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Effects of atomic ordering on the elastic properties of TiN- and VN-based ternary alloys
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.ORCID iD: 0000-0002-2837-3656
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
2014 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 571, no Part 1, 145-153 p.Article in journal (Refereed) Published
Abstract [en]

Improved toughness is one of the central goals in the development of wear-resistant coatings. Previous studies of toughness in transition metal nitride alloys have addressed the effects of chemical composition in these compounds. Herein, we use density functional theory to study the effects of various metal sublattice configurations, ranging from fully ordered to fully disordered, on the mechanical properties of VM2N and TiM2N (M2 = W, Mo) ternary alloys. Results show that all alloys display high incompressibility, indicating strong M-N bonds. Disordered atomic arrangements yield lower values of bulk moduli and C11 elastic constants, as well as higher values of C44 elastic constants, compared to ordered structures. We attribute the low C44 values of ordered structures to the formation of fully-bonding states perpendicular to the applied stress. We find that the ductility of these compounds is primarily an effect of the increased valence electron concentration induced upon alloying.

Place, publisher, year, edition, pages
Elsevier, 2014. Vol. 571, no Part 1, 145-153 p.
Keyword [en]
Nitrides, Density functional theory, Elastic properties, Ductility, Toughness
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-111949DOI: 10.1016/j.tsf.2014.09.048ISI: 000346053900024OAI: oai:DiVA.org:liu-111949DiVA: diva2:762289
Available from: 2014-11-11 Created: 2014-11-11 Last updated: 2017-12-05Bibliographically approved
In thesis
1. A theoretical study of mass transport processes on TiN(001) and mechanical properties of TiN- and VN-based ternaries
Open this publication in new window or tab >>A theoretical study of mass transport processes on TiN(001) and mechanical properties of TiN- and VN-based ternaries
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis concerns computer simulations, using classical molecular dynamics, of transport processes related to TiN(001) growth. It is motivated from the challenge to understand transport processes at the atomic scale responsible for crystal and film growth and their different growth modes. Not even the most advanced experimental techniques are capable of resolving the sub ps time and sub-Ångström length-scales required. TiN belongs to an important class of transition metal nitrides, and is chosen here as a model system for such fundamental studies of surface transport. The simulations show that on terraces, Ti adatoms exhibit much higher migration rates than N adatoms. For TiNx complexes, as x increases from 1 to 3, rotation becomes increasingly more prevalent than translation. This leads to surprisingly high mobilities of TiN2 trimers, higher than that of N adatoms. On islands, Ti adatoms experience a significant funneling effect, resulting in short residence times. TiN dimers and TiN2 trimers exhibit surprisingly high diffusivities and residence times even shorter than Ti adatoms. TiN3 trimers, however, are essentially stationary on both terraces and islands and serve as nucleation clusters. Overall, Ti adatoms and TiN2 trimers are the most efficient carriers of Ti and N atoms with and between TiN(001) surface layers. These results indicate that Ti/N flux ratios close to one promote layer-by-layer TiN(001) growth, whereas lower ratios result in surface roughening. Understanding of these phenomena enables experimentalists to tune  the growth processes to optimize material properties.

In this thesis I also carry out theoretical calculations to investigate the role of configurational order on the metallic sublattice in relation to toughness enhancement. My studies set out from the recent understanding that the toughness of transition metal nitrides can be enhanced by tuning the valence electron concentration. My results show that ordered alloys exhibit lower resistance to shear deformations than disordered alloys, and higher resistance to tensile deformation. The lower resistance to shear deformations is explained by the formation of fully bonding electronic states perpendicular to the applied stress. Using the Pugh-Pettifor criterion, it is shown that while configurational order has an effect on the ductility of the material, this is primarily governed by the valence electron concentration.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 22 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1686
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-111950 (URN)10.3384/lic.diva-111950 (DOI)978-91-7519-197-3 (ISBN)
Supervisors
Available from: 2014-11-11 Created: 2014-11-11 Last updated: 2016-08-31Bibliographically approved
2. Growth and Mechanical Properties of Transition Metal Nitrides and Carbides
Open this publication in new window or tab >>Growth and Mechanical Properties of Transition Metal Nitrides and Carbides
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The atomic-scale dynamical processes at play during film growth cannot be resolved by even the most advanced experimental methods. As such, computational methods, and chiefly classical molecular dynamics, are the only available research tools to study these processes. The investigation of key dynamical processes during thin film growth yields a deeper understanding of the film growth evolution, ultimately allowing for the optimization of experimental parameters and tailoring of film properties. This thesis details the study of fundamental surface dynamics processes, and the role played by primary diffusing species, during TiN film growth, here employed as a model system for transition metal nitrides in general. It is found that Ti adatoms and TiN2 admolecules are the fastest diffusing species, and the species which most rapidly descend from islands onto the growing film. Thus, they are the main contributors and players in driving the layer-by-layer growth mode. TiN3 admolecules, in contrast, are essentially stationary and thereby promote multilayer growth. Large-scale growth simulations reveal that tailoring the incident N/Ti ratio and N kinetic energy significantly affects the growth mode and film microstructure.

The mechanical properties of ternary transition metal nitride and carbide alloys, investigated using density functional theory, are also discussed herein, in comparison to recent experimental results. By optimizing the valence electron concentration in these compounds, the occupation of shear-compliant d‑t2g electronic states can be maximized. The investigation of M1M2N alloys, where M1 = Ti or V and M2 = W or Mo, with different structures demonstrates that this optimization leads to enhanced ductility, and thereby toughness, in transition metal nitride alloys regardless of the degree of ordering on the metal sublattice. Estimations based on the calculation of the mechanical properties of the corresponding M1M2C transition metal carbide alloys indicate that these materials remain brittle. However, charge density analysis and calculations of stress/strain curves reveal features commonly associated with ductile materials.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. 48 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1791
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-132272 (URN)10.3384/diss.diva-132272 (DOI)9789176856840 (ISBN)
Public defence
2016-11-30, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Available from: 2016-11-11 Created: 2016-10-25 Last updated: 2016-11-11Bibliographically approved

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