Effect of modulation period on structure and mechanical properties of WB2/a-WBC films deposited by direct-current magnetron sputtering

Abstract

WB2/a-WBC multilayer films with different modulation periods ranging from 145 to 830 nm were synthesized by direct-current magnetron sputtering. The effect of modulation period (Λ) on structure and tribo-mechanical properties of the WB2/a-WBC multilayer films was studied. The WB2 monolayer grows preferentially along (101) plane. However, the multilayered structure changes the growth of WB2 sublayers to (001) plane, and a transition layer with composition gradient distribution is formed at the interface between the two sublayers. Consequently, WB2, WB2(C) solid solution and a-WC are detected in the WB2 sublayer, while a-WBC, a-WC and a-C are found in the WBC sublayer. As the modulation period decreases, the residual compressive stress (-384∼-827 MPa) of the WB2/a-WBC films increases first and then decreases, and the hardness of the WB2/a-WBC films (18.8∼23.7 GPa) gradually increases under the joint action of solid solution strengthening and interface strengthening. Moreover, the fracture toughness of the WB2/a-WBC films is also improved greatly resulting from the multilayered structure with the soft a-WBC sublayers. The wear mechanism of the WB2/a-WBC films is a mixture of abrasion and lubrication, and the friction coefficient and wear rate of the WB2/a-WBC films are lower than those of the WB2 and WBC monolayer films because of their excellent toughness and lubricating effect as well as moderate hardness. Comprehensively, the WB2/a-WBC films with Ʌ=830 and 415 nm display lower friction coefficient (∼0.08) and wear rate (1.14∼1.24 × 10−7 mm3/mN).