The long-term isothermal oxidation behavior of bulk Ti 2 AlC in air and 100% water vapor has been investigated in the 1000-1300 • C temperature range. The kinetics, for oxidation up to 120 hours, follows a cubic rate law in both environments -air and water vapor. The kinetics was found to be slightly faster for hydrothermal oxidation compared to oxidation in air, especially below 1100 • C. However, there is little variation between the activation energies for oxidation in air (approximately 279 kJ/m 3 ) and in 100% water vapor (approximately 261 kJ/m 3 ). Scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction show that oxidation of Ti 2 AlC forms mostly a continuous and stable layer of α-Al 2 O 3 , along with a thin surface layer of rutile-TiO 2 in both environments. However, the thin TiO 2 layer volatilizes by forming gaseous TiO(OH) 2 in the presence of water vapor at high temperatures (>1200 • C). At short-term oxidation and lower temperatures, diffusion of hydroxyl ions is proposed to be the rate limiting mechanism under hydrothermal conditions. Above 1100 • C, the diffusion of oxide ions through Al 2 O 3 layer becomes the rate limiting step and humidity has little effect on the overall oxidation kinetics of Ti 2 AlC.

The reactivities of commercially available Ti2AlC or Ti3SiC2 powders with uncoated SiC fibers or SiC powders were evaluated in this paper. When Ti2AlC–SiC samples were hot pressed or hot isostatically pressed at temperatures up to 1500°C, fully dense composites were obtained. The latter were characterized by X‐ray diffraction and electron‐dispersive spectroscopy in a scanning and transmission electron microscope. Differential thermal analysis up to 1550°C was also carried out. In all cases, SiC reacted with the Ti2AlC powder resulting in the formation of Ti3(Al1−xSix)C2 TiC and Al1+xTi1−x, where x ranges from 0 to 1. In the limit x=1, pure Al forms. Conversely, Ti3SiC2 samples, reinforced with uncoated SiC fibers or powders, can be hot pressed in vacuum at temperatures as high as 1500°C to produce fully dense composites with no apparent reaction between the matrix and fibers. Based on these results, Ti3SiC2 can, but Ti2AlC cannot, be reinforced with SiC. Such reinforcements will be needed if the MAX phases are to be used as structural materials at very high temperatures.

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In this article, the reactivity of Ti2AlC powders, with 3 and 10 μm alumina, Al2O3, fibers during pressure‐assisted sintering is explored. Samples were fabricated by hot‐isostatic‐pressing (HIPed) or hot‐pressing (HPed), and characterized by X‐ray diffraction, differential thermal analysis, and electron microscopy—both scanning and transmission—equipped with energy dispersive X‐ray spectroscopes. Samples prepared at 1300°C were fully dense, with no apparent reaction between fiber and matrix. In samples HPed to 1500°C, even pure Ti2AlC powders dissociated to Ti3AlC2 according to: 2 Ti2AlC = Ti3AlC2 + TiAlx (l) + (1‐x) Al (l/v), with x < 1. More severe Al loss results in the formation of TiCy. The presence of the Al2O3 fibers delayed densification enough to allow most of the Al and some of the Ti to escape into the vacuum of the hot press or react with the encapsulating glass during HIPing a resulting in a more intensive dissociation of the Ti2AlC. Although, in principle Ti2AlC can be reinforced with Al2O3 fibers, the processing/use temperature will have to be kept below 1500°C, as, at that temperature the fibers, used here, sinter together.

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