Al-Si-N transparent hard nanostructured coatings

This thesis analyses the properties of Al-Si-N thin films. Based on the expected immiscibility of aluminum nitride (AlN) and silicon nitride (Si3 N4 ), the hardness and the optical transparency of each single phase, transparent nanostructured films with enhanced hardness are expected to form. Al-Si-N thin films were prepared by closed field unbalanced magnetron sputtering from pure Al and Si targets in an Ar/N2 reactive atmosphere. The films were deposited at 200◦ C and 500◦ C onto silicon, cemented carbide, glass and UV-grade fused silica, in order to investigate their chemical, morphological, structural, mechanical, optical and thermal properties. The chemical composition was varied from pure AlN to Al-Si-N with a Si concentration of 23 at.%, as determined by X-ray photoelectron spectroscopy. The deposition conditions were set to provide a sufficient amount of nitrogen resulting in fully nitrided layers, i.e. transparent and containing about 50 at.% of nitrogen. Despite an excellent base pressure of the deposition chamber (< 10−6 Pa), an oxygen concentration of 0.5–2.0 at.% was measured in the films. The structural and morphological properties of the films were studied by X-ray diffraction (XRD), scanning electron microscopy and transmission electron microscopy (TEM). Up to 15–20 at.% of Si, the films are crystalline with the hexagonal – wurtzite – structure of AlN and a strong (002) texture. Pure AlN films show a strong columnar morphology that progressively diminishes upon incorporation of silicon in the films. In parallel, the mean crystallite size within the columns decreases from 60 nm in pure AlN to about 30 nm at 6 at.% of Si and 5 nm at 12 at.% of Si. Altogether, it is shown that the investigated ternary material system consists in three composition regimes with different sets of properties that are determined by the silicon content and independent from the deposition temperature: for Si contents below 6 at.%, the Si atoms substitute Al atoms in the AlN wurtzite lattice, as revealed by a linear decrease of the c lattice parameter in XRD data. Exceeding the solubility limit of 6 at.%, the thin films consist of Al0.44 Si0.06 N0.5 nanocrystallites surrounded by a thin SiNy grain boundary (GB) layer: a nanocomposite structure is formed. In the silicon concentration range from 6 to about 12 at.% of Si, the SiNy layer thickness is constant and equals to 0.25 nm (∼ 1 monolayer), while the crystallite size decreases inversely proportional to the Si content. Above 12 at.% of Si, the growth mode changes and the SiNy layer thickness increases with the Si content. The films become progressively X-ray amorphous with chemical and mechanical properties approaching those of pure silicon nitride.
The hardness and biaxial residual stress of the films were studied by nanoindentation and mechanical profilometry, respectively. While hardness is about 20–25 GPa in pure AlN and SiNy , it reaches 32 GPa in Al-Si-N thin films at 8–12 at.% of Si, concentration at which a negligible residual compressive stress (< 0.5 GPa) is measured. This hardness enhancement over a broad composition range is explained by the interplay of several hardening mechanisms: on the one hand, the formation of an Al-Si-N solid solution and the grain refinement cause a reduction of dislocation activity within the crystalline phase; on the other hand, as the two-phase nanocomposite structure is formed, the presence of SiNy at the GBs hinders deformation by GB sliding. The optical properties of the films are moreover only little influenced by variations of the film composition: a refractive index in the range 2.00–2.12 at 633 nm wavelength, corresponding to 80 % light transmission, is measured. The structural, mechanical and optical properties of the films are stable upon annealing for 2h at 1000◦ C in argon. Al(-Si-)N/SiNy multilayer coatings were deposited as a model system to study interfacial properties which are difficult to access in 3D nanocomposite structures. XRD and high-resolution TEM reveal that up to 0.7–1.0 nm (∼ 2.5–3.5 monolayers) of crystalline silicon nitride can be epitaxially stabilized on an AlN (001) surface. This finding reinforces the assumption of an ordering of the SiNy GB layer and the formation of coherent interfaces in Al-Si-N nanocomposite coatings. It provides an explanation for the moderate hardness enhancement obtained in Al-Si-N coatings that probably results of a gradual structural transition at the interfaces. Moreover finite solubility in the Al-Si-N system is associated to a moderate thermodynamic driving force for phase separation. This probably calls for a composition gradient at the interfaces, all the more as a non-equilibrium deposition process is used. Interfaces in the Al-Si-N system are therefore expected to be penetrable obstacles e.g for dislocations upon plastic deformation. If incorporated at the grain boundaries, oxygen impurities may also lower the interface cohesion energy and facilitate GB sliding. The present work shows that optically transparent coatings can be produced of Al-SiN with a hardness exceeding 30 GPa at low deposition temperatures. The investigations presented in this thesis provide a thorough understanding of the material and of the evolution of its properties with the silicon content in the films.