Study On Template Synthesis Of Nano Tungsten Carbide Powders And Its Application In Wc/ni Composite Coatings

The preparation of nano tungsten carbide(WC)powders was studied.Porous carbon materials,tungstic acid and ammonia water were used to synthesize nano WC by in-situ reaction.Nickel coated nano WC powders were prepared by heterogeneous reaction.Different volume fractions of nickel coated WC powders were mixed with F102 powders to prepare feedstock materials of flame spraying.WC particle reinforced nickel matrix composite coatings were prepared by flame spraying and then laser remelting,and the wear-resistance of the coatings was studied.The impacts of different carbon materials,W/C ratio and reaction time on morphology and particle size of WC powders were investigated.The results showed that the reaction time of the expanded graphite was much longer,and some W₂C was observed in WC powders.However,powders prepared by active carbon were much purer;and their size was about several nanometers.The WC powders were obtained when the molar ratio of W/C was 1:2.When the reaction time was 90 minutes,the product was pure WC and the particle size was 92nm.According to the traditional thermodynamics theory,the Gibbs free energy was calculated.The results showed that when the temperature over 1200K,this reaction can spontaneously carry through,and this process was endothermic reaction.The study of kinetics showed that the mechanism of the reaction was diffusion.The effects of different preparation technology on the structure and morphology of the coatings were discussed.The results indicated that,the pores of micro WC/Ni coatings prepared by flame spraying were fewer than those of nano WC/Ni coatings', and WC particles distributed in WC/Ni coatings uniformly.The coatings treated by laser remelting were denser than those of untreated,but there were some cracks on the surface of micro WC/Ni coatings.The components of the nano WC/Ni coatings prepared by flame spraying were similar with the micro WC/Ni coatings prepared by two processes.The main phases of the coatings wereγ-Ni,BNi₃,WC,W₂C,M₆C, M₂₃C₆.And nano WC/Ni coatings after laser remelting had the same phases besides Cr₄Ni₁₅W.The influences of different WC volume fraction and particle size on hardness and wear resistance of coatings were studied.The results indicated that when the volume fraction of micro WC was 4%,the hardness and wear resistance of composite coatings were better than those of other volume fractions.Laser remelting could improve the hardness and wear resistance,and increased the volume fraction of micro WC to 6%.When the volume fraction of nano WC was 6%,the better wear resistance property of composite coatings was obtained.Because laser remelting could efficiently reduce the pores in nano WC/Ni coatings,the wear resistance of the coatings was improved.When the volume fractions of different kinds of WC powders were the same,and treated by laser remelting,the wear resistance of the nano WC/Ni coatings was better than that of the micro WC/Ni coatings'.