Improvement of MoSi2 oxidation resistance via boron addition: Fabrication of MoB/MoSi2 composite by mechanical alloying and subsequent reactive sintering

The purpose of this study is to determine the influence of boron addition on the oxidation resistance of MoSi2. Powder mixtures of Mo-Si with stoichiometric ratio of 1:2 and 30Mo-47Si-23B (at.%) were used, respectively to produce MoSi2 and MoB/MoSi2. First powder mixtures were separately milled for 20 h in an attritor ball mill. After degassing of as-mechanically alloyed powders at 450 degrees C, the powders were pressed into cylindrical samples under a pressure of 600 MPa. The final diameter of samples were 15 mm. Compacted samples were sintered in tube resistance furnace using Ar controlled atmosphere. 30Mo-47Si-23B alloy with a MoB/MoSi2 two-phase structure and monolithic MoSi2 were obtained by sintering at 1300 degrees C for 3 h and 1100 degrees C for 7 h, respectively. The sintered alloys were subjected to high temperature continuous oxidation test at 1000 degrees C. Microstructure of sintered alloys and as-mechanically alloyed powders, phase composition of sintered and oxidized samples were evaluated by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The results showed that in 30Mo-47Si-23B alloy no pest disintegration is observed. The presence of boron acted as an effective barrier against oxygen diffusion during oxidation test of MoB/MoSi2 composite. Excellent oxidation resistance of MoB/MoSi2 composite was achieved through homogeneously distributed and fine grained microstructure and also very high relative density. (C) 2014 Elsevier B.V. All rights reserved.