Aluminide coating formation on nickel-base superalloys by pack cementationprocess

A detailed study was carried out to investigate the effects of pack powder compositions, coating temperature and time on the aluminide coating formati on process on a superalloy CMSX-4 by pack cementation. With the aid of rece ntly developed thermodynamic analytical tools, powder mixtures that are act ivated by a series of fluoride and chloride salts were analysed and the eff ectiveness of these activators in transferring and depositing Al was evalua ted at a range of coating temperatures. The Al chloride vapours formed at c oating temperatures from 900 degreesC to 1100 degreesC were also analysed t hermodynamically as a function of Al concentration in the original pack for the powder mixtures activated by 4 wt% CrCl3. 6H(2)O. Based on the thermoc hemical calculations, a series of coating experiments was carried out. Alum inide coatings were formed at temperatures from 850 degreesC to 1100 degree sC for periods varying from 4 hours to 8 hours using powder mixtures activa ted by NH4Cl, NaCl and CrCl3. 6H(2)O and AlF3. The effects of changing Al c oncentration as well as adding small quantities of Cr in the powder mixture s on the coating formation process were also investigated. The aluminide co atings were analysed using a range of techniques including SEM, EDX and XRD . The relationships between the mass gain and coating thickness and structu re were investigated. The experimental results were compared with the predi ctions from thermochemical calculations. Based on the understandings establ ished, an effective approach to control the aluminide coating parameters an d structures was identified, which made it possible to optimise powder mixt ure compositions and coating conditions for different coating requirements. (C) 2001 Kluwer Academic Publishers.