Environmentally Resistant Mo-Si-B-Based Coatings

High-temperature applications have demonstrated aluminide-coated nickel-base superalloys to be remarkably effective, but are reaching their service limit. Alternate materials such as refractory (e.g., W, Mo) silicide alloys and SiC composites are being considered to extend high temperature capability, but the silica surfaces on these materials require coatings for enhanced environmental resistance. This can be accomplished with a Mo-Si-B-based coating that is deposited by a spray deposition of Mo followed by a chemical vapor deposition of Si and B by pack cementation to develop an aluminoborosilica surface. Oxidation of the as-deposited (Si + B)-pack coatings proceeds with partial consumption of the initial MoSi₂ forming amorphous silica. This Si depletion leads to formation of a B-saturated Mo₅Si₃ (T₁) phase. Reactions between the Mo and the B rich phases develop an underlying Mo₅SiB₂ (T₂) layer. The T₁ phase saturated with B has robust oxidation resistance, and the Si depletion is prevented by the underlying diffusion barrier (T₂). Further, due to the natural phase transformation characteristics of the Mo-Si-B system, cracks or scratches to the outer silica and T₁ layers can be repaired from the Si and B reservoirs of T₂ + MoB layer to yield a self-healing characteristic. Mo-Si-B-based coatings demonstrate robust performance up to at least 1700 °C not only to the rigors of elevated temperature oxidation, but also to CMAS attack, hot corrosion attack, water vapor and thermal cycling.     

Transient oxidation of Mo–Si–B alloys: Effect of the microstructure size scale

The composition and phase constituency of Mo–Si–B alloys are known to be important parameters in determining the oxidation response. For three phase Mo + T₂ + Mo₃Si alloys with constant composition and phase constituency, it is observed that a refined microstructure scale provides superior oxidation resistance. The transient stage of oxidation is shortened and the recession of the alloy is decreased with microstructural refinement. In order to identify the phase interaction during the transient stage, oxidation of each of the three alloy phases, Mo, Mo₃Si (A15) and Mo₅SiB₂ (T₂) has been investigated separately. Quantification of the separate phase size distributions by image analysis was coupled with the individual phase oxidation response to evaluate the overall oxidation behavior and phase interaction effects. A kinetic model for oxidation of Mo–Si–B alloys is proposed that incorporates the key role of microstructure scale on the transient stage and provides guidance for microstructure design.     

Extended Functionality of Environmentally-Resistant Mo-Si-B-Based Coatings

Multiphase Mo-Si-B alloys with compositions which yield the ternary intermetallic Mo₅SiB₂ (T₂) phase as a key microstructure constituent together with the Mo and Mo₃Si phases, offer an attractive balance of high melting temperature, oxidation resistance, and mechanical properties. The investigation of reaction kinetics involving the T₂ phase enables the analysis of oxidation in terms of diffusion pathways and the design of effective coatings. From this basis, kinetic biasing is used together with pack cementation to develop Mo-Si-B-based multilayered coatings with an aluminoborosilica surface and in situ diffusion barriers with self-healing characteristics for enhanced oxidation resistance. While a combustion environment contains water vapor that can accelerate an attack of silica-based coatings, the Mo-Si-B-based coatings provide oxidation resistance in water vapor up to at least 1,500°C. An exposure to hot ionized gas species generated in an arc jet confirms the robust coating performance in extreme environments. To extend the application beyond Mo-based systems, a two-stage process has been implemented to provide effective oxidation resistance for refractory metal cermets, SiC and ZrB₂ ultra-high-temperature composites.     

Effect of Intense Rolling and Folding on the Phase Stability of Amorphous Al-Y-Fe Alloys

A systematic examination of the effect of intense deformation on the crystallization behavior of amorphous Al₈₅Y₁₀Fe₅, Al₈₆Y₉Fe₅, and Al₈₈Y₅Fe₇ alloys demonstrated a strong composition dependence of the crystallization reactions at true strain levels of about −500 pct. Primary crystallization occurs during the deformation of the Al₈₈Y₅Fe₇ alloy, but for the Al₈₆Y₉Fe₅ and Al₈₅Y₁₀Fe₅ alloys, deformation-induced crystallization is not observed at a true strain of about −500 pct. At strain levels of the order of −1200 pct, the Al₈₅Y₁₀Fe₅ alloy develops regions with primary Al, which is not observed during thermal processing of the same amorphous alloy without deformation. In addition, at strain levels of −1200 pct, a deformed Al₈₈Y₇Fe₅ sample displays strong microstructural heterogeneities. Transmission electron microscopy (TEM) analysis showed the presence of nanocrystal dispersions adjacent to shear bands with a total width of about half a micrometer. The results demonstrate that the phase selection during deformation-induced crystallization can deviate from the thermally-induced phase selection. Novel phases and microstructures can thus be obtained from the deformation processing of amorphous alloys. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, FL, under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.

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