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

• Homogeneous and ultra-fine powder mixtures were produced by mechanical alloying.
• Pure MoSi2 was obtained by sintering as-milled Mo–67Si at 1100 ºC for 7 h.
• MoB–MoSi2 was obtained by sintering as-milled Mo–47Si–23B at 1300 °C for 3 h.
• MoB–MoSi2 had high oxidation resistance due to borosilicate protective scale.
• Pest disintegration was eliminated by boron addition to MoSi2 (MoB–MoSi2 composite).

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 °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 °C for 3 h and 1100 °C for 7 h, respectively. The sintered alloys were subjected to high temperature continuous oxidation test at 1000 °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.

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