidation at 1050 °C were highly influenced by the pO₂ of the atmosphere. High purity α-Al₂O₃ with large grain size was formed on the bond coats under a pO₂ of 10^− 12-10^− 9 Pa, which resulted in a lower growth rate of TGO and longer lifetime.     

Behavior of Calcia-Stabilized Zirconia Coating at High Temperature, Deposited by Air Plasma Spraying System

Thermal barrier coatings (TBCs) are employed to protect metallic components from heat, oxidation, and corrosion in hostile environments. In this paper Ni-20Cr bond coat followed by CaZrO₃ top coat was deposited on 316 stainless steel substrates by air plasma spray coating technique. Isothermal treatment of coated samples was carried out to investigate the effect of heat exposure on the microstructure and metallurgical phase changes of TBCs system. The fractured surface of as-sprayed and delaminated CaZrO₃ coatings was also studied to observe the splats morphology, structural defects, and lamellas internal microstructure. CaZrO₃ coating was found to be stable for 100 h at 700 °C but accelerated degradation was observed at 900 °C even at 20 h and lead to delamination after 60 h of exposure time. Chromium rich oxide formation was found to be responsible for the complete delamination of the top coat. Further, the formation of meta-stable monoclinic phase was also observed on the top surface of the top coat.     

The effects of heat treatment and gas atmosphere on the thermal conductivity of APS and EB-PVD PYSZ thermal barrier coatings

The effects of heat treatment and gas atmosphere on thermal conductivity of atmospheric plasma sprayed (APS) and electron beam physical vapor deposited (EB-PVD) partially Y₂O₃ stabilized ZrO₂ (PYSZ) thermal barrier coatings (TBCs) were investigated. Two-layer samples that had an EB-PVD coating deposited on bond coated nickel-base superalloy IN625 substrates, free-standing APS and EB-PVD coatings as well as a quasi-free-standing EB-PVD PYSZ coating (coating on semitransparent sapphire) were included in the study. Thermal diffusivity measurements for determining thermal conductivity were made from room temperature up to 1150 °C in vacuum and under argon gas using the laser flash technique. To investigate the effect of heat treatment on thermal conductivity, coatings were annealed at 1100 °C in air. For both the APS and EB-PVD PYSZ coatings the first 100 h heat treatment caused a significant increase in thermal conductivity that can be attributed to microstructural changes caused by sintering processes. Compared to the measurements in vacuum, the thermal conductivity of APS coatings increased by about 10% under argon gas at atmospheric pressure, whereas for the EB-PVD coatings, the influence of gas on thermal conductivity was relatively small. The effect of gas on the thermal conductivity of APS and EB-PVD PYSZ coatings can be attributed to amount, shape, and spatial arrangement of pores in the coating material.