Synthesis and mechanical properties of fully dense Ti2SC

Because the layered machinable ternary carbide, Ti2SC, has a significantly shorter c-lattice parameter-as compared with most of the 50+ other so-called M(n+1)AX(n) (MAX) phase family (M=early transition metal; A=A group element and X=C or N and n=1-3) to which it belongs-it was postulated that its mechanical properties would be significantly different than the other MAX phases. In this work, fine-grained (FG) and coarse-grained (CG) polycrystalline fully dense Ti2SC samples were fabricated. Hot pressing Ti2SC powders, at 1500 degrees C under a stress of similar to 45 MPa for 5 h resulted in FG (2-4 mu m) samples which and upon further annealing for 20 h at 1600 degrees C resulted in CG (10-20 mu m) ones. No peaks other than those associated with Ti2SC and an impurity anatase phase, with a volume fraction of similar to 6 vol% were observed in the XRD patterns and micrographs. The average Vickers hardness in the 2-300 N range is 8 +/- 2 GPa with the FG samples being slightly harder. This hardness is the highest of any of the MAX phases characterized to date. Also in contrast to all MAX phases, cracks extended from the corners of Vickers indents in the FG samples. From these cracks the fracture toughness was estimated and found to increase more or less linearly with load from approximate to 4 to 6 MPa.m(1/2) as the Vickers force was increased from 50 to 300 N, respectively. The room temperature compressive stress of the FG samples was 1.4 +/- 0.2 GPa; the failure mode was brittle. Its Young's modulus-also one of the highest for a M(2)AX phase measured to date-was 316 +/- 2 GPa. There was no evidence for incipient kink band formation during simple compression. The latter is attributed, in part, to the fine grain size of the hot-pressed material.