On the mechanism of formation of the granular structure and interfaces in self-reinforced aluminum nitride


I. M. Frantsevich Institute for Problems of Materials Science of the NAS of Ukraine, Kyiv
Usp. materialozn. 2021, 2:63-71


Results of an investigation by scanning electron microscopy methods of the microstructure of self-reinforced aluminum nitride obtained on the basis of aluminum nitride powder and containing 3% (mass.) oxygen by plasma-chemical synthesis in the temperature range 1700—2000 °С are presented. Initial aluminum nitride was represented by the wurtzite (2Н) phase. Samples were obtained by free sintering in a  nitrogen atmosphere. It was established that, during sintering of АlN in the indicated temperature range, three microstructural types of the material and six types of interfaces (three types of intergranular and three types of intragranular ones) formed. The features of the microstructure of the materials are fully determined by the development of intergranular crystal-oriented polytype transitions of 2Н АlN→multilayer polytypes (MP) in sintering. The sequence of successive structural transformations that determine the development of polytype transitions was established. 1. Formation of initial 2Н АlN grains of the solid solution 2Н АlN-О. The substitution of nitrogen by oxygen takes place. 2. The development of isomorphous delamination of the solid solution in every grain with the precipitation of an interlayer enriched in oxygen. 3. In interlayers, polytypes consisting of a series of polytypes with different number of layers (MP) form. Such an interlayer has developed base surfaces and propagates from one boundary to another in the grain, which determines the formation of a special structural state of fragments of boundaries, that border interlayers in the direction . 4. The high mobility of the indicated fragments of boundaries determines their break-off from the common boundary and formation of a grain nucleus of anisometric (plate-like) shape. This process begins already at a sintering temperature Tsint. = 1800 °С, and, at Tsint. = 2000 °С, polycrystals practically entirely consist of grains of plate-like shape. In this case, with increase in the sintering temperature, the aspect ratio (the length-to-width ratio) of such grains rises.

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