Search

Time-averaged and time-resolved ion fluxes during reactive HiPIMS deposition of Ti1-xAlxN thin films are thoroughly investigated for the usage of Ti1-xAlx composite targets – Al/(Ti + Al) ratio x = 0.4 and 0.6. Ion mass spectroscopy analysis revealed, that increasing x in the target material or reducing the N2 flow-rate ratio leads to a proportional increase of the Al+-ion count fraction, whereas that of Tin+-ions (n = 1, 2) remains unaffected despite of comparable primary ionisation energies between Al and Ti. In fact, energetic Ti2+-ions account for the lowest flux fraction incident on the substrate surface, allowing for a high Al-solubility limit in cubic-structured Ti1-xAlxN thin films (xmax ~ 0.63) at low residual stresses. In addition, time-resolved plasma analysis highlights the simultaneous arrival of metal- and process-gas-ions throughout the entire HiPIMS pulse duration. These ion-bombardment conditions, which were dominated by gas-ion irradiation with a significant contribution of Al+-ions (up to ~20 %) and negligible energetic Ti2+-ions, allowed for the growth of cubic Ti0.37Al0.63N coatings exhibiting high indentation hardness of up to ~36 GPa at a low compressive stress level (σ = −1.3 GPa).

Fe-alloyed intermetallic Al–Cr coatings as well as substoichiometric oxides (with respect to M 2 O 3 stoichiometry) thereof were synthesised by reactive arc evaporation in order to investigate the impact of Fe and the oxygen flow rate during deposition on the structural evolution and mechanical properties. All coatings prepared in non-reactive conditions crystallise in a γ 2 -type Al 8 Cr 5 structure with Fe substitutionally incorporated in the lattice featuring hardnesses in the range of 11 ± 1 GPa. By introducing oxygen to the process, the growth morphology changes from columnar to a nano-composite-like structure of small Cr-enriched crystallites embedded in an amorphous matrix. Simultaneously, the number of droplets and therewith associated defect structures increases and accordingly the hardness values of these films decrease to 8.5 ± 1 GPa. For oxygen flow rates close to the transition from substoichiometric to stoichiometric sesquioxides (~0.7–0.9 Pa), the crystallinity as well as the fraction of strong oxide bonds increase and highest hardnesses among all coatings investigated are obtained (>20 GPa). Furthermore, small additions of Fe to Al 0.7 Cr 0.3 cathodes effectively improve the structural and mechanical properties of substoichiometric oxide films.

The outstanding oxidation resistance, thermo-mechanical stability, and chemical inertness of alumina, but also the synthesis of phase pure polymorphs attract particular attention in academia and industry. Especially, the difficulties regarding the synthesis of α- or γ-structured Al2O3 by physical vapor deposition techniques are still strong limitations. Within this study, we investigated in detail the influence of 2 at.% tungsten in the Al-target on the process stability and phase formation during reactive DC magnetron sputtering as well as high power impulse magnetron sputtering (HiPIMS) of Al2O3-based coatings. The small addition of W to the Al target allows to increase the oxygen partial pressure by more than 200% while maintaining a stable deposition process. Ion mass spectroscopy measurements yield a promising high fraction of 16O+ and 32O2+, when operating the W-containing target in the metal-to-poisoned transition mode. A significant increase of 16O+ is further provided by the target surface oxide in poisoned mode. Detailed time-of-flight ion mass spectroscopy investigations during one HiPIMS pulse show a clear temporal separation of the individual ions arriving at the substrate plane during the pulse on-time, allowing for controlled ion attraction by synchronizing the bias pulse to the discharge impulse. Equal amounts of 27Al+ and 32O2+ can be attracted using a bias on-time between 400 μs and 900 μs in the “off-time” (after glow) leading to a dense and nano-crystalline coating. Detailed electron microscopy investigations show the presence of metallic phase fractions for higher duty cycles (7.5%). Decreasing the duty cycle to 3.75% leads to amorphous coatings when operating the Al-target at the highest oxygen partial pressure in metallic mode.

In the field of hard protective coatings, nano-crystalline Ti-B-N films are of great importance due to the adjustable microstructure and mechanical properties through their B content. Here, we systematically study this influence of B on Ti-B-N during reactive as well as non-reactive DC magnetron sputtering. The different deposition routes allow for an additional, very effective key parameter to modify bond characteristics and microstructure. Plasma analysis by mass spectroscopy reveals that for comparable amounts of Ti+, Ti2+, Ar+, and Ar2+ ions, the count of N+ ions is about 2 orders of magnitude lower during non-reactive sputtering. But for the latter, the N+/N2+ ratio is close to 1, whereas during reactive sputtering this ratio is only 0.1. This may explain why during reactive deposition of Ti-B-N, the B N bonds dominate (as suggested by X-ray photoelectron spectroscopy), whereas the B B and Ti B bonds dominate for non-reactively prepared Ti-B-N. Chemically, reactively versus non-reactively sputtered Ti-B-N coatings follow the TiN-BN versus TiN-TiB2 tie line, respectively. Detailed X-ray diffraction and transmission electron microscopy studies reveal, that up to 10 at.% B can be dissolved in the fcc-TiN lattice when prepared by non-reactive sputtering, whereas already for a B content of 4 at.% a BN-rich boundary phase forms when reactively sputtered. Thus, we could not only observe a higher hardness (35 GPa instead of 25 GPa) as well as a higher indentation modulus (480 GPa instead of 260 GPa), but also a higher fracture energy (0.016 instead of 0.009 J/m during cube-corner indentations) for Ti-B-N coatings with 10 at.% B, when prepared non-reactively.

Journal of vacuum science & technology / A 37(6), 060802 (1-23) (2019). doi:10.1116/1.5120793, The challenge of synthesizing protective alumina coatings in corundum structure at moderate temperatures inspired the development of novel strategies of both thin film processing and microstructural thin film design. While the growth of pure corundum coatings is still a domain of chemical vapor deposition, new concepts developed in physical vapor deposition addressed the growth of advanced solid solutions, nanocomposites, or multilayers. The system Al-Cr-O has received huge attention as thin films of the type (Al$_{1−x}$Cr$_x$)$_2$O$_3$ could be grown at substrate temperatures even below 500 °C in laboratory model experiments. These coatings offer particular potential for applications that require chemical inertness, thermal stability, mechanical strength, and excellent tribological properties at elevated temperatures. Mastering its low temperature deposition is one of the major aims of industrial research, as it would also enable the protection of temperature-sensitive components and consequently path the way for novel, yet not realizable applications. This work summarizes the state of the art and recent progress in the development and synthesis of such sesquioxide protective coatings prepared by physical vapor deposition. General coherencies are described for (Al$_{1−x}$Cr$_x$)$_2$O$_3$ coatings. Topics covered are important growth parameters (i.e., the oxygen gas flow and the substrate bias), the chemical composition and its impact on alloying concepts applicable to (Al$_{1−x}$Cr$_x$)$_2$O$_3$ coatings, specific architectural coating designs, and relevant properties such as the thermal stability and material response upon annealing in oxidative environments. In addition, a brief outlook into the development of other Cr-based sesquioxide thin films, (Cr,Zr)$_2$O$_3$ and (Cr,V)$_2$O$_3$, is given., Published by Inst., New York, NY

Detailed transmission electron microscopy analyses were performed to complement preceding investigations on Al0.675Cr0.275Fe0.05-based intermetallic coatings and substoichiometric oxides after ambient air annealing between 900 and 1300 °C. Oxidation of the intermetallic coating occurs uniformly via the formation of a metastable θ-Al2O3 scale at the surface and occasionally along the grain and defect boundaries deep into the coating, which then with higher oxidation temperature phase transform into the α structure. Substoichiometric films prepared with 50 sccm O2 per active source (p.a.s., ∼25 at. % O within the coatings; consisting of a nanocompositelike structure) show a complex elemental and structural separation with preferred oxidation of Al-rich regions to α-Al2O3 already at 900 °C. Cr- and Fe-containing domains remain mostly intermetallic up to Tox = 1300 °C, where coarse-grained solid solution α-(Al,Cr,Fe)2O3 phases are formed. Selective oxidation and phase separation are also observed for coatings having initially 40–50 at. % oxygen (prepared with 100 sccm O2 p.a.s.); however, the formation of α-structured (Al,Cr,Fe)2O3 grains is observed already at 1000 °C. The possibility to alter the phase formation sequence and microstructure of Al-Cr and Al-Cr-Fe coatings by adjusting the oxygen flow during synthesis (below the threshold to stoichiometric sesquioxides) was thus corroborated.Detailed transmission electron microscopy analyses were performed to complement preceding investigations on Al0.675Cr0.275Fe0.05-based intermetallic coatings and substoichiometric oxides after ambient air annealing between 900 and 1300 °C. Oxidation of the intermetallic coating occurs uniformly via the formation of a metastable θ-Al2O3 scale at the surface and occasionally along the grain and defect boundaries deep into the coating, which then with higher oxidation temperature phase transform into the α structure. Substoichiometric films prepared with 50 sccm O2 per active source (p.a.s., ∼25 at. % O within the coatings; consisting of a nanocompositelike structure) show a complex elemental and structural separation with preferred oxidation of Al-rich regions to α-Al2O3 already at 900 °C. Cr- and Fe-containing domains remain mostly intermetallic up to Tox = 1300 °C, where coarse-grained solid solution α-(Al,Cr,Fe)2O3 phases are formed. Selective oxidation and phase separation are also observed for coatings...