Mechanisms Of Nitric And Arsenic In The Preparation Process Of Ultrafine Tungsten Carbide Powders

Ultrafine grained WC-Co cemented carbides with high strength and high hardness are widely used in cutting tools,mining machinery,corrosion-resistant and other fields.The grain size and uniformity of WC in WC-Co cemented carbides depend to a great extent on the particle size and uniformity of WC powders.Therefore,the preparation of ultra-fine WC with uniform particle size and good dispersion is the key to the preparation of high-performance ultra-fine grain WC-Co cemented carbides.Previous studies of our group have shown that As element easily forms WAs₂ with W in the process of WO₃ hydrogen reduction to prepare tungsten powders,which provides nucleation core for W powders nucleation,thus nano-W powders and ultra-fine WC powders can be prepared.When P and As are added together,P-assisted As can further refine W powders and WC powders.Therefore,we are interested in whether As can refine WC powders in the process of preparing WC by direct carbonization of tungsten oxide,and whether N element,as a non-metallic element of the same main group as P and As,can assist As in the preparation of ultra-fine WC powders.In this paper,these problems have been studied systematically.The main research contents of this paper include the following two aspects:On the one hand,we add a certain amount of As element into ammonium paratungstate by wet doping method,and WO₃,WO_2.9.9 and WO_2.72.72 can be prepared by calcination under different conditions,then WC powders can be prepared by direct carbonization.The effects of As on phase composition,morphology and particle size distribution of WC powders prepared by different tungsten oxide and its direct carbonization were studied.On the other hand,intermediate products such as?NH4?_xWO₃ and WN containing As were prepared by calcining APT in NH₃ atmosphere,then carbonized directly to obtain ultra-fine WC powders with uniform particle size.XRD,DSC,SEM and TEM were used to characterize the phase composition,thermodynamic process,morphology and phase structure of the products at different stages.The main conclusions are as follows:?1?As element can reduce the particle size and improve the dispersion of WO₃,WO_2.9.9 and WO_2.72.72 powders.A part of arsenic will volatilizes in the form of As₂O₃during the thermal decomposition process of APT-As precursors,and exists in the form of W₂O₃?AsO₄?₂ in WO_2.72-As.W₂O₃?AsO₄?₂ adheres to or is embedded on the surface of rod-like WO_2.72-As,which promotes the fracture of rod-like WO_2.72-As during carbonization process,it also hinders the coalescence and growth of adjacent WC particles,thus,the uniformity can be improved by weakening the agglomeration of WC powders.?2?The calcined products of APT in NH₃ atmosphere are?NH4?_0.25WO₃ at 500?and?NH4?_0.42WO₃/WN two-phase composite powders at 600?.The average particle size of WC prepared by direct carbonization of single phase?NH4?_0.25WO₃ is 2.23?m,while that of WC prepared by direct carbonization of?NH4?_0.42WO₃/WN-As two-phase composite powders is 0.22?m,and the uniformity of WC powders are also significantly improved.Arsenic are easily enriched in the form of W₂O₃?AsO₄?₂ on the surface of?NH4?_0.42WO₃/WN or distributed between the two phases,which can hinders the growth of?NH4?_0.42WO₃/WN particles;?NH4?_0.42WO₃ and WN distribute evenly,which can hinders the migration and coalescence of WC grains through grain boundaries during carbonization,thus,ultrafine tungsten carbide powders can be produced.