Microwave sintering of chessboard-structured TiN–Si₃N₄  nanocomposites reinforced by nanofibers

Abstract

The microstructural design of composite nanomaterials for microwave sintering applying chessboard structurization is proposed. Since single-phase and composite nanoparticles and nanofibers are distributed according to the chessboard principle in the mixture and the Si₃N₄–TiN powder composites acquire a combined microstructure formed by components that significantly differ in the depth of microwave penetration into the volume, the consolidation process in the electromagnetic microwave field at a ratio of ~50 : 50 can be improved significantly. The effectiveness of the above principles was proved by producing high-density (~99% relative density) composites in the Si₃N₄–TiN system and composites reinforced with nanofibers. The chessboard-structured Si₃N₄–TiN composites were consolidated in a microwave furnace at a frequency of 2.45 GHz in a nitrogen flow at T = 1500 °C. In situ mixtures of TiN–40 wt.% Si₃N₄ and TiN–20 wt.% Si₃N₄ plasma-chemical powders with 7 and 20 wt.% silicon nitride nanofibers incorporated by mechanical mixing and preliminarily covered with titanium nitride were used. Microstructural analysis of the TiN–40 wt.% Si₃N₄ composite revealed that titanium nitride grains coarsened to 100–200 nm, while silicon nitride grains remained 30–50 nm in size. This indicates that the electromagnetic microwave energy is predominantly absorbed by titanium nitride grains resulting in their self-heating. The mechanical properties of the TiN–40 wt.% Si₃N₄ nanocomposite were as follows: HV = 21.2 ± 0.5 GPa and K_Ic = 4.9 MPa × m^1/2. Reinforcement of the composites with silicon nitride fibers, with their surface being covered with a titanium nitride layer, increases the fracture toughness to 5.5 MPa × m^1/2 at ~ 20 GPa hardness. Increase in the number of Si₃N₄ nanofibers from 7 to 20 wt.% did not improve the mechanical characteristics of the composite, indicating that the optimal amount of silicon nitride nanofibers needs to be determined in the composite.