ICMCTF 2026 Session MA1-2-MoA: Coatings for High Temperatures and Harsh Environment Applications II

Ti_1-xAl_xN is one of the most used coating materials applied in various applications, including i.e. machining and forming tools but also components, due to its excellent thermomechanical properties. However, as operating temperatures rise, new strategies are needed. Alloying Ti_1-xAl_xN with Ta or Si shows promise. In more detail, the incorporation of Ta shifts the onset of the spinodal decomposition towards higher temperatures. Furthermore, the formation of the unfavoured anatase phase during oxidation is suppressed and the rutile phase is stabilised [1,2]. Adding Si enhances the thermal stability and oxidation resistance, while concomitantly leading to the formation of a nanocomposite microstructure [3]. Recent research explores combined alloying with Ta and Si to improve both oxidation resistance and mechanical performance. [4,5]. Compared with Ti_1-xAl_xN coatings – which fully oxidize at 1000 °C (15 h, synthetic air) – Ti_1-x-y-zAl_xTa_ySi_zN thin films form oxide scales below 1 μm [5].
Furthermore, Ti_1-x-y-zAl_xTa_ySi_zN coatings exhibit excellent mechanical properties, making them promising candidates for high temperature applications [4].

The present study investigates a series of Ti_1-x-y-zAl_xTa_ySi_zN coatings deposited by cathodic arc evaporation using an industrial-scale Oerlikon Balzers INNOVA 1.0 system. Two distinct target compositions were utilised, along with varying deposition parameters. Long-term oxidation experiments were conducted in a conventional furnace at temperatures of 850 °C for durations of 24 h up to 500 h. After the oxidation processes, we conducted an analysis using X-ray diffraction (XRD), focused ion beam (FIB) techniques, and transmission electron microscopy (TEM). Additionally, we performed low-temperature hot corrosion (LTHC) experiments at 700 °C with a hot gas corrosion testing rig, varying the concentrations of SO₂.
In summary, the study demonstrates that Ti_1-x-y-zAl_xTa_ySi_zN coatings exhibit extremely low oxidation kinetics, suggesting long-term stability well beyond 500 hours, alongside excellent resistance to hot gas corrosion environments. Based on these results, cathodic arc-evaporated Ti-Al-N-based coating materials are also promising candidates for protective applications in the aviation and power generation sectors.

[1] R. Hollerweger et al., Surf. Coat. Technol. 257 (2014) 78–86.
[2] C.M. Koller et al., Surf. Coat. Technol. 259 (2014) 599–607.
[3] Z.R. Liu et al., J. Alloys Compd. 917 (2022) 165483.
[4] A.R. Shugurov et al., Vacuum. 216 (2023) 112422.
[5] X. Sun et al., Surf. Coat. Technol. 461 (2023) 129428.

Research into accident-tolerant fuels (ATFs) for light water reactors (LWRs) has focused on improving the safety of zirconium alloy fuel rod claddings and one of the more developed approaches is the use of chromium coatings deposited onto the claddings. In addition to performing in oxidising conditions, normal operation also causes fretting wear on the fuel rod surface, which requires tribological improvements.

The aim of this work, therefore, is to produce Cr coatings using the magnetron sputtering technique for Zr alloy nuclear fuel rod cladding material to enhance oxidation and mechanical resistance. The coatings were characterised, as a function of deposition conditions, in terms of their morphology, topography, hardness, reciprocating and fretting wear resistance, scratch test performance and oxidation resistance in autoclave and air oxidation tests.All the coatings provided excellent oxidation protection, in comparison to the uncoated samples. Mechanical testing indicated contrasting results with coatings with higher hardness showing enhanced wear protection, but lower coating hardnesses provided better scratch test performance and reduced fretting wear.

Scale up of these experiments has progressed from small flat coupons, through short (<20cm) rods, to full length (4m) fuel rods.

This study elucidates how second phases, particularly the β-Al-Ca intermetallic, influence the behavior and corrosion performance of the non-flammable Mg–8Al–4Ca alloy during cerium conversion coating and micro-arc oxidation (MAO) treatments. Scanning Kelvin probe force microscopy (SKPFM) and transmission electron microscopy (TEM) analyses reveal that the β phase exhibits a lower Volta potential and higher electrochemical activity than the α-Mg matrix, serving as a micro-galvanic anode that accelerates localized corrosion and hydrogen evolution. During the cerium conversion process, this activity disrupts film uniformity; however, a simple deionized-water pretreatment dissolves the exposed β phase into Al(OH)₃, promoting homogeneous Ce deposition with enhanced coating integrity and corrosion resistance. In MAO processing, the distribution and conductivity of β phases strongly affect discharge behavior and coating development. At low voltages, localized discharges near β regions lead to thinner and non-uniform films, while higher voltages facilitate the formation of Mg–Ca-rich silicate/oxide phases that improve corrosion resistance but hinder further thickening. Selective removal of surface β phases prior to MAO yields thicker and more uniform coatings. These findings clarify the mechanistic link between second-phase characteristics and coating evolution, providing effective strategies to engineer durable protection for non-flammable magnesium alloys.

The plasma spray process uses a high-temperature plasma jet to melt and propel coating materials onto a substrate to form a dense and durable coating in a short time, which has been widely applied in many industrial applications. The focus of this study is to examine the influence of plasma spray parameters, including current, distance, powder feed rate and substrate rotation, on thickness, corrosion resistance, hardness and porosity on the S45C substrate. Design of experiments (DOE) by the Taguchi method was employed to optimize four factors for plasma spray coating of 316LSS to enhance the mechanical properties. Additionally, optimal parameters were also recognized to improve thickness, corrosion resistance, hardness and porosity based on the Taguchi method and analysis of variance (ANOVA). Overall, the more influential key parameters are plasma current of 700 A, powder feed rate of 28 g/min, and the current is a more contributing factor for output response in the analysis of variance. The confirmation experiment was also conducted to verify the results of the ANOVA analysis.

The development of highly efficient and economical steel components in plant and process engineering is crucial for reducing CO2 emissions. To withstand the high combined corrosive, tribological, thermal, and mechanical stresses, wear-resistant coatings tailored to the application and steel grade are employed. The increasing demand to substitute conventional cobalt alloys with nickel alloys, coupled with the need for defined or functional surfaces of high integrity, necessitates the development of novel wear-resistant coatings.

The use of wear-resistant coatings is essential for highly efficient and economical steel components in equipment, process, and power plant engineering. Co-alloys are commonly used as wear-resistant coatings for steel components, tailored to the specific application. The substitutability of Co alloys with Ni-based wear protection systems, in addition to price and supply uncertainties, is facilitated by the combination of innovative welding and machining processes such as ultrasonic-assisted milling.

This study investigates the influence of the microstructure and precipitation morphology adjusted by means of alloy modification on the machinability of wear-resistant plasma cladded coatings. The wear protection alloy NiCrMoSiFeB (trade name: Colmonoy 56 PTA), typically used for screw machines, is employed as a model system. Metallurgical investigations and in-situ measurements of occurring process forces and temperatures at the tool cutting edge during milling, as well as subsequent investigations of tool wear and surface integrity, allow for a detailed analysis and correlation between microstructural properties and machinability.

The primary objective of this study is the statistical correlation between specific microstructural features, like precipitation size, shape and amount with the characteristic process forces of conventional and ultrasonic assisted milling of the claddings.

The addition of Al, Ti, or Nb to the cast samples results in a clear change in the microstructure, hardness and machinability. Al and Ti cause long-needled or star-shaped precipitations and hardness increases, which lead to higher cutting forces and increased tool wear. In the case of the modified alloys, the inclusion of the alloying element Nb results in the formation of a more refined hard phase and reduces the machining force required for C56.

In most cases, the wear resistance potential has been maintained. The statistical model allows to adjust the chemical composition to a better machinability of the hard facings.

Increasing aircraft engine temperature is one method, amongst others, to decarbonize aviation. But at high temperature, 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 data-driven approaches 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.

Nitride coatings obtained by PVD methods have been used in the machine tool and aerospace industries for many years. Binary nitride systems exhibit mechanical properties such as high hardness (20-25 GPa [1]). The addition of transition metals, such as Al or Si, improves physical and chemical properties like wear resistance, thermal stability and oxidation resistance [2].

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

It was decided to creates our own database, starting from simple binary coating with the progressive addition of elements. In this talk, we will present results on the oxidation of binary and ternary nitride coatings. These coatings are obtained by magnetron sputtering in a reactive atmosphere and they are annealed in air up to 900°C, to propose oxidation mechanisms.

[1] W. D. Sproul, M. E. Graham, M.-S. Wong, et P. J. Rudnik, « Reactive unbalanced magnetron sputtering of the nitrides of Ti, Zr, Hf, Cr, Mo, Ti-Al, Ti-Zr and Ti-Al-V », Surface and Coatings Technology, vol. 61, n^o 1, p. 139–143, déc. 1993, doi: 10.1016/0257-8972(93)90216-B.

[2] V. Novikov, N. Stepanov, S. Zherebtsov, et G. Salishchev, « Structure and Properties of High-Entropy Nitride Coatings », Metals, vol. 12, n^o 5, p. 847, mai 2022, doi: 10.3390/met12050847.