Enhancement of high-temperature strength of Ni-based films by addition of nano-multilayers and incorporation of W

Nanocrystalline Ni thin film exhibits poor thermo-mechanical properties due to its unstable microstructure at elevated temperature. Here, the paper endows a new approach to solve above issue via addition of nano-multilayers and incorporation of W for nanocrystalline Ni-based films, to provide novel Ni/Ni3Al-W nano-composite multilayered structure with high hardness and good thermal stability. The thermal evolution of microstructure and mechanical properties was investigated to reveal nanocrystalline stability and strengthening mechanisms for co-sputtered Ni/Ni3Al-W multilayers with varied W concentrations and annealing temperature. The lamellar structure and nonequilibrium phases are well maintained in 600 °C annealed multilayers, while nano-grains are further refined with increasing W addition. Annealing at 800 °C results in the appearance of elemental redistribution and phase separation in multilayers, leading to the layered structure dissolved and globular W-related particles precipitated. Annealing hardening is founded in most of annealed Ni/Ni3Al-W multilayers. Based upon microstructure observation, grain boundary relaxation and W-related phase precipitation are mainly responsible for the hardness enhancement of multilayers at 600 °C and 800 °C, respectively. Notably, the best hardness is achieved at the value of 15.6 GPa for 800 °C annealed 12.5 at% W doped Ni/Ni3Al-W multilayer, which shows the residual layer interfaces with larger precipitations in microstructure. This hardness increment for annealed Ni-based multilayers can be attributed that the high degree of strengthening is provided by a combination of hardening precipitation and survived lamellar structure via the Orowan mechanisms, offering a feasible insight to develop nano-metallic coatings for further increasing thermo-mechanical properties.

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