Synthesis and characterization of ternary layered Nb2SB ceramics fabricated by spark plasma sintering

In this paper, B-containing MAX phase of Nb2SB ceramics with high purity of 96 wt% (4 wt% NbB impurity) was successfully synthesized using the molar ratio of Nb: S: B = 2: 1.3: 1 by spark plasma sintering at 1350 degrees C under 30 MPa. The reaction path, microstructure, physical and mechanical properties were systematically studied. It is found that the formation of Nb2SB is from the combination of Nb, NbS2, and Nb5B6, or NbB and NbS. The full dense sample (RD 99.7%) possesses the fine grains about 6 mu m in length and 3.6 mu m in width. The grains of Nb2SB show a layered structure, which is same to other MAX phases. The thermal expansion coefficient is 7.1 x 10(-6) K-1 in the range of 24-1100 degrees C. In the temperature range of 25-800 degrees C, the thermal diffusivity of Nb2SB ceramic increases from 5.58 mm(2)/s to 7.07 mm(2)/s. At 25 degrees C, the heat capacity is 0.36 J.g(-1).K-1, the thermal conductivity is 13.79 W.m(-1).K-1, and the electrical conductivity is 1.17 x 10(6) Omega(-1).m(-1). Additionally, the obtained Nb2SB ceramics exhibit excellent mechanical properties of Vickers hardness of 11.89 GPa (10 N load), flexural strength of 249 MPa, fracture toughness of 4.76 MPa.m(1/2), and compressive strength of 1157 MPa. (C) 2021 Elsevier B.V. All rights reserved.