R. Kužel

The evolution of dislocation density during heat treatment and creep of tempered martensite ferritic steels

Abstract The evolution of dislocation density in two tempered martensite ferritic steels (a 12% and a 9% chromium steel, “X20” and “P91”) during heat treatment and creep is analyzed using transmission electron microscopy (TEM) and X-ray diffraction (XRD); both methods yield results which are in good agreement when rationalizing the XRD-data based on densities of free dislocations. It is shown that due to the intermediate formation of martensite, standard heat treatments produce very high dislocation densities in tempered martensite ferritic steels (TMFSs). Long term tempering and creep are characterized by a decrease of dislocation density; but dislocation densities are still high as compared to alloys where the formation of microstructure does not involve a martensitic transformation. A heterogeneous microstructure after long term tempering and creep is a characteristic feature of TMFSs. Micro grains with high dislocation densities co-exist next to micro grains without dislocations. The XRD method yields average data and cannot account for this microstructural heterogeneity; but it supports the TEM results which in isolation suffer from providing insufficient statistics. The results of the present study are discussed in the light of earlier work published in the literature and contribute to a better understanding of the role of free dislocations in TMFSs during tempering, creep and high temperature low cycle fatigue.

Structure of TiN coatings deposited at relatively high rates and low temperatures by magnetron sputtering

Abstract Titanium nitride coatings were deposited at an average deposition rate ranging from 0.1 to 0.5 μm min -1 onto steel substrates maintained at 50 or 150 °C. The hardest coatings in each of the three sets investigated contained 36–38 at.%N and are thus significantly substoichiometric. The microstructure of the hardest coatings is characterized by the highest compressive stress, the greatest amount of strain, the minimum preferred orientation and the minimum volume fraction of crystalline phase in each set. A compressive stress was observed in the cubic phase layers and a tensile stress in the hexagonal phase layers. The stress and strain observed in the hardest TiN x coatings were approximately one order of magnitude higher than those of ordinary metals. With increasing nitrogen content the 〈111〉 texture in the cubic phase layers is gradually replaced by the 〈100〉 texture. It was found that the lattice parameters of the TiN x coatings exceed the bulk values.

Ion-assisted sputtering of TiN films

Abstract Ion bombardment of growing films is one of the possible ways to produce films with specific properties. As yet there are no general rules for the production of these films. The quality of TiN films produced depends on the deposition conditions. A sharp transition from porous, black TiN films to compact, dense, bright gold TiN films is observed at a substrate bias U s of about -40 V. Recent experiments have indicated that the microstructure of TiN films and the transition mentioned above can be controlled by the ion energy delivered to the growing film per deposited particle E p = eU s v i / v m . This paper investigates the transition from porous, soft TiN films with a zone I microstructure to compact, hard TiN films with a zone T microstructure as a function of i s , U s and the deposition rate a D at constant temperature T s = 350 °C and pressure p T = 5 Pa. Correlations between the microhardness HV, the macrostress σ, the microstrain e , the lattice parameters, the intensities of the X-ray reflections and the colour and appearance of the film are discussed. The zone I to zone T transition is observed at E p ≈ 150 eV atom -1 .

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.

Dependence of microstructure of TiN coatings on their thickness

Abstract Two sets of TiN coatings containing 34.5 at.% N and 50.3 at.% N, on average, were investigated by X-ray diffraction. No systematic changes in strain and microhardness were observed as the layer thickness was varied from 1 to 12.5 μm, with the exception of the influence of the substrate on the microhardness measurements in layers thinner than about 4 μm. A general tendency of the lattice parameters to decrease with increasing layer thickness was found together with their systematic dependence on the orientation of grains in the coatings. These results and the observed decrease in stress with increasing layer thickness are explained by a gradual improvement in the structure of the layers with increasing thickness and by the heterogeneous nature of the structure. A change in the preferred orientation of crystallites with the layer thickness was also registered.

Complex XRD microstructural studies of hard coatings applied to PVD-deposited TiN films Part I. Problems and methods

Abstract Conventional X-ray powder diffractometry is often insufficient for the study of the specific microstructure of hard coatings. It should be complemented at least by XRD geometries giving other information. Therefore the following geometries are compared: conventional powder diffractometry in Bragg-Brentano geometry, asymmetric diffraction used for stress measurement in the so-called ohm- and ψ- goniometer geometries, and Seemann-Bohlin diffractometry. Advantages and drawbacks of the methods are briefly estimated. A short review of basic methods for the characterization of texture, stress and lattice defects is given. Specific effects frequently observed for hard coatings (e.g. the anisotropy of lattice parameters and XRD line broadening) are discussed, together with some of the possible reasons. Quantitative estimations are given for different types of lattice defects and elastic anisotropy. The crystallite group method is found to be useful for thin films.

Relation of deposition conditions of Ti-N films prepared by d.c. magnetron sputtering to their microstructure and macrostress

Abstract A systematic study of the relation of the deposition conditions of Ti-N films prepared by d.c. magnetron sputtering to their microstructure and macrostress has been performed using two sets of α-Ti(N) and δ-TiN films. The variable parameter in the deposition was the combined parameter S E representing the energy delivered to the growing film. The threshold energy S Etr necessary for the transition from zone I to zone T of Thorntons structural zone model was investigated for both α-Ti(N) and δ-TiN films. S Etr for δ-TiN films is about one order of magnitude higher ( S E = 0.9 MJ cm -3 ) than for α-Ti(N) films ( S E = 0.15 MJ cm -3 ) according to X-ray diffraction. This finding can be explained by difference in the melting temperature T m of TiN T m = 3200 K) and that of Ti T m = 1930 K). Values of the macrostress of Ti-N films as a function of S E are also given.

X-ray analysis of heat-treated titanium nitride films

Abstract Titanium nitride films deposited onto steel substrates maintained at 423 K were heat treated in the temperature interval 773–1173 K. Samples were studied by electron probe microanalysis and X-ray diffraction. The high values of the microhardness observed for the as-deposited films decreased after annealing for 3 h at 973 K to nearly bulk values. This decrease is mainly due to the improvement in the microstructure of the films. It is accompanied by strong decreases in strain, stress and the lattice parameter of δ-TiN x . Growth of the ϵ-Ti 2 N phase at the expense of the δ-TiN x phase was observed in a film with 34 at .% N when it was annealed for 3 h at 973 K. The lattice parameter and strain in the substrate increased after film deposition, most probably due to a dissolution of nitrogen or titanium atoms in the lattice of α-Fe.

XRD characterization of ion-implanted TiN coatings

Abstract A microstructure model of thin films is presented that treats large variations observed in real structure of hard coatings. The model takes into account the free expansion of grains in perpendicular direction, a limited expansion along the substrate surface and the existence of the crystallographic direction of easiest deformation. It can explain the anisotropy of cubic lattice parameters, the curvature of interplanar spacing in dependence on sin 2 ψψ in stress measurement, the anisotropy in diffraction line broadening and changes in intensities of diffraction lines. The penetration of foreign atoms and ions into the host structure is considered as one of the possible sources of real structure diversity of materials. The ion implantation was used as a tool changing the microstructure of thin films in a controlled way.

Complex XRD microstructural studies of hard coatings applied to PVD-deposited TiN films Part II. Transition from porous to compact films and microstructural inhomogeneity of the layers

Abstract Transition from porous to compact microstructure of magnetron deposited TiN films with increasing energy of incoming particles is associated with the changes of stress and strain. This is studied by conventional powder diffractometry in Bragg-Brentano geometry, by asymmetric diffraction used for stress measurement in the so-called Ω-and ψ-goniometer geometries and by the Seemann-Bohlin diffractometry. The changes of lattice parameters and preferred orientation are investigated as well. The results are discussed from several points of view: elasticity theory, lattice defects and heterogeneous microstructure of the films. The heterogeneity seems to be an important reason for specific effects of lattice parameters and X-ray diffraction line broadening anisotropy. The crystallite group method (CGM) is used for the evaluation of stress and crystallite size and reveals the microstructural inhomogeneity. The importance of the orientation of crystallite with respect to the surface for its real structure is further confirmed by the measurement on freestanding film.