Si-B co-deposition coatings on an Nb silicide based ultrahigh temperature alloy prepared by pack cementation technique

In order to improve the oxidation resistance of silicide coatings on an Nb silicide based alloy, Si-B co-deposition coatings were prepared at 1150°C for 8 h by using pack mixtures containing different amounts of B powders. The microstructural characteristics of the coatings were studied. The results showed that all coatings had a triple-layer structure. The middle layers of all coatings were composed of (Nb,X)Si₂ (X represents Ti, Cr and Hf elements) and the inner layers of all coatings were composed of (Nb,X) ₅Si₃, but the outer layers of the coatings had different phase constituents and morphologies when the amount of B powders in the pack mixture changed. When the amount of B powders in the pack mixture ranged from 0.5 wt% to 12 wt%, the outer layers were composed of (Nb,X)B₂, with a few CrB₂ particles. When the amount of B powders in the pack mixture was in the range of 0.05∼0.1 wt%, the outer layers were composed of (Nb,X)B₂. However, when the amount of B powders in the pack mixture was in the range of 0.02∼0.03 wt%, the outer layers were composed of (Nb,X)Si₂, with needle-like (Nb,X)B₂ particles dispersed in it.