Microstructural characterization and hardening behavior of reactive magnetron sputtered TiN/Si₃N₄ multilayer thin films
Abstract: 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.
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