Recently, a hardness similar to that of diamond has been reported for a quasiternary, nitride-based nanocomposite. The related, quasibinary nanocomposite "nc-TiN/a-Si3N4," which may be regarded as the prototype of the family of superhard nc-metal-N/a-Si3N4 systems, also exhibits a significant hardness enhancement. Extensive density-functional theory calculations indicate that the superhardness is related to the preferential formation of TiN(111) polar interfaces with a thin beta-Si3N4-derived layer. The strength of TiN in the 111 direction is similar to that of the weakest bonding direction in diamond. Oxygen impurities cause a significant reduction of the interface strength.