TEM analysis and mechanical strengthening mechanism of MnO2 sintering aid in selective laser sintered porous mullites

Abstract

A novel preparation method for significantly improving mechanical properties of SLS-formed porous mullites was proposed. MnO2 sintering aid was coated on mullite powder surface via a liquid phase co-precipitation reaction between MnC4H6O4·4H2O (Mn(Ac)2·4H2O) and KMnO4. With increasing Mn(Ac)2·4H2O content from 6 to 48 mL, the generated MnO2 content increased from 3.64 to 5.17 wt% which promoted the high-temperature sintering performance and accelerated the re-arrangement of mullite particles to enhance the densification of porous mullite. Then the microstructure evolution and grain growth of mullite grains and the effect of MnO2 sintering aid on mechanical strengthening mechanism of SLS-formed porous mullites were evaluated in depth from the perspective of TEM analysis. It was found that the aspect ratio of mullite grains decreases gradually with increasing Mn(Ac)2·4H2O content, and the accelerated radial growth of mullite grains contributed an increase of fracture toughness from 0.11 ± 0.04 to 0.53 ± 0.01 MPa m1/2. With increasing Mn(Ac)2·4H2O content, the MnO2 crystal changed from ramsdellite structure γ to a stable tetragonal structure β type. Moreover, the fine MnO2 particles preferentially precipitated between interwoven mullite grains at the initial stage and then grew into the large ones pinned between mullite grains, resulting in an improvement of compressive strength from 3.8 ± 0.1 to 22.1 ± 1.0 MPa. These values of mechanical properties were significantly improved by 1–2 orders of magnitude compared with that of mullites adding no MnO2.