Effect of modulation ratio on structure and mechanical properties of WB2/CrN films deposited by direct-current magnetron sputtering

Abstract WB2/CrN multilayer films with thick modulation period about 500 nm but different modulation ratios (tWB2:tCrN = 1.2, 2, 3 and 4) were deposited on steel substrates by magnetron sputtering, and the effect of modulation ratio on the structure, residual stress and tribo-mechanical properties of the WB2/CrN multilayer films was systematically studied. Additionally, WB2 and CrN monolayer films with various thicknesses were also prepared to discuss the growth behavior and the related properties of the WB2/CrN multilayer films. All the sample films present the columnar growth, and the particle size in the WB2 monolayers is smaller than that in the corresponding WB2 sublayers. The preferred orientation of WB2 (260–400nm) and CrN monolayers (132–220 nm) is (101) and (111), respectively. However, the multilayered structure changes the orientation of CrN sublayers to (002), and crystalline Cr2N phase is detected in WB2/CrN multilayers caused by the element diffusion at interface. Moreover, the multilayered structure reduces the compressive stress of the WB2 films greatly by diffusion of point defects to interfaces and an indirect effect of interfaces on stress via film structure with soft CrN, and the residual stress of the WB2, CrN and WB2/CrN films is −2.64∼-4.16, −0.85–0.57 and −1.49∼-2.61 GPa, respectively. Furthermore, film hardness ranging from 27.9 to 33.0 GPa mainly obeys the rule of mixture. The adhesive strength drops greatly from 26 to 17 N with the increasing tWB2:tCrN, and the tensile CrN bottom-layer deteriorates the adhesive strength of the WB2/CrN multilayers with tWB2:tCrN = 2. Overall, WB2/CrN films with tWB2:tCrN = 3 show better wear resistance with lower friction coefficient ∼0.35 and wear rate about 3.6 × 10−7 mm3/mN benefiting from their higher hardness, lower roughness, proper toughness and compressive stress.