Investigation of protective silicide coatings for niobium alloys

• Published in: Ultramicroscopy 1985, Vol.17 (2), p.160-160
• Main Authors: Kazinets, M., Kiriaty, A., Gafri, O.
• Format: Article
• Language: English
• ISSN: 0304-3991; 1879-2723
• DOI: 10.1016/0304-3991(85)90014-2

The structure and composition of protective coatings for Nb alloy specimens were investigated by electron microscopy, X-ray energy-dispersive analysis (EDA) and X-ray diffraction analysis. The specimens were coated with a silicide coating by the slurry diffusion process. Electron microscopic investigations of the metallographic structure of the coating showed that it was composed of five layers, each of which had a different structure and composition. The total thickness of the coating was about 150-200 mu m. The phase composition of each layer was determined by analysis of the X-ray diffraction pattern from the surface exposed by consecutive removal of the layers. All the layers consisted of two phases: NbSi sub 2 and a high-temperature tetragonal phase of Nb sub 5 Si sub 3 . The ratio between the quantities of these phases changed from one layer to the next. Additional X-ray studies of the surface layer of the coatings revealed, besides the two known phases (NbSi sub 2 and Nb sub 5 Si sub 3 ), a series of unidentified strong diffraction peaks. A thorough search of possible compounds in this complicated system led to the conclusion that the unidentified peaks belong to a new silicide phase. The approximate composition of the new phase is Me sub 2 Nb sub 4 Si sub 5 , where Me is Cr, Ti, Fe or Hf. All the diffraction peaks of the new phase could be indexed on the basis of a tetragonal lattice with a = 3.692 A and c = 6.394 A.--AA