ICMCTF 2021 Virtual Conference Session B1: PVD Coatings and Technologies

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The main challenge in hard coating manufacturing is developing dense, flawless, and smooth surface morphology coatings, which improve the performance of components exposed to severe service conditions. One of the most promising coating techniques to achieve this goal is the High-Power Impulse Magnetron Sputtering (HiPIMS), which produces highly ionized plasmas, resulting in definite improvements on the film microstructure and properties. The application of pulses (50 - 500 μs) during the deposition process attains a high ionization rate in HiPIMS. The pulse modification (e.g., duty-cycle, also known as τ) affects the current density, modifying the stoichiometry and chemical composition of the coating; this indicates that the pulse appropriate selection is a viable and practical alternative for obtaining graded or multilayer metal nitrides at a fixed gas flow, thus tailoring the coating internal stress distribution, microstructure, and mechanical properties.

In this work, the duty-cycle design to modify the chemistry, microstructure, and residual stresses of Cr_xN coatings is presented and discussed. The average current tends to increase from 2.9 to 12 % with the variation of τ during the HiPIMS process. These process conditions lead to a significant increment of the deposition rate and the chromium content in the Cr_xN coating. Various crystalline phases like α-Cr, α-Cr + h-Cr₂N, h-Cr₂N, and h-Cr₂N + c-CrN were obtained in the film, resulting in graded multilayer systems. Cr-rich samples presented faceted columns with intercolumnar pores and cauliflower-like surface morphology. The growth of the h-Cr₂N phase caused a decrease of the grain sizes, and their morphology changed to pyramidal or stacked pyramids at lower duty-cycles. The transformation of the h-Cr₂N to CrN leads to highly dense columnar microstructures, while the residual stresses of the coating increased with the higher duty-cycle.

Keywords: HiPIMS, duty-cycle, Cr_xN, graded microstructure.

The CO2 debate—a global challenge requires the necessity of renewed investments and innovations by manufacturers and suppliers with a strong focus on an increase of sustainability. Especially, the automotive sector (driven by the governmentally regulated CO2 emission limit), in detail the trends for the e-mobility, which has reached higher levels of importance. Consequently, high-performance materials and versatile material combinations with focus on weight reduction and energy-efficient automotive design are replacing conventional materials. The trend to a reduction of the cycle times of the production lines, leads to new challenges in the application of tools, such as higher mechanical and thermal loads exposure during operation.

The paper presents the results of the investigation into the formation of the nanolayer structure of the Ti-TiN-(Ti,Cr,Al)N coating and its influence on the thickness of coatings, their resistance to fracture in scratch testing, and the wear resistance of coated tools in turning 1045 steel. The structure of the coatings with the nanolayer thicknesses of 302, 160, 70, 53, 38, 24, 16, and 10 nm was studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution (HR) TEM. It is shown that the grain sizes in the nanolayers decrease to certain values with an increase in the thickness of the nanolayers, and then, with a further decrease in the nanolayer thickness, the grain sizes of the nanolayer grow as the interlayer interfaces cease to produce a restraining effect on the growth of the grains. The study found that the nanolayer thickness influenced the wear of carbide cutting tools and the pattern of fracture for the Ti-TiN-(Ti,Cr,Al)N coatings.

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/2/ H.-J. Scheibe et al, Laser-arc: a new method for preparation of diamond-like carbon films. Surf. Coat. Technol. 47, 455-464 (1991).

Reactive high-power impulse magnetron sputtering (HiPIMS) with a feedback pulsed reactive gas (oxygen and nitrogen) flow control and to-substrate reactive gas injection into a high-density plasma in front of the sputtered metal target was used for a low-temperature deposition of highly optically transparent Al-O-N films (substrate temperature of 120 °C) and thermochromic VO₂-based films (substrate temperature of 330 °C) onto unbiased substrates.

A modified version of HiPIMS, called Deep Oscillation Magnetron Sputtering, was used to produce high-quality Al-O-N films with a gradually changed elemental composition (from Al₂O₃ in AlN), structure and properties. We give the basic principles of this controlled deposition, maximizing the degree of dissociation of both O₂ and N₂ molecules in a discharge plasma, which leads to a replacement of very different reactivities of the O₂ and N₂ molecules with metal atoms on the surface of growing films by similar (high) reactivities of atomic O and N.

Tantalum nitride thin films were deposited onto steel substrates using Reactive High-Power Impulse Magnetron Sputtering (HiPIMS) allowing reaching a high ionization degree of the sputtered metallic material thanks to high power density applied to the target during few tens of microseconds pulse. The influence of target power density, N₂ partial pressure, total gas (Ar + N₂) pressure and target-to-substrate distance on film crystalline structure is reported. The structures obtained were investigated by X-ray diffraction and transmission electron microscopy. Cubic metastable phase or hexagonal stable phase can be successfully isolated in single phase continuous layer. The TaN crystalline phase obtained depends strongly on processing parameters especially pulse parameters. It is well known that TaN hexagonal single-phase continuous layer is difficult to isolate using conventional reactive magnetron sputtering. Our previous study, based on RF magnetron sputtering, has shown TaN hexagonal structure formation to be enhanced in growth conditions promoting adatoms mobility on the substrate surface. With HiPIMS, TaN hexagonal phase layer is much more difficult to obtain due to the increase number of process parameters to select, the specific composition and the energy of the plasma created. Comparison of mechanisms involved during the stabilization of each TaN structure depending on the process is presented as well as characterization of microstructure, and properties (mechanical, electrical and optical).

Without additional efforts, thin films deposited by magnetron sputtering are dense due to the bombardment by sputtered atoms and reflected neutrals. To overcome this intrinsic feature of magnetron sputtering, several routes have been explored to deposit a porous thin film, and still to benefit from the advantages of magnetron sputtering.

Increasing the deposition pressure and/or tilting the substrate belongs to the most common approaches, but these strategies are plagued by technical issues such as enhanced arcing, a low deposition rate, and limited scalability.

Another strategy is based on the deposition of a mixture of materials, or an alloy which is chemically and/or physically treated to remove one of the constituents. Dealloying by applying a heat treatment and subsequently electrochemical etching is one example that belongs to this group of methods. High temperature and/or (electro)chemical treatments limit the substrate choice, and are often environmentally harmful.

This paper suggests an alternative approach based on the sputter deposition of a powder mixture of a metal with NaCl [1]. The deposited layer is simply treated with water to dissolve the deposited salt, leaving a porous metal layer on the substrate. The thin films are deposited from a powder target composed of NaCl and metal powder. Due to the low thermal conductivity of the target, the target gets hot. As a result, the salt is sublimed while the metal is sputtered from the target. The film composition, and therefore the porosity, is controlled by the applied target power.

This one-source approach without the use of film annealing or aggressive chemicals overcomes the major obstacles of other synthesis routes without compromising the benefits of magnetron sputtering.

[1] Sputter deposition of porous thin films from metal/NaCl powder targets, R. Dedoncker, H. Ryckaert, D. Depla, Appl. Phys. Lett. 115(2019) 041601doi:10.1016/10.1063/1.51128225

The increasing demand for multi-element thin films and coatings for multifunctional purposes has pushed the target and cathode material industry to produce multi-element products. Nowadays, alloyed and composite targets are commonly employed in various physical vapor deposition technologies including magnetron sputtering and cathodic arc deposition. The targets, which serve as cathodes in the respective discharges, are exposed to the discharge plasma during the deposition process, which alters their surface properties such as phase and chemical composition. The surface modifications are particularly severe for cathodic arc deposition since the cathode spots impose countless melting-solidification cycles on the target material near the surface, leading to the formation of a network of craters and a several 10 µm thick modified layer. The formation mechanisms and properties of the modified layer, semi-empirically quantified by the cohesive energy of the constituent phases, influence the charge states and kinetic energies of the ions and, hence, the film growth conditions and coating properties.

In a first step, a Mo/Al multilayer cathode was designed to reveal information about the heat-affected zone below the craters as well as the evolution of material intermixing as the sequential cathode spot events take place. The modified layer is formed by micro-sized displacements of the cathode material during crater formation. In addition, the material intermixing predominantly occurs in liquid state while the mechanisms based on solid-state diffusion play an insignificant role due to the sharp temperature gradient (shallow heat-affected zone) below craters and very high cooling rates [1]. As a next step, the microstructure and phase composition of the modified layers of Al-Cr composite cathodes with varying grain sizes were studied. The results from high-resolution analysis techniques revealed that metastable phases, including quasicrystalline phases, are formed during the solidification of arc craters and, hence, are the constituting phases of the modified layers. The average cooling rate in these rapid solidification processes was estimated to be in the order of 10⁶ K/s. Accordingly, to optimize the plasma properties for film and coating depositions it is necessary to consider non-equilibrium phases of the alloy system as they might be present on the modified surface. This means that the target’s or cathode’s microstructure and constituent phases need to be designed to enable the formation of phases with optimized cohesive energy during the rapid solidification of arc craters.

1. Golizadeh, M., et al., J. Appl. Phys., 2020. 127(11).

Due to economical demands to further increase the efficiency of production processes, it is essential to exploit the full potential of wear resistant hard coatings. TiN and AlTiN-based coatings are widely used as protective material for cutting tools, molds, and mechanical components in mechanical industries. Low chemical reactivity of these hard coatings with workpiece materials protects against sticking and thus reduces the adhesive wear. The most widespread wear resistant coatings are those with the following chemical formula Ti-X-(N,C and B) (X = Al, Cr, C, Si, and B etc.) which have proven to have excellent properties for industrial applications in the cutting, forming and stamping fields.

In this study, the coating design, mechanical property, high temperature oxidation behavior and cutting performance of multicomponent and multilayered AlTi(X)N coatings, which X= Cr, Si and B etc., will be discussed. These high performance coatings can be deposited by using cathodic-arc deposition with arc cathodes or unbalanced magnetron sputtering. Various cathode targets, such as Ti, Cr, TiAl, TiAlSi, CrAlSi, and AlSi, are used for the deposition. The microstructure of the as-deposited and high temperature annealed coatings was characterized by field emission scanning electron microscope (FESEM), high resolution transmission electron microscope (HRTEM) and X-ray diffraction (XRD) using Bragg-Brentano and glancing angle parallel beam geometries. The mechanical properties including hardness and elastic modulus of the coatings were analyzed by a nanoindenter with Berkovich indenter tip.

Depending on the coating design, the deposited AlTi(X)N coatings showed B1-NaCl crystal structure and have multiple orientations of (111), (200), and (220). The nanohardness, which measured by nanoindentation, of these coatings possessed hardness higher than 30 GPa, depending on the gradient and multilayered structures. The high temperature oxidation test showed the oxidation rate during annealing depends on film composition and microstructure. The oxide layer formed on the AlTiSiN coatings consists of large TiO₂ and AlTiSiN grains at the oxide-coating interface, followed by a layer of protective Al₂O₃ in the near-surface region. Interestingly, after oxidation, the AlTiBN coating contained an oxide layer composed of nanocrystalline Al₂O₃ and TiO₂. No crystallite growth or phase transformation occurred in the unoxidized AlTiBN coating part after oxidation. The gradient, multilayered, and nanocomposite AlTi(X)N show significantly improvement of the lifespan of cutting tools and mechanical parts.

Keywords: Hard coating; Mechanical property; AlTiN; Multicomponent; Multilayer