Oxidation Behavior of Ti2AlC MAX Phase Ceramics at 800 and 1200℃ in Air

The MAX phase ceramics represented by Mn+1AXn in the general chemical formula are layered ternary transition metal carbides and nitrides. The MAX phase ceramics have excellent properties, such as good machinability with cemented carbide tools, high thermal and electrical conductivity as well as high thermal resistance, elastic stiffness and oxidation resistance. Ti2AlC ceramics have been studied as potential candidates for use in various mechanical components operating at high temperatures because of their good resistance against high-temperature oxidation. However, there are both reports of good and poor in oxidation resistance of Ti2AlC. Differences in chemical composition may affect the oxidation behavior of the material. In this study, the oxidation behavior of Ti2AlC ceramics with various Al concentrations and phase composition was investigated. Ti2AlC ceramic powder was synthesized using commercial Ti, Al and C powders. These powders were mixed to obtain the nominal chemical compositions of Ti2AlC0.9, Ti2Al1.2C0.9 and Ti2Al1.5C0.9. Almost fully-densified Ti2AlC samples with various Al concentrations were prepared by means of pulsed electric current sintering. The phase composition of the sintered samples was identified via XRD. The main peaks recorded for all samples were attributed to Ti2AlC. Minority phases in Ti2AlC0.9 were identified as Ti3AlC2 and TiC. In the cases of Ti2Al1.2C0.9 and Ti2Al1.5C0.9, Ti3AlC2 and TiAl3 were detected with X-ray diffraction. Samples for oxidation experiments were cut with a diamond saw to obtain a size of either 4x4x3 mm or 7.5x7.5x3 mm and polished with water-resistant papers with a grit size of 2000. The Ti2AlC0.9, Ti2Al1.2C0.9 and Ti2Al1.5C0.9 samples were heated in an electric furnace at 800℃ for 2, 4, 7 and 14 d and at 1200°C for 0.5, 1, 2 and 4 d in laboratory air. The surface of the oxidized samples was analyzed via XRD to identify the phases. The oxidized samples were placed in a resin and cut with a diamond cutting wheel to allow the observation of their cross-sections using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX). Ti2AlC0.9 exhibited non-protective oxidation in all samples for different experimental periods. The thickness of the non-protective oxide scale was over 20 μm. The phase composition of the non-protective oxide scale detected via XRD was identified as rutile-type TiO2 and α-Al2O3. In Ti2Al1.2C0.9 and Ti2Al1.5C0.9, all samples exhibited protective oxidation at 800℃. Some samples exhibited protective oxidation at 1200℃, but some did not. The thickness of the protective oxide scale was approximately 5 μm. The phase of the protective oxide scale was identified as α-Al2O3. The non-protective oxide scale near the surface had a stronger Ti intensity than near the scale/MAX phase interface. The results of XRD, SEM and EDX studies indicate that rutile with Al2O3 had formed as the non-protective oxide scale. Oxidation resistance of Ti2AlC ceramics changed with Al concentration. Non-protective oxidation for Ti2AlC ceramics occurred when the supply of Al from the sample interior had been insufficient. The supply of Al from TiAl3 grains might have been the main factor that ensured a continuous Al2O3 scale. The concurrence of Ti2AlC ceramics and TiAl3 lead to protective oxidation at 800℃. However, the Al supply at 1200℃ was insufficient for the formation of a protective Al2O3 scale on Ti2AlC ceramics.