Relationship between hardness and fracture toughness in Ti–TiB2 nanocomposite coatings

We have synthesized Ti–TiB2 nanocomposite coatings by dual-cathode magnetron sputtering in argon onto polished titanium and silicon substrates with the TiB2 volume fraction varying between 6 and 45%. Over this volume fraction range, TiB2 is present as nanoparticles with diameter ~ 3 to 5 nm. When deposited on titanium substrates, the coatings are relatively stress-free. Through most of the composition range, the coating hardness is enhanced over the rule-of-mixture value by 5 to 10 GPa. We used two methods to determine coating fracture toughness: nanoindentation and nanoscratch. Results from these two methods are consistent and demonstrate that Ti–TiB2 nanocomposite coatings exhibit a “flatter” hardness–toughness relationship compared with metal carbide/DLC and TiN/a–Si3N4 nanocomposite coatings, i.e., the hardness decreases slower with increasing toughness. We speculate that the improved toughness of Ti–TiB2 nanocomposite coatings is due to the formation of coherent Ti–TiB2 interfaces, which in turn produce stress fields activating the motion of nearby dislocations within the Ti matrix and hence improved fracture toughness.

► Ti–TiB2 nanocomposite coatings were made with TiB2 vol. fraction between 6 and 45%. ► Coating hardness enhancement can be explained by Orowan strengthening. ► Fracture toughness was measured using nanoindentation and nanoscratch testing. ► Hardness falls more slowly with rising toughness compared with previous coatings. ► Enhanced toughness may be due to formation of coherent Ti–TiB2 interfaces.