Formation Of Carbide Coatings On Steels’ Surface By Thermal Diffusion (TD) Processes In Fluoride Molten Salts

Carbide coatings which usually have high hardness,high wear,corrosion and oxidation resistance have a bright future in the field of surface coating.The thermal diffusion process（TD process）is an important deposition technique to obtaine carbide coatings.The mostly used molten salts in TD processes were borax salts and chloride salts.TD in borax have many problems,such as high corrosion,poor fluidity,high viscosity,difficulty in cleaning residues and aging of salt bath and limited kinds of carbide coated.TD in chloride salt have problems like,high corrosion and the sublimation of reactive elements.These problems restrict the development and application of TD technique in formation of carbide coatings.To conquer the disadvantanges of borax and chloride salt in TD processes and to improve the properties of carbide coatings,high purity LiF-NaF-KF（46.5-11.5-42 mol.%,FLiNaK）salts made by TMSR are used to study the formation of carbide coatings via TD processes.The main contents are summarized as bellow.Invent the facility used for TD processes in fluoride.This facility can not only satify the high temperature（usually higher than 800°C）needed for TD processes,but also can control the atmosphere of treatment environment.Besides it is convenient to put in and get out of samples in this facility.High quality carbide coatings were formed via TD processes in FLiNaK salt.The effect of treatment time,treatment temperature,composition of molten salt and substrates on the formation of chromium carbide coatings were studied.The samples after treatment were analyzed by weighting,XRD,SEM-EDX.The salt composition was tested by ICP-OES after TD processes.The results are listed below:in the certain treatment temperature,the thickness of the coating is increased with the increasement of activity of carbon in the substrate.It is favor to form chromium carbide coating in the molten salt containing high concentration of atomic Cr.With increasing treatment time,the thickness and the mass gain of samples increases.For ductile cast iron QT600-3,the microstructure of chromium carbide coating is Cr₂₃C₆,M₇C₃ and M₃C from the outermost layer to the substrate.As for the formation of chromium carbide coatings on ductile cast iron QT600-3in the fluoride salt,the variation of thickness of chromium coatings with the treatment temperature and treatment time can be expressed as follows:（?）The mechanical properties of chromium carbide coatings formed on gray cast iron HT250 were studied.The hardness of samples increase 6 times to 20.5 GPa after coated chromium carbide coatings.And the tribological behavior is also improved.The zirconium carbide and tungsten caibide coatings were formed on H13 steel via TD processes in FLiNaK salt.The zirconium carbide coating is brownish black flake-like shape with the thickness about 2μm and the structure is ZrC.The tungsten carbide coating in greyish-green granular shape with the thickness approximately 7μm and the structure is Fe₃W₃C.