Current status of thermal barrier coatings — An overview

The science and technology of thermal barrier coatings has advanced considerably since reports of the first test on turbine blades in a research engine in 1976. Today thermal barrier coatings are flying in revenue service in a low risk location within the turbine section of certain gas turbine engines. The state-of-the-art coating system for gas turbine applications is currently a plasma-sprayed ZrO₂-(6%–8%) Y₂O₃ ceramic layer over an MCrAlY (M ≡ Ni, Co or NiCo) bond coat layer plasma sprayed at low pressure.Although the potential for meeting current and short-term goals is high, longer-range goals may not be attainable with current coating concepts. These longer-range goals will involve high risk designs where coating loss could lead directly to component loss. Several steps must be taken to help meet these goals. Improved understanding of coating failure mechanisms is required. Models are needed to predict lifetimes. Process automation and quality control procedures must be instituted. Finally, new concepts in plasma-sprayed coatings must be developed and alternatives to the plasma- spraying process may be required.The current status of thermal barrier coatings and prospects for future progress in the above areas are summarized.     

Failure mechanism of EB-PVD thermal barrier coatings on NiAl substrate

Yttria stabilized zirconia（YSZ） was deposited on the line cut β-NiAl substrate by electron-beam physical vapour deposition（EB-PVD）, and the cyclic oxidation behaviors of thermal barrier coatings on β-NiAl substrate were investigated in 1 h thermal cycles at 1 200 ℃ in air. The results show that the samples fail after 80-100 cycles. Sub-interface cavitations in the substrate develop due to depletion of Al in forming thermally grown oxides（TGOs）. The collapse and closing up of cavities result in the ragged YSZ/TGO/substrate interface. Since the specific crack trajectories are quite sensitive to local geometry, cracks along the YSZ/TGO/substrate interfaces ultimately lead to YSZ spallation.     

Wavelet analysis of acoustic emission signals from thermal barrier coatings

The wavelet transform is applied to the analysis of acoustic emission signals collected during tensile test of the ZRO2-8% Y2O3 （YSZ） thermal barrier coatings （TBCs）. The acoustic emission signals are de-noised using the Daubechies discrete wavelets, and then decomposed into different wavelet levels using the programs developed by the authors. Each level is examined for its specific frequency range. The ratio of energy in different levels to the total energy gives information on the failure modes （coating micro-failures and substrate micro-failures） associated with TBCs system.     

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.     

Oxidation and thermal fatigue of EB-PVD thermal barrier coatings on tube superalloy substrate

Two-layer structure thermal barrier coatings （TBCs） （NiCoCrAIY （bond coat）＋（6%-8%, mass fraction） Y2O3-stabilized ZrO2（YSZ top coat）） were deposited by electron beam physical vapor deposition （EB-PVD） on tube superalloy substrates. The samples were investigated by isothermal oxidation and thermal shock tests. It is found that the mass gains of the substrate with and without TBCs are 0.165 and 7.34 mg/cm^2, respectively. So the TBCs system is a suitable protection for the substrate. In thermal shock tests the vertical cracks initiate at the top coat and grow into the bond coat, causing the oxidation of the bond coat along the cracks. Failure of the TBCs system occurs by the spallation of the YSZ from the bond coat, and some micro-cracks are found at the location where the fragment of the YSZ top coat spalled from.     

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.     

Microstructural features and properties of plasma sprayed YPSZ／NiCrAIY thermal barrier coating （TBC）

The plasma sprayed thermal barrier coating (TBC) consists of NiCrAlY bond coating and yttria partially stabilized zirconia (YPSZ) top coating. NiCrAlY coating mainly contains Ni solid solution with face centered cubic lattice, Al2 O3 oxides and pores. The most obvious feature of YPSZ coating with tetragonal zirconia is a lot of vertical microcracks in this coating. The thermal insulation capability of the TBC increased with an increase in YPSZ coating thickness, the temperature drop across the TBC increasing from 60℃ to 92℃ with increasing YPSZ coating thickness from 100 μm to 500 μm. The thermalshock resistance of the TBC decreased with increasing YPSZ coating thickness and cracks initiated mainly in original vertical microcrack tips of the YPSZ coating and propagated not only along YPSZ coating / NiCrAlY coating interface but also through NiCrAlY coating. The oxidation process of the TBC at 1 200℃ can be divided into two stages: traasieat oxidation stage with rapid oxidation rate and steady oxidation stage with slow oxidation. Their transition time was about 10 hours. The weight gain for 100 hours was 3. 222 mg/mm^2. It is favorable to increase YPSZ coating toughness and to decrease the pores and oxides of the TBC system for improving thermal shock resistance and oxidation resistance of the TBC.     

Microstructural features and properties of plasma sprayed YPSZ/NiCrAlY thermal barrier coating (TBC)

0　IntroductionThermalbarriercoatings(TBCs)arewidelyusedontheturbinebladesforaircraftpropulsionorpowergenerationtoreducethemetallicsubstratetemperature,whichleadstoincreasingengineefficiencyandloweringpollutantemissionsresultingfromallowableincreaseofoperationtemperature[1,2].Today,TBCsareattractingmoreattentionandhavewiderpotentialapplicationstoprotecthightemperaturecomponents.However,thermalbarriercoatingshaveatendencytocrackandspallinserviceduetothermalshockandthermalcyclingbetweenambient…     

Thermochemical compatibility of ytterbia–(hafnia/silica) multilayers for environmental barrier coatings

Environmental barrier coating (EBC) systems consisting of multiple layers tailored to address individual protection needs may offer improved performance relative to conventional architectures. If the requirements of thermochemical and thermomechanical compatibility are met, the deposition of a segmented thermal barrier coating on a dense rare earth silicate EBC could provide additional thermal protection and resistance to attack by molten deposits. The thermochemical compatibility between silicates in the YbO_1.5–SiO₂ system and phases in the YbO_1.5–HfO₂ system was investigated by equilibrating powder compacts of selected ternary compositions; diffusion couples were used to simulate interactions at the layer interfaces in the proposed architectures. The deduced 1500 °C ternary isothermal section reveals that the ordered δ-Yb₄Hf₃O₁₂ and H₃–Yb₆HfO₁₁ phases are only compatible with ytterbium monosilicate (Yb₂SiO₅) EBC. Implementation of these hafnates in contact with ytterbium disilicate (Yb₂Si₂O₇) leads to interfacial reactions that facilitate layer debonding. The results provide criteria to guide the design of future thermal/environmental barrier coating architectures.     

Flattening of droplets and formation of splats in thermal spraying: A review of recent work—Part 1

Properties of the coatings developed during thermal spraying are essentially determined by rapid solidification of splats formed as a result of impingement of the melted powder particles onto a substrate surface. The processes of flattening droplets and formation of splats in thermal spraying have been studied intensively during the last two decades. The last review on this topic was published at the end of 1994. Since then many papers have been dedicated to investigating splat formation, taking into account such important issues as roughness of the substrate surface, wetting phenomena, and splashing. This review, consisting of two parts, includes the main results obtained since 1994 and examines the influence of solidification of the lower part of the splat, substrate roughness, wetting at the substrate-coating interface, substrate deformation, oxidation, and splashing on the dynamics of flattening of droplets and the formation of splats. Flattening of composite powder particles, splat-substrate interaction, and development of splat-substrate adhesion and splat porosity are discussed. Part 1 of the review covers the following issues, which significantly influence the droplet flattening and splat formation: droplet solidification during flattening and roughness of the substrate surface, composite morphology of the powder particles, and oxidation processes. The results provide a better understanding of the thermal spray processes to increase their efficiency.     

Direct production of porous cathode material (La1−x Sr x MnO3) using a reactive DC thermal plasma spray system

The cathode material (La_0.8Sr_0.2MnO₃) in solid oxide fuel cells (SOFCs) was plasma sprayed on mild steel in a reactive DC thermal plasma spray process. This high-speed process of depositing thin films for the components of SOFCs was examined experimentally. The results showed that a coating layer of La_0.8Sr_0.2MnO₃ with a particular porosity could be obtained directly using both prereacted La_0.8Sr_0.2MnO₃ and mixed raw materials (La₂O₃, SrCO₃, and MnO or MnCO₃) as feed materials with or without a pore former. The heat treatment of the plasma coating material at 1073 K (800 °C) for 3 h significantly enhanced the desired crystallization of La_0.8Sr_0.2MnO₃ in the coated material.     

Fracture behavior at crack tip — a new framework for cleavage mechanism of steel

The fracture behavior at crack tip was analyzed based on: (1) observations of fracture surfaces and measurements of local critical parameters for cleavage of three point bending (3PB) precracked specimens of C-Mn steel, (2) detailed observations of configuration changes at precrack tips by metallographic cross sections in specimens unloaded at various applied loads, (3) sophisticated FEM calculations of distributions of stress, strain and triaxiality and simulations of short cracks initiated and extended at precrack tips. The results show that before a critical load (a critical COD) is reached, the crack tip is only blunted and in its vicinity three criteria for cleavage fracture (ϵ_p≥ϵ_pc for initiating a crack nucleus; σ_m/σ_e≥T_c for preventing the crack tip from blunting; and σ_yy≥σ_f for propagating the crack) are satisfied in different regions separated from each other, the nucleated cracks cannot be propagated and cleavage fracture cannot be triggered. As the applied load increases higher than this critical load, a short crack is initiated and extended at the precrack tip and then is blunted again. The plastic strain and the stress in front of the precrack are redistributed. While the plastic strain remains in front of the tip, the stress triaxiality is rebuilt. At a second critical load, the regions where the three criteria are satisfied overlap each other and a cleavage crack can be nucleated and propagated. The minimum distance for cleavage may be determined by the beginning of the overlapping of the mentioned regions. Combined with the three criteria previously suggested, the fracture behavior at crack tip and the corresponding changes of driving forces (ϵ_p, σ_m/σ_e,σ_yy) provide a complete physical model for cleavage of steels in the local scale.     

Dye adsorption—a new technique for the color metallography of oxide scales on steels

The possibility of producing contrast between phases in the oxide scale on steels by dye adsorption techniques is explored. A study of the major variables involved shows that acceptable etching and color contrast can be achieved on both scale and metal substrate simply by immersing the polished specimen in a suitable dye solution. The applicability of the technique to the metallography of metallic as well as nonmetallic surfaces is indicated.     

A study of the wear resistance of a TiC–SiC composite ceramic material prepared by spark plasma sintering

The effect of the silicon carbide content and the spark plasma sintering temperature on the porosity and wear resistance of a composite material is determined. An increase in the silicon carbide content leads to an increase in the porosity from 4 to 12%. The temperature effect on the porosity becomes apparent above 1200°C. The material consists of a matrix represented by titanium carbide grains with a size of 1–3 μm and silicon carbide grains with a size of 10–30 μm. It is found that the TiC–20%SiC compound exhibits the highest wear resistance, which is 12 times higher than that of hot-pressed silicon carbide.     

Vanadia-based coatings of self-repairing functionality for advanced magnesium Elektron ZE41 Mg–Zn–rare earth alloy

A smart vanadia protective coating of self-repairing functionality that has proven to provide superior corrosion resistance for several magnesium and aluminum alloys has successfully been designed by our group. A newly developed series of magnesium alloys, namely ZE41 alloy, has recently been proposed for automotive, electronics and aerospace applications. The advanced ZE41 alloy possesses very low density, high specific strength, and good castability and weldability characteristics compared to aluminum and steel based alloys. However, the corrosion resistance of ZE41 alloy in the presence of corrosive chloride environment is relatively low. The possibility of utilizing such coatings to add self-repairing functionalities to ZE41 alloy was discussed in this paper. The electrochemical corrosion behavior of the vanadia coatings over ZE41 alloy was investigated in 3.5% NaCl solution using EIS, linear polarization and cyclic voltammetry techniques. The optimum conditions for obtaining protective vanadia coatings of self-repairing abilities and improved localized corrosion resistance were determined. Surface examination of the coatings was investigated using SEM-EDS and macroscopic imaging.     

Chemical pretreatments at surface of WC—6% Cof for diamond coatings

The WC-6% Co(mass fraction)substrate surfaces were chemically pretreated with the two-step etching method,using Murakami reagent for 3-7min,and then an ψ（HNO3):ψ（HCl)=4:1 soluton for 1-15min,Diamond films were deposited on the substrates by a hot-filament chemical vapor deposition reactor.The results show that the Co content of the substrate surfaces can be reduced from 6% to 0.12% within the etching depth of 5-10μm ,the surface roughness of the substrates is increasesd up to Ra=1.0μm,as well as th substrates hardness is decresased from HRA 89..to HRA 84.2 after the two-step etching,A slight preference towarde {111} orientation can be observed from the XRD patterns and SEM micrograph of diamond film on WC-6%Co sample.The morphology of small rice-like ballas diamond was observed on the WC－6％Co substrates.A typical Raman spectrum with a sharp peak at 1332cm^-1 for the diamond film indicates that the deposited films are good-quality polycrystalline diamond.The indentation testing shows that the adhesion between diamond film and the substrate after HF CVD deposition is good.     

The effect of platinum on Al diffusion kinetics in β-NiAl: Implications for thermal barrier coating lifetime

First-principles density functional theory calculations are used to study Al diffusion in β-NiAl. The activation energy and diffusion constant pre-exponential factors are calculated for five previously postulated Al diffusion mechanisms: next-nearest-neighbor Al jumps, the triple defect mechanism and three variants of the six-jump cycle mechanism beginning with an Al vacancy. We predict that the triple defect mechanism has the lowest activation energy and is the mechanism by which Al diffusion occurs in NiAl. In order to elucidate why Pt has a beneficial effect on thermal barrier coating lifetime, the effect of Pt on each of these mechanisms is also examined. In all cases, Pt decreases the diffusion activation energy, which should enhance Al diffusion in the coatings.     

Cyclic oxidation resistance of Ni–Al alloy coatings deposited on steel by a cathodic arc plasma process

The cyclic oxidation resistance of nickel-aluminide coatings deposited on steel using a cathodic arc plasma (CAP) process has been investigated. Our results show that nickel-aluminide films can be successfully deposited on carbon steel and stainless steel substrates by this process; NiAl₃ is the major phase in the deposited films. The thermal cycling behaviour suggests that such coatings can resist oxidation through physical blocking of oxygen, either by the coating itself or by the aluminium oxide scale subsequently formed in-service. Aluminium diffusion inwards to the substrate may also be beneficial to the thermal oxidation resistance. The coating protects stainless steel substrate materials at 500°C by transforming the NiAl₃ phase into NiAl, producing aluminium oxide on the open substrate surface. At 800°C, oxide flaking is suppressed by the trace amounts of nickel or aluminium which have partially diffused into the substrate.     

Low porosity and fine coatings produced by a new type nozzle of high velocity arc spray gun

The new designed high-velocity arc spray gun with three different nozzles is developed to match the DZ400 arc spray system, which can produce the coatings with the structure of superfine and low porosity. This system can be used to spray three normal wires such as 4Cr13, FeCrAl and 7Cr13 （flux cored wires）. Using the scanning electron microscope （ SEM ） to analyze shape and particles size that sprayed by the nozzles with different parameters, as well as with the S-3500N SEM and the energy spectrum analytic （ESA） instrument to identify the content of the oxides, porosity and thickness of the coatings, we get the result that the porosity in the coatings of solid wire is less than 3%, of the flux-cored wires is less than 5%, and the distribution of the coatings sprayed by the nozzle with secondary supplementary airflow is typically shown in the form of highdensity lameUarsplat structure and the average lamellar thickness is around 5μm.     

Microstructure and thermal stability behavior of a rapidly solidified Al—Ti—Fe—Cr alloy

A rapidly solidified Al-2.5Ti-2.5Fe-2.5Cr(mass fraction in%) alloy was prepared by melt spinning.Asquenched and as-annealed microstructures were studied by X-ray diffractometry(XRD),transmission electron microseopy(TEM),high-resolution transmission electron microscopy(HREM) and energy dispersive spersive spectrum(EDS) analysis,The microhardness of the alloy at different annealing temperatures was measured.The results obtained indicate that the microhardness of the rapidly solidified Al-2.5Ti-2.5Fe-2.5Cr alloy does not vary with differnet annealing temperatures.The as-quenched microstructure of the alloy includes two kinds of dispersed primary phases:Al3Ti and Al13(Cr,Fe)2,After annealing at 400℃ for 10h,the stable phase Al13Fe4 appears in the microstructure.