Oxidation protection of γ-TiAl-based alloys – A review

Relationship between pack chemistry and aluminide coating formation for low-temperature aluminisation of alloy steels

Synthesis and characterisation of pack cemented aluminide coatings on metals

This study aims to investigate the feasibility of aluminising low alloy steels at temperatures below 700 °C by pack cementation process to increase their high temperature durability in oxidative and corrosive environments without adversely affecting their mechanical strength and creep resistance at elevated temperatures. Packs activated by AlCl 3 , AlF 3 , NaCl and NaF were used to carry out coating deposition experiments at 650 °C in an attempt to identify the most suitable activator for the intended pack aluminising process. Once this was achieved, a series of further experiments were undertaken to investigate the effects of pack composition, deposition temperature and time on the kinetics of coating growth process. The pack Al content was varied from 1 to 6 wt.%, deposition temperature from 600 to 750 °C and deposition time from 1 to 16 h. Within the ranges of these parameters, it was observed that these parameters only affected the coating thickness, but not the surface Al concentration and with the packs activated by AlCl 3 , the coatings formed via an inward reactive Al diffusion mechanism, which led to the formation of a surface Fe 14 Al 86 layer and an inner FeAl 3 layer. Thermochemical calculations were also undertaken to analyse the equilibrium vapour pressures of depositing halide species (AlF or AlCl) generated at deposition temperatures in packs activated by different halide salts. The results obtained were discussed in relation to the observed deposition tendencies of these packs and their influence on the kinetics of coating growth.

Pack aluminisation of low alloy steels at temperatures below 700 °C

A nanocrystalline Ni-30Cr-8Al-0.5Y coating was deposited on a Ni-base superalloy by magnetron sputtering. Post-aluminizing was performed on the sputtered coating to improve the corresponding high-temperature properties. The isothermal oxidation behavior at 1000 similar to 1100 degrees C and hot-corrosion behavior in the presence of 75 wt.% Na2SO4+25 wt.% K2SO4/NaCl film at 900 degrees C of the sputtered coating with and without aluminizing have been investigated using TGA, SEM/EDS, XRD and EPMA, respectively. The results indicated that the sputtered NiCrAlY coating possessed excellent oxidation resistance at 1000 degrees C due to the presence of extensive amount of chromium and good amount of aluminum. Because of the excessive consumption of Al in the coating, it lost protection at 1100 degrees C. The mass gain of the aluminized NiCrAlY coating was a little higher than that of the sputtered coating due to the formation of the rapid growth theta-Al2O3 phase at 1000 degrees C. At 1100 degrees C the oxidation resistance of the latter was better than that of the former. The sputtered coating provided a limited protection at the transient hot-corrosion stage in the molten salt film and degenerated completely with time. On the contrary, the aluminized NiCrAlY coating showed much better hot-corrosion resistance in the presence of 75 wt.% Na2SO4+K2SO4/NaCl film as a result of the formation of a continuous and protective Al2O3 scale. (c) 2005 Elsevier B.V. All rights reserved.

High-temperature oxidation and hot-corrosion behavior of a sputtered NiCrAlY coating with and without aluminizing

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.

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

Codeposition of Al and Si to form oxidation-resistant coatings on γ-TiAl by the pack cementation process

Low-temperature formation and oxidation resistance of nickel aluminide/nickel hybrid coatings on alloy steels

Both theoretical and experimental studies were carried out to identify pack powder compositions and coating conditions suitable for codeposition of Al and Cr by the pack cementation process. It has been demonstrated that for pack powders containing chloride salts as activators and elemental Al and Cr as source materials, codeposition of Al and Cr occurs when the partial pressures PAlCl and PCrCl are comparable or when PAlCl /PCrCl is close to one. But, only Al or Cr may be deposited when PAlCl /PCrCl is much larger or smaller than one. Pack powder compositions suitable for codeposition of Al and Cr depend very much on the type of activators used. For pack powders containing 25 wt-% of source elements of Al and Cr, codeposition of Al and Cr may occur when the Al content is within the range 1·25–1·90 wt-% for the packs activated by 3 wt-%NH4 Cl, and 1·85–1·95 wt-% for the pack activated by 4 wt-%CrCl3 . 6H2O. However, codeposition of Al and Cr may be not achievable with pack powders activated by Na...

Conditions for Codeposition of Al and Cr on Ni Base Superalloys 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 γ-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 γ-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 γ-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 γ-TiAl by the pack cementation process.

Z. D. Xiang

Papers

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

Codeposition of Al and Si to form oxidation-resistant coatings on γ-TiAl by the pack cementation process

Low-temperature formation and oxidation resistance of nickel aluminide/nickel hybrid coatings on alloy steels

Conditions for Codeposition of Al and Cr on Ni Base Superalloys by Pack Cementation Process

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