Investigations on the microstructure and hardening mechanism of TiN/Si3N4 nanocomposite coatings

A detailed TEM investigation on the microstructure of TiN/Si3N4 nanocomposite coatings, which is believed to be responsible for the coatings' remarkable mechanical properties, was carried out. Parallel simulation utilizing two-dimensional TiN/Si3N4 nanomultilayered coatings was further performed to study whether the variation of Si N-3(4) interlayer thickness has an influence on the coatings' microstructure and mechanical properties. The results revealed that, in nanocomposite coatings with high hardness, Si3N4 tissue has a thickness of about 0.5 - 0.7 nm and exists in the crystalline state. Low-energy coherent interfaces are formed between Si3N4 and neighbouring elongated TiN grains. For TiN/Si3N4 nanomultilayered coatings, Si3N4 modulation layers with thicknesses less than 0.7 nm were also found to crystallize and form coherent interfaces with the neighbouring TiN layers; at the same time, the hardness of the coatings is remarkably enhanced. When its thickness exceeds 1.0 nm, Si3N4 transformed its growth mode into amorphous and the coherent interfaces were damaged; as a consequence, hardness enhancements in the coatings vanished. The similarity of the microstructure and the mechanical properties response between nanocomposites and nanomultilayered coatings indicates that the crystallization of Si3N4 as well as the formation of coherent interfaces between TiN and Si3N4 is the main reason for the hardening of the nanocomposites.