Abstract
The microstructure and oxidation property of NbSi 2/α-Si 3N 4 nanocomposite coating formed on Nb substrate by a prior nitridation process followed by pack siliconizing process were investigated. The Nb-nitride layers formed by nitridation process at 1300 °C consisted of two layers, i.e. the outer layer of mixed nitrides of NbN and Nb 4N 3 and the inner layer of Nb 2N. While the monolithic NbSi 2 coating showed the typical columnar microstructure perpendicular to the Nb substrate, the NbSi 2/α-Si 3N 4 nanocomposite coating formed by the solid-state displacement reaction of the outer Nb-nitride layer with Si was composed of the equiaxed NbSi 2 grains and the α-Si 3N 4 particles with the oblate-spheroidal shape, which were mostly located at the grain boundaries of NbSi 2. The average size of equiaxed NbSi 2 grains and α-Si 3N 4 particles were about 44–125 and 33–45 nm, respectively. The nanocomposite coating formed by the solid-state displacement reaction of the inner Nb-nitride layer with Si showed the columnar microstructure. The average diameters of NbSi 2 grains and the α-Si 3N 4 particles were about 255 and 43 nm, respectively. The volume percentage of α-Si 3N 4 particles ranged from 16.8 to 24.4% with respect to nitrogen concentration in Nb-nitride layers. No cracks were observed in the nanocomposite coating, indicating that its thermal expansion coefficient was close to that of Nb substrate. The isothermal oxidation resistance of NbSi 2/α-Si 3N 4 nanocomposite coating in 80% Ar–20% O 2 atmosphere at 1100 °C was superior to that of monolithic NbSi 2 coating.
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