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Transition Metal Nitrides: Alloy Design and Surface Transport Properties using Ab-initio and Classical Computational Methods
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Enhanced toughness in brittle ceramic materials, such as transition metal nitrides (TMN), is achieved by optimizing the occupancy of shear-sensitive metallic electronic-states. This is the major result of my theoretical research, aimed to solve an inherent long-standing problem for hard ceramic protective coatings: brittleness. High hardness, in combination with high toughness, is thus one of the most desired mechanical/physical properties in modern coatings. A significant part of this PhD Thesis is dedicated to the density functional theory (DFT) calculations carried out to understand the electronic origins of ductility, and to predict novel TMN alloys with optimal hardness/toughness ratios. Importantly, one of the TMN alloys identified in my theoretical work has subsequently been synthesized in the laboratory and exhibits the predicted properties.

The second part of this Thesis concerns molecular dynamics (MD) simulations of Ti, N, and TiNx adspecies diffusion on TiN surfaces, chosen as a model material, to provide unprecedented detail of critical atomic-scale transport processes, which dictate the growth modes of TMN thin films. Even the most advanced experimental techniques cannot provide sufficient information on the kinetics and dynamics of picosecond atomistic processes, which affect thin films nucleation and growth. Information on these phenomena would allow experimentalists to better understand the role of deposition conditions and fine tune thin films growth modes, to tailor coatings properties to the requirements of different applications. The MD simulations discussed in the second part of this PhD Thesis, predict that Ti adatoms and TiN2 admolecules are the most mobile species on TiN(001) terraces. Moreover, these adspecies are rapidly incorporated at island descending steps, and primarily contribute to layer-by-layer growth. In contrast, TiN3 tetramers are found to be essentially stationary on both TiN(001) terraces and islands, and thus constitute the critical nuclei for three-dimensional growth.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. , 75 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1513
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-91379ISBN: 978-91-7519-638-1 (print)OAI: oai:DiVA.org:liu-91379DiVA: diva2:617410
Public defence
2013-05-23, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2013-04-23 Created: 2013-04-23 Last updated: 2016-08-31Bibliographically approved
List of papers
1. Electronic mechanism for toughness enhancement in TixM1-xN (M=Mo and W)
Open this publication in new window or tab >>Electronic mechanism for toughness enhancement in TixM1-xN (M=Mo and W)
2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 10, 104107-104113 p.Article in journal (Refereed) Published
Abstract [en]

Toughness, besides hardness, is one of the most important properties of wear-resistant coatings. We use ab initio density-functional theory calculations to investigate the mechanical properties of ternary metal nitrides TixM1-xN, with M=Mo and W, for x=0.5. Results show that Mo and W alloying significantly enhances the toughness of TiN. The electronic mechanism responsible for this improvement, as revealed by electronic structure calculations, stems from the changes in charge density induced by the additional transition-metal atom. This leads to the formation of a layered electronic arrangement, characterized by strong, respectively, weak, directional bonding, which enables a selective response to strain, respectively, shear, deformations of the structures and yields up to 60% decrease in C-44 values.

Place, publisher, year, edition, pages
American Physical Society, 2010
Keyword
cubic, transition metal nitrides, mechanical properties, ab initio, dft, ductility, toughness, electronic structure
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-54851 (URN)10.1103/PhysRevB.81.104107 (DOI)000276248700040 ()
Note
Original Publication: Davide Sangiovanni, Valeriu Chirita and Lars Hultman, Electronic mechanism for toughness enhancement in TixM1-xN (M=Mo and W), 2010, PHYSICAL REVIEW B, (81), 10, 104107. http://dx.doi.org/10.1103/PhysRevB.81.104107 Copyright: American Physical Society http://www.aps.org/ Available from: 2010-04-16 Created: 2010-04-16 Last updated: 2017-12-12
2. Supertoughening in B1 transition metal nitride alloys by increased valence electron concentration
Open this publication in new window or tab >>Supertoughening in B1 transition metal nitride alloys by increased valence electron concentration
2011 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 5, 212-2134 p.Article in journal (Refereed) Published
Abstract [en]

We use density functional theory calculations to explore the effects of alloying cubic TiN and VN with transition metals M = Nb, Ta, Mo, W in 50% concentrations. The obtained ternaries are predicted to become supertough as they are shown to be harder and significantly more ductile compared to the reference binaries. The primary electronic mechanism of this supertoughening effect is shown in a comprehensive electronic structure analysis of these compounds to be the increased valence electron concentration intrinsic to these ternaries. Our investigations reveal the complex nature of chemical bonding in these compounds, which ultimately explains the observed selective response to stress. The findings presented in this paper thus offer a design route for the synthesis of supertough transition metal nitride alloys via valence electron concentration tuning.

Place, publisher, year, edition, pages
Elsevier, 2011
Keyword
Cubic, transition metal nitrides, mechanical properties, ab initio, dft, ductility, toughness, electronic structure
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-63361 (URN)10.1016/j.actamat.2010.12.013 (DOI)000287775400026 ()
Funder
Swedish Research Council
Note
On the defence day the status of the article was: Accepted. Original Publication: Davide Giuseppe Sangiovanni, Lars Hultman and Valeriu Chirita, Supertoughening in B1 transition metal nitride alloys by increased valence electron concentration, 2011, Acta Materialia, (59), 5, 212-2134. http://dx.doi.org/10.1016/j.actamat.2010.12.013 Copyright: Elsevier Science B.V., Amsterdam. http://www.elsevier.com/ Available from: 2010-12-16 Created: 2010-12-16 Last updated: 2017-12-11Bibliographically approved
3. Structure and mechanical properties of TiAlN-WNI(x) thin films
Open this publication in new window or tab >>Structure and mechanical properties of TiAlN-WNI(x) thin films
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2011 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 205, no 20, 4821-4827 p.Article in journal (Refereed) Published
Abstract [en]

A combinatorial method was employed to grow TiAlN-WNx films by DC sputtering as well as by High Power Pulsed Magnetron Sputtering (HPPMS) where the W concentration was varied between 10-52 at.% and 7-54 at.%, respectively. Experiments were paired with ab initio calculations to investigate the correlation between composition, structure, and mechanical properties. During all depositions the time averaged power was kept constant. As the W concentration was increased, the lattice parameter of cubic TiAlN-WNx films first increased and then decreased for W concentrations above approximate to 29 at.% (DCMS) and approximate to 27 at.% (HPPMS) as the N concentration decreased. Calculations helped to attribute the increase to the substitution of Ti and Al by W and the decrease to the presence of N vacancies. Youngs modulus and hardness were around 385-400 GPa and 29-31 GPa for DCMS and 430-480 GPa and 34-38 GPa for HPPMS, respectively, showing no significant trend as the W concentration was increased, whereas calculations showed a continuous decrease in Youngs modulus from 440 to 325 GPa as the W concentration was increased from 0 to 37.5 at.%. The presence of N vacancies was shown to increase the calculated Youngs modulus. Hence, the relatively constant values measured may be understood based on N vacancy formation as the W concentration was increased. HPPMS-deposited films exceed DCMS films in Youngs modulus and hardness, which may be a consequence of the larger degree of ionization in the HPPMS plasma. It is reasonable to assume that especially the ionized film forming species may contribute towards film densification and N vacancy formation.

Place, publisher, year, edition, pages
Elsevier Science B.V., Amsterdam., 2011
Keyword
TiAl-WNx; HPPMS/HiPIMS; Combinatorial sputtering; Ab initio calculation; Structure; Elastic properties
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-69794 (URN)10.1016/j.surfcoat.2011.04.066 (DOI)000292361400012 ()
Available from: 2011-08-10 Created: 2011-08-08 Last updated: 2017-12-08
4. Toughness enhancement in TiAlN-based quarternary alloys
Open this publication in new window or tab >>Toughness enhancement in TiAlN-based quarternary alloys
2012 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 520, no 11, 4080-4088 p.Article in journal (Refereed) Published
Abstract [en]

Improved toughness in hard and superhard thin films is a primary requirement for present day ceramic hard coatings, known to be prone to brittle failure during in-use conditions. We use density functional theory calculations to investigate a number of (TiAl)(1-x)MxN thin films in the B1 structure, with 0.06 andlt;= x andlt;= 0.75 obtained by alloying TiAlN with M = V, Nb, Ta, Mo and W. Results show significant ductility enhancements, hence increased toughness, in these compounds. Importantly, these thin films are also predicted to be superhard, with similar or increased hardness values, compared to Ti0.5Al0.5 N. For (TiAl)(1-x)WxN the results are experimentally confirmed. The ductility increase originates in the enhanced occupancy of d-t(2g) metallic states, induced by the valence electrons of substitutional elements (V, Nb, Ta, Mo, W). This effect is more pronounced with increasing valence electron concentration, and, upon shearing, leads to the formation of a layered electronic structure in the compound material, consisting of alternating layers of high and low charge density in the metallic sublattice, which in turn, allows a selective response to normal and shear stresses.

Place, publisher, year, edition, pages
Elsevier, 2012
Keyword
Nitrides, Titanium aluminum nitride, Hardness, Toughness, Ductility, Density Functional Theory, Metals, Quarternaries
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-77336 (URN)10.1016/j.tsf.2012.01.030 (DOI)000302838800037 ()
Note
Funding Agencies|Swedish Research Council (VR)||Swedish Strategic Research Foundation (SSF)||Available from: 2012-05-11 Created: 2012-05-11 Last updated: 2017-12-07
5. Dynamics of Ti, N, and TiNx (x=1-3) admolecule transport on TiN(001) surfaces
Open this publication in new window or tab >>Dynamics of Ti, N, and TiNx (x=1-3) admolecule transport on TiN(001) surfaces
Show others...
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 15, 155443- p.Article in journal (Refereed) Published
Abstract [en]

We use classical molecular dynamics and the modified embedded atom method formalism to investigate the dynamics of atomic-scale transport on a low-index model compound surface, TiN(001). Our simulations, totaling 0.25 mu s for each case study, follow the pathways and migration kinetics of Ti and N adatoms, as well as TiNx complexes with x = 1-3, which are known to contribute to the growth of TiN thin films by reactive deposition from Ti, N-2, and N precursors. The simulations are carried out at 1000 K, within the optimal range for TiN(001) epitaxial growth. We find Ti adatoms to be the highest-mobility species on TiN(001), with the primary migration path involving jumps of one nearest-neighbor distance d(NN) between adjacent fourfold hollow sites along in-plane andlt; 100 andgt; channels. Long jumps, 2d(NN), are also observed, but at much lower frequency. N adatoms, which exhibit significantly lower migration rates than Ti, diffuse along in-plane andlt; 110 andgt; directions and, when they intersect other N atoms, associatively form N-2 molecules, which desorb at kinetic rates. As expected, TiN and TiN3 complexes migrate at even lower rates with complex diffusion pathways involving rotations, translations, and rototranslations. TiN2 trimers, however, are shown to have surprisingly high diffusion rates, above that of N adatoms and almost half that of Ti adatoms. TiN3 motion is dominated by in-place rotation with negligible diffusion.

Place, publisher, year, edition, pages
American Physical Society, 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-86129 (URN)10.1103/PhysRevB.86.155443 (DOI)000310130800008 ()
Note

Funding Agencies|Swedish Research Council (VR)|2008-6572|Swedish Government Strategic Research Area Grant in Materials Science|Mat-LiU 2009-00971|

Available from: 2012-12-07 Created: 2012-12-07 Last updated: 2017-12-07
6. Toughness Enhancement in Hard Ceramic Thin Films by Alloy Design
Open this publication in new window or tab >>Toughness Enhancement in Hard Ceramic Thin Films by Alloy Design
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2013 (English)In: APL MATERIALS, ISSN 2166-532X, Vol. 1, no 4, 042104- p.Article in journal (Refereed) Published
Abstract [en]

Hardness is an essential property for a wide range of applications. However, hardness alone, typically accompanied by brittleness, is not sufficient to prevent failure in ceramic films exposed to high stresses. Using VN as a model system, we demonstrate with experiment and density functional theory (DFT) that refractory VMoN alloys exhibit not only enhanced hardness, but dramatically increased ductility. V0.5Mo0.5N hardness is 25% higher than that of VN. In addition, while nanoindented VN, as well as TiN reference samples, suffer from severe cracking typical of brittle ceramics, V0.5Mo0.5N films do not crack. Instead, they exhibit material pile-up around nanoindents, characteristic of plastic flow in ductile materials. Moreover, the wear resistance of V0.5Mo0.5N is considerably higher than that of VN. DFT results show that tuning the occupancy of d-t2g metallic bonding states in VMoN facilitates dislocation glide, and hence enhances toughness, via the formation of stronger metal/metal bonds along the slip direction and weaker metal/N bonds across the slip plane.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2013
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-91373 (URN)10.1063/1.4822440 (DOI)000332277600006 ()
Available from: 2013-04-23 Created: 2013-04-23 Last updated: 2016-08-31Bibliographically approved
7. Ti and N adatom descent pathways to the terrace from atop two-dimensional TiN/TiN(001) islands
Open this publication in new window or tab >>Ti and N adatom descent pathways to the terrace from atop two-dimensional TiN/TiN(001) islands
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2014 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 558, 37-46 p.Article in journal (Refereed) Published
Abstract [en]

We use classical molecular dynamics and the modified embedded atom method to determine residence times and descent pathways of Ti and N adatoms on square, single-atom-high, TiN islands on TiN(001). Simulations are carried out at 1000 K, which is within the optimal range for TiN(001) epitaxial growth. Results show that the frequency of descent events, and overall adatom residence times, depend strongly on both the TiN(001) diffusion barrier for each species as well as the adatom island-edge location immediately prior to descent. Ti adatoms, with a low diffusion barrier, rapidly move toward the island periphery, via funneling, where they diffuse along upper island edges. The primary descent mechanism for Ti adatoms is via push-out/exchange with Ti island-edge atoms, a process in which the adatom replaces an island edge atom by moving down while pushing the edge atom out onto the terrace to occupy an epitaxial position along the island edge. Double push-out events are also observed for Ti adatoms descending at N corner positions. N adatoms, with a considerably higher diffusion barrier on TiN(001), require much longer times to reach island edges and, consequently, have significantly longer residence times. N adatoms are found to descend onto the terrace by direct hopping over island edges and corner atoms, as well as by concerted push-out/exchange with N atoms adjacent to Ti corners. For both adspecies, we also observe several complex adatom/island interactions, before and after descent onto the terrace, including two instances of Ti islandatom ascent onto the island surface.

Place, publisher, year, edition, pages
Elsevier, 2014
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-91377 (URN)10.1016/j.tsf.2014.02.053 (DOI)000334314100006 ()
Available from: 2013-04-23 Created: 2013-04-23 Last updated: 2017-12-06Bibliographically approved

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