Effect of Si additions on the Al2O3 grain refinement upon oxidation of Cr2AlC MAX phase

Nano-scale Si segregation and precipitation in Cr2Al(Si)C MAX phase coatings impeding grain growth during oxidation

“…A selected-area electron diffraction (SAED) pattern (inset of Figure 1) recorded from a sample with an identical chemical composition to that reported in Ref. [16] was consistent with the formation of the Cr 2 Al(Si)C MAX phase.…”

“…Although the segregations may or may not be crystalline, the volume fraction of the precipitations may be too small to be detected using XRD (assuming that they are crystalline). Furthermore, no positive phase identification of the precipitates could be obtained using SAED [16]. The Si rich precipitations, which have a thickness of ∼ 5-10 nm and a length on the order of 250 nm, should be visible using TEM and hence should be detectable using SAED.…”

“…MAX phases of Cr 2 AlC and Cr 2 Al(Si)C with 0.7 at.% Si were synthesized using DC magnetron sputtering in an industrial chamber (CC800/9, Cemecon AG, Germany). Further synthesis details can be found in Ref [16]. Oxidation was performed using a Carbolite HTF-1700 laboratory furnace at 1120°C for 4 h in air.…”

“…We recently showed that the addition of Si as a fourth alloying element in the minor amount of 0.7 at.% (determined by energy-dispersive X-ray spectroscopy (EDX)) to the Cr 2 AlC MAX phase can enhance the Al 2 O 3 layer thickness by ∼ 40% upon oxidation at 1120°C in air for 4 h [16,17]. It was also observed that the Cr 2 Al(Si)C MAX phase exhibited retarded grain growth during oxidation.…”

“…Detailed near atomic-scale characterization using atom probe tomography (APT) did not reveal the presence of Si, even in trace amounts, either in the Al 2 O 3 layer or in the chromium carbide layer located immediately below the Al 2 O 3 layer. However, APT revealed the incorporation of up to 1.3 ± 0.2 at.% Si into the MAX phase after oxidation [16]. Hence, the role of Si in the retardation of MAX phase grain growth is currently not understood.…”