ICMCTF2018 Session B6: Coating Design and Architectures

The demand for discovering new materials is, scientifically as well as industrially, a continuously present topic covering all fields of application. The recent scientific work on thin film materials (especially nitride based protective coatings) has shown, that computationally-driven understanding and modelling serves as a reliable trend-giver and can be used for target-oriented experiments.

Boride-, or diboride thin films in particular, show promising results when considering their use as hard protective coatings. Their drawback in ductility for binary systems calls for improvement via further alloying elements. Instead of trial and error, when designing novel ternary coating systems, we use a semi-automated High Throughput Search (HTS) calculating the basic properties (e.g., lattice parameters, bulk moduli, elastic constants, etc.) of all different TMB₂s´ in two stable/metastable hexagonal structures (space group 191 with prototype AlB₂ vs space group 194 with prototype W₂B_5-x) via density functional theory (DFT).

After analyzing the data on behalf of different criteria such as matching lattice constants, high bulk moduli, brittle/ductile transition and experimental feasibility, promising ternary boride thin films were synthesized via physical vapor deposition (DC magnetron sputtering). Analyzing their structure and mechanical properties reveal the potential of this material combination, by reaching superhardness-level and allowing for phase transformation induced toughening effects.

For next-generation integrated circuits, an ultrathin (< 2 nm) robust diffusion barrier layer between Cu wires and dielectric layers is demanded to inhibit the rapid diffusion of Cu atoms into Si devices. Due to the limitation of current thin-film deposition techniques, a “self-forming” or “barrierless” technique has alternatively been developed to generate an ultrathin barrier layer at Cu/dielectric interfaces, by annealing Cu alloy films at relatively low temperatures (~ 400°C) and segregating the solute elements of the Cu alloy films to the Cu/Si interfaces. In addition, multicomponent high-entropy alloys with severe lattice distortions and high resistance to Cu/Si interdiffusion have been suggested to be very promising diffusion barrier materials. Therefore in this study, Cu(V) to Cu(V,Nb,Mo,Ta,W) alloy films were prepared, and the self-forming technique was applied to fabricate ultrathin (< 2 nm) unitary V to Quinary V-Nb-Mo-Ta-W barrier layers. The segregation behaviors of the solute elements (V to V-Nb-Mo-Ta-W), thermodynamically driven by enthalpy and entropy-dominated free energy, were also investigated. High-resolution TEM observations and elemental mappings indicated that, owing to positive mixing enthalpies, the solute elements were separated from Cu during annealing (250-400°C). For the Cu(V) alloy film, the solute atoms segregated to the Cu/Si interface and formed a < 2 nm V layer. For the Cu(V,Nb), Cu(V,Nb,Mo) and Cu(V,Nb,Mo,Ta) alloy films, the solutes segregated to the grain boundaries of the Cu alloy films and formed intermetallic compound precipitates. For the Cu(V,Nb,Mo,Ta,W) alloy film, attributable to the high mixing entropy of the five solute elements, the solute atoms segregated to the interface and formed a < 2 nm V-Nb-Mo-Ta-W solid-solution alloy layer.

Improved toughness is one of the central goals in the development of wear-resistant coatings. Extensive theoretical and experimental work has revealed that single-crystal NaCl-structure VMoN ceramics possess inherently enhanced ductility, as well as high hardness (≈20 GPa) [Kindlund et al. APL Mat 2013]. These surprising findings demonstrate that VMoN-based materials are very promising candidates for replacing other ceramics in hard, refractory protective-coating applications. However, during applications, hard coatings inevitably oxidize which can compromise material properties. Herein, we use density functional theory to evaluate the mechanical properties, as well as the thermodynamical stability, of V_0.5Mo_0.5N_1‑xO_x, with x approximately equal to 0.05, 0.1, and 0.5. We study cubic V_0.5Mo_0.5N_1-xO_x solid solutions characterized by both high and low short-range cation/anion ordering. V_0.5Mo_0.5N_1-xO_x is predicted to be thermodynamically stable for x < 0.1, although higher oxygen ratios can possibly be achieved with non-equilibrium growth techniques such as physical vapor deposition. Our results show that oxygen concentrations x = 5% and 10% have little effect on the mechanical properties of random V_0.5Mo_0.5N_1-xO_x alloys, which retain both hardness and ductility. At x = 50%, bulk, elastic, and shear moduli, as well as Cauchy pressure, are reduced by ~25%, but the material is still predicted to remain ductile. For ordered V_0.5Mo_0.5N_1-xO_x, x = 6% already results in a drastic change in mechanical properties, likely due to disruption of the cubic symmetry. A further increase in the oxygen content yields significant reductions in Cauchy pressures, indicating reduced ductility. However, the Cauchy pressure remains positive for all oxygen concentrations, suggesting that none of the investigated alloys are brittle according to the Pugh and Pettifor criteria.

The Al-Si-O-N material system allows for the creation of various hard, transparent thin films by reactive unbalanced closed field direct current magnetron sputtering (R-UCFDCMS). Combinations of the possible binary materials AlN, Al₂O₃, Si₃N₄ and SiO₂ lead on to ternary materials such as Al-O-N and Al-Si-N, and combining the latter two yields quaternary Al-Si-O-N. As long as the films are fully nitrided and / or oxidized, they are transparent and thus interesting for optical applications.

In the work reported here, polycrystalline AlN was chosen as starting point. By gradually adding O and / or Si, the material system first transformed into a solid solution with the dopant dispersed in the crystallites. Upon transgressing a distinct solubility limit at 6-8 at% for the foreign atoms, a nanocomposite evolved with an amorphous matrix surrounding shrinked crystallites. Yet higher O and / or Si amounts lead to an entirely amorphous network.

Along with these microstructural changes, physical parameters such as hardness, elastic modulus and refractive index vary. This renders it possible to tune thin films to desired properties by simply choosing according deposition conditions. Applying R-UCFDCMS as a deposition technique far off thermodynamic equilibrium for this purpose allows for the generation of metastable, yet highly inert phases.

In this paper, mechanical and antistatic properties of magnetron sputtered, filtered arc ion plating (FAIP) and a hybrid PVD process (magnetron sputtered+FAIP) prepared TiO₂ based nanocomposite thin film were investigated. Both of the antistatic properties of different thin films were analysis by measuring resistance values and decay times. The results showed that for magnetron sputtered and hybrid process prepared TiO₂ based nanocomposite thin film showed resistance around 10⁶~10⁸ W/sq and a short decay time. But for the FAIP process, the resistance uniformity is at about 80 %. In terms of mechanical properties, the FAIP and hybrid process nanocomposite thin film showed adhesion and hardness at around 15 N , 14 N and 15 GPa, 12GPa. For the nanocomposite thin film prepared by magnetron sputtered the adhesion and hardness are only about 9 N and 10 GPa. Thus for a high quality of antistatic tin film the hybrid PVD process was recommend due to the excellent mechanical and antistatic properties.