David Rafaja

On friction layer formation in polymer matrix composite materials for brake applications

Abstract Due to the complexity of friction phenomena in polymer matrix composites, the friction mechanisms have not been fully understood. This paper concentrates on the characterization of friction layer formation and correlation of friction layer properties to the performance of a recently developed family of polymer matrix composites. It was demonstrated that character of the friction layer determines the friction performance of the investigated composite material. Structure and chemical composition of the friction layer generated on the friction surface significantly differs from the bulk. Mechano-chemical interaction occurring in the friction process is compared to a “non-friction” situation where an “equivalent” apparent temperature and compressive loading, respectively were applied to the same material. No simple relationship exists between composition of the friction layer and bulk material formulation. Phase stability and kinetics of interactions for “friction” and “equivalent non-friction” loading conditions significantly differ.

Critical review on the elastic properties of transition metal carbides, nitrides and carbonitrides

Presently available data on the elastic properties of transition metal carbides, nitrides and carbonitrides are critically reviewed. It is shown that a lot of information is available on binary systems, but the scatter of the data is quite large, due to different sample preparation methods, different measurement and evaluation methods and insufficient chemical and microstructural characterization of the materials studied. Furthermore, experimental data were used to compare the evaluation methods for Youngs modulus as a function of the sample porosity. Only few data are available for ternary and quaternary systems. For the nitrides the elastic constants were determined almost entirely at room temperature only, although the compounds are frequently used at much higher temperatures. Information about the temperature dependency of the carbides is scarce. Therefore, it is obvious that for the design and tailoring of composites additional research work is necessary.

Lattice parameters and thermal expansion of Ti(CxN1−x), Zr(CxN1−x), Hf(CxN1−x) and TiN1−x from 298 to 1473 K as investigated by high-temperature X-ray diffraction

Abstract The thermal expansion values of the fee phases Ti(CxN1−x), Zr(CxN1−x) and Hf(CxN1−x with [C+N]/[Me] ∼1 and TiN1−x with [N]/[Ti] = 1.0 to 0.67 were determined from lattice parameter measurements made with high-temperature X-ray diffraction (HTXRD) in the temperature range of 298–1473 K. Stoichiometric titanium carbonitrides, zirconium carbonitrides and hafnium carbonitrides obey the Vegard rule up to 1473 K quite well, i.e. at any temperature up to 1473 K the lattice parameter behaves linearly between TiNTiC, ZrNZrC and HfNHfC, respectively. The lattice parameter of titanium nitride is also a linear function of the nitrogen/metal ratio in the temperature range investigated.

X-ray residual stress measurement in TiN, ZrN and HfN films using the Seemann-Bohlin method

Abstract The equation for the effect of stress on the lattice parameters returned by the Seemann-Bohlin fine focus goniometer is generalized for cubic crystals to include the effect of the X-ray elastic constants (XECs) of the individual crystal planes. Using the equation, the analysis of data taken from some group IVB nitride films, TiN, ZrN and HfN, indicates that the residual stress derived when taking the XECs into account can be higher by as much as 20% in the case of ZrN. The analysis also returned values for the bulk Poissons ratio for all the films which are in good agreement with an earlier study. The study appears to indicate that the grazing angle of incidence in the Seemann-Bohlin method should not be set too low or the results can be affected by the native oxygen-containing layer on the sample surface.

Main-Group and Transition-Element IRMOF Homologues

A simple two-component procedure was developed to synthesize not only classical zinc-based IRMOFs represented by MOF-5 but also the cobalt and beryllium homologues of this most prominent MOF. The procedure is the first manifestation of mirroring the IRMOF series with other metal centers taken from main-group as well as transition-metal elements. Because of the existence of many suitable precursors, the procedure promises the generation of a large number of IRMOF homologues. Since the IRMOF series together with the MIL series is the MOF group with the largest number of representatives, the possibility of choosing the metal centers of the secondary building units from an extended set will tremendously expand the number of obtainable structures in a predictive, crystal-engineering-type way. Use of metal centers other than zinc will allow the addition of new features to the existing IRMOF structures, such as magnetic properties in the example of cobalt.

Stacking fault model of ∊-martensite and its DIFFaX implementation

Plastic deformation of highly alloyed austenitic transformation-induced plasticity (TRIP) steels with low stacking fault energy leads typically to the formation of ∊-martensite within the original austenite. The ∊-martensite is often described as a phase having a hexagonal close-packed crystal structure. In this contribution, an alternative structure model is presented that describes ∊-martensite embedded in the austenitic matrix via clustering of stacking faults in austenite. The applicability of the model was tested on experimental X-ray diffraction data measured on a CrMnNi TRIP steel after 15% compression. The model of clustered stacking faults was implemented in the DIFFaX routine; the faulted austenite and ∊-martensite were represented by different stacking fault arrangements. The probabilities of the respective stacking fault arrangements were obtained from fitting the simulated X-ray diffraction patterns to the experimental data. The reliability of the model was proven by scanning and transmission electron microscopy. For visualization of the clusters of stacking faults, the scanning electron microscopy employed electron channelling contrast imaging and electron backscatter diffraction.

Interference phenomena observed by x-ray diffraction in nanocrystalline thin films

An increase of the X-ray diffraction line broadening with increasing diffraction angle was observed experimentally in nanocrystalline thin films. Such a change of the line width is usually related to the microstrain in the sample, which, however, contradicts the assumptions that the microstrain is relatively low in nanocrystalline materials and that the line broadening is caused mainly by small crystallite size. For nanocrystalline thin films, the observed changes in the diffraction line broadening are explained by a partial coherence of adjacent crystallites, which is stronger at low diffraction angles than at high diffraction angles. Furthermore, it is found that the degree of coherence of the adjacent crystallites depends on their size and preferred orientation. Smaller crystallites show better coherence, because the corresponding reciprocal-lattice points are broadened compared with those related to large crystallites. A strong preferred orientation improves further the coherence of the adjacent crystallites. Theoretical results are confirmed by experimental data obtained on nanocrystalline (Ti,Al)N thin films using a combination of glancing-angle X-ray diffraction, high-resolution transmission electron microscopy and X-ray texture analysis.

Substrate influence on the optical and structural properties of pulsed laser deposited BiFeO3 epitaxial films

Epitaxial BiFeO3 films pulsed laser deposited on SrTiO3, Nb:doped SrTiO3, and DyScO3 were studied using variable angle spectroscopic ellipsometry, vacuum ultraviolet ellipsometry, micro-Raman spectroscopy, and x-ray diffraction. The energy band gap of the film deposited on DyScO3 is 2.75 eV, while the one for the film deposited on Nb:doped SrTiO3 is larger by 50 meV. The blueshift in the dielectric function of the BiFeO3 films deposited on Nb:doped SrTiO3 compared to the films deposited on DyScO3, indicates a larger compressive strain in the films deposited on Nb:doped SrTiO3. This is confirmed by Raman spectroscopy and by high resolution x-ray diffraction investigations.

Optical and magneto-optical study of nickel and cobalt ferrite epitaxial thin films and submicron structures

Epitaxial films and ordered arrays of submicron structures of nickel and cobalt ferrites were deposited on Nb doped SrTiO3 by pulsed laser deposition. X-Ray diffraction and atomic force microscopy showed that the films have a good crystalline quality and smooth surfaces. A larger number of phonon bands was observed in the polarization dependent Raman spectra of the ferrite films than expected for the cubic spinel structures. This is explained by short range ordering of the Ni2+ (or Co2+) and Fe3+ cations at the octahedral sites inducing a lowering of the symmetry. The same behavior was also observed in the Raman spectra measured for the submicron structures, suggesting the same cation distribution as in the films. The diagonal components of the dielectric function for nickel and cobalt ferrites are determined from ellipsometry in the 0.73–5 eV photon energy range. The absorption edge was analyzed using a bandgap model and the energies for the indirect and direct optical transitions were calculated. It was...

Phase equilibria and multiphase reaction diffusion in the Cr-C and Cr-N systems

The phase formation in the Cr-C and Cr-N systems was investigated using reaction diffusion couples. The carbides were prepared by reaction of chromium metal with graphite powder in the range 1143 to 1413 °C in argon atmosphere; the nitride samples by reaction of the metal with N2 (≤31 bar) in the range 1155 to 1420 °C. While the carbide samples showed the three chromium carbide phases in form of dense diffusion layers between 1100 and 1400 °C, porosity occurred at temperatures above 1400 °C. The composition of the phase bands was measured by the means of electron probe microanalysis. For the Cr23C6 phase, a slightly higher C composition was found than given in the literature. In Cr-N diffusion couples both the δCrN1−x and βCr2N formed phase bands at T≥1150 °C. Because decomposition processes occurred upon cooling, quenching experiments were carried out in the range 1370 to 1420 °C at 31 bar N2 to stabilize the phases. The EPMA investigations of the homogeneity ranges yielded a large increase of the homogeneity range for δCrN1−x with increasing temperature. The nonmetal diffusion coefficients in all phases of both systems were calculated from layer growth and/or from concentration profiles. In δCrN1−x the N diffusivity was found to be strongly dependent on the composition. The Vickers microhardnesses of the various phases were obtained by measuring the diffusion layers.