ICMCTF2015 Session B5-1: Hard and Multifunctional Nano-Structured Coatings

Increasing Al content of Ti_1-xAl_XN coatings with single phase cubic (c) structure is beneficial to their mechanical properties and oxidation resistance. Here, multilayer architecture combining Ti_0.52Al_0.34N and Al_0.66Ti_0.34N is used to increasing the solubility limitation of c-Ti_1-xAl_XN by controlling their coherent growth in order to optimize the properties of Ti-Al-N coating. Consequently, we use differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), cross-sectional scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), and nanoindentation measurements to investigate the oxidation resistance, thermal stability, structure evolution, and mechanical properties of Ti-Al-N monolithic and multilayer coatings. The c-Ti_0.52Al_0.34N and c/w-Al_0.66Ti_0.34N monolithic coatings have hardness values of ~27.7 and 30.3 GPa, respectively. The preparation of TiAlN/AlTiN multilayer coatings behave a structural change from cubic-wurtzite with bilayer period of 20 nm to single phase cubic with bilayer period below 14 nm, and exhibit a high hardness value of ~34.0 GPa with bilayer period of 8 nm. The decomposition process of multilayer coatings during thermal annealing is promoted by the interface-directed spinodal decomposition, and shows an earlier spinodal decomposition and w-AlN formation. Multilayer architecture has a beneficial effect on the oxidation resistance. And the structural change of TiAlN/AlTiN multilayer coatings from mixed cubic-wurtzite to single phase cubic results in a further improvement of their oxidation resistance. After 10 h oxidation at 850 ^oC, Ti_0.52Al_0.34N and c/w-TiAlN/AlTiN coatings are already completely oxidized, whereas c-TiAlN/AlTiN coatings still exhibit intact nitride layers. Additionally, the high Al content of multilayer coatings also assists the decomposition process and preferable oxidation resistance.

The financial support from National Natural Science Foundation of China (Grant nos. 51371201 and 51371199) and the Doctoral Scientific Fund Project of the State Education Committee of China (Grant No. 20120162110051) is highly acknowledged.

Wear resistant coatings based on the ternary Al-Ti-N (Ti-Al-N) system have been industrial introduced in large scale during the last two decades.

The most Al-rich coating of the chemical composition of two third Al and one third Ti of the total metallic content in at%, mostly termed Al_0,66Ti_0.34 N or Al_0,67Ti_0.33 N, will be discussed in more detail. This system combines high oxidation resistance (formation of aluminum oxide on the surface) and high hot hardness (hardening by spinodal decomposition).

Due to the metastable nature of Ti-Al-N coatings, the phase content and microstructure of the coatings can be very sensitive to changes in process parameters, especially in that high Al content regime. Different microstructural morphologies ranging from fine grained, almost glass-like, to coarse grained columnar can be deposited. The tailoring of coating properties to fit the requirements of different applications is still a hot topic in research and industrial applications. The influence of various alloying elements at the coating properties including the phase formation are shown. Multilayer concepts by depositing a second coating material to generate alternating multilayers are discussed too.

The application of the coatings cover a wide industrial field including cutting inserts, shaft tools, forming tools, functional decorative applications and show also a potential for wear parts. The wear characteristics of the coating in milling operations with inserts are discussed.

Substoichiometric solid solution cubic (Ti_0.5Al_0.5)N_y coatings with y=0.75, 0.87 and 0.92 were grown on WC:Co using an industrial scale reactive cathodic arc evaporation system in a mixed Ar/N₂ plasma and a total gas pressure of 2 Pa. The nitrogen content, y, and hence the amount nitrogen vacancies (V_N) in the as-deposited coatings was controlled by tuning the Ar and N₂ gas flow ratio during growth. The influence of V_N in (Ti_0.5Al_0.5)N_y on the thermal stability and hardness was studied by a combination of X-ray diffractometry, differential scanning calorimetry, scanning- and transmission electron microscopy, atom probe tomography and nanoindentation. Results show grain refinement and a structural transition from B1 towards a metallic fcc structure with a decreasing in nitrogen content of the as-deposited coatings. Regardless of the amount V_N, the coatings undergoes an isostructural phase decomposition into cubic c-TiN and c-AlN rich domains at elevated temperatures, consistent with the spinodal decomposition process taking place in stoichiometric (Ti,Al)N coatings [1]. For (Ti_0.5Al_0.5)N_y however, this up-hill diffusion process of the metallic species clearly slows down with increasing V_N. In fact, we observe a higher onset temperature for decomposition of the coatings grown with y=0.75 and 0.87 at about 1100 °C and 1000 °C, respectively, whereas an onset temperature of about 800 °C was observed for the reference coating (y=0.92). Accompanied with the onset, V_N affects the hardness through an age hardening at higher temperatures. The coatings with y=0.87 and 0.75 show a hardness of about 29 GPa up to a temperature between 1100 °C and 1200 °C after which a prominent age hardening occurs, while the reference sample (y=0.92) exhibit the characteristic drop in hardness at these temperatures due to the formation of wurtzite AlN [1]. Our study suggests that nitrogen vacancies reduce the driving force for spinodal decomposition, which is in accord with ab initio calculations predicting a reduction of the mixing enthalpy by increasing V_N in the Ti-Al-N system [2].

[1] A. Hörling et al., J. Vac. Sci. Technol. A20 (2002) 1815

[2] B. Alling et al., Applied Physics Letters92 (2008) 071903

The paper reports on the structure and mechanical properties of magnetron sputtered (Ti, Al, V) N_x films and their resistance to cracking in bending. The films were reactively sputtered from a Ti6Al4V alloy target (90 at.% Ti, 6 at.% Al, 4 at.% V) in an Ar+N₂ sputtering gas mixture on the steel, Si (100) and Mo substrates negatively biased with a DC power supply. The structure, mechanical properties (the hardness H, the effective Young’s modulus E^*, the elastic recovery W_e) of (Ti, Al, V) N_x films and their resistance to cracking in bending were characterized by (i) the X-ray diffraction (XRD), (ii) the diamond indentation test and (iii) the bending of a metallic strip coated by the tested film around a fixed cylinder of small radius r, respectively.

Detailed investigation of physical and mechanical properties of (Ti, Al, V) N_x nitride films showed that: (1) Highly elastic coatings with W_e ≥ 60% do not have to exhibit a high ratio H/E^* ≥ 0.1 only. It means that a high value of W_e ≥ 60% cannot be used as a simple indicator of the enhanced resistance of the film to cracking in bending. (2) The increase of the H/E^* ratio from low values < 0.1 to high values of about of 0.1 and greater requires to deliver a higher energy ε to the growing film by increasing (i) the substrate heating (T_s) and/or (ii) the ion bombardment. (3) The increase of the energy ε delivered to the growing film strongly influences not only its mechanical properties but also its structure. (4) The energy ε_i delivered to the growing film by bombarding ions, expressed in W/cm², is not the best parameter which controls its H/E^* ratio. Obtained results indicate that it is better to characterize the energy ε_i by separate values of the substrate bias U_s and the substrate ion current density i_s. To reach a high ratio H/E^* ≥ 0.1 the value of i_s has to exceed a critical minimum value.

An overview of recent activities with respect to AlN-based ternary and quaternary coatings will be given. Alloying has been explored in two directions: By adding a group 14 element, forming solid solution phases and / or provoking phase segregation to form nanocomposites, and also by adding oxygen to form oxynitride materials. Focus of the presentation will be on coating structure / microstructure, the material properties, and the correlation thereof.

Coatings were deposited by reactive magnetron sputtering, using elemental targets and varied ratios of the three process gases Ar, N₂ and O₂. Both traditional depositions with one composition per experiment, as well as a combinatorial approach with compositional spread depositions have been used. Deposition was generally carried out at low temperatures to allow formation of metastable solid solution phases. Coatings have been characterised using mainly X-ray diffraction (XRD), photoelectron spectroscopy (XPS) and scanning electron microscopy. Optical and mechanical properties have been evaluated using nanoindentation and UV-vis spectroscopy.

In this study, the effect of the cathode current on the formation and the stability of metastable AlN phases in nanoscaled TiN/(Ti,Al)N/AlN multilayers was investigated. The samples were deposited in a cathodic arc evaporation process from two separated cathodes containing titanium and aluminum. The multilayer structure was produced via sample rotation. The thickness of the periodic motif was approximately 9 to 11 nm. The thickness of the Ti-rich and Al-rich regions varied consequently with the cathode current. The glancing-angle X-ray diffraction (GAXRD) experiments, which were performed at the European Synchrotron Radiation Facility, revealed that the as-deposited multilayers consisted mainly of fcc-(Ti,Al)N. The composition gradients across the TiN/(Ti,Al)N/AlN stack were detected by GAXRD as variations of the lattice spacing and were confirmed by scanning transmission electron microscopy with high resolution (HRSTEM) combining electron dispersive X-ray spectroscopy (EDS) and electron energy-loss spectroscopy (EELS). Additionally, GAXRD identified a secondary cubic phase with the lattice parameter of about 0.4367 nm, which resembles the lattice parameter of the zinc blend modification of AlN reported first by Petrov in 1992 [1]. The secondary phase became especially pronounced at higher currents applied on the Ti cathode. The in situ high-temperature GAXRD experiments revealed a phase transition of the secondary cubic phase to the high pressure rock salt modification of AlN at the annealing temperature of 750°C. Further increase of the annealing temperature up to 950°C activated the spinodal decomposition of fcc-(Ti,Al)N, as it was concluded from the increase of the stress-free lattice parameter of the Ti-rich fcc-(Ti,Al)N. Aluminum nitride persisted in the fcc form, the formation of stable wurtzitic AlN was not observed.

[1] Petrov, I., et al. "Synthesis of metastable epitaxial zinc-blend-structure AlN by solid-state reaction." Applied Physics Letters 60(20). 1992. 2491-2493.

We report a study on mechanical properties of nanoscale multi-layered ZnO/Al2O3 coatings prepared by atomic layer deposition (ALD) method at temperature 120 °C. The ALD process was run in TFS200 (Beneq Oy, Finland) using precursors diethylzinc (DEZ), trimethylaluminium (TMA) and water at pressure 0.9 mbar. The coatings were deposited on Si-wafers (100) and glass substrates to investigate the influence of substrate on mechanical properties of films. We used nano-indentation method to measure hardness and elastic modulus of the coatings of various bi-layer period and various ratio between ZnO and Al2O3 thickness (e.g. 1:3, 1:4, etc.). The crystallinity of the coatings were investigated by X-ray diffractometer and the chemical composition and bonding of elements by X-ray photoelectron spectroscopy. Morphology of the coating as well as the morphology of the individual sub-layers was measured by atomic force microscope. Optical properties were investigated by uv-vis-nir spectrophotometer. We found that deposition of ZnO and Al2O3 into multi-layered super-lattice led to increase of hardness and overall mechanical performance mainly due to finer structure of ZnO crystals in the coating.