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

Abstract

WB2/CrN multilayer films with thick modulation periods over 50 nm (Λ = 1400, 315, 235, 150, 55 nm) were synthesized by direct-current magnetron sputtering, and the influence of modulation period on microstructure and mechanical properties for the multilayer films was systematically studied. In WB2/CrN multilayer films, CrN sublayers present the columnar microstructure. As Λ decreases, the structure of WB2 sublayers evolves from (110) orientation to (001) orientation to amorphous structure, and critical crystalline thickness for WB2 sublayers is over 150 nm here. A transition layer, which shows the columnar crystal with size of 10–11 nm high and 2.5–3.5 nm wide caused by the effect of the crystalline interface of the CrN sublayers, is detected in WB2 sublayers. Additionally, a-BN, WB2, WB2(N), CrN, Cr2N and Cr2O3 phase are formed in the multilayer films. Moreover, film hardness mainly obeys the rule of mixture. The maximum hardness of 31.2 GPa is obtained at Λ = 315 nm due to crystalline WB2 sublayers with (001) preferred orientation, and amorphous WB2 sublayers greatly reduce the film hardness to only 22.3–24.3 GPa at Λ ≤ 235 nm. Consequently, the poor hardness leads to the higher wear rates (5.7–7.8 × 10−7 mm3/mN) of multilayer films with Λ ≤ 235 nm compared with those (2.9–3.3 × 10−7 mm3/mN) of other films. However, both the fracture toughness and adhesive strength of the films present an increasing trend with decreasing Λ, resulting from the soft CrN and BN phases and a certain amount of interface. In conclusion, decreasing the critical crystal-thickness of the WB2 sublayers, controlling the N content in WB2 sublayers and getting sharp interfaces will play important roles in developing the higher-performance WB2/CrN multilayer films.