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According to the properties determined for the ternary end-members, MAX phases in the quaternary Cr-Ti-Al-C system could be of interest as protective coatings for nuclear fuel cladding in the case of severe accident conditions. In this study, syntheses of 211 and 312 MAX phase compositions were attempted using pressureless reactions starting from Cr, TiH 2 , Al, and C (graphite) powders. It was observed that both the Ti substitution by Cr in Ti 3 AlC 2 and the mutual solubility of Ti 2 AlC and Cr 2 AlC are limited to a few atomic percent. In addition, the remarkable stability of the (Cr 2/3 Ti 1/3 ) 3 AlC 2 MAX phase composition was confirmed. Due to the low miscibility of MAX phases in the Cr-Ti-Al-C system, most samples contained substantial amounts of TiC x and Al-Cr alloys as secondary phases, thus forming composite materials. After sintering, all samples were submitted to a single oxidation test (12 h at 1400°C in air) to identify compositions potentially offering high-temperature oxidation resistance and so warranting further investigation. In addition to (Cr 0.95 Ti 0.05 ) 2 AlC, composite samples containing substantial quantities of Al 8 Cr 5 and AlCr 2 formed a stable and passivating Al 2 O 3 scale, whereas the other samples were fully oxidized.

Rare-earth (RE) monosilicates are promising candidates as environmental barrier coating (EBC) materials for ceramic matrix composites for aerospace applications. Five rare-earth monosilicate materials have been investigated: Y2SiO5, Gd2SiO5, Er2SiO5, Yb2SiO5, and Lu2SiO5 produced from RE oxides and silica starting materials pressed and sintered at 1580°C under flowing air. Relative densities above 94% were obtained for all samples and ceramics were made containing 85–100 wt% of the RE monosilicate according to X-ray diffraction (XRD) with RE disilicates as the second phase in the Gd, Yb, and Lu silicate systems. Microstructures were characterized using scanning electron microscopy and XRD, and thermal properties measured including specific heat, thermal expansion, and thermal diffusivity. For the first time, specific heat capacity values are reported for the monosilicates [0.45–0.69 J·(g·K)−1]. Thermal expansion coefficients (TECs) of the dense samples ranged between 5.9 and 10.3 × 10−6 K−1 measured for 473 to 1473 K. All EBCs have low thermal conductivities [1.8 W·(m·K)−1 or less] making them excellent EBC insulators

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