ICMCTF 2025 Session MA4-3-WeA: High Entropy and Other Multi-principal-element Materials III

The hohlraum is a centimeter-scale sphero-cylindrical cannister used as the housing for a hydrogen fuel capsule in an indirect-drive inertial confinement fusion (ICF) target. The hohlraum is a critical component in increasing the ICF energy yield. Our simulations with the radiation hydrodynamics code LASNEX suggest that the fusion yield can be improved by using hohlraums made of Ta-W-Au-Bi high entropy alloys (HEAs). However, the magnetron sputter deposition of these HEAs with low porosity and submicron grains remains a challenge. Here, we examine how tailoring the main deposition process parameters, including the average plasma discharge power, working pressure, substrate bias, target-to-substrate distance, and substrate temperature, can be leveraged to enable the fabrication of Ta-W-Au-Bi films with a dense microstructure and high electrical resistivity, thus providing a promising path forward for the development of next-generation ICF targets.

This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344 and was supported by the LLNL-LDRD Program under Project No. 23-ERD-005.

Increasing aircraft engine temperature is one method, amongst others, to decarbonize aviation. But at high temperature, e.g., 1200 °C, metallic materials performances are drastically decreased due to the effect of hot corrosion. To limit this impact, metallic materials need to be protected with dedicated coatings with adequate properties, which Entropy-augmented ceramics could feature.

However, the composition space of complex ceramics is very wide, and comparatively very few bibliographical data are available as these specific ceramics have not been widely studied to date. While the use of a data-driven screening tools to identify relevant compositions appears necessary, it is not sufficient as (1) it requires data to be trained on, and (2) final properties should be experimentally assessed.

Due to considered temperatures, coatings based on refractory elements such as Zirconium (Zr), Niobium (Nb), Molybdenum (Mo), Hafnium (Hf) ; Tantalum (Ta), Tungsten (W), Ruthenium (Ru) or Rhenium (Re), combined with Carbon (C), Nitrogen (N) or Boron (B), are credible potential candidates [1]. Cheaper and more abundant elements, as Iron (Fe) and Aluminium (Al), could also be considered in the mix to comply with industrial and environmental constraints.

High Entropy Alloys (HEA) (or Complex Concentrated Alloys (CCA) for their multiphase counterparts) are single-phase multielementary alloys showing original combination of properties (chemical resistance, mechanical resistance…) over a wide temperature range. The relatively new paradigm of HEA design, translated into the space of ceramics, offers new opportunities to meet high temperature requirements [2].

Two main challenges have to be overcome: achieving a single solid solution films to guarantee both material and property homogeneity throughout the coatings, and assessing the long term mechanical and environmental stability of the materials.

In this talk, we will highlight our methodology to combine numerical and experimental studies. First results about binary and ternary nitrides will be shown, together with the prospective work to come.

[1] W. G. Fahrenholtz, « A Historical Perspective on Research Related to Ultra-High Temperature Ceramics », in Ultra-High Temperature Ceramics, John Wiley & Sons, Ltd, 2014, p. 6–32. doi: 10.1002/9781118700853.ch2

MoNbTaW is well known for its refractory high entropy properties which can maintain the same crystal structure even at very high temperatures without losing its mechanical properties. Nitrides of such (MoNbTaW)N will be a prime focus to get a hard and tough, mechanically stable high temperature withstanding coatings at room temperature.

In this study, (MoNbTaW)N hard coatings were deposited using a DC magnetron sputtering technique at a working pressure of 0.3Pa by varying substrate bias voltage from 0V to -200V. Optimized deposition parameters, including nitrogen flow and substrate temperature (400^oC), were employed to produce dense and homogenous coatings on Silicon (100) substrates. X-Ray diffraction studies revealed that all the deposited films have Face Centered Cubic (FCC) crystal structure. A significant decrease in intensity ratio of principal reflection peak (111) to (200), from 2.39 to 0.84, is observed with increasing bias voltage from 0V to -200V. AFM studies indicated all the films have a fine granular morphology, with a maximum film thickness of 636nm at 0V, reducing to 550nm as the bias voltage is increased.

Topological analysis demonstrated that higher bias voltage led to smoother coatings, achieving an RMS roughness of < 2nm. XPS studies revealed that the covalency due to the increased bonding of p(N)-d(TM) with the increase in bias voltage. Nanoindentation studies confirmed a maximum hardness of 32 ± 2 GPa and a modulus of 345 ± 18 GPa at -200V bias. Additionally, the coatings displayed improved toughness, with the highest H/E value of 0.09 achieved at -200V.

Equimolar medium-entropy alloy VCrCoNi thin films were deposited on tool steel substrates by way of unbalanced magnetron sputtering, under different substrate bias voltages ranging from -20V to -120V. The deposited films were typically ~5.4 um thick. Variations in chemical composition as a function of bias voltage were observed, showing fluctuations in the concentrations of V, Ni, and Cr, while Co remained constant. These compositional variations arose from the interaction between the sputtered metal cations and the kinetic energy differences of the adatoms induced by changes in bias voltage. The thin films exhibited strong crystallographic textures and a microstructure characterized by ultrafine (< 5 nm) equiaxed grains. Changes in phase composition were also observed with variations in bias voltage. Hardness values ranged from 11 GPa to 14 GPa, peaking at -100 V bias. Additionally, scratch resistance and wear performance were examined, revealing correlations between microstructural characteristics and tribological behaviour.