ICMCTF2014 Session B6: Coating Design and Architectures

This work summarizes recent developments on applying thin film structure and architecture concepts to hard coatings for optimized performance in various application fields. Hard coatings deposited by plasma-assisted vapour deposition are widely used to reduce friction and wear of tools and engineering components.

We will look in more detail into the correlation between growth processes, microstructure, mechanical properties and thermal stability of hard ceramic coatings. This is done for single-phase coatings and composition or phase modulated layers within the model systems Ti–Al–N, Cr–Al–N, and Zr–Al–N.

The hardness of materials rapidly decreases at elevated temperatures as generally the density of structural defects, such as point defects, dislocations, and grain boundaries, decreases. Additional strengthening can be provided by age-hardening mechanisms, which originate from decomposition-processes of supersaturated phases to form new obstacles retarding plastic deformation. Furthermore important is the resistance against oxidation. Here, we will show, that in addition of supporting the formation of a dense corundum structure on these ternary TM-Al-N coatings, it is of utmost importance to minimize or even completely suppress any phase changes taking place within the growing oxide scale.

By using ab initio calculations and sophisticated experimental methods we will have a detailed insight into various mechanisms responsible for excellent mechanical strength, thermal stability and oxidation resistance properties of Ti–Al–N, Cr–Al–N, and Zr–Al–N hard coatings. For these materials we will also compare the effect of various architecture and alloying concepts with e.g., Y, Zr, Hf, Nb, and Ta.

As the brittleness of such ceramic-like coatings often negatively influences their performance, especially when used in conditions with an increased need for crack resistance, we will also discuss this topic by using in-situ electron microscopy micro- and nano-mechanical investigations of CrN/AlN multilayers.

The various thin film structure and architecture concepts allow the utilization of multifunctional properties facilitating the development of next generation's hard coatings.

Multicomponent nitrides is one of the major classes of materials in use for protective and decorative coatings. In particular the Ti-Al-N system is in extensive use as wear resistant coatings on cutting tools owing to good mechanical properties, high thermal stability and good oxidation resistance. There is a huge scientific literature covering the synthesis, structure, and properties of Ti_1-xAl_xN thin films but in contrast very few investigations have been made on the neighbouring Ti-Mg-N system [1,2]. In this work theoretical first-principles methods based on the density functional theory and alloy theory have been employed to investigate the phase stability and electronic structure of Ti_1-xMg_xN_y solid solutions and ordered compounds. We demonstrate that the solution of Mg into TiN forming rock salt structure Ti_1-xMg_xN is thermodynamically stable at least up to x≤0.5 with respect to the TiN and Mg₃N₂ parent compounds in contrast to the TiAlN coatings which tend to phase separate. The electronic structure displays a transition from metallic TiN to semiconducting at x=0.5 explaining the drastic change in colour as observed experimentally [3]. The connection between the electronic structure and the phase stability as well as the possibility to synthesise ordered TiMgN₂ compounds with respect to disordered solid solutions is discussed.

[1] O. Banakh, M. Balzer, M. Fenker, and A. Blatter, Thin Solid Films, 455-456, 650 (2004)

[2] A. Hodroj, O. Chaix-Pluchery, P. Steyer, J.F. Pierson, Surface & Coatings Technology 205, 4547 (2011)

[3] M. Fenker,T, M. Balzer, H. Kappl, O. Banakh, Surface & Coatings Technology 200, 227 (2005)

This paper deals with the toughness optimization of hard, wear-resistant coatings by structural design on the nano- and microscale. The coatings were produced on the industrial π 411 platform, using lateral (LARC®) and central (CERC®) cylindrical rotating arc cathodes PVD technology. On the microscale, the new coatings were designed according to the Quad Coatings® multi-zone architecture, whereas on the nanoscale, their nanolayer- and nanocomposite structure was optimized.

Coating members from chemically different families were compared to each other with respect to their physical and structural properties both at ambient and elevated temperatures. It will be shown how processing parameters such as the choice of target arrangement and -composition can be used to alter the nanolayer structure, thereby influencing the coating properties such as toughness and cutting performance. In addition to the nanoscale design, the successful multi-zone Quad Coatings4® concept was used to optimize adhesion and fracture resistance through microscale structural design.

By performing annealing tests on a wide range of coating types and –structures, it was found that certain properties such as oxidation resistance are mainly governed by chemical effects, while under thermal load, grain growth, hardness– and stress changes can rather be controlled by applying a nanocomposite structure design.

The optimized concept coatings were applied on tools for high-performance cutting at intermediate temperatures. Interrupted cut applications such as hobbing of low-alloy steels, milling of tool steels using using indexable cutting inserts, and milling or aerospace alloys using solid carbide end mills pose different levels of challenge to the coatings’ toughness and their ability to retard crack propagation, especially in wet cutting.

Ti_1-xAl_xN is a typical protective coating to increase the lifetime of machining and forming tools especially during high temperature applications and under demanding tribological conditions. Due to thermal decomposition and severe oxidation at temperatures above ~800 °C the field of their applications is limited. To counteract these tendencies quaternary systems like Ti_1-x-yAl_xTa_yN were developed and successfully implemented. However, especially the mechanisms for increased oxidation resistance and the necessary ideal chemical composition for an optimized behavior are still not clarified and understood.

Therefore, we have reactively deposited Ti_1-x-yAl_xTa_yN coatings with a Ti/Al ratio of 51/49 and 35/65 and Ta contents of 0, ~8, and ~16 at%. By using isothermal Differential Scanning Calorimetry combined with Thermal Gravimetric Analysis we observe for single cubic phased Ti_0.32Al_0.60Ta_0.08N a mass gain of only ~5% after 5h at 950 °C in synthetic air, whereas Ti_0.35Al_0.65N is completely oxidized after 15 min (mass gain ~24%). Structural investigations by X-Ray Diffraction and Scanning Electron Microscopy reveal anatase-to-rutile phase transformations with increasing oxidation time and a porous scale for Ta-free Ti_0.35Al_0.65N. Contrary, Ti_0.32Al_0.60Ta_0.08N exhibits a highly dense and rutile dominated and protective scale.

Density Functional Theory simulations of the phase stabilities throughout the ternary system rutile (R) and anatase (A) (Ti,Al,Ta)O₂, corundum (α) type (Al,Ta,Ti)₂O₃, and orthorhombic (Ta,Ti,Al)₂O₅ show that in the case of Ti_0.35Al_0.65N the transformation A + α to A + R + α to R + α occurs. If Ta is alloyed a rutile phase field opens even at 0 K which allows for the direct formation of a R + α scale. This indicates that the phase transformation – which is accompanied by a volume change of 5-10% leading to the formation of pores and cracks within the scale – can be avoided for Ti_0.32Al_0.60Ta_0.08N.

Based on these results we can conclude that for increased oxidation resistance the coatings chemical composition has to be optimized to the respective oxide phase diagram to allow for alumina and a Ti-oxide-based phase without or minimal anatase to rutile phase transformation up to the application temperature of the coating.

Thin film growth from vapour phase starts by nucleation of atomic islands on the substrate surface. Those islands grow in size, impinge on each other, coalesce into larger islands and eventually form a continuous film. These film formation stages are particularly apparent during Volmer-Weber (i.e., 3-dimensional) growth. At the same time, film growth from the vapour phase proceeds far from thermodynamics equilibrium and dynamics and characteristics of the various formation stages are mainly dictated by growth kinetics. The initial film formation stages (i.e., island nucleation, growth and coalescence) largely set the film nanoscale architecture. They are, thus, decisive for film microstructural features and evolution which, in turn, determines the film physical attributes. Fundamental understanding of how each of the initial formation stages and their complex interplay affect microstructural evolution is paramount to knowledge-based design of film architecture at the nanoscale. In this talk, well defined pulsed vapour fluxes [1] are employed to tune the kinetic conditions during growth of Ag on SiO₂ – an archetype system for the study of Volmer-Weber growth. A research strategy that entails the combined use of in situ growth monitoring, post-growth imaging and growth simulations is compiled to unravel to role of the initial film formation stages on the growth evolution [2]. Growth simulations are also used to obtain a universal understanding of the relationships between deposition conditions and scaling behaviour of early film growth stages [3]. The implications of the findings for surface science and surface engineering communities are discussed.

[1] D. Magnfält, K. Sarakinos et al., J. Phys. D: Appl. Phys. 46 (2013) 215303.

[2] V. Elofsson, K. Sarakinos et al., unpublished data.

[3] B- Lü, K. Sarakinos et al., unbublished data.

Recently, many different aluminium-based hard coatings deposited by physical vapour deposition (PVD) have been introduced into the market. For the deposition of wear resistant coatings for tools cathodic arc evaporation and r.f., d.c. or high power impulse magnetron sputtering or even ion and electron beam supported processes are frequently used. AlCrN-based coatings produced by cathodic arc deposition are state-of-the-art, therefore in the present work we are focussing on AlCr-based coatings deposited by magnetron sputtering in an industrial coating machine.

The influence of the composition of the AlCr powder metallurgical sputtering targets and the influence of the deposition process parameters on the coatings are investigated. The mechanical properties of the coatings - hardness, elastic modulus, adhesion, crystallographic orientation and chemical composition – have been examined. These characteristics have been examined for a range of operating parameters like bias voltage, gas ratios and different power densities.

Another goal of this work is to investigate the impact of different aluminium and chromium ratios in the target on the deposited coatings. For this reason different AlCr targets have been used for sputtering tests.

Passive films, the primary mechanism for corrosion protection in materials such as stainless steel and titanium, form in oxidizing environments. Conventionally the films that form are a fixed chemistry, and have fixed thicknesses. While anodizing provides additional design flexibility in developing thicknesses that exceed those formed on native passive oxides, they still have limited chemistry variation. However, it is possible to grow oxides using pulsed laser oxidation, which leads to non-stochiometric oxides and/or nitrides which develop based on the underlying substrate material interacting with the surrounding environment. This presentation will focus on testing methodologies using nanomechancial probes of the surface of passivating metals to determine the toughness of the films. An approach that relies on selecting appropriate probe geometry in relationship to the film thickness and substrate properties will be described. The method and model will then be used to demonstrate the impact of environment and the exposure to corrosive environments on the resulting toughness of native passive films, anodized films, and laser-grown oxides.

A theory was proposed to explain the enhanced light absorption and, therefore, the enhanced heating effect due to light irradiation. At final, an example was given to explain the possible annealing of TaN-(Ag,Cu) thin films by using the enhanced photothermal effect.