Hans Lind

Improving thermal stability of hard coating films via a concept of multicomponent alloying

We propose a design route for the next generation of nitride alloys via a concept of multicomponent alloying based on self-organization on the nanoscale via a formation of metastable intermediate products during the spinodal decomposition. We predict theoretically and demonstrate experimentally that quasi-ternary (TiCrAl)N alloys decompose spinodally into (TiCr)N and (CrAl)N-rich nanometer sized regions. The spinodal decomposition results in age hardening, while the presence of Cr within the AlN phase delays the formation of a detrimental wurtzite phase leading to a substantial improvement of thermal stability compared to the quasi-binary (TiAl)N or (CrAl)N alloys.

Phase Stability and Elasticity of TiAlN

We review results of recent combined theoretical and experimental studies of Ti1−xAlxN, an archetypical alloy system material for hard-coating applications. Theoretical simulations of lattice parameters, mixing enthalpies, and elastic properties are presented. Calculated phase diagrams at ambient pressure, as well as at pressure of 10 GPa, show a wide miscibility gap and broad region of compositions and temperatures where the spinodal decomposition takes place. The strong dependence of the elastic properties and sound wave anisotropy on the Al-content offers detailed understanding of the spinodal decomposition and age hardening in Ti1−xAlxN alloy films and multilayers. TiAlN/TiN multilayers can further improve the hardness and thermal stability compared to TiAlN since they offer means to influence the kinetics of the favorable spinodal decomposition and suppress the detrimental transformation to w-AlN. Here, we show that a 100 degree improvement in terms of w-AlN suppression can be achieved, which is of importance when the coating is used as a protective coating on metal cutting inserts.

High pressure and high temperature stabilization of cubic AlN in Ti0.60Al0.40N

In the present work, the decomposition of unstable arc evaporated Ti0.6Al0.4N at elevated temperatures and quasihydrostatic pressures has been studied both experimentally and by first-principles ca ...

Systematic theoretical search for alloys with increased thermal stability for advanced hard coatings applications

State-of-the-art alloys for hard coating applications, such as TiAlN, are known to suffer from decreased hardness during heat treatment in excess of 900 C due to the formation of detrimental wurtzite AlN phases. Recent research has shown that multicomponent alloying with additional transition metals (TMs) such as Cr can shift the onset of the phase transformations to higher temperatures, but a search for new alloys is generally time-consuming due to the large number of processes that influence material properties along with the large number of alloy compositions that have to be synthesized. To overcome this difficulty we carry out systematic first-principles calculations aimed at finding potential new multicomponent TM aluminum nitride alloys for advanced hard coating applications. We direct our search towards a specific property, the thermal stability of the coating. In particular, we concentrate on the thermodynamic stability of the cubic B1 TM-Al-N phase relative to the wurtzite phase, and choose the enthalpy difference between them as our search descriptor. We perform ab initio calculations for all TMs, considered as impurities in AlN, and identify the most promising candidates that may improve the thermal stability. We present arguments that these elements should be targeted in future in-depth studies, theoretical, as well as experimental.

Anomalous epitaxial stability of (001) interfaces in ZrN/SiNx multilayers

Isostructural stability of B1-NaCl type SiN on (001) and (111) oriented ZrN surfaces is studied theoretically and experimentally. The ZrN/SiNx/ZrN superlattices with modulation wavelength of 3.76 nm (dSiNx∼0.4 nm) were grown by dc-magnetron sputtering on MgO(001) and MgO(111). The results indicate that 0.4 nm thin SiNx layers utterly influence the preferred orientation of epitaxial growth: on MgO(001) cube-on-cube epitaxy of ZrN/SiNx superlattices were realized whereas multilayers on MgO(111) surface exhibited an unexpected 002 texture with a complex fourfold 90°-rotated in-plane preferred orientation. Density functional theory calculations confirm stability of a (001) interface with respect to a (111) which explains the anomaly.

High temperature phase decomposition in TixZryAlzN

Through a combination of theoretical and experimental observations we study the high temperature decomposition behavior of c-(TixZryAlzN) alloys. We show that for most concentrations the high forma ...

Giant two-phonon Raman scattering from nanoscale NbC precipitates in Nb

High-purity niobium (Nb), subjected to the processing methods used in the fabrication of superconducting rf cavities, displays micrometer-sized surface patches containing excess carbon. High-resolution transmission electronmicroscopyandelectronenergy-lossspectroscopymeasurementsarepresentedwhichrevealthepresence of nanoscale NbC coherent precipitates in such regions. Raman backscatter spectroscopy on similar surface regions exhibit spectra consistent with the literature results on bulk NbC but with significantly enhanced twophononscattering.Theunprecedentedstrengthandsharpnessofthetwo-phononsignalhaspromptedatheoretical analysis, using density functional theory (DFT), of phonon modes in NbC for two different interface models of the coherent precipitate. One model leads to overall compressive strain and a comparison to ab initio calculations of phonon dispersion curves under uniform compression of the NbC shows that the measured two-phonon peaks are linked directly to phonon anomalies arising from strong electron-phonon interaction. Another model of the extended interface between Nb and NbC, studied by DFT, gives insight into the frequency shifts of the acoustic and optical mode density of states measured by first-order Raman spectroscopy. The exact origin of the stronger two-phononresponseisnotknown atpresentbutitsuggeststhepossibilityof enhanced electron-phonon coupling in transition-metal carbides under strain found either in the bulk NbC inclusions or at their interfaces with Nb metal. Preliminary tunneling studies using a point contact method show some energy gaps larger than expected for bulk NbC.

Theoretical description of pressure-induced phase transitions: A case study of Ti-V alloys

We discuss theoretical description of pressure-induced phase transitions by means of first-principles calculations in the framework of density functional theory. We illustrate applications of theoretical tools that allow one to take into account configurational and vibrational disorders, considering Ti–V alloys as a model system. The universality of the first-principles theory allows us to apply it in studies of different phenomena that occur in the Ti–V system upon compression. Besides the transitions between different crystal structures, we discuss isostructural transitions in bcc Ti–V alloys. Moreover, we present arguments for possible electronic transitions in this system, which may explain peculiar behaviour of elastic properties of V upon compression.