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Theoretical understanding of stability of alloys for hard-coating applications and design
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The performance of modern hard coating materials puts high demands on properties such as hardness, thermal stability and oxidation resistance. These properties not only depend on the chemical composition, but also on the structure of the material on a nanoscale. This kind of nanostructuring will change during use and can be both beneficial and detrimental as materials grown under non-equilibrium conditions transforms under heat treatment or pressure into other structures with significantly different properties. This thesis aims to reveal the physics behind the processes of phase stability and transformations and how this can be utilized to improve on the properties of this class of alloys. This has been achieved through the application of various methods of first-principles calculations and analysis of the results on the basis of thermodynamics and electronic structure theory.

Within multicomponent transition metal aluminum nitride alloys (TMAlN) a number of studies have been carried out and presented here on ways of improving high temperature stability and hardness. Most (TMAl)N and TMN prefer a cubic B1 structure while AlN is stable in a hexagonal B4 phase, but for the purposes of hard coatings the metastable cubic B1 AlN phase, isostructural with the TMN phase is desired. It will be shown how the introduction of additional alloying components, such as Cr, into (TiAl)N changes the thermodynamic stability of phases so that new intermediary and metastable phases are formed during decomposition. In the case of such a (CrAl)N phase it is shown to have greater thermodynamic stability in the cubic phase than the pure AlN, resulting in improved high temperature hardness. Also, the importance of treating not just the binodal decomposition through the formation energy relative to end products but also the impact of spinodal decomposition from its second derivative due to the topology of formation energy surfaces is emphasized in the thesis. The impact of pressure on the AlN phase has also been studied through the calculation of a P-T diagram of AlN as part of a (TiAl)N alloy.

During the study of chemical alloying of TM components into AlN the alloying of low concentrations of these TM were treated in great detail. What is generally referred to as the AlN phase in decomposition is not entirely pure and can be expected to contain traces of any alloying components, such as Ti and Cr or whatever other metals may be present. Low concentration alloying of Cr, on the order of 5-10% is also shown to be stable with regard to isostructural decomposition. Detailed analysis of the effect of Ti and Cr impurities in AlN has been carried out along with a systematic search of AlN alloyed with small amounts of other TM components. The impact of these impurities on the electronic structure and thermodynamic properties is analyzed and the general trends will be explained through the occupation of impurity states by d-like electrons.

Theoretical treatment of such impurities is not straightforward however. AlN is an s-p semiconductor with a wide band gap while TM impurities generate states of a d-like nature situated inside the band gap. Such localized impurity states are expected to give rise to magnetic effects due to spin dependent exchange, in addition strong correlation effects might have to be taken into account. For that reason the use of hybrid functionals with orbital corrections according to the mHSE+Vw scheme, developed specifically for this class of materials, has been used and shown to influence the results during calculation of impurities of Ti and Cr.

In nanocomposite multilayered structures, composed of very thin layers of one material sandwiched between slabs of another, such as layers of SiN between TiN or ZrN, the material properties are greatly affected by the interfaces. In addition to the thermodynamic effects and lattice strains of the interfaces one also has to consider the atomic vibrational motion in the interface structure. Hence, dynamical stability of these thin multilayers is of great importance. As part of this thesis, results on the thermodynamic and dynamical stability of both TiN-SiN layers and ZrN-SiN will be presented. It will be shown that due to considerable dynamical instability in the interface structure of monolayered B1 SiN sandwiched between isostructural layers of B1 ZrN along (111) interfaces this structure cannot be expected to grow, instead preferring the stable (001) direction of growth.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. , 73 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1647
National Category
Physical Sciences Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-115405DOI: 10.3384/diss.diva-115405ISBN: 978-91-7519-112-6 (print)OAI: oai:DiVA.org:liu-115405DiVA: diva2:795370
Public defence
2015-04-10, Planck, Fysikhuset, Campus Valla, Linköping, 13:15 (English)
Opponent
Supervisors
Available from: 2015-03-16 Created: 2015-03-16 Last updated: 2015-03-16Bibliographically approved
List of papers
1. Phase Stability and Elasticity of TiAlN
Open this publication in new window or tab >>Phase Stability and Elasticity of TiAlN
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2011 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 4, no 9, 1599-1618 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
MDPI, 2011
Keyword
hard coatings; spinodal decomposition; ab initio calculations; thermodynamics; multilayer; TiN
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:liu:diva-72927 (URN)10.3390/ma4091599 (DOI)000298245900006 ()
Funder
Strategic Initiative - Materials Science
Note

Funding agencies|Swedish Foundation for Strategic Research (SSF) via research center MultiFilms||Swedish Foundation for Strategic Research (SSF) via research center MS2E||Swedish Research Council (VR)||

Available from: 2011-12-11 Created: 2011-12-11 Last updated: 2017-12-08
2. Improving thermal stability of hard coating films via a concept of multicomponent alloying
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2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 99, no 9, 091903- p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-70747 (URN)10.1063/1.3631672 (DOI)000294489300018 ()
Note

Funding Agencies|SSF||Swedish Research Council||Gran Gustafsson Foundation for Research in Natural Sciences and Medicine||

Available from: 2011-09-16 Created: 2011-09-16 Last updated: 2017-12-08
3. High temperature phase decomposition in TixZryAlzN
Open this publication in new window or tab >>High temperature phase decomposition in TixZryAlzN
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2014 (English)In: AIP Advances, ISSN 2158-3226, E-ISSN 2158-3226, Vol. 4, no 12, 127147-1-127147-9 p.Article in journal (Refereed) Published
Abstract [en]

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 formation energy of (ZrAl)N causes a strong tendency for spinodal decomposition between ZrN and AlN while other decompositions tendencies are suppressed. In addition we observe that entropic  effects due to configurational disorder favor a formation of a stable Zr-rich (TiZr)N phase with increasing temperature. Our calculations also predict that at high temperatures a Zr rich (TiZrAl)N disordered phase should become more resistant against the spinodal decomposition despite its high and positive formation energy due to the specific topology of the free energy surface at the relevant concentrations. Our experimental observations confirm this prediction by showing strong tendency towards decomposition in a Zr-poor sample while a Zr-rich alloy shows a greatly reduced decomposition rate, which is mostly attributable to binodal decomposition processes. This result highlights the importance of considering the second derivative of the free energy, in addition to its absolute value in predicting decomposition trends of thermodynamically unstable alloys.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2014
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-110682 (URN)10.1063/1.4905138 (DOI)000347170100078 ()
Note

On the day of the defence date the status of this article was Manuscript.

Available from: 2014-09-18 Created: 2014-09-18 Last updated: 2017-12-05Bibliographically approved
4. High pressure and high temperature stabilization of cubic AlN in Ti0.60Al0.40N
Open this publication in new window or tab >>High pressure and high temperature stabilization of cubic AlN in Ti0.60Al0.40N
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2013 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, no 5Article in journal (Refereed) Published
Abstract [en]

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 calculations. High pressure and high temperature (HPHT) treatment of the samples was realized using the multi anvil press and diamond anvil cell techniques. The products of the HPHT treatment of Ti0.6Al0.4N were investigated using x-ray diffractometry and transmission electron microscopy. Complimentary calculations show that both hydrostatic pressure and high temperature stabilize the cubic phase of AlN, which is one of the decomposition products of Ti0.6Al0.4N. This is in agreement with the experimental results which in addition suggest that the presence of Ti in the system serves to increase the stability region of the cubic c-AlN phase. The results are industrially important as they show that Ti0.6Al0.4N coatings on cutting inserts do not deteriorate faster under pressure due to the cubic AlN to hexagonal AlN transformation.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2013
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-90201 (URN)10.1063/1.4790800 (DOI)000314746200028 ()
Note

Funding Agencies|Swedish Foundation for Strategic Research (SSF)||German Research Foundation (DFG)|SPP 1236|

On the day of the defence day the status of this articla was Manucsript and title of this was High pressure and high temperature behavior of Ti0.60Al0.40N.

Available from: 2013-03-21 Created: 2013-03-21 Last updated: 2017-12-06
5. Systematic theoretical search for alloys with increased thermal stability for advanced hard coatings applications
Open this publication in new window or tab >>Systematic theoretical search for alloys with increased thermal stability for advanced hard coatings applications
2013 (English)In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 15Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Institute of Physics: Open Access Journals / Institute of Physics (IoP) and Deutsche Physikalische Gesellschaft, 2013
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-98663 (URN)10.1088/1367-2630/15/9/095010 (DOI)000324462800002 ()
Note

Funding Agencies|Swedish Foundation for Strategic Research (SSF)||SRL|10-0026|Swedish Research Council|621-2011-4426|

Available from: 2013-10-10 Created: 2013-10-10 Last updated: 2017-12-06
6. Ti and Cr impurities in cubic and hexagonal AlN
Open this publication in new window or tab >>Ti and Cr impurities in cubic and hexagonal AlN
(English)Manuscript (preprint) (Other academic)
Abstract [en]

AlN is a wide band gap semiconductor that is used in many fields, e.g. as electronic material, for piezoelectric applications but also as a component material in high performance hard coating alloys. The stable structure under ambient conditions is a hexagonal wurtzite structure, but it has also been observed in the tetrahedrally bonded cubic zinc-blende structure as well as cubic rock-salt phases that become stable at high pressure. The metastable rock-salt phase of AlN also forms during decomposition processes in hard-coating alloys such as (TiAl)N, (CrAl)N and (TiCrAl)N. Even though thermodynamically unstable, one can expect some amount of Ti and Cr to be present in the c-AlN phase during the decomposition. Still, little study has been done for the dilute (TMAl)N alloys with cubic B1 crystal structure. We study the electronic structure of Ti and Cr impurities in B1 AlN. Because of the limitations of standard local and semi-local approximations within the density functional theory (DFT) in the treatment of wide band gap semiconductors, as well as conventional hybrid functionals for systems consisting of correlated localized and delocalized orbitals, we apply the mHSE+Vw method, which has been developed specifically to dealing with these kind of problems. Simulations are done by means of the supercell technique with single impurities, as well as for the impurity pairs. The effects of different atomic configurations of the TM-impurities on phase stability and magnetic properties of the cubic B1 AlN is studied and compared to the those in hexagonal B4 structures. Our results underline the importance of correlation and magnetic effects for the description of properties of cubic AlN doped with Ti and Cr.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-115402 (URN)
Available from: 2015-03-16 Created: 2015-03-16 Last updated: 2015-03-16Bibliographically approved
7. First-principles study of the SiNx/TiN(001) interface
Open this publication in new window or tab >>First-principles study of the SiNx/TiN(001) interface
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2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 10, 104106- p.Article in journal (Refereed) Published
Abstract [en]

The structure of the SiNx tissue phase in superhard TiN/SiNx nanocomposites has been debated in the literature. We present a theoretical investigation of the possibility of crystalline and coherent ( 001) interfaces that satisfies the two necessary criteria, stability with respect to lattice vibrations as well as to variations in stoichiometry. It is found that one monolayer of Si tetrahedrally coordinated by N in a B3-like geometry embedded between B1-TiN( 001) surfaces is both dynamically stable and thermodynamically stable with respect to vacancy formation. However, with increasing layer thickness the B3-type structure becomes unstable with respect to Si vacancy formation. Instead we suggest that a tetragonal D0(22)-like order of Si vacancies can stabilize the interface. These structures are in line with the experimental findings of the crystalline tissue phase which has coherent interfaces with TiN.

Place, publisher, year, edition, pages
American Physical Society, 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-76187 (URN)10.1103/PhysRevB.85.104106 (DOI)000301334700002 ()
Available from: 2012-03-31 Created: 2012-03-30 Last updated: 2017-12-07
8. Anomalous epitaxial stability of (001) interfaces in ZrN/SiNx multilayers
Open this publication in new window or tab >>Anomalous epitaxial stability of (001) interfaces in ZrN/SiNx multilayers
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2014 (English)In: APL Materials, ISSN 2166-532X, Vol. 2, no 4, 046106- p.Article in journal (Refereed) Published
Abstract [en]

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 similar to 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 degrees-rotated in-plane preferred orientation. Density functional theory calculations confirm stability of a (001) interface with respect to a (111) which explains the anomaly.

Place, publisher, year, edition, pages
American Institute of Physics, 2014
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-107456 (URN)10.1063/1.4870876 (DOI)000336083600007 ()
Available from: 2014-06-12 Created: 2014-06-12 Last updated: 2015-03-16Bibliographically approved
9. Giant two-phonon Raman scattering from nanoscale NbC precipitates in Nb
Open this publication in new window or tab >>Giant two-phonon Raman scattering from nanoscale NbC precipitates in Nb
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2015 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 91, no 9, 094302Article in journal (Refereed) Published
Abstract [en]

High purity niobium (Nb), subjected to the processing methods used in the fabrication of superconducting RF cavities, displays micron-sized surface patches containing excess carbon. High-resolution transmission electron microscopy and electron energy-loss spectroscopy measurements are presented which reveal the presence 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 two-phonon scattering. The unprecedented strength and sharpness of the two-phonon signal has prompted a theoretical 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. The exact origin of the stronger two-phonon response is not known at present but it suggests the possibility of 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.

Place, publisher, year, edition, pages
American Physical Society, 2015
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-115403 (URN)10.1103/PhysRevB.91.094302 (DOI)000351036900002 ()
Note

The authors thank G. Ciovati of Jefferson Laboratory for supplying Nb samples used in this study. Calculations (H.L.) were performed with financial support by the SSF-project Designed multicomponent coatings, MultiFilms and the Swedish Research Council. Calculations were carried out at the Swedish National Infrastructure for Computing (SNIC), Argonne LCRC and Argonne Center for Nanoscale Materials. The work at Argonne National Laboratory and the use of the Center for Nanoscale Materials and the Electron Microscopy center at Argonne National Laboratory were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357. This work was also supported by the Department of Energy, Office of Science, Office of High Energy Physics, early career award FWP#50335 to T.P.

Available from: 2015-03-16 Created: 2015-03-16 Last updated: 2017-12-04Bibliographically approved

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