Marie Sklodowska-Curie Action: RealMAX

 

Scope

The RealMAX project aims to provide novel material solutions for extending the lifetime of MAX phase parts aimed for applications for harsh environments.

MAX phases are inherently nanolaminated carbides and nitrides which are described by the general formula Mn+1AXn, where M is an early transition metal, A is an element from groups 13 to 16, and X is carbon and/or nitrogen, and n =1, 2 or 3. This class of ceramics has attracted a great deal of attention due to their unique combination of metallic and ceramic properties, and the broad range of applications for which they can be considered.

Al-based MAX phases in particular, are very promising for applications requiring high temperature oxidation resistant materials. Indeed, Al-containing MAX phases are able to self-protect (and self-repair) by releasing Al to the surface, and which then forms a protective oxide scale. While the release of Al allows the formation of stable oxide scales which can withstand high temperatures and which limits the inward oxygen diffusion, it also causes local decomposition of the MAX phase in the vicinity of the MAX phase/oxide interface. This is particularly true in the case of Cr2AlC.

The RealMAX project addresses the instability of MAX phase coatings when subjected to high temperature oxidation processes, in order to limit the decomposition of the MAX phase and prolong its lifetime.

Two approaches are investigated:

  • Evaluate the possibility of continuously supplying Al to Cr2AlC coatings by using a Cr2AlC substrate as Al-reservoir.
  • Incorporating reactive elements (such as Y) in the MAX phase structure.
 

Methods

Both bulk (spark plasma sintering) and thin film (magnetron sputtering) processing are used to alter the microstructures, compositions and textures of the MAX phases, which are routinely characterized. An important aspect of the project is to identify diffusion processes which occur in the MAX phases, and the oxide scales, using atom probe tomography (APT) and scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDS).

  STEM micrographs Copyright: © Azina

STEM micrographs (High angle annular dark field) of an oxidized Cr2AlC bulk substrate. The different contrasts represent different phases including Cr2AlC, Cr7C3, Al2O3 and (Cr,Al)2O3.

  SEM images Copyright: © Azina

Surface and cross-section micrographs of oxidized Cr2AlC coatings deposited on MgO, Cr2AlC obtained from solid state reaction (SSR) powders, and Cr2AlC obtained from molten salt shielded synthesized (MS3) powders. All cross-section specimens were prepared by Focused ion beam. The oxidation states of the coatings differ with respect to the substrate on which they were deposited.

  APT image Copyright: © MCh

APT specimens prepared from an oxidized Cr2AlC coating deposited on a Cr2AlC substrate. The lamellar structure observed corresponds to the atomic arrangement and inherent lamellar structure of the MAX phase. APT measurements and reconstruction carried out by Dr. Marcus Hans.