| 1. |
- Fekete, Matej, et al.
(författare)
-
On the determination of the thermal shock parameter of MAX phases : A combined experimental-computational study
- 2023
-
Ingår i: Journal of the European Ceramic Society. - : Elsevier. - 0955-2219 .- 1873-619X. ; 43:13, s. 5484-5492
-
Tidskriftsartikel (refereegranskat)abstract
- Thermal shock resistance is one of the performance-defining properties for applications where extreme temperature gradients are required. The thermal shock resistance of a material can be described by means of the thermal shock parameter RT. Here, the thermo-mechanical properties required for the calculation of RT are quantum-mechanically predicted, experimentally determined, and compared for Ti3AlC2 and Cr2AlC MAX phases. The coatings are synthesized utilizing direct current magnetron sputtering without additional heating, followed by vacuum annealing. It is shown that the RT of both Ti3AlC2 and Cr2AlC obtained via simulations are in good agreement with the experimentally obtained ones. Comparing the MAX phase coatings, both experiments and simulations indicate superior thermal shock behavior of Ti3AlC2 compared to Cr2AlC, attributed primarily to the larger linear coefficient of thermal expansion of Cr2AlC. The results presented herein underline the potential of ab initio calculations for predicting the thermal shock behavior of ionically-covalently bonded materials.
|
|
| 2. |
- Hans, Marcus, et al.
(författare)
-
Spinodal decomposition of reactively sputtered (V0.64Al0.36)(0.49)N-0.51 thin films
- 2020
-
Ingår i: Surface & Coatings Technology. - : ELSEVIER SCIENCE SA. - 0257-8972 .- 1879-3347. ; 389
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the decomposition mechanisms of metastable cubic (c-)(V0.64Al0.36)(0.49)N-0.51 thin films, grown by reactive high power pulsed magnetron sputtering, by combination of structural and compositional characterization at the nanometer scale with density functional theory (DFT) calculations. Based on thermodynamic considerations of partial derivative(2)Delta G/partial derivative x(2) < 0, spinodal decomposition is expected for c-V1-xAlxN with x >= 0.35. While no indications for spinodal decomposition are observable from laboratory and synchroton diffraction data after annealing in Ar atmosphere at 1300 degrees C, the formation of wurtzite (w-)AlN is evident after annealing at 900 degrees C by utilizing high energy synchrotron X-ray diffraction. However, the complementary nature of elemental V and Al maps, obtained by energy dispersive X-ray spectroscopy in scanning transmission electron microscopy mode, imply spinodal decomposition of c-(V0.64Al0.36)(0.49)N-0.51 into V- and Al-rich cubic nitride phases after annealing at 900 degrees C. These chemical modulations are quantified by atom probe tomography and maximum variations of x in V1-xAlxN are in the range of 0.36 to 0.50. The magnitude of the compositional modulations is enhanced after annealing at 1100 degrees C as x varies on average between 0.30 and 0.61, while the modulation wavelength remains unchanged at approximately 8 nm. Based on DFT data, the local x variation from 0.30 to 0.61 would cause lattice parameter variations from 4.111 to 4.099 angstrom. This difference corresponds to a shift of the (200) peak from 44.0 to 44.1 degrees. As the maximum decomposition-induced peak separation magnitude of 0.1 degrees is significantly smaller than the measured full width at half maximum of 0.4 degrees, spinodal decomposition cannot be unravelled by diffraction data. However, consistent with DFT predictions, spinodal decomposition in c-(V0.64Al0.36)(0.49)N-0.51 is revealed by chemical composition characterization at the nanometer scale.
|
|
| 3. |
- Holzapfel, Damian M., et al.
(författare)
-
Influence of ion irradiation-induced defects on phase formation and thermal stability of Ti0.27Al0.21N0.52 coatings
- 2022
-
Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 237
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of changes induced by ion irradiation on structure and thermal stability of metastable cubic (Ti,Al)N coatings deposited by cathodic arc evaporation is systematically investigated by correlating experiments and theory. Decreasing the nitrogen deposition pressure from 5.0 to 0.5 Pa results in an ion flux-enhancement by a factor of three and an increase of the average ion energy from 15 to 30 eV, causing the stress-free lattice parameter to expand from 4.170 to 4.206 Å, while the chemical composition of Ti0.27Al0.21N0.52 remains unchanged. The 0.9% lattice parameter increase is a consequence of formation of Frenkel pairs induced by ion bombardment, as revealed by density functional theory (DFT) simulations. The influence of the presence of Frenkel pairs on the thermal stability of metastable Ti0.27Al0.21N0.52 is investigated by scanning transmission electron microscopy, differential scanning calorimetry, atom probe tomography and in-situ synchrotron X-ray powder diffraction. It is demonstrated that the ion flux and ion energy induced formation of Frenkel pairs increases the thermal stability as the Al diffusion enabled crystallization of the wurtzite solid solution is retarded. This can be rationalized by DFT predictions since the presence of Frenkel pairs increases the activation energy for Al diffusion by up to 142%. Hence, the thermal stability enhancement is caused by a hitherto unreported mechanism - the Frenkel pair impeded Al mobility and thereby retarded formation of wurtzite solid solution.
|
|
| 4. |
- Kashani, Amir Hossein Navidi, et al.
(författare)
-
Synthesis and oxidation behavior of Ti0.35Al0.65By (y=1.7-2.4) coatings
- 2022
-
Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 442
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of B concentration on phase formation and oxidation resistance of (Ti0.35Al0.65)By coatings with y = 1.7, 2.0, 2.4 was investigated. Elemental B targets in radio frequency mode and a compound Ti0.4Al0.6 target in direct current mode were sputtered. The B concentration was varied systematically by adjusting the applied power to the respective magnetrons, while keeping the power supplied to the magnetron with the Ti0.4Al0.6 target constant. Measured lattice parameters and elastic properties are consistent with ab initio predictions. The oxidation resistance at 700 degrees C in air for up to 8 h was compared to a cathodic arc evaporated (Ti0.37Al0.63)0.49N0.51 coating with an Al/Ti ratio of 1.69 +/- 0.20 which is very similar to 1.84 +/- 0.40 for the boride coatings. Scanning transmission electron microscopy imaging revealed oxide scale thicknesses of 39 +/- 7 and 101 +/- 25 nm for (Ti0.35Al0.65)B2.0 and (Ti0.37Al0.63)0.49N0.51 after 8 h, respectively. Hence, the close to stoichiometric diboride outperforms the nitride coating. This behavior can be understood based on composition and structure analysis of the oxide scales: While the protective layer on the diboride is primarily composed of Al and O, the porous oxide layer on the nitride coating contains Ti, Al and O.
|
|
| 5. |
- Ondracka, Pavel, et al.
(författare)
-
Ab initio-guided X-ray photoelectron spectroscopy quantification of Ti vacancies in Ti1-dOxN1-x thin films
- 2022
-
Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 230
-
Tidskriftsartikel (refereegranskat)abstract
- Ab initio calculations were employed to investigate the effect of oxygen concentration dependent Ti vacancies formation on the core electron binding energy shifts in cubic titanium oxynitride (Ti1-delta OxN1-x). It was shown, that the presence of a Ti vacancy reduces the 1s core electron binding energy of the first N neighbors by ~& nbsp;0.6 eV and that this effect is additive with respect to the number of vacancies. Hence it is predicted that the Ti vacancy concentration can be revealed from the intensity of the shifted components in the N 1s core spectra region. This notion was critically appraised by fitting the N 1s region obtained via X-ray photoelectron spectroscopy (XPS) measurements of Ti1-delta OxN1-x thin films deposited by high power pulsed magnetron sputtering. A model to quantify the Ti vacancy concentration based on the intensity ratio between the N 1s signal components, corresponding to N atoms with locally different Ti vacancy concentration, was developed. Herein a random vacancy distribution was assumed and the influence of surface oxidation from atmospheric exposure after deposition was considered. The so estimated vacancy concentrations are consistent with a model calculating the vacancy concentration based on the O concentrations determined by elastic recoil detection analysis and text book oxidation states and hence electroneutrality. Thus, we have unequivocally established that the formation and population of Ti vacancies in cubic Ti1-delta OxN1-x thin films can be quantified by XPS measurements from N 1s core electron binding energy shifts. (C)& nbsp;2022 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc.& nbsp;
|
|
| 6. |
- Thörnberg, Jimmy, et al.
(författare)
-
Oxidation resistance and mechanical properties of sputter-deposited Ti0.9Al0.1B2-y thin films
- 2022
-
Ingår i: Surface & Coatings Technology. - : Elsevier Science SA. - 0257-8972 .- 1879-3347. ; 442
-
Tidskriftsartikel (refereegranskat)abstract
- Direct-current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) were used to deposit understoichiometric Ti(1-x)Al(x)B(2-y )diboride coatings by sputtering from a segmented TiB2-AlB2 target using Ar and Kr as sputtering gas. For films with a fixed Al/(Ti + Al) ratio of x = 0.1 (Ti0.9Al0.1B2-y), the B content was varied with y & ISIN; (0.1, 0.6 and 0.7). For films with a fixed y = 0.7 (Ti1-xAlxB1.3), the Al content was varied with x & ISIN; (0.1, 0.4 and 0.7). Evaluation of the mechanical properties of the Ti1-xAlxB1.3 samples showed a reduction in both hardness and elastic modulus with increasing Al concentration, while the Ti0.9Al0.1B2-y samples showed a hardness increase with decreasing B content. Thus, Ti0.9Al0.1B1.3 films exhibited a superior hardness of 46.2 +/- 1.1 GPa and an elastic modulus of 523 & PLUSMN; 7 GPa, compared to the values for Ti0.9Al0.1B1.4 and Ti0.9Al0.1B1.9, showing a hardness of 44 +/- 1 GPa and 36 +/- 1 GPa, and an elastic modulus of 569 +/- 7 GPa and 493 +/- 6 GPa, respectively. The oxidation behavior of the mechanically most promising Ti0.9Al0.1B2-y sample series was investigated through air-annealing at 600 C for durations from 1 h to 10 h. All films formed a mixed non-conformal Al2O3-TiO2 oxide scale which acts as an inward and outward diffusion barrier, significantly reducing the oxidation rate compared to TiBz films, which form an oxide scale consisting of porous TiO2. The thinnest oxide scale after 10 h was found in the B-deficient samples, Ti0.9Al0.1B1.3 and Ti0.9Al0.1B1.4, at ~200 nm, which is significantly below that for Ti0.9Al0.1B1.9 at 320 nm. The enhanced oxidation resistance of highly understoichiometric films is due to the elimination of the B-rich tissue phase that is present at the grain boundaries for higher B content, where the latter has been shown to enhance the rate of oxidation in borides.
|
|
