Growth of single-crystal TiN/VN strained-layer superlattices with extremely high mechanical hardness

Single-crystal TiN/VN strained-layer superlattices (SLS's) with layer thicknesses l_TiN =l_VN =λ/2 (where λ is the period of the superlattice) ranging from 0.75 to 16 nm have been grown on MgO(100) substrates by reactive magnetron sputtering. Cross-sectional transmission electron microscopy (TEM) and x-ray diffraction examinations showed that the films were single crystals exhibiting coherent interfaces and several orders of superlattice reflections. There was no evidence in either plan-view or cross-sectional TEM analyses of misfit interfacial dislocation arrays. The primary defects observed were dislocation loops with a diameter of 8-10 nm extending through several layers and small defects with a diameter of 1-2 nm that were confined within single layers. Microindentation hardness values H, measured as a function of λ in films with a total thickness of 2.5 μm, increased from 2035±280 kg mm^-2 for Ti _0.5V_0.5N alloys (i.e., λ=0) to reach a maximum of 5560±1000 kg mm^-2 at λ=5.2 nm and then decreased rapidly to 3950±550 kg mm^-2 at λ=7.5 nm. Further increases in λ resulted in a slower decrease in H to 3640±550 kg mm^-2 at λ=32 nm. The large error bars in the H values for the SLS samples were due to the difficulty in measuring such extremely high hardnesses in thin films. (H for pure single-crystal TiN and VN films were 2200±300 and 1620±200 kg mm^-2, respectively.) SLS samples grown with constant λ=6.5 nm, but different l_TiN /λ ratios, exhibited a maximum hardness at l_TiN /λ≅0.3.