High Temperature Oxidation of Mo-Si-B Alloys and Coatings

The challenges of a high temperature environment (T>1400°C) impose severe material performance constraints in terms of melting point, oxidation resistance and structural functionality. In metallic systems there are several high melting temperature intermetallics, but there are a much smaller number of intermetallic phases that offer a level of inherent environmental resistance. The multiphase microstructures that can be developed in the Mo-Si-B system offer useful options for high temperature applications. Alloys based upon the coexistence of the high melting temperature (>2100°C) ternary intermetallic Mo₅SiB₂ (T₂) phase with Mo and the Mo₃Si phase allow for in-situ toughening and offer some oxidation resistance that can be enhanced by coatings. In terms of oxidation performance, the Mo-Si-B ternary system offers an attractive option since boron additions enhance significantly the oxidation resistance of metal-rich binary silicides and the Mo phase together with the equilibrium ternary phase (Mo₅SiB₂) have demonstrated a ductile phase toughening. At the same time, at high temperature the Mo-Si-B system tends to produce favorable SiO₂ layers which exhibit useful oxidation resistance, comparable to other silicides. While there are several factors influencing the oxidation resistance that have been identified, it is clear that the B to Si ratio of the alloy is the dominant factor that  controls the constitution, the viscosity and oxygen diffusivity as well as the in-situ SiO₂-B₂O₃ passive layer upon oxidation. Since the alloy compositions that exhibit the lowest oxidation rate will most likely not yield optimum mechanical properties performance, it is important to develop robust and compatible oxidation resistant coatings. Thus, coating designs are necessary to provide enhanced oxidation protection. An effective strategy to address this challenge is based upon in-situ reaction processing to develop coating systems that are thermodynamically compatible with the base alloy and also incorporate an inherent capability for repair.

In order to control the B/Si ratio a pack cementation process was adapted for coating synthesis. The pack cementation process involves the elevated temperature deposition of Si or Si+B, carried by a volatile metal-halide vapor species to the substrate embedded in a mixed powder pack containing powder of the deposition element, a halide salt activator, and an inert filler. The resulting coating consists of an outer layer of MoSi₂ and a thin MoB layer underneath. During oxidation tests of the (Si+B)-pack alloys, the initial MoSi₂ outer layer is consumed by formation of the Mo₅Si₃ (T₁) phase as one consequence of the transient composition trajectory. Part of the initial transient stage of reaction that yields the T₁ phase from the inward flux of Si and B also leads to the development of the T₂ borosilicide and/or boride phase layer. The T₁ phase that is saturated with B has excellent oxidation resistance and the loss of Si is blocked by the underlying diffusion barrier (T₂). Further, any damage to the outer T₁ layer can be recovered from the underlying T₂ + MoB layer. In effect, the in-situ reaction that yields the T₂ + MoB layer also provides a kinetic bias that allows for the continued existence of the outer T₁ layer and also yields a self-healing characteristic of the coating.

The environmental resistance can be enhanced up to at least 1700°C and also extended against attack by water.  Silicon hydroxide formation during exposure to water vapor has been shown to accelerate oxidation through induced paralinear kinetics. The current Mo-Si-B coatings have proven resistant to water vapor attack up to 1400-1500°C, with estimated service lifetimes of greater than 10,000 hours at 1300°C.  Incorporation of Al into the coating is currently being investigated to further enhance water vapor resistance above 1400°C.

Moreover, the coating strategy can be adapted to apply to other refractory metal systems and ZrB₂ and SiC composites to provide excellent high temperature environmental resistance. A two-step process was adopted for ceramic substrates: 1) deposition of Mo through decomposition of Mo(CO)₆, and 2) codeposition of Si+B though pack cementation. The Mo-Si-B based coating has been proven to enhance the oxidation resistance of SiC/C composites from 800-1500°C. The coating has been further investigated to protect SiC-based materials from undergoing active oxidation at high temperature and low partial pressure of oxygen, environments that may be experienced during hypersonic vehicle flight trajectories. With these advances the multiphase microstructures in the Mo-Si-B ternary system offer useful options for ultrahigh temperature applications.