S. Hofmann

Preferential sputtering of oxides: a comparison of model predictions with experimental data

Abstract Experimental data on the composition of the altered layer of some oxides bombarded with noble gas ions are compared with different model predictions based on Sigmunds sputtering theory. It is shown that although the difference in mass of the metal atoms is the dominant factor in the preferential sputtering of oxygen, the additional consideration of the surface binding energies yields an improved agreement between theory and experiment.

Thermodynamics and structural aspects of grain boundary segregation

Abstract Physical and chemical properties of solid materials are strongly. influenced by the chemical composition of internal interfaces, One of the crucial parameters affecting interfacial chemistry is the atomic structure of the interface. Due to its importance. a considerable amount of work was done to elucidate the relationship between structure and chemical composition of interfaces. This article reviews the present understanding of an important and fundamental part of this relationship, namely, the structural aspects of grain boundary segregation. After a brief outline of grain boundary structure and geometry. thermodynamic approaches to describe grain boundary segregation are summarized and their application to materials is discussed. covering particular sites at a single grain boundary as well as the role of interfaces in polycrystals. Both the experimental evidence of grain boundary segregation anisotropy and the theoretical results of computer simulations of grain boundary segregation are summar...

Morphology and properties of sputtered (Ti,Al)N layers on high speed steel substrates as a function of deposition temperature and sputtering atmosphere

(Ti,Al)N layers were prepared by reactive dc and radio‐frequency (rf) magnetron sputtering onto polished flat high speed steel (HSS) surfaces. The rectangular samples were mounted on a special sample holder providing a temperature gradient in the range of 500 °C to room temperature along the length of the sample. The (Ti,Al)N layers were deposited at various N2 and Ar pressures. The target was a Ti–50 at. % Al alloy. The film morphology and composition was observed by scanning electron microscopy (SEM) and Auger electron spectroscopy (AES), respectively, and correlated to mechanical properties like hardness and critical load (scratch test). The results are discussed with the respect of the sputtering conditions.

The contribution of characteristic energy losses in the core-level X-ray photoelectron spectroscopy peaks of TiN and (Ti, Al)N studied by electron energy loss spectroscopy and X-ray photoelectron spectroscopy

Abstract The correct interpretation of the electronic relaxation of Ti compounds upon soft X-ray excitation, such as is used in X-ray photoelectron spectroscopy (XPS), is a notoriously difficult task. Previous studies of the structure and peak assignment of the core-level Ti 2p peaks have shown that the usual methods of background subtraction and peak fitting can bring about large errors in the XPS quantification of the Ti compounds TiN and (Ti,Al)N which are produced as coatings by reactive sputter deposition. Electron energy loss spectroscopy (EELS) was performed on TiN and (Ti,Al)N coatings at high energy resolution in a scanning Auger microprobe in the reflection mode. Results are presented which show that the primary electron beam reflected from these materials exhibits very intense and characteristic energy losses due mainly to intraband transitions. The EELS measurements in conjunction with valence-band XPS studies confirmed molecular orbital (MO) calculations of the occupied and unoccupied electron states near the Fermi level of TiN. The core-level Ti 2p photoelectrons measured by XPS suffer the same discrete electronic excitation losses identified in TiN as transitions from the tlg and t2u MOs (derived from N 2p, O 2p and Ti 3d/4s/4p levels) to the partially occupied 2t2g state at the Fermi level. Corrections of the Ti 2p XPS spectra with the relevant energy losses obtained by EELS allow a consistent interpretation of the Ti photoemission peak structure and a considerable improvement in the quantification of the corresponding Ti compound.

Surface and interface characterization of heat- treated (Ti, Al)N coatings on high speed steel substrates☆

Abstract (Ti, Al)N hard coatings have been shown to be more oxidation resistant than TiN coatings. Furthermore, they seem to be a promising alternative coating for high speed steel cutting tools. In order to obtain information on the oxidation behaviour and the adhesive properties, the surface and interface composition of heat-treated samples were determined by Auger electron spectroscopy (AES) as a function of temperature (500–800°C), time (15 min–16h) and surrounding atmosphere (air, vacuum). The AES results were obtained by sputter depth profiling and crater edge profiling. The oxidation always produces an aluminium oxide layer at the outermost surface followed by a mixed oxynitride and an aluminium-depleted zone in the (Ti, Al)N coatings. The coating-substrate interface shows no distinct change caused by the heat treatments.

Depth resolution and surface roughness effects in sputter profiling of NiCr multilayer sandwich samples using Auger electron spectroscopy

Abstract Concentration-depth profiles of two multilayer sandwich structures composed of alternate layers of Ni and Cr (ten layers 370 A thick and twenty layers 115 A thick respectively) with initial surface roughnesses of not greater than 100 A and approximately 1 μm were obtained by sputtering with 1 keV argon ions and simultaneous Auger electron spectroscopy. Comparison with calculations according to a sequential layer sputtering model shows that the information depth influence is negligible for a sputter depth of more than about 300 A. The original surface roughness has an appreciable influence on depth resolution, which leads to increasing interface broadening with sputter depth.

Solute segregation and classification of [100] tilt grain boundaries in α-iron: consequences for grain boundary engineering

A complete classification of [100] tilt grain boundaries into the categories of special, vicinal and general resulting from measurements of grain boundary segregation in α-iron is presented. In contrast to the coincidence site lattice approach at least one special grain boundary was found to exist for each misorientation of adjoining grains irrespective of the value of the reciprocal density of coincidence lattice sites. This finding raises a new challenge to grain boundary engineering. It is suggested that a configuration of typically brittle general boundaries in a polycrystal can be transformed into another one consisting of non-brittle special boundaries under suitable annealing conditions by boundary inclination via boundary migration without any change of grain orientations.

High temperature oxidation behaviour of (Ti1−xCrx)N coatings

Abstract (Ti1−xCrx)N coatings with different Ti/Cr ratios were deposited on polished high speed steel substrates by reactive magnetron sputtering and oxidized in air at temperatures between 350 and 800 °C, with reaction times between 10 and 300 min. The oxidized coatings were characterized by sputter depth profiling using Auger Electron Spectroscopy (AES). The oxide layers of all the coatings grow according to an oxygen and chromium diffusion controlled, parabolic time law with activation energies between 98 and 166 kJ mol−1. Composition of the oxide layers as well as the oxidation rate depend strongly on the chromium content of the Ti1−xCrx)N coatings. A minimum oxidation rate is observed for the (Ti0.6Cr0.4)N coating and is explained by the counterdiffusion effect of oxygen and chromium through the oxide layers.

The chemical analysis of TiN films: A round robin experiment

Abstract Two series of titanium nitride films have been prepared under different conditions by reactive d.c. bias magnetron sputtering. They have been studied by X-ray photoelectron spectroscopy (XPS), electron probe microanalysis (EPMA), Auger electron spectroscopy (AES), X-ray diffraction (XRD) and gravimetric oxide chemical analyses, in a round robin experiment, to compare the reliability of the techniques in characterizing films quantitatively. The EPMA, AES and chemical analyses (where applied) of the samples produce consistent data. However, a discrepancy was found between AES and XPS data in one series of samples. XRD shows considerable deviations from equilibrium in the lattice parameters, negating its use in analysis.

Auger electron spectroscopy and X-ray photoelectron spectroscopy studies of the oxidation of polycrystalline tantalum and niobium at room temperature and low oxygen pressures

Abstract Auger electron spectroscopy and X-ray photoelectron spectroscopy were used to characterize the initial stages of the oxidation of polycrystalline tantalum and niobium in an oxygen atmosphere at low pressures (10−6 Pa ⩽ po2 ⩽ 10−3 Pa) androomtemperature. Theoxidationofbothmetalsproceedsinasimilarway: thegrowthoftheoxidelayersiscomparableandfollowsthesamekinetics. Twodistinctstagesareobserved. Atfirstrapidoxygenuptaketakesplaceuptoanoxygenexposureofabout 1.5L (1L = 10[su−6 Torr s = 1.32 × 10−4 Pa s) followed by a continuous decrease in the oxygen sorption rate until an oxygen exposure of 7–10 L is reached. This stage is characterized by the formation of an oxide film 2–3 monolayers thick (TaO and TaO2 on tantalum and NbO and NbO2 on niobium). At oxygen exposures above 10 L a second stage with a low reaction rate is observed which is characterized by the formation of Ta2O5 (Nb2O5) according to a logarithmic growth law until an oxygen exposure of 104 L is attained.