ICMCTF 2023 Session B1-2-ThA: PVD Coatings and Technologies II

Decorative coatings have been increasingly used since 1990’s in various applications and end products. Originally applied to door handles and sanitary hardware from the premier segment, their use has expanded towards mobile appliances and automotive interior and exterior parts. Nowadays, decorative PVD and PECVD finishes are widely spread in a broad range of consumer products covering almost the whole price range. This spread comes with continuously growing demand for wider range of colors from one side and improved or new functional properties from other side. Originally limited to variations of golden and metal colors, today’s market demands deep black, rose gold, purple, brown, blue and other colors. Next to the appearance, properties like scratch, corrosion and fingerprint resistance become a must. For many applications, which are in direct contact with the human body, antimicrobial properties are highly desired. In the automotive industry, aesthetics often needs to be combined to the possibility of back lighting or radar transparency. To meet all this, the individual benefits of different PVD and PECVD technologies need to be combined in a single process.

In this talk, we are presenting the current state of the contemporary decorative industrial coatings combining technologies such as UBM sputtering, HiPIMS, arc evaporation and (microwave) PECVD. The paper discusses color development, radar transparency, antibacterial properties and hydrophobic performance.

In the context of Ag-based low-emissive glazing developed by glass manufacturers, magnetron sputtering deposition is the industrially relevant technique to grow thin films of a wide range of materials, ranging from metals to dielectrics and semiconductors. Microstructural control of the active Ag layer used for infra-red reflection is a crucial issue, since its electrical conductivity ultimately drives the glazing thermal insulation efficiency. It is thus of great importance to better understand the link between deposition process parameters and the Ag film growth mechanism.

In this work, the effect of O₂ addition on the microstructure and the electrical and chemical properties of Ag nanometre-thick films grown on SiO₂ was explored using a custom sputtering deposition setup, by means of in situ X-ray photoelectron spectroscopy (XPS) and real-time measurements of electrical film resistance and Surface Differential Reflectance Spectroscopy (SDRS).

The impact of O₂ addition turned out to be divided into three regimes, depending on its relative amount in the incoming gas flux into the deposition chamber. In situ XPS analysis showed that mixtures of metallic Ag and Ag oxides (both surface oxides and bulk-like Ag₂O) were obtained in varying proportions depending on the O₂ flux used during deposition. In parallel, real-time electrical resistance measurements allowed for the detection of the percolation threshold thickness, in which a conductive Ag network is formed on the surface of the substrate. Such a threshold was lowered by increasing O₂ flux (Fig-A), but at the expense of final film resistivity. Finally, SDRS measurements were used to characterize the Ag nanoparticles formed during the initial stages of the 3D “Völmer-Weber” growth mechanism, namely nucleation, growth and coalescence. Information on the film morphology evolution were inferred from the plasmonic response using simulations from the Granfilm software, with the conclusion that O₂ addition leads to changes in nanoparticle aspect ratio, surface oxidation state and density (Fig-B). The latter was also confirmed by a statistical analysis of Transmission Electron Microscopy images for each condition.

Hard coatings are commonly used in industrial applications to prolong the service lifetime of cutting tools. Tribological property is one of the important properties that could be enhanced by hard and protective coatings, by which wear rate can be decreased and better protection to the tools is provided. For environmental protection, dry cutting is becoming a requirement in the manufacturing processes, in which high-speed cutting at elevated temperature will increase the wear rate of hard coating and the underlying tool. Molybdenum nitride (Mo₂N) is regarded as one of the promising coating materials for the tools applied at elevated temperature, because Mo₂N will form a Magnѐli phase with self-lubricating property at high temperature such that the coating can maintain good wear resistance. However, our previous study found that the Mo₂N coatings deposited on D2 steel by the high-power pulsed magnetron sputtering (HPPMS) was with insufficient adhesion strength that induced delamination during wear test. In this study, Mo interlayer with various thicknesses was introduced to enhance the adhesion of the Mo₂N coating. The objective of this study was to investigate the influence of Mo interlayer on the adhesion and tribological properties of Mo₂N coatings on D2 steel substrate. Mo₂N coatings with different thicknesses of Mo interlayer were respectively deposited by HPPMS and DC magnetron sputtering on D2 steel substrate. Compared with the traditional DC magnetron sputtering, HPPMS can achieve higher ionization rate and plasma density due to the high instantaneous peak power, where the adatoms can obtain higher energy and thereby producing coatings with better crystallinity. After deposition, X-ray diffraction was used to identify the crystal structure. The cross-sectional and surface morphology were determined using field emission scanning electron microscope microscopy. The compositions of the specimens were measured by an electron probe microanalyzer. The residual stress of both coating and interlayer was separately measured using the average X-ray strain method. The hardness of specimens was determined by nanoindentation. The adhesion strength of the coatings was evaluated by scratch test, and the wear rate was measured by pin-on-disk test at temperature ranging from room temperature to 600 °C. From the experimental results, the effects of Mo interlayer on the adhesion strength and the tribological properties of Mo₂N coatings on D2 steel will be discussed.

Complex coatings with compositional, mechanical, and structural gradients over a wide area are powerful in seeking perspective materials with specific combination of properties. Multielement CuWCrTi alloy is an auspicious candidate for such aim as it contains elements, which well differ in their mechanical properties and tend to form various phases with unique microstructure and properties, depending on the total amount of each element.

We have selected 13 different CuWCrTi alloy compositions in total as they formed on a static 3” Si wafer during magnetron co-sputtering deposition from Cu, W, Cr and Ti targets operated at an optimized discharge power ensuring homogenous thickness of the alloy over the entire wafer area and nearly equiatomic composition of the alloy at its center. Due to large compositional variations and limited miscibility of the elements, solid solutions, nanocomposites, and metallic glasses have been found to form, having unexpected combinations of hardness and elastic modulus. Even alloys composed primarily of elements exhibiting low hardness and elastic modulus such as Ti and Cu (e.g., Cu₅₁W₂₅Cr₈Ti₁₆)achieved high values of hardness ranging between 7 and 8.5 GPa.

High Entropy Alloy (HEA) Nitrides are an interesting material system intended for sophisticated wear and high temperature applications, due to its core effects, such as entropic stabilization of solid solutions, severe lattice distortion, sluggish diffusion kinetics and the cocktail effect. Such materials could be easily synthesized by means of Cathodic Arc Evaporation (CAE) in various gas atmospheres, leading to metallic or ceramic HEAs. In previous works high hardnesses up to 37 GPa could be obtained [1-3].
In this contribution new results on different HEA nitride coatings (HfNbTaTiVZr-N and AlCrTaTiZr-N) will be presented, deposited by means of DC Cathodic Arc Evaporation from compound metal targets. The presentation will give an overview on the dependency of the deposition parameters, such as the nitrogen partial pressures, nitrogen flow and bias potential on the evaporation behaviour of the targets and the (spatial) chemical composition, mechanical properties and the coating structure as well. Analysis were carried out by different methods, for instance SEM, EDS, nanoindentation, XRD or TEM. Furthermore, peculiarities of the evaporation process, such as the evaporation behaviour of the metal compound targets will be discussed.

REFERENCES:
[1] Kuczyk, Krülle, Schmidt, dos Santos Maximo, Kaspar, Zimmer, Zimmermann, Leyens: Deposition of nitride hard coatings based on high entropy alloys by arc PVD process and their characterization. Year Book Surface Technology 2020, publisher Eugen G. Leuze GmbH.
[2] Kuczyk, Zawischa, Leonhardt, Krülle, Zimmer, Kaspar, Leyens, Zimmermann: Analysis of the damage tolerance of high entropy alloy based nitride coatings. Tagung Werkstoffprüfung 2021.
[3] Kuczyk, Krülle, Zawischa, Kaspar, Zimmer, Leonhardt, Leyens, Zimmermann: Microstructure and mechanical properties of high entropy alloy nitride coatings deposited via direct current cathodic vacuum arc deposition; doi.org/10.1016/j.surfcoat.2022.128916.

Compositional modulations through the thickness are reported in a near-equimolar AlCrTaTiZr high-entropy nitride (HEN) and a Al₅₇Ti₃₅Ta₈ nitride coating. The coatings were synthesized by reactive cathodic arc deposition with two-fold substrate rotation from single powder metallurgical targets. High Angle Angular Dark Field (HAADF) Transmission Electron Microscopy (TEM) images of a Focused Ion Beam (FIB) lamella show a repeating, layer-like pattern with nanometer-scale periodicity. While evident as Z-contrast, Energy-dispersive X-ray spectroscopy (EDX) was used to quantify the pronounced, off-nominal fluctuations of the metallic element concentrations through the thickness. The periodicity and pattern of the modulations are governed by the combination of two known effects: the substrate’s hypotrochoidal motion, and the non-homogeneous distribution of the film-forming species in the plasma. Using a widely accepted model for the cathodic arc plasma plume, the radial and angular flux distributions for the light and heavy elements from the target were determined experimentally by depositing on stationary substrates, followed by EDX and ball-cratering. A general line-of-sight, flux-tracking program was developed to model a substrate undergoing two-fold or three-fold rotation from one or more multi-element sources. The experimental average deposition rate was used to scale the simulated flux to coating thickness and secondary effects such as occlusion, re-sputtering and sticking probabilities were considered. The simulated line intensity profiles of the coatings are compared to those extracted from the HAADF lamella with excellent agreement and the relative concentrations match the EDX data. In the HEN, changing the table rotational speed decreased coating hardness and toughness, implying these modulations may enhance properties not unlike the engineered nano-multilayer coatings synthesized with multiple targets. A coating designer should consider the phenomena described herein when employing substrate rotation when the target contains elements with a large disparity in atomic masses.

To modify the microstructures and enhance the mechanical properties of CoCrNi medium-entropy alloys films (MEAFs), multiple strengthening will be multiple strengthening methods will be introduced in this study. We will add rare-earth, neodymium, and metalloid, boron, elements into the MEAFs system to introduce precipitation and solid solution strengthening. CoCrNiNd_xB_yMEAFs will be deposited onto silicon wafers using DC/RF magnetron three-target co-sputtering of CoCrNi, B and Nd targets. Nd and B concentrations in CoCrNi-based films were controlled by various DC/RF sputtering powers. The effects of multiple dopants on the phases, microstructures, and mechanical properties on these MEAFs will be investigated systematically. The influences of Nd and B contents on the microstructure of MEAFs were studied by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Meanwhile, mechanical properties were examined by a nanoindentation. Based on present results, when Nd and B concentrations was increased to approximately 0.5 and 1.0 at.%, respectively, hardness increased to 10.8 GPa from 9.0 GPa of CoCrNi MEAFs. A discussion on the phase structure and strengthening mechanisms will be given further in this study based on the experimental results.