Growth and characterization of TiN/SiNx multilayer thin films

Sammanfattning: The demands from industry for higher cutting speeds, feeding rates, and reduction of the use of cooling agents during turning and milling operations are increasing. Consequently the requirements on the cutting inserts are increasing and new advanced coatings that can withstand the higher temperatures and larger loads are highly sought after. Throughout the last decade a large amount of publications regarding the Ti-Si-N system have been published. The majority of publications treat nanocomposites, exhibiting high mechanical properties, i.e. hardness. There are even reports of ultrahard nanocomposites reaching hardness values of ~100 GPa. When these nanocomposites are grown under conditions for optimal mechanical properties they have been described to consist of TiN nanocrystallites encapsulated by a thin layer (1-2 monolayers) of amorphous Si3N4. Due to the small crystallite size, a large part of nanocomposite coatings macroscopic properties will be controlled by the vast amount of interfaces between the crystallites. Despite the large amount of research done on these types of nanocomposites, the interfacial structure is still largely unexplored due to the complex 3-dimensional microstructure. Therefore, within this thesis, multilayers have been used as model systems. The layered structure provides a 2-dimensional geometry more suited for electron microscopy observations. Furthermore, a multilayer model system also allows for a precise control of the 2-dimensional geometry. Additional scientific importance can be found in the layered structure itself. Publications on multilayers deposited from various material systems all show similar results, i.e. hardness increase as the layer thicknesses are decreased. Hardening effects have been discussed to arise due to e.g. decreasing crystallite sizes, more interfaces, coherency strains, and differences in elastic shear modulus between the layers (Koehler hardening). In this work monolithic, trilayer, and multilayer/superlattice coatings have been grown by dual reactive magnetron sputtering. The films have subsequently been characterized and tested by a number of analytical tools such as x-ray reflection (XRR) and diffraction (XRD), in situ reflection high energy electron diffraction (RHEED), transmission electron microscopy (TEM), and nanoindentation. When depositing TiN on Si wafers, which have a native oxide layer on the surface, the TiN films grow polycrystalline. With the addition of periodic SiNx layers the columnar polycrystalline growth is maintained as long as the SiNx layers are grown thin. This indicates that the SiNx interlayers transmit the crystal structure and act as a template for each successive TiN layer. I.e. at small SiNx layers a crystalline structure is formed, and not an amorphous one. Similar epitaxial stabilization of non- equilibrium phases are observed in several other material systems and their formation are made possible due to the non-equilibrium conditions during deposition. However with thicker SiNx layers the columnar growth is lost and instead equiaxed TiN crystals are grown within each TiN layer, layers which are separated by amorphous SiNx. Changing the substrate to MgO, which exhibit a good lattice match with TiN, epitaxial growth of single crystal TiN films is possible. The deposition of thin (<7-10 Å) SiNx interlayers resulted in the growth of high quality single crystals superlattices. As thicker SiNx layers are grown, amorphous SiNx starts to form, which leads to the evolution of polycrystalline TiN layers separated by amorphous SiNx layers. The epitaxial stabilization arises due to pseudomorphic forces that during the initial stages of SiNx deposition reduce the interfacial energy during nucleation. Amorphous tissue formation is governed by the increase in strain energy as the SiNx layers grows thicker. In addition, Si strives to form a four-folded coordination with N, which will contribute to the relatively fast crystalline-amorphous transition, which is supported by ab intio calculation of the TiN-SiN system and in situ RHEED observations. The highest hardness values are observed within samples with thinner, crystalline SiNx layers. However, all multilayered films exhibit a higher hardness than the monolithic TiN and SiNx films. TEM studies on indented coatings also revealed that the layered structure acts favorable during deformation as cracks are deflected at the interfaces. A higher hardness and more favorable deformation properties are obtained when a layered structure is grown compared to monolithic films. The origin of this superhardening is discussed in terms of hardening mechanisms such as Koehler hardening, coherency strain hardening, and Hall-Petch type hardening. The results provided within this thesis shows that the interfacial characteristics within TiN and SiNx multilayers are more complex than previously thought, exhibiting epitaxial stabilization of metastable crystalline SiNx, and also the formation of super structured surface reconstructions on the growing surface. This will have impact on the description of nanocomposites and how they should be described and designed.