ICMCTF 2025 Session IA2-1-MoA: Surface Modification of Components in Automotive, Aerospace and Manufacturing Applications I

There is currently an interest in the synthesis and applications of mnemonic structure materials for various applications, such as biomedical, aerospace, automotive, and robotics. Shape memory alloys (SMAs) are metallic materials that can return to their initial state after being subjected to deformation as a result of increased temperature, increased pressure, or other stress conditions. These materials have been used in thermoelastic actuators in space applications, such as antenna supports and solar panel deployments, and can impact the manufacture of polymorphic aircraft engines and fuselages. SMAs, such as Nitinol (equimolar alloy of Ni and Ti), are difficult to fabricate, and the powder metallurgy route, whether classical or using laser powder bed fusion (L-PBF), has been constantly improved to meet new application niches. This study proposes reactive sintering of samples with equiatomic compositions of Ni and Ti to form Nitinol using laser surface remelting. Elementary powders were inserted into a high-energy ball mill to fabricate mechanically alloyed Ni-Ti powders, which were subsequently pressed in the form of discs (20 mm diameter, 5 mm thickness). An experimental arrangement with a vacuum chamber and fiber laser beam manipulation was developed to induce sufficient heat for the reaction of elementary powders. Spiral scanning of the fiber laser beam produced surface remelting that ignited the pressed powder mixture. Macro- and microstructural analyses, the crystalline structure, and the composition of the sample surface remelted with a beam power ranging from 10 to 46 W were performed. In this power range, the sintering time was varied between 55 and 295 s when the laser power was varied from 46 W to 10 W. Although the presence of intermetallic phases was approximately the same, the microstructure in the laser-surface-remelted region was more homogeneous than that in the sintered volume. At the end of sintering, tablets were obtained with an apparent density of 61%–67% and a large number of intermetallic phases, such as NiTi₂, Ni₃Ti, and Ni₄Ti₃, together with unreacted elemental powders (Ni and Ti). The samples prepared in air also presented these phases in addition to TiO₂ and NiTi. The air-processed samples presented an equimolar Nitinol phase, as observed by X-ray diffractometry. According to mass spectrometry analyses of secondary ions, the presence of air oxidized the surface of the grains, which reacted at shorter distances and generated the Nitinol phase.

Magnesium-aluminum-calcium (Mg−Al−Ca) alloys have attracted significant attention due to their excellent strength-to-weight ratio, good castability, and potential for flame retardancy, owing to the presence of calcium and aluminum. However, the applications of these alloys are limited by their poor corrosion resistance. Micro-arc oxidation (MAO) is one of the most common techniques for corrosion protection of magnesium alloys; however, the formation mechanism of MAO coatings is extremely complex and influenced by numerous process parameters, including the substrate effect. In this study, the mechanism of MAO coating formation on the AZ31 and dual-phase AC84 (Mg−8Al−4Ca) alloys was comparatively examined. The preliminary results reveal that, during MAO treatment at a constant anodizing voltage of 150V, unreacted Al₂Ca secondary phases were observed in the micro-arc oxidation coatings of AC84 magnesium alloys, causing non-uniform surface structures and thicknesses, which led to poor corrosion resistance compared to the MAO coating formed on AZ31.Conversely, AC84 exhibited better corrosion resistance than AZ31 when the voltage was increased to 250V. Further increasing the voltage to 300V resulted in the involvement of secondary phases in the reaction, leading to more uniform microstructures and chemical compositions of the coatings on both alloys. These findings suggest that the anodizing voltage plays a crucial role in the reaction behavior of secondary phases and the properties of the MAO coatings.

We will discuss a recently built state-of-the-art ultra-high-vacuum (UHV) test system with high accuracy and precision to quantify hydrogen uptake, solubility and diffusion in various materials. The design was developed by Pacific Northwest National Laboratory (PNNL) in collaboration with Vacuum Technology Inc (VTI). This automated UHV system can be used for several studies of hydrogen-metal interactions including absorption/desorption kinetics, thermodynamics, isotherms, plateau pressures, isotope studies, gaseous impurity identification and permeation rate. We will present recent permeation rate data of ceramics coatings to reduce permeation through stainless steel.

The PVD (Physical Vapor Deposition) market is rapidly expanding into new application fields. To achieve these new applications, various PVD coating techniques are employed, with HIPIMS (High Power Impulse Magnetron Sputtering) being one of the most fascinating since its discovery. Over the past 25 years, numerous advancements have been made in latest Generation 3 - HIPIMS power supply technology, including modifications in bipolar mode, pulse shape, pulse length, pulse trains, and higher frequencies. Synchronization of cathodes and HIPIMS-based bias has also led to innovative PVD coating solutions.

Beyond the well-known performance improvements in HIPIMS-coated cutting tools, HIPIMS has demonstrated its potential for various other applications. We will showcase the ability to create different colors using HIPIMS technology and highlight its advantages for decorative applications on 3D products. For components, HIPIMS is an excellent tool for enhancing the wear and corrosion resistance of existing material systems. We will also present the latest coating development for cutting tools. Our presentation will illustrate how combining HIPIMS with new material systems can further expand and enhance potential application areas.

Decorative, tool and tribological markets are driven by production costs, making coating volume and size crucial factors. To meet these demands, we have scaled up our HIPIMS developments to deposit coatings on our largest industrial platforms, e.g. the Flexicoat 1500 to address market needs.

In cross-country skiing, reducing the friction coefficient between the skis and snow is essential for sportive performance [1]. Fluorinated waxes, i.e. containing perfluoroalkyl (PFA) are known for their hydrophobic properties and were remarkably efficient in wet snow conditions. However, the International Ski and Snowboard Federation (FIS) has banned fluorinated wax since winter 2023/2024 -for health and environmental reasons [2]. Since then, no alternative with equivalent performance has been found. This project aims to develop hard and hydrophobic coatings based on titanium nitride (TiN), aluminum nitride (AlN) and alumina (Al₂O₃) materials directly deposited on ski bases and UHMW polyethylene. The role of coating surface properties and structure in friction is investigated

Thin films were deposited using DC and RF magnetron sputtering. The surface (contact angle, roughness, chemical composition), mechanical and thermal properties of the coatings were investigated. Friction coefficient of coated samples was evaluated on snow with a linear tribometer (speed: 0.1 m/s, displacement: 130 mm, contact pressure: 50 kPa). The tribo-system is therefore a 10cm-long coated ski sliding on controlled man-made snow in a cold-room at 0°C and dry air. Snow with different liquid water content were used for the tests.

Results are encouraging as deposition on ski base is feasible at ambient temperature with adhesive and dense coatings. Coating thicknesses were evaluated by scanning electronic microscopy between 50 and 200 nm depending on process parameters. Chemical analysis with XPS indicates nitride films contain a relative high amount of carbon and oxygen. Coatings, selected for their hydrophobicity and structural properties, were investigated in gliding tests. AlN and Al₂O₃-based coatings presented very high friction coefficient (0.2-0.3). TiN-based coating had the lower friction coefficient with a value of 0.11 on very wet snow, whereas a ski waxed with PFA friction coefficient was measured at 0.072.

To sum up, deposition of sputtered coatings was realized with success and may be a promising technique for preparing competition skis. For winter sport application, titanium nitride seems to be the most promising: it is indeed known for better mechanical properties [3] and lower thermal conductivity [4] which will be further investigated.

References:

[1] Moxnes, J. F. et al, J Sports Med, 4, 127-139 (2013).

[2] Freberg, B. I. et al, Environ Sc & Techno, 44, 7723–7728 (2010).

[3] Glocker, D. A. et al, Bristol, UK: Inst of Phys (1995).

[4] Moraes V. et al, J Appl Phys, 119, 225304 (2016).

The atmospheric corrosive substances like Cl^- ions or SO₃^2- ions make it easier for soluble zinc salts to form. Consequently, the introduction of more magnesium into the Zn-Al alloy bath enhances the formation of basic zinc salts that are susceptible to environmental corrosion. Zinc-aluminum carbonate hydroxide and aluminum-magnesium carbonate hydroxide are particularly notable corrosion products because of their stable and compact characteristics. Local acidity influences the rate of transformation of zinc hydroxide from zinc oxide, while the formation of Mg(OH)₂ mitigates the surface reduction of the galvanizing coating.

Although the Zn-5Al-2Mg coating is effectively produced using the continuous galvanizing process, the maximum coating thickness achieved is 20 μm. Numerous studies present their experimental findings about batch galvanizing zinc alloys. The coating structure is nonuniform, with an enrichment of iron content resulting from the elevated operating temperature, which inhibits the formation of a dense and continuous layer of corrosion products. Consequently, two-step batch galvanizing is utilized to provide a zinc alloy coating of adequate thickness. The initial process involves immersing samples in pure zinc, followed by immersion in zinc alloy. The coating's microstructure consists of an outer layer formed from zinc alloy and internal layers including an iron-aluminum intermetallic compound combined with a eutectic phase.