The Influence of Bilayer Periods and Ratios on Mechanical and Tribological Properties of TiN/MoN Superlattice Thin Films

Abstract

Transition metal nitrides are commonly used in hard and protective coating industry, but still limited by low intrinsic fracture toughness. Encouraged by the previous study that superlattices (SLs) could remarkably improve strength and ductility, in this study, some TiN/MoN SL thin films are sputtered on (100) MgO substrates. These SLs are with bilayer periods (Λ) of 2–23 nm and a bilayer ratio (ℓTiN:ℓMoN) of 1:1, 1:0.5, 1:1, 1:2, and 1:2.7. This work is aim to explore the influence of bilayer periods and ratios, and the vacancies on TiN/MoN superlattice. All SLs – independent of bilayer period, bilayer ratio, and nitrogen content – present a rocksalt structure, with high-order satellite peaks during X-ray diffraction. But a weak tetragonal β-Mo2N signal is also detected for the SLs with Λ = 7.3 nm (ℓTiN:ℓMoN = 1:2). The SLs with bilayer ratios of 1:0.5 and 1:2 do not show superlattice effect and bilayer-period-dependent. The SLs with ℓTiN:ℓMoN = 1:2.7 provide highest hardness due to a higher nitrogen supply during deposition. Contrary, the SLs with ℓTiN:ℓMoN = 1:2 have the worst mechanical properties due to the tetragonal β-Mo2N phase exist. Among all SLs investigated, those with ℓTiN:ℓMoN = 1:1 provide the best blend of mechanical properties, such as H = 34.8 ± 1.6 GPa, KIC = 4.1 ± 0.2 MPa√m, and μ = 0.27, when Λ = 9.9 nm. This study extends our understanding of superlattice effect in general, especially on the influence of bilayer periods and bilayer ratios, as well as nitrogen supply and heterogeneous microstructures, which is benefit to next SL design.