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@PHDTHESIS{Hans:706947,
author = {Hans, Marcus},
othercontributors = {Schneider, Jochen M. and Mitterer, Christian},
title = {{M}etastable cubic transition metal aluminium nitride and
oxynitride coatings: {T}heoretical phase stability and
defect structure predictions and verification by
industrial-scale growth experiments},
volume = {28},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {Shaker},
reportid = {RWTH-2017-08939},
isbn = {978-3-8440-5553-5},
series = {Materials Chemistry Dissertation},
year = {2017},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, RWTH Aachen University, 2017},
abstract = {In the first part of this thesis, the crystallite
size-dependent metastable phase formation of nanocrystalline
TiAlN, an industrial benchmark coating material, is
demonstrated through correlative ab initio calculations and
advanced material characterization at the nanometer scale.
By relating calculated surface and volume energy
contributions to the total energy, the chemical
composition-dependent phase boundary between the two
metastable solid solution phases of cubic and wurzite
Ti1-xAlxN is predicted. This phase boundary is characterized
by the critical crystallite size dcritical. Crystallite
size-dependent phase stability predictions are in very good
agreement with experimental phase formation data where x was
varied by utilizing combinatorial vapor phase condensation.
The wide range of maximum Al solubilities for metastable
cubic Ti1-xAlxN from xmax = 0.4 to 0.9 reported in
literature and the sobering disagreement thereof with
density functional theory predictions can be rationalized
based on the here identified crystallite size-dependent
metastable phase formation. Furthermore, it is evident that
phase stability predictions are flawed, if the previously
overlooked surface energy contribution to the total energy
is not considered. In the second part, Ti-Al-O-N coatings
are synthesized by cathodic arc and high power pulsed
magnetron sputtering. The effect of oxygen incorporation on
stress-free lattice parameters and Young’s moduli of
Ti-Al-O-N coatings is investigated by X-ray diffraction and
nanoindentation, respectively. As nitrogen is substituted by
oxygen, implications for the charge balance may be expected.
A reduction in equilibrium volume with increasing O
concentration is identified by X-ray diffraction and density
functional theory calculations of Ti-Al-O-N supercells
reveal the concomitant formation of metal vacancies. Hence,
the oxygen incorporation-induced metal vacancy formation
enables charge balancing. Furthermore, nanoindentation
experiments reveal a decrease in elastic modulus with
increasing O concentration. Based on ab initio data, two
causes can be identified: Metal vacancy-induced reduction in
elasticity and, second, the formation of, compared to the
corresponding metal nitride bonds, relatively weak Ti-O and
Al-O bonds. In the third and last part, consequences induced
by reactive cathodic arc evaporation of Ti-Al-O-N in an
industrial deposition system with two-fold substrate
rotation are addressed experimentally. The formation of
alternating O- and N-rich sublayers is identified by atom
probe tomography and can be understood by considering the
substrate rotation-induced variation in plasma density and
fluxes of film-forming species. The effect of plasma density
and fluxes on the incorporation of reactive species is
studied in stationary deposition experiments and preferred N
incorporation occurs, when the growing coating surface is
facing the arc source. Thus, the growing surface is
positioned in a region of high plasma density characterized
by large fluxes of film forming-species. Preferred O
incorporation takes place in a region of low plasma density
where small fluxes are present, when the growing surface is
blocked from the arc source by the substrate holder. Hence,
compositional modulations are caused by substrate rotation
as the growing coating surface is periodically exposed to
regions of different plasma density and fluxes. In summary,
the combination of theoretical predictions by ab initio
calculations and experimental verification by
industrial-scale growth experiments enables to identify
physical and chemical mechanisms defining metastable cubic
phase formation. The evolution of the local chemical
composition is prerequisite for a meaningful model of the
phase formation. Furthermore, understanding of the chemical
composition-induced defect structure facilitates materials
design of protective coatings with tunable mechanical
properties. The selection of process parameters such as
substrate rotation speed and reactive gas mixtures allows
for developing chemically modulated architectures of
functional protective coatings.},
cin = {520000 / 521110},
ddc = {620},
cid = {$I:(DE-82)520000_20140620$ / $I:(DE-82)521110_20140620$},
typ = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
doi = {10.18154/RWTH-2017-08939},
url = {https://publications.rwth-aachen.de/record/706947},
}
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