Development of multilayer oxidation resistant coatings on niobium and tantalum

In order to improve the high temperature oxidation resistance of refractory metals, multilayers Al–Si composite coatings of about 50 μm and 40 μm thickness were formed on niobium and tantalum metals, respectively, by pack cementation technique. Substrate metals embedded in the pack consisting of Al, Si powders and activator NH4F intimately mixed in the prescribed proportions were heat treated at 1300 K for 16 h. Coated samples were characterized by SEM, XRD, Optical microscopy and EPMA. Mainly two layers of different intermetallic phases were formed in coating of both niobium and tantalum. The inner layers were thicker than the outer layers and comprise of NbSi2 and TaSi2 for niobium and tantalum, respectively. Outer layers were ternary phases of Al, Si, Nb and Al, Si, Ta. From EPMA analysis, the presence of a ternary phase is a possibility for the outer layer of the coating on niobium. The outer layer of the coating on tantalum consisted of a compound that contained Ta, Si and Al. All the coated layers were adherent and no cracks were observed in any layer of coating. Coated samples exhibited good resistance to oxidation during heating up to 1300 °C in air for 8 to 10 h.