Reversing the inverse hardness–toughness trend using W/VC multilayer coatings

•It is possible to synthesize metal-ceramic nanolayer coatings that are harder and tougher than the pure ceramics alone•W/VC multilayer coatings with bilayer thickness of 10 nm containing 30-90% W are harder and tougher than pure VC•H/E and H3/E2 are not good proxies for coating toughness.

It is desirable to increase the hardness of protective coatings for reduced abrasive wear and to increase the toughness for improved wear performance due to fatigue and formation of flaws or cracks. Unfortunately, there is an inverse relationship between hardness and toughness: the harder the coating, the lower the toughness. Using W/VC multilayer coatings, we demonstrated that it is possible to reverse this hardness–toughness trend. These coatings were synthesized by DC magnetron sputtering with bilayer periods of about 10 nm and different thickness fractions of W. They are crystalline, with hardness and toughness exceeding that of pure VC (25 GPa and 1.0 MPa-m1/2). In particular, the W/VC multilayer coating with thickness fraction of 90% W achieves hardness of 28.5 GPa, comparable to many ceramics-based hard coatings, and toughness of 7.5 MPa-m1/2, similar to many nanocrystalline metals. This investigation shows that one can synthesize coatings as hard as ceramics and as tough as metals. These results also indicate that H/E and H3/E2 (H = hardness and E = elastic modulus) are not good proxies for coating toughness.