P.H. Mayrhofer

Self-organized nanostructures in the Ti-Al-N system

The phenomenon of age hardening could be evidenced in thin film applications. A model system, Ti1-xAlxN was chosen as such coatings are known for their excellent wear resistance enabling improved m ...

Microstructure and mechanical/thermal properties of Cr–N coatings deposited by reactive unbalanced magnetron sputtering

Chromium nitride (CrN) is a hard material and a well-established coating for applications where severe corrosion and friction conditions are present. In this work, we report on the influence of nitrogen/argon flow rate ratio, ion energy and ion/atom flux ratio on the microstructure, hardness, residual stresses and thermal stability of magnetron sputtered chromium nitride coatings. The coatings were characterized with respect to thickness, morphology, chemical composition, microstructure and hardness. Hardness values up to 38.4 GPa could be obtained for stoichiometric CrN, which strongly depend on the grain size and residual stress. Thermal coating properties were evaluated using stress measurements during thermal cycling and XRD analyses after annealing at 500 and 700°C. Film stresses up to 700°C were measured from the bending of coated silicon specimens using the Stoney formula. Stress relaxation occurring during this temperature treatment strongly depends on the biaxial stresses in the as-deposited state. The interrelationships between growth conditions, microstructure, mechanical and thermal properties will be presented and discussed.

A comparative study on reactive and non-reactive unbalanced magnetron sputter deposition of TiN coatings

Abstract Titanium nitride (TiN) coatings were deposited by unbalanced D.C. magnetron sputtering via the non-reactive and reactive technique using a TiN or Ti target, respectively. The differences of these sputter techniques have been studied in detail. Main emphasis was laid on the characterization of the ion bombardment parameters for both techniques. The ion energy and the ion/atom flux ratio was varied in the range between 30 and 120 eV and 0.1 and 10, respectively. Coating characterization was done with respect to morphology, chemical composition, crystallographic structure, hardness, and macrostresses during thermal cycling. The use of an ion energy of 30 eV combined with an ion/atom flux ratio of 8.6 and 10 results in a microhardness of approximately 47 GPa for non-reactive and reactive TiN coatings, respectively. Their biaxial stresses and grain sizes also show comparable values for both techniques of approximately −2 GPa and 23 nm, respectively. The similar properties of TiN coatings deposited using non-reactive or reactive sputtering are, however, only valid for an intense ion bombardment. The transition from porous columnar to dense fibrous structures requires a more pronounced activation of film growth by ion bombardment in the case of reactive deposition as compared to non-reactive sputtering. Mainly, this is a result of the higher energy of the N atoms and the three times higher deposition rate in the non-reactive process compared to the reactive one. Moreover, during reactive sputtering, energy is also needed to dissociate the molecular nitrogen gas. The results obtained should serve as a fundamental basis for the understanding of the differences in growth conditions for non-reactive and reactive sputter techniques. Furthermore, an explanation of the high hardness values of the coatings is given and the influence of thermal annealing on the defect density, grain size and microhardness of the coatings is presented and discussed in detail.

Self-organized nanocolumnar structure in superhard TiB2 thin films

TiB2 thin films are well known for their high hardness which makes them useful for wear-resistant applications. Overstoichiometric TiB2 deposited at 300 °C by nonreactive sputtering has been shown to exhibit superhardness (H⩾40GPa), while the hardness of their bulk stoichiometric counterparts is ∼25GPa. We show, using high-resolution transmission electron microscopy, that overstoichiometric TiB2.4 layers have a complex self-organized columnar nanostructure. The ∼20nm wide columns, encapsulated in excess B and oriented along 0001, consist of a bundle of ∼5nm diameter TiB2 subcolumns separated by an ultrathin B-rich tissue phase. The nanocolumnar structure, which is thermally stable to postannealing temperatures up to 700 °C, inhibits nucleation and glide of dislocations during hardness indentation measurements, while the high cohesive strength of the B-rich tissue phase prevents grain-boundary sliding. The combination of these effects results in the observed superhardness of ∼60GPa.

Structure–property relationships in single- and dual-phase nanocrystalline hard coatings

Abstract The structure of hard coatings deposited by PVD (physical vapor deposition) strongly depends on the growth parameters. In this work, the influence of microstructure and chemical composition on mechanical properties was investigated in detail for several nanocrystalline hard coatings. Single-phase TiN, CrN and TiB2 as well as dual-phase CrN–Cr2N, TiN–TiB2 and TiC–TiB2 coatings were prepared by reactive or non-reactive unbalanced dc magnetron sputtering, respectively. The hardness of the coatings investigated will be discussed with respect to the average grain size and residual biaxial stresses. By optimizing both nanostructure and biaxial stresses of stoichiometric TiN coatings, their microhardness (H) could be improved from 33 to 56 GPa, whereas the reduced Youngs modulus (E*) changed from 402 to 480 GPa. Thus, the ratio H3/E*2 which is representative for the resistance to plastic deformation could be increased from 0.222 to 0.806 GPa. For Cr–N coatings, H3/E*2 values up to 0.521 GPa were obtained, depending on their chemical composition and nanostructure. The nanocrystalline dual-phase coatings TiN–TiB2 and TiC–TiB2 even yielded H3/E*2 values up to 1.332 and 1.575 GPa, respectively. Comparing the different single- and dual-phase coatings investigated, the establishment of correlations between structure and mechanical properties enables the development of advanced coatings with high hardness and high resistance against plastic deformation. In addition, this work shows the tremendous importance of a controlled deposition process for optimizing coating properties.

High-temperature properties of nanocomposite TiBxNy and TiBxCy coatings

Abstract High-temperature investigations on nanocomposite TiB x N y and TiB x C y coatings were performed to determine their thermal stability. All coatings investigated were prepared by means of unbalanced DC magnetron co-sputtering using either a segmented TiN/TiB 2 or TiC/TiB 2 target. Recovery and recrystallization behavior were characterized by means of stress measurements during thermal annealing and differential scanning calorimetry (DSC). The nanocomposite coatings used for DSC measurements were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD of these coatings showed a pronounced growth of the individual phases after the DSC measurements. Therefore, the total enthalpy change during DSC can be attributed to grain growth of the nanocrystalline phases. The grain sizes after the DSC treatment as analyzed by XRD and SEM are in excellent agreement with the grain sizes calculated from the exothermic peaks of the DSC measurements. Grain growth occurred for the individual phases in TiB 1.2 N 0.5 and TiB 1.2 C 0.6 coatings during heating up to 1400°C from approximately 4 to 15 nm and 4 to 5 nm, respectively. Recovery effects of the coatings investigated are strongly influenced by the ion bombardment during film growth as well as by the chemical composition of the coatings.

Oxidation kinetics of sputtered Cr–N hard coatings

Hard coatings are nowadays successfully applied for applications where they are subjected to high temperatures. Although the oxidation threshold of hard coatings has been investigated by several authors, there is only limited information on the oxidation kinetics. Within this paper, the oxidation behavior and kinetics of sputtered Cr–N coatings were studied systematically. All coatings investigated were prepared by means of reactive unbalanced DC magnetron sputtering using a Cr target and argon/nitrogen discharges. The N/Cr ratio was varied systematically to achieve coatings with phase compositions of single phase Cr2N or CrN, respectively. For the single phase CrN coatings, the macrostresses were adjusted to values between −211 and 400 MPa using different ion bombardment conditions to evaluate the stress influence. The oxidation behavior of coatings deposited onto hot-working tool steel was investigated using thermo-gravimetric analysis in an argon/oxygen atmosphere. Dynamical measurements were carried out to determine the oxidation threshold and to provide basic information on suitable temperatures (between 725 and 825°C) for isothermal measurements. The activation energies for oxidation calculated by means of Arrhenius plots range between 102 and 465 kJ mol−1 for the different Cr–N coatings. The oxidation products were characterized using scanning electron microscopy, energy-dispersive X-ray analysis and X-ray diffraction and were found to be Cr2O3. Based on the results obtained in this study it can be concluded that the activation energy for oxidation provides a suitable basis for comparison of the oxidation behavior of different hard coatings.

Ab initio calculated binodal and spinodal of cubic Ti1−xAlxN

During annealing, metastable NaCl-structured (c) Ti1−xAlxN films initially exhibit spinodal decomposition which results in age hardening. Based on ab initio calculations, we show that the chemical requirement for spinodal decomposition in the quasibinary c‐TiN–c‐AlN system is fulfilled over a wide composition and temperature range. The enthalpy change for the decomposition of c‐Ti0.34Al0.66N is ∼26.4kJmol−1, which is in good agreement with previously reported experiments. The obtained results enable materials design of Ti1−xAlxN-based coating systems for high-temperature applications.

Low-stress superhard TiB films prepared by magnetron sputtering

The article reports on structure and mechanical properties of Ti-B alloy films sputter deposited from a sintered TiB 2 target using an unbalanced dc magnetron. We present results of a systematic investigation of the effect of negative substrate bias, U s , substrate ion current density i s , and substrate temperature, T s , on properties of Ti-B films. The X-ray diffraction (XRD) analysis shows that the Ti-B films consist of the hexagonal TiB 2 phase with the typical (0001) texture only. The TiB x films are over-stoichiometric with the ratio x=B/Ti=2.4. All Ti-B films sputter ion plated in argon magnetron discharge are superhard films with hardness H>40 GPa and exhibit high values of (i) effective Youngs modulus E*=E/(1 - v 2 ) up to approximately 600 GPa and (ii) elastic recovery, W e , up to approximately 82%; here E and v are the Youngs modulus and the Poissons ratio, respectively. Besides, it was found that the value of the Braggs angle 20 of the (0001) reflection line can be easily controlled by the energy delivered to the film during its growth by (I) the substrate heating T s and (2) ion bombardment (U s , i s ). The angle 20 of the (0001) reflection increases with increasing T s from 300 to 550 °C and decreasing U s from -150 to -50 V. In this range of process parameters, the energy E p delivered to the growing film per condensing atom by ion bombardment can be adjusted to a value, at which the (0001) reflection from sputtered films is close to that of the TiB 2 (0001) powder standard. These films exhibit a low macrostress, which approaches to zero. It enables to sputter thick (up to 8 μm) superhard (H>40 GPa) Ti-B films. The optimum value of E p is achieved when the Ti-B film is sputtered at U s =-50 V, i s = 1 mA/cm 2 , T s =550 °C with a deposition rate a D =52 nm/min. The Ti-B film prepared under these conditions exhibits a maximum hardness of H77 GPa, measured using a computer controlled microhardness tester Fischerscope H100 at the Vickers diamond indenter load L=50 mN.

In situ observation of rapid reactions in nanoscale Ni–Al multilayer foils using synchrotron radiation

The observation of rapid reactions in nanoscale multilayers present challenges that require sophisticated analysis methods. We present high-resolution in situ x-ray diffraction analysis of reactions in nanoscale foils of Ni0.9V0.1–Al using the Mythen II solid-state microstrip detector system at the Material Science beamline of the Swiss Light Source Synchrotron at Paul Scherrer Institute in Villigen, Switzerland. The results reveal the temperature evolution corresponding to the rapid formation of NiAl intermetallic phase, vanadium segregation and formation of stresses during cooling, determined at high temporal (0.125 ms) and angular (0.004°) resolution over a full angular range of 120°.