This study shows how the impact resistance of WC-Cr 3 C 2 -Ni coatings can be improved by as much as an order of magnitude through the addition of an undercoat with suitable hardness. It also investigates the effect of powder modifications and substrate hardness. Undercoats with Vickers hardness ranging from400-600 HV provided the biggest increases in impact resistance, but at around 700 HV and above, they are shown to have the opposite effect. The influence of the process used to apply the undercoat and the magnitude of the impact load used for testing are also considered in the study.

Cavitation erosion is one of the serious problems that affect lifetime and performance on hydraulic machineries and its components. Surface modification techniques is required to apply to hydraulic components, such as water turbine and radial flow pump, in general. Here, the techniques, which can easily be applied to large area, is must be required for actual applications in the fields. Thus, thermal sprayed WC cermet coatings with high abrasion resistance under wet conditions are one of the promising solutions as the protective coatings against cavitation erosion. In this study, WC/Cr 3 C 2 /Ni and WC/Co/Cr coatings prepared using a high-velocity oxygen fuel (HVOF) spraying have been evaluated by a magneto-striction vibratory apparatus to investigate the cavitation erosion resistance. The microstructures of the sprayed coatings after the cavitation erosion test have been analyzed by SEM to clarify the erosion mechanism. It has been found that the cavitation erosion resistance of the WC cermet coatings is significantly affected by primary particle size of WC, powder compositions and spray conditions. In the WC/10wt%Co/4wt%Cr, the coating with WC average particle size of 6.3 µm sprayed by optimized spray conditions achieved the highest cavitation erosion resistance than the other WC coatings as well as base materials of stainless steel used in the actual components.

High velocity oxy-fuel (HVOF) sprayed cermet coatings are required in various industrial fields due to their excellent properties, such as combination of wear resistance, corrosion resistance, high hardness, high bonding strength and stability under high temperature. In order to utilize them in the fields, optimization of composition and structure of the coatings are essentially important meaning that both spray powders and spray conditions are key process parameters. In this paper, developed spray powders of cermet materials are introduced for the specific applications, where 1. cavitation erosion, 2. mechanical impact, 3. corrosion by molten alloy and 4. general abrasive wear are major factors that damage the coatings. In order to solve these factors, HVOF coatings of 1. WC/Co/Cr with large WC particle, 2. WC/Cr 3 C 2 /Ni with addition of metal or alloy, 3. MoB/CoCr composed of double boride and 4. WC particle size in WC based cermet, are proposed and these merits are described.

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.

MoB/CoCr, a novel thermal spray material, with high durability in molten Al and/or Al-Zn alloys has been developed to utilize for die casting parts of Al alloy, and for hot continuous dipping roll in Zn and Al-Zn plating lines. The durability of the MoB/CoCr coatings prepared by high velocity oxy-fuel (HVOF) spraying has been investigated using a molten metal immersion tester. The immersion tests revealed that the MoB/CoCr coating has much higher durability without dissolution in the Al alloys than conventional spray coatings, such as WC/12%Co and WC/10%Co/4%Cr, as well as various other surface modification methods. The MoB/CoCr coating also has showed higher durability in the molten Al-Zn alloy when the coefficient of thermal expansion (CTE) of the substrate is similar to that of the MoB/CoCr. The lifetime of the coating on stainless 316L substrate, widely used as the hot continuous dipping roll, was insufficient because of the generation of cracks due to a large difference in the CTE between the MoB/CoCr coating and substrate. Basic anneal examinations have revealed that optimization of coating structure by changing undercoat material and its thickness is effective to improve the lifetime.