Replenishing the limited Aluminium reservoir of MAX phase coatings in harsh environments

Project Information

REALMAX

Grant agreement ID: 892501

DOI

10.3030/892501

Project closed

EC signature date 13 March 2020

Start date 17 August 2020

End date 16 August 2022

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 162 806,40

EU contribution

€ 162 806,40

Coordinated by

RHEINISCH-WESTFAELISCHE TECHNISCHE HOCHSCHULE AACHEN
Germany

Periodic Reporting for period 1 - REALMAX (Replenishing the limited Aluminium reservoir of MAX phase coatings in harsh environments)

Reporting period: 2020-08-17 to 2022-08-16

Summary of the context and overall objectives of the project

MAX phases constitute a family of layered ceramics exhibiting a unique combination of ceramic and metallic properties. These materials have attracted significant attention because they are promising candidates for applications that require resistance to harsh environments, such as high temperatures and oxidizing/corrosive environments. In fact, MAX phases are particularly interesting because of their self-protecting capabilities, which result from the weakly bonded A-elements of the MAX phase. For example, in the case of high-temperature oxidation of the Cr2AlC MAX phase, the crystallographic structure of the MAX phase allows for the Al to diffuse out of the structure, react with the oxidizing surroundings and form a protective aluminium oxide scale at the surface. While leading to the protection of the MAX phase, the depletion in Al also causes local decomposition, of the MAX phase into binary carbides, in the vicinity of the newly formed oxide scale. The decomposition is often accompanied by the formation of pores as the exposure to the oxidizing environment continues leading to the catastrophic failure of the MAX phase component.

The production of high-temperature materials able to withstand extreme environments is crucial for the energy transition. By improving the thermal stability and the oxidation/corrosion resistance of such materials, the lifetime of components is increased. Furthermore, MAX phases are self-healing materials and therefore damage tolerant. Aside from sustainable impact, MAX phases can also improve the performance of for example gas turbines for aerospace and concentrated solar power systems.

The REALMAX project dealt with improving the high-temperature oxidation resistance of the Cr2AlC MAX phase by employing different strategies. First, compositional design was considered in order to prevent excessive Al-consumption. Second, microstructural design was implemented in order to decrease the direct diffusion paths for either O or Al ion diffusion. Finally, the possibility of continuously supplying Al to a MAX phase coating was implemented.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The REALMAX project was organized in 3 scientific work packages (WPs).
WP1 dealt with Cr2AlC in both bulk and thin film form. The microstructures of bulk Cr2AlC samples were varied by using different powder precursors for sintering. The texture of the samples was also investigated. Cr2AlC thin films were deposited on conventional substrates but also on the bulk Cr2AlC samples which were previously produced. The oxidation resistance, at high temperature, of bulk and bulk/thin film assemblies were then investigated.

Outcomes of WP1 include:
• Confirming that the microstructural design of Cr2AlC samples is possible by varying the precursors.
• Identifying the relationship between microstructure and oxidation resistance of bulk Cr2AlC samples.
• Continuous Al supply appears to be possible but requires further investigations.

WP2 was meant to deal with the synthesis of “high entropy” MAX phases by magnetron sputtering during the scheduled secondment at Linköping University. Unfortunately, due to covid-related regulations, the 6-month secondment could not be carried out. Therefore, the scope of WP2 was slightly modified. Solid solutions of (Crx,Y1-x)2AlC were instead considered. Ab initio calculations were carried out to identify the stability domains of the solid solution with respect to Y contents. The metastability of the solid solution was then explored experimentally. To this end, Cr-Al-C was co-sputtered with Y at high temperature to allow for the direct synthesis of the MAX phase solid solution and at room temperature. The room temperature deposited samples were then annealed in order to track the crystallization process of the (Crx,Y1-x)2AlC phase. Finally, the oxidation resistance of the Y-containing MAX phase was also explored.

WP2 was further complemented by tracking the outward diffusion of Al ions and the inward diffusion of Ge and Cu ions. To do so, Cr2AlC was deposited on conventional substrates and was then coated by either Ge or Cu. After annealing, transmission electron microscopy was carried out on selected samples to identify whether elemental exchange between Al and Ge (or Cu) occurs. By doing so, diffusion mechanisms in Cr2AlC, and in MAX phases in general, were investigated.

Outcomes of WP2 include:
• The successful prediction and synthesis of (Crx,Y1-x)2AlC solid solutions
• Evaluation of the effect of Y on the oxidation resistance of Cr2AlC
• Determining whether an elemental exchange is feasible in Cr2AlC.

WP3 was also modified. In this WP the aim was to consider the technological transfer of the MAX phase materials which were produced. Instead of burner rig testing which was originally considered, the materials were instead considered as absorbers for concentrated solar power systems. To do so, corrosion of Cr2AlC and other MAX phases was carried out in solar salt at high temperature for durations up to one month.

Outcomes of WP3 include:
• Determining whether MAX phases are promising candidates for concentrated solar power systems.

The results are currently being compiled into research articles and have, in part, been presented at international conferences and invited seminars.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The overarching goal of the REALMAX project was mostly fundamental. In fact, great importance was given to diffusion phenomena occurring in MAX phases when confronted with high-temperature oxidizing environments. The research involved went beyond the state-of-the-art in understanding how microstructural and compositional design employing diffusion mechanisms allow the synthesis of Cr2AlC but also allow improving their oxidation resistance.

Microstructural design was shown to not only be possible by using different carbon sources, but also by using texturing approaches. The composition of Cr2AlC can also be tailored to needs by incorporating either Y or Ge.

Overall, the main impact of the REALMAX project lies in furthering the understanding of the synthesis and stability of MAX phases. By doing so, material design is facilitated in the case of MAX phases but also in the case of MXenes. MAX phases are also expected to be considered for different applications than originally planned. For example, concentrated solar power systems may greatly benefit from the use of certain MAX phases which exhibit promising optical properties and corrosion resistance and join ultrahigh temperature ceramics as alternatives to SiC. Finally, the origin of the project contemplated the use of MAX phases as Al reservoirs, which is supported by the results obtained to this date.

STEM images in HAADF mode of oxidized MAX/MAX assemblies, illustrating the effect of Al-reservoir