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

Abstract

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.