Controllable Preparation Of Ultrafine WC Powder Induced By Co And Its Regulation Mechanism

There is a inherent contradiction between hardness and strength of tradition YG cemented carbides,in other words,high hardness leads to low strength and high strength leads to low hardness.It has been shown that the mechanical properties of cemented carbides increase with the decrease of WC grain size.When the grain size reaches ultlafine or nanoscale,the hardness and strength of cemented carbides can be improved simultaneously.Therefore,the ultrafine and nano-sized WC grains in the alloy can effectively solve this contradiction and obtain double-high cemented carbides with high hardness and high strength.At present,ultrafine or nanocrystalline cemented carbides have become the development direction of cemented carbides in the world.The synthesis of high quality WC-Co composite powders is the key to prepare ultrafrne or nanocrystalline WC-Co cemented carbide.This subject aims at some problems about low efficiency,uneven particle size and serious agglomeration of WC powder prepared by direct reduction carbonization of tungsten oxide.The WC is prepared by a stage-reduction carbonization process,in which the WO3 obtained by decomposing ammonium metatungstate(AMT)is firstly reduced to WO2.72 and then further reduced in carbonization.The WO2.72 with high aspect ratio was induced by adding cobalt.Further,the WC-Co composite powders were controllable preparation,and the mechanism of cobalt in the preparation of powder was studied.Details are as follows:The hydrogen reduction process of tungsten oxide is calculated based on thermodynanics,and the thermodynamics conditions for the formation of WO2.72 and its phase are determined.On this basis,the effects of reduction time,temperature and cobalt content on powder morphology and phase in the process of hydrogen reduction to prepare WO2.72 are studied.The results showed that the AMT-Co(CH3COO)2 precursor powder synthesized by spray drying is completely converted to WO3-CoWO4 after calcining at 600? for 1 h;By controlling the hydrogen flow rate,hydrogen reduction temperature and time to influence the rod-like diameter,length and purity of WO2.72,and the optimum process for obtaining WO2.72 powder by hydrogen reduction is 800? for 25 min.Different raw materials have great influence on the morphology of the prepared WO2.72.For example,the diameter of rod-like WO2.72 prepared from AMT as raw material is about 340 nm,while the diameter of rod-like WO2.72 prepared from the two materials with cobalt content of 8wt.%of ATM-C4H6CoO4 and cemented carbide recovery powder have no significant difference,with an average diameter of 250 nm and 230 nm,respectively.However,after high-energy ball milling of the latter,the powder particles are in an unstable state with high energy,and the increase of reaction activity promotes the preferential growth of WO2.72 along the(010)crystal plane,which makes the particle length much larger than the former.The effect of cobalt content on the growth of WO2.72 is studied systematically.The results show that the diameter of rod-like WO2.72 decreases first and then increases with the increase of cobalt content.When the cobalt content is 6wt.%s the diameter of rod-like WO2.72 is about 100 nm,and its aspect ratio reaches 22.The reduction carbonization of WO2.72 was calculated by thermodynamics to predict the reduction carbonization path and thermodynamic initiation conditions of the reaction.The effects of different cobalt content,reduction carbonization temperature and time on the phase,Particle size and morphology of the powder are studied to obtain an optimized process for preparing ultrafine WC powder.The experimental results show that the addition of cobalt decreases the reduction carbonization temperature of WO2.72(about 50?)and promotes its reaction.The W2C and WC are formed in pure WO2.72 samples only when the carbonization temperature reached 800?,while WC is formed in specimens at 750?.The 8wt.%cobalt samples are completely transformed after holding time at 800? for 2.5 h,the WC-Co composite powder with pure phase,good dispersibility,narrow particle size distribution and average size of about 110 nm can be obtained.The XRD analysis showed tihat the reduction carbonization path of WO2.72 is WO2.72?WO2?W2C?WC,and the reaction process does not produce W,which is the same as the thermodynamic analysis.Furthermor,with the increase of cobalt content,the reduction carbonization rate of WO2.72 accelerated.When the 2wt.%cobalt is added to the sample,the carbon-deficient phase still exists after holding at 800?C for 5 h.The samples with 4wt%,6wt.%and wt.8%cobalt content react completely after 4h,3h and 2.5h,respectively.The effect of cobalt content on the reduction carbonization of WO2.72 was analyzed from the dynamics.That is,with the increase of cobalt content,the reaction rate constant(k)increases and the activation energy(E)decreases.