A logical method of increasing the oxidation resistance of refractory metal alloys and superalloys is to coat them with some protective material. Coating materials can be weaker but more resistant to oxidation than the metal to be coated. Little work has been done on coating superalloys since they already have good oxidation resistance. However, proprietary diffusion coatings of nickel and aluminum or iron-aluminum reduce intergranular attack and raise useful operating temperatures to as high as 2300 F for Alloy No. “713C”, for example. No promising coatings have been developed for chromium and vanadium alloys.
Considerable research has been done and is in progress on the problem of protecting tungsten, tantalum, molybdenum, and columbium alloys from oxidation at high temperatures. Molybdenum as the metal or alloybase substrate has received the most attention. Additional research will be required on oxidation-resistant coatings for these refractory metals before reliable coating systems will be available for service at high temperatures in air. However, coated molybdenum-alloy jet-engine buckets have been satisfactory on an experimental basis.
A number of processes have been used for applying protective coatings. These processes include electrodeposition, flame spraying, vapor and pack deposition, cladding, and enameling. More recently, flame spraying with a plasma torch has been used for applying coatings. Progress has been made in overcoming many of the original problems, such as matching thermal expansion of the coating to the base metal and compatibility of the coating and base metal. However, many of the coatings are brittle, or require firing at temperatures above the recrystallization temperature of the base metal, or offer protection only to 2000 F, or are porous.
For molybdenum and molybdenum alloys, nickel-chromium-boron, nickel-silicon-boron, aluminum-chromium- silicon, and molybdenum disilicide modifications have been applied by flame spraying with considerable success. Siliconized and chromized coatings are the foremost among coatings that can be applied by vapor deposition. Excellent coverage, uniformity of coating, and control of thickness are characteristics of vapor-deposited coatings. Modification of the chromized coating has resulted in a reliable coating for use up to about 2700 F.
These coatings and combinations of them have been tried on components used in gas turbines, ramjet engines, and other applications with marked improvement in service performance. However, further improvements in coatings are needed for molybdenum alloys to achieve the high-temperature service that is indicated by the mechanical properties.
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