ICMCTF2010 Session B6-1: Hard and Multifunctional Nano-Structured Coatings

Coating structure and morphology often play a dominating rule in applications (i.e. cutting tools, components). The morphological and structural development of AlTiN coatings deposited by cathodic vacuum arc are high lightened. The observed morphologies include the fine columnar growth and the super fine growth. Deposition conditions (pressure, bias, voltage, temperature) are influencing both the coating morphology and the phase composition, e.g. the existence of a minor hcp phase for coatings with Al/Ti-content larger than 1. It will be shown that the Al content in the coating and the type of evaporators influence the coating growth. Experimental results will be shown for different Al/Ti compositions at different growth conditions. Experimental methods used were X-ray investigations and SEM. The possibility to generate super fine growth will be discussed.

Oxidation resistant and wear resistant coatings for environmental protection of light weight materials such as gama-TiAl alloys for future applications in aero and automotive engines are in high demand. CrAlYN/CrN coatings utilising nanoscale multilayer structure with a typical bi-layer thickness of 4.2nm were successfully produced on large scale by utilising the High Power Impulse Magnetron Sputtering (HIPIMS) technology. HIPIMS was employed for surface pre-treatment as well as for coating deposition. The surface pre-treatment was carried out by bombardment with Cr⁺ ions. For comparison CrAlYN/CrN was deposited by Unbalanced Magnetron Sputtering, (UBM) as well.

Scanning Transmission Electron Microscopy, (STEM) revealed that the coating/substrate interface was extremely clean and sharp. Large areas of coating grown epitaxially were observed. STEM-Energy Dispersive Spectroscopy (EDS) profile analysis further showed that during the HIPIMS ion bombardment Cr had been implanted into the substrate to a depth of 5 nm. HIPIMS deposited coatings showed extremely sharp interfaces between the individual layers in the nanolaminated material and almost no layer waviness. This improved structure resulted in further enhancement of the coatings barrier properties.

CrAlYN/CrN showed potential for reliable protection of gama-TiAl alloys against wear and aggressive environmental attack. For coated gama-TiAl alloys, thermo gravimetric quasi-isothermal oxidation tests in air at 750⁰ C after 2000 hours exposure showed four times smaller weight gain compared to the uncoated material. HIPIMS coatings were superior to UBM deposited coatings. Tested even at higher, 850⁰ C temperature the HIPIMS coatings showed by factor of 2 lower mass gain as compared to the UBM deposited coatings.

In sulphidation tests after 1000 hours exposure to aggressive H₂ /H₂S/H₂O atmosphere the CrAlYN/CrN protected gama-TiAl alloys showed reduced weigh gain by factor of four as compared to the uncoated substrate. High temperature pin-on-disc tests revealed that CrAlYN/CrN reduces its friction coefficient from 0.56 at room temperature to 0.4 at 650⁰C, which demonstrates the excellent high temperature tribological behaviour of the coating. HIPIMS deposited coatings showed extremely low wear coefficient of K_C= 1.83 E-17 m³ N^-1m^-1 at this temperature. Importantly HIPIMS deposited coatings retained the ultimate tensile strength of the gama-TiAl and reduced the fatigue strength only by 9%, compared to 20 % measured for the UBM deposited coatings, which opens perspectives for turbine blade application.

The tribomechanical environment during cutting operations differs highly between machining techniques and cutting conditions. One important circumstance regards the temperature at the cutting edge, which varies between values close to room temperature for applications such as thread tapping and values higher than 1000°C for continuous cutting applications.

Generally, temperature dependent coating properties have to be considered in the characterization and later on in the selection of coatings for cutting tools. Mechanisms such as oxidation, phase transformation, recovery and chemical reactions in the contact area can occur. The measurement of the hardness at different temperatures is a characterization method which indicates presence of mentioned effects in form of changing the coating properties.

Hence, the hardness in relation to the temperature is discussed based on measurements which were conducted on TiAlN, AlCrN and alloyed AlCrN coatings deposited on polished cemented carbide (WC / 6% Co) substrates. The micro-hardness indents were performed under a non-oxidizing gas atmosphere at temperatures between 25°C and 1000°C. Furthermore, residual hardness after the annealing sequence was determined to distinguish between permanent and reversible transformations.

We show that the AlN incorporation in single crystal Hf_1-xAl_xN(001) films can be controllably manipulated between ~ 0 and 100% by varying the ion energy (E_i) incident at the growing film over a narrow range, 10 – 40 eV. The layers are grown on MgO(001) at 450°C using ultrahigh-vacuum reactive magnetically-unbalanced magnetron sputtering from a single Hf/Al 70/30 (at. %) alloy target in 5% N₂/Ar mixtures at a total pressure of 20 mTorr. The ion-to-metal flux ratio incident at the growing film is maintained constant at 8, while E_i is varied from 10 to 80eV. Film compositions vary from x = 0.3 with E_i = 10 eV to 0.27 with E_i = 20 eV, 0.17 with E_i = 30 eV, and ≤ 0.02 with E_i ≥ 40 eV. Thus, the AlN incorporation probability decreases by greater than two orders of magnitude! This extraordinary range in real-time manipulation of film chemistry during film deposition is due to the efficient resputtering of Al atoms (27 amu) by Ar ions (and fast Ar atoms, both 40 amu) scattered off heavy Hf atoms (178.5 amu) in the film. This effect can be used to grow planar heterostructures and superlattices with abrupt interfaces at high deposition rates from a single target by controllably switching E_i. The choice of E_i values determines the layer compositions and the switching speed controls the layer thickness. Here, we present the effects of film micro- and nanostructural properties on film hardness in Hf_0.70Al_0.30N/HfN superlattices with bilayer thicknesses ranging from 1 to 7 nm using high-resolution transmission electron microscopy (HR-TEM), HR-STEM, HR-XRD and nanoindentation.

For TiN/NbN(001) superlattices, dislocation glide within the layers is the dominant deformation mechanism. That confirms the present models for superhardening that presumes plasticity and postulates dislocation hindering at interfaces between layers of different shear modulus. In consequence, these coatings also exhibit crystal rotation during deformation. [2]

Superhardening occurs in TiN/Si₃N₄ nanocomposites due to Si segregation forming a few-monolayer (ML)-thick SiN_x tissue phase, which can be amorphous or crystalline. In the latter case, we show the existence of a cubic-SiN_x layer that is epitaxially stabilized to TiN. A hardness maximum at 34 GPa is observed in TiN/SiN_x(001) superlattices at the epitaxial break-down limit for the epitaxial SiN_x layers of 2-5 ML, well above the percolation limit. [3]

Our concept of age hardening in supersaturated cubic-phase transition metal nitride alloy systems is presented for the pseudobinary MeAlN systems. Secondary phase transformation including spinodal decomposition of TiAlN into isostructural (cubic-phase) nm-size domains of TiN and AlN is thus demonstrated. [4] The as-formed domains hinder dislocation glide in films annealed to temperatures corresponding to cutting tool operations. The effects of pressure [5], temperature, composition, disorder, and stoichiometry (vacancies) will also be discussed.

[1] B. Alling, et al., Surf. Coat. Technol. 203 (2008) p. 883.

[2] L. Hultman, in Nanostructured Coatings, ed. A. Cavaleiro, et al, Plenum 2006, Ch. 13, pp. 539-552.

[3] L. Hultman et al., Phys. Rev. B75 (2007) p. 155437

[4] P.H. Mayrhofer, C. Mitterer, L. Hultman and H. Clemens, Progress in Mater. Sci., 51 (2006) pp. 1033-1114.

[5] B. Alling et al., Appl Phys. Lett. 95 (2009)p. 181906

[1] S. H. Sheng, R. F. Zhang, S. Veprek, Acta Mater. 56, 968 (2008).

[2] B. Alling, A. Karimi, I. A. Abrikosov. Surf. Coat. Technol., 203:883-886 (2008).

[3] R. Sanjinés, C. S. Sandu, R. Lamni, F. Lévy. Surf. Coat. Technol., 200:6308 (2006).

[4] L. Rogström, L. Johnson, M.P. Johansson, M. Ahlgren, L. Hultman, M. Odén, Age hardening in arc evaporated ZrAlN films, Submitted (2009).

Microstructure features like phase composition, crystallite size and lattice strain are usually related to the formation and mobility of microstructure defects and influence always the properties of protective hard coatings. Among the parameters of the deposition process, which are employed to adjust the microstructure of the Ti-Al-N coatings deposited by cathodic arc evaporation (CAE), the aluminium contents and the bias voltage play a crucial role. In this study, the influence of both parameters on selected microstructure parameters of the CAE Ti_1-xAl_xN coatings deposited from the Ti-Al cathodes was investigated by using X-ray diffraction and transmission electron microscopy.

The microstructure of the coatings was described in terms of the chemical and phase composition, local distribution of the phases, crystallite size and lattice strain, which results from the interaction between neighbouring crystallites and from the presence of the dominant microstructure defects. The microstructure model based on the knowledge of the above microstructure parameters allowed the microstructure formation during the deposition process to be explained and correlated to the parameters of the deposition process. The correlation between the microstructure formation and the parameters of the deposition process contributed substantially to the understanding of the effect of the aluminium contents and the bias voltage on the microstructure formation in CAE Ti_1-xAl_xN coatings.

This study was performed on coatings with the chemical compositions Ti_0.60Al_0.40N, Ti_0.50Al_0.50N and Ti_0.40Al_0.60N, which were deposited at the bias voltages (U_B) ranging between -20 V to -120 V. Disregard the chemical composition, the coatings deposited at U_B ≤-40 V contained face centred cubic (Ti,Al)N as a single phase. With increasing U_B, Al segregated from the (Ti,Al)N and created AlN. The segregation of Al contributed to the reduction of the crystallite size and to the increase of the lattice strains in the coatings. Both these phenomena are related to the increase of the hardness. A departure from the monotonous reduction of the crystallite size and from the monotonous increase of the lattice strain was observed at the highest bias voltage (U_B = -120 V).

The non-monotonous effect of the bias voltage on the phase composition, on the crystallite size and on the kind and density of the microstructure defects will be discussed with respect to several competing processes, which are related to the energy of impinging particles, to the mobility of the deposited species, to the formation, arrangement and mobility of microstructure defects and to the mechanical interaction of neighbouring crystallites.

State of the art three-dimensional atom probe tomography (3D-APT) enables the simultaneous investigation of morphology and chemical composition with near atomic resolution. Since spinodal decomposition in Ti_1-xAl_xN thin films results in the formation of a nanostructure and an increase in hardness at high temperatures, this process has attracted increasing interest and is in the focus of many research activities on the design and modification of chemical and phase-modulated nanostructured thin films.

We employ a local electrode 3D-atom probe in laser-mode to study the morphological and chemical development of Ti_1-xAl_xN coatings. The results obtained clearly indicate compositional fluctuations of Ti and Al atoms of cubic Ti_0.46Al_0.54N coatings, prepared by dc magnetron sputtering at 500 °C, already in the as-deposited state [1]. This is in good agreement to results reported for epitaxially grown Ti_0.50Al_0.50N coatings [2]. Vacuum annealing to 900 °C induces an increase in the compositional amplitude of the fluctuations and the formation of a 3D-interconnected network of cubic Ti- and Al-rich domains. Detailed 3D-APT studies exhibit diffuse domain boundaries, which highlight the spinodal character of the decomposition process. During vacuum annealing to 1450 °C the aluminium enriched domains phase-transform in the stable modification, hexagonal (B4) AlN, whereas the titanium enriched domains maintain the cubic (B1) structure while transforming into TiN.

Based on our studies, we can conclude that starting from an almost randomly distributed input of 22.9 at.% Ti, 26.4 at.% Al and 50.5 at.% N in the as-deposited single-phase cubic Ti_0.46Al_0.54N, the complexity of the system increases upon annealing to temperatures above 900 °C. The decomposition process into TiN and AlN via spinodally formed Ti- and Al-rich domains also causes a rearrangement of the initially randomly incorporated 0.13 at.% oxygen impurities. After decomposition, the oxygen is preferentially incorporated in the AlN phases. Consequently, 3D-APT studies will have a strong impact on future developments of super-saturated, chemically and phase-modulated hard coatings and thin films, especially involving investigations on the effect of minority trace elements.

[1] R. Rachbauer, E. Stergar, S. Massl, M. Moser, P.H. Mayrhofer, Scripta Materialia 61 (2009) 725-728.