Processing, Microstructure and Mechanical Properties of TiB[sub.2]-MoSi[sub.2]-C Ceramics

Titanium boride (TiB[sub.2]) is a material classified as an ultra-high-temperature ceramic. The TiB[sub.2] structure is dominated by covalent bonds, which gives the materials based on TiB[sub.2] very good mechanical and thermal properties, making them difficult to sinter at the same time. Obtaining dense TiB[sub.2] polycrystals requires a chemical or physical sintering activation. Carbon and molybdenum disilicide (MoSi[sub.2]) were chosen as sintering activation additives. Three series of samples were made, the first one with carbon additives: 0 to 4 wt.%; the second used 2.5, 5 and 10 wt.% MoSi[sub.2]; and the third with both additions of 2 wt.% carbon and 2.5, 5 and 10 wt.% MoSi[sub.2]. On the basis of the dilatometric sintering analysis, all additives were found to have a favourable effect on the sinterability of TiB[sub.2], and it was determined that sintering TiB[sub.2] with the addition of carbon can be carried at 2100 °C and with MoSi[sub.2] and both additives at 1800 °C. The polycrystals were sintered using the hot-pressing technique. On the basis of the studies conducted in this work, it was found that the addition of 1 wt.% of carbon allows single-phase TiB[sub.2] polycrystals of high density (>90%) to be obtained. The minimum MoSi[sub.2] addition, required to obtain dense sinters with a cermet-like microstructure, was 5 wt.%. High density was also achieved by the materials containing both additives. The samples with higher MoSi[sub.2] content, i.e., 5 and 10%, showed densities close to 100%. The mechanical properties, such as Young’s modulus, hardness and fracture toughness (K[sub.Ic]), of the polycrystals and composites were similar for samples with densities exceeding 95%. The Vickers hardness was 23 to 27 GPa, fracture toughness (K[sub.IC]) was 4 to 6 MPa·m[sup.0.5] and the Young’s modulus was 480 to 540 GPa. The resulting TiB[sub.2]-based materials showed potential in high-temperature applications.