Search for: All records

The niobium hemicarbide (Nb2C) has at least three known polymorphs: a (Pnma or Pbcn), b (P31mÞ, and c (P63/mmc) as a function of temperature. Identification of these phases has been notoriously difficult particularly for the lower-temperature variations (a and b) because of their long-range vacancy ordering. In the current study, an overall Nb2C composition has been processed by hot isostatically pressing NbC and Nb powders together which did not fully homogenize. Using neutron diffraction and selected area electron diffraction, the C6 (P3m1) structure was identified in the Nb2C. The formation pathway for this phase is postulated from the high density of stacking faults observed in the NbC. 

Microwave plasma chemical vapor deposition (MPCVD) was used to diffuse boron into tantalum using plasma initiated from a feedgas mixture containing hydrogen and diborane. The role of substrate temperature and substrate bias in influencing surface chemical structure and hardness was investigated. X-ray diffraction shows that increased temperature results in increased TaB 2 formation (relative to TaB) along with increased strain in the tantalum body-centered cubic lattice. Once the strained tantalum becomes locally supersaturated with boron, TaB and TaB 2 precipitate. Additional boron remains in a solid solution within the tantalum. The combination of precipitation and solid solution hardening along with boron-induced lattice strain may help explain the 40 GPa average hardness measured by nanoindentation. Application of negative substrate bias did not further increase the hardness, possibly due to etching from increased ion bombardment. These results show that MPCVD is a viable method for synthesis of superhard borides based on plasma-assisted diffusion.