Investigation of hardness and fracture toughness properties of Fe/VC multilayer coatings with coherent interfaces

• Fe/VC multilayer coatings with a fixed period (Λ = 8 nm) containing 60-90% Fe were deposited via dc magnetron sputtering.
• Fe0.9/VC0.1 coating has almost twice the fracture toughness of other Fe/VC coatings, while the high hardness are remained.
• The enhanced toughness is due to the generation and activation of dislocations in the Fe/VC multilayer coatings.
• For the Fe/VC multilayer coatings, the H/E and H3/E2 vary inversely as the toughness.

Fe/VC multilayer coatings with a fixed bilayer period (Λ = 8 nm) and variable Fe fraction (ΛFe/Λ ranging from 0.6 to 0.9) were deposited on Si substrates via dc magnetron sputtering. X-Ray diffraction and transmission electron microscopy showed the nanoscale layer structure of these coatings. High-resolution transmission electron microscopy imaging revealed the formation of coherent interfaces between Fe and VC (100). Throughout the entire range of Fe fractions investigated in this study, the hardness was enhanced over the rule-of-mixture trend line. Even at Fe fraction of 0.9, the hardness value was 16.3 GPa, enhanced by about 80% over the rule-of-mixture value of 9 GPa. While all Fe/VC coatings investigated in this work have statistically the same hardness, Fe/VC coating with Fe fraction of 90% has almost twice the fracture toughness of the other Fe/VC coatings. We attribute this enhanced toughness to the favorable generation of dislocations in the Fe layers and their activation by coherency strain emanating from Fe/VC interfaces. This study also demonstrates that H/E and H3/E2 are not good proxies for the toughness of these coatings.