| Hot-dip galvanizing is widely used as one of the most important methods for resisting corrosion in metal products. However, the immersed bath hardware (e.g. the sink and stabilizer rolls) is subject to corrosive attack by the molten bath material (Zn or Zn-Al alloy) and often lead to premature failure. In addition to the corrosive effects of the molten bath material, the bath hardware is subject to erosive and abrasive attack from the hard intermetallic particles formed within the bath, so the working condition is very agressive. Thermal sprayed coatings are often used to protect the immersed bath hardware because of its excellent abrasive and erosion resistance. However, the intrinsic brittleness of the ceramic coating often lead to spallation failure, which limits its application range. In this paper, a novel ternary borides based ceramic coating containing amorphous and nanocrystalline phase was developed and does not contain metal binding phase by analyzing the research status of corrosion protection technique against molten Zn or Zn-Al, and combining with the corrosion mechanism of bath hardware against molten Zn or Zn-Al. Research results indicates that the ceramic coating contain amorphous phase and nano-crystal. The brittleness of ceramic coating is expected to be resolved because that nano-crystal can improve the toughness of materials and poor corrosion resistance of cermet coating also is expected to be effectively overcome. So the work has important theoretical and practical values.The technical routine of this research is as follows: firstly, we calculate the phase equilibrium thermodynamics of Co-Mo-B three system using CALPHAD (Calculation of Phase Diagram) technique, in order to guide the design for the composition of original powder. Following, the ternary borides based ceramic powder was prepared by the process of wet ball-milling, spray drying, high temperature sintering and calssifying. The sintered powder has good spherical shape and small size distribution so as to have good fluidity and high loose packing density, the mean particle size is about 32μm. Ternary borides based ceramic coating was prepared on the surface of 316L stainless steel by high velocity oxy-fuel (HVOF) thermal spray. The microstructures, physical and mechanical properties of the ceramic coating and powder were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential thermal analysis (DTA), differential scanning calorimerty (DSC), thermal dilatometer, micro-indentation and mercury intrusion porosimeter (MIP). The formation of amorphous phase and nanocrystalline phase were analyzed by thermodynamics principle. Finally, the corrosion resistance and mechanism of the ternary borides based coating against molten 55%Al-43.5%Zn-Si alloy were studied. The main work and conclusions in this study are described as the following:The composition of the original powder is: Co: 22.92wt.%,MoB: 54.32wt.%,CrB: 18wt.%. The borides-Co slurry exhibits characteristic of pseudoplastic fluid within basic scope. As the shear rate increases, the viscosity decreases. The optimum content of binder system was 0.5wt.% referred to solid, the best content of A15 as dispersant was 2.4wt.%, and the solid concentration is about 45wt.%. The optimized slurry shows good dispersity and stability and fit for spray pelletization.The suitable temperature range for the spray pelletization is 300-325℃. In this scope, the maximum collection rate of dry agglomerations is 73% and the agglomerations have good spherical shape and good fluidity. The optimum sintering temperature for the agglomerations is 1290℃,the sintered powder is composed of CoMo2B2, CoMoB, MoB and CrB,but no oxide and others phase were detected. The morphology of the sintered powder is characterized by a porous internal structure of particles and no conglutination phenomena were observed.The HVOF sprayed ternary borides basesd coating is a very dense coating with homogeneously dispersed micropores, while cracking or large amounts of unmelted particles are not observed. The surface connected porosity and mean pore diameter of the coating are 6.38% and 0.16μm, respectively. The surface connected porosity in the coating presents a typical bimodal pore size distribution. XRD results indicate that the coating consisted of CoMo2B2(45.8wt.%), CoMoB(19.7wt.%), MoB(2.3wt.%), CrB(16.4wt.%) and amorphous phase(15.8wt.%), the particle size of the nanocrystalline phase is less than 10nm.The formation of the amorphous phase in the coating was attributed to the high cooling rates of molten droplets and proper powder composition. The nanocrystalline phase could result from annealing of the pervious amorphous matrix during subsequent deposit of molten droplets, existing in the inner of amorphous phase and the interface between amorphous phase and borides by homogeneous and heterogeneous nucleation, respectively.The coating exhibit high microhardness, elastic modulus and fracture toughness due to composing of amorphous, nano-crystal and borides. Micro-mechanical test results show that anisotropy exists in microhardness, elastic moduls and fracture toughness between the cross section and the surface of the ceramic coating. The ceramic coating exhibits a bimodal distribution of microhardness values, as evidenced by their weibull plots. The adhesive strength of the coating decreases with increasing the thickness of the coating.?Coefficient of thermal expansion (CTE) for the ternary borides based coating, bond coating and substrate are 8.9×10-6/K, 14.3×10-6/K and 17.2×10-6/K, respectively.The immersed test shows that the lifetime of the ceramic coating is as high as 600h in the molten Zn-Al alloy and is more higher than those of WC-Co and low-carbon WC-Co cermet coating. The equilibrium contact angle of Zn-Al alloy melt on the non heat-treated ceramic coating (TB) is smaller than that of the heat treated ceramic coating (HTTB), which is attributed to the amorphous phase contained in the TB coating.After 50h immersion test, an erosion area with the thickness of 20μm appears at the TB coating while erosion area was not found at the HTTB coating. After 600h immersion test, the thickness of the erosion area at TB coating was not enlarged and the erosion area still does not appear at the HTTB coating. The corrosion mechanism of the TB coating is associated with the amorphous phase contained in the ceramic coating. Although it is generally accepted that the atomic bonding of amorphous alloy is primarily metallic, atoms in amorphous alloy are comparatively more active than those in crystalline alloy because the amorphous alloys are in thermodynamically metastable states. Atoms in amorphous alloy easily dissolve and diffuse in the molten alloy during wetting process. In molten Zn-Al alloy of 600℃, the amorphous phase would crystallized at a certain time, the erosion of the molten Zn-Al also stopped.After immersed for 600h, the failure modes of the TB coating exhibit as spallation and delamination at the ceramic coating. Horizontal microcracks parallel to the ceramic coating/bond coating interface are generated at the ceramic coating, which result in the failure of the ceramic coating. The microcracks in the ceramic coating are mainly caused by two reasons:1. Damage caused by collision between the ceramic coating and hard intermetallic particles formed within the bath during the rotation process;2. Thermal stress damage originated from mismatch of CTE of ceramic coating and that of substrate.
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