Molybdenum (Mo) plates are recognized as exceptional high-temperature structural materials among refractory metals, maintaining high strength and hardness at temperatures up to 1500°C. However, their application is limited to vacuum, inert, or reducing atmospheres. Despite their high melting point, low thermal expansion coefficient, excellent electrical and thermal conductivity, and superior corrosion resistance, molybdenum and its alloys undergo rapid oxidation in high-temperature oxidative environments, leading to a loss of their exceptional properties. Therefore, enhancing the oxidation resistance of molybdenum plates is crucial for broadening their application scope and improving durability.
Oxidation Mechanisms and Challenges
In high-temperature oxidative conditions, molybdenum and molybdenum alloys readily form volatile molybdenum trioxide (MoO3), which degrades their performance. The formation of this oxide layer disrupts the protective oxide film typically present on molybdenum surfaces, allowing oxygen molecules to penetrate deeper into the material, accelerating the oxidation process.
Strategies for Enhancing Oxidation Resistance
Several strategies have been explored to enhance the oxidation resistance of molybdenum plates, primarily focusing on surface coatings, alloying, and controlling oxidation conditions.
Surface Coatings
Silicides and Composite Coatings: Molybdenum disilicide (MoSi2) has emerged as a promising coating material due to its ability to form a protective silica (SiO2) film at temperatures exceeding 1200°C. Research has shown that MoSi2 coatings prepared using atmospheric plasma spraying and in-situ chemical vapor deposition (CVD) methods can significantly improve the oxidation resistance of molybdenum plates. Additionally, MoSi2/MoB composite coatings exhibit enhanced durability by blocking and delaying the diffusion of silicon at high temperatures.
Other Coatings: Coatings such as alumina (Al2O3), aluminum nitride (AlN), and silicates can also form dense protective films on molybdenum surfaces, further improving their oxidation resistance. The choice of coating material depends on the specific application environment.
Alloying
- Alloying molybdenum with other metals, such as aluminum (Al) and cobalt (Co), can enhance its oxidation resistance by forming stable oxide films on the surface. Aluminum, in particular, has been shown to improve the antioxidant properties of molybdenum when alloyed in low concentrations (0.17 wt.% Al). High-alloyed binary molybdenum alloys also exhibit significant improvements in oxidation resistance and lifetime.
Controlling Oxidation Conditions
-Strictly controlling the oxidation temperature is another key factor in protecting the oxidation resistance of molybdenum alloys. Excessive oxidation temperature can damage the protective film on the surface of molybdenum alloy, causing oxygen molecules to penetrate into the interior of molybdenum alloy for oxidation reaction, thereby reducing the material’s oxidation resistance.
Silicon’s Role in Enhancing Oxidation Resistance
Silicon (Si) plays a significant role in enhancing the oxidation resistance of molybdenum plates. When molybdenum plates are exposed to high-temperature oxidative environments, silicon reacts with oxygen to form a dense silicon oxide (SiO2) protective film, which acts as a barrier against further oxidation. This protective layer is crucial for maintaining the stability and durability of molybdenum plates in high-temperature applications, such as furnace linings, where they can effectively resist oxidation and extend the service life and efficiency of the furnace.
Enhancing the oxidation resistance of molybdenum plates is essential for improving their durability and expanding their application scope. Surface coatings, alloying, and controlling oxidation conditions are effective strategies for achieving this goal. Among them, silicides and composite coatings, particularly MoSi2 and MoSi2/MoB, have shown great promise in improving the oxidation resistance of molybdenum plates. Future research should focus on optimizing coating preparation techniques, exploring new coating materials, and developing more efficient alloying strategies to further enhance the durability of molybdenum-based materials in high-temperature environments.