SHS-metallurgy of aluminum oxynitride and its subsequent purification from impurities
A promising direction in design of new materials is the development of high-strength transparent ceramics. Examples of fabricating transparent ceramics out of alumomagnesium spinel or leucosapphire are known. However, alumomagnesium ceramics have significantly lower strength than leucosapphire, while the latter is too expensive due to a complexity and low productivity of its technology. One of leading materials for manufacturing high-strength polycrystalline transparent ceramics is aluminum oxynitride composition Al<sub>23</sub>O<sub>27</sub>N<sub>5</sub> (or Al<sub>5</sub>O<sub>6</sub>N). Introduction of nitrogen into aluminum oxide results in a formation of an isotropic cubic crystal structure of oxynitride, stabilizes its matrix and gives it a high strength and crack toughness. The present study aims to establish the scientific bases for a new high-performance low power high-purity aluminum oxynitride powder technology Al<sub>23</sub>O<sub>27</sub>N<sub>5</sub> (Alon), which includes synthesis of ingots by SHS-metallurgy, subsequent disintegration of ingots to obtain powders as well as magnetic and chemical removal of impurities. In experiments in the process reactor SHS-30, it was found that an increase of the weight of a starting mixture from 100 g to 3 kg leads to a noticeable change in the desired product composition: nitrogen content increases from 7.7 to 10.8 % N, and Al and O content correspondingly decreases. According to X-ray phase analysis, the main phase in cast oxynitrides is Al<sub>5</sub>O<sub>6</sub>N. An appreciable content of Al<sub>7</sub>O<sub>3</sub>N<sub>5</sub> was also detected. A method for disintegration of cast aluminum oxynitride has been developed, and optimal grinding parameters have been determined that make it possible to obtain powders with a particle size of less than 100 μm. After grinding, the particles are fragmented. In the integral composition of the particles there are traces of milled steel balls, the material of which is spread over the surface of Alon particles. The magnetic and chemical cleaning practically completely removes impurities (Mo, Al, Fe, AlN) arising due to the incompleteness of chemical transformation, product contamination and milling of steel when grinding.