Microstructure and oxidation property of NbSi2/Si3N4 nanocomposite coating formed on Nb substrate by nitridation process followed by pack siliconizing process

The microstructure and oxidation property of NbSi2/α-Si3N4 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 Nb4N3 and the inner layer of Nb2N. While the monolithic NbSi2 coating showed the typical columnar microstructure perpendicular to the Nb substrate, the NbSi2/α-Si3N4 nanocomposite coating formed by the solid-state displacement reaction of the outer Nb-nitride layer with Si was composed of the equiaxed NbSi2 grains and the α-Si3N4 particles with the oblate-spheroidal shape, which were mostly located at the grain boundaries of NbSi2. The average size of equiaxed NbSi2 grains and α-Si3N4 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 NbSi2 grains and the α-Si3N4 particles were about 255 and 43 nm, respectively. The volume percentage of α-Si3N4 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 NbSi2/α-Si3N4 nanocomposite coating in 80% Ar-20% O2 atmosphere at 1100 °C was superior to that of monolithic NbSi2 coating.