AVS 70 Session SE-MoA: Surface Engineering Solutions for Sustainable Development

The sustainable development goal (SDG) 12 'Responsible Consumption and Production' of the United Nations deals with the necessary change of consumption and production patterns towards a more sustainable future. Plasma-assisted physical vapor deposition (PVD) is increasingly employed to address global challenges, such as energy efficiency and reduction of CO₂ emissions. Two important questions are critically evaluated in this context:

1) How sustainable are state-of-the-art PVD processes and materials?

2) Which pathways are needed for future responsible surface engineering?

While our modern world and human life benefit from surface engineering, the consideration of energy and mass balances demonstrates that state-of-the-art PVD processes and products are not necessarily sustainable, leaving space for innovation. Responsible surface engineering comprises pathways to enhance the sustainability of processes as well as materials. Impurities will be discussed with respect to tolerable levels and even exploitation of 'impurities', which can be beneficial for the performance of a coating. Moreover, the microstructural design of chemically simple coatings offers opportunities to avoid economically and ecologically expensive elements. Prospective product cycles of emerging technologies and future products will enable the evaluation of ecological, economical as well as societal costs and benefits. Finally, responsible surface engineering involves a change in mindset of materials scientists, process engineers and of all stakeholders involved in garnering innovation.

In the past, we have studied the system (Hf,Ta,Ti,V,Zr)B-N with exceptional hardness and thermal stability, but the coatings contained a significant amount of C impurities, which may have influenced the properties [1]. To investigate the impact of C in this system, we have deposited new coatings with a TiN target, on which we placed diboride and/or carbide pieces of the metals Hf, Ta, V, and Zr. We have varied the composition by using either only diborides, only carbides, or different mixtures of the two material types to make 5 coatings containing either N and B, N and C, or all three. The B concentration varies between 42 and 0 at%, the C content between 25 and 0 at%, and the N content is stable at around 30 at% in all coatings. The Ti makes up roughly 20 at%, while the other metals are in the range between 2 and 5 at%. X-ray diffraction (XRD) shows a weakly textured single-phase fcc solid solution in all coatings. The FWHM of the 200 reflex ranges from 2 ° in the C-free coating down to 0.5 ° in the B-free coating, indicating different grain sizes. This is confirmed by transmission electron microscopy, revealing fine columnar growth in the 2.3 to 3.2 µm thick coatings, with especially fine grains in the B-richer coatings. Electron diffraction confirms that no secondary phases are present. We annealed the coatings in a vacuum furnace at 1000, 1200, and 1400 °C for 10 min, followed by XRD and nanoindentation. The coatings stay stable up to 1200 °C and start decomposing at 1400 °C. The as-deposited hardness of all coatings lies between 36 and 38 GPa, and is maintained after annealing at 1000 °C. After annealing at 1200 °C, the coatings containing only C or only B both soften to ~34 GPa, while the coatings with both C and B do not lose any hardness at this temperature. Only after annealing at 1400 °C does the hardness of all coatings drop below 30 GPa. We confirmed the thermal stability by atom probe tomography on the most promising sample, hereby we show that despite the high B content of 22 at%, decomposition into a diboride phase initiates only after annealing at 1400 °C. We further investigated the fracture-toughness by in-situ micromechanical cantilever bending. The best performing coating yields 4.0±0.5 MPa*m^(1/2), thus surpassing other similar coatings not only in hardness and thermal stability, but also fracture toughness.

[1] Kretschmer, A., et al. (2022). Materials & Design, 218, 110695. https://doi.org/10.1016/j.matdes.2022.110695

The oxidation behavior of stoichiometric Ti_0.12Al_0.21B_0.67 coatings is investigated by high resolution Transmission Electron Microscopy (TEM) after oxidizing for 1, 4 and 8 h at 700 °C and at 800 and 900 °C.

In the as deposited state, a ~ 4 nm thick, native, amorphous oxide layer covers the surface of the coating while the magnitude of incorporated O along the column boundaries decreases with depth. During oxidation, the formation of scale layers consisting predominantly of Al, O and B is observed that appear to be amorphous at 700 °C in which, after oxidation at 900 °C for 8h, nanocrystalline Al₅(BO₃)O₆ regions form. Concurrently, underneath the scale, the formation of Al- and Ti-rich boride regions, consistent with spinodal decomposition, is observed. Chemical environment dependent DFT predictions of the energies required for mass transport on the metal sublattice indicate that that Al diffusion is initiated before Ti. Hence, as the temperature is increased, the migration of Al is initiated first, leading to the formation of the oxide scale observed already after oxidation at 700 °C after 1 h. Below the oxidized region, the formation of Al-rich and Ti-rich regions by spinodal decomposition require the concurrent migration of Al and Ti. The fact that decomposition takes place at 900 °C and hence at larger temperatures than the Al diffusion mediate scale formation is consistent with DFT predictions as the averages of the predicted energies required for both, vacancy formation and migration for Ti are larger than for Al.

Friction and wear-related failures remain the greatest problems in moving mechanical assemblies operating under various conditions. This study demonstrate lubricity achieved by spray-coating solution-processed multilayer Ti3C2Tx-MoS2 blends onto rough 52100-grade steel surfaces. Blends exhibited lower frictional performance for individual pristine materials, MoS₂ and Ti₃C₂T_x, under high pressure, sliding speed. Study investigated the processing, structure, and property correlation to gain a deeper understanding of the underlying phenomena. Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy results revealed the formation of an in-situ robust tribolayer responsible for the outstanding performance observed at high contact pressures and sliding speeds. This study has broad implications for the development of solid lubricants that can operate under extreme conditions and low viscosity fuel environment, inspiring further research and development in this field.

Binary alloy systems such as Pt-Au have been shown to exhibit ultra-low wear and high hardness enabled by its intrinsically thermally stable nanocrystalline microstructure making them suitable candidates for electrical contact materials. When the unique mechanical properties are combined with the catalytic behavior of platinum under cyclic shearing in inert environments, adventitious carbon can be mechano-chemically transformed into lubricious surface films (µ < 0.05). While ideal for solid lubrication applications, carbon deposits can inhibit electrified interfaces and compromise end-use performance. By balancing the catalytic and mechanical properties of Pt-Au thin films, the friction behavior and surface film morphology can be controlled to either promote/prevent lubricious carbon film formation.In this work, we explore 448 compositions of sputter deposited Pt and Au thin films spanning 0-100 at% and the intertwined role of hardness and composition on the resulting friction and wear behavior. We highlight the use of custom high-throughput tribological platforms with robotic automation to rapidly test large material spaces and for the creation of in-depth structure-property relationships that will play a crucial role in the development of next-generation tribological coatings. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Liquid metals (LMs) are of interest for the development of several applications related to the energy sector. However, the presence of LMs creates concerns about the compatibility with standard structural materials since, generally, LMs affects their integrity and mechanical properties through different phenomena [1]. Therefore, the mitigation of LM corrosion emerges as a crucial step. This especially applies if the aim is the integration of LMs in a nuclear energy system which, being characterized by high temperatures and strong radiation fields, introduces constraints on the choice of possible structural materials.

The deposition of nanocrystalline or metallic-glass protective coating is an appealing approach to engineer the structural material surface and, consequently, to control the interaction with LMs. In this respect, the fine tailoring of the coating properties at the nanoscale represents a key aspect to optimize its performances in the harsh LM environment. To this end, the use of an of advanced Physical Vapor Deposition methods such as High Power Impulse Magnetron Sputtering (HiPIMS) [2] can play a key role to produce films with controlled properties.

In this framework, a notable example is represented by the magnetic confinement fusion research area where innovative divertor designs based on liquid tin (Sn) are under investigation [3,4]. Nevertheless, the action of liquid Sn in detrimental for many structural materials typically used in fusion systems. Specifically, liquid Sn heavily corrodes the heath-sink components which are constituted by CuCrZr alloy to satisfy the thermomechanical and cooling requirements imposed by the fusion power flux.

Prof. Yeh has been developing high entropy alloys (HEAs) for twenty years.The research on bulk HEA materials has attracted much attention due to their unique properties being better than those of traditional alloys. On the other hand, the HEA coatings fabricated by magnetron sputtering methods have been ensured for improving substrate materials' mechanical properties, corrosion resistance, oxidation resistance, and wear resistance. In this work, several strengthening mechanisms, including nitridation, carburization, solid solution hardening, and grain refinement,were adopted to study their effects on the mechanical properties improvements of VNbMoTaW, VNbMoTaWAl, VNbMoTaWCr, TiZrNbTaFe, TiZrNbTaFeB, and ZrTiNbSiFe high entropy alloy coatings grown by magnetron sputtering technique. The chemical compositions, phase structures, microstructures, and surface roughness of these HEA coatings were examined. The nanohardness, reduced elastic modulus, and wear resistance of HEA coatings were measured by nanoindenter and pin-on-disk wear tester, respectively.

We can conclude that good mechanical properties, including higher hardness and lower wear rate, can be obtained for those HEA coatings through the proper selection of strengthening mechanisms and the addition of several constituents, such as nitrogen, carbon, TiB₂, and Cr elements. This work evaluated some strengthening mechanisms and promising results of HEA coatings that can be used as protective coatings in harsh environments.

Glass is an indispensable part of display technologies, and knowing how to incorporate multiple useful properties into a single optical surface will enhance their performance. For example, nature-inspired designs can deliver high transparency and clarity, broadband and omnidirectional optical response, self-cleaning capabilities, and mechanical resistance. However, fabricating these surfaces with the desired properties can be complex, often requiring multistep lithography methods, which are costly and not easy to scale. In this talk, we discuss our team’s recent developments in utilizing thermal dewetting of ultrathin metal films (Cu, Ag, Ni) as a lithography-free method to create durable, nanostructured optical surfaces with tailored multifunctional features.

In the first part, we demonstrate a transparent anti-microbial coating on glass surface based on dewetted copper (Cu) nanoparticles encapsulated by conformal SiO₂ and fluorosilane functional layers. The coatings kill more than 99.9% of Staphylococcus aureus within 2 hours due to the released Cu ions and can maintain their anti-microbial properties after wiping tests. The relatively flat transmission of 70-80% in the 380-750 nm range along with color neutrality and non-conductivity make them suitable for high-touch surfaces in medical and public settings where hygiene is important.

Dopants and alloying elements are commonly introduced in amorphous carbon (a-C) materials to tailor their mechanical and tribological properties. While most published studies have focused on doping and/or alloying a-C coatings with metals or metalloids, doping a-C films with rare-earth elements has only recently been explored. Notably, our understanding of the shear-induced structural changes occurring in rare-earth element-containing a-C films is still elusive, even in the absence of any liquid lubricants. Here, the friction response of Eu- and Gd-containing a-C films with low hydrogen content deposited by high-power impulse magnetron sputtering (HiPIMS) on silicon was evaluated in open air and at room temperature. The load-dependent friction measurements indicated that the introduction of Gd ((2.3 ± 0.1) at.%) and Eu ((2.4 ± 0.1) at.%) into the a-C matrix results in a significant reduction of the shear strength of the sliding interfaces ((41 ± 2) MPa for a-C, (16 ± 1) MPa for a-C:Gd2.3 at.%, and (11 ± 2) MPa for a-C:Eu2.4 at.%.). Near-edge X-ray absorption fine structure (NEXAFS) spectromicroscopy experiments provided evidence that no stress-assisted sp3-to-sp2 rehybridization of carbon atoms was induced by the sliding process in the near-surface region of undoped a-C, while the amount of sp2-bonded carbon progressively increased in a-C:Gd2.3 at.% and a-C:Eu2.4 at.% upon increasing the applied normal load in tribological tests. The formation of an sp2-bonded carbon-rich surface layer in a-C:Gd2.3 at.% and a-C:Eu2.4 at.% films was not only proposed to be the origin for the reduced duration of the running-in period in tribological tests, but was also postulated to induce shear localization within the sp2-carbon rich layer and the transfer film formed on the countersurface, thus decreasing the interfacial shear strength. These findings open the path for the use of Gd- and Eu-containing a-C even under critical conditions for nearly hydrogen-free a-C films (i.e., humid air).

As aircraft engine operating temperatures increase, so must the thermal insulation capabilities of the thermal barrier coatings (TBCs) used to shield metallic components in the combustion chamber and high-temperature turbine areas. Heat transfer from the hot gases to the engine components occurs through two main mechanisms: conduction and radiation. Considerable efforts have been deployed over the years to ensure TBCs have low thermal conductivity, thanks to a porous microstructure generally achieved by thermal spray or EB-PVD techniques. The radiative component of heat transfer, however, has been largely ignored in TBC design. This aspect is however very important due to an exponential increase in radiative heat transfer for higher gas temperature. While TBCs are naturally reflective of radiative heat due to light scattering, this property is very vulnerable to degradation.

The application of protective organic coatings is one of the most effective and commonly used corrosion mitigation strategies. To maintain the protective nature of coatings on long-term-exposed surfaces such as steel bridge components, coatings must be periodically removed and reapplied. A relatively new method called laser ablation coating removal (LACR), which incorporates a high energy nanosecond pulsed laser beam (1 kW, 1064 nm, Nd:YAG) in combination with a high efficiency filtration system, allows for safe and effective removal of coatings and contamination from metal surfaces and provides excellent re-coating adhesion.

LACR cleaning has been applied to legacy bridge steel components to investigate its effect on substrate cleanliness, adhesion, and steel mechanical properties. Legacy bridges were never grit blasted and contain a 20–100 μm thick mill-scale layer (Fe₃O₄, Fe₂O₃, FeO) below several coating layers (including lead-rich coatings). The surface oxide layer thermally insulates the underlying steel, preventing melting of the metallic substrate. Characterization of the LACR surfaces with electron microscopy, hardness, tensile, and fatigue testing has shown that LACR does not cause any measurable detrimental effects to the bulk mechanical properties of the bridge steel.