CRYSTALLIZATION OF SiC-AlN SOLID SOLUTION IN THE SHS PROCESS

Control of composition and microstructure in the SiC-AlN system has been shown to potentially increase ballistic performance. Additionally, the energy band gap of the solid solution can be controlled, enabling unique semiconductor applications. However, consolidation at up to 2200^oC results in SiC-rich and AlN-rich phases, rather than a homogeneous solid solution, because of sluggish diffusion. In situ formation of SiC-AlN solid solutions is expected to result in better homogeneity; One such reactive method is Self-propagating High-temperature Synthesis (SHS).

Ceramic SiC-AlN powders with 40 mol.%AlN were produced by SHS, from powders of the elemental constituents (Si, Al and C) and under N₂ pressure. The exothermic reaction proceeded at a velocity of about 1 mm/s, and with temperatures exceeding 2100^oC. Powder samples were taken from various locations of the spongy product.

Powder XRD showed that the spongy material is a (SiC)_1-x (AlN)_x solid solution with the wurtzite structure, with
x ≅ 0.40. Some peak broadening was observed (but no peak splitting), suggesting that practically all the material was in the solid solution state, with some variations in composition.

SEM studies revealed several typical particle morphologies, e.g., relatively large (2 – 10 μm) crystallites with clear facets tended to grow one on top of the other, forming dense polycrystalline "sintered" bodies, up to ~50 μm in size, with x ~ 0.35-0.40, determined by EDS. Another typical morphology comprised of agglomerates of nano/sub-micron particles, with turbulent appearance, sometimes containing spherical agglomerates, with x ~ 0.25-0.55.

The results are discussed in terms of the complicated sequence of solid solution formation within the propagating reaction front, involving possible gas phase reaction (Al + N₂), as well as liquid/solid state reactions and diffusion.