| Ternary carbide, Cr2AlC, has an even better oxidation and corrosion resistance than Ti3SiC2 and Ti3AlC2 in MAX family at high temperatures. So, Cr2AlC is expected to be a more promising candidate for high temperature applications.In addition, the thermal expansion of Cr2AlC is 12～13×10-6/K, close to the superalloys. Hence Cr2AlC has potential applications in proctective coatings on the superalloys.There are several methods including hot pressing, hot isostatic pressing and pulse discharge sintering used to produce Cr2AlC bulk ceramics. However, problems such as high synthesis temperature (1300-1450℃), relatively low density (-95%) and impurities (Cr7C3 or Al4C3) were always found during synthesis of Cr2AlCTo solve the above problems, mechanically activated hot-pressing technology was used to synthesize a fine-crystalline Cr2AlC ceramics at relatively low temperatures. A mixture of Cr, Al and C powders with a molar ratio of 2:1.2:1 was mechanically alloyed for 3 h, and then subjected to hot-pressing at 30 MPa at 600-1100℃for 1 h in Ar atmosphere. The results show that a dense and pure Cr2AlC ceramic with a grain size of about 2μm can be synthesized at a relatively low temperature of 1100℃(denoted as the fine-grained Cr2AlC). Compared to the sintering temperature of 1450℃for achieving Cr2AlC ceramics (grain size of about 35μm, denoted as the coarse-grained Cr2AlC) by hot pressing the conventional mixture of Cr, Al and C powders, the present synthesis temperature is decreased by about 350℃. The synthesized fine grained Cr2AlC has a high density of 99 %, which is high than 95% for the coarse grained Cr2AlC. The flexural strength and Vickers hardness for the fine grained Cr2AlC are 513 MPa and 6.4 GPa, respectively. These values are much higher than those for the coarse grained Cr2AlC, viz.305 MPa and 3.5GPa, respectively.Oxidation behavior of the fine-crystalline Cr2AlC ceramic was investigated in a temperature range of 900-1200℃in simulated air (N2 64 mL/min and O2 16 mL/min) for up to 100 h. Cr2AlC displays an excellent oxidation resistance by forming a dense layer of a-Al2O3 oxide, which prevents the further oxidation. The morphology and phase composition were characterized by scanning electron microscope and X-ray diffraction.In addition, A new Cr2Al(Si)C solid solution has been first synthesized by hot pressing a mixture of Cr, Al, Si and C powders with a molar ratio of 2:1.1:0.2:1 at 1450 ℃with 30 MPa for 1 h in Ar atmosphere. The flexural strength, fracture toughness and Vickers hardness of the Cr2Al(Si)C solid solution was determined to be 354MPa, 6.6MP.am1/2 and 3GPa, compared with the values for synthesized Cr2AlC:305MPa, 6.2MP.am1/2 and 3.4GPa. Buckling and kinking of the layered structure as well as grain delamination crack deflection were extensively observed around indentations and on the fracture surfaces. These multiple energy-absorbing mechanisms endow the sold solution with damage tolerance and micro-scale plasticity at room temperature.
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