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Development of Durable Spray Coatings in Molten Aluminum Alloy


 A novel thermal spray material of MoB/CoCr with high durability in molten aluminum and/or zinc alloys has been developed to utilize for die casting parts and for galvanizing bath parts. The durability of the MoB/CoCr coatings prepared by high velocity oxy-fuel (HVOF) spraying has been investigated using a molten-metal immersing tester. It has been found that durability of the MoB/CoCr coating in the molten aluminum and Al-45%Zn alloys is much higher than that of the conventional surface treatments, such as physical vapor deposition, nitridation and spray coatings of conventional materials, such as WC/12%Co, WC/10%Co/4%Cr, Cr3C2/NiCr, Al2O3 and ZrO2-8%Y2O3. Preservation of the crystal structure and no decrease in coating thickness due to dissolution or delamination of the MoB/CoCr coating were seen after long term immersing test of about 600 hours. Further, the molten alloys were easily dropped from the specimens of the MoB/CoCr coatings during the immersing test, suggesting much lower reactivity. On the other hand, an adhesion of the alloys on the coating surface of other specimens was clearly observed. This means that the MoB/CoCr coating is excellent in increasing the lifetime of mechanical parts, which come into contact with molten metal and/or alloys.