Formation of Fe-Cr-Mo Alloy Metallic Glass Coating Using High-Velocity Oxy-Fuel Apparatus with Hydrogen Gas and Its Microstructural Transition at Elevated Temperatures

To improve high-velocity oxy-fuel (HVOF) sprays and reduce CO2 emission, an iron-based metallic glass coating produced using HVOF apparatus with hydrogen gas without a fusing process was investigated. Crystallization phenomena of the metallic glass were also evaluated at elevated temperatures. The Fe-Cr-Mo-based alloy was sprayed on a mild steel substrate using a specific gun with hydrogen gas, and metallographic observation revealed that the alloy was successfully coated on the substrate even when using the hydrogen gas. In addition, the corrosion resistance was investigated by performing a combined cyclic corrosion test. Significant corrosion was prevented until 1578 h by a sealing treatment even without a fusing process. The metallic glass coating was heat-treated at 500 to 800 degrees C, and then X-ray diffraction analysis was performed. In the X-ray diffraction profile, the intensity of the observed broad peak from the metallic glass decreased with increasing temperature and holding time, while sharp peaks from the crystal phase appeared. The crystallization process was successfully predicted from the Johnson-Mehl-Avrami equation regarding nucleation and growth of crystal grains from the glass phase. Although the Vickers hardness of the as-sprayed specimen was 778 HV, it was improved to 1029 HV at approximately 80% crystallinity; thus, the nanoscale crystals enhanced the hardness of the metallic glass.