Sputter Deposition, Mechanical Properties, and Thermal Stability of Nitride Based Superlattices

Sammanfattning: Investigations concerning deposition, mechanical properties and thermal stability of nitride-based superlattice films have been performed. X-ray diffraction and transmission electron microscopy techniques were adopted for microstructural characterization, while nanoindentation hardness and abrasive wear resistance measurements were utilized for evaluation of mechanical properties.A dual-target sputter deposition system with a solenoid coil serving to increase plasma density in near substrate position was designed. A high ion bombardment intensity at the substrate was achieved for each depositing magnetron, and the coil current strength and direction may be altered in synchrony with each depositing magnetron and its deposition time. The plasma density could exhibit a 25-fold increase of the ion flux to the substrate, with a typical ion-to-neutral arrival rate ratio of ~ 20. The system described offers prospects of realizing layered structures consisting of materials with a large difference in melting temperatures and/or large difference in atomic mass, where the materials need different ion-assisted growth conditions for controlled texture and film density at low deposition temperatures.Microstructure, residual stress, nanoindentation hardness, and abrasive wear resistance were investigated for polycrystalline nanolayered TiN/NbN thin films deposited by dual target reactive magnetron sputtering. The hardness showed only a small variation with layer period A, with an overall high value in between the corresponding homogenous nitride films (25-40 GPa). The residual stress in the nanolayer structures (<1.6 GPa), however, was significantly lower than the homogeneous or alloyed films (≥3.2 GPa). Homogeneous hexagonal β-Nb2N films exhibited the largest wear resistance, and alloyed TiNbN films were more wear resistant than the nanolayered TiN/NbN films. The absence of superlattice hardening in the polycrystalline TiN/NbN multilayers was related to rnicrocracking at grain boundaries under the indenter tip.A new class of non-iso structural metal/nitride superlattices were investigated, where a bee-metal (Mo, W) was combined with a NaCl-structure nitride (NbN). A substantial hardness enhancement was observed, as compared to the rule-of-mixture, with maximum hardnesses of ~30 GPa at a superlattice period Λ of ~2-3 nm. For Λ above 3 nm, the hardness decreased with a Hall-Petch-type dependence, but with a power law of ~O.3. These superlattices provide features that may be of value for hardcoating applications; the combination of a brittle ceramic with a more-ductile metal which may provide hard coatings with a varying degrees of ductility.The interdiffusion behavior of TiN/NbN and Mo/NbN systems was determined. The interdiffusion was investigated by electron microscopy and by studying the evolution of superlattice satellite peaks in x-ray diffraction. In situ x-ray diffraction spectra were recorded, using synchrotron light and a linear detector, during isothermal anneals. The results pointed to a non-linear diffusion behavior in TiN-NbN couples, where the structure maintained abrupt interfaces throughout annealing, while the position of the interfaces was continuously shifted into the TiN layers. A model is proposed, where Ti diffuses into the NbN-layer to form a NbTiN alloy, while the diffusion of Nb in the opposite direction is restricted due to high activation energies of Nb in TiN. Within the temperature range from 750 to 875° C, activation energies for metal interdiffusion were limited to 1.2 eV for the lower temperature end, and 2.5 eV for the higher temperature end.For the epitaxial non-iso structural Mo/NbN system, superlattice films with Λ=1.4 and 6.5 nm were investigated. The as-deposited films showed increased nanoindentation hardness values compared to the rule-of mixtures value. Upon annealing in vacuum at 1000°C for up to 3 h, the superlattice layering gradually diminished, while there was no loss in hardness. Remnants of the layering structure were still observed after 3 h at 1000°C. However, annealing of these superlattices promotes the formation of a composite structure with a tetragonal-MoNbN phase, which is as resistant to elasto-plastic deformation as the as-deposited Mo/NbN superlattice.