Deposition of silicon modified aluminide coatings on nickel base superalloys by pack cementation process

Abstract

The equilibrium partial pressures of vapour species generated in halide activated pack powder mixtures at high temperatures were calculated for a series of compositions using thermochemical analysis tools. The results obtained were applied to identify suitable activators and pack compositions for codepositing Al and Si to form diffusion coatings on nickel base superalloys by the pack cementation process. The calculation results suggested that compositions of the packpowder mixtures activated by CrCl₃.6H₂O may be adjusted to create deposition conditions favourable for codepositing Al and Si, but, those activated byAlF₃or AlCl₃ may only deposit Al.A series of coating deposition experiments were also carried out at 1000 ° C and 1100 ° C and the results obtained confirmed that, with adequate control of pack compositions and deposition conditions, codeposition of Al and Si can be achieved with CrCl₃.6H₂O activated pack powder mixtures. A mixture of elemental Al and Si powders may be used as a depositing source instead of using Al-Si master alloy powders as conventionally recommended. The coatings could be formed either through the inward diffusions of Al and Si or through the outward diffusion of Ni together with other substrate elements such as Cr and Co, depending on the deposition temperature used. Prolonged deposition at 1100 ° C ledtothe formationofa coatingwith amultilayeredstructure consistingofanouter nickelsilicide layerand a middle Simodified NiAl layer followed by a diffusion zone. The pack compositions and deposition conditions may be adjusted to control the microstructure of the coatings formed by the codeposition process.     

Pack codeposition of Al and Cr to form diffusion coatings resistant to high temperature oxidation and corrosion for γ-TiAl

Abstract

Thermochemical analyses were carried out for a series of pack powder mixtures formulated for codepositing Al with Cr to form diffusion coatings on γ-TiAl resistant to high temperature oxidation by the pack cementation process. Based on the results obtained, experimental studies were undertaken to identify optimum pack powder mixtures for codepositing Al with Cr to form diffusion coatings with an adherent and coherent coating structure. The results of the thermochemical calculations performed indicated that codeposition of Al and Cr is possible with CrCl₃.6H₂O and AlCl₃ activated pack powders containing elemental Al and Cr as depositing sources. However, experimental results obtained at 1100°C revealed that CrCl₃.6H₂O is not suitable for use as an activator for codepositing Al with Cr on γ-TiAl. It caused a significant degree of degradation indicated by weight losses instead of coating deposition to the substrate. However, adherent coatings with excellent structural integrity consisting of an outer Cr doped TiAl₃ layer containing Al₆₇Cr₈Ti₂₅ phase and an inner layer containing TiAl₃ and TiAl₂ phases were successfully codeposited at 1100°C using pack powder mixtures activated by AlCl₃. It is suggested that such coatings were formed via a sequential deposition mechanism through inward diffusion of aluminium and chromium. Conditions that affect the pack codeposition process, and hence need to be carefully controlled, are discussed in relation to the mechanism of the formation of diffusion coatings with an integral structure free from microcracking on γ-TiAl.     

Aluminide coating formation on nickel-base superalloys by pack cementation process

A detailed study was carried out to investigate the effects of pack powder compositions, coating temperature and time on the aluminide coating formation process on a superalloy CMSX-4 by pack cementation. With the aid of recently developed thermodynamic analytical tools, powder mixtures that are activated by a series of fluoride and chloride salts were analysed and the effectiveness of these activators in transferring and depositing Al was evaluated at a range of coating temperatures. The Al chloride vapours formed at coating temperatures from 900°C to 1100°C were also analysed thermodynamically as a function of Al concentration in the original pack for the powder mixtures activated by 4 wt% CrCl₃·6H₂O. Based on the thermochemical calculations, a series of coating experiments was carried out. Aluminide coatings were formed at temperatures from 850°C to 1100°C for periods varying from 4 hours to 8 hours using powder mixtures activated by NH₄Cl, NaCl and CrCl₃·6H₂O and AlF₃. The effects of changing Al concentration as well as adding small quantities of Cr in the powder mixtures on the coating formation process were also investigated. The aluminide coatings were analysed using a range of techniques including SEM, EDX and XRD. The relationships between the mass gain and coating thickness and structure were investigated. The experimental results were compared with the predictions from thermochemical calculations. Based on the understandings established, an effective approach to control the aluminide coating parameters and structures was identified, which made it possible to optimise powder mixture compositions and coating conditions for different coating requirements.     

Formation and cyclic oxidation resistance of Hf–Co-modified aluminide coatings on nickel base superalloys

The codeposition of Co, Al and Hf on nickel base superalloys by pack cementation in a single-step process was investigated in this work. Thermochemical analyses were applied to search for suitable conditions including pack composition and deposition temperature. Co, Al, Hf, NH₄Cl, NH₄I and Al₂O₃ made up the pack powder mixture. According to a series of thermochemical calculations, the pack powder mixture of 20Co–10Al–2Hf–4NH₄Cl–4NH₄I–60Al₂O₃ (wt.%) was adopted. Further experimental results demonstrated that the codeposition of Co, Al and Hf could be achieved practically. The coating consisted of a diffusion zone and an outer layer. The outer layer was mainly composed of Al_0.9Ni_1.1 where a part of Ni was replaced by Co or Hf. The trace element Hf was enriched in the interface between the outer layer and the diffusion zone. The Co–Al–Hf coating exhibited excellent cyclic oxidation resistance due to improvement in adhesion between the oxide scale and the coating.     

Effects of pack composition on the formation of aluminide coatings on alloy steels at 650°C

This is a detailed study aimed to understand the effects of pack composition on the formation and growth of aluminide coatings on alloy steels by pack aluminisation at 650°C, a temperature below the melting point of Al (660°C), using pack powders consisting of Al as depositing source, a halide salt as an activator and Al₂O₃ as inert filler. The packs activated by AlCl₃, NH₄Cl, AlF₃ and NH₄F were used to investigate the effects of the type of halide salt on the coating formation and growth process and subsequently to identify the most suitable activator for pack aluminising alloy steels at 650°C. The effects of pack Al content on the rate of coating growth were then studied by varying the pack Al content from 1.4 wt% to 10 wt% whilst fixing the pack activator content at 2 wt%. It was observed that among the halide salts studied, AlCl₃ is the only suitable activator for pack aluminising alloy steels at 650°C and the rate of coating growth increases with the pack Al content. The equilibrium partial pressures of vapour species generated at the deposition temperature in packs activated by different types of halide salts were calculated and the results were discussed in relation to the observed deposition tendency of packs activated by different types of activators. A vapour phase transportation model was applied to elucidate the relationship between the rate of coating growth and the pack Al content. It was also demonstrated that by combining the low temperature pack aluminising parameters identified in this study with electroless or electro Ni plating, coherent nickel aluminide coatings free of microcracking can be produced on alloy steels at 650°C.     

Aluminization of nickel-formation of intermetallic phases and Ni2Al3 coatings

The occurrence and growth mechanisms of the various intermetallic phases of the Al-Ni system formed during pack aluminization of unalloyed nickel have been investigated with respect to the aluminium activity in the pack. Several types of coatings were obtained: (1) a Ni₂Al₃ coating formed by inward aluminium diffusion in a high activity cement of pure aluminium; (2) a Ni-rich NiAl coating formed by outward nickel diffusion in a low activity pack constituted by an Al-Ni alloy; (3) a mixed type of coating exhibiting the phases Ni₂Al₃, Al-rich NiAl, Ni-rich NiAl and Ni₃Al in four superposed layers, formed in a pack containing an Al-Cr alloy; (4) a high temperature, high activity type of coating formed above 950° C with an outer layer exhibiting a hypereutectic structure of NiAl₃ grains in a eutectic matrix due to precipitation from the liquid state. The optimum cementation conditions, for the production of maximum thickness and quality Ni₂Al₃ coatings were determined. The influence of surface reactivity and pack activity on the coating quality parameters was investigated.     

Co-deposition of aluminide and silicide coatings on γ-TiAl by pack cementation process

Thermochemical analyses were carried out for a series of pack powder mixtures for deposition of aluminide and for co-deposition of aluminide and silicide coatings on -TiAl by the pack cementation process. Based on the results obtained, experimental studies were undertaken to identify optimum pack powder mixtures for depositing adherent and coherent aluminide and silicide coatings. Pack powder mixtures activated by 2 wt% AlCl₃ was used to aluminise -TiAl at 1000°C. With proper control of pack compositions and coating conditions, an aluminide coating of TiAl₃ with a coherent structure free from microcracking was deposited on the substrate surface via inward diffusion of aluminium. The results of thermochemical calculations indicated that co-deposition of Al and Si is possible with CrCl₃ · 6H₂O and AlCl₃ activated pack powders containing elemental Al and Si as depositing sources. Experimental results obtained at 1100°C revealed that CrCl₃ · 6H₂O is not suitable for use as an activator for co-depositing aluminide and silicide coatings on -TiAl. It caused a significant degree of degradation instead of coating deposition to the substrate. However, adherent coatings with excellent structural integrity consisting of an outer TiSi₄ layer and an inner TiAl₃ layer were successfully co-deposited at 1100°C and 1000°C using pack powder mixtures activated by AlCl₃. IT is suggested that such coatings were formed via a sequential deposition mechanism through inward diffusion of aluminium and silicon. Discussion is presented on the issues that need to be considered to ensure the deposition of aluminide and silicide coatings with coherent structure free from microcracking on -TiAl by the pack cementation process.     

Study on behavior of NiAl coating with different Ni/Al ratios

β-NiAl coatings with different Ni/Al ratios were deposited on K403 superalloy substrates via magnetron sputtering. The phase transformation and diffusion phenomenon of the NiAl/Ni-based superalloy system after vacuum annealing at 900 and 1000 °C were analyzed using X-Ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectrometry (EDS). The effect of coating concentrations on the outward diffusion behavior of substrate elements was discussed. The high Cr concentrations in the Al-rich NiAl coatings were caused by the intense interdiffusion between Al and Cr. The Ti, W and Mo partitioned to γ′-Ni₃Al in the coatings. Several possible reasons for the formation of γ′-Ni₃Al at the surface of Ni-rich NiAl coating were identified, including: diffusion behavior of W and Mo in β-NiAl, destabilizing effect of substrate elements on β-NiAl, and diffusion rates of Ni and Al in β-NiAl. The volume change in β ⇛ γ′ transformation process shows Ni uphill diffused to the γ′-Ni₃Al islands at the surface of Ni-rich NiAl coatings. The IDZ (interdiffusion zone) thickness and precipitates in IDZ were related to the Al initial concentrations in the coatings.     

Inter-diffusion and oxidation behavior in electron-beam evaporated NiAl coatings

In this work, by electron-beam physical vapour deposition (EB-PVD), grit blasting and vacuum heat treatment, a two-phase [NiAl+Ni₃Al] coating was fabricated. Rather than the column grain boundaries, the small voids formed on the surface of the columnar grains serve as the fast diffusion route for chromium of the substrate. In order to evaluate the oxidation resistance of this coating, an isothermal oxidation test at 1100 °C in air was conducted. The oxidation resistance of the as-coated specimen was improved approximately twice that for a substrate without coating.     

Formation of aluminide coatings on low alloy steels at 650°C by pack cementation process

Abstract

The pack aluminising process is normally conducted on alloy steels at temperatures higher than 900°C at which mechanical properties of steels would degrade. This study aims to investigate the feasibility of pack aluminising a commercial 9Cr – 1Mo alloy steel at 650°C in an attempt to increase its high temperature oxidation and corrosion resistance without adversely affecting its mechanical properties and consequently to increase its long-term structural operating temperatures to up to 700°C. It was demonstrated that this could be achieved using packs containing AlCl₃ as an activator and elemental Al as a depositing source. The coatings formed under these conditions consisted of an outer Fe₁₄ Al₈₆ layer and an inner FeAl₃ layer with an abrupt interface between the coating and substrate, suggesting that the coating is formed via a mechanism of the inward Al reaction – diffusion. The pack Al content was varied from 1 to 6 wt-% to investigate its effects on the coating formation process. It was found that the pack Al content in this range affected only the coating thickness and therefore the growth rate of the coating, but not the surface Al concentration. A post-aluminising heat treatment study was also undertaken for an aluminised specimen at 650°C under an argon atmosphere to investigate the kinetics of converting the brittle Fe₁₄ Al₈₆ and FeAl₃ phase layers to a more ductile FeAl phase layer. It was observed that this was a slow process requiring 1132 h for an initial coating layer thickness of 33μm. The coating after the conversion consisted of a uniform top FeAl layer with all other alloying elements in the solid solution and a diffusion zone underneath.     

Chemical vapour deposition of chromium onto nickel

The chemical vapour deposition (CVD) of chromium onto a pure nickel substrate was investigated in the presence of hydrogen at three temperatures (950, 1000 and 1050°C) using a pack cementation method.Of the thermochemical reactions generally involved in CVD, such as thermal decomposition, reduction, displacement and disproportionation, the only significant reaction which occurs for this system is a reduction of CrCl₂ by hydrogen according to

$\text{CrCl}{}_2\text{(g)+H}{}_2\text{(g)→Cr(s)+2HCl(g)}$

Two diffusion layers appear during the process. (1) In the early stages of deposition a nickel-rich layer with a face-centred cubic structure develops. (2) Depending on the time and the temperature of the coating, an outer chromium-rich layer with a body-centred cubic structure develops progressively on the sample surface and grows from or together with the inner layer.The deposition rate is controlled by diffusion of the chromium and the nickel atoms through the coating layers, mainly through the inner layer.     

Influence of alkali content and alkali mixing on the chemical durability of fluorozirconate glasses

The corrosion of alkali-containing fluorozirconate glasses in water and acidic and alkaline solutions was studied with glasses of the compositions (100–x) (0.6ZrF₄·0.1AlF₃·0.3BaF₂)· xLiF and 48ZrF₄·8AlF₃·24BaF₂·xLiF·(20–x)NaF. The corrosion of the glasses in deionized water and 0.1 n HCl solution is mainly controlled by diffusion with an inductive period due to the passage from reaction-controlled to diffusion-controlled mechanisms, and the weight loss of glass increases with increasing LiF content in the single-alkali glasses. There was no appreciable effect of alkali mixing on the corrosion of glasses in deionized water and 0.1 n HCl solution. The glasses exhibited good stability in alkaline solution. The weight loss due to corrosion of the glasses in acidic buffered solution increased exponentially with increasing LiF content.     

A phenomenological model for lifetime design of Ni2Al3/Ni hybrid coating formed on creep resistant ferritic steels

The phase layer transformation kinetics in the Ni₂Al₃/Ni hybrid coating formed on creep resistant steel P92 has been studied via a series of prolonged isothermal annealing experiments at 650 °C. All the intermediate phase layers of NiAl, Ni₅Al₃ and Ni₃Al formed in the coating by interdiffusion during isothermal annealing process. The phase layers of NiAl and Ni₃Al formed at the very beginning of isothermal annealing at the interface between Ni₂Al₃ and Ni, but the Ni₅Al₃ phase layer formed at the interface between the NiAl and Ni₃Al phase layer only at an annealing time at which the outer Ni₂Al₃ phase layer was completely consumed. The growth and consumption of the Ni₂Al₃ and NiAl phase layers and the growth of the Ni₃Al₅ phase layer were all parabolic, but the growth of the Ni₃Al phase layer obeyed the power rate law d = kt ^1/n. The growth kinetics of an intermediate phase layer was found to be faster than the kinetics of its subsequent consumption. The rate constants in both the growth and consumption kinetics need to be determined for each of the intermediate phase layers at a particular temperature. The lifetime of the coating with an outer Ni₂Al₃ phase layer of any specified initial thickness and a sufficiently thick inner Ni layer can then be estimated using the lifetime design model delineated in this study.     

In situ chemical vapour co-deposition of Al and Si to form diffusion coatings on TZM

Multilayer alumino-silicide and silicide coatings were formed by in situ chemical vapour co-deposition of Al and Si on TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy for improving its high-temperature oxidation resistance. MoSi₂ and Mo (Si, Al)₂ layers were formed in the inner and the outer layers, respectively in the case of alumino-silicide coating. Whereas silicide coating consisted of Mo₅Si₃ and MoSi₂ phases in the inner and the outer layers, respectively. 24–100-μm thick coatings were formed by optimizing the pack mixture of Al and or Si, NH₄F and Al₂O₃ powders and conducting the experiments at 1000 °C for 8–36 h. MoSi₂ layer showed a faster growth rate and presence of columnar grains. A small weight gain at the initial stages was observed during the oxidation tests of the coated samples under continuous or cyclic heating at 1300 °C in air. Neither cracks nor peeling of the coating layers were noticed after oxidation tests.     

AlCl_3-EMIC室温离子液体制备Al-Al_2O_3复合涂层

首次在室温下用含10g/L Al2O3颗粒的AlCl3-EMIC室温离子液体电沉积制备出Al-Al2O3复合镀层。沉降试验表明,Al2O3颗粒在酸性AlCl3-EMIC室温离子液体中能形成稳定的悬浮液。通过SEM观察镀层表面和断面形貌,发现Al2O3颗粒均匀地分布在镀层中。显微努氏硬度检测结果表明,Al-Al2O3复合镀层的硬度高于纯Al镀层,其中Al镀层的硬度随着电流密度的增大而增高,而Al-Al2O3复合镀层的硬度却随着电流密度的增大而呈降低的趋势。本文还讨论了Al2O3颗粒在离子液体中的分散机制,以及和颗粒共沉积的过程。     

Nickel aluminide coating produced on γ-TiAl alloy by high activity aluminising process

Abstract

Ni aluminide diffusion coatings on the surface of γ-TiAl alloy were produced by electroplating a Ni layer followed by a single step high activity aluminising carried out in Ar+H₂ atmosphere with a mixture of Al, NH₄Cl and Al₂O₃ powders at 1000°C for 5 h. The effect of initial thickness for Ni layer on microstructure of produced Ni aluminide coating was highlighted. The thickness of initial Ni layer was changed to 4–20 μm. In the case of the Ni layer with thickness of 4 μm, only a little amount of NiAl phase was formed in a TiAl₃ matrix. However, the microstructure of coating, in the case of the Ni layer with thickness of 8 μm, consisted of an outer layer of two phases (NiAl+TiAl₃), an intermediate layer of TiAl₃ and an interdiffusion layer. For thicker initial Ni layers (16 and 20 μm), beside the latter coating microstructure, a continuous surface layer of NiAl phase was observed. Isothermal oxidation tests on these aluminide coatings reveal that the oxidation resistance of the aluminide coatings increases with increase in initial thickness of Ni layer.     

Sliding wear behaviors of electrodeposited nickel composite coatings containing micrometer and nanometer La2O3 particles

Micrometer and nanometer La₂O₃ particles were codeposited with nickel by electroplating from a nickel sulfamate bath. The wear behaviors of the composite coatings were evaluated sliding against AISI 1045 steel under non-lubricated conditions. It was found that the incorporation of the La₂O₃ particles enhances the microhardness and wear resistance of Ni coatings. The wear resistance of the Ni composite coating containing nano-sized La₂O₃ particles is higher than that of the Ni composite coating containing micro-sized La₂O₃ particles. The codeposition of the smaller nanometer La₂O₃ particles with Ni effectively reduces the size of Ni crystals and significantly increases the hardness of the composite coatings, resulting in significantly improved wear resistance of the nano-sized La₂O₃/Ni composite coating.     

The kinetics of gas transport in halide-activated aluminizing packs

The kinetics of diffusion of gaseous halides in an aluminizing pack are analyzed assuming the presence of an activator, as well as an aluminum-depleted zone. The influence of activator concentration on the aluminum transport rate is calculated for a wide range of activator concentrations in an AlF₃-activated pack. Theoretical results are compared with experimental data for the aluminization of nickel.     

HP40Nb钢热浸镀Al-Si高温氧化行为及组织研究

为提高高温抗氧化性能,对HP40Nb钢进行了热浸镀Al-10%(质量分数)Si,并进行不同温度扩散处理,研究了不同扩散处理试样在1000℃条件下的高温氧化行为,通过SEM,EDS和XRD分析了经不同扩散处理后的渗层在高温氧化过程中的组织结构变化.结果表明:经800℃/4h扩散处理,渗层由内层(NiAl+ Cr3 Si)...     

Low temperature aluminisation of alloy steels by pack cementation process

Abstract

The pack aluminisation process is normally applied at temperatures >973 K at which the mechanical properties of alloy steels would degrade. Thus, the present study was undertaken to apply this process to aluminising the alloy steels at temperatures <973 K in order to increase their high temperature oxidation resistance while maintaining their microstructure and hence mechanical strength and creep resistance. A type of commercial alloy steel P92 (9Cr–1Mo) was used for the present study. Pack powder mixtures consisting of Al, AlCl₃ (anhydrous) or NH₄Cl and Al₂O₃ were used to carry out the process. The aluminising temperature was varied from 773 to 973 K, pack Al content from 1 to 30 wt-% and aluminising time from 1 to 16 h to investigate their effects on the coating growth kinetics in the AlCl₃ activated packs. It was observed that all the coatings formed in the AlCl₃ activated packs were of a single layer structure with Fe₂Al₅ as the main coating phase. It was established that the interrelationship between the thickness h (in μm) of this coating layer and aluminising temperature T (in K), time t (in h) and pack Al content W (in wt-%) can be described by h=83005·9W^1/2t^1/2e^?73330/(RT). In the NH₄Cl activated packs, it was found that coating formation and dissolution took place simultaneously at 923 K and stable growth of a coating layer was only possible when the pack Al content was sufficiently high. However, the coatings formed in these packs had highly uneven regions.