Through nanoscratch experiments with a spherical diamond indenter, a contrastive study of the nanotribological properties of Ga- and N-faced gallium nitride (GaN) samples was carried out. Nanoindentation results revealed that the elastic modulus of the Ga-faced GaN sample was slightly higher than that of N-faced GaN sample. Particularly, Ga- and N-faced GaN samples exhibited rather different nanotribological properties, and the Ga-faced sample showed a stronger wear resistance. The study indicated that the critical normal load required to cause material removal of N-faced GaN sample was almost two times that of Ga-faced GaN sample. Both Ga- and N-faces exhibited a rather low frictional coefficient at the elastic and elastoplastic stages of material, e.g., ~0.06 for Ga-face and ~0.075 for N-face when scratching under the progressive normal load. Combined with transmission electron microscopy and X-ray photoelectron spectroscopy, we speculated that, except for the intrinsic atomic arrangements attributed to the non-reverse crystallographic symmetry of c-plane wurtzite GaN, the difference of nanotribological properties between Ga- and N-faces may also be related to the preferential formation of a native oxide layer and a slight lattice damage layer on the N-faced GaN surface. This study can enrich the understanding of the nanotribological properties of Ga- and N-polar-faced bulk monocrystalline GaN materials fabricated by the conventional technique.
Keywords: frictional coefficient; gallium nitride (GaN); nanoscratch; nanotribological properties; wear resistance.