Effect of increased water vapor levels on TBC lifetime with Pt-containing bond coatings

To investigate the effect of increased water vapor levels on thermal barrier coating (TBC) lifetime, furnace cycle tests were performed at 1150 °C in air with 10 vol.% water vapor (similar to natural gas combustion) and 90 vol.%. Either Pt diffusion or Pt-modified aluminide bond coatings were applied to specimens from the same batch of a commercial second-generation single-crystal superalloy and commercial vapor-deposited yttria-stabilized zirconia (YSZ) top coats were applied. Three coatings of each type were furnace cycled to failure to compare the average lifetimes obtained in dry O₂, using the same superalloy batch and coating types. Average lifetimes with Pt diffusion coatings were unaffected by the addition of water vapor. In contrast, the average lifetime of Pt-modified aluminide coatings was reduced by more than 50% with 10% water vapor but only slightly reduced by 90% water vapor. Based on roughness measurements from similar specimens without a YSZ coating, the addition of 10% water vapor increased the rate of coating roughening more than 90% water vapor. Qualitatively, the amount of β-phase depletion in the coatings exposed in 10% water vapor did not appear to be accelerated.     

钴基超合金铝化物涂层的高温氧化寿命

通过分析钴基超合金外扩散型铝化物涂层的高温氧化及退化行为 ,并采用近似方法推算 ,得出了这类涂层氧化和退化与时间的关系。研究表明 ,涂层外侧退化主要由氧化反应驱动力所控制 ,涂层的外侧退化速率正比于其氧化速率 ;而涂层的氧化寿期近似正比于其主体层厚的平方。运用氧化退化与时间的关系式 ,进行简便的辅助实验 ,即可预测出这类涂层的高温氧化寿命。     

Laser cladding of titanium alloy coating on titanium aluminide alloy substrate

1　INTRODUCTIONInthedesignandmanufactureofaviationindus try ,differentpartsoftheaircraftengineshouldmeetthedifferentrequirementsonoperatingtemperatureandserviceperformance .Thesoundbondofdissimi laralloysisthekeytechniquefordevelopingnovele quipmentsandimprovingtheirintegralperformance .Lowdensityandexcellenthigh temperaturepropertiesofTiAlalloysmakethem promisinghigh temperaturestructuralmaterials .Successfuljoiningofthesematerialswillincreasetheirutilityinengineer ing[1,2 ] .Somejoinin…     

Ceramic coating on ceramic with metallic bond coating

The change in structure and adhesion strength of the interface by heating in air has been investigated for a plasma- sprayed alumina coating on a ceramic substrate with a 50Ni- 50Cr alloy bond coating. A veined structure composed of NiO, NiCr ₂O₄, and NiAl₂O₄ oxides grew from the bond coating into cracks or pores in the top coating and the alumina substrate after heating at 1273 K for 20 h in air. The NiAl₂O₄ spinel may have formed by the oxidization of nickel, which subsequently reacted with the alumina coating or the substrate. The mechanism of the penetration of the spinel oxides into the cracks or pores is not clear. The adhesion strength of the coating is increased to about 15 MPa after heating at 1273 K for 20 h in air, compared to an as- sprayed coating strength of only 1.5 MPa.     

The role that bond coat depletion of aluminum has on the lifetime of APS‐TBC under oxidizing conditions

Bond coat oxidation as well as bond coat depletion of Al are still believed to be a major degradation mechanism with respect to the lifetime of thermal barrier coating (TBC) systems. In this study the top coat lifetime is described as being limited by both bond coat depletion of Al and mechanical failure of the top coat. The empirical results are introduced by considering three spallation cases, namely, Al depletion failure, thermal fatigue failure, and thermal aging failure. Al depletion failure occurs when the Al content within the bond coat reaches a critical value. In this paper bond coat depletion of Al is modeled by considering the diffusion of Al into both the thermally grown oxide (TGO) and substrate. The diffusion model results are compared to Al concentration profiles measured with an electron beam microprobe. These measured results are from oxidized air plasma sprayed TBC systems (APS‐TBC) with vacuum plasma sprayed (VPS) bond coats for exposures up to 5000 h in the temperature range of 950–1100 °C. This paper focuses on the Al depletion failure and how it relates to top coat spallation.     

一种镍基高温合金基体表面的新型高辐射涂层

在高温条件下由于高辐射涂层辐射传热的有效性,引起了广泛的兴趣。本文通过空气喷涂方法制备了一种新型的高辐射涂层。涂层主要成分包括无定型硼硅玻璃粉,Mg2B2O5, MoSi2 和SiB4。涂层的厚度约50 μm。通过实验发现,涂层具有优良的抗热震性能（能够经受超过100次的从950℃到水的冷热循环）。在950℃时涂层的平均辐射率为0.905?.024。经过100次的热循环后,涂层的辐射系数有轻微的减小。     

Transient thermography testing of unpainted thermal barrier coating (TBC) systems

Test piece surfaces are sometimes coated with a black, energy absorbing paint before transient thermography is applied. This practice is not acceptable to some thermal barrier coating (TBC) manufacturers and servicers of these systems since thermal barrier coatings are porous so the paint contaminates the coating and it is very difficult and costly to remove. Unfortunately, unpainted TBC surfaces have low emissivity, and after service their colour is usually uneven. The low emissivity gives low signal levels and also problems with reflections of the incident heat pulse, while the variation in emissivity over the surface gives strong variation in the contrast obtained even in the absence of defects. Additionally, the TBC is translucent to mid-wavelength IR radiation which negatively affects the location of disbonds based on the thermal responses. This paper investigates the effects of uneven discolouration of the surface and of IR translucency on the thermal responses. It has been shown that unpainted TBC systems can be inspected reliably by using higher power flash heating equipment assembled with an IR glass filter and a long wavelength IR camera. The paper also shows that the problem with uneven surface emissivity can be overcome by applying 2nd time derivative processing of the log–log surface cooling curves.     

Parameters affecting TGO growth rate and the lifetime of TBC systems with MCrAlY‐bondcoats

In the present work different parameters which affect the oxide growth on MCrAlY‐bondcoats have been studied, in an attempt to find a reliable way to define the critical oxide thickness to failure of an EB‐PVD thermal barrier coatings (TBC). It was found that the variation of selected parameters such as oxidation temperature, surface roughness, and bondcoat thickness certainly alters the thermally grown oxide (TGO) growth rate. However, simultaneously the morphology, composition, and/or microstructure of the oxide are changed, which can affect the critical TGO thickness to failure in a TBC system. In contrast, the variation of the oxygen partial pressure of the oxidizing atmosphere led to different TGO growth rates without significantly changing the oxide morphology and composition. Comparing the TGOs grown at different rates in the specimens exposed to high pO₂ and low pO₂ atmospheres, it was estimated that at failure the oxide scales in both specimens have reached a similar critical thickness.     

真空热处理双粘结层热障涂层抗氧化性能

传统单层结构粘结层热障涂层抗氧化性能不足寿命短,采用超音速火焰喷涂(high velocity oXy-fuel,HVOF)和大气等离子喷涂(atmosphere plasma spray,APS)制备双层结构粘结层,对粘结层进行真空热处理,研究热障涂层的抗氧化性能.结果 显示,经过1050℃×3 h真空热处理,粘结层...     

Effect of modification by Pt and manufacturing processes on the microstructure of two NiCoCrAlYTa bond coatings intended for thermal barrier system applications

Few studies have already shown that Pt influences the diffusion of aluminium and therefore the microstructure of β-NiAl or γ-Ni/γ′-Ni₃Al materials. Besides, several works have revealed that the addition of Pt to MCrAlY (M = Ni and/or Co) improves the oxidation/corrosion behavior of the material. Nevertheless, very few data have been published on the microstructure of such modified MCrAlYs. Then, the present work deals with the addition of Pt to two NiCoCrAlYTa coatings that differ by their manufacturing process. Characterization is carried out in order to understand the influence of Pt diffusion but also the effect of the manufacturing process on the final microstructure. The collected data from XRD, SEM, EDS and TEM analyses reveal that an Al uphill diffusion occurs during heat treatment due to the presence of the Pt layer. The Al diffusion from the NiCoCrAlYTa bulk to the Pt-rich surface is so extensive that no more β-phase remains within the core of the coating. Pt may also dissolve TaC, precipitates largely present in the non-modified NiCoCrAlYTa coatings. In addition to Pt, the microstructure of the NiCoCrAlYTa prior to Pt deposition and heat treatment, dependent on the NiCoCrAlYTa manufacturing process, greatly influences the final microstructure.     

Improvement of the adhesion of a ceramic coating on a ceramic substrate

The structure and adhesion of an alumina coating on a ceramic substrate with NiCrAlY alloy bond coating was investigated by heating at 1573 and 1673 K in the air. Phases of NiO, NiCrO₃, NiAl₂O₄, αAl₂O₃, and Ni were revealed in a 100 μm thick bond coating on heating at 1573 and 1673 K. A veined structure was also detected in the coating heated at 1573 K. The adhesion strength of the coating was improved and reached approximately 20 MPa on heating at 1573 and 1673 K for 14.4 ks in air although the strength of the as-sprayed coating was only 2 MPa. The improvement of adhesion strength may arise from the formation of NiAl₂O₄ with a spinel structure at the interfaces of the top coating/bond coating/substrate coating system. The adhesion strength of the coating improved on decreasing the bond coating thickness and reached approximately 45 MPa for a 20 μm thick bond coating which was heated at 1673 K. Only NiAl₂O₄ oxide was formed in the bond coating.     

Comparison of classical oxidation and laser oxidation of a chromium PVD coating on a pure-iron substrate

The oxidation of a chromium PVD coating on pure iron by a continuous 5-kW CO₂ laser beam in pure oxygen at 700°C for 20 min was compared with classical furnace oxidation. Laser oxidation induces faster oxidation kinetics, especially at the beginning of oxidation, without modifying the oxide nature (Cr₂O₃) and morphology. Oxygen-isotopic-exchange tests show that oxygen grain-boundary diffusion does not depend on the oxidation conditions, at least after 12 min oxidation. The effec of the laser treatment is discussed with respect to oxide nucleation, metastable-oxide formation, and oxide-formation-entropy evolution. The oxidation kinetics follow a parabolic law, and the oxide-growth mechanism was attributed to countercurrent oxygen and chromium diffusion along grain boundaries. Oxygen diffusion occurred by oxygen interstitials. The oxidation constant calculated from grain-boundary diffusion in the chromia scale is smaller than the experimental oxidation constant, suggesting the presence of particular short circuits (e.g., microcracks).     

基于第一性原理揭示活性元素铪对热障涂层关键界面的强化机理

本文总结回顾作者近年来从第一性原理出发,以应用广泛的高温热障涂层中γ-Ni(Al)/α-Al2O3关键界面为基础,通过建立合理的热力学模型,结合电子密度泛函计算,针对界面结合强度与温度、原子化学配比和活度等的相关性所开展的一系列理论研究实践.结果表明在感兴趣的温度区间内(1300～1600K),界面平衡相为富Al相,但靠近理想化学配比相的相界;富Al相界面的结合强度较高,约3倍于理想化学配比相界面的结合强度.杂质S可向界面强烈富集,并严重削弱界面强度的60%～70%;活性元素Hf具有在基体中有效钉扎S、直接参与界面成键和在界面处置换S的3种界面强化效应.     

Structure and cyclic oxidation resistance of Pt, Pt/Pd-modified and simple aluminide coatings on CMSX-4 superalloy

The coatings were prepared by the means of Pt and Pt/Pd galvanizing, followed by vapor phase aluminizing at 1050 °C. Microstructural and phase analysis revealed that all the investigated coatings consisted mainly of β-NiAl phase, however the Pt-modified aluminide coating also contained PtAl₂ phase and pure platinum precipitates. The cross-sectional microstructure of the coatings was zonal and composed of β-NiAl phase zone and the diffusion zone. The Pt modified aluminide coating's cross-section also incorporated an outermost zone consisting of β-NiAl and PtAl₂ phases. The concentration profiles proved that both Pt and Pd contents decrease gradually inwards the modified coatings. Cyclic oxidation tests at 1100 °C proved that Pt/Pd-modified aluminide coatings exhibit the best performance under cyclic conditions. The analysis of oxidation kinetics curves showed that the course of simple aluminide coating's oxidation is slightly different from that of Pt- and Pt/Pd-modified aluminide coatings.     

Effect of alloying element segregation on the work of adhesion of metallic coating on metallic substrate: Application to zinc coatings on steel substrates

A thermodynamic model based on the ‘Macroscopic Atom’ approach is proposed to assess the effect of alloying element segregation on the adhesion of metallic coating on metallic substrate. The interfaces that occur in hot-dip galvanized steels are considered, which include: Zn/Fe, Zn/Fe₂Al₅, Zn/FeZn₁₃, FeZn₁₃/Fe₂Al₅, and Fe₂Al₅/Fe. The effect of the alloying element on the work of adhesion of these interfaces is investigated, which includes Mg, Al, Si, P, Ti, V, Cr, Mn, Fe, Ni, Zn, Nb, Mo, Sn and Bi. Among these elements, Bi, Sn and Mg are predicted to decrease the work of adhesion of the Zn/Fe interface, whereas P, Nb, Mo, V, Ti and Ni tend to enhance this adhesion. The effect of element M (M = Al, Si, Cr, Mn) is positive when it exists in the zinc coating or negative when it occurs in the iron substrate. Among these interfaces, the Fe₂Al₅/Fe interface with a value of 3.8 J m⁻² is the strongest, whereas the Zn/FeZn₁₃ interface with of a value of 1.7 J m⁻² is the weakest. Delamination of the coating upon deformation is predicted to occur along the FeZn₁₃/Fe₂Al₅ and Zn/Fe₂Al₅ interfaces. This agrees with microscopic observations of hot dip galvanized steel after tensile testing.     

Oxidation and alkali sulfate‐induced corrosion of aluminide diffusion coating with and without platinum

A platinum‐free (MDC210) and a platinum‐rich (MDC150L) aluminide diffusion coating applied to a CMSX4 single crystal Ni‐based superalloy were investigated after exposure to sulfate‐induced corrosion and oxidation at 900 °C for a total of 100 h. Weight changes, microstructural, and microchemical analyses of the products were performed by the means of gravimetry, scanning electron microscopy (SEM), energy dispersive X‐ray (EDX), and X‐ray diffraction (XRD). Platinum was found to have a beneficial effect on both the oxidation and corrosion resistance of the coating. In both cases the scale was thicker on the platinum‐free coating. However, the difference in the extent of oxidation of the two coatings was small.     

Effect of substrate temperature on adhesion strength of plasma-sprayed nickel coatings

We plasma-sprayed nickel coatings on stainless steel and cobalt alloy coupons heated to temperatures ranging from room temperature to 650 °C. Temperatures, velocities, and sizes of spray particles were recorded while in-flight and held constant during experiments. We measured coating adhesion strength and porosity, photographed coating microstructure, and determined thickness and composition of surface oxide layers on heated substrates. Coating adhesion strength on stainless steel coupons increased from 10–74 MPa when substrate temperatures were raised from 25–650 °C. Coating porosity was lower on high-temperature surfaces. Surface oxide layers grew thicker when substrates were heated, but oxidation alone could not account for the increase in coating adhesion strength. When a coupon was heated to 650 °C and allowed to cool before plasma-spraying, its coating adhesion strength was much less than that of a coating deposited on a surface maintained at 650 °C. Cobalt alloy coupons, which oxidize much less than stainless steel when heated, also showed improved coating adhesion when heated. Heating the substrate removes surface moisture and other volatile contaminants, delays solidification of droplets so that they can better penetrate surface cavities, and promotes diffusion between the coating and substrate. All of these mechanisms enhance coating adhesion.     

Evaluation of aluminide diffusion coatings for thermal cyclic oxidation protection of a nickel‐base superalloy

Active element modified aluminide diffusion coatings on IN738 substrates were produced by a new route using continuously cast, aluminum alloy wires consisting of Al‐Y, Al‐Ce, Al‐La and Al‐Si‐Y. The cast wires were used as evaporation sources for ion‐vapour deposition followed by diffusion heat treatments to form nickel aluminide coatings. In order to examine the oxidation resistance of these coatings at elevated temperatures, thermal cyclic oxidation experiments were carried out in air at 1050°C. While all coatings were found to provide significant protection, the Al‐La modified coatings provided the greatest resistance to cyclic oxidation. On the other hand, with coatings based on Al‐Si‐Y alloys, while silicon has a strong ability to reduce the outward diffusion of aluminum, the adverse effect of silicon on mechanical properties of the coating, together with the formation of volatile silicon monoxide, led to catastrophic localized oxidation of the protective coatings.     

K4104镍基高温合金渗Al-Si涂层氧化动力学

用无机盐料浆法在K4104镍基高温合金表面渗AlSi涂层。改变粘结剂中CrO3的添加量制备了三种不同成分的AlSi涂层。采用静态氧化增重法进行1000℃×200h抗氧化性试验。用扫描电镜(带能谱分析仪)对涂层表面形貌及成分进行观察分析,并绘制氧化动力学曲线和拟合氧化动力学方程。结果表明,料浆渗AlSi涂层改善了合金K4104的抗高温氧化性及延长了其使用寿命。粘结剂中添加少量CrO3制备的AlSi涂层具有最佳的抗高温氧化性。三种AlSi涂层都属于完全抗氧化级。