Cr₂AlC MAX phase as bond coat for thermal barrier coatings: Processing, testing under thermal gradient loading, and future challenges

Abstract

Cr2AlC layers with thickness up to 100 µm were deposited by high‐velocity‐atmospheric plasma spray (HV‐APS) on Inconel 738 substrates to analyze the potential of MAX phases as bond coat in thermal barrier coating systems (TBCs). The deposited Cr2AlC layers showed high purity with theoretical densities up to 93%, although some secondary phases were detected after the deposition process. On top of this MAX phase layer, a porous yttria‐stabilized zirconia (YSZ) was deposited by atmospheric plasma spraying. The system was tested under realistic thermal loading conditions using a burner rig facility, achieving surface and substrate temperatures of 1400°C and 1050°C, respectively. The system failed after 745 cycles mainly for three reasons: (i) open porosity of the bond coat layer, (ii) oxidation of secondary phases, and (iii) inter‐diffusion. Nevertheless, these results show a high potential of Cr2AlC and other Al‐based MAX phases as bond coat material for high‐temperature applications. Furthermore, future challenges to transfer MAX phases as eventual bond coat or protective layer are discussed.

Processing of MAX phases: From synthesis to applications

“…High velocity atmospheric plasma spray (HV-APS) and liquid plasma spray (LPS) have been used to deposit Cr 2 AlC and Ti 3 AlC 2 . 248,249 Cr 2 AlC coatings with thicknesses up to 100 µm and relative densities up to 93% were deposited on stainless steel and Inconel 738 by HV-APS (Figure 10 C). Coatings exhibited good adhesion, but Cr 2 AlC was partially decomposed.…”

“…The Inconel/Cr 2 AlC/YSZ system was tested in a burner rig at 1400°C (surface temperature) and survived 745 cycles, indicating that Cr 2 AlC has a high potential as bond-coat. 248 Regarding the deposition of Ti 3 AlC 2 powders by LPS, 249 coatings were 10-20 µm thick with high density and purity despite an expected temperature higher than 2200 K during deposition. The solvent and the pH play a determinant role, but more investigation is required to confirm and improve these promising results.…”

“…Recently, Cr 2 AlC bond-coats have been deposited on Inconel 738 substrates, followed by deposition of a porous yttria-stabilized zirconia as external TBC. 248 The system was tested under thermal loading conditions similar to a gas turbine environment, achieving surface and substrate temperatures of 1400°C and 1050°C, respectively. The system failed after 745 cycles (conventional bond-coat systems under similar conditions around 1000-1200 cycles) mainly for three reasons: (i) open porosity of the Cr 2 AlC bond-coat layer, (ii) oxidation of secondary phases, and (iii) inter-diffusion.…”

“…Furthermore, future challenges to specifically transfer MAX phase as bond-coats are described elsewhere. 248 In this case, Cr 2 AlC was tested, but Ti 2 AlC presents even more potential because of better CTE match.…”

“…In general the "A" element diffuses into the metal, meanwhile element such as Co, Ti, Mo, Zr or Ni are detected in the MAX phase side. 248,293,294 Diffusion of Al from Cr 2 AlC into Zr substrates has been even detected during the annealing treatment at 800°C in vacuum to crystallize the coatings. 295 The diffusion can be suppressed using ZrN and AlN diffusion barriers, which was thermodynamically calculated and experimentally validated for one hour at 1000°C.…”