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

Abstract WB 2 /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 WB 2 /CrN multilayer films, CrN sublayers present the columnar microstructure. As Λ decreases, the structure of WB 2 sublayers evolves from (110) orientation to (001) orientation to amorphous structure, and critical crystalline thickness for WB 2 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 WB 2 sublayers. Additionally, a-BN, WB 2 , WB 2 (N), CrN, Cr 2 N and Cr 2 O 3 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 WB 2 sublayers with (001) preferred orientation, and amorphous WB 2 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 WB 2 sublayers, controlling the N content in WB 2 sublayers and getting sharp interfaces will play important roles in developing the higher-performance WB 2 /CrN multilayer films. Highlights • WB 2 /CrN multilayer films with different bilayer periods over 50 nm are prepared. • A transition layer with nano-columnar crystal exists between WB 2 and CrN sublayers. • Critical crystalline thickness of WB 2 sublayers is over 150 nm in this work. • Decreasing bilayer period to ≤235 nm causes a reduction in hardness and wear rate. • Multilayer architecture promotes the film toughness and adhesive strength. [ABSTRACT FROM AUTHOR]