One-monolayer-thick ${\text{SiN}}_x$ interfacial layer in superhard nanocomposites, consisting of 3–4 nm size TiN nanocrystals joined by that layer, is stronger than a bulk ${\text{SiN}}_x$ crystal due to valence charge transfer from the metallic TiN, thus providing the nanocomposites with significant hardness enhancement. However, this enhancement is lost when the thickness of the interfacial SiN increases to $\ge 2$ monolayers and the hardness decreases. We show that the softening of the nanocomposites with thicker ${\text{SiN}}_x$ interface is caused by the weakening of the TiN bonds close to that interface, that increases with increasing of the ${\text{SiN}}_x$ thickness. Other possible mechanisms of the softening are briefly discussed and ruled out. This finding may open up possible way of preparing new, even stronger superhard nanocomposites.

• Received 13 November 2009

DOI:https://doi.org/10.1103/PhysRevB.81.245418