Cyclic-Oxidation Behavior of Multilayered Pt/Ru-Modified Aluminide Coating

Multilayered Pt/Ru modified aluminide coating for thermal barrier coating(TBC) systems has been investigated.2 μm Pt+2 μm Ru+2 μm Pt was first deposited on nickel-base superalloy DZ125 by electrodeposition,and then the coating was treated by annealing and a conventional pack-cementation aluminizing process.The cyclic oxidation tests were carried out at 1423 K in air.It was found that the thermal cyclic oxidation resistance of Pt/Ru-modified aluminide coating was comparable to that of Pt-modified aluminide coating,which was much better than simply aluminized DZ125.The addition of Ru to Pt-modified aluminide coating increased the resistance to rumpling.The microstructures and phase constitutions of the coating before and after oxidation were investigated.     

Benefits of Zr addition to oxidation resistance of a single-phase (Ni,Pt)Al coating at 1373 K

A single-phase (Ni,Pt)Al coating with lean addition of Zr was prepared by co-electroplating of Pt-Zr composite plating and subsequent gaseous aluminization treatments. Isothermal and cyclic oxidation behavior of the Zr-doped (Ni,Pt)Al coating samples was assessed at 1373 K in static air in comparison with plain nickel aluminide (NiAl) and normal (Ni,Pt)Al coatings. Results indicated that Zr-doped (Ni,Pt)Al coating demonstrated a lower oxidation rate constant and reduced tendency of oxide scale spallation as well as surface rumpling, in which the enhanced oxidation performance was mainly attributed to the segregation of Zr at oxide scale grain boundaries and the improved Young’s modulus of the coating. Besides, the addition of Zr effectively delayed oxide phase transformation of Al₂O₃ from θ phase to α phase in the early oxidation stage and coating degradation of β-NiAl to γ'-Ni₃Al in the stable oxidation stage.     

Damage mechanisms of coated systems under thermomechanical fatigue

Abstract

The damage mechanisms of several kinds of coatings on a single crystal nickel base superalloy under thermomechanical fatigue (TMF) are described. The systems investigated were diffusion platinum aluminide coatings, Co–Ni–Cr–Al–Y overlay coatings, and thermal barrier coatings (TBCs). The TMF experiments were carried out on hollow specimens over a temperature range from 300 to 1050°C, at strain ranges Δ? = 0·5 and 0·7%, and at a strain ratio R = -∞. No coating cracking was found for the platinum aluminide coating. Instead, specimens failed owing to oxidation induced crack initiation from the uncoated inner surface of the hollow testpieces, although coating surface roughening caused by non-homogeneous oxidation was observed. For the overlay coating, roughening in terms of coating rumpling and coating cracking occurred, resulting in reduced TMF life. For TBC specimens with a thin ceramic coating processed by electron beam–physical vapour deposition (EB–PVD), TMF life was comparable with that of specimens with the overlay coating. Failure once again occurred owing to Co–Ni–Cr–Al–Y bond coat cracking and propagation into the substrate. In this system, some bond coat cracks penetrated through the top ceramic coat although others did not. In contrast with specimens coated with the overlay alone, no significant rumpling on the bond coat surface was observed and the crack density was low.     

Evaluation of cyclic oxidation of thermal barrier coatings exposed to NaCl vapor by finite element method

The cyclic oxidation of NiCrAlY + YSZ coating exposed to NaCl vapor has been investigated under atmospheric pressure at 1050 °C, 1100 °C and 1150 °C. The result showed that the cyclic oxidation life of NiCrAlY + YSZ coating in the presence of NaCl vapor was shortened compared with that in air. The failure of the TBC exposed to NaCl vapor occurred within the top coat and close to the YSZ/thermal growth oxide (TGO) interface. A finite element analysis was employed to analyze the stress distribution in the coatings. The computed result showed that maximum stresses occurred at the interface between the bond coat and TGO near the edge of the sample and the increased thickness of TGO caused the value of stress in TGO/YSZ interface to increase. The comparison of the maximum stresses indicated that the spinel TGO resulted in significantly higher stresses than Al₂O₃ TGO. This implies that the formation of spinel plays a dominant role in shortening the coating cycling lifetime.     

Thermal cycling behaviour of thermal barrier coating systems based on first- and fourthgeneration Ni-based superalloys

Abstract

This study deals with the cyclic oxidation behaviour of thermal barrier coating systems. The systems consist of an yttria-stabilised zircona ceramic top coat deposited by EB-PVD, a β-(Ni,Pt)Al bond coat and a Ni-based superalloy. Two different superalloys are studied: a first-generation one and a fourthgeneration one containing Re, Ru and Hf. The aim of this work is to characterise the microstructural evolution of those systems and to correlate it to their resistance to spallation. Thermal cycling is carried out at 1100°C in laboratory air, with the number of cycles ranging between 10 and 1000. Each cycle consists of a 1 h dwell followed by forced-air cooling for 15 min down to room temperature. Among the main results of this work, it is shown that the MCNG-based system is significantly more resistant to spallation than the AM1-based one. Up to 50 cycles, both systems exhibit similar oxidation rate and phase transformations but major differences are observed after long-term ageing. In particular, a Ru-rich β-phase is formed in the bond coat of the MCNG-based system while the AM1- based one undergoes strong rumpling of the TGO/bond coat interface due to the loss of the thermal barrier coating.     

镀锌钢板蒸镀镁层的耐腐蚀机理

采用真空蒸镀技术在镀锌钢板上蒸镀镁,制得了蒸镀镁层,将其在5%NaCl溶液中浸泡,运用XRD,SEM和电化学方法等对其腐蚀产物进行了分析.对2种材料的的腐蚀行为进行了研究,探讨了其耐腐蚀机理.结果表明:镀锌钢板蒸镀镁层是以MgZn2和Mg2Zn11.金属间化合物的形式存在;其腐蚀产物是以致密且具有良好绝缘性的ZnCl2·4Zn(OH)2·H2O和致密的Zn4CO3(OH)6·H2O为主体,而纯锌镀层的腐蚀产物是以疏松且具有N型半导体性质的ZnO为主体;蒸镀镁层的形成可以抑制Zn(OH)2向ZnO的转化,能促进Zn(OH)2向ZnCl2·4Zn(OH)2·H2O和Zn4CO3(OH)6·H2O的转化,后二者能够牢固地覆盖在基体表面,从而延缓了锌的腐蚀进程;在宏观上表现为镀锌钢板蒸镀镁层腐蚀产物相对于纯锌镀层更加致密,在电化学行为上表现为镀锌钢板蒸镀镬层具有更小的腐蚀电流和更高的极化电阻.     

Characterization of the degraded microstructures of a platinum aluminide coating

The degradation of a platinum modified aluminide (PtAl) coating and a CMSX-4 superalloy substrate were investigated for cyclic and quasi-isothermal heating to 1200 °C. To accelerate the oxidation of the specimens, the thermally grown oxide (TGO) was removed at 10 h intervals. For up to 80 h of exposure, comparisons of specimens with periodic oxide removal and those without oxide removal were made. Qualitatively, the major changes to the bond coat were associated with phase changes from β-(Ni,Pt)Al to γ′-(Ni₃Al) and precipitate coarsening. This evolution was quantified through backscatter scanning electron microcopy and image analysis. With instrumented indentation, the room temperature coating modulus was also measured at 10 h intervals. Additional results include observations of differences in waviness of the bond coat surface for cyclic, quasi-isothermal, and true-isothermal heating, and observations of rafting near the substrate/coating interface. The differences between cyclic and quasi-isothermal heating indicate that stresses associated with cooling and heating significantly alter microstructural evolution.     

Oxidation behavior of aluminide coatings on carbon steel with and without electrodeposited Ni-CeO2 film by low-temperature pack cementation

A CeO₂-dispersed aluminide coating was fabricated through aluminizing the electrodeposited Ni-CeO₂ nanocomposite film on carbon steel using pack cementation method at 700 °C for 4 h. The isothermal and cyclic oxidation behavior of the CeO₂-dispersed aluminide coating at 900 °C, including the growth of oxide scale and the microstructure of the coatings, have been investigated comparing with the aluminide coating on carbon steel. The results show enhanced oxidation performance of the CeO₂-dispersed aluminide coating, which is concerned with not only CeO₂ effect on the microstructure and oxidation, but also decreased interdiffusion between the aluminide and the Ni film. The CeO₂ benefit effects and interdiffusion are discussed in detail.     

Ca~(2+)和臭氧对A3碳钢表面磷化膜的影响

在低温磷化条件下, 在磷化液中加入Ca ²⁺并以臭氧作为促进剂, 在A3碳钢表面制备了磷化膜。通过SEM、
XRD、EDS、FT--IR以及腐蚀电化学测试等手段对磷化膜进行表征, 研究了Ca ²⁺和臭氧对磷化膜的结构和性能的影响。结果表明, 在磷化液中添加Ca ²⁺所得磷化膜的质量随着Ca ²⁺浓度的提高而减小, 添加Ca ²⁺可细化磷化膜的晶粒、提高磷化膜的致密度和耐蚀性能; 溶解在磷化液中的臭氧具有细化磷化膜晶粒和促进晶粒生长的作用, 能大幅提高磷化膜晶粒的形核率和磷化膜的主体形成速度。当磷化液的pH=2.70、Ca ²⁺浓度为1.8 g/L、臭氧含量为2.50 mg/L时, 磷化膜的质量为5.46 g/m², 其耐硫酸铜点滴腐蚀时间超过122 s, 在5% NaCl溶液中的腐蚀电流为0.50 μA/cm²。     

Oxidation behavior of aluminide coatings on carbon steel with and without electrodeposited Ni–CeO2 film by low-temperature pack cementation

A CeO₂-dispersed aluminide coating was fabricated through aluminizing the electrodeposited Ni–CeO₂ nanocomposite film on carbon steel using pack cementation method at 700 °C for 4 h. The isothermal and cyclic oxidation behavior of the CeO₂-dispersed aluminide coating at 900 °C, including the growth of oxide scale and the microstructure of the coatings, have been investigated comparing with the aluminide coating on carbon steel. The results show enhanced oxidation performance of the CeO₂-dispersed aluminide coating, which is concerned with not only CeO₂ effect on the microstructure and oxidation, but also decreased interdiffusion between the aluminide and the Ni film. The CeO₂ benefit effects and interdiffusion are discussed in detail.     

The effect of micro-structure on the oxidation behavior of a Ni-based superalloy in water vapor plus oxygen

The oxidation behavior of a Ni-based superalloy with polycrystalline, single-crystalline (SC) and nanocrystalline (NC) structures was studied at 1000 °C in water vapor (20.12 vol.%) plus oxygen. The oxidation behavior of the SC alloy was similar to that of the cast alloy, while nanocrystallization increased the corrosion resistance. The oxides scale on the SC alloy and cast alloy consisted of external TiO₂, Cr₂O₃ and internal Al₂O₃, while only external Al₂O₃ scale formed on the NC coating in water vapor plus oxygen. Meanwhile, the morphologies of oxides scale on three samples are significantly different from one another. The effect of micro-structures on the oxidation behavior of this Ni-based superalloy is discussed.     

Microstructural characterization of the failures of thermal barrier coatings on Ni-base superalloys

Abstract

Typical thermal barrier coating (TBC) systems consist of a nickel-base superalloy substrate coated with a MCrAlY or diffusion aluminide bond coat, onto which is deposited a yttria-stabilized zirconia (YSZ) TBC. The bond coats are usually deposited via diffusion aluminizing processes or low pressure plasma spray processes (LPPS). The YSZ can be deposited by air plasma spraying (APS) or electron beam physical vapor deposition (EBPVD). A layer of thermally-grown oxide (TGO), which is usually alumina, forms between the bond coat and YSZ during TBC deposition and subsequent high-temperature exposure. The conventional wisdom is that APS coatings tend to fail in the YSZ and that EBPVD coatings tend to fail at the interface between the TGO and bond coat. However, current research has shown that the situation is much more complex and that the actual fracture path can be a function of the type of bond coat, the type of high-temperature exposure, and coating process parameters. This paper describes the results of a study of the failure of state-of-the-art EBPVD TBCs deposited on NiCoCrAlY and platinum-modified diffusion aluminide bond coats. The failure times and fracture morphology are described as a function of bond coat type. The failure times were found to be a strong function of temperature for both bond coats. The failure for NiCoCrAlY bond coats was found to initiate at defects in the coating, particularly at the TGO/YSZ interface, but the fracture propagated primarily along the TGO–bond coat interface. The failure times and morphologies for platinum-modified diffusion aluminide bond coats depended strongly on bond coat surface preparation. The mechanisms for failure of the two bond coats are described. Also, the effects of modifications to the bond coats and variations in processing parameters on these mechanisms are presented.     

Composite coatings of CoCrAlY plus platinum

The addition of noble metals has been reported to improve the corrosion resistance of aluminide and overlay coatings for gas turbine airfoils. The production and structure of a CoCrAlY + Pt coating are described. This coating is superior in oxidation and hot corrosion resistance to a coating made of the same CoCrAlY material without platinum. The 0.6% ductility temperature T_d for these two coating is similar. However, mechanical damage greatly raises the T_d value of CoCrAlY + Pt. This is attributed to a favorable residual stress system locally destroyed by the damage.     

Corrosion Behavior of Aerosol Thermal Sprayed ZrO2 Coatings

ZrO2 coatings of 2–7 m thickness were obtained by the aerosol thermal spraying process on 316L stainless steel. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) showed that these films consisted of nanocrystalline ZrO2 aggregates with a monoclinic structure. Corrosion tests were performed in aqueous NaCl (0.5 M) and H2SO4 (20 wt%) using potentiodynamic cyclic polarization curves. Surfaces and cross-sections of coated samples were compared with the uncoated substrate. This evaluation indicated decreased pitting of the steel with coating. The protection against corrosion in NaCl solution depended strongly on coating thickness. No protection was found for thicknesses less than 5 m. Meanwhile, a marked reduction of the current density was produced by thicker coatings that acted as an effective corrosion barrier. Coatings thicker than 5 m also provided significant protection against corrosion in H2SO4.     

Effect of platinum on the oxide-to-metal adhesion in thermal barrier coating systems

An investigation was conducted to determine the role of Pt in a thermal barrier coating system deposited on a nickel-base superalloy. Three coating systems were included in the study using a layer of yttria-stabilized zirconia as a model top coat, and simple aluminide, Pt-aluminide, and Pt bond coats. Thermal exposure tests at 1,150 °C with a 24-h cycling period to room temperature were used to compare the coating performance. Additional exposure tests at 1,000, 1,050, and 1,100 °C were conducted to study the kinetics of interdiffusion. Microstructural features were characterized by scanning electron microscopy and transmission electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction. Wavelength dispersive spectroscopy was also used to qualitatively distinguish among various refractory transition metals. Particular emphasis was placed upon: (i) thermal stability of the bond coats, (ii) thickening rate of the thermally grown oxide, and (iii) failure mechanism of the coating. Experimental results indicated that Pt acts as a “cleanser” of the oxide-bond coat interface by decelerating the kinetics of interdiffusion between the bond coat and superalloy substrate. This was found to promote selective oxidation of Al resulting in a purer Al₂O₃ scale of a slower growth rate increasing its effectiveness as “glue” holding the ceramic top coat to the underlying metallic substrate. However, the exact effect of Pt was found to be a function of the state of its presence within the outermost coating layer. Among the bond coats included in the study, a surface layer of Pt-rich γ′-phase (L1₂ superlattice) was found to provide longer coating life in comparison with a mixture of PtAl₂ and β-phase.     

M38合金表面防护涂层的热腐蚀行为

通过粉末包埋渗的方法在M38高温合金表面制备了4种改性铝化物涂层：NiCr-CrAI、Al-Si、Al-Ti和Co-Al涂层,对比研究了4种涂层在900℃下的涂盐（25％NaCl＋75％Na2SO4质量分数）热腐蚀行为。结果表明,4种改性的铝化物涂层中Al-Ti涂层抗热腐蚀性最好,腐蚀产物连续、致密;Al-Si涂层与Al-Ti涂层抗热腐蚀性相当,但腐蚀表面有局部剥落现象、氧化较严重;Co-Al涂层和NiCr—CrAl涂层抗热腐蚀性能依次降低,其中Co-Al涂层裂纹较严重,NiCr-CrAl涂层氧化较严重。     

NaCl-induced accelerated oxidation of 304 stainless steel and Fe-Mn-Al alloy at 900°C

The high-temperature corrosion behavior of cold-rolled and annealed 304 stainless steel (304SS) and Fe-29Mn-8Al-2.5Si-2Cr-0.74C alloy coated with 0.002 g cm^–2 NaCl initially were studied at 900°C in air. The corrosion kinetics of the two alloys follow the parabolic rate law. The initial NaCl coating accelerates oxidation of these alloys by oxychlorination and chlorination/oxidation cyclic reactions, and catalytic actions of chloride or chlorine are thought to be the principal causes. A bulky, layered scale as well as some intergranular attack is noted on the annealed 304SS, and intergranular attack distributes over the alloy substrate of the cold-rolled 304SS during a 144 h exposure. With the formation of a compact Al₂O₃ scale to decrease further chlorine attack, the corrosion resistance of Fe-Mn-Al alloy is superior to that of 304SS in this study.     

High-temperature degradation mechanism of aluminide coatings on titanium alloys under cyclic oxidation at 750°C

Aluminide coatings prepared on Ti-6Al-4V substrate were able to improve oxidation resistance of the alloy under cyclic oxidation at 750°C both in dry and moist air conditions due to aluminide’s ability to form a stable alumina oxide scale. However, degradation of the coating due to spallation, cracking, internal oxidation and formation of voids with increased cyclic oxidation reduced the lifespan of the coating and the underneath substrate. The main cause of coating degradation for hot-dip specimens is cracks that initiated and propagated perpendicular to the surface. For the plasma spray specimens, the cracks are parallel to the surface. Initiation of cracks in hot-dip coatings are more accredited to residual stresses due to cooling and presence of brittle intermetallic phases TiAl₂ and TiAl. For plasma spray coatings, initiation and propagation of cracks are attributed to presence of entrapped oxides, pores and grain boundaries of the deposited splats whose flattened edges are parallel to the surface of the coating. Presence of water vapor, too, acts as an oxygen carrier and thus promotes oxidation internally, inhibits growth of continuous protective alumina oxide scales and weakens the scale/alloy interfacial toughness. Water vapor therefore accelerates degradation by increasing spallation and cracking rate of the coating.     

Aluminizing nickel foam by a slurry coating process

A simple, one-step slurry coating technique was used to aluminize open cell nickel metal foam at low temperature and short hold-down time. Three slurries of different composition, heat-treated at 650 °C for 2 h, were used to investigate the possibility of developing an aluminide coating on a commercially produced Ni foam. In all cases a dense, well-adhered to the Ni substrate aluminide coating of several μm thickness was produced. The thickness and aluminide phase and composition (NiAl and/or Ni₃Al) of the coating strongly depend on Al content and the mix of activators in the slurry.     

Effect of microstructure on corrosion behavior of TiN hard coatings produced by a modified two-grid attachment magnetron sputtering process

The introduction of an electrically biased two-grid attachment inside a conventional physical vapor deposition process system produces a reactive coating deposition and increases the metal ion-to-neutral ratio in the plasma, resulting in denser and smoother films.The corrosion behavior of titanium nitride (TiN) coatings was investigated by electrochemical methods, such as potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) in deaerated 3.5% NaCl solution. Electrochemical tests were used to evaluate the effect of microstructure on the corrosion behavior of TiN coatings exposed to a corrosive environment. The crystal structure of the coatings was examined by X-ray diffractometry and the microstructure of the coatings was investigated by scanning electron microscopy.In the potentiodynamic polarization test and EIS measurement, the corrosion current density of TiN deposited by the modified two-grid attachment magnetron sputtering process was lower than for TiN deposited by conventional magnetron type and also presented higher charge-transfer resistance values during 240 h total immersion time. The modified process promotes the grain refinement, which yields lower porosity.