Synthesis of MAX phase coatings by magnetron sputter deposition: Investigation of the oxidation behavior

Kurzfassung

Development of MAX phase coatings by magnetron sputter deposition Ternary carbide and nitride MAX phases form a novel materials class that bridges the gap between ceramics and metals, thanks to their distinctive nanolaminated structure. They possess several attractive properties, including high resistance to thermal shocks, oxidation and corrosion, along with a favorable resistance to fatigue and creep. The general formula for these materials is Mn+1AXn, where n can be 1, 2 or 3. M represents an early transition metal, while A stands for an A-group element along with either carbon or nitrogen, denoted as X [1]. In this work, MAX phases were synthesized by the mean of physical vapor deposition techniques. The Cr2AlC and Ti2AlC MAX phase coatings were manufactured by an industrial sized coater using DC magnetron sputter deposition. For each MAX phase, three pure targets, Cr or Ti, Al and C were used. The two-fold rotation allowing a homogenous coating all around the substrate. The substrates were placed in front of the C target to compensate its low sputtering yield [2]. The Ti2AlN MAX phase was produced in a smaller lab coater using reactive sputter deposition. Two metallic targets, Ti and Al, were placed inside while N was introduced as reactive gas during the process. The three MAX phase coatings were all X-ray amorphous and dense with columnar structures, see Fig. 1. In order to determine the right composition different analytical methods like glow-discharge optical emission spectroscopy (GDOES), EDS and WDS were used. The preferred crystallization treatment was identified by testing different atmospheres from vacuum, Ar-atmosphere and laboratory air and evaluating in terms of coating adhesion, interdiffusion with the substrates and phase formation. To determine the crystallization temperatures of each MAX phase coating, high temperature XRD was used. Oxidation behavior of MAX phase coatings Since MAX phases exhibit strong M-X bond and relatively weak M-A bond, they inhibit potentially great oxidation properties by forming a protective oxide layer out of the A element, which is Al2O3 in this work presented. Al2O3 is known to form a slow growing and dense oxide especially at high temperatures forming α-Al2O3. The formation temperature of α-Al2O3 starts at a temperature range of 800 to 1100°C [3]. Therefore, these coatings are of interest as oxidation protection for turbine engine materials such as titanium aluminides (γ-TiAl), Ni-based superalloys and SiC/SiC ceramic matrix composites (CMC). The oxidation behavior of three MAX phase coatings, namely, Ti2AlC-based, Cr2AlC-based, and Ti2AlN-based, was initially examined on an inert substrate material. In this study, the oxidation kinetics were analyzed exclusively, without any interference from interdiffusion processes. Two temperature regimes, 800°C and 1200°C, were established to correspond to the high-pressure and low-pressure turbine temperature regimes. The selective oxidation of Al2O3 and the formation of α-Al2O3 were specifically analyzed at 800°C and 1200°C. In the following step, γ-TiAl and SiC were coated and tested under isothermal and cyclic conditions. The temperature regime of interest for γ-TiAl was 800°C, and for SiC, applied at 1200°C. Following deposition, crystallization treatment and oxidation tests, all coatings exhibited excellent adhesion to γ-TiAl. The purity of the coating and the stability of the coating process are critical factors in the selective oxidation of Al2O3. This study has shown that the formation of TiN, TiC or CrC can interfere with the protective thermally grown oxide layer (TGO) of Al2O3. Additionally, coatings made of Ti2AlN and Cr2AlN are prone to interdiffusion zones in the alloy, which alters the coating's composition. Only Ti2AlC did not form an interdiffusion zone so far. The protective TGO was successfully formed for all three coatings, which were tested for at least 100 hours at 800°C. The two Ti-based MAX phases were coated on SiC material. Due to the higher coefficient of thermal expansion (CTE) mismatches, the Ti2AlC coating experiences adhesion problems. Both coatings form a TGO layer of Al2O3, and thus far can endure the temperature regime of 1200°C for a few hours. The tests were all supported by analytic instruments, such as SEM, EDS, and XRD.