17 research outputs found
Bond formation at polycarbonate | X interfaces (X = AlO, TiO, TiAlO) studied by theory and experiments
Interfacial bond formation during sputter deposition of metal oxide thin
films onto polycarbonate (PC) is investigated by ab initio molecular dynamics
simulations and X-ray photoelectron spectroscopy (XPS) analysis of PC | X
interfaces (X = AlO, TiO, TiAlO). Generally, the predicted bond
formation is consistent with the experimental data. For all three interfaces,
the majority of bonds identified by XPS are (C-O)-metal bonds, whereas C-metal
bonds are the minority. Compared to the PC | AlO interface, the PC |
TiO and PC | TiAlO interfaces exhibit a reduction in the measured
interfacial bond density by ~ 75 and ~ 65%, respectively. Multiplying the
predicted bond strength with the corresponding experimentally determined
interfacial bond density shows that AlO exhibits the strongest
interface with PC, while TiO and TiAlO exhibit ~ 70 and ~ 60% weaker
interfaces, respectively. This can be understood by considering the complex
interplay between the metal oxide composition, the bond strength as well as the
population of bonds that are formed across the interface
On the effects of microstructural orientation on fracture toughness in (V,Al)-nitride and -oxynitride thin films
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Valence electron concentration- and N vacancy-induced elasticity in cubic early transition metal nitrides
Motivated by frequently reported deviations from stoichiometry in cubic
transition metal nitride (TMNx) thin films, the effect of N-vacancy
concentration on the elastic properties of cubic TiNx, ZrNx, VNx, NbNx, and
MoNx (0.72<x<1.00) is systematically studied by density functional theory (DFT)
calculations. The predictions are validated experimentally for VNx
(0.77<x<0.97). The DFT results indicate that the elastic behavior of the TMNx
depends on both the N-vacancy concentration and the valence electron
concentration (VEC) of the transition metal: While TiNx and ZrNx exhibit
vacancy-induced reductions in elastic modulus, VNx and NbNx show an increase.
These trends can be rationalized by considering vacancy-induced changes in
elastic anisotropy and bonding. While introduction of N-vacancies in TiNx
results in a significant reduction of elastic modulus along all directions and
a lower average bond strength of Ti-N, the vacancy-induced reduction in [001]
direction of VNx is overcompensated by the higher stiffness along [011] and
[111] directions, resulting in a higher average bond strength of V-N. To
validate the predicted vacancy-induced changes in elasticity experimentally,
close-to-single-crystal VNx (0.77<x<0.97) are grown on MgO(001) substrates. As
the N-content is reduced, the relaxed lattice parameter a0, as probed by X-ray
diffraction, decreases from 4.128 A to 4.096 A. This reduction in lattice
parameter is accompanied by an anomalous 11% increase in elastic modulus, as
determined by nanoindentation. As the experimental data agree with the
predictions, the elasticity enhancement in VNx upon N-vacancy formation can be
understood based on the concomitant changes in elastic anisotropy and bonding.Comment: 30 pages, 8 figures in the manuscript, 1 figure in supplementary
material
Large-area deposition of protective (Ti,Al)N coatings onto polycarbonate
Polycarbonate (PC) and protective (Ti,Al)N coatings exhibit extremely
different material properties, specifically crystal structure, thermal
stability, elastic and plastic behavior as well as thermal expansion
coefficients. These differences present formidable challenges for the
deposition process development as low-temperature synthesis routes have to be
explored to avoid a thermal overload of the polymer substrate. Here, a
large-area sputtering process is developed to address the challenges by
systematically adjusting target peak power density and duty cycle. Adhering
(Ti,Al)N coatings with a critical residual tensile stress of 2.2 +/- 0.2 GPa
are obtained in the pulsed direct current magnetron sputtering range, whereas
depositions at higher target peak power densities, realized by high power
pulsed magnetron sputtering, lead to stress-induced adhesive and/or cohesive
failure. The stress-optimized (Ti,Al)N coatings deposited onto PC with a target
peak power density of 0.036 kW cm-2 and a duty cycle of 5.3% were investigated
by cross-cut test confirming adhesion. By investigating the bond formation at
the PC | (Ti,Al)N interface, mostly interfacial CNx bonds and a small fraction
of (C-O)-(Ti,Al) bonds are identified by X-ray photoelectron spectroscopy,
indicating reactions at the hydrocarbon and the carbonate groups during
deposition. Nanoindentation reveals an elastic modulus of 296 +/- 18 GPa for
the (Ti,Al)N coating, while a Ti-Al-O layer is formed during electrochemical
impedance spectroscopy in a borate buffer solution, indicating protective
passivation. This work demonstrates that the challenge posed by the extremely
different material properties at the interface of soft polymer substrates and
hard coatings can be addressed by systematical variation of the pulsing
parameters to reduce the residual film stress
A Proposal for a Composite with Temperature-Independent Thermophysical Properties : HfV2-HfV2O7
The HfV2-HfV2O7 composite is proposed as a material with potentially temperature-independent thermophysical properties due to the combination of anomalously increasing thermoelastic constants of HfV2 with the negative thermal expansion of HfV2O7. Based on literature data, the coexistence of both a near-zero temperature coefficient of elasticity and a coefficient of thermal expansion is suggested for a composite with a phase fraction of approximately 30 vol.% HfV2 and 70 vol.% HfV2O7. To produce HfV2-HfV2O7 composites, two synthesis pathways were investigated: (1) annealing of sputtered HfV2 films in air to form HfV2O7 oxide on the surface and (2) sputtering of HfV2O7/HfV2 bilayers. The high oxygen mobility in HfV2 is suggested to inhibit the formation of crystalline HfV2-HfV2O7 composites by annealing HfV2 in air due to oxygen-incorporation-induced amorphization of HfV2. Reducing the formation temperature of crystalline HfV2O7 from 550 degrees C, as obtained upon annealing, to 300 degrees C using reactive sputtering enables the synthesis of crystalline bilayered HfV2-HfV2O7