Christina Wüstefeld

Magnetic response of (Cr,Al,Si)N nanocrystallites on the microstructure of Cr—Al—Si—N nanocomposites

Abstract A combination of microstructure analysis and ab initio calculations helped us to describe the interplay between the microstructure of Cr—Al—Si—N thin film nanocomposites and the ordering of the magnetic moments in the chromium-rich phase of (Cr,Al)N. The microstructure of the Cr—Al—Si—N nanocomposites was modified through the degree of ionisation of the deposited species in three physical vapour deposition processes – cathodic arc evaporation, unbalanced magnetron sputtering and high power impulse magnetron sputtering. According to the results of the ab initio calculations, the magnetic ordering was concluded from the expansion of the elementary cell and from the change of the crystal anisotropy of the elastic constants of (Cr,Al)N; these microstructure features were obtained from X-ray diffraction experiments. The microstructure of the Cr—Al—Si—N nanocomposites was furthermore characterised using the combination of X-ray diffraction and transmission electron microscopy with high resolution in order to obtain information about the phase composition of the thin films, distribution of individual elements and the crystallite size.

Capability of X-ray diffraction for the study of microstructure of metastable thin films

The capability of X-ray diffraction for the microstructure investigations of metastable systems is illustrated on supersaturated and partially decomposed thin films of titanium aluminium nitrides with high aluminium content. The anisotropy of the elastic constants and their role in these investigations is discussed.

Microstructure Investigation of the PVD Thin Films of TRIP Steels

The combination of a TRIP steel with the MgO stabilized ZrO2 ceramics (MgO•ZrO2) is regarded as a promising way to increase the energy absorption in engineering materials. An additional contribution to the energy absorption in the counterparts, i.e. in the TRIP steel and in MgO•ZrO2, is expected to arise at the interfaces between the individual materials. However, the mutual crystallographic orientation of the TRIP steel and MgO•ZrO2 at their interface plays a crucial role both for the adhesion of the counterparts and for the energy absorption process. In this work, the interfaces between the TRIP steel and MgO•ZrO2 were studied on simplified systems, which were prepared in form of the TRIP steel thin films that were deposited using the magnetron sputtering on various substrates, e.g. Si wafer, MgO•ZrO2 and the Al2O3/ZrO2 composites. The substrates were both single-crystalline (Si wafer) and polycrystalline (MgO•ZrO2, Al2O3/ZrO2). The basic characteristics of the thin films (morphology, thickness, chemical composition) were obtained from the scanning electron micrographs and from the energy dispersive analysis of the characteristic X-rays (EDX). X-ray diffraction (XRD) and transmission electron microscopy with high-resolution (HRTEM) that was complemented by the Fast Fourier Transform (FFT) of the HRTEM micrographs were employed as the crucial experimental methods for the microstructure analysis of these thin films. XRD was used for the phase analysis and for the global texture analysis. The global texture analysis was performed via the pole figure measurements. FFT/HRTEM was used for the characterisation of the local orientation relationships between the TRIP steel and the respective substrate and for the visualisation of the interfaces between individual crystallites.

Formation and high-temperature stability of metastable (Cr,Zr)2O3/(Zr,Cr)O2 nanocomposites

Successive crystallization of amorphous Cr-Zr-O thin films, formation of the (Cr,Zr)2O3/(Zr,Cr)O2 nanocomposites and thermally induced changes in the hexagonal crystal structure of metastable (Cr,Zr)2O3 were investigated by means of in situ high-temperature synchrotron diffraction experiments up to 1100°C. The thin films of Cr-Zr-O were deposited at room temperature using reactive ion beam sputtering from zonal Cr-Zr targets under oxygen flow. The resulting amorphous Cr-Zr-O solid solutions contained up to 15 at.% Zr. During the annealing in vacuum, the Cr-Zr-O solid solutions decomposed into two metastable phases, Cr-rich (Cr,Zr)2O3 and Zr-rich (Zr,Cr)O2, which crystallized in hexagonal and tetragonal structure, respectively. With increasing Zr content in amorphous Cr-Zr-O, the start of the phase segregation and crystallization was shifted from 600°C at 3 at.% Zr to 1000°C at 15 at.% Zr. With the aid of the in situ high-temperature synchrotron powder diffraction experiments, it was found that the metastable Cr2-2xZrxO3-x can accommodate up to approx. 3 at.% Zr. The zirconium atoms occupy partially the Wyckoff positions 6b in the corundum-like crystal structure of Cr2O3 that are empty in the stoichiometric chromium oxide. The incorporation of Zr into the crystal structure of Cr2O3 inflated the elementary cell and modified the thermal expansion of Cr2-2xZrxO3-x. The tetragonal structure of zirconia was stabilized by chromium. The phase segregation during the crystallization led to the formation of (Cr,Zr)2O3/(Zr,Cr)O2 nanocomposites. The size of crystallites in these nanocomposites decreased with increasing Zr content from 60 nm to 30 nm and increased only slightly at the highest annealing temperatures. In summary, this contribution illustrates the microstructure design in nanocomposites on the example of metastable chromium and zirconium oxides.