Hermann A. Jehn

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.

Multicomponent and multiphase hard coatings for tribological applications

Abstract Plasma vapour deposition (PVD) processes are often used to deposit coatings and thin films in order to reduce wear and friction of tools and machine parts. These techniques especially allow the deposition of a broad spectrum of materials, particularly covering the nitrides and carbides of the transition metals and some borides and oxides as well as carbon-containing coatings like diamond, diamond-like, metal-carbon and carbon-nitrogen coatings. On the basis of the binary compounds TiN and TiC, various strategies were developed to improve or adapt the hard coatings to a specific tribological problem. This was first realized in multicomponent coatings by adding other metallic or non-metallic elements — also with graded concentrations. Further developments were focussed on the deposition of multiple-layer and multilayer coatings and most recently on nanostructured coatings where in addition to hard components also soft and lubricating materials are simultaneously deposited. The paper reviews the recent developments of tribological coatings giving examples of their structure, properties and performances.

The molybdenum-nitrogen phase diagram

Abstract In high and low pressure experiments in the Mo-N system the solidus line (α + L) and the composition and temperature of the eutectic (L = α-Mo + γ-Mo 2 N) have been determined. Mo dissolves 1.08 at.% N at the eutectic temperature of 1860°C and at the equilibrium pressure of 670 atm (6.7 × 10 7 Pa). The eutectic composition is 19 at.% N and the corresponding N content of γ-Mo 2 N is 27 at.% N. The solubility of N 2 in Mo(1) and the liquidus line (α + L) L have been calculated on the basis of existing data. For γ-Mo 2 N a melting temperature of 2000°C has been estimated. An Mo-N phase diagram is presented and the phases are discussed in detail. Equations for the solubility of N 2 in solid and liquid Mo, the solid solubility limit and the dissociation and plateau pressures are given together with the Gibbs free energy of the corresponding reactions. The special behaviour of the metal-gas system Mo-N is additionally treated in a p - c diagram.

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.

High temperature behaviour of platinum group metals in oxidizing atmospheres

Abstract The high temperature behaviour of the platinum group metals in oxidizing atmospheres is characterized by the formation of volatile oxides in addition to the evaporation of pure metal. The pressure- and temperature-dependent oxide evaporation can considerably increase the metal losses when compared with high vacuum or inert atmosphere conditions. Solid or liquid oxides are not formed on the surface because of their high dissociation pressure. In this paper the high temperature oxidation behaviour of the six platinum group metals (ruthenium, rhodium, palladium, osmium, iridium and platinum) is reviewed and the dependence on temperature, pressure and experimental conditions as well as the oxidation mechanism are discussed.

Improvement of the corrosion resistance of hard wear resistant coatings by intermediate plasma etching or multilayered structure

PVD hard coatings can be a serious alternative to electrochemical deposited coatings in the field of corrosion protection, if they are dense and without pores. This study is part of a series of experiments with the aim to deposit thin films onto high-speed steel by magnetron sputtering which possess a high corrosion resistance. Two different methods were chosen: (1) deposition of single layers (CrN, NbN) and (2) deposition of multilayer films (Cr/CrN). Additionally the effect of intermediate plasma etching was studied for these coating systems. That means for single layer systems, that the film growth was stopped several times and the surface of the as-grown film was exposed to an ion bombardment. For multilayer systems the intermediate plasma etching was applied between the different layers. It is assumed that the plasma etching of the film leads to a denser coating, due to defect-induced renucleation and improved adatom mobility. The corrosion resistance of the coatings was studied by potentiodynamic corrosion tests in sodium chloride solution. Further film properties were investigated by SEM, XRD, Scratch testing, Rockwell indentation tests and microhardness measurements. The results reveal that intermediate etching improves the corrosion resistance of monolithic CrN films, whereas for the multilayered coatings no further improvement could be observed.

Reactions of niobium and tantalum with gases at high temperatures and low pressures

The gas-metal reactions of niobium and tantalum solid solutions with such gases as hydrogen, nitrogen, oxygen, water vapour, carbon monoxide, and methane at high temperatures and low pressures are briefly discussed. The equilibrium pressures and the conditions for steady states are presented in figures and tables. These data enable one to estimate the gas content of niobium and tantalum after vacuum annealing processes, when the temperature of the specimen and the partial pressures of the gases are known.

Thermodynamics and phase relations in refractory metal solid solutions containing carbon, nitrogen, and oxygen

The refractory metals Nb, Ta, Mo, and W dissolve C, N, and O by forming interstitial solid solutions. The concentration of these components can be increased or decreased by annealing treatments, depending on the partial pressure of gases such as N2, O2, H2O, CH4, or CO and on the temperature of the specimen. New results in binary and ternary systems combined with those obtained in the last few years now provide most of the data needed for the thermochemical analysis of the solid solution phases and for the establishment of p-T-c phase diagrams. The mechanisms of the gas-metal reactions and the general feature of the constitution diagrams are similar for all refractory metals. However, marked differences exist between group Va metals niobium and tantalum and group VIa metals molybdenum and tungsten in the absolute values and temperature dependence of the equilibrium pressure of gaseous species and therefore also in the amount of gas absorbed. The data now available for the estimation of the final content of interstitials in solid solution after annealing treatments are compiled and discussed.

Corrosion studies with hard coating-substrate systems

Abstract After reviewing the fundamentals of corrosion, the various electrochemical and technical corrosion tests will be discussed with special emphasis on their problems and evidence. In addition to the corrosion behavior of the coating material, the effect of the substrate is also important. The experimental potential-current density curves reflect the electrochemical properties of the coating and the influence of the underlying substrate when micropores, pinholes and other defects are present. Examples are given for nitride hard coating-substrate and hard coating-interlayer-substrate systems. The interlayers can act as barrier layers improving corrosion resistance. The nitride hard coatings are deposited by various physical vapor deposition methods and the interlayers by electrochemical or physical vapor deposition.

High temperature solid-solubility limit and phase studies in the system tantalum-oxygen

Abstract The solid solubility limit of oxygen in tantalum has been determined in the temperature range 1100 °C–2500 °C. The terminal solubility increases from 3 at.%O at 1100 °C to 5.7 at.%O at 1880 °C; the solubility line shows a slight kink at 1550 °C and 5.1 at.%O. From the determinations of the terminal solubility and from quantitative metallographic examination of arc-melted alloys containing 20–70 at.%O a partial TaTa 2 O 5 phase diagram is proposed. The system shows a liquid immiscibility gap and hence a monotectic at 1880° C ± 30 ° C with the equilibrium phases α-Ta (5.7 at.%O), liquid L I (≈43 at.%O), and liquid L II (≈65 at.%O). Liquid L II decomposes by an eutectic reaction at 1550° C ± 30 ° C with the equilibrium phases α-Ta (5.1 at.%O). L II (≈71at.%O), and Ta 2 O 5 .