Studies On Thermal Stability And Strenghening Mechanism Of Multilayerded W Doped Nanocrystalline Ni-based Thin Flims At Elevated Temperature

Recently,with expanded the application of microelectromechanical systems?MEMS?,more MEMS sensors are used in harsh environments.This will require new demands of material selections for the structural element of many MEMS devices.Pure nickel film as a traditional material has been widely studied in majority of commercial MEMS for load-bearing application.Unfortunately,pure Ni film suffers from microstructure instabilities and significant property degradation due to rapid growth of nanocrystalline grains upon thermal exposure.This limits the application of pure Ni film in MEMS applications at elevated temperature.In order to improve the thermal stability and high-temperature performance of Ni films,one promising route is alloying with W elements at larger concentrations into Ni films.However,the internal stress is raised and some of intrinsic physical properties are compromised in Ni-based films if the amount of W is excessive.Another viable approach is deposition of multi-component nanocrystalline Ni-based superalloy films.The challenges are the weakened suppression of grain growth and the prevalence of overaged?-phase,which is adversed to the elevated temperature mechanical properties.Hence,the urgentest challenge for material design of MEMS Ni-based films is to produce better excellent thermo-mechanical properties without the presence of a large concentration of W.To solve the abovementioned issue,Ni/Ni₃Al multilayers with individual layer thickness?h?of 10¹60 nm,Ni₃Al-W monolayered films and Ni/Ni₃Al-W multilayers at W concentrations?20.6 at%,as three kinds of films were co-sputtered on a Si?100?wafer with a 300 nm thick SiO₂ diffusion barrier layer by direct-current?DC?magnetron sputtering.Subsequently,as-sputtered samples were annealed in encapsulated vacuum quartz tube at range from 500°C to 800°C in order to simulate extreme temperature cycles that MEMS device can stand in normal service.The phase constitutions,the microstructured morphology and the interface structures were charactered and analyzed by X-ray diffusion?XRD?,atomic force microscope?AFM?,scanning electron microscopy?SEM?and transmission electron microscopy?TEM?,respectively.The results reveal that the grain growth during annealing at high temperature is retarded in Ni/Ni₃Al multilayers with smaller h.For Ni₃Al-W films and Ni/Ni₃Al-W multilayers,W addition suppresses grains growth significantly as enhancement of thermal stability of grain sizes in annealed films.In terms of thermal-induced phase transformations,the size-dependent chemical ordering occurs,with formation of ordered L₁₂ Ni₃Al intermetallics in annealed Ni/Ni₃Al multilayers.For Ni₃Al-W films,W addition delays the chemical ordering and crystallization at 500?,and lead to precipitate?-W phase at 700?.The phase separation is promoted by more W dopant for Ni/Ni₃Al-W multilayers at elevated temperature,leading that many kinds of W-related phases are precipitated.The Ni₄W and?-W phases are regarded as mainly precipitated phases for 800?annealed Ni/Ni₃Al-W?12.5 at%?and?20.6 at%?multilayers,respectively.With respect to morphology of microstructure,the multilayered structure shows poor thermal stability without constraint of grain growth for 700?annealed Ni/Ni₃Al multilayers at both larger h?80nm and smaller h?10nm.When h=20-40 nm,although the layer interfaces still maintain relative sharp with the presence of considerable annealing twins.On contrary,no obvious changes are detected in morphology of both 500?annealed Ni₃Al-W films and 600?annealed Ni/Ni₃Al-W multilayers.At temperature range of 500?-600?,these films exhibit high thermal stability due to the drag force for layer interfaces and grain boundary motions provided by W atoms and triple junctions?TJ?.At 700?,the spherical nano-sized?-W particles are precipated in?-Ni solid solution and the grain coarsen are inhibited via Zener pinning for Ni₃Al-W films.Upon annealing at 800?at 12.5 at%W content of Ni/Ni₃Al-W multilayers,layer interfaces can be reserved due to low density of precipations,which provided the pinning of grain boundary.This leads to increase thermal stability of microstructure of 800?annealed multilayers.The results of nano-hardness test of films under different annealing conditions shows that the hardness of Ni/Ni₃Al multilayers decreases with increasing annealing temperature.For as-deposited and 500?annealed multilayers,the hardness shows the strength behavior at smaller h.At 700?,40 nm Ni/Ni₃Al multilayer exhibits a little thermal-induced drop of hardness,which attributes to strengthening via combination of refined grain,high density of layer interface and formation of ordered Ni₃Al phase.Hence,based on above results,40 nm is adopted as h for the Ni/Ni₃Al-W multilayers in this study in order to obtain the optimal thermo-mechanical properties for the multilayered geometry.In contrast,the hardness of both Ni₃Al-W films and Ni/Ni₃Al-W multilayers is enhanced with increament of annealing temperature,as presence of strong anneal hardening.At 500?and 600?,grain boundary relaxation triggered in nanocrystalline films as a dominating mechanism is responsible for the hardness increments.In addition,the hardness is also increased slightly via corresponding strengthening mechasiam such as chemical ordering,initial crystallization,incoherent interface and initial precipitation for films with different W contents and layer structures.At 700?,the higher hardness of Ni₃Al-W films at value of 13.2 GPa can be attributed to precipitation of nano-sized?-W particles via Orawan strengthening mechanism.Further increasing temperature to 800?,the peak hardness is achieved at the value of 15.6 GPa for Ni/Ni₃Al-W?12.5 at%?multilayers.Based on grain refinement induced by W addition,the combination of hardening precipitation and survived lamellar structure via the Orowan mechanisms is responsible for the high degree of strengthening at elevated temperature,offering a feasible insight to develop nano-metallic coatings for further increasing thermo-mechanical properties.