Oxidation resistance of a Mo-W-Si-B alloy at 1000-1300 °C: The effect of a multicomponent Mo-Si-B coating

The oxidation behavior and microstructures of a three phase Mo50W20Si15B15 alloy containing (Mo, W) solid solution, (Mo, W)(5)Si-3 and (Mo, W)(5)SiB2 were investigated over the 1000-1300 degrees C temperature range. At 1300 degrees C, the alloy exhibited a large initial mass loss and eventually transitioned to a steady state behavior associated with the development of an amorphous borosilica scale. The oxidation resistance at lower temperature was un-satisfactory due to the pesting of Mo and W. Grazing Incident Angle X-ray Diffraction (GIXRD) measurements during the initial transient stage of oxidation identified the evolution of surface oxide products that were responsible for the large weight loss and pesting behavior. In order to address the large initial weight loss and the low temperature pesting behavior, the alloy was coated with a Mo-Si-B based coaling by employing the pack cementation process followed by a conditioning treatment at 1450 degrees C. The microstructures of the coatings and the interlayer between the base alloy and the coaling were studied, and the oxidation resistance of the coated alloys was evaluated over the 800-1300 degrees C temperature range. The coated alloy showed improved oxidation resistance with nearly zero mass change for extended period of time over the entire range of temperatures studied.