We report the layer structure and composition in recently discovered TiN/SiN(001) superlattices deposited by dual-reactive magnetron sputtering on MgO(001) substrates. High-resolution transmission electron microscopy combined with Z-contrast scanning transmission electron microscopy, x-ray reflection, diffraction, and reciprocal-space mapping shows the formation of high-quality superlattices with coherently strained cubic TiN and SiN layers for SiN thickness below 7-10 Å. For increasing SiN layer thicknesses, a transformation from epitaxial to amorphous SiNx (x 1) occurs during growth. Elastic recoil detection analysis revealed an increase in nitrogen and argon content in SiNx layers during the phase transformation. The oxygen, carbon, and hydrogen contents in the multilayers were around the detection limit (∼0.1 at.%) with no indication of segregation to the layer interfaces. Nanoindentation experiments confirmed superlattice hardening in the films. The highest hardness of 40.4 ± 0.8 GPa was obtained for 20-Å TiN with 5-Å-thick SiN(001) interlayers, compared to monolithic TiN at 20.2 ± 0.9 GPa.

Nanostructured materials-the subject of much of contemporary materials research-are defined by internal interfaces, the nature of which is largely unknown. Yet, the interfaces determine the properties of nanocomposites and nanolaminates. An example is nanocomposites with extreme hardness70-90 GPa, which is of the order of, or higher than, diamond. The Ti-Si-N system, in particular, is attracting attention for the synthesis of such superhard materials. In this case, the nanocomposite structure consists of TiN nanocrystallites encapsulated in a fully percolated SiNx "tissue phase" (1 to 2 monolayers thick) that is assumed to be amorphous. Here, we show that the interfacial tissue phase can be crystalline, and even epitaxial with complex surface reconstructions. Using in situ structural analyses combined with ab initio calculations, we find that SiNx layers grow epitaxially, giving rise to strong interfacial bonding, on both TiN(001) and TiN(111) surfaces. In addition, TiN overlayers grow epitaxially on SiNx/TiN(001) bilayers in nanolaminate structures. These results provide insight into the development of design rules for new nanostructured materials.

TiN/SiNx/TiN(0 0 1) trilayers have been deposited on MgO(0 0 1) substrates using ultra-high vacuum based reactive magnetron sputtering and studied by in situ reflection high energy electron diffraction (RHEED). Depositions were carried out at 500 °C and 800 °C, with SiNx layer thicknesses between 3 and 300 Å. Here, we find that SiNx(0 0 1) layers grown at 800 °C exhibit 1 × 4 surface reconstructions along orthogonal (1 1 0) directions up to a critical thickness of ≈9 Å, where an amorphous phase forms. Growth of TiN overlayers on the reconstructed SiNx(0 0 1) layers yield RHEED patterns indicating the growth of (0 0 1)-oriented epitaxial layers with a 1 × 1 reconstruction. For the case of amorphous SiNx layers the TiN overlayers grow polycrystalline.

The formation of cubic-phase SiNx is demonstrated in TiN/SiNx multilayers deposited by reactive dual magnetron sputtering. Transmission electron microscopy examination shows a transition from epitaxially stabilized growth of crystalline SiNx to amorphous growth as the layer thickness increases from 0.3 to 0.8 nm. The observations are supported by ab initio calculations on different polytypes, which show that the NaCl structure has the best lattice match to TiN. Calculations also reveal a large difference in elastic shear modulus between NaCl-SiNx and TiN. The results for phase structure and shear modulus offer an explanation for the superhardening effect determined by nanoindentation experiments.

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 (

Multilayer thin films consisting of titanium nitride (TiN) and silicon nitride (SiNx) layers with compositional modulation periodicities between 3.7 and 101.7 nm have been grown on silicon wafers using reactive magnetron sputtering. The TiN and SiNx layer thicknesses were varied between 2-100 nm and 0.1-2.8 nm, respectively. Electron microscopy and x-ray diffraction studies showed that the layering is flat with distinct interfaces. The deposited TiN layers were crystalline and exhibited a preferred 002 orientation for layer thicknesses of 4.5 nm and below. For larger TiN layer thicknesses, a mixed 111/002 preferred orientation was present as the competitive growth favored 111 texture in monolithic TiN films. SiNx layers exhibited an amorphous structure for layer thicknesses ≥0.8 nm; however, cubic crystalline silicon nitride phase was observed for layer thicknesses ≤0.3 nm. The formation of this metastable SiNx phase is explained by epitaxial stabilization to TiN. The microstructure of the multilayers displayed a columnar growth within the TiN layers with intermittent TiN renucleation after each SiNx layer. A nano-brick-wall structure was thus demonstrated over a range of periodicities. As-deposited films exhibited relatively constant residual stress levels of 1.3±0.7 GPa (compressive), independent of the layering. Nanoindentation was used to determine the hardness of the films, and the measurements showed an increase in hardness for the multilayered films compared to those for the monolithic SiNx and TiN films. The hardness results varied between 18 GPa for the monolithic TiN film up to 32 GPa for the hardest multilayer, which corresponds to the presence of cubic SiNx. For larger wavelengths, ≥20 nm, the observed hardness correlated to the layer thickness similar to a Hall-Petch dependence, but with a generalized power of 0.4. Sources of the hardness increase for shorter wavelengths are discussed, e.g., epitaxial stabilization of metastable cubic SiNx, coherency stress, and impeded dislocation activity.

This licentiate thesis adds a new piece to the puzzle that describes how the microstructural characteristics influence the hardness behavior of a multilayer coating. It contains a presentation of the manufacturing and the subsequent characterization of multilayer thin films. These multilayers consist of alternating layers of crystalline titanium nitride (TiN) and amorphous silicon nitride (Si3N4), deposited with a physical vapor deposition technique referred to as reactive magnetron sputtering. The microstructure of as-deposited films was examined with cross-sectional transmission electron microscopy (XTEM) and x-ray diffraction (XRD). XRD studies revealed a transition in preferred orientation for TiN, from a pure 002 orientation to a mixed 111/002 orientation as the TiN layer thickness increased from 4.5 nm to 9.8 nm. XTEM studies showed a microstructure consisting of equiaxed or elongated TiN grains, depending on layer thickness, limited in size by the amorphous interlayers. Selected area diffraction verified the observed transition in preferred orientation in TiN. For small silicon nitride layer thicknesses (~0.3 nm) an epitaxial stabilization of Si3N4 to the crystalline TiN lattice was observed through high resolution electron microscopy studies. Instead of amorphous interlayers a cubic silicon nitride rich phase (SiNx) was observed. This is to the present knowledge of the author the first time this phenomenon has been observed within this material system. In order to explain the observed behavior a model based on the involved energies were developed. Nanoindentation was performed to evaluate the mechanical behavior of the coatings as the layer thicknesses varied. All multilayers were harder than the monolithic TiN film, which had a hardness of 18 GPa compared to 32 GPa for the hardest multilayer. An interesting observation was that the hardest multilayer corresponds to the presence of cubic silicon nitride. Curvature measurements were performed and showed that the residual stresses within the multilayers were compressive and relatively constant, 1.3±0.7 GPa. In addition to the XTEM studies of as-deposited samples, XTEM studies of deformed multilayers were also conducted. The 300 mN load produced plastic deformation in the substrate under the indent. Cracks within the multilayer normally propagated along TiN/Si3N4 interfaces, which suggest that a lower energy is needed for cracking along an interface compared to intralayer cracking. The observed hardness increase can be ascribed to the multilayered structure of these films. By the interruption of TiN growth with intermittent Si3N4 layers the produced microstructure consisted of small TiN grains, separated in the growth direction by amorphous or crystalline interlayers. Small grains are known to contribute to hardening, but the interlayers also contribute, acting as dislocation obstacles either due to the amorphous tissue or to coherency stresses.