Transmission electron microscopy on Zr- and Hf-borides with MoSi 2 addition: Densification mechanisms
- PDF / 1,112,599 Bytes
- 7 Pages / 584.957 x 782.986 pts Page_size
- 77 Downloads / 123 Views
Hans-Joachim Kleebe, Stefan Lauterbach, and Mathis Mu¨ller Technical University of Darmstadt–Institute of Applied Geosciences (TUD-IAG), D-64287 Darmstadt, Germany
Diletta Sciti National Council of Research–Institute of Science and Technology for Ceramics (CNR-ISTEC), I-48018 Faenza, Italy (Received 3 June 2009; accepted 29 September 2009)
The microstructures of two pressureless sintered ceramics, ZrB2 and HfB2 with 20 vol% MoSi2 added, were analyzed by scanning and transmission electron microscopies. Carbides and oxides of the transition metals and MoB were observed to be well dispersed within the boride matrix. Mo5Si3 and Mo5SiB2, with Zr or Hf impurities, were observed at triple grain junctions and showed a partial wetting of the matrix. It was also noticed that the borides had a core-shell structure, which was especially pronounced in the ZrB2-based composite. The experimental results suggest the formation of a Mo–Si–B liquid phase at high temperature, which strongly promoted the densification. The densification mechanisms are discussed in light of the microstructure evolution on sintering, thermodynamic considerations, and the phase diagrams of the species involved. I. INTRODUCTION
Intermetallic borides such as ZrB2 and HfB2 are of great interest as high-temperature structural materials, since they possess high hardness and refractory characteristics, in addition to their good oxidation and corrosion resistance. Potential fields of application for these ceramics are in aerospace as leading edges, nose caps, and protective coatings for hypersonic re-entry vehicles or nozzle inserts and other components in propulsion systems.1 These materials have also potential use in the foundry and refractory industries. Conventionally, these materials have been densified by various pressureassisted techniques such as hot pressing or spark plasma sintering, because of their highly covalent bonds and high melting points. The possibility to obtain high density ceramics without the application of pressure represents an important technological advantage, because it allows production of near-net-shaped components, which do not require additional expensive machining. It has been shown that additions of SiC,2 HfN,3 B4C, and C4 are useful in lowering the densification temperatures and times of such composites. Moreover, it was reported that MoSi2 has a beneficial effect for pressureless densiAddress all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0126
http://journals.cambridge.org
II. EXPERIMENTAL
The compositions examined are listed in Table I. Details on the processing of powders and sintering conditions are reported elsewhere.5
a)
828
fication, improves the oxidation resistance, and does not provoke high-temperature degradation of flexural strength.5–7 In a previous work, the role of MoSi2 in the densification of Zr- and Hf-carbides was reported.8 Nevertheless, the mechanisms that govern densification of borides are still unclear and a matter of debate. Transmission electron m
Data Loading...
