Nb silicide coatings processed by double pack cementation: Formation mechanisms and stability

Niobium silicide coatings were processed on Nb metal using double pack cementation procedures at temperatures in the range 1000 °C–1200 °C for up to 6 h. Coating thickness and hardness were affected by processing parameters. Lower processing temperatures resulted in single-layer NbSi2 coatings while...

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Bibliographic Details
Published in:Surface & coatings technology 2021-03, Vol.409, p.126913, Article 126913
Main Authors: Pinto, Beatriz A., d'Oliveira, Ana Sofia C.M.
Format: Article
Language:English
Subjects:
Coatings
Density
Diffusion coatings
Interdiffusion
Intermetallic compounds
Nb5Si3
NbSi2
Niobium oxides
Oxidation
Oxidation resistance
Pack cementation
Process parameters
Scale (corrosion)
Silicides
Silicon dioxide
Temperature exposure
Vacancies
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Summary:Niobium silicide coatings were processed on Nb metal using double pack cementation procedures at temperatures in the range 1000 °C–1200 °C for up to 6 h. Coating thickness and hardness were affected by processing parameters. Lower processing temperatures resulted in single-layer NbSi2 coatings while the highest processing temperature (1200 °C) produced coatings with two layers, NbSi2 and Nb5Si3, the latter at the NbSi2/Nb interface. Analysis of the results shows that Nb5Si3 forms by decomposition of NbSi2 and interdiffusion of Si and Nb. The influence of processing parameters on vacancy density and of vacancy density on the coating formation mechanisms and properties is discussed. Silicide-coated Nb had superior oxidation resistance to pure Nb. However, under the conditions tested a two-layer scale of Nb2O5 and SiO2 was formed. Analysis of the data indicates that Nb2O5 forms rapidly on the surface of the coatings at lower temperatures whereas SiO2 forms at the top of the oxide scale. It is concluded that pre-oxidation treatment is essential to ensure formation of an SiO2 scale, which is associated with enhanced oxidation performance. •Double pack-cementation allowed to process silicide diffusion coatings on Nb•Processing and post-processing above 1100 °C favoured the synthesis of Nb5Si3.•Si vacancy concentration and interdiffusion of Si/Nb occurred near the substrate.•Parabolic oxidation kinetics is associated with synthesis of a protective SiO2 scale.•Oxidation above 1200 °C forms a protective continuous scale of SiO2.
ISSN:0257-8972
1879-3347
DOI:10.1016/j.surfcoat.2021.126913