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 8SPS) 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 8III) 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.