Growth mechanism of in situ diamond-shaped mullite platelets and their effect on the properties of Al2O3-SiC-C refractories

The microstructure evolution and properties of Al2O3-SiC-C refractories with different Fe2O3 content (0, 0.5, 1.0 wt%) heat-treated at 1400 degrees C were investigated. Also, the in-situ growth mechanism of diamond-shaped mullite platelets was discussed. The experimental results indicate that there were only mullite and SiC whiskers formed in the absence of the additive. However, both whisker-shaped and diamond-shaped mullite phases could be generated aside from SiC whiskers, which were satisfactorily correlated with Fe2O3 addition. Interestingly, the introduction of Fe2O3 changed the morphology of mullite whiskers from the curved structure (without additive) to the straight and elongated shape (with additive). Furthermore, these two-dimensional mullite platelets were better developed resulting in the larger size and higher yield with 1.0 wt% Fe2O3. XRD, SEM, TEM and HRTEM analyses reveal that with the addition of Fe2O3, mullite phases in refractory samples grew preferentially toward the crystal facets along (130) and (4 2 0) directions of mullite nanocrystals, thus, a diamond-shaped structure was generated. As the well-developed mullite platelets were formed, the cold modulus of rupture (CMOR) and the cold crushing strength (CCS) increased from 6.5 MPa to 10.3 MPa and from 38.7 MPa to 55.7 MPa, respectively. Meanwhile, after three thermal shock cycles, the CCS of samples containing 1.0 wt% Fe2O3 only decreased by 2.6 MPa, and the residual strength ratio of CCS (CCSst) reached the maximum value of 95.3%. These results suggest that the in situ synthesized platelet-shaped mullite phase observably improved the thermal shock resistance of Al2O3-SiC-C refractories.