Controllable Fabrication Of Co-coated Coarse Grained WC Powder By Fluidized Bed Chemical Vapor Deposition And Its Application

Compared with fine grained cemented carbide,coarse grained cemented carbide has higher toughness and red hardness,higher thermal shock resistance and thermal fatigue properties due to the characteristics of coarse WC grains.Thus they have been widely used in impact tools,wear-resistant and corrosion-resistant parts and cemented carbide coatings.However,there always exist some common industrial application problems such as coarse grain property loss,uneven structure,and pore defects in cemented carbide tools and wear-resistant coatings,which severely restrict their service performance.The root cause of the above-mentioned common problems lies in the conventional ball milling technology process used to prepare composite powder.In order to obtain high uniformity,long-term high-energy load mixing is often required,which seriously damages the original grain size of WC.So,how to synergistically solve the contradiction between the maintenance of WC coarse grain and the high homogeneity of the two phases is a key issue in the preparation of high-performance cemented carbide tools and coatings.Up to now,although a series of physical modification and chemical synthesis methods have been developed,these technologies cannot simultaneously solve the maintenance of WC coarse grain and the high homogenization of the two phases.Therefore,no fundamental breakthrough has been made.In view of the above problems,this thesis proposed a new idea based on FBCVD technology to realize the controllable preparation of high-quality coarse grained Co-coated WC composite powder.The aim was to utilize the characteristics of FBCVD that can realize uniform distribution of the two phases witthout changing the original WC grain size to solve the limitations of the existing methods for preparing high-quality WC-Co composite powder.The main innovative findings achieved are as follows:(1)The feasibility of FBCVD technology for the controllable preparation of high-quality coarse grained WC-Co composite powder was proposed and verified.The CoCl2-H2-Ar reaction system with CoCl2 as the precursor was optimized.The deposition temperature ranged from 750 to 850? and the molar ratio of CoCl2 and H2 should be controlled below 1:5.The deposition and growth mechanism of Co was proved.It was found that Co preferentially nucleated and deposited at the edges,protrusions,steps of WC particles,and the enrichment of Co impurity introduced by ball milling with a Volmer-Weber growth mode.(2)It was revealed that the adhesion of the metal Co deposited on the surface of the WC particles at high temperature was the fundamental cause of defluidization.The complex relationship between the deposition of Co and the fluidization of WC particles was synergistic and competitive.The details were as follows:the long-term fluidization of WC particles was beneficial to increasing of the deposition content of Co,but the increase of the Co content would cause defluidization quickly.It was found that deposition temperature was the main factor for coordinating this competitive relationship.Lowering the deposition temperature was beneficial to increase the fluidization time of WC particles,but it would decrease the Co deposition efficiency.Increasing the deposition temperature could significantly increase the Co deposition rate,but it would reduce the fluidization time of WC powder.The optimal deposition temperature was determined to be 800?.For the conditions tested,the maximum Co content of 3.44 wt.%was obtained.(3)It was found that the Co catalyst deposited on the surface of WC particles by FBCVD had a strong autocatalytic ability in electroless plating.Co catalyst with content of 0.05-0.72 wt.%and particle size of 15-50 nm could be prepared by adjusting the FBCVD temperature and time.Co particle size was the main factor affecting the electroless plating rate.The smaller the particle size,the faster the catalytic reaction rate.The optimal deposition temperature and time for the preparation of Co catalyst by FBCVD was 750? and 3 minutes respectively,corresponding to the Co content of about 0.09 wt.%and the particle size of about 20 nm.The optimized parameters of electroless plating conditions were as follows:temperature of 80?,pH value of 12,complexing agent concentration of 55.0 g/L and reducing agent concentration of 100.0 g/L.The electroless plating rate of Co under this condition was as high as 2.34 mg·g-1·min-1.(4)The grain growth behavior during the hot pressed sintering process of the new composite powder was revealed,and a new sintering system was established with the main purpose of maintaining the coarse grain characteristics.Co-coated WC composite powder greatly reduced the contact probability of WC particles,which effectively inhibit the aggregation and recrystallization of WC grains caused by mutual contact and bonding during the solid phase sintering stage.Compared with the ball milled composite powder,the average grain size of the alloy was reduced by about 8%,The optimal hot pressured sintering temperature and pressure for the preparation of cemented carbide was 1350? and 10-15 MPa,respectively.The cemented carbide prepared under these conditions had excellent mechanical properties of hardness as 1267 MPa,fracture toughness as 14.19 MPa·m1/2 and transverse rupture strength as 2383 MPa.(5)It was proved that the new composite powder had good adaptability in laser cladding and plasma spraying,and a new preparation process of high-performance coating was developed.The metal Co coated on WC particles could effectively avoid the contact among WC particles,reduce the direct oxidation of WC at high temperature and reduce the powder splash caused by the direct contact of the hard phase WC with the substrate.Therefore,it would effectively maintain the coarse grain characteristics,reduce the generation of pores and cracks in the coating,improve the uniformity of the coating structure,inhibit carbon loss and maintain a good interface.The hardness of the laser cladding coating was nearly 5 times higher than that of the substrate,and the wear rate was only 5%of the substrate.The hardness of the plasma spraying coating under this condition was 3.9 times that of the substrate,and the wear rate was only 10%of the substrate,which greatly improved the wear resistance of the substrate.