Mechanical Properties of Superhard Nanocompo.sites with High Thermal Stability

摘要

Superhard nanocomposiles, nc-M_nN/a-X_xN_y (M = Ti, W, V, Zr, (Al_(1-x)Ti_x)N; X = Si, B) with hardness of 40-100 GPa are prepared by plasma CVD or PVD under a sufficiently high nitrogen activity and deposition temperature that allow the formation of a stable nanostructure by self-organization upon strong thermodynamically driven, spinodal phase segregation. These nanocomposiles display an extraordinary combination of a high hardness, high elastic recovery, high resistance against brittle fracture and tensile strength of 5 to 40 GPa approaching the ideal strength of flaw-free materials. These properties can be understood in terms of conventional fracture physics scaled appropriately down lo crystallite sizes of few nm. The interfa-cial monolayer of Si_3N_4 or BN with strong bonding to the nanocrystallites and high structural flexibility avoids grain boundary sliding. With increasing thickness of this interface the hardness decreases, possibly due to an increase of this "liquid-like" component in which plastic transformation can be triggered.