Effect of microstructure and mechanical properties difference between sub-layers on the performance of alternate hard and soft diamond-like carbon multilayer films

Alternate hard and soft diamond-like carbon (DLC) multilayer films with different modulus ratio and residual stress ratio between adjacent sub-layers were prepared by magnetron sputtering through alternating deposition bias. The microstructure, hardness and toughness of DLC films as well as their tribological properties under dry and water-lubricated sliding conditions were systematically studied in relation to the effects of modulus ratio and residual stress ratio between adjacent sub-layers. For multilayer DLC films with similar microstructure of adjacent sub-layers, the higher bias dominates the transformation from sp2 to sp3 carbon. As to multilayer DLC films with much different microstructure between adjacent sub-layers, the compact and ordered sub-layer structures formed under lower bias keeps growing even under higher deposition bias. Although multilayer strengthening effect is insignificant in multilayer DLC films, multilayer toughening effect is outstanding. Multilayer DLC films deposited at a substrate bias alternating between - 120 and - 160. V have the highest toughness which is almost double of that of monolayer DLC films, because the multilayer structure with low modulus ratio contributes to reduce the stress concentration in harder sub-layer thereby inhibiting crack initiation. Moreover, multilayer DLC films show better wear resistance than monolayer DLC films under both dry and water-lubricated sliding conditions. It was found that the higher toughness inhibited abrasive wear on the sliding steel counterface and contributed to the formation of transfer film.