The development of new bond coat compositions for thermal barrier coating systems operating under industrial gas turbine conditions

Environmental and economic issues are driving the development of increasingly efficient gas turbines. An important step in achieving this is to engineer components which can operate with longer lifetimes and at higher metal temperatures. Inlet temperatures for gas turbines now exceed the melting temperatures of nickel-based superalloys (i.e. 1300–1350 °C). The use of advanced air cooling systems coupled with thermal barrier coatings (TBCs) reduces the temperature of the underlying superalloy substrate. The bond coating, an important part of the TBC system, oxidizes to form a slow growing protective oxide layer, while also providing adhesion between the ceramic topcoat and the substrate. NiCoCrAlY overlay coatings are some of the most commonly used bond coatings for industrial gas turbines and extensive research has been undertaken over many years to find the best bond coating composition.This paper reports upon the production of new, model bond coatings with a wide range of different compositions. The focus is on their oxidation behavior at a temperature typically experienced by bond coatings on industrial turbine blades (950 °C). A physical vapor deposition technique, magnetron sputtering, has been used to deposit a range of Ni–Co–Cr–Al coatings onto 10 mm diameter sapphire substrates. This was achieved through co-sputtering two targets: a Ni–10%Cr, Ni–20%Cr, Ni–50%Cr, Ni–20%Co–40%Cr or Ni–40%Co–20%Cr target and a pure Al target. About a hundred samples with varying compositions were produced by this method. The coatings were then oxidized in air for 500 h at 950 °C.All samples were assessed by measuring the change in coating thickness, using pre- and post-exposure metrology only, and also the change in specimen weight. This approach has shown that magnetron sputtering successfully deposited 20 to 30 μm thick coatings and allowed the calculation of oxide growth rates. Energy dispersive X-ray (EDX) analysis was used to characterize the exact composition of each sample. Additionally, X-ray diffraction (XRD) has been used to identify the major oxides formed during exposure. The selective growth of protective Cr₂O₃ or Al₂O₃ or other less protective mixed oxides (depending on the initial coating composition) was observed. This influenced the oxide scale growth rate, indicating which coatings produced more protective oxides and allowing future optimization of the bond coating composition, for service within the turbine section of industrial gas turbines to be planned.     

Life time dependency on the pre-coating treatment of a thermal barrier coating under thermal cycling

A thermal barrier coating system consisting of the single crystalline Ni-based superalloy CMSX 4, a Pt aluminide bond coat and an EB-PVD processed ceramic top coat was thermally cycled in order to study the influence of three different treatments prior the deposition of the ceramic top coat. Besides the standard treatment, one type of treatment was annealing in vacuum, while the other was annealing in an O containing ArH atmosphere; in both cases for 4 h at 1080 °C.Compared to the standard treatment, annealing in vacuum almost doubled and annealing in ArH atmosphere almost tripled the cyclic life time of the ceramic coating. The improvement was related to the creation of a defined alumina scale before and during TBC deposition.     

Finite-element evaluation and improvement of a test procedure for coating shear bond strength determination

In the research and development of thermal spraying coating systems for wear-resistance applications, it is essential to determine coating/substrate bond strength with a proper test procedure. This article describes mechanical evaluations of a widely adopted coating shear bond strength test procedure conducted via the finite-element method. Analyses of the stress distributions on the coating/substrate interface indicate that significant errors will be introduced if the standard test procedure is used to determine coating shear bond strength. A new test procedure with modified specimen geometry is proposed and then verified for effectiveness.     

Influence of thermal cycle on surface evolution and oxide formation in a superalloy system with a NiCoCrAlY bond coat

A material system comprising a NiCoCrAlY bond coat deposited on a superalloy substrate has been subjected to thermal cycling. The assessment contrasts the influence of simple and stepwise (intermediate temperature hold) thermal cycles on the undulation of the surface and on the evolution of residual compressive stress in the thermally-grown oxide (TGO) layer. Stress-mapping of the TGO was performed using luminescence spectroscopy. Regions of interest were cross-sectioned using focused ion beam techniques to enable sub-surface examination by scanning electron microscopy. The investigation revealed that the surface develops undulations upon stepwise cycling, but not for either simple cycling or isothermal exposure (at comparable TGO thickness). This behavior has been related to the rapid creep displacements occurring in the bond coat during the intermediate temperature hold, because it is subject to large stress at this temperature. When the undulations attain sufficient amplitude, creep cracks form along the ridges, causing the stress to locally relax. For situations that do not cause undulations, areas of reduced residual compression appear in the TGO. Yttria-rich particles were invariably present in these regions.     

Effects of coating thickness and residual stresses on the bond strength of ASTM C633-79 thermal spray coating test specimens

Wire-arc-sprayed nickel-aluminum is widely used in the aircraft industry for dimensional restoration of worn parts and as a bond coat for thermal barrier coatings and other top coats. Some repair applications require thick coatings, which often result in lower bond strength. A mechanism being investigated to ex-plain this decrease in bond strength is the free edge effect, which includes both coating residual stresses and coating thickness. The layer-removal method was used to determine experimentally the residual stresses in wire-arc-sprayed nickel-aluminum coatings of different thicknesses. Bond strength evalu-ations were performed using an improved ASTM C 633-79 test specimen. Finite-element analysis and fracture mechanics were used to investigate the effects of coating thickness and residual stress state on coating bond strength.     

LY12铝合金微弧氧化/树脂填料复合涂层的组织与防热性能

为提高LY12铝合金的隔热与抗火焰烧蚀性能,采用微弧氧化及涂覆复合工艺在其表面制备了底层微弧氧化/外层树脂填料复合涂层。用自制隔热装置及氧-乙炔烧蚀装置分别评价涂层的隔热与抗烧蚀性能。隔热测试表明试样暴露在450℃恒温6 min时,微弧氧化/莫来石空心微球树脂复合涂层隔热温度为245℃。微弧氧化/微球树脂复合涂层在2200℃氧-乙炔火焰下烧蚀持续25 s,质量烧蚀率为0.0425 g.s-1,线烧蚀率为0.0478 mm.s-1,复合涂层烧蚀区域背面均无变化,烧蚀条件下微弧氧化/微球树脂涂层隔热温度为1859℃,其隔热性能与450℃静态隔热测试结果一致。铝合金表面微弧氧化/树脂填料复合涂层表现出优异的隔热与抗烧蚀性能。     

The in situ synthesis and wear performance of a metal matrix composite coating reinforced with TiC-TiB2 particulates, formed on Ti-6Al-4V alloy by a low oxygen partial pressure fusing technique

A metal matrix composite coating reinforced with TiC-TiB₂ particulates has been successfully fabricated utilizing the in situ reaction of Al, Ti and B₄C by the low oxygen partial pressure fusing technique to improve the wear resistance of Ti-6Al-4V alloy. The results show that increasing the B₄C content is adverse to forming the coating for the formation of interfacial stress; however, the addition of TiC powder as a diluent can favor the formation of this coating and the addition of small amounts of Y₂O₃ can greatly improve the adhesion of the coating. After a pin-on-disc wear test, the wear mass loss of the coating is only about 1/12 that of the Ti-6Al-4V alloy and the wear mechanism of coating is a mixed type of slight peeling-off, adhesion and abrasion.     

Influence of ceramic thickness on residual stress and bonding strength for plasma sprayed duplex thermal barrier coating on aluminum alloy

NiCoCrAlY/8wt.%Y₂O₃–ZrO₂ coating was plasma sprayed on aluminum alloy to evaluate the effect of ceramic thickness on residual stress and bonding strength. A new stress calculation method based on Stoney equation and substrate-removal technique was proposed. Stress in both bond coat and ceramic was studied. With the increase of ceramic thickness, the residual stress in both layers was firstly compressive then turned tensile. The large thermal expansion coefficient of the substrate played an important role in residual stress formation when the ceramic was thin. However, the intrinsic deposition stress took a dominant position when the ceramic coating turned thicker. The bonding strength decreased and the location of the fractured surface moved toward the ceramic surface. The moving of the surface was mainly resulted from the variation of stress gradient and the weakness of high porosity zone near the bond coat–ceramic interface.