The influence of amorphous phase to plastic hardness of super- and ultrahard nanocomposites

Hardness and yield stress of (polycrystalline) materials typically increase with decreasing grain size. However, in nanocomposites materials which consist of nanocrystalline phase (filler) embedded in amorphous phase (matrix), the hardness relation to the crystallite size curve shifts due to amorphous fraction. Experimental data show that some coatings have very different hardness (H_v > 40-105 GPa) even though they have similar grain (nanocrystalline) size. The shifting of hardness versus crystallite size curve in nanocomposites is of particular interest. Here we use mechanical system modeling to determine the plastic hardness based on displacement of nanocrystallines during indentation process in nanocomposite nc-TiN/a-Si ₃N₄. Our hypothesis is that the nanocrystalline behave like a rigid body that move relative to each other under shear condition by indentation load. This relative movement is retarded by drag force from the surface friction between nanocrystals and amorphous phase. Our findings indicated that the influence of amorphous content in nanocomposites shift the plastic hardness as the amorphous fraction is varied. In the range of 3-13 nm grain size, one can construct several hardness versus crystallite size curve by varying the amorphous fraction. The experimental data of nanocomposite nc-TiN/a-Si₃N₄/a-&nc-TiSi₂ agree well with our calculation results.