ICMCTF2012 Session B5-2: Hard and Multifunctional Nano-Structured Coatings

High hardness combined with a low friction coefficient against a counterpart and high temperature stability (up to several hundreds of degree Celsius) are properties which are of main importance for hard protective thin films. A disadvantage of a lot of developed coating materials, like e.g. diamond-like carbon coatings, is that they fulfill only one or two of the three coating properties.

Crystalline and nanocrystalline transition metal nitrides have attracted a lot of interest over the years, and they are well known to have a wide range of outstanding properties that makes them suitable in many different applications. In comparison with crystalline transition metal nitrides, and also compared to corresponding carbides and oxides, very little has been reported on amorphous transition metal nitrides and most of their properties are still unknown. Nevertheless they are potentially attractive, e.g., as wear resistant coatings, due to their homogeneous structure. We propose amorphous multicomponent transition metal nitrides as a new class of refractory materials.

Using kinetically limited growth techniques - including low growth temperatures and high deposition rates - is one of the keys to growing amorphous transition metal nitride films. In this study, we investigate the HfN-AlN-Si₃N₄ system, where the difference in atomic size and bond coordination between the constituent elements, in combination with kinetically limited growth, promotes the formation of an amorphous structure. Hf_1-x-yAl_xSi_yN thin films were grown on Si(001) substrates by reactive magnetron sputtering from a single Hf_0.6Al_0.2Si_0.2 target. We show that we can control the film morphology, from nanocrystalline to amorphous, by varying the ion flux and growth temperature, and also the film composition by varying the ion energy.

Compositional analysis of the as-deposited films was performed by energy dispersive x-ray spectroscopy (EDS), and the structural information was gained by x-ray diffraction and analytical transmission electron microscopy. We will report on hardness and elastic properties of the films, as well as temperature dependent resistivity.

Coatings in the Al-Ge-N system have been synthesized using reactive DC magnetron sputtering and characterized with the goal to explore the structure and properties, as well as their potential use as hard optical coatings. The composition was varied from pure AlN to pure GeN_y. Also experiments varying substrate temperature and sample bias were conducted. Coatings were analyzed using X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and scanning and transmission electron microscopy. Besides the binary reference samples, ternary samples with Ge-contents from 5 to 30 at.% were synthesized, and found to be nanocomposites of a nanocrystalline, Ge-containing AlN-phase, nc-(Al_1-xGe_x)N, and an amorphous, N-deficient GeN_y-phase. The grain size of the (Al_1-xGe_x)N phase decreased with increasing Ge-content from about 30 to 15nm. Additionally the (Al_1-xGe_x)N -phase was found to exhibit two different textures depending on the Ge-content: at low Ge content a (001) preferred orientation was observed, while at increased Ge-content the (110) orientation became dominant. The GeN_y phase was found to be highly susceptible to sputter damage during sputter-cleaning prior to XPS analysis.

A nanocomposite-hardening, similar to what has been observed for e.g. the AlN/Si₃N₄ system, was observed: the hardness increased from 19 GPa (for the AlN reference sample) to 25 GPa for the hardest ternary sample. Samples were found to have a high transparency in the visible region, and the index of refraction shows a slight dependence of Ge-content, increasing from about 2.0 to 2.2 as Ge-content increases from 0 to 30 at.%. Thus these coatings have a potential use as protective coatings for optical components operating in the visual or near IR range.

Thin film metallic glasses (TFMGs) possess many excellent and unique properties, including high strength, ductility, annealing-induced amorphization and good adhesion between the substrate and film. Like ordinary metallic glasses, the TFMGs in general undergo the glass transition and crystallization at temperatures of T_g and T_x, respectively, upon annealing. Another exceptional property of TFMGs is the shape-recovery ability during annealing at temperatures between T_g and T_x (or so-called the supercooled liquid region, ΔT) due mainly to the low viscosity flow. Surface defects such as indentation marks on the scales up to a few micrometers tend to shrink in size or even disappear without crystallization in ΔT.

In this study, 200 nm-thick TFMGs with different compositions are deposited on Si substrates. Nanoindentation tests are performed to evaluate the film properties. Surface morphology and microstructure before and after annealing within ΔT are examined and reveal that the size of indentation mark decreases after annealing. As a result, the shape-recovery behavior is confirmed and detailed characterization results of various TFMGs as a function of annealing time will be discussed.

Keyword: thin film metallic glass, annealing, shape recovery

A new group of thin film metallic glasses (TFMGs) have been reported to exhibit properties different from conventional crystalline metal films, though their bulk forms are already well-known for the high strength and toughness, large elastic limits, excellent corrosion and wear resistances because of the amorphous structure. In recent decades, bulk metallic glasses (BMGs) have gained a great deal of interest due to the substantial improvements in specimen sizes. On the other hand, much less attention has been devoted to the TFMGs, despite the fact that they have many properties and characteristics which are not readily achievable with other types of metallic or oxide films. Furthermore, these TFMGs have been progressively used for engineering applications and thus deserve to be recognized in the field of thin film coatings.

In addition, while the BMGs are still difficult to use because of their brittle macroscopic nature and difficulty of processing, TFMGs are a possible solution to make use of their great properties of high strength, large plasticity, and excellent wear resistance. In this presentation, many advantages and properties of TFMGs are discussed. These are such as mechanical properties, tribological properties, annealing-induced amorphization and resulting smooth surface, some of which lead to useful applications, for example, for substrate fatigue property enhancements. In addition, potential applications in microelectronics and optoelectronics are mentioned. Ironically, there have been enormous research efforts dedicated to developing metallic glasses with large critical sizes only to reveal that the best use for these materials may, in fact, be in thin film applications. It is thus hoped that this talk serves the purpose of calling attention to the importance of TFMGs such that many more studies and applications may be explored.

The aim of the present work was to obtain dense thin films that present simultaneously good corrosion resistance and hardness. For this, we propose the deposition of nanocomposite thin films where tantalum nitride (TaN) nanocrystals were embedded in an amorphous silicon nitride (SiN_x) phase. The deposition was done using two magnetrons; one of pure Si and the other of pure Ta and the amount of Si into the samples was varied by changing the RF power (20 - 340 W) applied to the Si target. The N₂/Ar ratio was fixed at the value optimized for the stoichiometric deposition of tantalum nitride films (6/14). The TaN-SiN_x samples were deposited on silicon and their composition, structure and hardness were evaluated by X-ray photoelectron spectroscopy, X.ray diffraction and nanoindentation, respectively. The results indicated that the silicon content increased linearly from 1 to 12 at% as the RF power was increased. Meanwhile, the hardness showed a maximum around 5.4 at% of silicon attaining 40 GPa, representing a 20 % hardness enhancement in comparison to the fcc phase of the TaN film. From the structural analysis, it was observed that the TaN d -fcc phase was stabilized due to the addition of Si. However, for the largest Si at%, a quasi-amorphous phase and softer film (23 GPa) was obtained.

The TaN-SiNx samples were deposited on AISI 304L stainless steel and the corrosion resistance was evaluated using DC and AC electrochemical techniques. The DC techniques include potentyodynamic polarization and polarization resistance, data that were analyzed using the Tafel method. The electrochemical impedance spectroscopy was used to evaluate the properties of the different interfaces present using electronic equivalent circuits to model the variations of the coatings parameters as a function of the silicon content. The results showed that the polarization resistance (R_p) of the TaN-SiN_x film that presents the highest hardness value was double than the R_p of both the AISI304L and the TaN film. Similarly, the corrosion current density (I_corr), which is proportional to the corrosion rate of the material, was approximately one order of magnitude lower than the substrate and the TaN films.

Acknowledgements: We wish to acknowledge the financial support from DGAPA-UNAM IN103910. G. Ramírez acknowledges CONACYT for his PhD scholarship.

We adopted a combinatorial approach to develop a series of Ti-Cr oxide films with a full composition spread. The films were deposited on silicon and quartz substrates by co-sputtering a metallic titanium target and a chromium target simultaneously onto a stationary long-strip of substrate in a gas mixture of argon and oxygen at 250 °C and without substrate bias. The location of substrates in relation to targets is found to be very crucial for the result. At the two ends of the substrate strip corresponding to Ti and Cr metal targets, crystalline rutile-structured TiO₂ and corundum-structured Cr₂O₃ are formed, respectively, while in the middle region, the films are amorphous. No film of mixed phases is observed. The nanoindentation hardness values of the rutile-structured TiO₂ and the corundum-structured Cr₂O₃ films are 18 and 23 Gpa, respectively, while those of the amorphous films vary with the lowest down to 13 Gpa. As the dopant content varies from the both ends, the film structure and crystallinity transform from crystalline to amorphous-liked phase and its morphologies and microstructures change from rough surface of columnar grains to smooth surface of nanocomposite. Composition mapping of the rutile-structured TiO₂ and the corundum-structured Cr₂O₃ films shows that the dopant element is present along grain boundaries, which indicate the two oxides have low solubility for each other or are even immiscible.

Keywords：Combinatorial approach, chromia, titania, mixed oxide, nanoindentation.