ICMCTF2015 Session B5-2: Hard and Multifunctional Nano-Structured Coatings

Following to Bückle¹ it is commonly assumed that hardness of hard coatings on softer substrate is correctly measured when the indentation depth does not exceed 10 % of the thickness of the coating. However, the applied load and resulting indentation depth must be sufficiently large to assure that the system is operating in the regime of fully developed plasticity where the measured hardness is load-invariant². We use the non-linear Finite Element Modeling³ (FEM) to study the deformation of the softer substrate during indentation into superhard coatings to show that the Bückle’s rule does not apply in this case. Depending on the hardness of the coating and of the substrate (and also on their elastic moduli), the ratio of the thickness of the coatings to the indentation depth where the deformation of the substrate does not falsify the correct hardness, increases with increasing hardness of the coatings. In many cases, plastic deformation of the substrate occurs already for indentation depth of only few % of the coating thickness. Thus, to correctly measure the hardness of e.g. 50-60 GPa on steel substrate, the thickness of the coatings must be larger than 7-8 µm. We shall present data which allow one to estimate the minimum thickness needed for correct measurement of the hardness of superhard coatings of different hardnesses on substrates made of steel, silicon and cemented carbide.

In the second part we shall present FEM modeling of the indentations into ultrahard coatings vs. maximum applied load up to 500 mN to show an excellent agreement of the previously reported combined hardnesses of the coatings and the substrate with the present FEM data. It will be further shown that already Li Shizhi et al.⁴ prepared ultrahard coatings with hardness around 100 GPa.

¹ H. Bückle, im: J.H. Westbrook, H. Conrad, Eds., The Science of Hardness Testing and its Research Applications, Amer. Soc. For Metals, Metals Park, Ohio 1973, p. 453.

² S. Veprek, invited review, J. Vac. Sci. Technol. A 31(2013)050822-1-33.

³ M.G.J. Veprek-Heijman et al., Surf. Coat. Technol. 203(2009)3385.

⁴ Li Shizhi et al., Plasma Chem. Plasma Process. 12(1992)287.

The load-displacement behavior of three different geometries of Al/a-Si nanostructures is studied using instrumented nanoindentation to investigate the role that Al confinement plays on the deformation behavior of these structures. These geometries are hemispherical Al nanodots with a diameter of 100 nm, 200-nm-long horizontally-aligned Al nanorods, and a 100-nm-thick Al thin film fabricated on Si substrates and then conformally coated with either 100 or 300 nm of PECVD a-Si, representing 3-, 2-, and 1-dimensional Al confinement, respectively. For the hemispherical nanostructures, under load-controlled indentations up to 300 μN, displacement excursions at constant load (i.e., pop-ins and pop-outs) are evident in the load-displacement behavior; for displacement-controlled indentations up to 80 nm, load drops/jumps are observed during loading/unloading. Deformation resistance and recovery are observed in conjunction with these indentation signatures. In the nanorod structures, these indentation signatures become suppressed due to the 2-dimensional Al confinement in these structures, while the thin-film structure fails primarily due to cracking at the Al/a-Si interface. This dependence of the deformation behavior on the degree of Al confinement indicates that different deformation mechanisms are involved in each type of structure and could lead to a better design of the nanomechanical behavior of these structures.

Transition metal nitrides have been extensively studied in the last decades, owing to their excellent performance as hard, wear- and corrosion resistant coatings but also as suitable template layers for group-III-nitride wide band-gap semiconductors or as biomaterials, and recently as reflecting back contacts and barrier film in solar cells. For most of the applications, the thermal stability and the subsequent high-temperature mechanical properties are of great interest.

The effect of temperature on thin film elastic constants can be studied by high-temperature acoustic measurements. The Brillouin light scattering (BLS) laser-based technique allows measuring sound velocity of a few kind of surface acoustic waves in thin films and coatings and thus estimating effective elastic constants if the mass density of the film is known. This non-conventional technique for thin films has been recently combined with a high-temperature chamber dedicated to optical techniques; samples can be heated up to 1600°C, in air or under controllable vacuum (pressure down to 5.10^-7 Pa). This original experimental set-up has been used to study in-situ the evolution of polycrystalline binary nitride coatings elastic constants (shear constant C₄₄) with temperature.

In this talk, we will show the general principle of the technique, the applicability in terms of materials and atmosphere conditions and give first results recently obtained.

Over the recent years the research areas of self-organized nanocomposites and multilayered structures of metastable transition metal nitride alloys have witnessed an increasing complexity in developing superhard materials. We have explored a range of obtainable self-organized structure in magnetron sputterd ZrN-AlN alloys, which has one of the largest positive enthalpies of mixing among the transition metal aluminum nitride systems and discover a unique, highly regular two-dimensional nanolabyrinthine structure of two intergrown different single-crystal phases [1]. In a parallel study on arc-evaporated ZrAlN/TiN multilayered structures we have illustrated hardness enhancement upon isothermal annealing due to a formation of a secondary phase cubic ZrTi(Al)N structure at the sublayer interfaces[2].

Here, we take a step forward to combine above structures to further tune the film hardness and toughness properties. We grow self-organized nanocomposites ZrN-AlN and TiN multilayerd structure on single-crystal substrates using magnetron sputtering with varying ZrAlN layer composition, modulation period, and substrate temperature. We elucidate phase evolution, decomposition, and mechanical properties of the films as a function of structural design and growth parameters. We also discuss the hardness/toughness enhancement mechanisms of the coatings upon structural rearrangements.

References

[1] N. Ghafoor, L. Johnson, D. Klenov, J. Demeulemeester, P. Desjardins, I. Petrov, L. Hultman, M. Odén, APL Materials 1 (2) (2013) 022105

[2] L. Rogström, N. Ghafoor, M. Ahlgren, M. Odén, Thin Solid Films 520 (21) (2012) 6451.

CrN based coatings, which are widely used on forming tools, are good candidates for extreme environments since they possess good oxidation resistance, anti-corrosive and anti-adhesive properties. Nonetheless, the single-nitride presents an inadequate hardness and wear resistance.

The nanolayering approach is a good solution for counteracting its limitations [i]. The addition of vanadium can enhance mechanical properties as well as oxidation resistance [ii]. As a matter of fact, vanadium can be oxidized at high temperature forming a highly lubricious V₂O₅ which drastically reduces the friction coefficient during dry sliding [iii].

In this study, the mechanical properties of arc evaporated CrN-VN nanolayered coatings were studied by varying the bilayer period, the bias voltage and the deposition temperature. The bias voltage control allowed a hardness evolution from 15 GPa up to 35 GPa, while deposition temperature (floating and 400°C) tailored the residual stresses thus allowing a hardness increase from 30 GPa up to 35 GPa, respectively. These mechanical features evolved differently as a function of the testing temperature.

For this reason, the research was focused on the tribological behaviour by means of ball-on-disc tests from RT and 600°C. In this context, the so-called Magnéli-phases V_nO_2n-1 formation was studied as a function of temperature.

The mechanical-tribological characterization was completed by microstructural observations with the aim of comparing the different behaviour during mechanical tests.

References

[i] F. Lomello et al. Surf. Coat.Technol.238 (2014) 216.

[ii] Y. Qiu et al. Surf. Coat. Technol. 231 (2013) 357.

[iii] R. Franz et al. Surf. Coat. Technol. 228 (2013) 1.

Thin hard coatings are widely known as key factors in many fields of industry, from equipment for metals machining to dental implants and orthopedic prosthesis. Tools for machining and cutting are mainly made of hard steels, cemented carbides (WC-Co) and in special cases, such as aluminum machining, of diamond or polycrystalline cubic boron nitride. The main issue affecting tool steels is their relatively low hardness (below 10 GPa), which strongly decreases upon annealing above about 600 K. Therefore the role of thin hard coatings is to: (i) increase the hardness, (ii) decrease the coefficient of friction, and (iii) protect the tools against oxidation. Multi-component and nanostructured materials represent a promising class of protective hard coatings due to their enhanced mechanical and thermal oxidation properties.

Aim of this work was to evaluate the tribological performance of three different thin hard coatings deposited by physical vapor deposition (PVD) techniques, in order to assess the feasibility of their implementation in industrial processes different from the one for which they were designed.

Three different thin hard nitrogen-rich coatings have been considered in the present study: (i) a 2.5 micron-thick nano-layered CrN-NbN, (ii) a 11.7 micron-thick monolayer TiAlN (SBN, super booster nitride), and (iii) a 2.92 micron-thick multilayer AlTiCr_xN_y. The main feature of the CrN-NbN coating is the fabrication by the alternate deposition of 4 nm thick-nanolayer of NewChrome (new type of CrN, with strong adhesion and low coating temperature).

All the coatings were deposited on S600 tool steel and were subjected to pin-on-disc tribological tests at room temperature (293 K) and at high temperature (873 K), using alumina (Al₂O₃) ball as counter body. The normal load was set at 12 N, while rotating speed was 538 rpm. During each test, a linear distance of 5000 m was covered. Then, all the samples were subjected to microstructural characterization by means of high resolution scanning electron microscopy (HRSEM) and energy dispersive spectroscopy (EDS). All the three coatings can reach values of hardness and elastic modulus exceeding 20 and 250 GPa, respectively.

Nanostructuring transition metal nitride coatings have been in use as wear resistant coatings on metal cutting tools since the 1990’s, with the TiAlN family of coatings as the most widespread and successful example, and the industrial field has enjoyed a continual improvement in the performance of such coatings up to today. At the same time, our understanding of the fundamental materials science of these, and related, coatings has been significantly improved in recent years. For example, the spinodal decomposition of (Ti,Al)N solid solutions is now quite well understood, and the field is moving on towards synthesis and analysis of quaternary or even more complex nitride alloys. What remain to be explained, however, are the connections between application performance in metal cutting and the structures and properties of such new nitrides. As the options for new coatings rapidly increase, any such understanding will be of great help in reducing the development work for new products. One thing that is known is that the same coating may exhibit widely different levels of performance, depending on the particulars of the cutting application, such as choice of cutting parameters, work piece material, or kind of operation. In this talk, such cases are used as a probe into the mechanisms underlying the observable wear patterns, and, through coatings chosen as typical examples of (Ti,Al)N used in industry today, discussed for their implications regarding the design of nanostructuring thin films.

Magnesium is an attractive engineering metal, possessing very low density and (with careful alloying and heat treatment) a moderately high strength/weight ratio, which enable lighter engineering products to be manufactured. Moreover, the excellent castability and weldability of magnesium alloys contribute to prospective markets in a wide range of industrial sectors, including construction, automotive, aerospace, and communications. Unfortunately, several undesirable properties of magnesium alloys preclude their more widespread use, the most significant of these often being insufficient resistance to corrosion and wear. Depositing a protective coating onto the surface of Mg-alloys is a good strategy to enhance corrosion and wear performance; in this regard, PVD metallic nanostructured coatings hold significant promise.

In this study, Al-Cu-Zr-Mo and Al-Ni-Ti-Si based alloy coatings of varying composition were deposited onto a proprietary high-strength Mg alloy (WE43-T6) using an unbalanced magnetron sputtering technique. By the introduction of amorphous/nanocrystalline interfacial layers, with elastic modulus closer to that of the underlying Mg-alloy substrate, it is possible to accommodate significant substrate elastic strain under mechanical loading. In addition, on top of the (mainly) amorphous interface layer, a hard and dense ceramic coating can be created by introducing nitrogen reactive gas which reacts with nitride-forming elements within the coating system.

Compared to directly depositing a hard coating onto magnesium alloy surface, a design strategy of layering and grading coating chemical composition improves the matching of coating/substrate elastic properties so that the coating toughness and durability is enhanced. Desirable coating mechanical properties such as high hardness (up to 18GPa) and low elastic modulus (typically between 50 and 150GPa) have been measured by nanoindentation. The nanostructure of the coatings has been examined by glancing-angle XRD and high resolution optical microscopy. OCP and potentiodynamic polarisation corrosion tests have also been performed.