Study On Calcination And Reduction-carbonization Processes Of WC-Co Thermal Spray Powders

The WC-Co coating can maintain good friction resistance and wear resistance at high temperature.In the related fields,it has successfully replaced the high-polluting,hard chromium electroplating process.The traditional process has disadvantages of long process flow,large energy consumption,high cost and unstable powder quality.The precursor powders with good spherical structure can be prepared by controlling the spray granulation parameters.Because of its hereditary structure,WC-Co powder has good fluidity and can be directly used in thermal spraying process.Therefore,the WC-Co thermal spraying powder produced by this method has the advantages of short process flow,low energy consumption,simple process and high efficiency.Based on this,the precursor powders produced by spray drying of ammonium tungstate,cobalt acetate,carbon black or organic carbon were used as raw materials.The influence of carbon source and synthesis atmosphere activity on thermal spraying powder was mainly studied in the reduction carbonization stage.The process of preparing thermal spraying powder with good performance was explored.In this thesis,the composite salt precursor powder prepared by spray drying,carbon black or organic carbon as carbon source,was used as the raw material.The effect of two types of carbon source in H₂ atmosphere on the thermal spraying powder was studied.Because of the high activity of organic carbon,the WC-Co thermal spraying powder prepared in the atmosphere of H₂ is easy to decarbonize.Therefore,the synthesis atmosphere of organic carbon as carbon source was changed from H₂ to N₂,in order to research into the process of preparing WC-Co thermal spraying powder in N₂ atmosphere.In order to supplement influences from the reactivity of reactants,the most suitable process for preparing WC-Co thermal spraying powder was obtained by using carbon black as the reference.Through X-ray diffraction,SEM,carbon-sulfur analyzer and laser particle size meter,the present study analyzes the microstructure,carbon content and particle size distribution of thermal spraying powders obtained.The results indicate that different carbon sources have different carbonization rates in two atmospheres.In the H₂ atmosphere,carbon black is easier to prepare thermal spraying powders than organic carbon.In the N₂ atmosphere,organic carbon is more easily prepared WC-Co thermal spraying powder than carbon black.In the H₂ atmosphere,the WC-Co thermal spraying powder is easy to form clastic structure.As a result,the powder of small particles increased which reduced the powder proportion applied to thermal spraying process.In the N₂ atmosphere,the WC-Co thermal spraying spherical powder has clastic structure,and the particle size of small particles is smaller.The average particle size of secondary particles is larger than that of H₂.The distribution of powder particles is concentrated,and the normal distribution trend is obvious.Once particle size becomes small,and the particle size fluctuation becomes small.It is beneficial to prepare ultrafine crystal WC-Co thermal spraying powder.The best technology of preparation of WC-Co thermal spraying powder is to adopt organic carbon as carbon source in N₂ atmosphere.This process needs low energy consumption and has high safety coefficient.The powder particle size is small and the secondary particles distribution is concentration.It is suitable for high powder proportion of thermal spraying process.Therefore,under the conditions of the process,950oC for 120minutes,using organic carbon as carbon source,it can prepare thermal spraying powder with the primary particle size less than 100 nm.