Deposition of aluminide and silicide based protective coatings on niobium

• Published in: Applied surface science Vol. 257; no. 2; pp. 635 - 640
• Main Authors: Majumdar, S., Arya, A., Sharma, I.G., Suri, A.K., Banerjee, S.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.11.2010; Elsevier
• Subjects: Ab initio calculations; Adhesive bonding; Aluminides; Alumino-silicide; Aluminum; Coating; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Intermetallics; Mathematical analysis; Mechanical and acoustical properties; Niobium; Niobium aluminide; Oxidation resistance; Physical properties of thin films, nonelectronic; Physics; Silicon; Structure and morphology; thickness; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Thin film structure and morphology; Work of adhesion; Niobium aluminide; Alumino-silicide; Coating; Ab initio calculations; Work of adhesion; Intermediate state; Scanning electron microscopy; Mechanical strength; Ternary compounds; Pseudopotential methods; Hardness; Adhesion; Niobium; Niobium Aluminides; Binary alloys; Oxidation resistance; Composite materials; Aluminium alloys; Electron microprobe analysis; Composite coating; Density functional method; Carburizing; Protective coatings
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2010.07.055

We compare aluminide and alumino-silicide composite coatings on niobium using halide activated pack cementation (HAPC) technique for improving its oxidation resistance. The coated samples are characterized by SEM, EDS, EPMA and hardness measurements. We observe formation of NbAl3 in aluminide coating of Nb, though the alumino-silicide coating leads to formation primarily of NbSi2 in the inner layer and a ternary compound of Nb–Si–Al in the outer layer, as reported earlier (Majumdar et al. [11]). Formation of niobium silicide is preferred over niobium aluminide during alumino-silicide coating experiments, indicating Si is more strongly bonded to Nb than Al, although equivalent quantities of aluminium and silicon powders were used in the pack chemistry. We also employ first-principles density functional pseudopotential-based calculations to calculate the relative stability of these intermediate phases and the adhesion strength of the Al/Nb and Si/Nb interfaces. NbSi2 exhibits much stronger covalent character as compared to NbAl3. The ideal work of adhesion for the relaxed Al/Nb and Si/Nb interfaces are calculated to be 3226mJ/m2 and 3545mJ/m2, respectively, indicating stronger Nb–Si bonding across the interface.