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 5nm. 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 10GPa. 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.