TY - BOOK

T1 - Non-homogeneous structural and mechanical properties of nanocrystalline thin films characterized by advanced X-ray diffraction techniques

AU - Bartosik, Matthias

N1 - no embargo

PY - 2012

Y1 - 2012

N2 - Mechanical and functional properties of thin films are predetermined by their microstructure and strain state. This thesis deals with the characterization of microstructural properties and strains in various types of hard coatings. Three thematic areas are addressed.(I) A novel X-ray nanodiffraction technique to evaluate the thickness dependence of microstructure, strain and composition in thin films is introduced. The approach is based on wide angle X-ray scattering (WAXS) using a monochromatic synchrotron beam with a diameter of about 100 nm. First, the film/substrate composite is aligned so that its interface is parallel to the incident beam. Subsequently, the specimen is moved stepwise opposite to the film growth direction. At each point of the film cross-section a Debye-Scherrer diffraction pattern is collected by a charge-coupled device (CCD) area detector placed behind the specimen.The nanodiffraction studies concentrated on the characterization of 15 µm CrN and a 6 µm compositionally graded CrxN1-x coating used as model systems. The coatings were deposited by reactive magnetron sputtering on Si (100) at 350 °C. In the case of the 15 µm CrN film, three consecutive CrN sublayers with a thickness of 5 µm each were deposited applying a sequence of three bias voltages of −40, −120 and −40 V in the deposition process. Along with the bias voltage levels the energy of the incident ions changed. The nanodiffraction data revealed a complex strain depth-profile with a step-like shape, smooth texture gradients and grain size oscillations at the film cross-section. The graded CrxN1-x coating was deposited applying a constant negative bias voltage of −80 V. The compositional gradient in the film growth direction was adjusted via the N2 and Ar partial pressures in the deposition chamber. By using the nanodiffraction technique coexisting depth-gradients of macrostrain, composition and texture could be determined and accurately separated. The new approach overcomes typical short comings of conventional diffraction techniques and represents a unique tool for local thin film characterization.(II) High-temperature X-ray diffraction experiments were performed in-situ from room temperature to 1100 °C using a dedicated heating stage to elucidate the influence of solid state reactions in thin films on their residual stress state. In this thesis, to the knowledge of the author, for the first time the in-situ residual stress evolution during the decomposition of metastable face-centered cubic (fcc)-Ti-Al-N was characterized. It was found that above the deposition temperature a relaxation of the as-deposited compressive stresses sets it, which was attributed to recovery processes and effects coming along with the decomposition in its early stage (when nm-sized fcc-TiN and fcc-AlN rich domains develop in the Ti-Al-N matrix). Above 800 °C a steep compressive residual stress increase up to a few GPa was observed which was mainly attributed to the molar volumes expansion of ∼20% associated with the phase transformation from fcc- to hexagonal (hex.)-AlN. The residual stress-temperature dependence can be used for effective residual stress engineering by performing optimized post-deposition heat treatments.(III) Laser heating of selected thin film systems was successfully applied to experimentally simulate local thermal loads occurring in the application of hard coatings. The irradiated specimens were characterized applying a variety of position-resolved techniques including energy dispersive X-ray diffraction, nanoindentation, raman scattering and high-resolution transmission electron microscopy.

AB - Mechanical and functional properties of thin films are predetermined by their microstructure and strain state. This thesis deals with the characterization of microstructural properties and strains in various types of hard coatings. Three thematic areas are addressed.(I) A novel X-ray nanodiffraction technique to evaluate the thickness dependence of microstructure, strain and composition in thin films is introduced. The approach is based on wide angle X-ray scattering (WAXS) using a monochromatic synchrotron beam with a diameter of about 100 nm. First, the film/substrate composite is aligned so that its interface is parallel to the incident beam. Subsequently, the specimen is moved stepwise opposite to the film growth direction. At each point of the film cross-section a Debye-Scherrer diffraction pattern is collected by a charge-coupled device (CCD) area detector placed behind the specimen.The nanodiffraction studies concentrated on the characterization of 15 µm CrN and a 6 µm compositionally graded CrxN1-x coating used as model systems. The coatings were deposited by reactive magnetron sputtering on Si (100) at 350 °C. In the case of the 15 µm CrN film, three consecutive CrN sublayers with a thickness of 5 µm each were deposited applying a sequence of three bias voltages of −40, −120 and −40 V in the deposition process. Along with the bias voltage levels the energy of the incident ions changed. The nanodiffraction data revealed a complex strain depth-profile with a step-like shape, smooth texture gradients and grain size oscillations at the film cross-section. The graded CrxN1-x coating was deposited applying a constant negative bias voltage of −80 V. The compositional gradient in the film growth direction was adjusted via the N2 and Ar partial pressures in the deposition chamber. By using the nanodiffraction technique coexisting depth-gradients of macrostrain, composition and texture could be determined and accurately separated. The new approach overcomes typical short comings of conventional diffraction techniques and represents a unique tool for local thin film characterization.(II) High-temperature X-ray diffraction experiments were performed in-situ from room temperature to 1100 °C using a dedicated heating stage to elucidate the influence of solid state reactions in thin films on their residual stress state. In this thesis, to the knowledge of the author, for the first time the in-situ residual stress evolution during the decomposition of metastable face-centered cubic (fcc)-Ti-Al-N was characterized. It was found that above the deposition temperature a relaxation of the as-deposited compressive stresses sets it, which was attributed to recovery processes and effects coming along with the decomposition in its early stage (when nm-sized fcc-TiN and fcc-AlN rich domains develop in the Ti-Al-N matrix). Above 800 °C a steep compressive residual stress increase up to a few GPa was observed which was mainly attributed to the molar volumes expansion of ∼20% associated with the phase transformation from fcc- to hexagonal (hex.)-AlN. The residual stress-temperature dependence can be used for effective residual stress engineering by performing optimized post-deposition heat treatments.(III) Laser heating of selected thin film systems was successfully applied to experimentally simulate local thermal loads occurring in the application of hard coatings. The irradiated specimens were characterized applying a variety of position-resolved techniques including energy dispersive X-ray diffraction, nanoindentation, raman scattering and high-resolution transmission electron microscopy.

KW - X-ray nanodiffraction

KW - nanobeam

KW - residual strain

KW - CrN

KW - TiAlN

KW - HT-XRD

KW - in-situ

M3 - Doctoral Thesis

ER -