High temperature environmental resistant Mo-Si-B based coatings

For high temperature applications nickel base superalloys with aluminide coatings are reaching the service limit. Alternate materials such as refractory metal silicide alloys can extend high temperature capability, but the silica surfaces require coatings for enhanced environmental resistance. We present a review of an effective strategy to achieve the environmental resistance through the use of an integrated Mo-Si-B based coating that is applied by a co-deposition of Si and B by pack cementation. During oxidation of the (Si + B)-pack alloys, the initial MoSi2 outer layer is consumed by formation of the Mo5Si3 (T1) phase and the development of the underlying Mo5SiB2, T2 borosilicide and/or MoB boride phase layer. The T1 phase with B has excellent oxidation resistance and the loss of Si to the substrate is blocked by the underlying diffusion barrier (T2). Any damage to the outer T1 layer can be recovered from the T2 + MoB layer to yield a self-healing characteristic. The Mo-Si-B based coating on Mo based alloys exhibits robust performance up to at least 1700 °C not only to high temperature oxidation, but also to water vapor and corrosive deposit attack. Some of the applications are discussed for use of the coating strategy on ceramic materials.