Recrystallization and grain growth of nanocomposite Ti–B–N coatings

Nanocomposite Ti–B–N coatings with different chemical composition were prepared by non-reactive co-sputtering of a segmented TiN–TiB₂ target. The coatings investigated are primarily composed of nanocrystalline TiN and TiB₂ phases. Increasing boron content results in a decreasing grain size from approximately 6 to 2 nm. During a thermal treatment of such coatings solely recovery and recrystallization with subsequent grain growth would appear, since the two phases are in thermodynamic equilibrium. Differential scanning calorimetry (DSC) and X-ray diffraction analysis were used to investigate the recrystallization behavior and subsequent grain growth of the nanocomposite Ti–B–N coatings. On heating the coating samples, which were removed chemically from their low alloyed steel substrates, an exothermal peak appeared during the DSC measurements indicating grain growth. From the onset temperature of this peak the recrystallization temperature was found which increases with increasing boron content from 1032 to 1070 °C. Activation energies for grain growth are obtained from Kissinger plots and yield values decreasing from 7.9 to 4.4 eV with increasing boron content. After heat treatment up to 1400 °C during the DSC measurements the coatings showed grain sizes within the range of 15–30 nm. It was found that the highest recrystallization temperature does not imply the highest activation energy for grain growth.