Preparation Of Thermal Diffusion Coatings By Molten Fluorides

Molten salt thermal diffusion surface alloying is a method to prepare surface alloys or compounds in molten salt environment,which is a combination of chemical deposition and solid diffusion.The molten salt thermal diffusion process is convenient,which is suitable for all kinds of complex substrates.The coating is metallurgical bonded with substrate and owns better adhesion.So far,borax molten salt is generally used for molten salt thermal diffusion process and chloride molten salt is used in a small amount.Some problems have been found in traditional melts,such as high viscosity,difficult to handle microporous structural parts,difficulty in cleaning residual salts and easy volatilization.In contrast,fluoride molten salt has good fluidity,low saturated vapor pressure,good thermal conductivity,and strong ability to dissolve active elements.It is more suitable for the surface metallization of materials under milder conditions.Based on the above background,this project conducts research on the preparation of thermal diffusion coating by molten fluoride,including the research on iridium hafnium intermetallic compound coatings and carbide coatings.The noble metal iridium has a high melting point and a very low oxygen permeability coefficient,and is used as an ultra-high temperature thermal protection material.However,iridium tends to form volatile oxides under high temperature oxidation conditions,leading to material thinning failure.In order to reduce the volatilization loss of iridium and prolong the service life of the material,the iridium-hafnium alloy coating was prepared by molten salt thermal diffusion method at 950-1200℃for 5 hours.The thickness of the Ir-Hf diffusion coating varied from 0.7μm to5μm,and the phase composition of the coating was Hf₂Ir,Hf Ir and Hf Ir₃.The formation of alloy phase can be attributed to the reaction diffusion.This study shows it is feasible to prepare iridium-hafnium alloy coatings at relatively low temperature and in a short time.The maximum thickness of the coating can reach 1.7 microns at 1000℃for 9hours.The coating thickens gradually with the increase of treating time,but the growth rate becomes slower,and the coating thickness changes with time in a parabolic trend.This clarifies the diffusion growth mechanism of iridium-hafnium alloy coatings.Compared with the treatment time,the effect of the treatment temperature on the coating is more significant.The higher the treatment temperature is,the faster the iridium-hafnium interdiffusion rate and the faster the coating growth.Accordingly,the growth rate of the coating can be controlled.Electrochemical experimental results showed that the current generated by the iridium-hafnium system at 950℃decayed to nearly 0 m A after 10 minutes,which further confirms that the growth of the iridium-hafnium alloy coating is controlled by diffusion.Cemented carbides have been widely used as cutting tools because of superior properties such as high hardness,mechanical strength.In the long-term cutting process,the cemented carbide tool will occur attrition wear on the one hand,and high temperature oxidation will occur on the other hand,which seriously restricts the service life of the cemented carbide tool.In order to improve the service life of cemented carbide,chromium carbide and niobium carbide coatings were prepared.The initial stage of growth of chromium carbide coating on cemented carbide surface was studied by molten salt thermal diffusion method at 900℃.The research showed that chromium carbide can be formed in a few seconds,and the entire sample surface can be covered after 5 to 10 minutes.The hardness of chromium carbide coating is related to its composition.The composition of the chromium carbide coating can be controlled by controlling the reaction time.In the early stage of coating formation,the main components of the coatings were Cr₃C₂ and Cr₇C₃,and Cr₂₃C₆ began to form and increased with the extension of the treatment time,which has significant importance for improving the performance of the chromium carbide coating.The variation of coating thickness with time shows a parabolic trend,and the growth rate constant K was 1.36×10^-15 m²/s,indicating the growth process of chromium carbide coating follows the diffusion mechanism.The nanoindentation hardness of chromium carbide coating prepared at 900°C for 0.5 h was about 23 GPa,which is the maximum experimental value of hardness on metal base materials so far.The initial stage of growth of wear-resistant niobium carbide coating on cemented carbide surface at 900°C was studied.It was shown niobium carbide was formed by thermal diffusion of molten salt at 900°C for 5 seconds,and the niobium carbide coating can cover the entire surface in 10 to 15 minutes.The composition of the niobium carbide coating can be controlled by controlling the reaction time.In the initial stage of coating formation,the main component of the coating was Nb C.With the extension of the treatment time,Nb₂C began to form and gradually increased,which is of great significance for controlling the composition of the coating.The variation of coating thickness with time shows a parabolic trend,and the growth rate constant K was 0.90×10^-15 m²/s,indicating the growth of niobium carbide coating is controlled by diffusion.The nanoindentation hardness of niobium carbide coating prepared at 900°C for 1 hour was about 25 GPa,which was 25%higher than that of the substrate.Wear-resistant niobium carbide and chromium carbide coated cemented carbide have certain industrial application value.