HVOF Coatings of MoB/CoCr Against Erosion by Molten Al-Zn Alloy

Abstract

Abstract MoB/CoCr with excellent durability against molten metal has been developed to be applied to aluminum die casting parts in automobile and to hot continuous dipping rolls in Zn and Al- Zn plating lines for steel industries. Our previous study has revealed that the MoB/CoCr coating prepared by high velocity oxy-fuel (HVOF) spraying has much higher resistance against attack of the molten Al-45wt%Zn alloys compared to conventional WC/12wt%Co coating from evaluation by laboratory scaled molten metal immersion test. However, performance with no damage is only limited when applying onto the substrate with smaller thermal expansion similar to the MoB/CoCr coatings. This paper describes the solution to prepare the MoB/CoCr coating without damage onto austenitic stainless steel of AISI316L with larger thermal expansion, whose material is commonly utilized for the hot continuous dipping rolls. It has been found that use of undercoat is effective to reduce the negative influence of large difference in thermal expansion between the MoB/CoCr coating and substrate of AISI316L. Optimized thickness of topcoat and undercoat and its combination showed intrinsic performance of low reactive MoB/CoCr without any mechanical damage such as crack and/or peeling.