Microstructure and High Temperature Oxidation Performance of Silicide Coating on Nb-Based Alloy C-103

Abstract

The microstructure and oxidation resistance of NbSi 2 coating formed on Nb-based alloy C-103 by a pack siliconization process have been studied. The as-formed coating consists of an outer NbSi 2 layer and an inner Nb 5 Si 3 layer. A NbSi 2 -Nb 5 Si 3 two-phase zone is also present between the above two layers. Weight-change data obtained under isothermal and cyclic oxidation in air at 1100 and 1300°C, suggests that the coating gives oxidation protection up to about 4 h. The oxide scale that formed on the coating during oxidation exposure consists of an outer glassy silica layer and an inner Nb 2 O 5 -silica mixed layer. Nb 2 O 5 phase is also present in the outer silica scale in the form of elongated particles. Oxidation protection is achieved primarily by the presence of the glassy silica layer on the surface. Spallation of this layer during thermal cycling causes significant reduction in the protective life of the coating.

Mo–Si–B Coating for Improved Oxidation Resistance of Niobium

“…While the oxidation of niobium is complex, the most stable oxide formed is Nb 2 O 5 . This agrees with previous research on silicide coatings for Nb‐based Alloy C‐103 that shows that above 900 °C, only Nb 2 O 5 is formed . Figure (b) and (e) shows the cross‐section of the samples after oxidation at 1200 °C for 5 h. The Si–B (Figure (e)) coating that produces a much thicker silica/niobium oxide layer compared to the Mo–Si–B coating (Figure (b)), indicating that more of the coating is being oxidized in the Si–B coated samples.…”

“…Previous research from Alam et al . reported a layer of Nb 5 Si 3 of 8 μm after an oxidation test at 1100 °C for 50 h . Other reports show that the formation of Nb 5 Si 3 is very slow at oxidation temperatures below 1200 °C .…”

Microstructure formation of silicide coating on Nb–W–Mo–Zr alloy

“…Thus, an Nb 5 Si 3 -NbSi 2 two-phase layer and an Nb 5 Si 3 inner layer formed through the reactions at the NbSi 2 /Nb 5 Si 3 and Nb 5 Si 3 /substrate interfaces. Similar results were also found on silicide coatings prepared on C-103 alloy by a pack siliconisation process [5,12].…”

“…This technique is a diffusion-controlled process, which could form metallurgically bonded coatings. In the simple case, single deposition of Si was performed on Nb-based alloys to form NbSi 2 coatings [12][13][14][15]. To modify the silicide coatings, co-deposition of two or three elements have also been achieved by pack cementation [8,9,11,[16][17][18][19][20][21][22][23][24], and this process is usually complex including complicated relations among the activators, the deposition elements and the substrates [9].…”

High-temperature stability of yttria-stabilized zirconia thermal barrier coating on niobium alloy—C-103

“…Although such kind of TBC is beneficial for the component life; but may make the process tedious and time consuming. Traditionally, silicide coatings have been used on Nb alloys for scramjet combustor and vehicle acreage applications [19][20][21]. However, silicide coatings are generally used for oxidation protection; but not as TBC due to their higher thermal conductivity compared with YSZ [21].…”