Thermal Failure of Nanostructured Thermal Barrier Coatings with Cold-Sprayed Nanostructured NiCrAlY Bond Coat

Nanostructured thermal barrier coatings (TBCs) were deposited by plasma spraying using agglomerated nanostructured YSZ powder on Inconel 738 substrate with cold-sprayed nanostructured NiCrAlY powder as bond coat. The isothermal oxidation and thermal cycling tests were applied to examine failure modes of plasma-sprayed nanostructured TBCs. For comparison, the TBC consisting of conventional microstructure YSZ and conventional NiCrAlY bond coat was also deposited and subjected to the thermal shock test. The results showed that nanostructured YSZ coating contained two kinds of microstructures; nanosized zirconia particles embedded in the matrix and microcolumnar grain structures of zirconia similar to those of conventional YSZ. Although, after thermal cyclic test, a continuous, uniform thermally grown oxide (TGO) was formed, cracks were observed at the interface between TGO/BC or TGO/YSZ after thermal cyclic test. However, the failure of nanostructured and conventional TBCs mainly occurred through spalling of YSZ. Compared with conventional TBCs, nanostructured TBCs exhibited better thermal shock resistance.     

Plasma spraying of functionally graded yttria stabilized zirconia/NiCoCrAlY coating system using composite powders

Pre-alloyed and plasma spheroidized composite powders were used as the feedstock in the plasma spraying of functionally graded yttria stabilized zirconia (YSZ)/NiCoCrAlY coatings. The ball milling parameters of the composite powders and the plasma spraying parameters for preparing functionally graded materials (FMGs) coatings were optimized to obtain the best performance for the thermal barrier coatings (TBCs). Microstructure, physical, mechanical, and thermal properties of YSZ/NiCoCrAlY FGMs coatings were investigated and compared with those of traditional duplex coatings. Results showed that the advantages of using pre-alloyed composite powders in plasma spraying were to ensure chemical homogeneity and promote uniform density along the graded layers. Microstructure observation showed the gradient distribution of YSZ and NiCoCrAlY phases in the coating, and no clear interface was found between two adjacent different layers. Oxidation occurred during plasma spray and the resultant aluminum oxide combines with YSZ in a wide range of proportions. The bond strength of functionally graded coatings was about twice as high as that of the duplex coatings because of the significant reduction of the residual stresses in the coatings. The thermal cycling resistance of functionally graded coating was much better than that of duplex coating.     

Plasma-sprayed duplex and graded partially stabilized zirconia thermal barrier coatings: deposition process and properties

Atmospheric plasma spraying of duplex and graded ZrO₂ (8% Y₂O₃) thermal barrier coatings (TBCs) on Inconel 617 substrate with a NiCrAlY bond coat is described in terms of a deposition process of con-trolled coating structure. Special attention is devoted to the dominant spray parameters and the injector configuration for powder feeding, which play a fundamental role in graded coating deposition with con-trolled formation of a graded metal-ceramic (GMC) intermediate zone. The results of the graded coating spraying allow: (a) suppression of step-interface effects, (b) suppression of large differences (misfit) be-tween physical and mechanical constants of the coating and those of the substrate material, and (c) favor-able intergrowth of crystallites for a microstructurally integrated structure. Sprayed TBCs were investigated and compared with regard to their thermal cycling, oxidation behavior, and mechanical properties. The influence of crystal anisotropy changes on the resulting coating structure and properties is shown. On the basis of finite element (FE) calculations, the stress distribution within thermally cycled coating systems was analyzed. It is confirmed that the graded coating structure relaxes considerably the stresses resulting from the internal constraint due to thermal expansion difference between both metallic and ce-ramic materials. This stress distribution also decreases the gradient of elastic deformation and/or resid-ual stresses between the metal bond coat and top ceramic coating, and hence leads to a better thermal cycling behavior of the graded TBC systems. However, this advantage is not practical in every case, since the rapid oxidation of the metallic lamellae causes the ceramic phase in the GMC zone to undergo tensile stresses within a short thermal exposure time. The lifetime of duplex TBC systems that are under steady-state thermal load conditions is much higher than that of graded ones.     

Some aspects of thick thermal barrier coating lifetime prolongation

Much research has been performed in the field of thermal barrier coatings (TBCs) deposited by atmospheric plasma spraying of ZrO₂-Y₂O₃. The necessity of efficient thermal insulation, corrosion resistance, and sufficient lifetime under thermomechanical loads promotes the development of TBCs of several millimeters in thickness. However, some problems arise with the production of thick TBCs, such as poor adhesion and low thermal shock resistance. These problems are not observed clearly when the TBCs are, for example, 300 μm thick. This article presents strategies of thick TBC lifetime optimization by different cooling systems. Attempts have been made to improve thermal shock resistance (TSR) by applying thicker coatings with graded porosity, but they failed. Besides metallographical evaluation and scanning electron microscopy (SEM) analysis, microcracks and porosity were determined. Furthermore, the results of bond strength and burner rig tests are presented, and forthcoming experimental tasks are outlined.     

Failure of physical vapor deposition/plasma-sprayed thermal barrier coatings during thermal cycling

ZrO₂-7 wt.% Y₂O₃ plasma-sprayed (PS) coatings were applied on high-temperature Ni-based alloys precoated by physical vapor deposition with a thin, dense, stabilized zirconia coating (PVD bond coat). The PS coatings were applied by atmospheric plasma spraying (APS) and inert gas plasma spraying (IPS) at 2 bar for different substrate temperatures. The thermal barrier coatings (TBCs) were tested by furnace isothermal cycling and flame thermal cycling at maximum temperatures between 1000 and 1150 °C. The temperature gradients within the duplex PVD/PS thermal barrier coatings during the thermal cycling process were modeled using an unsteady heat transfer program. This modeling enables calculation of the transient thermal strains and stresses, which contributes to a better understanding of the failure mechanisms of the TBC during thermal cycling. The adherence and failure modes of these coating systems were experimentally studied during the high-temperature testing. The TBC failure mechanism during thermal cycling is discussed in light of coating transient stresses and substrate oxidation.     

等离子喷涂热障涂层的热循环氧化行为研究

本文采用大气等离子喷涂在HA188合金基材上制备NiCrAlY+YSZ热障涂层，并进行了1100 oC、1120 oC和1150 oC三个温度点的高温循环氧化行为对比研究。结果表明，随着考核温度的升高，热障涂层热循环失效寿命显著下降，失效主要是由YSZ/NiCrAlY界面附近YSZ 层中裂纹形成和扩展导致。循环失效后的YSZ与制备态的相结构一样，均为非平衡四方相t"-ZrO2，未发生t"→c+m相变。在热循环过程中，YSZ/NiCrAlY界面形成的热生长氧化物层（Thermally Grown Oxide, TGO）增厚基本符合“抛物线”规律，并且YSZ中裂纹的产生和扩展与TGO的增厚直接相关。     

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.     

Ultrafast thermal plasma physical vapor deposition of yttria-stabilized zirconia for novel thermal barrier coatings

This research aims to develop advanced thermal plasma spraying technology for the next-generation thermal barrier coatings (TBCs) with a high power hybrid plasma spraying system. By using thermal plasma physical vapor deposition (TP-PVD), various functional structured yttria-stabilized zirconia (YSZ) coatings were deposited. Parameters, such as powder feeding rate, hydrogen gas concentration, and total mass flow rate of the plasma gas, were optimized, and their influences on the evaporation of YSZ powder were investigated. Ultrafast deposition of a thick coating was achieved at a rate of over 150 μm/min. The deposited porous coating has a low thermal conductivity of 0.7W/mK and the dense coating with interlaced t′ domains possesses a high nanohardness of 27.85 GPa and a high reflectance. These characteristics show that the TP-PVD technique is a very valuable process for manufacturing novel TBCs.     

High temperature properties of thermal barrier coatings obtained by detonation spraying

NiCrAlY/YPSZ and NiCrAlY/NiAl/YPSZ thermal barrier coatings (TBCs) were successfully deposited by detonation spraying. The results indicated that the detonation sprayed TBCs included a uniform ceramic coat containing a few microcracks and a bond coat with a rough surface. The lamellar structure and the presence of cracks and impurities could reduce the thermal conductivity of the ceramic coat. Oxidation kinetics at 1000–1150 °C of detonation sprayed TBCs have been measured and discussed. The role of a Ni–Al intermediate layer in improving the oxidation resistance of duplex TBCs has also been studied.     

Thermal cycling failure of new LaMgAl11O19/YSZ double ceramic top coat thermal barrier coating systems

New LaMgAl₁₁O₁₉ (LaMA)/YSZ double ceramic top coat thermal barrier coatings (TBCs) with the potential application in advanced gas-turbines and diesel engines to realize improved efficiency and durability were prepared by plasma spraying, and their thermal cycling failure were investigated. The microstructure evolutions as well as the crystal chemistry characteristics of LaMA coating which seemed to have strong influences on the thermal cycling failure of LaMA and the new double ceramic top coat TBCs based on LaMA/YSZ system were studied. For double ceramic top coat TBC system, interface modification of LaMA/YSZ by preparing thin composite coatings seemed to be more preferred due to the formations of multiple cracks during thermal cycling making the TBC to be more strain tolerant and as well as resulting in an improved thermal cycling property. The effects of the TGO stresses on the failure behavior of the TBCs were discussed through fluorescence piezo-spectroscopy analysis.     

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.     

Formation and behavior of thermal barrier coatings on nickel-base superalloys

Plasma-sprayed thermal barrier coatings (TBCs) have been used to extend the life of combustors. Electron beam physical vapor deposited (EB-PVD) ceramic coating has been developed for more demanding rotating as well as stationary turbine components. Here 3 kW RF magnetron sputtering equipment was used to gain zirconia ceramic coatings on hollow turbine blades and vanes, which had been deposited NiCrAIY by cathodic arc deposition. NiCrAlY coating surface was treated by shot peening; the effects of shot peening on the residual stress are presented. The results show that RF sputtered TBCs are columnar ceramics, strongly bonded to metal substrates. NiCrAlY bond coat is made of β, γ‘ and Cr phases, ZrO2 ceramic layer consists of t‘ and c phases. No degradation occurs to RF ceramic coatings after 100 h high temperature oxidation at 1150℃ and 500 thermal cycles at 1150℃ for 2 min, air-cooling.     

Hot-corrosion behavior of graded thermal barrier coatings formed by plasma-spraying process

The hot-corrosion behavior of thermal barrier coatings (TBCs) has been studied by comparing double-layer coatings and graded coatings. Two types of oxide ceramics, 2CaO·SiO₂-15mass%CaO·ZrO₂ (C₂S-15CZ) and 8 mass% Y₂O₃·ZrO₂ (8YSZ), with a bond coating of NiCrAlY, were applied to metallic substrates in this study. After hot-corrosion testing with V₂O₅-Na₂SO₄ corrosive ash for 3 h at 1273 K, the TBCs were investigated by visual inspection, a scanning electron microscope, x-ray diffraction, and electron probe microanalysis. The findings for the resulting coating of C₂S-15CZ reacted with V₂O₅ only where it was in direct contact with the corrosive ash. The affected area from the reaction was limited to the coating surface where V₂O₅ was present. The coating showed adequate hot-corrosion resistance against V₂O₅-Na₂SO₄ corrosive ash for 3 h at 1273 K. The findings for the 8YSZ coating were that Y₂O₃, the stabilizing component, particularly reacted with V₂O₅ and lost its function, which led to partial spalling of the coating. It was observed that the hot-corrosion resistance of the double-layer TBC was largely influenced by the performance of a corrosion-resistant NiCrAlY bond coat, which provided protection against corrosive components penetrating through the ceramic topcoat. Last, the graded coating degraded due to the oxidation of NiCrAlY particles that existed near the topcoat surface and affected the durability of the TBC.     

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.     

Microstructure formation in thermally-sprayed duplex and functionally graded NiCrAlY/Yttria-Stabilized Zirconia coatings

NiCrAlY bond coats were deposited on a number of Inconel-738LC specimens using HVOF spray technique. For duplex coating, a group of these specimens were coated with Yttria Stabilized Zirconia (YSZ) using plasma spray technique. Functionally graded NiCrAlY/YSZ coatings were fabricated by plasma spray using co-injection of the two different powders in a single plasma torch. The amount of Zirconia in functionally graded coatings were gradually increased from 30 to 100 vol.%. Duplex and functionally graded coatings were then characterized using optical microscope, Scanning Electron Microscopy (SEM), Energy Dispersive x-ray Spectrometry (EDS), map analysis and X-ray Diffraction (XRD). The strength of the Adhesive coatings of the substrate was also measured.The results show that microstructure, porosity and compositions are gradually varied in the functionally graded coatings. EDS analyses revealed that oxidation of aluminum, chromium and yttrium in the NiCrAlY alloy are occur in the high-temperature plasma-spray stream during deposition. The oxidized products, mixed with zirconia at high temperature in a wide composition range, produce ceramic composites and increase the cohesion strength between the layers. The results also show a better performance in as-sprayed functionally graded coatings comparing with duplex coatings, especially regard to the adhesion strength of the coatings.     

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.     

High-temperature thermo-mechanical behavior of functionally graded materials produced by plasma sprayed coating: Experimental and modeling results

Thermal barrier coatings are widely used in aerospace industries to protect exterior surfaces from harsh environments. In this study, functionally graded materials (FGMs) were investigated with the aim to optimize their high temperature resistance and strength characteristics. NiCrAlY bond coats were deposited on Inconel-617 superalloy substrate specimens by the low vacuum plasma spraying technique. Functionally graded Ni-yttria-stabilized zirconia (YSZ) coatings with gradually varying amounts of YSZ (20%-100%) were fabricated from composite powders by vacuum plasma spraying. Heat shield performance tests were conducted using a high- temperature plasma torch. The temperature distributions were measured using thermocouples at the interfaces of the FGM layers during the tests. A model for predicting the temperature at the bond coating–substrate interface was established. The temperature distributions simulated using the finite element method agreed well with the experimental results.     

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.     

Evaluating the oxidation resistance of bond coats on thermal barrier coatings

Coatings are being increasingly used as an engineering alternative for advanced projects. Various techniques and processes are available for applying coatings, depending on the specific situation they are intended for. Thermal barrier coatings, known as TBCs, form part of a particular series of metal-ceramic coatings that are traditionally used in the aeronautical industry and are being increasingly applied in the automotive industries and for industrial turbines. One of the main issues with TBCs is degradation owing to the oxidation of the bond coat in high temperatures, resulting in the failure of the coating due to peel off. This study investigates and compares the behaviour of the oxidation of the TBC bond coat when the material used is NiAl alloy; this alloy is commonly used because of its characteristics at high temperatures and to ensure strong adhesion on various substrates. The bond coat was applied on an AISI 1020 Steel substrate using the flame spraying process. In order to carry out isothermal oxidation tests, the furnace used was regulated at a temperature of 1000 °C in static air. The samples were exposed for 24, 48 and 96 h and cooling was carried out in atmospheric air at ambient temperature. The analysis of the thermally grown oxide for each sample was carried out based on the exposure times and the oxide rates were evaluated by measuring the mass gained by the samples with oxidized coatings and using Scanning Electron Microscopy and Optical Microscopy.     

Strain analysis of plasma sprayed thermal barrier coatings under mechanical stress

The stiffness of air plasma sprayed (APS) thermal barrier coatings (TBCs) was determined from bending experiments combining strain analysis on a microstructural level with macroscopic mechanical parameters. Tests were performed with freestanding and attached TBCs, the latter either loaded in tension or in compression. Relationships are derived, which describe the TBC stiffness in a multilayer composite (attached TBC) and for a bimodular material that possess a lower stiffness in tension than in compression (stand-alone TBC). The increase of in-plane stiffness with increasing compressive stress emphasizes the importance of the spraying defects for the elastic response of the coating.