Synthesis and high temperature oxidation of Mo-Si-B-O pseudo in situ composites

In this paper, the new concept of 'pseudo in situ composites' is introduced into artificial composites for ultra-high temperature applications, composed of five phases, Mo, Mo₃Si, Mo₅Si₃, Mo₅SiB₂ and SiO₂. Among these phases, Mo and Mo₅Si₃ are not thermodynamically stable with each other, but they are locally equilibrated in the composites due to the formation of Mo₃Si as their reactant. Using the spark plasma sintering (SPS) technique, the Mo-Si-B-O pseudo in situ composites are successfully synthesized from Mo₃Si/Mo₅Si₃/Mo₅SiB₂ in situ composite powder, Mo and/or SiO₂ powders. The consolidated compacts are sound and fully dense, indicating that the SPS is a promising technique to synthesize the Mo-Si-B-O pseudo in situ composites. High temperature oxidation properties of the composites were examined up to 1673 K. The temperature range is divided into three with respect to the oxidation behavior; i.e. (I) below 1000 K, (II) between 1000 and 1400 K, and (III) above 1400 K. In the range II, the oxidation resistance of the composites is significantly improved by SiO₂ addition. In the range III, the oxidation resistance of the composites is good enough even at 1673 K in spite of the existence of Mo, displaying high potential for ultra-high temperature applications.