In situ synthesis of melt-grown mullite ceramics using directed laser deposition

While mullite is a significant oxide ceramic material, its current preparation process has many limitations such as complicated process, low efficiency, and high cost. In this paper, directed laser deposition (DLD) was used to prepare mullite ceramics directly using α-Al2O3 and SiO2 powder as raw materials. The phase composition, microstructure, and primary defects of prepared samples were investigated by XRD, Raman spectroscopy, and SEM/EDS. Vickers indentation and three-point bending experiments were used to evaluate mechanical properties. The results showed that microstructure was mainly composed of a mullite crystal phase and Si-rich glass phase. It is shown that mullite ceramics were successfully synthesized in situ during the DLD process with the use of Al2O3 and SiO2 powder. Morphology of mullite crystal inside cylindrical sample was particular “tabular cellular,” which transformed into “rod cellular” at the edge. Two mullite crystals were arranged along deposition direction basically, surrounded by the Si-rich glass phase. The defects included pores with various sizes and crack, in which large pores were mainly distributed at the edge of the sample, and small pores were throughout the section. The crack was distributed in the center of the sample and spread to the edge. Mechanical properties test showed flexural strength, microhardness, and fracture toughness were 62.8 ± 30.3 MPa, 12.7 ± 0.34 GPa, and 1.84 ± 0.14 MPa·m1/2, respectively. It is considered that pores and crack were responsible for poor mechanical properties. This research is expected to provide a new option for the rapid preparation of mullite ceramics.