Processing, microstructure, and mechanical properties of zirconium diboride-molybdenum disilicide ceramics and dual composite architectures
"This research had two objectives: characterization of processing-microstructure-mechanical property relationships of conventional ZrB2-MoSi2 ceramics at room temperature (RT) and 1500⁰C in air, and fabrication of ZrB2-MoSi2 dual composite architectures (DCAs) for use near 1500⁰C. Elastic moduli, fracture toughness, and flexure strength were measured at RT and 1500⁰C for 15 ZrB2-MoSi2 ceramics hot pressed using fine, medium, or coarse ZrB2 starting powder with 5-70 vol.% MoSi2, referred to as FX, MX, and CX respectively where X is the nominal MoSi2 content. MoSi2 decomposed during sintering, resulting in microstructures with ZrB2 cores and (Zr1-xMox)B2 shells via surface and grain boundary diffusion. Flexure strength at RT (700-800 MPa for FX, 560-720 MPa for MX, and 440-590 MPa for CX) was controlled by the maximum ZrB2 grain size, and toughness (2.7-3.9 MPa·m1/2) did not trend with MoSi2 content. At 1500⁰C toughness increased with MoSi2 content and ZrB2 grain size, and strength of FX and MX was controlled by oxidation damage at 1500⁰C. Strength of CX followed the opposite trend, with C10 exhibiting a strength of ~600 MPa. Four ZrB2-MoSi2 DCAs were fabricated by dispersing granules of selected ZrB2-MoSi2 compositions in matrices of different ZrB2-MoSi2 compositions. Strength limitation at 1500⁰C by differential oxidation of granules and matrix was resolved by compositional adjustment, but microcracking due to granule-matrix CTE mismatch limited strength to ~140 MPa at RT and ~360 MPa at 1500⁰C. The granule-matrix interface did not deflect cracks, and the toughness at 1500⁰C was 6.1-6.9 MPa·m1/2, similar to that of conventional ZrB2-MoSi2 ceramics. CTE matching via addition of a third phase and use of a weak granule-matrix interface are recommended areas of focus for future development of high-temperature DCAs"--Abstract, page iv.
High-Temperature Ceramics, Mechanical Properties, Microstructure, Molybdenum Disilicide, Processing, Zirconium Diboride, Energy Systems, Materials Science and Engineering
Missouri University of Science and Technology
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