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.     

Pack cementation process for the formation of refractory metal modified aluminide coatings on nickel-base superalloys

The vapour phase compositions of a series of pack powder mixtures containing elemental Al and Hf or W powders as depositing sources and CrCl₃·6H₂O or AlF₃or CrF₃as activators were analysed in an attempt to further develop the pack cementation process to codeposit Al and Hf or W to form diffusion coatings on nickel base superalloys. The results suggested that Al could be codeposited with Hf, but not with W, from the vapour phase. Compared with both AlF₃and CrF₃, CrCl₃·6H₂O has been shown to be a more suitable activator for codepositing Al with Hf. The optimum coating temperature was identified to be in the range of 1050°C to 1150°C. Based on the thermochemical analysis, a series of coating deposition studies were undertaken, which confirmed that codeposition of Al and Hf could be achieved at a deposition temperature of 1100°C in the CrCl₃·6H₂O activated packs containing elemental Al and Hf powders. The coating obtained had a multilayer structure consisting of a Ni₇Hf₆Al₁₆top layer and a NiAl layer underneath, followed by a diffusion zone, which revealed that the coating was formed by the outward Ni diffusion. It is suggested that the compositions suitable for codeposition of Al and Hf could be effectively identified by comparing the vapour pressures of HfCl₄and HfCl₃with that of AlCl in the packs activated by chloride salts. It has also been experimentally demonstrated that, although W could not be deposited from the vapour phase, a high volume of fine W particles can be entrapped into the outer NiAl coating layer formed by the outward Ni diffusion using a modified pack configuration. This leads to the formation of a composite coating layer with W particles evenly distributed in a matrix of NiAl. It is suggested that this modified pack process could be similarly applied to develop nickel aluminide coatings containing other refractory metals that may not be codeposited with Al from the vapour phase.     

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.     

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.     

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.     

Preparation and oxidation of an Al2O3-modified aluminide coating

An Al₂O₃-modified aluminide coating was produced by aluminizing an as-electrodeposited Ni-Al₂O₃ nanocomposite film using pack cementation method at 1100  C for 4 h. For comparison, aluminizing was also performed in the same condition on an as-deposited Ni film without Al₂O₃ nanoparticles. SEM/EDAX and TEM results indicated that the co-deposited Al₂O₃ nanoparticles were homogeneously dispersed in the finer-grain nanocrystalline Ni grains. The isothermal and cyclic oxidation in air at 900 °C indicated that the Al₂O₃-modified aluminide coatings were profoundly oxidation resistant as compared to the Al₂O₃-free aluminide coatings due to the faster formation of a continuous adherent α-Al₂O₃ scale. The effect of Al₂O₃ on the microstructure and the oxidation of the aluminide coating are discussed in the detail.     

Formation of nickel aluminide/nickel hybrid coatings on alloy steels by two step process of nickel plating and low temperature pack aluminisation

Abstract

The present study investigates the conditions required for forming a hybrid coating consisting of an outer nickel aluminide layer and an inner nickel layer on alloy steels. A commercial alloy steel of 9Cr–1Mo was used as a substrate. Electroless and electronickel plating processes were used to form an initial nickel layer on the steel. The AlCl₃ activated packs containing pure Al as a depositing source were then used to aluminise the nickel deposit at temperatures ≤650°C. The effect of phosphorus or boron content in the initial nickel layer deposited with the electroless nickel plating solutions using hypophosphite or boron–hydrogen compound as reducing agent was investigated in relation to the spallation tendency of the coating either immediately after the aluminising process or during the thermal annealing post-aluminising process. Under the aluminising conditions used, the outer nickel aluminide layer formed was Ni₂Al₃. For the electroplated nickel deposit, the growth kinetics of the outer Ni₂Al₃ layer during the pack aluminising process was found to obey the parabolic rate law with a parabolic rate constant being 12·67 μm at 650°C for 2 wt-%AlCl₃ activated pack containing 4 wt-% pure Al as a deposition source.     

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.     

Economical synthesis of nano alumina powder using an aqueous sol-gel method

Nano alumina powder can be widely used in the fields of electronics, catalyst supports and high temperature applications. In the present article, a nano Al₂O₃ powder was synthesized by a simple aqueous sol-gel method using inexpensive AlCl₃·6H₂O and Al powder as raw materials. It was shown that the gel calcined at 1100 °C resulted in the formation of a crystalline α-Al₂O₃ nano powder. It had a particle size distribution ranging from 32 to 100 nm after heat treatment at 1200 °C.     

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.     

In‐situ composite coating on powder metallurgy master alloy fabricated by vacuum hot‐pressing sintering technology

A material consisting of an in‐situ titanium carbide reinforced nickel‐aluminide (Ni₃Al) coating and a powder metallurgy master alloy was fabricated by vacuum hot‐pressing sintering technology. A metallurgical bonded, pores‐free interface between composite coating and powder metallurgy master alloy was formed at the sintering temperature of 1050 °C, pressure of 10^‐4 Pa and pressing pressure of 40 MPa. The phase, microstructure and wear behavior of composite coating were investigated. The results showed that polygonal titanium carbide particulates with various sizes were homogeneously distributed in nickel‐aluminide matrix. The sintering temperature, pressing pressure and heat from as‐reactions‐formed coating green compact facilitated the pore infiltration with transiently generated liquid phases and ensured the high‐intensity metallurgical bonding between composite coating and powder metallurgy master alloy. Due to the abnormal elevated‐temperature properties of nickel‐aluminide matrix, titanium carbide particulates reinforcement and the mechanically mixed layer protection, TiC/Ni₃Al‐coated parts demonstrated superior wear resistance and lower friction coefficient while compared with Ni₃Al‐coated parts and H13 steel.     

Effect of heat treatment on the microstructure and property of cold-sprayed nanostructured FeAl/Al2O3 intermetallic composite coating

It is difficult to deposit dense intermetallic compound coatings by cold spraying directly using compound feedstock powders due to their intrinsic low temperature brittleness. A method to prepare intermetallic compound coatings in-situ employing cold spraying was developed using a metastable alloy powder assisted with post heat treatment. In this study, a nanostructured Fe(Al)/Al₂O₃ composite alloy coating was prepared by cold spraying of ball-milled powder. The cold-sprayed Fe(Al)/Al₂O₃ composite alloy coating was evolved in-situ to FeAl/Al₂O₃ intermetallic composite coating through a post heat treatment. The effect of heat treatment on the phase formation, microstructure and microhardness of cold-sprayed Fe(Al)/Al₂O₃ composite coating was investigated. The results showed that annealing at a temperature of 600 °C results in the complete transformation of the Fe(Al) solid solution to a FeAl intermetallic compound. Annealing temperature significantly influenced the microstructure and microhardness of the cold-sprayed FeAl/Al₂O₃ coating. On raising the temperature to over 950 °C, diffusion occurred not only in the coating but also at the interface between the coating and substrate. The microhardness of the FeAl/Al₂O₃ coating was maintained at about 600HV_0.1 at an annealing temperature below 500 °C, and gradually decreased to 400HV_0.1 at 1100 °C.     

NbSi基原位复合材料表面包埋渗制备SiCrY共渗涂层

通过热力学计算分析了分别使用NH₄Cl和CrCl₃·6H₂O两种不同激活剂时, 各个共渗元素的卤化物蒸汽压分压的变化, 得到了实现Nb-Si基原位复合材料上Si-Cr-Y三元包埋共渗的最佳条件。采用包埋共渗法在Nb-Si基原位复合材料表面制备了Si-Cr-Y共渗涂层, 研究了涂层的组织形貌、 成分及其相组成。结果表明: 使用NH₄Cl做激活剂, 通过调整包埋渗料的成分, 可以在适当的温度下实现Si-Cr-Y的三元共渗。当渗料成分为12Cr-6Si-0.75Y₂O₃-5NH₄Cl-76.25Al₂O₃(质量分数)时, 在1350℃可以实现Si-Cr-Y三元共渗。制备的涂层具有多层结构, 分为外层、 内层和明显的互扩散层。互扩散层的存在, 表明涂层的形成是一个连续生长过程, 伴随着Cr、 Si、 Y元素向基体内的扩散。涂层的主要成分由Cr₂(Nb,Ti)、 (Nb,Ti)₅Si₃和HfSi₂组成, Y元素的添加起到了细化涂层的作用。     

Synthesis and densification of lutetium pyrosilicate from lutetia and silica

Cerium-doped lutetium pyrosilicate (Lu₂Si₂O₇:Ce) powder was synthesized by solid state reaction of Lu₂O₃ and SiO₂. Stoichiometric mixtures of the starting materials were heat treated at various different temperatures and their phase contents were measured by XRD technique. It was found that the first step in the formation of Lu₂Si₂O₇ (LPS) is the appearance of Lu₂SiO₅ (LSO). This takes place at 1100 °C, fully 300 °C below the first appearance of LPS. Between 1400 and 1500 °C both LSO and LPS coexist in the calcined batch, but by 1550 °C all LSO is completely converted to LPS. LPS formation temperature does not have appreciable effect on the density of the hot pressed samples. Hot pressed samples obtained from powder synthesized at 1650 °C are nearly transparent, although the particle size of the starting powder is higher than that of the powder formed at lower temperatures.     

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.     

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.     

Thermal stability of a platinum aluminide coating on nickel-based superalloys

An investigation was carried out to determine the thermal stability of a platinum aluminide coating on the directionally solidified alloy MAR M 002 and its single-crystal version alloy, SRR 99, at 800, 1000 and 1100°C. The morphology, structure and microchemical composition of the coating were characterized using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. In the as-deposited condition, the coating was found to consist of two layers. Most of the platinum was concentrated in the outer coating layer which consisted of a fine dispersion of PtAl₂ in a matrix of β-(Ni, Pt)Al containing other elements in solid solution, such as cobalt and chromium. The inner coating layer was relatively free of platinum and consisted essentially of β-NiAl. Exposure at 800°C was found to have no significant effect on the structure and composition of the coating on each alloy. At temperatures ?1000°C, however, PtAl₂ became thermodynamically unstable and significant interdiffusion occurred between the coating and alloy substrate. After exposure at 1000°C, the components of the outer coating layer were NiAl and Ni₃Al. However, after exposure at 1100°C, the outer coating layer consisted only of Ni₃Al. Also, after exposure at both temperatures, the composition of the outer coating layer approached that of the inner layer due to interdiffusion. Although the coating on both alloys exhibited similar structural stability at all temperatures investigated, the coating on alloy MAR M 002 was found to develop a more protective scale. This behaviour was correlated with differences in alloy substrate composition particularly rare-earth elements such as hafnium.     

Cyclic Corrosion Behavior of Pt/Ru-Modified Bond Coatings Exposed to NaCl Plus Water Vapor at 1050°C

In the present investigation, Pt/Ru-modified bond coating consisted of 2 μm Pt+2 μm Ru was deposited on a nickel-based superalloy by electroplating method and followed by conventional Al pack cementation. The cyclic corrosion behavior of Pt/Ru-modified bond coating exposed to NaCl plus water vapor has been investigated under atmospheric pressure at 1050°C. The result shows that the cyclic corrosion life of Pt/Ru-modified bond coating is longer than that of the conventional Pt-modified aluminide coating in the presence of NaCl plus water vapor. The addition of Ru makes the coating possess the increased strength and suppress the rumpling behavior. The absence of rumpling may be responsible for the improved corrosion resistance of Pt/Ru-modified aluminide coating.     

Chemical vapour deposition of silicon onto iron: influence of silicon vapour phase source on the composition and nature of the coating

Armco iron was siliconized with two different gas mixtures in the temperature range of 750 to 1100 °C. The Ar-SiH₄-H₂ mixture leads mainly to the formation of a non-porous and adherent solid solution with a maximum silicon content of 6% by weight. For high concentrations of silane in the vapour phase or low treatment temperatures, non adherent pure silicon powder or flakey iron silicides may also depose. The use of Ar-SiH₄-SiCl₄-H₂ mixture leads to the immediate nucleation of Fe₃Si, the growth of which occurring from and around the open porosity of the coating. The influence of various parameters such as vapour phase composition and flow rate, hydrogen dilution and treatment temperature, is investigated and analysed in terms of limiting factors of the overall process: gas phase transport, interfacial reactions and diffusion velocity of different species in solid phase.