Processing, Microstructure and Mechanical Properties of TiB2-MoSi2-C Ceramics

Abstract

Titanium boride (TiB2) is a material classified as an ultra-high-temperature ceramic. The TiB2 structure is dominated by covalent bonds, which gives the materials based on TiB2 very good mechanical and thermal properties, making them difficult to sinter at the same time. Obtaining dense TiB2 polycrystals requires a chemical or physical sintering activation. Carbon and molybdenum disilicide (MoSi2) 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.% MoSi2; and the third with both additions of 2 wt.% carbon and 2.5, 5 and 10 wt.% MoSi2. On the basis of the dilatometric sintering analysis, all additives were found to have a favourable effect on the sinterability of TiB2, and it was determined that sintering TiB2 with the addition of carbon can be carried at 2100 °C and with MoSi2 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 TiB2 polycrystals of high density (>90%) to be obtained. The minimum MoSi2 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 MoSi2 content, i.e., 5 and 10%, showed densities close to 100%. The mechanical properties, such as Young’s modulus, hardness and fracture toughness (KIc), of the polycrystals and composites were similar for samples with densities exceeding 95%. The Vickers hardness was 23 to 27 GPa, fracture toughness (KIC) was 4 to 6 MPa·m0.5 and the Young’s modulus was 480 to 540 GPa. The resulting TiB2-based materials showed potential in high-temperature applications.