Grain size softening in nanocrystalline TiN

The effect of grain size d on the hardness H of TiN is considered, with focus on the nanometer grain size range. H increases from 22 GPa to 32 GPa with decrease in d from single crystals to d = 50 nm. Three sets of data show that H decreases with further reduction in d below 50 nm, i.e., grain size softening occurs. The softening is not in complete or unequivocal accord with any of the three models normally proposed for such softening, namely Coble creep, grain boundary shear and dislocation line tension modification. Factors which could have contributed to the softening are: (a) changes in texture and in turn the corresponding value of the Taylor orientation factor M and (b) the presence of weakening imperfections produced during fabrication.