MAX phase based PVD coatings as protection for lightweight materials in high temperature environments

Abstract

MAX-phases are of increasing interest as coating material for high temperature applications due to their unique combination of metallic and ceramic properties. Especially the alumina forming MAX phases of Cr2AlC, Ti2AlC or Ti2AlN are promising as oxidation resistant coatings. Unfortunately, degradation of MAX phases is observed when applied on various Ti- or Ni-based alloys by interdiffusion processes between coating and alloy and the associated Al-depletion. This degradation is not present when MAX-phases are applied on the Al-rich γ-TiAl based alloys, which leads to an inward diffusion of Al from the substrate alloy into the coating and finally to a stabilization of the thermally grown alumina layer. In the present work, the coating deposition process to get the MAX phases was DC magnetron sputtering using pure elemental targets of Ti or Cr, Al and C and in case of the Ti2AlN MAX-phase based coating, nitrogen as reactive gas. No additional heating was applied during the sputtering process, the obtained substrate temperature was self-adjusted due to the target power. Prior to coating deposition, an Ar-plasma etching process for surface cleaning using a bias voltage of 500V and a frequency of 100 kHz was carried out for 15min. Using a threefold rotation, homogenous all-around coatings of about 10 µm were achieved with the desired stochiometric composition of the MAX-phases. The formation of the MAX-phase in the sputtered coatings was characterized during a post-annealing process at 800°C by in situ HT-XRD measurements as well as by SEM equipped with EDS and WDS, as well as by TEM with electron diffraction. The MAX-phase coatings were tested under cyclic oxidation conditions. They provide a good oxidation protection of the γ-TiAl alloys due to the development of a protective alumina layer up to 850°C for up to 300 hrs in laboratory air. The performance of the MAX-phases is strongly depended on the substrate material and the accompanying interdiffusion processes between coating and substrate. Therefore, the Ti-Al-C based coating is more favored on TiAl alloys due to the thermodynamic stability of the Ti2AlC MAX phase in particular in the presence of the γ-TiAl phase. In comparison, the Cr2AlC MAX phase degrades after just 100 hrs at 850°C due to the formation of chromiumcarbides next to alumina.